DOI: 10.1002/chem.201405643

Full Paper

& Donor–Acceptor Systems

Tuning Electron Donor–Acceptor Hybrids by Alkali Metal Complexation Marcus Lederer,[a] Uwe Hahn,[b, c] Javier Fernndez-Ariza,[b] Olga Trukhina,[b, e] M. Salom Rodrguez-Morgade,[b] Claudia Dammann,[d] Thomas Drewello,*[d] Toms Torres,*[b, e] and Dirk M. Guldi*[a] Dedicated to Professor Atsuhiro Osuka on the occasion of his 60th birthday

Abstract: A zinc phthalocyanine endowed with four [18]crown-6 moieties, ZnPcTeCr, has been prepared and self-assembled with either pyridyl-functionalized perylenebisimides (PDI-Py) or fullerenes (C60-Py) to afford a set of novel electron donor–acceptor hybrids. In the case of ZnPcTeCr, aggregation has been circumvented by the addition of potassium or rubidium ions to lead to the formation of monomers and cofacial dimers, respectively. From fluorescence titration experiments, which gave rise to mutual interactions between the electron donors and the acceptors in the excited state, the association constants of the respective ZnPcTeCr mono-

mers and/or dimers with the corresponding electron acceptors were derived. Complementary transient-absorption experiments not only corroborated photoinduced electron transfer from ZnPcTeCr to either PDI-Py or C60-Py within the electron donor–acceptor hybrids, but also the unexpected photoinduced electron transfer within ZnPcTeCr dimers. In the electron donor–acceptor hybrids, the charge-separated-state lifetimes were elucidated to be close to 337 ps and 3.4 ns for the two PDI-Pys, whereas the longest lifetime for the photoactive system that contains C60-Py was calculated to be approximately 5.1 ns.

Introduction

sitions of phthalocyanines (Pcs), a class of materials with great potential in organic photovoltaic applications or photodynamic therapy.[3–7] One of the most remarkable functional systems comprised of Pcs and [18]-crown-6 ethers has been shown to give rise to coiled-coil aggregates with tunable helicity.[8, 9] The chiral tails appended at the outside of the crown ether units led to the formation of long fibers of molecular diameter and micrometer length upon self-assembly. Notably, the presence of potassium induced striking differences with respect to the supramolecular organization. In addition, the crown ether motif has also been exploited in the formation of self-assembled photoactive devices.[10–13] The assembly of molecular components by means of noncovalent rather than covalent interactions constitutes an elegant approach as the building blocks might be assembled in the last step. In this way, the photoactive entities form electron donor–acceptor systems by virtue of just mixing the building blocks in solution. The stability of the final associate strongly depends on the recognition motif, that is, hydrogen-bonding, p–p, electrostatic, metal-to-ligand interactions, and so forth.[14–16] Additional forces might, however, be at work to overall strengthen the association. A leading example is an ammonium-functionalized methanofullerene. It exhibits increased stability upon self-assembly with an oligophenylenevinylene/ crown ether host owing to the presence of additional recognition motifs.[17–19] Such cooperativity in electron donor–acceptor arrays was even stronger in dendritic systems that featured several fullerenes.[20] Likewise, pseudorotaxane-derived systems

Coordinating metal ions by crown ethers has been an intriguing motif for self-assembly since its discovery.[1, 2] In the late 1980s, crown ethers were also introduced at the peripheral po[a] M. Lederer, Prof. D. M. Guldi Department of Chemistry and Pharmacy Interdisciplinary Center for Molecular Materials (ICMM) Friedrich-Alexander-Universitt Erlangen-Nrnberg Egerlandstrasse 3, 91058 Erlangen (Germany) E-mail: [email protected] [b] Dr. U. Hahn, J. Fernndez-Ariza, O. Trukhina, Dr. M. S. Rodrguez-Morgade, Prof. T. Torres Departamento de Qumica Orgnica, Universidad Autnoma de Madrid Cantoblanco, 28049 Madrid (Spain) E-mail: [email protected] [c] Dr. U. Hahn Laboratoire de Chimie des Matriaux Molculaires Universit de Strasbourg et CNRS (UMR 7509) Ecole Europenne de Chimie, Polymrs et Matriaux (ECPM) 25 rue Becquerel, 67087 Strasbourg Cedex 2 (France) [d] C. Dammann, Prof. T. Drewello Department of Chemistry and Pharmacy Friedrich-Alexander-Universitt Erlangen-Nrnberg Egerlandstrasse 3, 91058 Erlangen (Germany) E-mail: [email protected] [e] O. Trukhina, Prof. T. Torres IMDEA-Nanociencia, c/ Faraday 9, Campus de Cantoblanco 28049 Madrid (Spain) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201405643. Chem. Eur. J. 2015, 21, 1 – 11

