Multichromophoric electrochromic polymers: colour tuning of conjugated polymers through the side chain functionalization approach.

Published on 05 March 2014. Downloaded by UNIVERSITY OF ALABAMA AT BIRMINGHAM on 24/10/2014 23:08:51.

ChemComm View Article Online

FEATURE ARTICLE

Cite this: Chem. Commun., 2014, 50, 5413

View Journal | View Issue

Multichromophoric electrochromic polymers: colour tuning of conjugated polymers through the side chain functionalization approach L. Beverina,* G. A. Pagani and M. Sassi Organic electrochromic materials have gained constantly increasing interest over the years with respect to their inorganic counterpart due to essentially two distinctive characteristics: their processability through solution based low cost processes and their wide colour palette. Such characteristic features enabled their application in displays, smart windows, electronic paper and ophthalmic lenses. Alongside the established concept of donor–acceptor polymers, side chain functionalized multichromophoric

Received 8th January 2014, Accepted 28th February 2014

polymers are gaining attention as a highly performing and synthetically feasible alternative, particularly

DOI: 10.1039/c4cc00163j

material. The primary aim of the present article is to review all the results involving the tuning of the

relevant to applications requiring a complete colourlessness in one of the accessible redox states of the native electrochromic properties of simple conjugated polymers through the introduction of a discrete

www.rsc.org/chemcomm

electrochromic molecule as a side chain substituent.

1 Introduction Organic electrochromic materials (OEM) are redox active compounds, characterized by a change in their optical properties upon reduction or oxidation. Many aromatic organic molecules, monodisperse or polymeric, are electrochromic, with performances in some cases exceeding those of their inorganic, technologically well-established, counterparts. Amongst the most attractive, specific characteristics of organics, their solution processability and extreme chromatic diversity represent an invaluable asset for technological applications. In the last 15 years the use of organic electrochromic devices for smart windows, electronic paper, displays and ophthalmic evolved from an intriguing possibility to a technological reality now leading to commercial applications. OEM can be classified into two main families: molecular materials and polymers. With few exceptions, the former are soluble, in one or both redox states in the electrolyte and are hence classified as Type II and Type I respectively.1,2 Viologens represent the prototypical and most studied example of this class of compounds. On the other hand, polymers are classified as Type III materials, as they are solid in all their redox states. This characteristic is reflected in higher write–erase efficiency, longer cycle lives and better memory effect making them particularly suitable for incorporation in the commercial all-solid-state electrochromic devices (ECDs) without the need University of Milano-Bicocca, Department of Material Science, via Cozzi 55, Milano, Italy I-20125. E-mail: [email protected]; Fax: +39 0264485403; Tel: +39 0264485109

This journal is © The Royal Society of Chemistry 2014

for the complex immobilization strategies required by molecular materials. In this case poly[3,4-(ethylenedioxy)thiophene] (PEDOT) surely is the most valuable and extensively studied example of type III material, thanks to its unique combination of thermal, electrochemical and environmental stability, processability and optical properties. The advantages and disadvantages of both polymers and molecular electrochromic materials have been extensively studied and recently reviewed.2–5 The present feature article focuses on a peculiar class of polymeric materials featuring molecular electrochromes as side chain substituents in an attempt to synergistically combine the characteristics of both to enhance final performances.

2 Conjugated polymers After the discovery and rationalization of the properties of polyacetylene, the interest in the study of these fascinating materials rose rapidly. They soon found several applications, based on their unique transport and electrochemical properties: sensors, solar cells, transistors, organic light emitting devices (OLEDs) and, of course, ECDs. Conjugated polymers mostly are p-type electroactive compounds, since upon oxidation positive charges are generated on the polymer backbone, balanced by negatively charged counterions. The positive charges are able to migrate along the polymeric chain and to nearby chains being responsible for conductivity of the doped polymer. The case of n-type conjugated polymers is not unknown but definitely less common.6,7 In conjugated polymers with a non-degenerate

Chem. Commun., 2014, 50, 5413--5430 | 5413

View Article Online


Feature Article

ground state (like poly(heterocycles)), these positively charged states are termed bipolarons, and correspond to localized levels inside the gap of the polymer.8 At low and intermediate doping levels, radical cationic states called polarons populate the polymer chain. As shown by EPR, at the high doping level, spinless bipolaronic states dominate. These represent the first step of the oxidation process. Their formation is accompanied by a structural relaxation that leads to a local distortion of the chain in the proximity of the charge that can be pictured as a quinoid-like structure in which single bonds assume double bonding character (Fig. 1a). Further oxidation leads to the formation of bipolarons, a process driven by their larger binding energy compared to that of polaron couples, despite the coulomb repulsion between similar charges.9 The presence of mid-gap levels in doped (oxidized) polymers deeply influences their spectroscopic properties that become dominated by two sub-gap (low energy) transitions from the valence band to the localized bipolaronic states (Eb1 and Eb2 in Fig. 1). This behaviour is the origin of the electrochromism in conjugated polymers since it results in a huge change in their absorption spectrum. The main optical absorption band in the reduced neutral conjugated polymers is determined by a p–p* transition through the band gap, hence falling in the UV/Vis

Luca Beverina was born in 1975 in Milano, Italy. He graduated in Materials Science from the University of Milano-Bicocca in 1999 working at an experimental thesis on Nonlinear Optical Properties of heteroaromatic containing conjugated materials under the supervision of Prof. Giorgio A. Pagani. He received his PhD in Materials Science from the same University in 2002 discussing a thesis on Multiphoton Absorption L. Beverina phenomena in organic conjugated materials, again under the supervision of Prof. Giorgio A. Pagani. He later joined the group of Prof. Seth R. Marder at the Georgia Institute of Technology for a one year Post Doc period. Later he came back to the University of Milano-Bicocca for another Post Doc year. In 2004 he got a permanent faculty position as Assistant Professor of Organic Chemistry at the same university. In 2007 he was confirmed in the role. Dr Beverina’s present research interests cover organic photovoltaics, electrochromism, rechargeable organic batteries, photonics and biophotonics with occasional excursions in photoand thermoresists. Apart from chemistry, he enjoys hiking, swimming, growing exotic chilli pepper varieties and listening to alternative music. He particularly supports the Norwegian band Ulver.

5414 | Chem. Commun., 2014, 50, 5413--5430

ChemComm

Fig. 1 Band diagrams of bipolarons and polarons, in poly(heterocycles), showing the localized gap states and their occupancy.

region, while the low energy bipolaronic transitions give absorption bands at lower energies. The observed colour change depends upon the energy gap of the polymer. Thin films of polymers with Eg 4 3 eV are colourless in the undoped states while their doped form is coloured because of absorption in the visible region. In contrast, those with lower band gaps (Eg B 1.5 eV) show coloured reduced forms, but possess faintly coloured or colourless doped forms, as an effect of absorption

Giorgio A Pagani is presently a retired Professor of Organic Chemistry and Material Science. He received his PhD Degree in Industrial Chemistry from the University of Milano in 1963. From 1963 to 1980 he was a Chief Researcher at the Italian National Council of Research. During 1972–1973 he was a visiting Professor (Associate) at the College of Arts and Sciences of Northwestern University, EvanG. A. Pagani ston, Ill., USA. From 1980 he was a full Professor in Heterocyclic Chemistry at Milan University, then he joined in 1994 the University of Milano-Bicocca. He is the Italian National Representative at IUPAC (Commission on Physical Organic Chemistry). In 2002 he received the Award of the Italian Chemical Society (Organic Division) for Mechanistic and Theoretical Aspects of Organic Chemistry. His current interests include design, synthesis and characterization of polar (zwitterionic) organic materials for nonlinear optics and multiphotonics, synthesis of heterocycles and their physical organic chemistry, carbanions and nitranions, synthesis and preparation of conducting polymers from polymerogenic heterocycles, organic nanostructured materials, polymeric and sol–gel materials, self assembled multilayers for nonlinear optics, frequency-upconverted dye lasers, optical limiters, and electrochromic organic materials. He is author or co-author of ca 250 papers, many of which are related to Organic Materials.


View Article Online


ChemComm

bands shifted to the NIR region with just small residual absorption in the visible region. The most relevant example of the latter case is offered by PEDOT which possesses a relatively stable and highly transmissive sky-blue oxidized state, a suitable characteristic for a polymer based ECD. This was demonstrated with the realization of the first PEDOT-based ¨s et al.10 As can be noted from Fig. 2, the ECDs by Ingana absorption band of reduced PEDOT lies in the NIR region of the visible spectrum giving its distinctive dark blue-colour, and its doped form (very light blue) has minimal tailing in the visible region from its charged carrier transitions. The main advantages of conjugated polymers over other electrochromic systems are: high colouration efficiency, good mechanical properties, ease of colour tuning via structural modifications on the monomers, low switching times, potentially low cost, and ease of processing. The ease of colour hue tuning is certainly one of the most desired properties and the possibility of accomplishing this task is surely the most remarkable characteristic of conjugated polymers. Many research efforts have been devoted, over years, to the expansion of the ‘‘colour palette’’ of electrochromic conjugated polymers, and several different approaches have been investigated: modification of the HOMO and LUMO positions of the building blocks incorporated in the repeating unit variation of the backbone planarity (p-overlap) using sterically hindering substituents increase of the conjugation lengths using ring fused heterocycles that change the bond length alternation use of a donor–acceptor structure in the repeating unit. A detailed review on electrochromic conjugated polymers and the different strategies to tune the optical transitions associated

Mauro Sassi was born in 1982 in Varese, Italy. In 2007, he graduated in Materials Science from the University of MilanoBicocca with an experimental thesis on luminescent perfluorinated lathanide complexes under the supervision of Prof. Giorgio A. Pagani. In 2011, he received a PhD in Material Science from the same University with a thesis on high performance heteroaromatic M. Sassi electrochromic organic materials under the supervision of Dr Luca Beverina. During his PhD he joined the group of Prof. Milko Van der Boom at the Weizmann Institute of Science for a 6-month period. In 2011, he started a Post Doc period at the University of Milano-Bicocca working on organic batteries and organic photovoltaics. His research interests cover organic photovoltaics, rechargeable organic batteries and electrochromism. In his spare time he enjoys playing the French horn, and hiking.


Feature Article

Fig. 2 Spectroelectrogram of poly[3,4-(ethylenedioxy)thiophene] (PEDOT) film on ITO/glass. The film was electrochemically deposited from 0.3 M EDOT in 0.1 M Bu4ClO4/PC and switched in 0.1 M Bu4ClO4/ACN. Inset shows absorbance at lmax versus potential. Adapted from Thomas.11

with their main chain has been recently published by Reynolds et al.,4,5 and consequently we will not discuss them in detail. That said, we will focus on a less popular yet promising and never reviewed before subclass of conjugated polymers, those incorporating electrochromes as side chain substituents. This peculiar strategy can in principle represent an easy approach to colour tuning with some unique advantages over the others and will be discussed in Section 4. Since it relies deeply on the type and nature of the discrete molecular electrochromes, we will first discuss this class of materials paying attention to structure– property relationships.

