High-potential perfluorinated phthalocyanine-fullerene dyads for generation of high-energy charge-separated states: formation and photoinduced electron-transfer studies.

CHEMPHYSCHEM ARTICLES DOI: 10.1002/cphc.201402118

High-Potential Perfluorinated Phthalocyanine–Fullerene Dyads for Generation of High-Energy Charge-Separated States: Formation and Photoinduced Electron-Transfer Studies Sushanta K. Das, Andrew Mahler, Angela K. Wilson, and Francis D’Souza*[a] High oxidation potential perfluorinated zinc phthalocyanines (ZnFnPcs) are synthesised and their spectroscopic, redox, and light-induced electron-transfer properties investigated systematically by forming donor–acceptor dyads through metal– ligand axial coordination of fullerene (C60) derivatives. Absorption and fluorescence spectral studies reveal efficient binding of the pyridine- (Py) and phenylimidazole-functionalised fullerene (C60Im) derivatives to the zinc centre of the FnPcs. The determined binding constants, K, in o-dichlorobenzene for the 1:1 complexes are in the order of 104 to 105 m1; nearly an order of magnitude higher than that observed for the dyad formed from zinc phthalocyanine (ZnPc) lacking fluorine substituents. The geometry and electronic structure of the dyads are determined by using the B3LYP/6-31G* method. The HOMO and LUMO levels are located on the Pc and C60 entities, respectively; this suggests the formation of ZnFnPcC + –C60ImC

and ZnFnPcC + –C60PyC (n = 0, 8 or 16) intra-supramolecular charge-separated states during electron transfer. Electrochemical studies on the ZnPc–C60 dyads enable accurate determination of their oxidation and reduction potentials and the energy of the charge-separated states. The energy of the charge-separated state for dyads composed of ZnFnPc is higher than that of normal ZnPc–C60 dyads and reveals their significance in harvesting higher amounts of light energy. Evidence for charge separation in the dyads is secured from femtosecond transient absorption studies in nonpolar toluene. Kinetic evaluation of the cation and anion radical ion peaks reveals ultrafast charge separation and charge recombination in dyads composed of perfluorinated phthalocyanine and fullerene; this implies their significance in solar-energy harvesting and optoelectronic device building applications.

1. Introduction Photoinduced electron transfer in donor–acceptor models is of great interest for researchers designing artificial photosynthetic systems for light energy harvesting, solar fuel production, and building opto-electronic devices.[1–13] Porphyrins (P)[14] and phthalocyanines (Pc)[15] have been studied extensively due to their roles in mimicking biological systems. Additionally, these macrocycles offer high chemical and thermal stabilities, absorption and redox features, and the possibility of tuning these properties by inserting different metals into the macrocycle central cavity or by introducing different substituents at their ring peripheries. As a result of their remarkable absorption properties in the visible region, they have been highly sought out photosensitisers in photovoltaic applications, and redox-based fluorescence applications.[13] Recently, employing high (oxidation) potential sensitisers to build donor–acceptor dyads or immobilise semiconductor sur[a] Dr. S. K. Das, A. Mahler, Prof. A. K. Wilson, Prof. Dr. F. D’Souza Department of Chemistry, University of North Texas 1155 Union Circle, #305070, Denton, TX 76203-5017 (USA) Fax: (+ 1) 940-565-4318 francis E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201402118.

2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

faces has gained some interest. This is primarily due to the possibility of generating high-energy charge-separated states suitable for developing water oxidation/proton reduction catalysts.[16–18] In this context, recently, we reported donor–acceptor dyads composed of high potential P–fullerene (C60) dyads by employing P with either fluoro or chloro substituents on the meso-aryl entities.[18] As expected, the presence of halogens on the periphery of P made the oxidation processes difficult; however, it was possible to observe ultrafast photoinduced electron transfer in these dyads. Encouraged by these findings, in the present study, we have employed perfluorinated Pcs as high oxidation potential photosensitisers and built donor– acceptor dyads by using fulleropyrrolidines (C60Pys) with either a pyridine or phenylimidazole axial ligating functionality. Two perfluorinated ZnPc derivatives, namely, zinc 2,3,9,10,16,17,23,24,-octafluoro-29H,31H-phthalocyanine (ZnF8Pc) with 8 fluorine substituents on the macrocycle ring periphery and zinc 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-hexadecafluoro-29H,31H-phthalocyanine (ZnF16Pc) with 16 fluorine substituents on the macrocycle ring periphery were synthesised. Donor–acceptor dyads were formed by the metal–ligand axial coordination approach with either pyridine- (C60Py) or phenylimidazole-functionalised (C60Im) fulleropyrrolidine (see Figure 1 for structures). Zinc 2,9,16,23-tetra-tert-butyl-29 H,31HChemPhysChem 2014, 15, 2462 – 2472

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Figure 1. Structure of the ZnPc, ZnF8Pc, ZnF16Pc, C60Im and C60Py compounds used in this study.