These are not the final page numbers! ÞÞ

1

 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

&

&

Full Paper gave rise to the formation of microsecond-timescale radical ionpair states,[21, 22] the radical ionpair state lifetimes of which were two orders of magnitude longer relative to covalently linked zinc phthalocyanine (ZnPc)–fullerene hybrids. Also, D’Souza et al. described photoactive systems,[23, 24] in which the close proximity between self-assembled electron donors and acceptors enabled photoinduced electron transfer. As a matter of fact, the charge-separated-state lifetimes were as long as 6.7 ms. Such a long lifetime has been rationalized by the cofacial arrangement of the two ZnPcs playing a decisive role in stabilizing them. Herein, we report on a ZnPc that features four [18]-crown-6 moieties at its periphery, namely, ZnPcTeCr (Figure 1). Considering that the former is prone to coordinate alkali ions, potassium ions lead to the formation of ZnPcTeCr monomers, whereas the presence of rubidium ions leads to ZnPcTeCr dimers. To tune the supramolecular interactions between electron donors and acceptors we opted for a recogni- Figure 1. Structures of the molecular building blocks. tion motif that relies on the well-known complexation of pyridyl functionalities by ZnPc.[25, 26] From titration assays with seving Information) with 2,6-diidopropylaniline in propanoic acid eral electron acceptors—namely, perylenebisimides (PDI) and heated under reflux conditions. fullerenes (C60), to which pyridyl groups are covalently appended—the association constants were determined. Transient-abSteady-state absorption and emission spectroscopy sorption spectroscopy was employed to corroborate formation of the corresponding charge-separated states. Initially, the absorption and the fluorescence of the individual building blocks—namely, ZnPcref, ZnPcTeCr, PDI-Py-1, PDI-Py-2, and C60-Py—were probed. The absorption spectra of ZnPcref Results and Discussion and ZnPcTeCr feature Soret-band and Q-band maxima in the blue and the red region of the solar spectrum, respectively. We Materials and synthesis ascribe the absorption spectrum of ZnPcref to monomers and that of ZnPcTeCr to a mixture of monomers and aggregates. The The structures of the different molecular building blocks used in the present study are depicted in Figure 1. maxima of PDI-Py-1 and PDI-Py-2 in the visible region are due The synthesis of tetra-[18]-crown-6 ether functionalized to monomers and nicely complement the absorption spectra ZnPcTeCr[27, 28] was performed by following reported procedures, of ZnPcref and ZnPcTeCr. In contrast to the latter, C60-Py absorbs and its characterization is in good agreement with the literarather weakly throughout the visible part of the solar spectrum ture.[23] Reference ZnPc (ZnPcref) was purchased from Sigma–Alwith an onset of absorption close to the near-infrared. PDI-Py-1 and PDI-Py-2 give rise to the strongest fluorescence drich. The two potential acceptor building blocks PDI-Py-2[29] with fluorescence maxima at 618 and 554 nm, respectively, and C60-Py[30] were also prepared following literature procefluorescence quantum yields of 54 and 86 %, respectively, and dures. PDI-Py-1 was prepared in 56 % yield by treating the apfluorescence lifetimes of 6.4 and 5.5 ns, respectively. Somewhat propriate anhydride intermediate PMI-Py[31] (see the Support&

&

Chem. Eur. J. 2015, 21, 1 – 11

www.chemeurj.org

2

 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ÝÝ These are not the final page numbers!

Full Paper lower are the fluorescence quantum yields for ZnPcref with 28 % and for ZnPcTeCr with 3 %. In addition, the fluorescence maxima are redshifted to 687 nm for ZnPcref and to 683 nm for ZnPcTeCr. The weakest fluorescence is seen for C60-Py at 703 nm with a quantum yield of 1.1  103.

Table 1. Binding constants (K), charge-separation time (CS), and chargerecombination time (CR) of donor–acceptor complexes discussed in this work. Complex ZnPcref·PDI-Py-1 ZnPcref·PDI-Py-2 ZnPcref·C60-Py ZnPcTeCr·C60-Py ZnPcTeCr·PDI-Py-1[a] ZnPcTeCr·PDI-Py-2[a] ZnPcTeCr·C60-Py[a] ZnPcTeCr·PDI-Py-1[b] ZnPcTeCr·PDI-Py-2[b] ZnPcTeCr·C60-Py[b]

Fluorescence titration experiments To investigate the association between the electron donors and the acceptors, titration experiments were performed by means of steady-state absorption and fluorescence assays. To this end, the relative fluorescence intensities (If/I0) were plotted versus the electron-acceptor or -donor concentrations. Finally, the corresponding binding constants were estimated by means of nonlinear curve fittings according to Equation (1):[32] If 1 ¼1þ I0 2c0 "

ffi#  sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi   1 1 2 c0 þ cadd þ 4c0 cadd ðylim  1Þ  c0 þ cadd þ K K

These are not the final page numbers! ÞÞ

1.1  10 4.4  105 1.6  106 1.9  105 9.6  104 7.7  104 5.8  106 9.4  105 1.3  106 n.c.[c]

[c]

n.c. n.c.[c] n.c.[c] n.c.[c] 124  13 53  7 32  4 n.c.[c] 60  13 41  5

CR [ps] 1188  50 183  30 2633  281 n.c.[c] 3471  237 337  46 5112  1200 194  50 169  25 1236  50