3 Molecular electrochromes Many different types of organic and metal–organic compounds are able to undergo reversible redox processes. In some of them this process is accompanied by a modification of the UV/Vis absorption spectrum, making them electrochromic. Such compounds have been used for more than a century as indicators for titration procedures, mostly on a semi-empirical basis. A special position is occupied by those compounds that can undergo reversible redox reactions without involvement of hydrogen ions since these compounds do not have a pH dependent standard potential, and can be easily used with aprotic organic solvents. In fact, the use of these solvents is highly desirable because they present a larger potential window and minimize the possible side reaction of the electrochromic material with O2 and H2 coming from water electrolysis. For these reasons the forthcoming discussion will be restricted to these types of electrochromes. 3.1

Violenes

The first report of a compound belonging to this class dates back to 1879 when Wurster discovered the dye depicted in Fig. 4d, whose structure was lately recognized by Weitz in 1925.12–15

Chem. Commun., 2014, 50, 5413--5430 | 5415

View Article Online


Feature Article

ChemComm

Fig. 3 General structures of two-step reversible redox systems. Adapted 16 from Hu ¨ nig et al.

This dye was the very first example of the now large class of aromatic redox active compounds called ‘‘violenes’’ which were ¨nig et al. and described in an impressive later studied by Hu number of papers published over 35 years, from 1964 to 1999.16,17 In particular, they were able to derive a general structural principle for compounds that have the capacity of reversible two-stage electron transfer. It has been found that a great variety of organic compounds, derived from the general structural types depicted in Fig. 3, are able to transfer two electrons in a stepwise fashion. The intermediate oxidation state SEM represents radical cations, radical anions and neutral radicals for A, B and C type respectively. The structural requirements for such a compound to reversibly transfer two electrons in two stages can be summarized as follows: the end groups of the reduced form (RED) have free electron pairs or p-systems available (X,Y = N, O, S, Se, P, and p-systems); the end groups are connected by vinylene groups (n = 0, 1, 2. . .), which can be substituted, partially or completely, by ring systems. Aza substitution of methine groups is also possible. Although the structures of the intermediate radical cations in pictures 4d and 4m are depicted by a fixed unpaired electron, they are strongly delocalized. As a consequence, low energy absorption bands are present in the UV/Vis spectra of the SEM form, and a thermodynamic stabilization is expected in comparison to similar undelocalized radicals. In the case of type C violenes (Fig. 3), formally obtained by ‘‘crossing’’ systems A and B, a neutral SEM state is present. In systems of this type the radical is said to be merostabilized, and a lower stability in comparison with types A and B is expected as an effect of its dissymmetry.18 Among compounds possessing these structural motifs, a further classification17 can be made: 1. Open-chain redox systems: neither the end groups nor the vinylene groups belong to a cyclic p-system; 2. Wurster-type redox systems: end groups are located outside a cyclic p-system that has aromatic character in the reduced form; 3. Inverse Wurster-type redox systems: end groups are located outside a cyclic p-system that has aromatic character in the oxidized form; 4. Weitz-type redox systems: end groups are part of a cyclic p-system that has aromatic character in the oxidized form; 5. Inverse Weitz-type redox systems: end groups are part of a cyclic p-system that has aromatic character in the reduced form;

5416 | Chem. Commun., 2014, 50, 5413--5430

Fig. 4 Examples of the different types of violene systems.

6. Semi-Weitz redox systems: one end group is part of a cyclic p-system that has aromatic character in the oxidized form. A few examples, taken from the different subgroups, are depicted in Fig. 4. Among them, TCNQs (compound f in Fig. 4) and TTFs (compound n in Fig. 4), worthy of mention, are widely used as acceptor and donor compounds, respectively, in materials science and supramolecular chemistry.43–45 Viologens (structure m), which are simple and easily accessible Weitz type electrochromes, represent another important group.46 They will be discussed further in the forthcoming section. The reactivity of violenes varies widely with their redox state but since the equilibrium has to be moved during electrochemical switching of ECDs, one must pay attention to the characteristics of all of them. In particular, the RED form is sensitive to electrophiles, as a consequence of the presence of non-bonded electron pairs and an electron rich p-system. This property is expected to be more pronounced when the potential of the RED/ SEM couple is more negative. The opposite is valid for the OX form, which is sensitive to nucleophiles; an effect that is increased


View Article Online


ChemComm

Fig. 5

Methyl viologen (MV) electrochemistry.

at higher SEM/OX potentials. Moreover, the radical form is prone to undergo homocoupling reactions to form dimeric species, unless properly stabilized. 3.1.1 Viologens. Viologens, 1,10 -disubstituted-4,40 -bipyridinium dications, have special importance among other Weitz type violene compounds. Since their discovery in 1933,47 these redox active organic compounds have been studied in detail over the years. The interest in these compounds and their applications is mainly related to their remarkable electrochemical properties. Besides ECDs, viologens have been applied as redox switchable moieties in supramolecular chemistry,48,49 or more generally as acceptor units.50,51 The simplest viologen is 1,1 0 -dimethyl-4-4 0 bipyridinium, known as methyl viologen (MV). A radical cation, stabilized by delocalization throughout the p-system, is formed upon reduction (Fig. 5). This form is intensely coloured, as an effect of the presence of a charge transfer transition, with an absorption band that can be slightly modulated by changing the 1 and 1 0 substituents. Simple alkyl groups promote a blue/ violet colour of the radical cation, whereas aryl groups tend to impart a green hue (as in 1,1 0 -cyanophenyl-4,4 0 -bipyridinium, e = 83 300 Lmol1 cm1 at lmax = 674 nm in MeCN). Viologen radical cations can dimerize in solution to form a diamagnetic dimer. This species possesses different spectral properties from the monomer (e.g. MV dimer is red) and has a slower oxidation kinetics, showing a quasi-reversible behaviour.52,53 The completely reduced redox state is weakly coloured, lacking any intense optical transition in the visible region. The simplest viologen based ECDs make use of a solution phase Type I system. To attain better performances, viologens with longer alkyl chains were adopted in order to obtain insoluble radical cation species. In this way a type II system with enhanced write– erase efficiency was obtained, using heptyl viologen dibromide (HV).54 These devices had a maroon colour due to incorporation of dimeric species in the deposited insoluble material, but were affected by ageing problems due to the crystallization of the radical cation inside the film.55 The formation of the HV0 species was also found to affect the write–erase efficiency.56 The Gentex Corporation patented Night Vision Safety (NVS) mirrors employing viologen electrochemistry.57 In this system a proprietary gel formulation, containing a viologen and an anodically colouring electrochrome (thiazine or phenylene diamine), is placed between an ITO-glass electrode and a metallic mirror counterelectrode. This type of device has no memory effect and requires a continuous passage of current since the coloured species can diffuse in the electrolyte, and erase the formed colour through electron transfer (inverse redox reaction). More complex viologen based electrochromic systems make use of a surface modified electrode (using silanization)58


Feature Article

or polyviologen systems. In the latter case, a preformed polymeric viologen system is prepared by the reaction of 4,4 0 -bipyridyl and a difunctional haloalkane in a suitable solvent. The obtained product is usually deposited by spin-coating on the electrode surface.59 Another deposition method employs polyviologen and polystyrenesulfonate (PSS) in an alternate layer-by-layer deposition procedure, to build a polyelectrolyte multilayer.60,61 Another approach to viologen based ECDs makes use of anchoring groups to adsorb the molecules on the surface of electrodes coated with nanostructured TiO2 in a structure that resembles the one used for dye sensitized solar cells (DSSCs), the same type of electrodes are also used.62 In this way, a high colouration efficiency and a fast switching time can be obtained, thanks to the high internal surface area and the surface confinement of the redox process.63 These characteristics boosted the interest of different research groups,64–70 eventually leading to commercialization by NTera Limiteds as NanoChromicst Ink System for the production of printed displays on flexible supports.71 These devices (Fig. 6b) are based on a viologen molecule, carrying a phosphonic acid group (compound i in Fig. 6a), chemisorbed on a 4 mm thick TiO2 layer deposited on an ITO/PET substrate. The counterelectrode is a 3 mm thick film of high surface area SnO2:Sb on a carbon/PET substrate. An ion-permeable TiO2 layer can be placed between the electrodes to act as a diffuse reflector for obtaining the effect of ink on white paper. In another configuration, phosphonated phenothiazine molecules (compound ii in Fig. 6) can be adsorbed on the counterelectrode for obtaining a dual electrochromic device able to change from colourless to purple.72,73 3.1.2 Phenothiazine, phenazine, phenoxathiin, and thianthrene. The title compounds cannot be clearly classified since they can be considered both Weitz type and Wurster electrochromes. Phenothiazine derivatives are known to form very stable, red coloured radical cations, and have already been mentioned in Section 3.1.1 for their application in conjunction with viologens in electrochromic devices based on TiO2adsorbed dyes. Application of phenothiazine alone in simple electrochromic devices has also been reported.74 3.1.3 Rylene and related diimides. Compounds of this class have been extensively used as acceptors, thanks to their low LUMO level, and hence they can easily form stable radical

