DOI: 10.1002/chem.201304611
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& Electron Transfer
Functionalization of a Ruthenium–Diacetylide Organometallic Complex as a Next-Generation Push–Pull Chromophore Samuel De Sousa,[a] Laurent Ducasse,[a] Brice Kauffmann,[b] Thierry Toupance,*[a] and Cline Olivier*[a]
choring carboxylic acid function, was further adsorbed onto a semiconducting metal oxide porous thin film to serve as a photosensitizer in hybrid solar cells. The resulting photoactive material, when embedded in dye-sensitized solar cell devices, showed a good spectral response with a broad incident photon-to-current conversion efficiency profile and a power conversion efficiency that reached 7.3 %. Thus, this material paves the way to a new generation of organometallic chromophores for photovoltaic applications.
Abstract: The design and preparation of an asymmetric ruthenium–diacetylide organometallic complex was successfully achieved to provide an original donor–p–[M]–p–acceptor architecture, in which [M] corresponds to the [Ru(dppe)2] (dppe: bisdiphenylphosphinoethane) metal fragment. The charge-transfer processes occurring upon photoexcitation of the push–pull metal–dialkynyl s complex were investigated by combining experimental and theoretical data. The novel push–pull complex, appropriately end capped with an an-
Introduction
ethane), as the metal fragment showed exceptional propensity to serve as efficient electron relays in nanoscale oligomeric structures.[9] By taking advantage of the linear geometry of the trans-ditopic ruthenium–diacetylide unit and its p-unsaturated character, we anticipated that such a motif may act as an efficient relay within a “push–pull” architecture and, therefore, may find application in the field of dye-sensitized solar cells (DSCs), which are hybrid organic–inorganic devices that are able to convert light into electricity.[10] DSCs are nowadays considered as one of the most promising technologies towards solarenergy exploitation, with a record power conversion efficiency (PCE) of up to 12.3 % at the laboratory scale.[11] To achieve efficient light harvesting, the DSC device relies on pigment molecules covalently anchored onto a semiconductor metal oxide, which is a porous and high-surface-area thin film of TiO2. In this context, significant attention has been given to the synthesis of visible-light absorbers based on ruthenium coordination complexes.[12] Despite their good efficiencies, such metal– organic compounds exclusively consist of polypyridyl derivatives and thus limit the development of metal-complex-based sensitizers for DSC applications. In the meanwhile, many efforts have been devoted to the design and preparation of fully organic dyes showing a donor–p–acceptor (D–p–A) structure, such as indoline, triarylamine, or carbazole derivatives.[13] Organometallic complexes have been shown to exhibit higher conductance and weaker length dependence than those of fully organic counterparts of comparable length.[14] Thus, we postulated that the [Ru(dppe)2] metal fragment, if directly incorporated in the same plane as an organic p-conjugated backbone, may tune the optical and electronic properties of dyes and, therefore, enhance the performance of DSC devices. With the exception of platinum–acetylide polymers,
Transition-metal complexes are important targets for the investigation of charge-transfer processes.[1] In particular, organometallic complexes containing carbon-rich p-conjugated ligands present outstanding electronic and structural properties that lead to a wide range of applications, such as nonlinear optics,[2] photoluminescence,[3] and molecular electronics.[4] Metal–alkynyl s complexes[5] have been readily employed to design molecular wires that are simple electronic components with the ability to transport electronic charges between two points. Such complexes notably allowed the study of intramolecular charge-transport mechanisms over long distances.[6] However, these studies mainly focused on end-capped bimetallic species, while the introduction of one or more metal centers, based on various transition metals,[7] within extended organic p-conjugated systems, also allowed surveys of electronic communication through the metal fragment. In particular, the ability of ruthenium to operate as a connector allowing electron flow to occur between different elements in trans-ditopic architectures was demonstrated.[8] Examples with molecular wires, including [Ru(dppe)2] (dppe: bisdiphenylphosphino[a] S. De Sousa, Dr. L. Ducasse, Prof. T. Toupance, Dr. C. Olivier Universit de Bordeaux UMR-CNRS 5255 Institut des Sciences Molculaires 351 cours de la Libration, 33405 Talence (France) E-mail: [email protected] [email protected] [b] Dr. B. Kauffmann Institut Europen de Chimie et Biologie Universit de Bordeaux, UMS-3033 CNRS-INSERM 2 rue Robert Escarpit, 33600 Pessac (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304611. Chem. Eur. J. 2014, 20, 1 – 9
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Full Paper which have been used as donors in polymer-based solar cells,[15] few examples of metal–alkynyl s complexes have been reported for solar-cell applications. A rodlike diruthenium–acetylide complex was recently utilized as a donor in bulk heterojunction solar cells and led to 0.1 % PCE upon blending with a methanofullerene derivative.[16] The unique series of diacetylide–metal complexes used as photosensitizers in DSCs was recently described by Tian and co-workers.[17] These platinumbased dyes, with a D–p–[M]–p–A structure, in which [M] was [Pt(Et3)2], D was triphenylamine, and A was cyanoacrylic acid, afforded up to 4.2 % PCE. In the light of our previous studies on organic chromophores for DSCs,[18] we explored the preparation of an original ruthenium–diacetylide push–pull complex featuring a carbazole motif as the electron-donor part and a cyanoacrylic acid moiety as the electron-withdrawing and anchoring group. Although many examples of unsymmetrical diacetylide–transition-metal complexes have already been described,[19–21] asymmetric diacetylide s complexes featuring strong donor and/or acceptor groups have scarcely been reported.[17, 22] Indeed the synthesis of such a push–pull complex, especially one functionalized with a carboxylic acid anchoring group, can be somewhat delicate and represents a great challenge in the field of organometallics. Herein, we report the preparation and optoelectronic characterization of the first example of a photosensitizer for DSCs based on the D–p–[M]–p–A design concept, in which [M] is the [Ru(dppe)2] metal fragment. The photovoltaic properties of the new complex embedded in solar cell devices were also investigated.
plex. Stable ruthenium vinylidene complexes are easily obtained by treating the 16-electron species [(dppe)2RuCl][TfO] ([1][TfO]) with the appropriate alkyne,[23] in this case N-(4-ethynylphenyl)carbazole (2). The subsequent reaction of [3][TfO] with 4-ethynylbenzaldehyde, in the presence of the noncoordinating NaPF6 salt and Et3N as a base, led to the formation of the bis(s-arylacetylide) intermediate complex 4. The latter was endowed with an aldehyde function to allow the further introduction of the electron-withdrawing cyanoacrylic acid function. Knoevenagel condensation was achieved by using piperidine and cyanoacetic acid in the protected form of 2-trimethylsilylethyl cyanoethanoate. Protection of the carboxylic acid function was necessary to avoid side reactions with the complex during the condensation step. Silyl ester deprotection of 5 under mild reaction conditions, by using Bu4NF in THF at room temperature, led to the desired diacetylide complex 6 in good yield. All of the new compounds were fully characterized by means of different techniques. In particular, 31P NMR spectrum of the target complex 6 displays a sharp singlet at d = 53.3 ppm, which is characteristic of the trans-ditopic geometry of the ruthenium–diacetylide unit. FTIR analysis also allowed characterization of 6, the spectrum of which shows an intense absorption band at n˜ = 2045 cm1 corresponding to the n(CC) stretch of the s-diacetylide–metal fragment. In addition, typical vibrational stretchings for the cyanoacrylic acid motif were clearly identified: a peak of low intensity at n˜ = 2218 cm1 for the n(CN) stretch and two peaks at n˜ = 1713 and 1172 cm1 corresponding to the n(C=O) and n(CO) stretches of the carboxylic acid function.
Optoelectronic properties
Results and Discussion
The electronic absorption spectrum of the new push–pull complex 6, recorded in dichloromethane, shows intense highenergy absorption bands that can be ascribed to p!p* intraligand charge transfers (ILCTs) involving the diphosphine ligands and the electron-rich carbazole motif (Figure 1 and Table 1). More interestingly, the spectrum shows an intense band in the visible region, with a maximum centered at l = 520 nm and
Synthesis and characterizations The synthetic strategy to obtain the dissymmetric ruthenium– diacetylide complex 6 is depicted in Scheme 1. The first step consists of the formation of a vinylidene moiety, [3][TfO], featuring the electron-donor part of the target push–pull com-
Scheme 1. Synthetic route to complex 6: a) CH2Cl2, RT, 20 h; b) 4-ethynylbenzaldehyde, NaPF6, Et3N, CH2Cl2, RT, 20 h; c) trimethylsilylethyl cyanoethanoate, piperidine, CHCl3, reflux, 68 h; d) Bu4NF, THF, RT, 20 h. TfO : trifluoromethanesulfonate; TMSE: trimethylsilylethyl.
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Full Paper energy level of the push–pull chromophore, is located at a particularly well-suited position with regard to the standard potential of iodine/iodide (approximately + 0.45 V vs. NHE), that is, the redox shuttle contained in the electrolyte of DSC devices. An optimal potential difference of 0.28 V between the oxidation potential of 6 and the I3/I redox couple should ensure efficient driving force for the regeneration of the dye.[26] The LUMO level was deduced from the HOMO position and the optical band-gap energy (Eg = 2.1 eV), estimated from the intercept between the normalized absorption and emission spectra. As a result, the LUMO level of 6 (1.37 V vs. NHE) is more negative than the conduction band edge of TiO2 (approximately 0.50 V vs. NHE), which should allow electron injection from the complex in its excited state to the semiconductor.