quenched to 8 % of the initial intensity (Figure S15 in the Supporting Information). In this particular case, a binding constant of 1.6  106 m1 was determined on the basis of the nonlinear fluorescence quenching of ZnPcref (Figure S16 in the Supporting Information). Next, we probed ZnPcTeCr and C60-Py. In the absence of any other additive (see below), adding between 1.0  106 and 2.3  105 m of C60-Py to 1.5  106 m of ZnPcTeCr in chlorobenzene evoked blueshifts of the Q-band maxima of about 5 nm and 80 % fluorescence quenching. The corresponding binding constant of 1.9  105 m1 is rather moderate and is most likely due to ZnPcTeCr aggregation. This led us to add 500 equivalents of potassium in the form of KB(C6H4Cl)4 and to use ortho-dichlorobenzene (o-DCB), which exerted a stabilizing impact on the absorption and the fluorescence. In terms of absorption, a maximum at 358 nm for the Soret band and a maximum at 674 nm for the Q band are visible. In terms of fluorescence, a maximum at 681 nm is discernible. Under these conditions, the presence of 1.0  106 to 8.0  106 m of C60-Py led to a redshift of the Soret band from 358 to 364 nm (Figure 2) and to a fluorescence quenching with values as low as 14 % of the initial value (Figure 3). Please note that the binding constant of 5.8  106 m1 is comparable to that found for ZnPcref (Figure 4). Next, we added PDI-Py-2 at concentrations as low as 5.4  107 m and as high as 1.4  105 m to a solution that contained 5.8  107 m ZnPcTeCr and 93 equivalents of potassium. This resulted, on the one hand, in no discernable changes in the absorptions (Figure 5) and, on the other hand, in 67 % fluorescence quenching (Figure 6). The correspondingly determined binding constant for a 1:1 PDI-Py-2/ZnPcTeCr stoichiometry was 7.7  104 m1. Independent confirmation for such a stoichiometry came from Job’s plot analysis, which revealed a maximum at 0.5 (Figure 8). The binding constant is appreciably lower than that found for the combination of PDI-Py-2 and ZnPcref (Figure 7). In the case of PDIPy-1, the overall trend is virtually identical to what has been noted for PDI-Py-2, thus affording a binding constant of 9.6  104 m1 (Figures S17–S19 in the Supporting Information). In addition to potassium, which breaks up ZnPcTeCr aggregates and stabilizes ZnPcTeCr monomers (see below), we tested

ð1Þ

www.chemeurj.org

CS [ps] 5

[a] In the presence of potassium. [b] In the presence of rubidium. [c] Not calculated.

in which I0 refers to the normalized initial fluorescence intensity, c0 is the total concentration of the fluorophore, cadd is the total concentration of the added fluorescent quencher, K is the binding constant, and (ylim1) considers non-quantitative quenching. Owing to the fact that ZnPcTeCr aggregates, all experiments were initially performed with ZnPcref followed by titrations with ZnPcTeCr. For example, the addition of increasing concentrations of PDI-Py-1 in the range from 3.0  107 to 1.5  105 m to a solution that contained 1.0  106 m ZnPcref led to no appreciable changes in the absorption spectra (Figure S8 in the Supporting Information). In stark contrast, the fluorescence of ZnPcref decreases throughout the added PDI-Py-1 range to reach 83 % of the initial value (Figure S9 in the Supporting Information). Analyses of the corresponding nonlinear quenching afforded a binding constant of 1.1  105 m1 (Figure S10 in the Supporting Information; see Table 1 for binding constants, charge-separation time, and charge-recombination time of the donor–acceptor complexes discussed in this work). Similarly, the addition of variable concentrations of PDI-Py-2 within the concentration range from 0.9  107 to 3.5  106 to 5.7  107 m of ZnPcref did not affect the absorption (Figure S11 in the Supporting Information) of the latter but it does affect its fluorescence (Figure S12 in the Supporting Information). Here, from a nonlinear quenching, which levels off at 70 % of the initial value, a binding constant of 4.4  105 m1 was derived (Figure S13 in the Supporting Information). A likely rationale for the stronger binding for PDI-Py-2 relative to PDI-Py-1 is based on its better electron-accepting character. Finally, C60-Py and ZnPcref were probed with concentrations in the range from 3.0  107 to 2.1  105 m and 1.1  106 m1, respectively. Once again, the ZnPcref absorption was not notably impacted by the presence of C60-Py (Figure S14 in the Supporting Information). Instead, the ZnPcref fluorescence was Chem. Eur. J. 2015, 21, 1 – 11

K [m1]

3

 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

&

&

Full Paper

Figure 2. Absorption spectra of ZnPcTeCr (1.5  106 m, 500 equiv. KB(C6H4Cl)4) recorded upon addition of different concentrations of C60-Py (0, 1.0  106, 2.0  106, 3.0  106, 4.0  106, 5.3  106, 6.0  106, and 8.0  106 m) in oDCB at room temperature.

Figure 5. Absorption spectra of ZnPcTeCr (5.8  107 m, 93 equiv. KB(C6H4Cl)4) recorded upon addition of different concentrations of PDI-Py-2 (0, 5.4  107, 1.0  106, 2.6  106, 4.1  106, 5.4  106, 6.9  106, 1.0  106, and 1.4  105 m) in o-DCB at room temperature.

Figure 3. Steady-state fluorescence spectra of ZnPcTeCr (1.5  106 m, 500 equiv. KB(C6H4Cl)4) recorded at different concentrations of C60-Py (0, 1.0  106, 2.0  106, 3.0  106, 4.0  106, 5.3  106, 6.0  106, and 8.0  106 m) in o-DCB at room temperature upon 630 nm photoexcitation.