Fig. 6

Scheme of a NanoChromicst device. Adapted from Corr et al.71

Chem. Commun., 2014, 50, 5413--5430 | 5417

View Article Online


Feature Article

anions and dianions upon reduction at relatively high potentials.75,76 They can be considered to be Wurster type violenes with the imide groups acting as redox centers. They show a two-step reversible reduction, with two reversible cathodic waves in CVs. The first peak can be attributed to the formation of the radical anion, while the second arise from the dianion formation. The peaks separation is dependent on the size of the aromatic core as an effect of the better delocalization of the radical anion and the lower coulombic repulsion in the dianion. As a consequence, the difference is higher for the smaller pyromellitic diimides and naphthalene diimides than for the perylene ones. As an example di-n-amyl-pyromellitimide shows its two reduction waves at 0.74 and 1.43 V (vs. SCE) (which correspond to B1.14/ 1.82 V vs. Fc/Fc+), di-sec-butyl-naphthalene bisimide at 1.14 and 1.56 V (vs. Fc/Fc+), while di-sec-butyl perylene diimide at 1.08 and 1.27 V (vs. Fc/Fc+).75,77 Diimides with very large cores, such as terrylene or quaterrylene, show a single two-electron reduction process.78 The electrochemical and spectroelectrochemical properties of thin films obtained from polymeric pyromellitic diimides and naphthalene diimides have been studied but no reports of their application in electrochromic devices are present in the literature.77,79 3.1.4 Aromatic dicarboxylic acid esters. The electrochromic properties of these violene redox systems (Wurster type) have been reported in 1987 by Nakamura et al.,80 but apparently no further developments followed their preliminary findings. The investigation on the electrochromism of these compounds was then reopened in 2004 by Kobayashi et al. who characterized the electrochemistry and spectroelectrochemistry of dimethyl terephthalate solutions in NMP.81 The compound is reduced at B2.0 V vs. Ag/AgCl showing an increase in absorption at 530 nm. In a following paper, they built a solution phase 8 8 matrix ECD employing diacetylbenzene, dimethylterephthalate and biphenyl dicarboxylic acid diethyl ester cells using ferrocene as the redox counterpart.82 Attempts to build flexible devices using the PVB-based gel electrolyte have been carried out but the data for these devices show a very long switching times (around 100 s).83 The electrochromism of 5-substituted 1,3-isophthalates has been studied by Lee et al. who demonstrated the possibility of tuning the colour of the reduced state by changing the substituents.84 Biphenyl dicarboxylic acid diethyl ester shows a two-step reduction changing the colour of the solution from colourless to yellow and finally to red. An electrochemical cell using a solution of this compound and a NiO counterelectrode showed high colouration efficiency for the first reduction step (377 cm2 C1), thanks to the adoption of a solid state counterelectrode material. Nevertheless, a 50% decay in contrast (DT) was observed after 1000 cycles. 3.1.5 Phenylenediamine aromatic polyamides and polyimides. Phenylenediamine derivatives or analogous extended systems are widely used as hole-transport donor compounds in organic materials, TPD (N,N0 -bis(3-methylphenyl)-N,N0 -diphenylbenzidine) being one of the most popular.85 The charged states in these materials are stable highly delocalized radical cations with quinoid character, they coherently behave as Wurster type violenes.86–88

5418 | Chem. Commun., 2014, 50, 5413--5430

ChemComm

Fig. 7 Examples of polyimides and polyamides incorporating N,N,N 0 ,N 0 tetraphenyl-1,4-phenylenediamine moieties developed by Liou et al.

Nonconjugated polymeric phenylenediamine based systems, similar to these, have been extensively studied, since 2005, by Liou et al.89–95 They prepared and fully characterized a lot of different polyimides and polyamides with N,N,N0 ,N0 -tetraphenyl1,4-phenylenediamine moieties and a variety of different aromatic and non-aromatic spacers in the main chain (see Fig. 7). The obtained polymers are soluble in most of the polar aprotic solvents (NMP, DMSO, DMAc, DMF, m-cresol etc.) allowing to prepare amorphous films by a drop-casting technique. These films are three step electrochromes, showing a colourless green - blue transition upon oxidation, and exhibit high contrast values (460% in the visible region) and good cycling stabilities.89 The introduction of methoxyl substituents into the electrochromic unit (b in Fig. 7) in the p-position lowered the oxidation potential and allowed to obtain more stable radical intermediates preventing dimerization reactions at these positions.92 These improved materials show contrast values around 75% in the visible region and cycling stabilities up to 1000 cycles with a decay in colouration efficiencies of 5% for the colourless " green transition and 10% for the colourless " blue one. Switching times (colouring/ bleaching) of these two transitions are 3.04 s/1.87 s and 4.02 s/ 2.02 s respectively, which are rather high values in comparison to those achieved with conjugated polymers (o0.5 s).96 A further improvement on the electrochromic properties of these polymers was obtained using N,N 0 -bis(4-aminophenyl)-N,N 0 -di(4methoxyphenyl)-1,4-phenylenediamine as a functional monomer (c in Fig. 7).93 The obtained polymers retain 99% of their electroactivity after switching 10 000 times between 0 and 0.70 V (vs. Ag/AgCl) (colourless " green) with a decay in the colouration efficiency of 4.89%. Similar good results are obtained upon switching to 1.10 V (vs. Ag/AgCl) with a decay of 5.60% after 3000 cycles.


View Article Online

ChemComm


3.2

Feature Article

Violene–cyanine hybrids

The violenes (see Section 3.1) represent the majority of the known organic electrochromes. The main drawback of these systems is that the intensely coloured redox state (long wavelength absorbing and with high e) is a radical ion. These open-shell states, even the most stable ones, have the tendency to be more reactive when compared to closed-shell ones. As a consequence, a similar redox system switching between closed-shell states is expected to be less prone to decomposition. In order to obtain reversible redox systems of this kind, with a highly coloured closed-shell state, ¨nig et al.97–99 a new general structure has been proposed by Hu (see Fig. 8). This new general structure is clearly inspired by the violene one, but contains, as end groups, systems that in the reduced or oxidized form can be described as polymethine dyes (cyanines, oxonols and merocyanines; X = C–Y in Fig. 8). Highly delocalized closed-shell systems of this class are known to possess strong thermodynamic stabilization.100 As a consequence, in such a system the fully reduced or fully oxidized states in the RED ! SEM ! OX equilibrium are stabilized at the expense of the SEM radical species, with the result of a redox process that approaches a two-electron one. Furthermore, the cyaninic nature of one of the two accessible states makes it strongly absorptive in the visible region (log e C 5). As an example, tetrakis(dimethylaminophenyl)ethene (TDAPE, compound b in Fig. 9), known since 1906,101 shows a reversible RED - OX2+ two-electron redox process at C0.30 V (vs. Fc/Fc+) in its cyclic voltammogram (Fig. 10a).98 This process has been closely studied revealing a very small equilibrium concentration of SEM + species.98 In contrast, the second oxidation wave at C+0.70 V (vs. Fc/Fc+), shows a broadened peak with shoulders, and can be interpreted as closely spaced unresolved single electron processes. The spectroelectrochemistry of the RED - OX2+ transition, depicted in Fig. 10b, shows the appearance of broad and intense absorption bands in the visible region. Although this complex spectrum cannot be simply described as derived from one of its cyaninic substructures (Michler’s hydrol blue), it demonstrates the possibility of obtaining a highly coloured closed-shell state following the violene–cyanine approach. Further studies on these types of systems have been made by Ito et al. who developed more complex systems based on azulene moieties.102–106 Most of their ¨nig’s simple violene–cyanine hybrid structures do not fit in Hu structure depicted in Fig. 8 and new types of hybrids were developed (Fig. 11). It can be noted that some of these systems possess more than one cyaninic oxidation state, giving them intriguing electrochromic properties (multichromic and highly absorptive). Moreover, since the structural rearrangement following

Fig. 8 General structure of a violene–cyanine redox system (n can be any 97 integer). Adapted from Hu ¨ nig et al.


Fig. 9

Examples of violene–cyanine hybrids.

the reduction or oxidation processes is expected to be very fast (and by no means different from the case of conjugated polymers), we feel that such a peculiar class of derivatives deserves further study in the field of ECDs and will likely play an important role in the future as long as solutions are provided for the relevant issues of immobilization at the electrode and counterion diffusion.

3.3 Electrochromic systems based on intramolecular redox switching of bonds Although the violene type and the related violene–cyanine hybrid systems account for most of the organic electrochromic compounds, a further class has to be considered. In contrast to violenes and similar systems, where reduction or oxidation is accompanied by a small conformational change and a modification in the bond lengths, in these systems the formation of new intramolecular bonds is observed as a consequence of valence tautomerization occurring during the electrochemical process. This is usually accomplished through a multistep process involving an overall two-electron transfer and a bond formation/cleavage. The underlining mechanism is often of the ECE type (electron transfer–chemical event–electron transfer), where the valence tautomerization takes place after the transfer of the first electron. ¨nig et al. reported in 1977 one of the first compounds of Hu this class.107,108 In this case, compounds with a 1,3-dimethylenecyclobutane skeleton, show a homoconjugative interaction between the two chromophoric units, that lead to the formation of a bicyclo[1.1.0]butane skeleton upon oxidation to the dicationic forms. The overall chemical process is reversible since CV patterns are retained upon multiscanning, but the electron transfer is irreversible as is apparent from the large separation between the oxidation and reduction waves. This characteristic is recurrent in this type of systems. A series of systems of this type, based on a tetraaryldihydrophenanthrene skeleton, was

Chem. Commun., 2014, 50, 5413--5430 | 5419

View Article Online


Feature Article

ChemComm

Fig. 11 Examples of violene–cyanine hybrid structural motifs.

98 Fig. 10 Electrochemistry of TDAPE. Adapted from Hu ¨nig et al. (a) Cyclic voltammogram of TDAPE (b in Fig. 9) in 0.1 M nBu4NPF6/CH2Cl2; potentials vs. Fc/Fc+, at a scan rate of 100 mV s1. (b) Spectroelectrogram of the RED OX2+ oxidation step of TDAPE in 0.1 M nBu4NPF6/CH3CN; potentials vs. Ag/AgCl. (c) Spectroelectrogram of the OX2+ - OX4+ oxidation step of TDAPE in 0.1 M nBu4NPF6/CH3CN; potentials vs. Ag/AgCl.

published in 1997 by Suzuki et al.109,110 These molecules exploit the easy bond dissociation of hexaarylethane derivatives into trityl radicals to form a bistable redox active system that can be switched upon a two-electron transfer (with an ECE mechanism, see Fig. 12a). The cyclic voltammograms (Fig. 12b for Ar = 4-Me2NC6H4) show an

5420 | Chem. Commun., 2014, 50, 5413--5430

irreversible oxidation process centred at +0.77 V (absence of the corresponding cathodic peak), along with the appearance of new peaks in the cathodic region (at 0.45 V) assigned to the reduction of the OX dicationic species formed as a consequence of the combined redox and intramolecular steps. This indicates that following the first oxidation step and the bond cleavage chemical step the formed intermediate cation radical species possess a lower oxidation potential, and hence irreversibly undergo the second oxidation process well above the corresponding equilibrium potential. In this way, a reversible overall chemical process coexists with microscopically irreversible redox processes as in the previous case. In order to further investigate the overall reversibility process the authors attempted the chemical reduction of the oxidized forms (for both Ar = 4-Me2NC6H4 and Ar = 4-MeOC6H4) with Mg in MeCN and successfully isolated the reduced forms as stable solids with 86% and 93% yields respectively. Moreover, oxidation of the reduced forms with two equivalents of ( p-BrC6H4)3N +SbCl6 in CH2Cl2 led to the isolation of the corresponding dications as SbCl6 salts in 85% and 97% yields respectively. The switching between the two forms is accompanied by drastic colour changes, with an oxidized form showing strong absorption bands in the visible region (lmax(log e): 661 nm (4.92) and 604 nm (5.05); for Ar = 4-Me2NC6H4, Fig. 12c). It is noteworthy that a clear isosbestic point is observed in Fig. 12c. This indicates a quantitative conversion of the OX2+ form into the RED one along with a negligible steady-state concentration of intermediates. Similar systems, using cyanine radicals, were later studied by ¨nig et al.,112,113 who proposed the two general structural motifs Hu depicted in Fig. 13. In this scheme the X = C–Y system indicates a cyaninic type substructure that is able to stabilize the charge,