Figure 1. UV/Vis absorption spectra of 6 in CH2Cl2 solution (1.7 105 m; solid line) and anchored on a 3 mm thick transparentTiO2 film (dashed line; the cutoff in the UV region is due to TiO2 absorption).
a high extinction coefficient of e = 30 600 L mol1 cm1. The broad shape of this band indicates that multiple transitions close in energy are involved, but with a major contribution from the allowed HOMO!LUMO transition. The molecular frontier orbitals of acetylide s complexes result from a considerable mixing of the Ru dp orbitals with the alkynyl p orbitals of the carbon-rich ligands. In consequence, the broad absorption band observed in the visible region is likely to present a strong metal-to-ligand charge transfer (MLCT) character. The appealing optical properties of 6 are comparable to those of organic sensitizers used in DSCs; better yet, the introduction of the [Ru(dppe)2] metal center into the conjugated pathway of the push–pull chromophore induces significant redshift of the absorption band and increases the extinction coefficient relative to that of the analogous fully organic carbazole-based dyes.[24] The electrochemical properties of 6 were investigated by means of cyclic voltammetry in CH2Cl2 solution (Figure S1 in the Supporting Information) and the corresponding data are summarized in Table 1. The ruthenium–diacetylide complex 6 undergoes one monoelectronic reversible oxidation process (Eox = + 0.60 vs. FeCp*2 (Cp*: pentamethylcyclopentadienyl) = + 0.73 V vs. NHE)[25] followed by an irreversible process at higher potential, which is consistent with a chemical reaction of the second oxidized species. Due to the electronic enrichment stemming from the introduction of the metal fragment, the first oxidation potential of 6 is significantly more cathodic than those of fully organic analogues.[20] As a consequence, the oxidation potential of 6, which can be related to the HOMO
Crystallographic studies Attempts to grow crystals suitable for X-ray structure determination of complex 6 remained unsuccessful, insofar as carboxylic acids are notoriously unfavorable to crystallogenesis. However, single crystals of the unsymmetrical diacetylide complex in the protected form, 5, were grown by slow diffusion of hexane into concentrated solutions of the complex in a dichloromethane/acetonitrile mixture, and the crystal structure was solved by X-ray diffraction analyses. Complex 5 crystallized in the C2/c monoclinic space group. The unit cell includes eight molecules of the complex with as many molecules of hexane and thus shows a very large volume of 17 024 3 with cell dimensions a = 74.43, b = 12.16, and c = 18.94 . Perspective drawings of the molecular structure of 5 are depicted in Figure 2 and the crystal data are summarized in the Supporting Information. The molecular structure of 5 confirms the linear arrangement of the molecular backbone with a trans configuration of the carbon-rich ligands on either side of the [Ru(dppe)2] fragment, and an angle of CaRuCa’ = 174.5(3)8. Both carbon chains show a net acetylide character with bond length values of 2.055 (2.077), 1.221 (1.206), and 1.431 (1.451) for the RuCa (RuCa’), CaCb (Ca’Cb’), and Cb
Table 1. UV/Vis absorption data and electrochemical properties of 6. lmax [nm][a] (e [m1 cm1])
lmax [nm][b]
HOMO [V][c] (vs. NHE)
Eg [eV][d]
LUMO [V][e] (vs. NHE)
520 (30 600)
470
+ 0.73
2.1
1.37
[a] Absorption maximum in CH2Cl2. [b] Absorption maximum on a 3 mm thick TiO2 film. [c] Oxidation potential measured in CH2Cl2. NHE: Normal hydrogen electrode. [d] The optical band-gap energy (Eg) estimated from the intersection between the normalized absorption and emission spectra. [e] The LUMO level calculated by subtracting the Eg value from the HOMO level.
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Figure 2. Crystal structure of 5 (top and side views). Protons and solvent molecules were removed for clarity.
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Full Paper Cg (Cb’Cg’) bonds, respectively. The terminal carbazole moiety presents a torsion angle of 74.78 with the adjacent benzyl ring, and the two acetylide chains are arranged in a quasiplanar fashion. This allows for extensive delocalization of electrons over the entire carbon-rich p-conjugated sequence, which is about 23 long.
(HOMO!LUMO + 3 and HOMO!LUMO + 4 transitions) and of the carbazole unit (HOMO4!LUMO + 1). To gain more insight into the charge-transfer properties of the complex, time-dependent (TD) DFT calculations were performed to provide the electronic absorption spectrum of 6 in CH2Cl2 (Figure S4 in the Supporting Information). Although in close agreement with the experimental result, the calculated spectrum exhibits a global blueshift of about 50 nm. We attribute this inconsistency to the fact that TD-DFT calculations exclusively depend on the functional and on the basis set. The MPW1K pseudopotential employed in the present study had already provided reliable results for push–pull organic molecules.[18, 28] Besides, after consideration that 1) the first allowed transition mainly relies on the HOMO/LUMO frontier orbitals (Table S1 in the Supporting Information) and 2) the HOMO, by showing a substantial weight on the Ru atom (Figure 3), is sensitive to the basis set used to describe the metal center, the difference observed between the calculated and experimental spectra might therefore be ascribed to some inadequacy of the basis set (LANL2DZ) in describing a metal-based fragment embedded in a push–pull system. Finally, by applying recently described computational methods,[29] extra parameters can be considered to characterize the charge-transfer (CT) processes occurring upon photoexcitation of push–pull systems. Notably, the spatial overlap (L) between the occupied and unoccupied molecular orbitals involved in the main excitation was calculated to be L = 0.465 (Figure S5 in the Supporting Information). This quantity, which takes a value between 0 and 1, describes the matching between two energy levels, with small values reflecting long-range excitation and large values indicating short-range excitation. The rather low value of spatial overlap calculated for 6 echoes the experimental and calculated lmax absorption at around 500 nm (Table 1, Figure S2 in the Supporting Information). Additionally, the distance (DCT) and quantity (qCT) of transferred charge associated with the ground-to-excited-state transition were assessed. The former corresponds to the distance between the barycenters of the density-depletion and density-increment zones related to the CT excitation, whereas the latter quantifies the amount of charge transferred during the process. A qCT value of 1 corresponds to a pure CT process, whereas a value close to 0 equals a local phenomenon. Calculations related to complex 6 led to DCT = 4.6 and qCT = 1.0, which is higher than the calculated value obtained for a model organic compound of comparable length.[24b] From these data, we infer that the visible-range electronic transition observed for compound 6 shows a net charge-transfer character, strongly localized within the molecular backbone surrounding the [Ru(dppe)2] metal fragment.
Theoretical data The optimized geometry of ruthenium complex 6 again testifies to the linearity of the structure with the acetylide ligands in the trans position on either side of the metal center, as already determined for ruthenium complexes of this type.[8a–b, 9d, 27] Pictures of the frontier molecular orbitals are given in Figure 3. The LUMO level is well localized on the electron-withdrawing acetylide ligand with an important weight on the cyanoacrylic acid group. More interestingly, the HOMO level possesses a strong Ru t2g character, and the orbital is indeed spread over the whole p-conjugated skeleton with a sizable contribution from the metal–s-diacetylide fragment. Furthermore, isodensity surface plots of the transition-involved molecular orbitals and the corresponding energy diagram are shown in Figure S2 and S3 in the Supporting Information, and detailed transition assignments along with calculated oscillator strengths are gathered in Table S1 in the Supporting Information. The intense absorption band in the visible range can be considered as the result of several contributions involving electronic transitions from the first three HOMOs to the LUMO level. The HOMO and HOMO1 orbitals are of a fairly similar nature (that is, delocalized over the whole molecule), whereas HOMO2 exhibits a distinctive feature with distribution on the carbazole motif. Absorption bands at higher energies stem from excitations of the [Ru(dppe)2] fragment
Sensitization of porous titania thin films As a prelude to in-depth photovoltaic measurements, the capacity of new complex 6 for chemisorbing onto nanoparticulate TiO2 films was evidenced by means of UV/Vis absorption and attenuated total reflectance (ATR) FTIR spectroscopy studies.
Figure 3. Isodensity surface plots of the frontier molecular orbitals of 6. Left: HOMO; Right: LUMO.
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Full Paper The UV/Vis absorption spectrum was recorded after chemisorption of 6 on a transparent titania thin film (thickness 3 mm). As shown in Figure 1, the shape of the main absorption band remained similar to the one in solution, although with the maximum shifted to a lower wavelength by 50 nm. Such a blueshift has already been observed upon grafting organic push–pull dyes onto TiO2[18] and can be attributed, to some extent, to dye aggregation, but also to the deprotonation of the carboxylic function.[28] The ATR-FTIR spectra of 6 show the appearance of two bands upon grafting onto TiO2, at n˜ = 1575 and 1360 cm1 for the asymmetric and symmetric vibrational stretchings of the carboxylate groups, and the concomitant collapse of the carboxylic acid n(C=O) stretching peak of the free complex, at n˜ = 1713 cm1 (Figure 4). These data imply that the rutheniumcomplex molecules are anchored on the metal-oxide surface through a bidentate chelation or bridging mode, rather than through an ester-type linkage.
Figure 5. a) IPCE and b) current density–voltage profiles of a DSC with 6 as the dye, under 100 mW cm2 illumination (solid line) and in the dark (dashed line).
to note that the IPCE profile shows an increased red response compared with the absorption spectrum of the sensitizer. This feature can be attributed to interactions occurring between the dye molecules on the surface of the oxide layer. Overall, the effective action spectrum of 6 exceeds 70 % in the spectral range of l = 410–570 nm (above 80 % from l = 440–530 nm), and the tail of the curve extends until l = 700 nm. The photovoltaic performance of ruthenium–diacetylide complex 6 was measured under 100 mW cm2 illumination in AM1.5G conditions; the cell active area, defined by using a black mask, was fixed to 0.159 cm2. For the best solar cell, the overall power conversion efficiency reached 7.32 %, with a short-circuit photocurrent density of Jsc = 15.56 mA cm2, an open-circuit photovoltage of Voc = 0.68 V, and a fill factor of ff = 69.2 %. Under the same conditions, a reference cell realized with the commercial dye N3 (Figure S6 in the Supporting Information) led to a PCE value of 9.07 %, with Jsc = 16.71 mA cm2, Voc = 0.75 V, and ff = 71.9 %. These values clearly demonstrate the ability of the ruthenium–diacetylide complex 6 to work efficiently as a photosensitizer in DSCs. Moreover, in standard devices with an iodinebased electrolyte, compound 6 outperforms fully organic carbazole-based dyes of similar structure, reported by us and others.[18b, 24] In addition, the performance of DSCs incorporating complex 6 remained stable for several weeks under ambient conditions (Figure S7 in the Supporting Information). A slow decrease of the overall efficiency substantially stemmed from a gradual decrease in the Jsc value, despite concomitant increases in the Voc and ff values. The loss in current can be attributed either to slight dye degradation or to slow evaporation of the volatile solvent contained in the electrolyte.
Figure 4. ATR-FTIR spectra of free complex 6 (dashed line) and complex 6 anchored on a TiO2 film (solid line). The spectra were normalized on the CN signal.
The intense band at n˜ = 2045 cm1, attributed to the n(CC) vibration stretch of the s-diacetylide–metal fragment, remained unchanged, which indicates that the trans-ditopic structure of 6 was not altered upon chemisorption onto TiO2. This feature provides evidence that transition-metal–s-alkynyl complexes allow clean surface modification of metal-oxide thin films with preservation of the integrity of the whole functional organometallic core. Photovoltaic properties The ruthenium–diacetylide complex 6 was further embedded in DSCs along with an electrolyte based on the iodine/iodide couple as a redox shuttle. The detailed procedure for device fabrication is given in the Experimental Section, and characteristic incident photon-to-current conversion efficiency (IPCE) and photocurrent density–voltage (J–V) curves are given in Figure 5. Photoanodes were obtained by staining TiO2 films (thickness 15 mm) in CH2Cl2 solutions of 6 containing chenodeoxycholic acid as a coadsorbent. As shown in Figure 5 a, the IPCE profile of the device is consistent with the absorption spectrum of complex 6 after grafting onto TiO2, with the curve reaching a maximum value of 85 % at l = 475 nm. It is worthy Chem. Eur. J. 2014, 20, 1 – 9
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Full Paper Conclusion
133.8 (CH), 132.2 (m, Cq(dppe)), 131.9 (m, Cq(dppe)), 131.8 (CH), 131.3 (CH), 129.2 (CH), 128.7 (CH), 128.5 (CH), 126.7 (CH), 126.3 (CH), 125.7 (Cq), 123.6 (Cq), 120.7 (CH), 120.4 (CH), 109.9 (CH), 109.5 (CH(Ru=C=CH), 29.3 ppm (m, CH2(dppe)); FTIR (KBr): n˜ = 1635–1623 (n=C= 1 (nPPh); HRMS (FD +): m/z calcd for [C72H60ClNP4Ru] + C), 1095 cm + [MH] : 1199.24082; found: 1193.24250.