Figure 6. Steady-state fluorescence spectra of ZnPcTeCr (5.8  107 m, 93 equiv. KB(C6H4Cl)4) recorded at different concentrations of PDI-Py-2 (0, 5.4  107, 1.0  106, 2.6  106, 4.1  106, 5.4  106, 6.9  106, 1.0  106, and 1.4  105 m) in o-DCB at room temperature upon 630 nm photoexcitation.

Figure 4. Plot (I/I0) of the fluorescence intensity of ZnPcTeCr (500 equiv. KB(C6H4Cl)4 and 681 nm excitation) versus concentration of C60-Py (0– 8.0  106 m) used to determine the binding constant according to Equation (1).

&

&

Chem. Eur. J. 2015, 21, 1 – 11

www.chemeurj.org

Figure 7. Plot (I/I0) of the fluorescence intensity of ZnPcTeCr (93 equiv. KB(C6H4Cl)4 and 679 nm excitation) versus concentration of PDI-Py-2 (0– 1.4  105 m) used to determine the binding constant according to Equation (1).

4

 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ÝÝ These are not the final page numbers!

Full Paper em in which Iem c is the corrected fluorescence intensity, whereas I m is the measured emission intensity at a given wavelength, ODex is the optical density of the sample at the excitation wavelength, and ODem is the optical density at the chosen fluorescence wavelength.[33] In the case of 4.0  107 m PDI-Py-1, the addition of 6.0  108 to 4.8  106 m ZnPcTeCr dimers changed the PDI-Py-1 fluorescence (Figure S22 in the Supporting Information) but not its absorption (Figure S21 in the Supporting Information). On the basis of a nonlinear fluorescence quenching, which amounted to only 4 %, a binding constant of 9.4  105 m1 was determined (Figure S23 in the Supporting Information). More or less the same trends were noted when 4.0  107 m PDI-Py-2 was subjected to interactions with 6.0  108 to 4.8  106 m of the ZnPcTeCr dimers. On the one hand, the PDI-Py-2 absorption remains virtually unchanged and the resulting spectra are best described as the liner superimposition of the individual components (Figure S24 in the Supporting Information). On the other hand, the PDI-Py-2 fluorescence is quenched by about 9 % (Figure S25 in the Supporting Information). From the latter we determined the binding constant of PDI-Py-2 with ZnPcTeCr dimers to be 1.3  106 m1 (Figure S26 in the Supporting Information).[34]

Figure 8. Job’s plot analysis of the optical densities at 370, 371, 372, 373, 374, and 375 nm versus the molar fraction of ZnPcTeCr (328 equiv. KB(C6H4Cl)4) (0, 0.2, 0.25, 0.3, 0.4, 0.45, 0.50, 0.55, 0.6, 0.7, 0.8, 0.85, 0.9, 0.95, 1.0) based on the spectra gathered in Figure S20 of the Supporting Information. Further analysis (Gauss Fit) resulting in maxima around 0.5 showing the formation of a 1:1 complex between ZnPcTeCr and PDI-Py-1 in o-DCB at room temperature (constant overall concentration: 1.2  106 m).

rubidium in the form of RbB(C6H5)4. This is likely to favor the formation of ZnPcTeCr dimers (see below). To this end, the addition of 100 equivalents of rubidium led to marked changes in the absorption and fluorescence features. In the context of the former, the typical monomeric ZnPc absorption transforms into that of dimer with a new maximum that evolves at 637 nm. In the context of the latter, the fluorescence is nearly quantitatively lost. Beyond this, we investigated a solution of 4.3  107 m ZnPcTeCr that contained 438 equivalents of rubidium and added 240.000 equivalents of pyridine (Py). Addition of Py redshifted the Soret band from 347 to 364 nm and the Q band from 634 to 677 nm as well as intensifying them. The latter absorption spectrum is in sound agreement with that of ZnPcref. Interestingly, the strongly quenched ZnPcTeCr fluorescence owing to the presence of rubidium intensified by a factor of 160 and blueshifted from 686 to 684 nm. On the basis of the aforementioned results, we conclude that an excess amount of Py disintegrates the ZnPcTeCr dimers. Please note that small amounts of up to, for example, 30 equivalents of Py have no impact on the absorption spectrum. Implicit is that the ZnPcTeCr dimer is rather stable in the presence of rubidium. Owing to the weak fluorescence of ZnPcTeCr dimers, we used constant concentrations of PDI-Py-1 and PDI-Py-2, which were titrated with variable concentrations of ZnPcTeCr, to which 100 equivalents of rubidium were added. On account of some spectral overlap the corresponding fluorescence had to be corrected according to Equation (2):