View Article Online


ChemComm

Feature Article

Fig. 13 General structures of electrochromics based on single-bond 112 redox switching. Adapted from Hu ¨ nig et al.

limiting states.114 The absence of applications in ECDs is likely due to the very high chemical complexity of these materials. Moreover, problems related to the kinetics and reversibility of the bond formation/cleavage step have to be expected, especially if these systems are immobilized on the electrodes or embedded in a polymeric matrix, as it is required for a type III device. 3.4

Fig. 12 (a) Interconversion of RED and OX forms in tetraaryldihydrophenanthrene systems (Ar = 4-Me2NC6H4, 4-MeOC6H4; Ar2C+ = 9-xanthenylium) through an ECE type mechanism. Adapted from Suzuki et al.111 (b) Cyclic voltammogram of the RED form (Ar = 4-Me2NC6H4) in 0.1 M nBu4NBF4/ CH2Cl2; Pt vs. SCE. Scan rate 500 mV s1. The reduction wave at 0.45 V is absent when the system is first scanned cathodically. Adapted from Suzuki et al.111 (c) Continuous changes in UV-Vis spectra of OX2+(BF4)2 (Ar = 4-Me2NC6H4) (1.1 105 M) upon constant-current electrochemical reduction (10 mA) in MeCN. Adapted from Suzuki et al.111

T is a saturated or unsaturated tether and Z indicates N or C–R. Due to the presence of cyaninic substructures, these systems possess at least one strongly coloured, long wavelength absorbing state. In the case of (a) type system, this corresponds to the open form, while the opposite is verified for the (b) type system. By virtue of these characteristics, the electrochromic properties of these closed-shell systems are very interesting since they can in principle achieve very high contrast values but even in this case the application in actual ECDs has not yet been reported. The maximum achievable contrast, as in the former case, can thus only be roughly evaluated from the solution spectra looking at the very high ratio between molar extinction coefficients of the two


Metal coordination complexes

Metal coordination complexes appear as promising electrochromic materials as a result of their intense colouration and redox activity combined with the possibility of modulating their properties through easy modification of the organic ligands.115 In this class of compounds, the transitions involved in the chromophoric properties are mainly: metal-to-ligand charge transfer (MLCT), intervalence charge transfer and intra-ligand excitations; thus involving valence electrons. It follows that these transitions are altered or eliminated upon reduction or oxidation of the metal center, making these materials electrochromic. The incorporation of electrochromic metal coordination complexes in all-solid-state ECDs is of potential interest, provided they are obtained as thin-films or as polymers. 3.4.1 Polypyridyl complexes. Polypyridyl ligands are known to form stable complexes with a variety of transition metals, and some of them show interesting electrochromic properties. An example is represented by the bipyridyl complexes [MII(bipy)3]2+ (M = iron, ruthenium, and osmium; bipy = 2,2 0 -bipyridine) which appear as red, orange and green, respectively, as a consequence of the presence of a MLCT band which is suppressed upon switching to the MIII redox state. Bipyridine based complexes in which ligands are functionalized with electron-withdrawing groups also show some ligand-based electrochemical processes, as an effect of the anodic shift of the redox potentials, that lead to multicolour electrochromism.116,117 The incorporation of polypyridyl complexes inside thin-films is usually accomplished through polymerization of properly functionalized ligands (carrying polymerogenic functionalities). Two approaches are found in the literature: oxidative polymerization and reductive polymerization. Reductive electropolymerization of [MII(bipy)3]2+ complexes is usually employed with vinyl-substituted pyridyl ligands and exploits the ligand-centered nature of the three sequential reductions shown by such ligands. Radical species generated upon reduction lead to oligomeric metallopolymers through C–C bond formation. The oligomeric species grow near the electrode surface and are deposited onto it once a critical size is reached (due to their insolubility).118

Chem. Commun., 2014, 50, 5413--5430 | 5421

View Article Online


Feature Article

ChemComm

Electroactive thin-film formation by oxidative electropolymerization has been described for FeII and RuII complexes obtained from bipyridyl ligands substituted with amino119 moieties and aniline functionalized pendants.120 Recently, electrochromic films based on OsII and RuII supramolecular assemblies prepared following a layer-by-layer deposition method have been reported. This approach uses polytopic vinylpyridyl based ligands to form metal complexes that are subsequently crosslinked on the surface using PdCl2(PhCN)2 by an alternate deposition procedure, leading to porous metal– organic assemblies.121,122 3.4.2 Metallophthalocyanines. Phthalocyanines (Pc) can be considered as tetraazatetrabenzo derivatives of porphyrins, which form coordination complexes with ions derived from more than 70 different elements of the periodic table. The metal ion in metallophthalocyanines is located either at the center of a single Pc ligand or between two of them forming a sandwich-type complex. The first type of complexes is common for transition metal ions while the latter is usually encountered in lanthanides. The colour of metallophthalocyanine derivatives is due to the presence of a very intense (e 4 105 Lmol1 cm1) absorption band located in the red region of the visible spectrum (at around 670 nm), accompanied by weaker MLCT transitions in the region at around 500 nm. These transitions make possible to modulate the resulting hue by changing the metal ion. The first electrochromic phthalocyanine based thinfilm was reported in 1970.123 This material was obtained by vacuum sublimation of [Lu(Pc)2] and was extensively studied in the following years. Films obtained in this way show a brilliant green colour (lmax = 605 nm) and possess a complex electrochromism (eqn (1)–(3) and Fig. 14). Upon oxidation the film changes to a yellow/tan form (eqn (1)) and then to a red one of unknown composition,125–127 upon reduction it switches firstly to a blue form and then to a violet-blue one.128 Limiting the colour transition to yellow-tan " green, cycles greater than 5 105 have been reported in suitable solvents with a switching speed of C40 ms, but the contrast between these two redox states is not very good.129 Devices containing Pc complexes also suffer from degradation problems correlated to the mechanical stress caused by the inward/outward motion of counterions in the electrochromic film during cycling. This factor, connected to the rigidity of the structure, can be minimized using smaller anions.130,131 le

½Pc2 LuHþ ðsÞ Ð ½Pc2 LuHþ ðsÞ þ 1Hþ ; þle

green

þle

½Pc2 LuHðsÞ; ½Pc2 LuHþ ðsÞ Ð le

green

(1)

yellow=tan

(2)

blue

þle

½Pc2 LuH ðsÞ ½Pc2 LuHðsÞ Ð blue

le þle

violet

(3)

½Pc2 LuH ðsÞ Ð ½Pc2 LuH2 ðsÞ le

Another problem, that prevents application of [Lu(Pc)2] films in devices, is that they are usually prepared by vacuum sublimation. The intrinsic difficulties associated with this technique

5422 | Chem. Commun., 2014, 50, 5413--5430

Fig. 14 Absorption spectra of Lu(Pc)2 in aqueous 1 M KCl. The spectra of oxidized [Pc2Lu]+ (yellow/tan ), intermediate [Pc2LuH]+ (green ) and reduced [Pc2LuH] (blue ) forms are shown. Adapted from Somani et al.124

(need for a vacuum apparatus) are accompanied by a slow deposition rate, partial decomposition of the material and the presence of electrochemically inaccessible sites in the resulting film.124 Attempts to solve these problems include film preparation using the Langmuir–Blodgett technique132–134 and the use of polymerogenic phthalocyanines. The first has demonstrated to allow the synthesis of thin-films with extended cyclability (up to 106 cycles), although with a very low contrast (low thickness).135 The latter strategy uses polymerogenic functionalities attached to the phthalocyanine complexes, similarly to those used for polypyridyl complexes (see Section 3.4). These include pendant aniline and hydroxyl groups on ligands that can be electropolymerized on the electrode surface.136,137 An interesting example of the use of pyrrole, connected to the Pc via an alkylene spacer, as an oxidatively polymerizable unit has also been reported.138 It has to be clarified that the sharply peaked absorption bands of these kinds of materials make them unsuitable for transmissive devices such as smart windows or sunglasses, where a neutral hue is desired. As a consequence, their possible applications are in the field of electrochromic displays.

4 Composite multichromophoric systems based on conjugated polymers Conjugated polymers have proved to represent a fascinating class of electrochromic systems, showing very interesting properties in terms of cycling stability, colouration efficiency, ease of processing and hue modulability through functionalization. However, these latter characteristics appear to be rather limited if one looks at the literature examples. In particular, conjugated polymer based systems switching from a colourless highly transmissive state to one possessing a neutral hue (brown, grey, and green) are still missing. The advent of the donor– acceptor approach solved the problem only in part since the known examples lack the highly transmissive state of the best PEDOT analogues. Furthermore, their coloured state exhibits


View Article Online


ChemComm

pure hues that are unsuitable for applications like smart windows or sunglasses since they lead to distortion in colour perception, as a consequence of uneven filtering of some spectral regions. The only exception is represented by the PProDOT based copolymer incorporating DA units developed by Reynolds et al. that shows a transition from bluish-black to highly transmissive.139 Nevertheless, the transmittance of the bleached form in the visible region ranges from 45% to 67% that are unacceptable values for the aforementioned applications. These limitations are clearly apparent while looking at Fig. 15 comparing the performances of the latter electrochromic polymer to those of the state-of-the-art photochromic lenses (Transition VIt by Essilor). It is evident that the absorptive reduced state of this DA conjugated polymer presents a little too low absorption at around 450 nm, while the transmissive state appears to be very absorptive. A comparison with a ProDOTEt2 derivative (also shown in Fig. 15) highlights the reduction in transmissivity ‘‘paid’’ in order to obtain the panchromatic absorption (absorption covering all the visible range) in the reduced state. One possible solution to modify the colours of the existing electrochromic polymers is to incorporate a second, non-interacting, electrochrome inside their structure. Suppose that the two systems, the polymer and the embedded discrete electrochromic system, show independent redox behaviours, an easy way to colour tuning can be obtained. Furthermore, if both the polymer and the discrete molecular electrochromic system possess a colourless state in the same potential range, a colourless to coloured transition could in principle be observed. In light of these considerations, and looking again at Fig. 15, one can speculate that a molecular electrochrome absorbing in the 450–500 nm range can modify the colour of the reduced state of a PEDOT-like polymer obtaining the desired neutral hue. A few different strategies to incorporate redox active molecules in a conjugated polymer can be imagined. The first one is to simply dissolve the compound in the polymeric matrix. This approach is not feasible, especially in the case of violenes, for

Feature Article

many reasons. It has to be considered that most of the violene systems possess low solubility and even after functionalization with solubilizing alkyl chains, they still retain a tendency for phase segregation. These characteristics favour the formation of separated phases that lead to a drastic reduction of the fraction of the redox active compound inside the film and/or to poor optical properties (hazy films). Furthermore, the molecular electrochrome can diffuse away from the electrode upon prolonged cycling, leading to a drastic reduction in the performance. An alternative approach, demonstrated by Palmore et al.,140 uses a properly functionalized eletrochrome to form a polyelectrolyte that, acting as a counterion for PEDOT chains, is incorporated in the polymeric matrix. A more complex immobilization technique aims at covalently bonding electrochromic units, through a tether, to the conjugated polymeric backbone as schematically depicted in Fig. 16. Such a composite system can be obtained both with a postfunctionalization and a pre-functionalization approach. In the former case, a formed polymer film carrying reactive side chain functional groups is reacted with the discrete electrochromic unit to form a covalent bond between the two. As an example, Segura et al.141 polymerized an azide functionalized EDOT monomer, and exploited the azide–alkyne cycloaddition to incorporate redox active compounds in the polymeric layer. The main disadvantages of this procedure are: the poor control over the functionalization density and homogeneity and a more complex synthetic procedure. A more general and reliable approach uses modified monomers functionalized, through a tether, with the desired discrete electrochromic system. Monomers of this kind can be electrochemically polymerized to obtain thin films of the modified polymer. However, the electrochemical polymerization of electroactive monomers carrying bulky substituents has proved to be more difficult, as a consequence of lower reactivity and slow diffusion. To circumvent this problem pulsed techniques and copolymerization have been applied.142 In 1984, Bidan et al.143 reported the electrooxidation of a pyrrole

Fig. 15 Comparison of the best known panchromatic absorptive electrochromic polymer, developed by Reynolds et al.139 (structure 1, black—), with Transition VIt photochromic lenses (red ) and a p(ProDOTEt2) derivative96 (structure 2, blue ).