Asymmetric functionalization of a ruthenium–diacetylide organometallic complex led to a new generation of push–pull chromophores. The first example was designed and successfully obtained through a straightforward synthetic procedure. The molecular structure of s-alkynyl intermediate complex 5 was confirmed by X-ray analyses, and the optoelectronic properties of 6 were determined by combining experimental and theoretical data. The results confirmed the exceptional ability of the [Ru(dppe)2] metal fragment to act as an efficient electron relay in nanoscale systems. Chromophore 6, end capped with an anchoring carboxylic acid function, was readily grafted onto TiO2 mesoporous thin films and employed as a photosensitizer in DSC devices. The new dye operated efficiently and reached up to 7.3 % PCE under standard conditions. That result outperforms fully organic dyes of analogous structure and represents the best efficiency reported so far for s-alkynyl organometallic complexes in solar cells. Overall, this study highlights the beneficial effect of the introduction of an electron-rich metal fragment in a p-conjugated molecular backbone, which allows tuning of the optical and electronic properties of a chromophore. Such a push–pull system is of great interest because it opens up new prospects, both in the field of organometallics and dye-sensitized solar cells. In consideration of further elaboration of organometallic complexes featuring more than one metal center in alternation with visible-light absorbing motifs, the concept of a ruthenium–s-diacetylide complex as the photoactive system therefore represents a versatile approach for the improved collection of solar energy.
trans-[9-Carbazole-(p-C6H4)-CCRu(dppe)2-CC-(p-C6H4)-CHO] (4): Distilled Et3N (0.22 mL, 1.5 mmol, 3 equiv) was added to a solution of [3][TfO] (0.7 g, 0.5 mmol, 1 equiv), 4-ethynylbenzaldehyde (80 mg, 0.6 mmol, 1.1 equiv), and NaPF6 (180 mg, 1 mmol, 2 equiv) in dry CH2Cl2 (75 mL) under an inert atmosphere. The reaction mixture was stirred for 24 h at room temperature. The organic solution was washed with degassed water (3 30 mL) and evaporated to dryness. Precipitation from a CH2Cl2/pentane mixture afforded pure complex 4 as a yellow powder in 80 % yield (0.5 g, 0.4 mmol). 31 P NMR (243 MHz, CD2Cl2): d = 52.98 ppm (s, PPh2); 1H NMR (600 MHz, CD2Cl2): d = 9.90 (s, 1 H), 8.18 (d, 3JHH = 7.6 Hz, 2 H), 7.70– 7.66 (m, 8 H), 7.65 (d, 3JHH = 8.1 Hz, 2 H), 7.50 (d, 3JHH = 7.6 Hz, 2 H), 7.47 (t, 3JHH = 7.6 Hz, 2 H), 7.45–7.41 (m, 8 H), 7.36 (d, 3JHH = 8.4 Hz, 2 H), 7.31 (t, 3JHH = 7.6 Hz, 2 H), 7.26 (t, 3JHH = 7.5 Hz, 4 H), 7.22 (t, 3 JHH = 7.5 Hz, 4 H), 7.08 (t, 3JHH = 7.5 Hz, 8 H), 7.05 (d, 3JHH = 8.4 Hz, 2 H), 6.99 (t, 3JHH = 7.5 Hz, 8 H), 6.79 (d, 3JHH = 8.1 Hz, 2 H), 2.71 ppm (m, 8 H); 13C NMR (150 MHz, CD2Cl2): d = 191.5 (CH(CHO)), 148.1 (quint., Cq(RuC)), 141.5 (Cq), 137.4 (m, Cq(dppe)), 137.2 (Cq), 137.1 (m, Cq(dppe)), 134.7 (CH), 134.4 (CH), 133.3 (quint, Cq(RuC)), 132.9 (Cq), 131.6 (Cq), 131.4 (CH), 130.6 (CH), 130.1 (Cq), 129.6 (CH), 129.4 (CH), 129.2 (CH), 127.6 (CH), 127.5 (CH), 126.6 (CH), 126.2 (CH), 123.6 (Cq), 120.6 (CH), 120.1 (CH), 118.9 (Cq(RuCC)), 117.5 (Cq(RuCC)), 110.3 (CH), 31.8 ppm (m, CH2(dppe)); FTIR (KBr): n˜ = 2045 (nCC), 1684 (nC=O), 1585 (nC=C(Ph p-conj.)), 1096 cm1 (nPPh); HRMS (ESI +): m/z calcd for [C81H65NONaP4Ru] + [M + Na] + : 1316.2952; found: 1316.2966; exact mass deduced from the most intense peak of the isotopic pattern. trans-[9-Carbazole-(pC6H4)-CCRu(dppe)2-CC-(pC6H4)-CH= C(CN)CO2TMSE] (5): Piperidine (80 mg, 0.8 mmol, 4 equiv) was added to a solution of 4 (0.25 g, 0.2 mmol, 1 equiv) and 2-trimethylsilylethyl cyanoethanoate (70 mg, 0.4 mmol, 2 equiv) in dry CHCl3 (35 mL) under an inert atmosphere. The reaction mixture was heated to reflux for 68 h. The organic solution was washed with degassed water (3 20 mL) and evaporated to dryness. Precipitation from a CH2Cl2/pentane mixture afforded pure complex 5 as a red powder in 84 % yield (0.23 g, 0.16 mmol). 31P NMR (243 MHz, CD2Cl2): d = 52.92 ppm (s, PPh2); 1H NMR (600 MHz, CD2Cl2): d = 8.20–8.14 (m, 3 H), 7.83 (d, 3JHH = 8.4 Hz, 2 H), 7.67 (m, 8 H), 7.51– 7.44 (m, 4 H), 7.41 (m, 8 H), 7.36 (d, 3JHH = 8.3 Hz, 2 H), 7.30 (t, 3JHH = 7.2 Hz, 2 H), 7.26 (t, 3JHH = 7.5 Hz, 4 H), 7.22 (t, 3JHH = 7.5 Hz, 4 H), 7.07 (t, 3JHH = 7.5 Hz, 8 H), 7.05 (d, 3JHH = 8.4 Hz, 2 H), 6.99 (t, 3JHH = 7.5 Hz, 8 H), 6.74 (d, 3JHH = 8.3 Hz, 2 H), 4.42 (m, 2 H), 2.70 (m, 8 H), 1.17 (m, 2 H), 0.13 ppm (s, 9 H); 13C NMR (150 MHz, CD2Cl2): d = 164.0 (Cq(COOTMSE)), 154.4 (CH(ethylenic)), 152.7 (quint, Cq(RuC)), 141.5 (Cq), 137.3 (m, Cq(dppe)), 137.0 (m, Cq(dppe)), 136.7 (Cq), 134.7 (CH), 134.4 (CH), 133.0 (quint, Cq(RuC)), 132.9 (Cq), 131.6 (CH), 131.4 (CH), 130.9 (CH), 130.0 (Cq), 129.4 (CH), 127.6 (CH), 127.6 (CH), 126.6 (CH), 126.4 (Cq), 126.2 (CH), 123.6 (Cq), 120.8 (Cq(acrylic)), 120.6 (CH), 120.1 (CH), 117.9 (Cq(RuCC)), 117.3 (Cq(RuCC)), 110.3 (CH), 98.5 (Cq(CN)), 65.0 (CH2), 31.8 (m, CH2(dppe)), 17.7 (CH2), 1.4 ppm (CH3(TMS)); FTIR (KBr): n˜ = 2218 (nCN), 2043 (nCC), 1717 (nC=O), 1569 (nC=C(Ph p-conj.)), 1096 (nP 1 (nSiC); HRMS (ESI +): m/z calcd for Ph), 1171 (nCO), 836 cm [C89H78N2O2NaSiP4Ru] + [M + Na] + : 1483.3718; found: 1483.3866; exact mass deduced from the most intense peak of the isotopic pattern.
Experimental Section Chemistry General: The reactions were carried out under an inert atmosphere by using Schlenk techniques. Solvents were dried over appropriate drying agents (sodium for pentane, diethyl ether, and THF; calcium hydride for dichloromethane, chloroform, and methanol) and freshly distilled under nitrogen before use. All reagents were obtained from commercially available sources and used without further purification, otherwise stated. [(dppe)2RuCl][[TfO] ([1][TfO])[23] and 4ethynylbenzaldehyde[30] were prepared as previously reported. trans-[Cl(dppe)2Ru=C=CH-(pC6H4)-9-carbazole][TfO] ([3][TfO]): In a Schlenk tube under an inert atmosphere, [RuCl(dppe)2][TfO] ([1][TfO]; 1.3 g, 1.2 mmol, 1 equiv) and 9-(4-ethynylphenyl)carbazole (2; 0.65 g, 2.4 mmol, 2 equiv) were dissolved in dry CH2Cl2 (70 mL). The mixture was stirred for 20 h at room temperature. After removal of the solvent, the crude product was washed with freshly distilled Et2O (40 mL) and pentane (2 40 mL). Precipitation from a CH2Cl2/pentane mixture afforded pure [3][TfO] as a dark yellow-green powder in 92 % yield (1.5 g, 1.1 mmol). 31P NMR (243 MHz, CD2Cl2): d = 37.67 ppm (s, PPh2); 1H NMR (600 MHz, CD2Cl2): d = 8.13 (d, 3JHH = 7.7 Hz, 2 H), 7.44 (m, 2 H), 7.41–7.31 (m, 24 H), 7.29 (t, 3JHH = 7.4 Hz, 2 H), 7.27 (d, 3JHH = 8.2 Hz, 2 H), 7.20–7.13 (m, 16 H), 6.88 (d, 3JHH = 8.4 Hz, 2 H), 5.90 (d, 3JHH = 8.4 Hz, 2 H), 3.92 (m, 1 H), 3.01 (m, 4 H), 2.86 ppm (m, 4 H); 13C NMR (150 MHz, CD2Cl2): d = 356.3 (quint, Cq(Ru=C=)), 141.0 (Cq), 135.3 (Cq), 134.5 (CH),
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trans-[9-Carbazole-(pC6H4)-CCRu(dppe)2-CC-(pC6H4)-CH= C(CN)COOH] (6): Bu4NF (1 m in THF, 0.28 mL, 0.28 mmol, 2 equiv) was added to a solution of 5 (0.2 g, 0.14 mmol, 1 equiv) in dry THF
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Full Paper get an appropriate thickness of 10 mm. A diffusing layer, constituted of 250–400 nm TiO2 particles (Solaronix, Ti-Nanoxide R/SP), was deposited on top of the working electrode in order to backscatter the unabsorbed photons towards the transparent layer. This layer reached a thickness of 5 mm. Terpineol and organic binders contained in the titania pastes were removed by gradual thermal treatment under an air flow at 325 (5), 375 (5), 450 (15), and 500 8C (15 min). The as-obtained films were further treated with 40 mm TiCl4 aqueous solution at 70 8C for 30 min; this was followed by annealing at 500 8C for 30 min. After cooling to 40 8C, the electrodes were immersed in 0.3 mm dye solutions in CH2Cl2 containing 1 mm of chenodeoxycholic acid as a coadsorbent and deaggregating agent. Under these conditions, the sensitization time was optimized to 16 h in the dark. Counter electrodes were prepared by drop casting a solution of H2PtCl6 (5 mm in ethanol) onto an FTOcoated glass substrate (TCO22–7, 7 ohm sq1, 2.2 mm thickness, Solaronix). The complex was thermally decomposed in air at 500 8C for 30 min to leave Pt nanoparticles. The photoanode and counter electrode were assembled by using a hot-melt Surlyn polymer gasket (25 mm, DuPont). The electrolyte (0.03 m iodine, 0.05 m lithium iodide, 1.0 m 1,3-dimethylimidazolium iodide, 0.8 m 4-tert-butylpyridine, and 0.1 m guanidinium thiocyanate in acetonitrile/valeronitrile 85:15) was introduced into the cell by vacuum backfilling through a hole drilled in the counter electrode, which was finally sealed with a Surlyn gasket and a glass cover plate.