Icem ¼ Imem  10^



ODex  ODem 2

Chem. Eur. J. 2015, 21, 1 – 11

Mass spectrometry To corroborate the potassium and rubidium complexation of ZnPcTeCr, we analyzed the corresponding solutions by means of mass spectrometry. In the case of potassium, signals that corresponded to ZnPcTeCr monomers include, for example, m/z 709 [ZnPc+3 K + +KB(C6H4Cl)4], m/z 796 [ZnPc+2 K + ], and m/z 1292 [ZnPc+2 K + +2 KB(C6H4Cl)4]. In this instance, m/z 709 is the most intense signal in the mass spectra (Figure 9). In the case of rubidium, we found that ZnPcTeCr dimers in various combinations with other ions dominate the mass spectra. As such, we are confident that ZnPcTeCr dimers are the main species in solution. For example, the mass spectra include signals at m/z 1230 [2 ZnPc+3 Rb + +RbB(C6H5)4], m/z 1095 [2 ZnPc+3 Rb + ], and m/z 1600 [2 ZnPc+2 Rb + ]. Hereby, m/z 1230 is the most intense signal in the mass spectra. In summary, the mass spectrometric findings are in sound agreement with the absorption analysis (Figure 10). Transient-absorption spectroscopy Next we turned to femtosecond transient absorption spectroscopy with ZnPcTeCr monomers and ZnPcTeCr dimers induced by crown ether complexation of potassium and rubidium, respectively. These were subsequently subjected to coordination with electron-accepting C60-Py, PDI-Py-1, and PDI-Py-2. Altogether, this is meant to corroborate the fluorescence quenching seen in the steady-state experiments and to elucidate the nature of the fluorescence quenching. Please note that all of the experiments were performed upon exclusive ZnPc excitation at 660 nm. In the absence of any electron acceptors, ZnPcTeCr gives rise, upon 660 nm excitation, to the well-known ZnPc singlet excit-

 ð2Þ

www.chemeurj.org

These are not the final page numbers! ÞÞ

5

 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

&

&

Full Paper

Figure 9. The positive-ion ESI mass spectra of ZnPcTeCr (5.0  106 m, 400 equiv. KB(C6H4Cl)4) in o-DCB.

Figure 10. The positive-ion ESI mass spectra of ZnPcTeCr (3.4  106 m, 95 equiv. RbB(C6H5)4) in o-DCB.

ed-state characteristics. For example, when potassium is complexed, maxima at 473, 575, 634, 732, and 814 nm as well as minima at 612 and 678 nm evolve. We did not note any appreciable differences with respect to ZnPcref. As such, we postulate that in the ZnPcTeCr monomers, the ZnPc singlet excited state decays over (2700  400) ps to the corresponding triplet excit&

&

Chem. Eur. J. 2015, 21, 1 – 11

www.chemeurj.org

ed state. Characteristics of the latter are a maximum at 517 nm as well as minima at 612 and 678 nm (Figures S27 and S28 in the Supporting Information). When rubidium is complexed, the ZnPc singlet excited state in ZnPcTeCr dimers decays rapidly and monoexponentially over (183  50) ps through a transient intermediate (Figures 11 and 12). 6

 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ÝÝ These are not the final page numbers!

Full Paper investigated ZnPcTeCr solutions gave no evidence for dimerization as was seen for the rubidium-containing solutions. As a consequence, ZnPcTeCr monomers should be the most dominant species in solution next to a minor amount of aggregated ZnPcTeCr. However, following excitation at 660 nm, the ZnPc singlet excited state was found to decay through a ZnPcC + /ZnPcC charge-separated state to the triplet excited state. In particular, the transient maximum at 486 nm owing to singlet–singlet transitions disappears with the concomitant growth of the ZnPcC + features at 553 and 838 nm as well as the ZnPcC features at 575 and 960 nm. A kinetic analysis affords (46  9) ps for the charge separation and (635  30) ps for the charge recombination. Moreover, at the conclusion of the femtosecond timescale, that is, 7.7 ns, only the ZnPc triplet excited state features—in the form of a maximum at 527 nm and two minima at 621 and 687 nm—are appreciable (Figures 13 and 14).

Figure 11. Differential absorption spectra (visible and near-infrared) obtained upon femtosecond flash photolysis (660 nm) of ZnPcTeCr (3.0  105 m, 114 equiv. RbB(C6H5)4) in argon-saturated o-DCB with several time delays between 0.1 and 6750 ps (see key for details).

Figure 13. Differential absorption spectra (visible and near-infrared) obtained upon femtosecond flash photolysis (660 nm) of ZnPcTeCr (2.6  105 m) in argon-saturated o-DCB with several time delays between 0.1 and 6750 ps (see key for details). Figure 12. Time-absorption profiles upon femtosecond flash photolysis (660 nm) of ZnPcTeCr (3.0  105 m, 114 equiv. RbB(C6H5)4) in argon-saturated o-DCB monitored at 566 and 640 nm.

The intermediate reveals characteristics of the one-electronreduced ZnPcTeCr with a maximum at 965 nm and of the oneelectron-oxidized ZnPcTeCr with maxima at 567 and 828 nm. These are similar to those seen in spectroelectrochemical reduction (Figure S29 in the Supporting Information) and oxidation experiments (Figure S30 in the Supporting Information) that feature maxima at 953 and 550/827 nm, respectively. Relative to signals seen for ZnPcTeCr in the absence of any alkali cations, these features appear notably broadened and weaker. We conclude from the latter that the rubidium-induced ZnPcTeCr dimer facilitates an intramolecular electron transfer to afford a ZnPcC + /ZnPcC  charge-separated state. For potassium, on the contrary, no particular evidence of an intramolecular electron transfer is discernable. The aforementioned led us to revisit ZnPcTeCr, but in the absence of alkali metals. Notably, the absorption spectra of the Chem. Eur. J. 2015, 21, 1 – 11

www.chemeurj.org

These are not the final page numbers! ÞÞ

Figure 14. Time-absorption profile upon femtosecond flash photolysis (660 nm) of ZnPcTeCr (2.6  105 m) in argon-saturated o-DCB monitored at 577 nm showing charge-separation and charge-recombination dynamics.