Chem. Commun., 2014, 50, 5413--5430 | 5423

View Article Online


Feature Article

ChemComm

Fig. 16 Schematic representation of the colour modification of a conjugated polymer by incorporation of discrete electrochromic side chain redox units. The colour hue is modified to a neutral tint (closer to the white point of the CIE diagram).

N-substituted with an aliphatic chain bearing a 4,4 0 -bipyridinium (viologen, V2+) on a Pt electrode. The cyclic voltammogram shows the presence of the polypyrrole redox process along with the two sharp redox waves of the viologen unit. Interestingly, a pre-peak superimposed onto the poly(pyrrole) (pPy) oxidation wave is also present. The authors state that the amount of charge attributed to this pre-peak is identical to the observed difference between the integrated charges for reduction and oxidation of the V2+ unit. This modified electrode appears to be not so stable, since a 15% charge drop in the viologen reduction peak was observed after 450 cycles between 0 and 0.9 V vs. Ag/Ag+ (corresponding to the transition from the pPya+–V2+ to the pPy0–V + state). Similar systems, in which the viologen unit was anchored to the 3-position of a thiophene unit through an alkyl tether, were üerle and Gaudl144 in 1990 and their application reported by Ba to ECD was studied by Lee et al.145 in 2002. Even in this case the cyclic voltammograms of the polymers showed an additional peak superimposed onto the polymer oxidation process. Quantitative consideration on the charges involved in the redox waves, and additional experiments limiting the scan range, confirmed a similar behaviour to the one observed by Bidan et al. The authors explained this odd behaviour by the presence of a viologen mediated electron transfer process. Their hypothesis is that, upon reduction of the conjugated polymer backbone, charge trapping occurs, leaving residual charges. These species are then reduced at a lower potential through an electron transfer process mediated by the viologen radical cationic form, V +. Reoxidation occurs at the same potential as the rest of the polymeric chain. Nonetheless, the system appeared to have a fairly good cycling stability when the potential window is limited to the region of the viologen electroactivity (1.8/0.2 V vs. Ag/AgCl), as shown by dynamic spectroelectrochemical data under square wave potential. Conversely, when the potential window was switched to 0.0/0.6 V, a decrease in both current and magnitude of absorbance change, attributed to a degradation of the polythiophene backbone, was observed. Spectroelectrograms of the

5424 | Chem. Commun., 2014, 50, 5413--5430

system in its different oxidation states were also recorded. Similar redox behaviors were reported with different redox active systems attached to conjugated polymers: viologen– poly(cyclopentadithiophene),146 viologen–polycarbazole,147 148 titanocene–polypyrrole, titanocene–PEDOT,149 and anthra150 quinone–PEDOT. However, in most of these systems, the presence of pre-peaks is associated with a loss in the electroactivity of the attached redox-active unit. As an example, the cyclic voltammograms of the anthraquinone–PEDOT system studied by Otero et al.,150 and the viologen–poly(cyclopentadithiophene) system studied by Lee et al.146 are reported in Fig. 17 and 18 respectively. The presence of pre-peaks and non-equilibrium phenomena is a common feature of these kinds of polymeric composite multi-redox systems. Different explanations exist but there is no general understanding of the involved processes in such complex systems, and the observed behaviour can be a weighted combination of different factors depending on the specific sample under

Fig. 17 CVs of p(AQ-EDOT) in monomer-free 0.1 M Bu4NPF6 solution in CH3CN, scan rate: 50 mV s1. 1st , 2nd , 3rd , 4th , 7th and 25th--- cycle. Adapted from Otero et al.150


View Article Online


ChemComm

Feature Article

Fig. 18 CVs of p(CPDT-V2+-Me)(4) in a monomer-free 0.1 M Bu4NPF6 solution in CH3CN/CH2Cl2 (1 : 3 by volume), scan rate: 50 mV s1. Adapted from Lee et al.146

examination. Since conjugated polymers with pendant redox centres are multi-redox-state systems, the conduction mechanism and the film resistance vary abruptly with the applied potential. As a consequence, when scanning anodically, the re-oxidation of the embedded redox centres can be, at least partially, delayed to the onset of the polymer backbone electroactivity when the film becomes more conductive. The film is charged rapidly to the equilibrium value with the resultant additional peak.148,151 A similar behaviour has been observed in other types of multi-redox systems like bilayer,118,152 layer-by-layer (LbL) deposited multilayers,153 and phase segregated systems.154,155 Moreover, since these composite systems combine an apolar p-extended backbone with a polar ionic side chain, they are inherently inhomogeneous. This property can reflect itself in large spatial dispersion of local redox potentials with regions that lose electrical or ionic connectivity during the charge– discharge process. As pointed out by Vorotyntsev et al.,148 since polymeric films are dynamic systems, the conformational change imposed by the additional redox process has also to be considered. Each redox step, in fact, is accompanied by the inward/outward migration of counterions from the film, rearrangement of film structure and local conformation around redox centres. These structural modifications can be metastable until a potential value able to induce the reverse process is reached. Such an event can thus be associated with local modifications of redox potentials and/or the exposed surface area with occurrence of capacitive current peaks. As a matter of fact, looking at literature examples, the presence of nonequilibrium phenomena like these is often associated with rapid loss in electroactivity of the side chain redox centres as illustrated in Fig. 17 and 18. In 2004, both Lee et al.156 and Łapkowski et al.157 reported the synthesis and electrochemical characterization of PEDOT films incorporating a pendant viologen unit (Fig. 19a). The two monomers are almost identical, the only difference between the two being a butyl chain on one of the viologen nitrogen atoms (in Lee’s system) instead of a methyl group. Unluckily the first authors do not present data on the cycling stability of the synthesized polymeric film 5 and hence it is not possible to make an in-depth comparison of the two systems. However, Łapkowski et al. reported a drastic reduction in both cathodic and anodic current densities


Fig. 19 (a) Cyclic voltammogram of a poly(EDOT-V2+) film in a monomer-free solution (0.1 M Bu4NPF6 in MeCN, 50 mV s1). The polymer was obtained by electropolymerization of 2 mM EDOT-V2+ in 0.1 M Bu4NPF6 solution in MeCN/CH2Cl2 (1 : 5) at a scan rate of 50 mV s1. Adapted from Lee et al.156 (b) Spectroelectrochemistry of p(EDOT-V2+) on ITO/glass at various potentials: (a) 0.5, (b) 0.0, (c) 0.4, (d) 0.6, (e) 0.7, (f) 0.8 to 1.0, (g) 1.2 and (h) 1.4 to 1.6 V vs. Ag/Ag+. The inset shows pictures of a simple ECD based on the polymer. Adapted from Lee et al.156

corresponding to viologen electroactivity upon cycling the polymeric film in a monomer free solution in the 0.50 V/0.50 V and 0.70 V/0.00 V ranges (vs. Ag/Ag+), a behaviour that closely resembles that of the previously reported systems. The latter authors formerly analysed the presence of dimerized viologen radical cations in polypyrrole–viologen hybrid systems and suggested the dependence of the amount of these species on the mobility of the viologen moiety.158 This could suggest a similar mechanism to be operative in this EDOT based system, with an enhanced stability of the bulkier system by Lee et al., in light of the reversibility issues associated with viologen diamagnetic dimers (see Section 3.1.1). However, the dependence of the degradation of the redox moiety electroactivity on film thickness combined with the ineffectiveness of the copolymerization of the viologen functionalized monomer with EDOT (with different feed ratios) indicates that some further explanation is required. The spectroelectrogram of both viologen functionalized PEDOTs clearly shows a contrast enhancement and a slight modification of the hue of reduced PEDOT to a darker one, as a

Chem. Commun., 2014, 50, 5413--5430 | 5425

View Article Online


Feature Article

ChemComm

consequence of the absorption of V + species, after half-reduction of the viologen (Fig. 19b). A closely related system that exploits a polycarbazole backbone (6) was prepared by Lee et al. by electrochemical polymerization of the corresponding monomer by a potentiodynamic method. A peak attributed to the oxidation of the carbazole moiety is observed at B0.8 V (vs. Ag/AgCl), along with an increase of the current densities of the first viologen V2+/V + redox process (Ep,a = 0.63; Ep,c = 0.72 V) and the one related to the formed polycarbazole backbone (Ep,a = 0.55; Ep,c = 0.49 V). The obtained polymeric film proved to be electrochemically stable when cycled in the 1.0/1.2 V range (vs. Ag/Ag+) showing a reversible viologen process at 0.7 V and two superimposed waves, attributed to polycarbazole oxidation, above 0.3 V (Fig. 20a); while a gradual decrease of electroactivity is observed when the potential is swept more negatively to the one corresponding to the V +/V0 reduction process. The authors enclosed the electrochromic film of 6 in singletype and dual-type simple ECD devices with two ITO electrodes and a gel electrolyte. The UV/Vis spectra recorded when a potential of 3, 3 and 0 V is applied to the single type device are reported in Fig. 20b; a corresponding green - colourless violet colour change is observed. A sum of the absorption

characteristics of the reduced and oxidized states is obtained in the dual-type device with a bleached state at 0 V and a deep blue colour at 3 V with enhanced electrochromic contrast and a more neutral hue. Unluckily the authors reported a limited stability (B10 min) of both devices when the 3 V potential is applied continuously. The authors connected this degradation phenomenon with the oxygen adsorbed on the polymeric layer during device fabrication. In our opinion, the lack of control on the electrode potential in this type of device geometry is likely to play a role. In fact, in both the single and dual type devices described no reference electrode was employed. As a consequence, overreduction can lead to the V + - V0 process that the authors reported be associated with a decrease in film electroactivity. Conjugated polymers incorporating rylene diimides159–164 were also reported by several groups. As an example, Lee et al. reported the synthesis and electrochemical deposition of a pyrrole monomer N-substituted with a perylene diimide unit (PDI).159 The polymer was electrochemically prepared by the CV technique but, as an effect of the presence of the bulky PDI substituent judging from the current density around 0.25 V in monomer-free solution, the degree of polymerization was very low. The response times of the electrochromic film upon