(25 mL) under an inert atmosphere. The reaction was stirred overnight at room temperature. After solvent removal, the resulting solid was dissolved in CH2Cl2 (25 mL), and the solution was washed with degassed citric acid aqueous solution (10 %, 2 20 mL) and water (20 mL). The organic solution was evaporated to dryness, and the solid was further washed with pentane (2 20 mL). Slow crystallization from a CH2Cl2/pentane mixture afforded pure complex 6 as a deep red powder in 85 % yield (0.16 g, 0.12 mmol). 31 P NMR (162 MHz, [D8]THF): d = 53.31 ppm (s, PPh2); 1H NMR (600 MHz, [D8]THF): d = 8.16–8.12 (m, 3 H), 7.85 (d, 3JHH = 8.2 Hz, 2 H), 7.74–7.68 (m, 8 H), 7.47–7.34 (m, 14 H), 7.25–7.16 (m, 10 H), 7.09 (d, 3JHH = 8.2 Hz, 2 H,), 7.04 (t, 3JHH = 7.5 Hz, 8 H), 6.96 (t, 3JHH = 8.5 Hz, 8 H), 6.77 ppm (d, 3JHH = 8.2 Hz, 2 H); 13C NMR (150 MHz, [D8]THF): d = 164.5 (Cq(COOH)), 153.7 (CH(ethylenic)), 150.5 (quint, Cq(RuC )), 141.8 (Cq), 137.9 (m, Cq(dppe)), 137.5 (m, Cq(dppe)), 136.5 (Cq), 135.1 (CH), 134.8 (CH), 133.5 (quint., Cq(RuC)), 133.4 (Cq), 131.8 (CH), 131.5 (CH), 131.2 (CH), 130.5 (Cq), 129.5 (CH), 129.4 (CH), 127.8 (CH), 127.8 (CH), 127.3 (Cq), 126.7 (CH), 126.3 (CH), 124.1 (Cq), 120.7 (CH), 120.6 (Cq(acrylic)), 120.2 (CH), 118.2 (Cq(RuCC)), 117.2 (Cq(RuCC), 110.5 (CH), 100.2 (Cq(CN)), 32.1 ppm (m, CH2(dppe)); FTIR (KBr): n˜ = 2218 (nCN), 2045 (nCC), 1713 (nC=O), 1570 (nC=C(Ph p-conj.)), 1096 (nPPh), 1172 cm1 (nCO); HRMS (ESI +): m/z calcd for [C84H66N2O2N2P4Ru] + [M] + : 1360.3112; found: 1360.3046; exact mass deduced from the most intense peak of the isotopic pattern; elemental analysis: calcd (%) for C84H66N2O2N2P4Ru: C 74.15, H 4.89, N 2.06; found: C 73.75, H 4.80, N 1.98.
Photovoltaic and cell characterizations Electrochemical measurements
The photovoltaic performances of the devices were measured by using a black mask with an aperture area of 0.159 cm2. The device was illuminated with an AM1.5G solar simulator calibrated with a radiometer (IL 1400BL) to provide an incident irradiance of 100 mW cm2 at the surface of the solar cells. The J–V measurements were performed by using a Keithley model 2400 digital source meter (Keithley) with application of an independent external voltage to the cell and measurement of the photogenerated current out from the cell. Action spectra of the incident photon-tocurrent conversion efficiency were realized by using a Xe lamp associated with a monochromator (Triax 180, Jobin Yvon) to select and increment wavelength irradiation to the cell. No bias light was employed to illuminate the cell. The current produced was measured by steps of 5 nm after 2 s of radiation exposure with a Keithley 6487 picoammeter to be in steady-state conditions. The incident photon flux was measured with a 6 inch diameter calibrated integrated sphere (Labsphere) and a silicon detector.
Cyclic voltammetry analyses were carried out on an Autolab PGSTAT100 potentiostat/galvanostat. Measurements were performed in a CH2Cl2 solution containing 0.1 m Bu4NPF6 as a salt support, at scan rate of 100 mV s1 by using a three-electrode system: the working electrode was a Pt disc, the reference electrode was Ag/AgCl (calibrated with FeCp*2 as an internal reference), and the counter electrode was a Pt mesh. Potentials were afterward referred to NHE by addition of 130 mV.[21]
Computational details DFT and TD-DFT calculations were performed with Gaussian 09.[31] The molecular geometry was optimized by using the B3LYP functional and the LANL2DZ basis set in vacuo conditions. TD-DFT calculations were realized with the MPW1K XC functional containing 42.8 % of HF exchange,[32] with the solvent taken into account within the integral equation formalism of the polarizable continuum model (IEF-PCM).[33] This method had already provided UV/Vis absorption spectra of push–pull molecules in agreement with experimental data.[28]
Acknowledgements This work was supported by the CNRS and the Rgion Aquitaine (Ph.D. grant to S.D.). The authors thank Dr. G. Le Bourdon (ISM, UMR-CNRS 5255) and Dr. L. Hirsch (IMS, UMR-CNRS 5218) for providing access to ATR-FTIR and photovoltaic characterization facilities, respectively.
Electrode preparation and device fabrication Fluorine-doped tin oxide (FTO) coated conducting glass substrates (NSG10, 10 ohm sq1, 3.2 mm thickness, XOPFisica) were cleaned by using successive ultrasonic treatments, first in an alkaline detergent solution and then in ethanol. The conducting glass substrates were afterward treated with 40 mm TiCl4 aqueous solution at 70 8C for 30 min, in order to increase photoanode adhesion while reducing recombination between FTO and I3 at this interface. The photoanodes were prepared by the screen-printing method by using commercially available titania pastes. The transparent conducting oxide substrates were first coated with a transparent layer, composed of 20 nm anatase TiO2 nanoparticles (Solaronix, Ti-Nanoxide T/SP). The screen-printing procedure was repeated three times to Chem. Eur. J. 2014, 20, 1 – 9
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Keywords: electron transfer · organometallic compounds · ruthenium · sensitizers · solar cells [1] a) D. Astruc, Electron Transfer and Radical Processes in Transition-Metal Chemistry, Wiley-VCH, Weinheim, 1995; b) J.-P. Launay, C. Coudret, in Electron Transfer in Chemistry, Vol. 5: Molecular-Level Electronics, Imaging and Information, Energy and Environment (Ed.: V. Balzani), Wiley-VCH, Weinheim 2011, p. 3.
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Jin, Y. Hue, J. Hua, C. Jing, Y. Long, H. Tian, J. Mater. Chem. 2012, 22, 5382 – 5389. a) C. Olivier, F. Sauvage, L. Ducasse, F. Castet, M. Grtzel, T. Toupance, ChemSusChem 2011, 4, 731 – 736; b) S. De Sousa, C. Olivier, L. Ducasse, G. Le Bourdon, L. Hirsch, T. Toupance, ChemSusChem 2013, 6, 993 – 996. a) D. Touchard, P. Haquette, S. Guesmi, L. Le Pichon, A. Daridor, L. Toupet, P. H. Dixneuf, Organometallics 1997, 16, 3640 – 3648; b) D. Touchard, S. Guesmi, L. Le Pichon, A. daridor, P. H. Dixneuf, Inorg. Chim. Acta 1998, 280, 118 – 124; c) C. Lebreton, D. Touchard, L. Le Pichon, A. Daridor, L. Toupet, P. H. Dixneuf, Inorg. Chim. Acta 1998, 272, 188 – 196. a) N. Gauthier, G. Argouarch, F. Paul, M. G. Humphrey, L. Toupet, S. Ababou-Girard, H. Sabbah, P. Hapiot, B. Fabre, Adv. Mater. 2008, 20, 1952 – 1956; b) P. J. West, T. Schwich, M. P. Cifuentes, M. G. Humphrey, J. Organomet. Chem. 2011, 696, 2886 – 2893; c) P. J. West, M. P. Cifuentes, T. Schwich, M. D. Randles, J. P. Morrall, E. Kulasekera, S. Petrie, R. Stranger, M. G. Humphrey, Inorg. Chem. 2012, 51, 10495 – 10502. a) C. Olivier, B. Kim, D. Touchard, S. Rigaut, Organometallics 2008, 27, 509 – 518; b) E. Di Piazza, L. Norel, K. Costuas, A. Bourdolle, O. Maury, S. Rigaut, J. Am. Chem. Soc. 2011, 133, 6174 – 6176. a) K. Onitsuka, N. Ohara, F. Takei, S. Takahashi, Dalton Trans. 2006, 3693 – 3698; b) N. Gauthier, C. Olivier, S. Rigaut, D. Touchard, T. Roisnel, M. G. Humphrey, F. Paul, Organometallics 2008, 27, 1063 – 1072; c) N. Gauthier, G. Argouarch, F. Paul, L. Toupet, A. Ladjarafi, K. Costuas, J.-F. Halet, M. Samoc, M. P. Cifuentes, T. C. Corkery, M. G. Humphrey, Chem. Eur. J. 2011, 17, 5561 – 5577; d) G. Grelaud, M. P. Cifuentes, T. Schwich, G. Argouarch, S. Petrie, R. Stranger, F. Paul, M. G. Humphrey, Eur. J. Inorg. Chem. 2012, 65 – 75. M. A. Fox, J. E. Harris, S. Heider, V. Prez-Gregorio, M. E. Zakrzewska, J. D. Farmer, D. S. Yufit, J. A. K. Howard, P. Low, J. Organomet. Chem. 2009, 694, 2350 – 2358. C. Teng, X. Yang, S. Li, M. Cheng, A. Hagfeldt, L. Wu, L. Sun, Chem. Eur. J. 2010, 16, 13127 – 13138. J. Ruiz Aranzaes, M.-C. Daniel, D. Astruc, Can. J. Chem. 2006, 84, 288 – 299. H. Snaith, Adv. Funct. Mater. 2010, 20, 13 – 19. a) S. Rigaut, K. Costuas, D. Touchard, J.-Y. Saillard, S. Gohlen, P. H. Dixneuf, J. Am. Chem. Soc. 2004, 126, 4072 – 4073; b) F. Meng, Y.-M. Hervault, L. Norel, K. Costuas, C. Van Dyck, V. Geskin, J. Cornil, H. H. Hng, S. Rigaut, X. Chen, Chem. Sci. 2012, 3, 3113 – 3118. L. Ducasse, F. Castet, R. Mreau, S. Nnon, J. Id, T. Toupance, C. Olivier, Chem. Phys. Lett. 2013, 556, 151 – 157. a) M. J. G. Peach, P. Benfield, T. Helgaker, D. J. Tozer, J. Chem. Phys. 2008, 128, 044118; b) T. Le Bahers, C. Adamo, I. Ciofini, J. Chem. Theory Comput. 2011, 7, 2498 – 2506; c) D. Jacquemin, T. Le Bahers, C. Adamo, I. Ciofini, Phys. Chem. Chem. Phys. 2012, 14, 5383 – 5388. S. Thorand, N. Krause, J. Org. Chem. 1998, 63, 8551 – 8553. Gaussian 09, Revision A.02, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, . Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian, Inc., Wallingford CT, 2009. B. J. Lynch, P. L. Fast, M. Harris, D. G. Truhlar, J. Phys. Chem. A 2000, 104, 4811 – 4815. a) J. Tomasi, M. Persico, Chem. Rev. 1994, 94, 2027 – 2094; b) J. Tomasi, B. Mennucci, R. Cammi, Chem. Rev. 2005, 105, 2999 – 3093.