7

 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

&

&

Full Paper of kinetic features, lifetimes of (194  50) ps indicate processes that are slowed down (Figure S40 in the Supporting Information). Stronger binding between PDI-Py-2 and ZnPcTeCr changed the electron-transfer reactivity. In particular, evidence for the one-electron-reduced PDI-Py-2 is based on the 930 nm feature, from which charge separation and charge recombination kinetics of (60  13) and (169  25) ps, respectively, were deduced (Figures 15 and 16). Finally, we focused on the C60-Py acceptor. Starting with the potassium-complexed ZnPcTeCr monomers, once again the aforementioned ZnPc singlet excited-state features were rapidly replaced by a transient. The latter featured maxima at 550, 715, 829, and 1025 nm, next to minima at 610 and 680 nm (Figure S41 in the Supporting Information). In addition to the aforementioned characteristics of the one-electron-oxidized

With these experiments at hand, we probed the impact of just Py on ZnPcTeCr. In short, weakening the intermolecular forces by means of Py coordination disables the electron transfer and, in turn, reactivates the intersystem crossing. When using a large excess amount of Py, the transient absorption spectra for ZnPcTeCr are superimposable over those of ZnPcref. In fact, no corroboration of the aforementioned ZnPcC + or ZnPcC transients is discernable. Instead, the ZnPc singlet excited state decays over (2860  100) ps and transforms into the triplet excited state. Maxima at 492/816 nm and at 526 nm confirm the kinetic assignment. A small excess amount of Py (12 equivalents) led to the coexistence of both decays, namely, an intramolecular electron transfer of (28  3) ps for charge separation and of (604  20) ps for charge recombination as well as an intersystem crossing of (3593  300) ps (Figures S31 and S32 in the Supporting Information).[35] In the presence of PDI-Py-1 and PDI-Py-2, we note the ZnPc singlet excited state features upon 660 nm excitation. They evolve as in the reference experiments with potassium-induced ZnPcTeCr monomers and ZnPcref, which both lack any electron acceptor. In particular, maxima at 491, 631, and 734 nm as well as minima at 611 and 680 nm relate to ZnPcTeCr monomers/PDIPy-1 (Figures S33 and S34 in the Supporting Information). Similarly, the attributes for ZnPcTeCr monomers/PDI-Py-2 are maxima at 467, 734, and 827 nm as well as minima at 612 and 679 nm (Figures S35 and S36 in the Supporting Information). All of them decay, however, rather quickly with the simultaneous formation of new transients. In the case of PDI-Py-1, the new transient includes 511, 793, 993, and 1098 nm maxima. In the case of PDI-Py-2, the maxima are at 559, 766, 827, 930, and 1026 nm. On the basis of spectroelectrochemical reduction and/or oxidation experiments, we infer that the maxima at 554 and 827 nm, as well as the minima 612 and 680 nm agree well with the features of the one-electron-oxidized ZnPc (see above). On the one hand, the maxima at 793, 993, and 1098 nm correlate with the attributes of the one-electron-reduced PDI-Py-1 (Figure S37 in the Supporting Information). On the other hand, the one-electron-reduced PDI-Py-2 featured maxima at 764, 922, and 1012 nm (Figure S38 in the Supporting Information). In other words, ZnPcTeCr monomers transform together with PDI-Py-1 and PDI-Py-2 upon photoexcitation into ZnPcC + /PDIC charge-separated states. On the basis of multiwavelength analyses of (ZnPcTeCr)C + /(PDI-Py-1)C  , charge-separation and charge-recombination dynamics of (124  13) and (3471  237) ps, respectively, were determined. The (ZnPcTeCr)C + /(PDI-Py-2)C lifetimes are (53  7) and (337  46) ps. For PDI-Py1 and PDI-Py-2 the product of the charge recombination is the triplet excited state and the singlet ground state, respectively. With the information about potassium-induced ZnPcTeCr monomers at hand, rubidium-induced ZnPcTeCr dimers were tested in combination with PDI-Py-1 and PDI-Py-2. Interestingly, PDI-Py-1, which binds weaker to ZnPc, was seen to cancel the intramolecular electron transfer that affords the ZnPcC + /ZnPcC charge-separated states. In terms of spectroscopic features, no evidence for the one-electron-reduced PDI-Py1 evolves (Figure S39 in the Supporting Information). In terms &

&

Chem. Eur. J. 2015, 21, 1 – 11

www.chemeurj.org

Figure 15. Differential absorption spectra (visible and near-infrared) obtained upon femtosecond flash photolysis (660 nm) of ZnPcTeCr (3.0  105 m, 13 equiv. PDI-Py-2, 103 equiv. RbB(C6H5)4) in argon-saturated o-DCB with several time delays between 0.1 and 6750 ps (see key for details).

Figure 16. Time-absorption profiles upon femtosecond flash photolysis (660 nm) of ZnPcTeCr (3.0  105 m, 13 equiv. PDI-Py-2, 103 equiv. RbB(C6H5)4) in argon-saturated o-DCB monitored at 555, 630, and 900 nm showing charge-separation and charge-recombination dynamics.

8

 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ÝÝ These are not the final page numbers!