Fig. 20 (a) Cyclic voltammogram of film 6 in monomer-free 0.1 M Bu4NBF4 solution in CH2Cl2 at a scan rate of 50 mV s1. Adapted from Lee et al.147 (b) Electronic spectra of film 6 on FTO/glass in a single type (one electrochromic layer) ECD at various potentials (vs. bare ITO). The gel electrolyte is composed of CH2Cl2, PMMA, propylene carbonate, and Bu4NBF4 (70 : 7 : 20 : 3 w/w). Adapted from Lee et al.147

Fig. 21 (a) Cyclovoltammogram of film 7 in monomer-free 0.1 M Bu4NPF6 solution in MeCN/CH2Cl2 (1 : 1). The polymer was obtained by electropolymerization of 2 mM PDI-2Py in 0.1 M Bu4NPF6 solution in MeCN/CH2Cl2 (1 : 1) at a scan rate of 50 mV s1 for 3 cycles. Adapted from Lee et al.159 (b) Spectroelectrochemistry of the same film as in Fig. 21a at various potentials: (a) 0, (b) 1.00, (c) 1.20, (d) 1.45, and (e) 1.80 V vs. Ag/Ag+. Adapted from Lee et al.159

5426 | Chem. Commun., 2014, 50, 5413--5430


View Article Online


ChemComm

switching the potential from 0 V to 1.8 V vs. Ag/Ag+, corresponding to p(PDI–2Py) and p(PDI–2Py)2 respectively, were 160 s and 190 s. This process was accompanied by a colour change of the film from red (lmax: 573 nm) to emerald (lmax: 788 nm) (Fig. 21). The authors state that the slow kinetics might be due to the very low conductivity between the PDI units, as a result of their separation in the polymer matrix and the insulating nature of the polypyrrole chains in the operating potential range. However, they did not comment on the possible role played by inward–outward diffusion of bulky counterions (Bu4N+ and PF6) in such a compact cross-linked structure and did not include data on the resulting long-term cycling stability which is relevant to electrochromics. A related system that makes use of a substituted naphthalene diimide (NDI) core in the place of PDI has been reported in 2012 by Beverina et al.164 In this case, a naphthalene diimide based monomer,

Feature Article

functionalized with two electropolymerizable EDOT units has been synthesized. Several electrodeposition techniques were tested but the pulsed galvanostatic method gave the best results in terms of homogeneity and overall film quality. Moreover, the authors were able to produce polymeric films using in situ chemical polymerization with Fe(OTs)3 as the oxidizer. The synthesized material shows remarkable electrochromic properties: the presence of a transparent colourless state and an absorptive one with a neutral (grey) hue, short switching time and outstanding cycling stability. The CV plot recorded in a monomer-free solution (Fig. 22a) clearly shows a reversible redox process above 0.5 V related to the PEDOT backbone combined with two asymmetric redox waves at 1.0 and 1.4 V vs. Fc/Fc+. Interestingly, while the current density of the PEDOT related process scale linearly with the scan rate (n), one of the two waves at 1.0 and 1.4 V is proportional to n1/2 suggesting a diffusion limited n-doping process. A small charge trapping peak at 0.55 V is also present but in this case this phenomenon is not accompanied by a reduction of electroactivity of the discrete redox unit as observed previously with other systems and an outstanding cycling stability over the whole potential range is demonstrated (B100% capacity retention after 1000 cycles). This outcome is apparently a result of the chemical stability of the discrete redox unit and the polymer backbone combined with a structural stability imposed by the cross-linked structure. The latter effect was confirmed by the authors comparing the cycling stability of polymer 8 with that of an analogous derivative carrying a single electropolymerizable EDOT unit.

5 Conclusions

Fig. 22 (a) Cyclic voltammograms of a poly(EDOT-NTCI) film in monomer-free 0.1 M Bu4ClO4 in MeCN solution as a function of cycle number: 25 (violet ), 50 (brown ), 100 (blue ), 250 (dark red ), 500 (magenta ), and 1000 (green ); 50 mV s1 scan rate. The film was deposited on FTO/glass by the PGS technique (deposition charge: 15 mC cm2). Adapted from Beverina et al.164 (b) Spectroelectrochemistry of a poly(EDOT-NTCI) (8) on FTO/glass in 0.1 M Bu4ClO4 in MeCN solution at various potentials: (a) 0.40, (b) 0.60, (c) 1.45, and (d) 1.80 V vs. Fc/Fc+. The inset shows pictures of the film in redox states a–c. Adapted from Beverina et al.164


OEMs are the key components of technologically relevant applications like displays, electrochromic devices, electronic inks and smart windows. The majority of the research efforts reported in the literature focuses on conjugated polymers structurally connected with PEDOT and most recently on donor acceptor polymers. Both classes of compounds demonstrated high stability to extensive redox switching, good electrochromic contrast and wide colour tunability. Even though such polymers comply with most of the requirements of the target technologies, they still suffer from limitations including residual absorption in the bleached state and complex synthesis/purification. The present review shows that multichromophoric side chain polymers combining the electrochromic performances of a conjugated polymer and a small molecule could at least rival and in some cases outperform donor acceptor polymers. The main advantages of such multichromophoric systems become apparent in the context of applications requiring high transmissivity in the bleached state and a neutral hue in the coloured one. The spectroelectrochemical response of a multichromophoric system is the sum of the responses of the two constituting units: the polymer and the small molecules. If the said components possess complementary colours, the resulting electrochromic behaviour will provide a neutral hue. Remarkably, the two components can be designed and optimized independently of

Chem. Commun., 2014, 50, 5413--5430 | 5427

View Article Online


Feature Article

each other, thus greatly simplifying the achievement of panchromaticity. Moreover, representative examples of such polymers also possess very high transparency and complete colourlessness in the bleached state. All these features, along with a generally simple and easy to scale-up approach make multichromophoric side chain electrochromic polymers a viable alternative to simple PEDOT and Donor Acceptor polymers, particularly regarding the relevant fields of ophthalmic lenses and smart windows.

References 1 P. Monk, R. Mortimer and D. Rosseinsky, Electrochromism and Electrochromic Devices, Cambridge University Press, 2007. 2 I. F. Chang, B. L. Gilbert and T. I. Sun, J. Electrochem. Soc., 1975, 122, 955–962. 3 R. J. Mortimer, Annu. Rev. Mater. Res., 2011, 41, 1–28. 4 P. M. Beaujuge and J. R. Reynolds, Chem. Rev., 2010, 110, 268–320. 5 C. M. Amb, A. L. Dyer and J. R. Reynolds, Chem. Mater., 2011, 23, 397–415. 6 R. Chiechi, G. Sonmez and F. Wudl, Adv. Funct. Mater., 2005, 15, 427–432. 7 J. E. Anthony, A. Facchetti, M. Heeney, S. R. Marder and X. Zhan, Adv. Mater., 2010, 22, 3876–3892. 8 A. J. Heeger, S. Kivelson, J. R. Schrieffer and W. P. Su, Rev. Mod. Phys., 1988, 60, 781–850. 9 J. L. Bredas and G. B. Street, Acc. Chem. Res., 1985, 18, 309–315. ¨s, M. R. Andersson, C. Booth, 10 J. C. Gustafsson-Carlberg, O. Ingana A. Azens and C. G. Granqvist, Electrochim. Acta, 1995, 40, 2233–2235. 11 C. A. Thomas, Phd thesis, University of Florida, 2002. 12 C. Wurster and R. Sendtner, Ber. Dtsch. Chem. Ges., 1879, 12, 1803–1807. 13 C. Wurster and E. Schobig, Ber. Dtsch. Chem. Ges., 1879, 12, 1807–1813. 14 C. Wurster, Ber. Dtsch. Chem. Ges., 1879, 12, 528–530. 15 E. Weitz, Angew. Chem., 1954, 66, 658–677. ¨nig and H. Berneth, Organic Chemistry, Springer, Berlin/ 16 S. Hu Heidelberg, 1980, vol. 92, pp. 1–44. ¨nig, Angew. Chem., Int. Ed. Engl., 1978, 17, 17 K. Deuchert and S. Hu 875–886. 18 R. W. Baldock, P. Hudson, A. R. Katritzky and F. Soti, J. Chem. Soc., Perkin Trans. 1, 1974, 1422–1427. 19 D. M. Lemal and K. I. Kawano, J. Am. Chem. Soc., 1962, 84, 1761–1762. 20 R. M. Harnden, P. R. Moses and J. Q. Chambers, J. Chem. Soc., Chem. Commun., 1977, 11–12. ¨nig, F. Linhart and D. Scheutzow, Justus Liebigs Ann. Chem., 21 S. Hu 1975, 2089–2101. ¨nig, F. Linhart and D. Scheutzow, Justus Liebigs Ann. Chem., 22 S. Hu 1975, 2102–2115. ïnig and F. Linhart, Justus Liebigs Ann. Chem., 1975, 23 S. Hu 2116–2129. 24 L. Michaelis and E. S. Hill, J. Am. Chem. Soc., 1933, 55, 1481–1494. 25 A. Ronlan, J. Coleman, O. Hammerich and V. D. Parker, J. Am. Chem. Soc., 1974, 96, 845–849. 26 D. S. Acker, R. J. Harder, W. R. Hertler, W. Mahler, L. R. Melby, R. E. Benson and W. E. Mochel, J. Am. Chem. Soc., 1960, 82, 6408–6409. 27 A. Yildiz and H. Baumgrtel, Bunsen-Ges. Phys. Chem., Ber., 1977, 81, 1177–1182. 28 H. Almen, T. Bauer, S. Hnig, V. Kupik, U. Langohr, T. Metzenthin, K. Meyer, H. Rieder, J. U. von Schtz, E. Tillmanns and H. C. Wolf, Angew. Chem., Int. Ed. Engl., 1991, 30, 561–563. 29 V. Enkelmann, Angew. Chem., Int. Ed. Engl., 1991, 30, 1121–1123. 30 A. H. Maki and D. H. Geske, J. Am. Chem. Soc., 1961, 83, 1852–1860. 31 C. Chan-Leonor, S. L. Martin and D. K. Smith, J. Org. Chem., 2005, 70, 10817–10822. 32 H. Ehrhardt and S. Hnig, Tetrahedron Lett., 1976, 17, 3515–3518. 33 S. Hnig and W. Schenk, Liebigs Ann. Chem., 1979, 1523–1533. 34 D. L. Coffen, J. Q. Chambers, D. R. Williams, P. E. Garrett and N. D. Canfield, J. Am. Chem. Soc., 1971, 93, 2258–2268.