Received: November 25, 2013 Revised: February 2, 2014 Published online on && &&, 0000
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FULL PAPER & Electron Transfer S. De Sousa, L. Ducasse, B. Kauffmann, T. Toupance,* C. Olivier* && – && The next generation: Asymmetric functionalization of a ruthenium–diacetylide organometallic complex afforded an innovative push–pull structure (see figure; D: donor; A: acceptor) with excellent
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optoelectronic properties and a good power conversion efficiency rate when it was employed as a photosensitizer in dye-sensitized solar cells.
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Functionalization of a Ruthenium– Diacetylide Organometallic Complex as a Next-Generation Push–Pull Chromophore
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Functionalization of a Ruthenium–Diacetylide Organometallic Complex as a Next-Generation Push–Pull Chromophore Samuel De Sousa,[a] Laurent Ducasse,[a] Brice Kauffmann,[b] Thierry Toupance,*[a] and Cline Olivier*[a]
choring carboxylic acid function, was further adsorbed onto a semiconducting metal oxide porous thin film to serve as a photosensitizer in hybrid solar cells. The resulting photoactive material, when embedded in dye-sensitized solar cell devices, showed a good spectral response with a broad incident photon-to-current conversion efficiency profile and a power conversion efficiency that reached 7.3 %. Thus, this material paves the way to a new generation of organometallic chromophores for photovoltaic applications.
Abstract: The design and preparation of an asymmetric ruthenium–diacetylide organometallic complex was successfully achieved to provide an original donor–p–[M]–p–acceptor architecture, in which [M] corresponds to the [Ru(dppe)2] (dppe: bisdiphenylphosphinoethane) metal fragment. The charge-transfer processes occurring upon photoexcitation of the push–pull metal–dialkynyl s complex were investigated by combining experimental and theoretical data. The novel push–pull complex, appropriately end capped with an an-
Introduction
ethane), as the metal fragment showed exceptional propensity to serve as efficient electron relays in nanoscale oligomeric structures.[9] By taking advantage of the linear geometry of the trans-ditopic ruthenium–diacetylide unit and its p-unsaturated character, we anticipated that such a motif may act as an efficient relay within a “push–pull” architecture and, therefore, may find application in the field of dye-sensitized solar cells (DSCs), which are hybrid organic–inorganic devices that are able to convert light into electricity.[10] DSCs are nowadays considered as one of the most promising technologies towards solarenergy exploitation, with a record power conversion efficiency (PCE) of up to 12.3 % at the laboratory scale.[11] To achieve efficient light harvesting, the DSC device relies on pigment molecules covalently anchored onto a semiconductor metal oxide, which is a porous and high-surface-area thin film of TiO2. In this context, significant attention has been given to the synthesis of visible-light absorbers based on ruthenium coordination complexes.[12] Despite their good efficiencies, such metal– organic compounds exclusively consist of polypyridyl derivatives and thus limit the development of metal-complex-based sensitizers for DSC applications. In the meanwhile, many efforts have been devoted to the design and preparation of fully organic dyes showing a donor–p–acceptor (D–p–A) structure, such as indoline, triarylamine, or carbazole derivatives.[13] Organometallic complexes have been shown to exhibit higher conductance and weaker length dependence than those of fully organic counterparts of comparable length.[14] Thus, we postulated that the [Ru(dppe)2] metal fragment, if directly incorporated in the same plane as an organic p-conjugated backbone, may tune the optical and electronic properties of dyes and, therefore, enhance the performance of DSC devices. With the exception of platinum–acetylide polymers,
Transition-metal complexes are important targets for the investigation of charge-transfer processes.[1] In particular, organometallic complexes containing carbon-rich p-conjugated ligands present outstanding electronic and structural properties that lead to a wide range of applications, such as nonlinear optics,[2] photoluminescence,[3] and molecular electronics.[4] Metal–alkynyl s complexes[5] have been readily employed to design molecular wires that are simple electronic components with the ability to transport electronic charges between two points. Such complexes notably allowed the study of intramolecular charge-transport mechanisms over long distances.[6] However, these studies mainly focused on end-capped bimetallic species, while the introduction of one or more metal centers, based on various transition metals,[7] within extended organic p-conjugated systems, also allowed surveys of electronic communication through the metal fragment. In particular, the ability of ruthenium to operate as a connector allowing electron flow to occur between different elements in trans-ditopic architectures was demonstrated.[8] Examples with molecular wires, including [Ru(dppe)2] (dppe: bisdiphenylphosphino[a] S. De Sousa, Dr. L. Ducasse, Prof. T. Toupance, Dr. C. Olivier Universit de Bordeaux UMR-CNRS 5255 Institut des Sciences Molculaires 351 cours de la Libration, 33405 Talence (France) E-mail: [email protected] [email protected] [b] Dr. B. Kauffmann Institut Europen de Chimie et Biologie Universit de Bordeaux, UMS-3033 CNRS-INSERM 2 rue Robert Escarpit, 33600 Pessac (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304611. Chem. Eur. J. 2014, 20, 1 – 9
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Full Paper which have been used as donors in polymer-based solar cells,[15] few examples of metal–alkynyl s complexes have been reported for solar-cell applications. A rodlike diruthenium–acetylide complex was recently utilized as a donor in bulk heterojunction solar cells and led to 0.1 % PCE upon blending with a methanofullerene derivative.[16] The unique series of diacetylide–metal complexes used as photosensitizers in DSCs was recently described by Tian and co-workers.[17] These platinumbased dyes, with a D–p–[M]–p–A structure, in which [M] was [Pt(Et3)2], D was triphenylamine, and A was cyanoacrylic acid, afforded up to 4.2 % PCE. In the light of our previous studies on organic chromophores for DSCs,[18] we explored the preparation of an original ruthenium–diacetylide push–pull complex featuring a carbazole motif as the electron-donor part and a cyanoacrylic acid moiety as the electron-withdrawing and anchoring group. Although many examples of unsymmetrical diacetylide–transition-metal complexes have already been described,[19–21] asymmetric diacetylide s complexes featuring strong donor and/or acceptor groups have scarcely been reported.[17, 22] Indeed the synthesis of such a push–pull complex, especially one functionalized with a carboxylic acid anchoring group, can be somewhat delicate and represents a great challenge in the field of organometallics. Herein, we report the preparation and optoelectronic characterization of the first example of a photosensitizer for DSCs based on the D–p–[M]–p–A design concept, in which [M] is the [Ru(dppe)2] metal fragment. The photovoltaic properties of the new complex embedded in solar cell devices were also investigated.
plex. Stable ruthenium vinylidene complexes are easily obtained by treating the 16-electron species [(dppe)2RuCl][TfO] ([1][TfO]) with the appropriate alkyne,[23] in this case N-(4-ethynylphenyl)carbazole (2). The subsequent reaction of [3][TfO] with 4-ethynylbenzaldehyde, in the presence of the noncoordinating NaPF6 salt and Et3N as a base, led to the formation of the bis(s-arylacetylide) intermediate complex 4. The latter was endowed with an aldehyde function to allow the further introduction of the electron-withdrawing cyanoacrylic acid function. Knoevenagel condensation was achieved by using piperidine and cyanoacetic acid in the protected form of 2-trimethylsilylethyl cyanoethanoate. Protection of the carboxylic acid function was necessary to avoid side reactions with the complex during the condensation step. Silyl ester deprotection of 5 under mild reaction conditions, by using Bu4NF in THF at room temperature, led to the desired diacetylide complex 6 in good yield. All of the new compounds were fully characterized by means of different techniques. In particular, 31P NMR spectrum of the target complex 6 displays a sharp singlet at d = 53.3 ppm, which is characteristic of the trans-ditopic geometry of the ruthenium–diacetylide unit. FTIR analysis also allowed characterization of 6, the spectrum of which shows an intense absorption band at n˜ = 2045 cm1 corresponding to the n(CC) stretch of the s-diacetylide–metal fragment. In addition, typical vibrational stretchings for the cyanoacrylic acid motif were clearly identified: a peak of low intensity at n˜ = 2218 cm1 for the n(CN) stretch and two peaks at n˜ = 1713 and 1172 cm1 corresponding to the n(C=O) and n(CO) stretches of the carboxylic acid function.