Full Paper Acknowledgements Financial support from Solar Technologies Go Hybrid (SolTech), SFB953, Comunidad de Madrid, Spain (S2013/MIT-2841, FOTOCARBON), and Spanish MEC (CTQ2011-24187/BQU) and MICINN (PRI-PIBUS-2011-1128) is acknowledged. Keywords: alkali metals · charge transfer · phthalocyanines · donor–acceptor systems · crown ethers · perylenbisimides · fullerenes

Figure 17. Energy-level diagram illustrating the relaxation pathways of the supramolecular complexes.

[1] G. W. Gokel, W. M. Leevy, M. E. Weber, Chem. Rev. 2004, 104, 2723. [2] J. S. Bradshaw, R. M. Izatt, Acc. Chem. Res. 1997, 30, 338. [3] Handbook of Porphyrin Science (Eds.: K. M. Kadish, K. M. Smith, R. Guilard), World Scientific, Singapore, 2013. [4] M. V. Martnez-Daz, G. de la Torre, T. Torres, Chem. Commun. 2010, 46, 7090 – 7108. [5] C. G. Claessens, U. Hahn, T. Torres, Chem. Rec. 2008, 8, 75. [6] K. Ishii, Coord. Chem. Rev. 2012, 256, 1556. [7] N. N. Sergeeva, M. O. Senge, in CRC Handbook of Organic Photochemistry and Photobiology, 3rd ed. (Eds.: A. Griesbeck, M. Oelgemoller, F. Ghetti), 2012, p. 831. [8] J. A. A. W. Elemans, R. van Hameren, R. J. M. Nolte, A. E. Rowan, Adv. Mater. 2006, 18, 1251. [9] H. Engelkamp, S. Middelbeek, R. J. M. Nolte, Science 1999, 284, 785. [10] G. Bottari, G. de la Torre, D. M. Guldi, T. Torres, Chem. Rev. 2010, 110, 6768. [11] H. Imahori, T. Umeyama, K. Kurotobi, Y. Takano, Chem. Commun. 2012, 48, 4032 – 4045. [12] F. D’Souza, O. Ito, Chem. Commun. 2009, 4913 – 4928. [13] G. de la Torre, G. Bottari, M. Sekita, A. Hausmann, D. M. Guldi, T. Torres, Chem. Soc. Rev. 2013, 42, 8049. [14] D. M. Guldi, G. M. A. Rahman, V. Sgobba, C. Ehli, Chem. Soc. Rev. 2006, 35, 471. [15] G. H. Sarova, U. Hartnagel, D. Balbinot, S. Sali, N. Jux, A. Hirsch, D. M. Guldi, Chem. Eur. J. 2008, 14, 3137 – 3145. [16] A. Mateo-Alonso, C. Sooambar, M. Prato, C. R. Chim. 2006, 9, 944. [17] J.-F. Nierengarten, U. Hahn, T. M. Figueira Duarte, F. Cardinali, N. Solladi, M. E. Walther, A. Van Dorsselaer, H. Herschbach, E. Leize, A.-M. AlbrechtGary, A. Trabolsi, M. Elhabiri, C. R. Chim. 2006, 9, 1022. [18] M. Elhabiri, A. Trabolsi, F. Cardinali, U. Hahn, A.-M. Albrecht-Gary, J.-F. Nierengarten, Chem. Eur. J. 2005, 11, 4793 – 4798. [19] U. Hahn, M. Elhabiri, A. Trabolsi, H. Herschbach, E. Leize, A. Van Dorsselaer, A.-M. Albrecht-Gary, J.-F. Nierengarten, Angew. Chem. Int. Ed. 2005, 44, 5338; Angew. Chem. 2005, 117, 5472. [20] J.-F. Nierengarten, U. Hahn, A. Trabolsi, H. Herschbach, F. Cardinali, M. Elhabiri, E. Leize, A. Van Dorsselaer, A.-M. Albrecht-Gary, Chem. Eur. J. 2006, 12, 3365. [21] M. V. Martnez-Daz, N. S. Fender, M. S. Rodrguez-Morgade, M. GmezLpez, F. Diederich, L. Echegoyen, J. F. Stoddart, T. Torres, J. Mater. Chem. 2002, 12, 2095. [22] D. M. Guldi, J. Ramey, M. V. Martnez-Diaz, A. de la Escosura, T. Torres, T. Da Ros, M. Prato, Chem. Commun. 2002, 2774. [23] F. D’Souza, E. Maligaspe, A. S. D. Sandanayaka, N. K. Subbaiyan, P. A. Karr, T. Hasobe, O. Ito, J. Phys. Chem. A 2010, 114, 10951 – 10959. [24] F. D’Souza, E. Maligaspe, K. Ohkubo, M. E. Zandler, N. K. Subbaiyan, S. Fukuzumi, J. Am. Chem. Soc. 2009, 131, 8787 – 8797. [25] A. M. V. M. Pereira, A. Hausmann, A. R. M. Soares, J. P. C. Tom, O. Trukhina, M. Urbani, M. G. P. M. S. Neves, J. A. S. Cavaleiro, D. M. Guldi, T. Torres, Chem. Eur. J. 2012, 18, 3210. [26] Y. Rio, W. Seitz, A. Gouloumis, P. Vzquez, J. L. Sessler, D. M. Guldi, T. Torres, Chem. Eur. J. 2010, 16, 1929. [27] O. E. Sielcken, M. M. Tilborg, M. F. M. Roks, R. Hendriks, W. Drenth, R. J. M. Nolte, J. Am. Chem. Soc. 1987, 109, 4261. [28] N. Kobayashi, A. B. P. Lever, J. Am. Chem. Soc. 1987, 109, 7433.