5428 | Chem. Commun., 2014, 50, 5413--5430

ChemComm 35 F. Wudl, D. Wobschall and E. J. Hufnagel, J. Am. Chem. Soc., 1972, 94, 670–672. 36 E. M. Engler, F. B. Kaufman, D. C. Green, C. E. Klots and R. N. Compton, J. Am. Chem. Soc., 1975, 97, 2921–2922. 37 M. R. Bryce, E. Fleckenstein and S. Hunig, J. Chem. Soc., Perkin Trans. 2, 1990, 1777–1783. ¨nig and H.-C. Steinmetzer, Justus Liebigs Ann. Chem., 1976, 38 S. Hu 1039–1059. ¨nig and H.-C. Steinmetzer, Justus Liebigs Ann. Chem., 1976, 39 S. Hu 1060–1089. ¨nig and F. Linhart, Justus Liebigs Ann. Chem., 1976, 317–335. 40 S. Hu 41 R. A. Mackay, J. R. Landolph and E. J. Poziomek, J. Am. Chem. Soc., 1971, 93, 5026–5030. 42 M. Itoh and S. Nagakura, Bull. Chem. Soc. Jpn., 1966, 39, 369–375. 43 J. R. Kirtley and J. Mannhart, Nat. Mater., 2008, 7, 520–521. 44 M. B. Nielsen, C. Lomholt and J. Becher, Chem. Soc. Rev., 2000, 29, 153–164. 45 M. R. Bryce, Adv. Mater., 1999, 11, 11–23. 46 P. M. S. Monk, The Viologens: Physicochemical Properties, Synthesis and Applications of the Salts of 4,4 0 -Bipyridine, Wiley VCH, 1998. 47 L. Michaelis and E. S. Hill, J. Gen. Physiol., 1933, 16, 859–873. ´, V. Balzani, A. Credi, S. Silvi and J. F. Stoddart, Science, 48 J. D. Badjic 2004, 303, 1845–1849. 49 R. A. Bissell, E. Cordova, A. E. Kaifer and J. F. Stoddart, Nature, 1994, 369, 133–137. 50 C. G. Claessens and J. F. Stoddart, J. Phys. Org. Chem., 1997, 10, 254–272. 51 E. H. Yonemoto, R. L. Riley, Y. I. Kim, S. J. Atherton, R. H. Schmehl and T. E. Mallouk, J. Am. Chem. Soc., 1992, 114, 8081–8087. 52 R. J. Mortimer, Electrochim. Acta, 1999, 44, 2971–2981. 53 C. L. Bird and A. T. Kuhn, Chem. Soc. Rev., 1981, 10, 49–82. 54 C. J. Schoot, J. J. Ponjee, H. T. van Dam, R. A. van Doorn and P. T. Bolwijn, Appl. Phys. Lett., 1973, 23, 64–65. 55 A. Yasuda, H. Mori, Y. Takehana, A. Ohkoshi and N. Kamiya, J. Appl. Electrochem., 1984, 14, 323–327. 56 P. M. Monk, J. Electroanal. Chem., 1997, 432, 175–179. 57 H. J. Byker and M. I. Holland, US Pat., US4902108, 1990. 58 R. N. Dominey, T. J. Lewis and M. S. Wrighton, J. Phys. Chem., 1983, 87, 5345–5354. 59 H. Akahoshi, S. Toshima and K. Itaya, J. Phys. Chem., 1981, 85, 818–822. 60 J. Stepp and J. B. Schlenoff, J. Electrochem. Soc., 1997, 144, L155–L158. 61 G. Decher, Science, 1997, 277, 1232–1237. ¨tzel, Nature, 1991, 353, 737–740. 62 B. O’Regan and M. Gra 63 X. Marguerettaz, R. O’Neill and D. Fitzmaurice, J. Am. Chem. Soc., 1994, 116, 2629–2630. 64 R. Cinnsealach, G. Boschloo, S. N. Rao and D. Fitzmaurice, Sol. Energy Mater. Sol. Cells, 1998, 55, 215–223. 65 R. Cinnsealach, G. Boschloo, S. N. Rao and D. Fitzmaurice, Sol. Energy Mater. Sol. Cells, 1999, 57, 107–125. ˆte, E. Gogniat, F. Campus, L. Walder and M. Gra ¨tzel, 66 P. Bonho Displays, 1999, 20, 137–144. 67 M. O. M. Edwards, G. Boschloo, T. Gruszecki, H. Pettersson, R. Sohlberg and A. Hagfeldt, Electrochim. Acta, 2001, 46, 2187–2193. 68 M. Edwards, M. Andersson, T. Gruszecki, H. Pettersson, R. Thunman, G. Thuraisingham, L. Vestling and A. Hagfeldt, J. Electroanal. Chem., 2004, 565, 175–184. 69 P. Periyat, N. Leyland, D. E. McCormack, J. Colreavy, D. Corr and S. C. Pillai, J. Mater. Chem., 2010, 20, 3650–3655. 70 M. Freitag and E. Galoppini, Langmuir, 2010, 26, 8262–8269. 71 D. Corr, U. Bach, D. Fay, M. Kinsella, C. McAtamney, F. O’Reilly, S. N. Rao and N. Stobie, Solid State Ionics, 2003, 165, 315–321. 72 D. Cummins, G. Boschloo, M. Ryan, D. Corr, S. N. Rao and D. Fitzmaurice, J. Phys. Chem. B, 2000, 104, 11449–11459. ¨tzel, Nature, 2001, 409, 575–576. 73 M. Gra 74 X. Tu, X. Fu and Q. Jiang, Displays, 2010, 31, 150–154. 75 P. Gawrys, D. Boudinet, M. Zagorska, D. Djurado, J.-M. Verilhac, ćaud, S. Pouget and A. Pron, Synth. Met., 2009, G. Horowitz, J. Pe 159, 1478–1485. 76 X. Zhan, A. Facchetti, S. Barlow, T. J. Marks, M. A. Ratner, M. R. Wasielewski and S. R. Marder, Adv. Mater., 2011, 23, 268–284. 77 S. Mazur, P. S. Lugg and C. Yarnitzky, J. Electrochem. Soc., 1987, 134, 346–353. ¨llen and A. J. Bard, 78 S. K. Lee, Y. Zu, A. Herrmann, Y. Geerts, K. Mu J. Am. Chem. Soc., 1999, 121, 3513–3520.


View Article Online


ChemComm 79 P. Gawrys, G. Louarn, M. Zagorska and A. Pron, Electrochim. Acta, 2011, 56, 3429–3435. 80 K. Nakamura, Y. Oda, T. Sekikawa and M. Sugimoto, Jpn. J. Appl. Phys., 1987, 26, 931–935. 81 H. Urano, S. Sunohara, H. Ohtomo and N. Kobayashi, J. Mater. Chem., 2004, 14, 2366–2368. 82 N. Kobayashi, S. Miura, M. Nishimura and H. Urano, Sol. Energy Mater. Sol. Cells, 2008, 92, 136–139. 83 N. Kobayashi, S. Miura, M. Nishimura and Y. Goh, Electrochim. Acta, 2007, 53, 1643–1647. 84 W. Sharmoukh, K. C. Ko, J. H. Ko, H. J. Nam, D.-Y. Jung, C. Noh, J. Y. Lee and S. U. Son, J. Mater. Chem., 2008, 18, 4408–4413. 85 U. Mitschke and P. Bauerle, J. Mater. Chem., 2000, 10, 1471–1507. ´das, Chem. Phys. Lett., 2000, 327, 13–17. 86 M. Malagoli and J. L. Bre 87 P. J. Low, M. A. J. Paterson, H. Puschmann, A. E. Goeta, J. A. K. Howard, C. Lambert, J. C. Cherryman, D. R. Tackley, S. Leeming and B. Brown, Chem.–Eur. J., 2004, 10, 83–91. 88 A. V. Szeghalmi, M. Erdmann, V. Engel, M. Schmitt, S. Amthor, ¨ll, R. Stahl, C. Lambert, D. Leusser, D. Stalke, V. Kriegisch, G. No M. Zabel and J. Popp, J. Am. Chem. Soc., 2004, 126, 7834–7845. 89 S.-H. Cheng, S.-H. Hsiao, T.-H. Su and G.-S. Liou, Macromolecules, 2005, 38, 307–316. 90 C.-W. Chang, G.-S. Liou and S.-H. Hsiao, J. Mater. Chem., 2007, 17, 1007–1015. 91 S.-H. Hsiao, G.-S. Liou, Y.-C. Kung and H.-J. Yen, Macromolecules, 2008, 41, 2800–2808. 92 G.-S. Liou and C.-W. Chang, Macromolecules, 2008, 41, 1667–1674. 93 H.-J. Yen and G.-S. Liou, Chem. Mater., 2009, 21, 4062–4070. 94 H.-J. Yen and G.-S. Liou, J. Mater. Chem., 2010, 20, 9886–9894. 95 H.-J. Yen, H.-Y. Lin and G.-S. Liou, Chem. Mater., 2011, 23, 1874–1882. 96 C. L. Gaupp, D. M. Welsh and J. R. Reynolds, Macromol. Rapid Commun., 2002, 23, 885–889. üerle, ¨nig, M. Kemmer, H. Wenner, I. F. Perepichka, P. Ba 97 S. Hu A. Emge and G. Gescheid, Chem.–Eur. J., 1999, 5, 1969–1973. ¨nig, M. Kemmer, H. Wenner, F. Barbosa, G. Gescheidt, 98 S. Hu üerle, A. Emge and K. Peters, Chem.–Eur. J., I. F. Perepichka, P. Ba 2000, 6, 2618–2632. üerle and ¨nig, I. F. Perepichka, M. Kemmer, H. Wenner, P. Ba 99 S. Hu A. Emge, Tetrahedron, 2000, 56, 4203–4211. 100 N. Tyutyulkov, Polymethine dyes: structure and properties, St. Kliment Ohridski University Press, 1991. ¨tter and M. Goldmann, Ber. Dtsch. Chem. Ges., 1906, 39, 101 R. Willsta 3765–3776. 102 S. Ito, S. Kikuchi, T. Okujima, N. Morita and T. Asao, J. Org. Chem., 2001, 66, 2470–2479. 103 S. Ito, A. Nomura, N. Morita, C. Kabuto, H. Kobayashi, S. Maejima, K. Fujimori and M. Yasunami, J. Org. Chem., 2002, 67, 7295–7302. 104 S. Ito, H. Inabe, N. Morita, K. Ohta, T. Kitamura and K. Imafuku, J. Am. Chem. Soc., 2003, 125, 1669–1680. 105 S. Ito and N. Morita, Eur. J. Org. Chem., 2009, 4567–4579. 106 T. Shoji, J. Higashi, S. Ito, T. Okujima and N. Morita, Eur. J. Org. Chem., 2011, 584–592. 107 M. Horner and S. Huenig, J. Am. Chem. Soc., 1977, 99, 6120–6122. 108 M. Horner and S. Huenig, J. Am. Chem. Soc., 1977, 99, 6122–6124. 109 T. Suzuki, J.-i. Nishida and T. Tsuji, Angew. Chem., Int. Ed. Engl., 1997, 36, 1329–1331. 110 T. Suzuki, J.-i. Nishida and T. Tsuji, Chem. Commun., 1998, 2193–2194. 111 T. Suzuki, E. Ohta, H. Kawai, K. Fujiwara and T. Fukushima, Synlett, 2007, 0851–0869. üerle and A. Emge, Chem.–Eur. J., 2001, ¨nig, C. A. Briehn, P. Ba 112 S. Hu 7, 2745–2757. üerle, C. Briehn, M. Scha ¨ferling, ¨nig, S. Aldenkortt, P. Ba 113 S. Hu I. Perepichka, D. Stalke and B. Walfort, Eur. J. Org. Chem., 2002, 1603–1613. 114 J. Y. Lim, H. C. Ko and H. Lee, Synth. Met., 2005, 155, 595–598. 115 R. Mortimer and N. Rowley, Comprehensive Coordination Chemistry II: From Biology to Nanotechnology, Elsevier Pergamon, Oxford, UK, 2003, vol. 9, pp. 581–619. 116 C. M. Elliott and J. G. Redepenning, J. Electroanal. Chem., 1986, 197, 219–232. 117 F. Pichot, J. H. Beck and C. M. Elliott, J. Phys. Chem. A, 1999, 103, 6263–6267.