Optoelectronic properties
Results and Discussion
The electronic absorption spectrum of the new push–pull complex 6, recorded in dichloromethane, shows intense highenergy absorption bands that can be ascribed to p!p* intraligand charge transfers (ILCTs) involving the diphosphine ligands and the electron-rich carbazole motif (Figure 1 and Table 1). More interestingly, the spectrum shows an intense band in the visible region, with a maximum centered at l = 520 nm and
Synthesis and characterizations The synthetic strategy to obtain the dissymmetric ruthenium– diacetylide complex 6 is depicted in Scheme 1. The first step consists of the formation of a vinylidene moiety, [3][TfO], featuring the electron-donor part of the target push–pull com-
Scheme 1. Synthetic route to complex 6: a) CH2Cl2, RT, 20 h; b) 4-ethynylbenzaldehyde, NaPF6, Et3N, CH2Cl2, RT, 20 h; c) trimethylsilylethyl cyanoethanoate, piperidine, CHCl3, reflux, 68 h; d) Bu4NF, THF, RT, 20 h. TfO : trifluoromethanesulfonate; TMSE: trimethylsilylethyl.
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Full Paper energy level of the push–pull chromophore, is located at a particularly well-suited position with regard to the standard potential of iodine/iodide (approximately + 0.45 V vs. NHE), that is, the redox shuttle contained in the electrolyte of DSC devices. An optimal potential difference of 0.28 V between the oxidation potential of 6 and the I3/I redox couple should ensure efficient driving force for the regeneration of the dye.[26] The LUMO level was deduced from the HOMO position and the optical band-gap energy (Eg = 2.1 eV), estimated from the intercept between the normalized absorption and emission spectra. As a result, the LUMO level of 6 (1.37 V vs. NHE) is more negative than the conduction band edge of TiO2 (approximately 0.50 V vs. NHE), which should allow electron injection from the complex in its excited state to the semiconductor.
Figure 1. UV/Vis absorption spectra of 6 in CH2Cl2 solution (1.7 105 m; solid line) and anchored on a 3 mm thick transparentTiO2 film (dashed line; the cutoff in the UV region is due to TiO2 absorption).
a high extinction coefficient of e = 30 600 L mol1 cm1. The broad shape of this band indicates that multiple transitions close in energy are involved, but with a major contribution from the allowed HOMO!LUMO transition. The molecular frontier orbitals of acetylide s complexes result from a considerable mixing of the Ru dp orbitals with the alkynyl p orbitals of the carbon-rich ligands. In consequence, the broad absorption band observed in the visible region is likely to present a strong metal-to-ligand charge transfer (MLCT) character. The appealing optical properties of 6 are comparable to those of organic sensitizers used in DSCs; better yet, the introduction of the [Ru(dppe)2] metal center into the conjugated pathway of the push–pull chromophore induces significant redshift of the absorption band and increases the extinction coefficient relative to that of the analogous fully organic carbazole-based dyes.[24] The electrochemical properties of 6 were investigated by means of cyclic voltammetry in CH2Cl2 solution (Figure S1 in the Supporting Information) and the corresponding data are summarized in Table 1. The ruthenium–diacetylide complex 6 undergoes one monoelectronic reversible oxidation process (Eox = + 0.60 vs. FeCp*2 (Cp*: pentamethylcyclopentadienyl) = + 0.73 V vs. NHE)[25] followed by an irreversible process at higher potential, which is consistent with a chemical reaction of the second oxidized species. Due to the electronic enrichment stemming from the introduction of the metal fragment, the first oxidation potential of 6 is significantly more cathodic than those of fully organic analogues.[20] As a consequence, the oxidation potential of 6, which can be related to the HOMO
Crystallographic studies Attempts to grow crystals suitable for X-ray structure determination of complex 6 remained unsuccessful, insofar as carboxylic acids are notoriously unfavorable to crystallogenesis. However, single crystals of the unsymmetrical diacetylide complex in the protected form, 5, were grown by slow diffusion of hexane into concentrated solutions of the complex in a dichloromethane/acetonitrile mixture, and the crystal structure was solved by X-ray diffraction analyses. Complex 5 crystallized in the C2/c monoclinic space group. The unit cell includes eight molecules of the complex with as many molecules of hexane and thus shows a very large volume of 17 024 3 with cell dimensions a = 74.43, b = 12.16, and c = 18.94 . Perspective drawings of the molecular structure of 5 are depicted in Figure 2 and the crystal data are summarized in the Supporting Information. The molecular structure of 5 confirms the linear arrangement of the molecular backbone with a trans configuration of the carbon-rich ligands on either side of the [Ru(dppe)2] fragment, and an angle of CaRuCa’ = 174.5(3)8. Both carbon chains show a net acetylide character with bond length values of 2.055 (2.077), 1.221 (1.206), and 1.431 (1.451) for the RuCa (RuCa’), CaCb (Ca’Cb’), and Cb
Table 1. UV/Vis absorption data and electrochemical properties of 6. lmax [nm][a] (e [m1 cm1])
lmax [nm][b]
HOMO [V][c] (vs. NHE)
Eg [eV][d]
LUMO [V][e] (vs. NHE)
520 (30 600)
470
+ 0.73
2.1
1.37
[a] Absorption maximum in CH2Cl2. [b] Absorption maximum on a 3 mm thick TiO2 film. [c] Oxidation potential measured in CH2Cl2. NHE: Normal hydrogen electrode. [d] The optical band-gap energy (Eg) estimated from the intersection between the normalized absorption and emission spectra. [e] The LUMO level calculated by subtracting the Eg value from the HOMO level.
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Figure 2. Crystal structure of 5 (top and side views). Protons and solvent molecules were removed for clarity.
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Full Paper Cg (Cb’Cg’) bonds, respectively. The terminal carbazole moiety presents a torsion angle of 74.78 with the adjacent benzyl ring, and the two acetylide chains are arranged in a quasiplanar fashion. This allows for extensive delocalization of electrons over the entire carbon-rich p-conjugated sequence, which is about 23 long.
(HOMO!LUMO + 3 and HOMO!LUMO + 4 transitions) and of the carbazole unit (HOMO4!LUMO + 1). To gain more insight into the charge-transfer properties of the complex, time-dependent (TD) DFT calculations were performed to provide the electronic absorption spectrum of 6 in CH2Cl2 (Figure S4 in the Supporting Information). Although in close agreement with the experimental result, the calculated spectrum exhibits a global blueshift of about 50 nm. We attribute this inconsistency to the fact that TD-DFT calculations exclusively depend on the functional and on the basis set. The MPW1K pseudopotential employed in the present study had already provided reliable results for push–pull organic molecules.[18, 28] Besides, after consideration that 1) the first allowed transition mainly relies on the HOMO/LUMO frontier orbitals (Table S1 in the Supporting Information) and 2) the HOMO, by showing a substantial weight on the Ru atom (Figure 3), is sensitive to the basis set used to describe the metal center, the difference observed between the calculated and experimental spectra might therefore be ascribed to some inadequacy of the basis set (LANL2DZ) in describing a metal-based fragment embedded in a push–pull system. Finally, by applying recently described computational methods,[29] extra parameters can be considered to characterize the charge-transfer (CT) processes occurring upon photoexcitation of push–pull systems. Notably, the spatial overlap (L) between the occupied and unoccupied molecular orbitals involved in the main excitation was calculated to be L = 0.465 (Figure S5 in the Supporting Information). This quantity, which takes a value between 0 and 1, describes the matching between two energy levels, with small values reflecting long-range excitation and large values indicating short-range excitation. The rather low value of spatial overlap calculated for 6 echoes the experimental and calculated lmax absorption at around 500 nm (Table 1, Figure S2 in the Supporting Information). Additionally, the distance (DCT) and quantity (qCT) of transferred charge associated with the ground-to-excited-state transition were assessed. The former corresponds to the distance between the barycenters of the density-depletion and density-increment zones related to the CT excitation, whereas the latter quantifies the amount of charge transferred during the process. A qCT value of 1 corresponds to a pure CT process, whereas a value close to 0 equals a local phenomenon. Calculations related to complex 6 led to DCT = 4.6 and qCT = 1.0, which is higher than the calculated value obtained for a model organic compound of comparable length.[24b] From these data, we infer that the visible-range electronic transition observed for compound 6 shows a net charge-transfer character, strongly localized within the molecular backbone surrounding the [Ru(dppe)2] metal fragment.
Theoretical data The optimized geometry of ruthenium complex 6 again testifies to the linearity of the structure with the acetylide ligands in the trans position on either side of the metal center, as already determined for ruthenium complexes of this type.[8a–b, 9d, 27] Pictures of the frontier molecular orbitals are given in Figure 3. The LUMO level is well localized on the electron-withdrawing acetylide ligand with an important weight on the cyanoacrylic acid group. More interestingly, the HOMO level possesses a strong Ru t2g character, and the orbital is indeed spread over the whole p-conjugated skeleton with a sizable contribution from the metal–s-diacetylide fragment. Furthermore, isodensity surface plots of the transition-involved molecular orbitals and the corresponding energy diagram are shown in Figure S2 and S3 in the Supporting Information, and detailed transition assignments along with calculated oscillator strengths are gathered in Table S1 in the Supporting Information. The intense absorption band in the visible range can be considered as the result of several contributions involving electronic transitions from the first three HOMOs to the LUMO level. The HOMO and HOMO1 orbitals are of a fairly similar nature (that is, delocalized over the whole molecule), whereas HOMO2 exhibits a distinctive feature with distribution on the carbazole motif. Absorption bands at higher energies stem from excitations of the [Ru(dppe)2] fragment
Sensitization of porous titania thin films As a prelude to in-depth photovoltaic measurements, the capacity of new complex 6 for chemisorbing onto nanoparticulate TiO2 films was evidenced by means of UV/Vis absorption and attenuated total reflectance (ATR) FTIR spectroscopy studies.
Figure 3. Isodensity surface plots of the frontier molecular orbitals of 6. Left: HOMO; Right: LUMO.
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Full Paper The UV/Vis absorption spectrum was recorded after chemisorption of 6 on a transparent titania thin film (thickness 3 mm). As shown in Figure 1, the shape of the main absorption band remained similar to the one in solution, although with the maximum shifted to a lower wavelength by 50 nm. Such a blueshift has already been observed upon grafting organic push–pull dyes onto TiO2[18] and can be attributed, to some extent, to dye aggregation, but also to the deprotonation of the carboxylic function.[28] The ATR-FTIR spectra of 6 show the appearance of two bands upon grafting onto TiO2, at n˜ = 1575 and 1360 cm1 for the asymmetric and symmetric vibrational stretchings of the carboxylate groups, and the concomitant collapse of the carboxylic acid n(C=O) stretching peak of the free complex, at n˜ = 1713 cm1 (Figure 4). These data imply that the rutheniumcomplex molecules are anchored on the metal-oxide surface through a bidentate chelation or bridging mode, rather than through an ester-type linkage.