ZnPc in the visible region, the near-infrared fingerprint at 1025 nm relates to the one-electron-reduced C60-Py. Thus, the (ZnPcTeCr)C + /(C60-Py)C charge-separated state is formed upon photoexcitation (Figure 17). Dynamics of (32  4) and (5112  1200) ps were derived from multiwavelength analysis for the charge separation and charge recombination, respectively (Figure S42 in the Supporting Information). For rubidium-complexed ZnPcTeCr dimers, evidence for a (ZnPcTeCr)C + /(C60-Py)C charge-separated state came from maxima at 736 and 1023 nm, which were formed with a lifetime of (41  5) ps and that decay with a lifetime of (1236  50) ps (Figures S43 and S44 in the Supporting Information). Nevertheless, the rubidium-induced intramolecular electron transfer between two ZnPcs still seems active – although, with a value of less than 10 %, only to a minor extent.

Conclusion In summary, C60 (C60-Py) rather than PDI (PDI-Py-1 and PDI-Py2) as electron acceptor accelerates the charge separation but decelerates the charge recombination. The presence of ZnPcTeCr monomers in general favors the electron transfer to the electron acceptor. As such, the addition of potassium seems to evolve as the best option en route to the expected electron transfer that features charge-separated-state lifetimes that range from (337  46) ps for PDI-Py-2 to (5112  1200) ps for C60-Py. To assess the stabilization of the (ZnPcTeCr)C + /(PDI-Py-1)C  , (ZnPcTeCr)C + /(PDI-Py-2)C , and (ZnPcTeCr)C + /(C60-Py)C charge-separated states in the ZnPcTeCr monomers and dimers, we complemented our investigations by probing ZnPcref in the presence of PDI-Py-1 (Figure S45 in the Supporting Information), PDI-Py2 (Figure S46 in the Supporting Information), and C60-Py (Figure S47 in the Supporting Information). Here the corresponding charge-separated-state lifetimes were (1188  50) ps for PDI-Py-1, (183  30) ps for PDI-Py-2, and (2633  281) ps for C60Py. Chem. Eur. J. 2015, 21, 1 – 11

www.chemeurj.org

These are not the final page numbers! ÞÞ

9

 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

&

&

Full Paper from 6.0  107 to 2.4  105 m to PDI-Py-2 (4.0  106 m). However, strongly overlapping absorption and fluorescence features hampered a meaningful determination of the binding constants despite the fact that evidence emerges for the complexation of PDI-Py-1 and PDI-Py-2. [35] Addition of Py to ZnPcTeCr monomers and dimers stabilized by potassium and rubidium, respectively, and ZnPcref, led to transient absorption spectra that are virtually superimposable in terms of intermediates and kinetics to those in the absence of Py.

[29] A. Troeger, M. Ledendecker, J. T. Margraf, V. Sgobba, D. M. Guldi, B. F. Vieweg, E. Spiecker, S.-L. Suraru, F. Wrthner, Adv. Energy Mater. 2012, 2, 536. [30] S. R. Wilson, S. MacMahon, F. T. Tat, P. D. Jarowski, D. I. Schuster, Chem. Commun. 2003, 226. [31] D. K. Panda, F. S. Goodson, S. Ray, R. Lowell, S. Saha, Chem. Commun. 2012, 48, 8775. [32] B. Valeur, Molecular Fluorescence, Wiley-VCH, Weinheim, 2002. [33] J. R. Lakowicz, Principles of Fluorescence Spectroscopy, 3rd ed., Springer, New York, 2006, p. 56. [34] In reference experiments, ZnPcTeCr in the presence of 350 equivalents of potassium was added in the concentration range from 6.0  106 to 2.4  105 m to PDI-Py-1 (4.0  106 m) and in the concentration range

&

&

Chem. Eur. J. 2015, 21, 1 – 11

www.chemeurj.org

Received: October 14, 2014 Revised: January 2, 2015 Published online on && &&, 0000

10

 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ÝÝ These are not the final page numbers!

Full Paper

FULL PAPER & Donor–Acceptor Systems M. Lederer, U. Hahn, J. Fernndez-Ariza, O. Trukhina, M. S. Rodrguez-Morgade, C. Dammann, T. Drewello,* T. Torres,* D. M. Guldi* Being spoilt for choice: New supramolecular electron donor–acceptor hybrids based on a zinc phthalocyanine endowed with four [18]-crown-6 ether subunits and three different pyridylfunctionalized electron acceptors were

Chem. Eur. J. 2015, 21, 1 – 11

investigated. Several combinations were tested by tuning the appearance and optical properties of the phthalocyanine, which were found to be dependent on the present alkali metal cation (K + /Rb + ; see figure).

www.chemeurj.org

These are not the final page numbers! ÞÞ

11

&& – && Tuning Electron Donor–Acceptor Hybrids by Alkali Metal Complexation

 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

&

&

Tuning electron donor-acceptor hybrids by alkali metal complexation.

A zinc phthalocyanine endowed with four [18]-crown-6 moieties, ZnPcTeCr, has been prepared and self-assembled with either pyridyl-functionalized peryl...
888KB Sizes 1 Downloads 9 Views