Feature Article 118 H. D. Abruna, P. Denisevich, M. Umana, T. J. Meyer and R. W. Murray, J. Am. Chem. Soc., 1981, 103, 1–5. 119 C. D. Ellis, L. D. Margerum, R. W. Murray and T. J. Meyer, Inorg. Chem., 1983, 22, 1283–1291. 120 C. P. Horwitz and Q. Zuo, Inorg. Chem., 1992, 31, 1607–1613. 121 L. Motiei, M. Lahav, A. Gulino, M. A. Iron and M. E. van der Boom, J. Phys. Chem. B, 2010, 114, 14283–14286. 122 L. Motiei, M. Lahav, D. Freeman and M. E. van der Boom, J. Am. Chem. Soc., 2009, 131, 3468–3469. 123 P. Moskalev and I. Kirin, Opt. Spectrosc., 1970, 29, 220. 124 P. R. Somani and S. Radhakrishnan, Mater. Chem. Phys., 2003, 77, 117–133. 125 M. M. Nicholson and F. A. Pizzarello, J. Electrochem. Soc., 1980, 127, 2617–2620. 126 M. M. Nicholson and F. A. Pizzarello, J. Electrochem. Soc., 1980, 127, 821–827. 127 M. M. Nicholson and F. A. Pizzarello, J. Electrochem. Soc., 1979, 126, 1490–1495. 128 M. M. Nicholson and F. A. Pizzarello, J. Electrochem. Soc., 1981, 128, 1740–1743. 129 G. C. S. Collins and D. J. Schiffrin, J. Electrochem. Soc., 1985, 132, 1835–1842. 130 F. A. Pizzarello and M. M. Nicholson, J. Electrochem. Soc., 1981, 128, 1288–1290. 131 G. Collins and D. Schiffrin, J. Electroanal. Chem., 1982, 139, 335–369. 132 M. L. Rodriguez-Mendez, R. Aroca and J. A. DeSaja, Chem. Mater., 1993, 5, 933–937. 133 M. L. Rodriguez-Mendez, R. Aroca and J. A. DeSaja, Chem. Mater., 1992, 4, 1017–1020. 134 C. Granito, L. M. Goldenberg, M. R. Bryce, A. P. Monkman, L. Troisi, L. Pasimeni and M. C. Petty, Langmuir, 1996, 12, 472–476. 135 M. L. Rodriguez-Mendez, J. Souto, J. A. de Saja and R. Aroca, J. Mater. Chem., 1995, 5, 639–642. 136 H. Li and T. F. Guarr, J. Electroanal. Chem., 1991, 297, 169–183. 137 D. J. Moore and T. F. Guarr, J. Electroanal. Chem., 1991, 314, 313–321. 138 N. Trombach, O. Hild, D. Schlettwein and D. Wohrle, J. Mater. Chem., 2002, 12, 879–885. 139 P. M. Beaujuge, S. Ellinger and J. R. Reynolds, Nat. Mater., 2008, 7, 795–799. 140 J. Fei, K. G. Lim and G. T. R. Palmore, Chem. Mater., 2008, 20, 3832–3839. ¨tz, E. Reinold, A. Vogt, S. Schmid, J. L. Segura, 141 H.-B. Bu, G. Go ´mez and P. Peter Ba üerle, Tetrahedron, 2011, 67, R. Blanco, R. Go 1114–1125. ¨ger and J. Strohmeier, 142 W. Schuhmann, C. Kranz, H. Wohlschla Biosens. Bioelectron., 1997, 12, 1157–1167. 143 G. Bidan, A. Deronzier and J.-C. Moutet, J. Chem. Soc., Chem. Commun., 1984, 1185–1186. üerle and K.-U. Gaudl, Adv. Mater., 1990, 2, 185–188. 144 P. Ba 145 H. C. Ko, S. ah Park, W. kie Paik and H. Lee, Synth. Met., 2002, 132, 15–20. 146 H. C. Ko, J. Yom, B. Moon and H. Lee, Electrochim. Acta, 2003, 48, 4127–4135. 147 J. Y. Lim, H. C. Ko and H. Lee, Synth. Met., 2006, 156, 695–698. 148 M. Vorotyntsev, M. Casalta, E. Pousson, L. Roullier, G. Boni and C. Moise, Electrochim. Acta, 2001, 46, 4017–4033. 149 M. Skompska, M. A. Vorotyntsev, M. Refczynska, J. Goux, E. Lesniewska, G. Boni and C. Moise, Electrochim. Acta, 2006, 51, 2108–2119. 150 J. Arias-Pardilla, T. Otero, R. Blanco and J. Segura, Electrochim. Acta, 2010, 55, 1535–1542. 151 S. Gottesfeld, A. Redondo, I. Rubinstein and S. W. Feldberg, J. Electroanal. Chem., 1989, 265, 15–22. 152 P. Denisevich, K. W. Willman and R. W. Murray, J. Am. Chem. Soc., 1981, 103, 4727–4737. 153 D. M. DeLongchamp, M. Kastantin and P. T. Hammond, Chem. Mater., 2003, 15, 1575–1586. 154 C. T. Hable, R. M. Crooks and M. S. Wrighton, J. Phys. Chem., 1989, 93, 1190–1192. 155 C. T. Hable, R. M. Crooks, J. R. Valentine, R. Giasson and M. S. Wrighton, J. Phys. Chem., 1993, 97, 6060–6065. 156 H. C. Ko, M. Kang, B. Moon and H. Lee, Adv. Mater., 2004, 16, 1712–1716.

Chem. Commun., 2014, 50, 5413--5430 | 5429

View Article Online


Feature Article ˙ak and M. Łapkowski, Pol. J. Chem., 2004, 78, 157 A. Czardybon, J. Z 1533–1541. 158 M. Łapkowski and G. Bidan, J. Electroanal. Chem., 1993, 362, 249–256. 159 W. Choi, H. C. Ko, B. Moon and H. Lee, J. Electrochem. Soc., 2004, 151, E80–E83. ´mez, E. Reinold and P. Ba üerle, Org. Lett., 2005, 160 J. L. Segura, R. Go 7, 2345–2348.

5430 | Chem. Commun., 2014, 50, 5413--5430

ChemComm ´mez, R. Blanco, E. Reinold and P. Ba üerle, Chem. 161 J. L. Segura, R. Go Mater., 2006, 18, 2834–2847. 162 N. C. Tansil, E. A. B. Kantchev, Z. Gao and H.-h. Yu, Chem. Commun., 2011, 47, 1533–1535. 163 H.-H. Yu, J. Y. Ying and E. A. B. Kantchev, US Pat., WO 2008/057054 A1, 2008. 164 M. Sassi, M. M. Salamone, R. Ruffo, C. M. Mari, G. A. Pagani and L. Beverina, Adv. Mater., 2012, 24, 2004–2008.


Blending conjugated polymers without phase separation for fluorescent colour tuning of polymeric materials through FRET.

Tuning the thermal conductivity of solar cell polymers through side chain engineering.

Conjugated Oligomers and Polymers Sheathed with Designer Side Chains.

Functionalization of boron diiminates with unique optical properties: multicolor tuning of crystallization-induced emission and introduction into the main chain of conjugated polymers.

Fully conjugated ladder polymers.

Side Chain Influence on the Morphology and Photovoltaic Performance of 5-Fluoro-6-alkyloxybenzothiadiazole and Benzodithiophene Based Conjugated Polymers.

Electrochemical post-functionalization of conducting polymers.

Use of side-chain for rational design of n-type diketopyrrolopyrrole-based conjugated polymers: what did we find out?

Polymerization of low molecular weight hydrogelators to form electrochromic polymers.

Microfluidic Crystal Engineering of π-Conjugated Polymers.

Orienting semi-conducting π-conjugated polymers.

Preparation and Chemical Properties of π-Conjugated Polymers Containing Indigo Unit in the Main Chain.

Effect of side-chain carbonyl groups on the interface of vinyl polymers with water.

Conjugated polymers: Long and winding polymeric roads.

Dependence of crystallite formation and preferential backbone orientations on the side chain pattern in PBDTTPD polymers.

Influence of side-chain regiochemistry on the transistor performance of high-mobility, all-donor polymers.

The glass transition of polymers with different side-chain stiffness confined in free-standing thin films.

Nanoparticles of conjugated polymers prepared from phase-separated films of phospholipids and polymers for biomedical applications.

Aqueous Processing for Printed Organic Electronics: Conjugated Polymers with Multistage Cleavable Side Chains.

Nucleic acid chemistry in the organic phase: from functionalized oligonucleotides to DNA side chain polymers.

Recent advances in metathesis-derived polymers containing transition metals in the side chain.

Multi-Functionalization of Polymers by Strain-Promoted Cycloadditions.

Very low band gap thiadiazoloquinoxaline donor-acceptor polymers as multi-tool conjugated polymers.

The Optical Signature of Charges in Conjugated Polymers.