Figure 5. a) IPCE and b) current density–voltage profiles of a DSC with 6 as the dye, under 100 mW cm2 illumination (solid line) and in the dark (dashed line).
to note that the IPCE profile shows an increased red response compared with the absorption spectrum of the sensitizer. This feature can be attributed to interactions occurring between the dye molecules on the surface of the oxide layer. Overall, the effective action spectrum of 6 exceeds 70 % in the spectral range of l = 410–570 nm (above 80 % from l = 440–530 nm), and the tail of the curve extends until l = 700 nm. The photovoltaic performance of ruthenium–diacetylide complex 6 was measured under 100 mW cm2 illumination in AM1.5G conditions; the cell active area, defined by using a black mask, was fixed to 0.159 cm2. For the best solar cell, the overall power conversion efficiency reached 7.32 %, with a short-circuit photocurrent density of Jsc = 15.56 mA cm2, an open-circuit photovoltage of Voc = 0.68 V, and a fill factor of ff = 69.2 %. Under the same conditions, a reference cell realized with the commercial dye N3 (Figure S6 in the Supporting Information) led to a PCE value of 9.07 %, with Jsc = 16.71 mA cm2, Voc = 0.75 V, and ff = 71.9 %. These values clearly demonstrate the ability of the ruthenium–diacetylide complex 6 to work efficiently as a photosensitizer in DSCs. Moreover, in standard devices with an iodinebased electrolyte, compound 6 outperforms fully organic carbazole-based dyes of similar structure, reported by us and others.[18b, 24] In addition, the performance of DSCs incorporating complex 6 remained stable for several weeks under ambient conditions (Figure S7 in the Supporting Information). A slow decrease of the overall efficiency substantially stemmed from a gradual decrease in the Jsc value, despite concomitant increases in the Voc and ff values. The loss in current can be attributed either to slight dye degradation or to slow evaporation of the volatile solvent contained in the electrolyte.
Figure 4. ATR-FTIR spectra of free complex 6 (dashed line) and complex 6 anchored on a TiO2 film (solid line). The spectra were normalized on the CN signal.
The intense band at n˜ = 2045 cm1, attributed to the n(CC) vibration stretch of the s-diacetylide–metal fragment, remained unchanged, which indicates that the trans-ditopic structure of 6 was not altered upon chemisorption onto TiO2. This feature provides evidence that transition-metal–s-alkynyl complexes allow clean surface modification of metal-oxide thin films with preservation of the integrity of the whole functional organometallic core. Photovoltaic properties The ruthenium–diacetylide complex 6 was further embedded in DSCs along with an electrolyte based on the iodine/iodide couple as a redox shuttle. The detailed procedure for device fabrication is given in the Experimental Section, and characteristic incident photon-to-current conversion efficiency (IPCE) and photocurrent density–voltage (J–V) curves are given in Figure 5. Photoanodes were obtained by staining TiO2 films (thickness 15 mm) in CH2Cl2 solutions of 6 containing chenodeoxycholic acid as a coadsorbent. As shown in Figure 5 a, the IPCE profile of the device is consistent with the absorption spectrum of complex 6 after grafting onto TiO2, with the curve reaching a maximum value of 85 % at l = 475 nm. It is worthy Chem. Eur. J. 2014, 20, 1 – 9
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133.8 (CH), 132.2 (m, Cq(dppe)), 131.9 (m, Cq(dppe)), 131.8 (CH), 131.3 (CH), 129.2 (CH), 128.7 (CH), 128.5 (CH), 126.7 (CH), 126.3 (CH), 125.7 (Cq), 123.6 (Cq), 120.7 (CH), 120.4 (CH), 109.9 (CH), 109.5 (CH(Ru=C=CH), 29.3 ppm (m, CH2(dppe)); FTIR (KBr): n˜ = 1635–1623 (n=C= 1 (nPPh); HRMS (FD +): m/z calcd for [C72H60ClNP4Ru] + C), 1095 cm + [MH] : 1199.24082; found: 1193.24250.
Asymmetric functionalization of a ruthenium–diacetylide organometallic complex led to a new generation of push–pull chromophores. The first example was designed and successfully obtained through a straightforward synthetic procedure. The molecular structure of s-alkynyl intermediate complex 5 was confirmed by X-ray analyses, and the optoelectronic properties of 6 were determined by combining experimental and theoretical data. The results confirmed the exceptional ability of the [Ru(dppe)2] metal fragment to act as an efficient electron relay in nanoscale systems. Chromophore 6, end capped with an anchoring carboxylic acid function, was readily grafted onto TiO2 mesoporous thin films and employed as a photosensitizer in DSC devices. The new dye operated efficiently and reached up to 7.3 % PCE under standard conditions. That result outperforms fully organic dyes of analogous structure and represents the best efficiency reported so far for s-alkynyl organometallic complexes in solar cells. Overall, this study highlights the beneficial effect of the introduction of an electron-rich metal fragment in a p-conjugated molecular backbone, which allows tuning of the optical and electronic properties of a chromophore. Such a push–pull system is of great interest because it opens up new prospects, both in the field of organometallics and dye-sensitized solar cells. In consideration of further elaboration of organometallic complexes featuring more than one metal center in alternation with visible-light absorbing motifs, the concept of a ruthenium–s-diacetylide complex as the photoactive system therefore represents a versatile approach for the improved collection of solar energy.
trans-[9-Carbazole-(p-C6H4)-CCRu(dppe)2-CC-(p-C6H4)-CHO] (4): Distilled Et3N (0.22 mL, 1.5 mmol, 3 equiv) was added to a solution of [3][TfO] (0.7 g, 0.5 mmol, 1 equiv), 4-ethynylbenzaldehyde (80 mg, 0.6 mmol, 1.1 equiv), and NaPF6 (180 mg, 1 mmol, 2 equiv) in dry CH2Cl2 (75 mL) under an inert atmosphere. The reaction mixture was stirred for 24 h at room temperature. The organic solution was washed with degassed water (3 30 mL) and evaporated to dryness. Precipitation from a CH2Cl2/pentane mixture afforded pure complex 4 as a yellow powder in 80 % yield (0.5 g, 0.4 mmol). 31 P NMR (243 MHz, CD2Cl2): d = 52.98 ppm (s, PPh2); 1H NMR (600 MHz, CD2Cl2): d = 9.90 (s, 1 H), 8.18 (d, 3JHH = 7.6 Hz, 2 H), 7.70– 7.66 (m, 8 H), 7.65 (d, 3JHH = 8.1 Hz, 2 H), 7.50 (d, 3JHH = 7.6 Hz, 2 H), 7.47 (t, 3JHH = 7.6 Hz, 2 H), 7.45–7.41 (m, 8 H), 7.36 (d, 3JHH = 8.4 Hz, 2 H), 7.31 (t, 3JHH = 7.6 Hz, 2 H), 7.26 (t, 3JHH = 7.5 Hz, 4 H), 7.22 (t, 3 JHH = 7.5 Hz, 4 H), 7.08 (t, 3JHH = 7.5 Hz, 8 H), 7.05 (d, 3JHH = 8.4 Hz, 2 H), 6.99 (t, 3JHH = 7.5 Hz, 8 H), 6.79 (d, 3JHH = 8.1 Hz, 2 H), 2.71 ppm (m, 8 H); 13C NMR (150 MHz, CD2Cl2): d = 191.5 (CH(CHO)), 148.1 (quint., Cq(RuC)), 141.5 (Cq), 137.4 (m, Cq(dppe)), 137.2 (Cq), 137.1 (m, Cq(dppe)), 134.7 (CH), 134.4 (CH), 133.3 (quint, Cq(RuC)), 132.9 (Cq), 131.6 (Cq), 131.4 (CH), 130.6 (CH), 130.1 (Cq), 129.6 (CH), 129.4 (CH), 129.2 (CH), 127.6 (CH), 127.5 (CH), 126.6 (CH), 126.2 (CH), 123.6 (Cq), 120.6 (CH), 120.1 (CH), 118.9 (Cq(RuCC)), 117.5 (Cq(RuCC)), 110.3 (CH), 31.8 ppm (m, CH2(dppe)); FTIR (KBr): n˜ = 2045 (nCC), 1684 (nC=O), 1585 (nC=C(Ph p-conj.)), 1096 cm1 (nPPh); HRMS (ESI +): m/z calcd for [C81H65NONaP4Ru] + [M + Na] + : 1316.2952; found: 1316.2966; exact mass deduced from the most intense peak of the isotopic pattern. trans-[9-Carbazole-(pC6H4)-CCRu(dppe)2-CC-(pC6H4)-CH= C(CN)CO2TMSE] (5): Piperidine (80 mg, 0.8 mmol, 4 equiv) was added to a solution of 4 (0.25 g, 0.2 mmol, 1 equiv) and 2-trimethylsilylethyl cyanoethanoate (70 mg, 0.4 mmol, 2 equiv) in dry CHCl3 (35 mL) under an inert atmosphere. The reaction mixture was heated to reflux for 68 h. The organic solution was washed with degassed water (3 20 mL) and evaporated to dryness. Precipitation from a CH2Cl2/pentane mixture afforded pure complex 5 as a red powder in 84 % yield (0.23 g, 0.16 mmol). 31P NMR (243 MHz, CD2Cl2): d = 52.92 ppm (s, PPh2); 1H NMR (600 MHz, CD2Cl2): d = 8.20–8.14 (m, 3 H), 7.83 (d, 3JHH = 8.4 Hz, 2 H), 7.67 (m, 8 H), 7.51– 7.44 (m, 4 H), 7.41 (m, 8 H), 7.36 (d, 3JHH = 8.3 Hz, 2 H), 7.30 (t, 3JHH = 7.2 Hz, 2 H), 7.26 (t, 3JHH = 7.5 Hz, 4 H), 7.22 (t, 3JHH = 7.5 Hz, 4 H), 7.07 (t, 3JHH = 7.5 Hz, 8 H), 7.05 (d, 3JHH = 8.4 Hz, 2 H), 6.99 (t, 3JHH = 7.5 Hz, 8 H), 6.74 (d, 3JHH = 8.3 Hz, 2 H), 4.42 (m, 2 H), 2.70 (m, 8 H), 1.17 (m, 2 H), 0.13 ppm (s, 9 H); 13C NMR (150 MHz, CD2Cl2): d = 164.0 (Cq(COOTMSE)), 154.4 (CH(ethylenic)), 152.7 (quint, Cq(RuC)), 141.5 (Cq), 137.3 (m, Cq(dppe)), 137.0 (m, Cq(dppe)), 136.7 (Cq), 134.7 (CH), 134.4 (CH), 133.0 (quint, Cq(RuC)), 132.9 (Cq), 131.6 (CH), 131.4 (CH), 130.9 (CH), 130.0 (Cq), 129.4 (CH), 127.6 (CH), 127.6 (CH), 126.6 (CH), 126.4 (Cq), 126.2 (CH), 123.6 (Cq), 120.8 (Cq(acrylic)), 120.6 (CH), 120.1 (CH), 117.9 (Cq(RuCC)), 117.3 (Cq(RuCC)), 110.3 (CH), 98.5 (Cq(CN)), 65.0 (CH2), 31.8 (m, CH2(dppe)), 17.7 (CH2), 1.4 ppm (CH3(TMS)); FTIR (KBr): n˜ = 2218 (nCN), 2043 (nCC), 1717 (nC=O), 1569 (nC=C(Ph p-conj.)), 1096 (nP 1 (nSiC); HRMS (ESI +): m/z calcd for Ph), 1171 (nCO), 836 cm [C89H78N2O2NaSiP4Ru] + [M + Na] + : 1483.3718; found: 1483.3866; exact mass deduced from the most intense peak of the isotopic pattern.
Experimental Section Chemistry General: The reactions were carried out under an inert atmosphere by using Schlenk techniques. Solvents were dried over appropriate drying agents (sodium for pentane, diethyl ether, and THF; calcium hydride for dichloromethane, chloroform, and methanol) and freshly distilled under nitrogen before use. All reagents were obtained from commercially available sources and used without further purification, otherwise stated. [(dppe)2RuCl][[TfO] ([1][TfO])[23] and 4ethynylbenzaldehyde[30] were prepared as previously reported. trans-[Cl(dppe)2Ru=C=CH-(pC6H4)-9-carbazole][TfO] ([3][TfO]): In a Schlenk tube under an inert atmosphere, [RuCl(dppe)2][TfO] ([1][TfO]; 1.3 g, 1.2 mmol, 1 equiv) and 9-(4-ethynylphenyl)carbazole (2; 0.65 g, 2.4 mmol, 2 equiv) were dissolved in dry CH2Cl2 (70 mL). The mixture was stirred for 20 h at room temperature. After removal of the solvent, the crude product was washed with freshly distilled Et2O (40 mL) and pentane (2 40 mL). Precipitation from a CH2Cl2/pentane mixture afforded pure [3][TfO] as a dark yellow-green powder in 92 % yield (1.5 g, 1.1 mmol). 31P NMR (243 MHz, CD2Cl2): d = 37.67 ppm (s, PPh2); 1H NMR (600 MHz, CD2Cl2): d = 8.13 (d, 3JHH = 7.7 Hz, 2 H), 7.44 (m, 2 H), 7.41–7.31 (m, 24 H), 7.29 (t, 3JHH = 7.4 Hz, 2 H), 7.27 (d, 3JHH = 8.2 Hz, 2 H), 7.20–7.13 (m, 16 H), 6.88 (d, 3JHH = 8.4 Hz, 2 H), 5.90 (d, 3JHH = 8.4 Hz, 2 H), 3.92 (m, 1 H), 3.01 (m, 4 H), 2.86 ppm (m, 4 H); 13C NMR (150 MHz, CD2Cl2): d = 356.3 (quint, Cq(Ru=C=)), 141.0 (Cq), 135.3 (Cq), 134.5 (CH),
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trans-[9-Carbazole-(pC6H4)-CCRu(dppe)2-CC-(pC6H4)-CH= C(CN)COOH] (6): Bu4NF (1 m in THF, 0.28 mL, 0.28 mmol, 2 equiv) was added to a solution of 5 (0.2 g, 0.14 mmol, 1 equiv) in dry THF
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Full Paper get an appropriate thickness of 10 mm. A diffusing layer, constituted of 250–400 nm TiO2 particles (Solaronix, Ti-Nanoxide R/SP), was deposited on top of the working electrode in order to backscatter the unabsorbed photons towards the transparent layer. This layer reached a thickness of 5 mm. Terpineol and organic binders contained in the titania pastes were removed by gradual thermal treatment under an air flow at 325 (5), 375 (5), 450 (15), and 500 8C (15 min). The as-obtained films were further treated with 40 mm TiCl4 aqueous solution at 70 8C for 30 min; this was followed by annealing at 500 8C for 30 min. After cooling to 40 8C, the electrodes were immersed in 0.3 mm dye solutions in CH2Cl2 containing 1 mm of chenodeoxycholic acid as a coadsorbent and deaggregating agent. Under these conditions, the sensitization time was optimized to 16 h in the dark. Counter electrodes were prepared by drop casting a solution of H2PtCl6 (5 mm in ethanol) onto an FTOcoated glass substrate (TCO22–7, 7 ohm sq1, 2.2 mm thickness, Solaronix). The complex was thermally decomposed in air at 500 8C for 30 min to leave Pt nanoparticles. The photoanode and counter electrode were assembled by using a hot-melt Surlyn polymer gasket (25 mm, DuPont). The electrolyte (0.03 m iodine, 0.05 m lithium iodide, 1.0 m 1,3-dimethylimidazolium iodide, 0.8 m 4-tert-butylpyridine, and 0.1 m guanidinium thiocyanate in acetonitrile/valeronitrile 85:15) was introduced into the cell by vacuum backfilling through a hole drilled in the counter electrode, which was finally sealed with a Surlyn gasket and a glass cover plate.
(25 mL) under an inert atmosphere. The reaction was stirred overnight at room temperature. After solvent removal, the resulting solid was dissolved in CH2Cl2 (25 mL), and the solution was washed with degassed citric acid aqueous solution (10 %, 2 20 mL) and water (20 mL). The organic solution was evaporated to dryness, and the solid was further washed with pentane (2 20 mL). Slow crystallization from a CH2Cl2/pentane mixture afforded pure complex 6 as a deep red powder in 85 % yield (0.16 g, 0.12 mmol). 31 P NMR (162 MHz, [D8]THF): d = 53.31 ppm (s, PPh2); 1H NMR (600 MHz, [D8]THF): d = 8.16–8.12 (m, 3 H), 7.85 (d, 3JHH = 8.2 Hz, 2 H), 7.74–7.68 (m, 8 H), 7.47–7.34 (m, 14 H), 7.25–7.16 (m, 10 H), 7.09 (d, 3JHH = 8.2 Hz, 2 H,), 7.04 (t, 3JHH = 7.5 Hz, 8 H), 6.96 (t, 3JHH = 8.5 Hz, 8 H), 6.77 ppm (d, 3JHH = 8.2 Hz, 2 H); 13C NMR (150 MHz, [D8]THF): d = 164.5 (Cq(COOH)), 153.7 (CH(ethylenic)), 150.5 (quint, Cq(RuC )), 141.8 (Cq), 137.9 (m, Cq(dppe)), 137.5 (m, Cq(dppe)), 136.5 (Cq), 135.1 (CH), 134.8 (CH), 133.5 (quint., Cq(RuC)), 133.4 (Cq), 131.8 (CH), 131.5 (CH), 131.2 (CH), 130.5 (Cq), 129.5 (CH), 129.4 (CH), 127.8 (CH), 127.8 (CH), 127.3 (Cq), 126.7 (CH), 126.3 (CH), 124.1 (Cq), 120.7 (CH), 120.6 (Cq(acrylic)), 120.2 (CH), 118.2 (Cq(RuCC)), 117.2 (Cq(RuCC), 110.5 (CH), 100.2 (Cq(CN)), 32.1 ppm (m, CH2(dppe)); FTIR (KBr): n˜ = 2218 (nCN), 2045 (nCC), 1713 (nC=O), 1570 (nC=C(Ph p-conj.)), 1096 (nPPh), 1172 cm1 (nCO); HRMS (ESI +): m/z calcd for [C84H66N2O2N2P4Ru] + [M] + : 1360.3112; found: 1360.3046; exact mass deduced from the most intense peak of the isotopic pattern; elemental analysis: calcd (%) for C84H66N2O2N2P4Ru: C 74.15, H 4.89, N 2.06; found: C 73.75, H 4.80, N 1.98.
Photovoltaic and cell characterizations Electrochemical measurements
The photovoltaic performances of the devices were measured by using a black mask with an aperture area of 0.159 cm2. The device was illuminated with an AM1.5G solar simulator calibrated with a radiometer (IL 1400BL) to provide an incident irradiance of 100 mW cm2 at the surface of the solar cells. The J–V measurements were performed by using a Keithley model 2400 digital source meter (Keithley) with application of an independent external voltage to the cell and measurement of the photogenerated current out from the cell. Action spectra of the incident photon-tocurrent conversion efficiency were realized by using a Xe lamp associated with a monochromator (Triax 180, Jobin Yvon) to select and increment wavelength irradiation to the cell. No bias light was employed to illuminate the cell. The current produced was measured by steps of 5 nm after 2 s of radiation exposure with a Keithley 6487 picoammeter to be in steady-state conditions. The incident photon flux was measured with a 6 inch diameter calibrated integrated sphere (Labsphere) and a silicon detector.
Cyclic voltammetry analyses were carried out on an Autolab PGSTAT100 potentiostat/galvanostat. Measurements were performed in a CH2Cl2 solution containing 0.1 m Bu4NPF6 as a salt support, at scan rate of 100 mV s1 by using a three-electrode system: the working electrode was a Pt disc, the reference electrode was Ag/AgCl (calibrated with FeCp*2 as an internal reference), and the counter electrode was a Pt mesh. Potentials were afterward referred to NHE by addition of 130 mV.[21]
Computational details DFT and TD-DFT calculations were performed with Gaussian 09.[31] The molecular geometry was optimized by using the B3LYP functional and the LANL2DZ basis set in vacuo conditions. TD-DFT calculations were realized with the MPW1K XC functional containing 42.8 % of HF exchange,[32] with the solvent taken into account within the integral equation formalism of the polarizable continuum model (IEF-PCM).[33] This method had already provided UV/Vis absorption spectra of push–pull molecules in agreement with experimental data.[28]
Acknowledgements This work was supported by the CNRS and the Rgion Aquitaine (Ph.D. grant to S.D.). The authors thank Dr. G. Le Bourdon (ISM, UMR-CNRS 5255) and Dr. L. Hirsch (IMS, UMR-CNRS 5218) for providing access to ATR-FTIR and photovoltaic characterization facilities, respectively.
Electrode preparation and device fabrication Fluorine-doped tin oxide (FTO) coated conducting glass substrates (NSG10, 10 ohm sq1, 3.2 mm thickness, XOPFisica) were cleaned by using successive ultrasonic treatments, first in an alkaline detergent solution and then in ethanol. The conducting glass substrates were afterward treated with 40 mm TiCl4 aqueous solution at 70 8C for 30 min, in order to increase photoanode adhesion while reducing recombination between FTO and I3 at this interface. The photoanodes were prepared by the screen-printing method by using commercially available titania pastes. The transparent conducting oxide substrates were first coated with a transparent layer, composed of 20 nm anatase TiO2 nanoparticles (Solaronix, Ti-Nanoxide T/SP). The screen-printing procedure was repeated three times to Chem. Eur. J. 2014, 20, 1 – 9
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Full Paper
FULL PAPER & Electron Transfer S. De Sousa, L. Ducasse, B. Kauffmann, T. Toupance,* C. Olivier* && – && The next generation: Asymmetric functionalization of a ruthenium–diacetylide organometallic complex afforded an innovative push–pull structure (see figure; D: donor; A: acceptor) with excellent
Chem. Eur. J. 2014, 20, 1 – 9
optoelectronic properties and a good power conversion efficiency rate when it was employed as a photosensitizer in dye-sensitized solar cells.
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Functionalization of a Ruthenium– Diacetylide Organometallic Complex as a Next-Generation Push–Pull Chromophore
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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