Figure 2. a) Absorption spectra of ZnPc (i), ZnF8Pc (ii) and ZnF16Pc (iii); and b) steady-state fluorescence spectra of ZnPc, lex = 680 nm (i); ZnF8Pc, lex = 710 nm (ii) and ZnF16Pc, lex = 730 nm (iii). All of the compounds were prepared in degassed DCB.

phthalocyanine (ZnPc), with no fluorine substituents, was also used to form the dyads with C60. Photoinduced electron transfer in these dyads was investigated systematically by using various spectroscopic, electrochemical, computational and transient spectral studies to unravel the significance of high potential Pcs in electron-transfer reactions.

and 680 nm, that is, a gradual redshift of both the Soret and visible bands of ZnPcs with increasing fluorine substituents was observed. Interestingly, ZnF8Pc revealed a broad absorption in the l = 475 nm region and a peak at l = 860 nm, which slightly decreased in intensity with time in solution to suggest appreciable decomposition. However, no such results were seen for ZnF16Pc, revealing its higher stability. Consequently, fresh solutions of the studied compounds were prepared prior to performing spectral and photophysical measurements. A similar spectral trend was also observed in the fluorescence spectrum of these compounds. Upon excitation at the wavelength maxima of the most intense visible band, the emission maxima were found to be at l = 745 nm for ZnF16Pc, l = 727 nm for ZnF8Pc and l = 692 nm for ZnPc. In addition, the fluorescence intensity of the fluorinated Pcs also revealed drastic changes, that is, a 30 % reduction in intensity for ZnF8Pc and 63 % reduction intensity for ZnF16Pc compared with the intensity of the ZnPc emission band was observed. Notably, the redshifted absorbance and emission bands of ZnF8Pc and ZnF16Pc reveal their significance in harvesting light from the near-infrared (NIR) portion of the electromagnetic spectrum. Figure 3 shows the time-resolved fluorescence decay curves obtained by the TCSPC of various fluorinated Pc derivatives in degassed toluene. All of the ZnPc derivatives revealed monoexponential decay with lifetimes of 3.14, 2.85 and 2.40 ns, respectively, for ZnPc, ZnF8Pc and ZnF16Pc. The lifetimes tracked the fluorescence intensities shown in Figure 2 b.

2. Results and Discussion 2.1. Characterisation of the Perfluorinated ZnPc Derivatives: Absorbance and Fluorescence Studies Detailed synthetic details for the perfluorinated Pcs is provided in the Experimental Section. The one-step synthesis involved the reaction of either 4,5-difluorophthalonitrile or tetrafluorophthalonitrile in the presence of zinc chloride in dimethylethanolamine (DMAE) at elevated temperature followed by chromatographic purification on a silica gel column with a mixture of chloroform/methanol as the eluent. The newly synthesised compounds were stored in dark prior spectral and photochemical investigations. Figure 2 a shows the normalised absorption spectrum of the Pc sensitisers in o-dichlorobenzene (DCB). For ZnF16Pc, the bands were located at l = 380, 656 and 734 nm, whereas for ZnF8Pc these bands were located at l = 352, 477(br), 640 and 712 nm. The position of these bands compared with the control ZnPc, the bands for which were located at l = 350(sh), 614 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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www.chemphyschem.org involving ZnFnPc and C60Im or ZnFnPc and C60Py can be represented by Equations (1) and (2), in which the symbol “:” represents a coordinate bond between zinc–imidazole or zinc–pyridine entities; Fn represents fluorine substituents (n = 0, 8 or 16) on the ZnPc ring.

Figure 3. Time-correlated single photon counting (TCSPC) fluorescence decay curves of prompt/scatter (i), ZnF16Pc (ii), ZnF8Pc (iii) and ZnPc (iv) recorded in degassed toluene. The samples were excited at l = 674 nm by means of a nano-light-emitting diode (nanoLED) with a pulse width of about 100 ps and the emission was collected at the emission peak maxima of ZnPcs. The time calibration factor was 0.439 ns per channel.

2.2. Formation and Characterisation of ZnPc-C60 Dyads SelfAssembled by Axial Co-ordination Axial coordination of the electron acceptors C60Im and C60Py to various perfluorinated ZnPcs was first studied by optical absorption methods. Figure 4 shows the spectral changes observed during the titration of ZnF16Pc with C60Py in DCB. For the sake of simplicity, ZnF16Pc and C60Py have been chosen here to illustrate both the titrations for UV/Vis absorbance and fluorescence emission titrations. Absorbance and fluorescence spectral changes for the other ZnPcs during the formation of the dyads is given in Figures S1–S7 in the Supporting Information. Upon increasing the addition of a calculated concentration of C60Py to ZnF16Pc, peaks at l = 430 and 700 nm appeared, whereas the visible band of ZnF16Pc at l = 730 nm diminished in intensity. The sharp peak at l = 430 nm could be attributed to C60Py, which increased with increasing concentration of C60Py. An isosbestic point at l = 710 nm confirmed the existence of an equilibrium process. The equilibrium process

C60 Im þ ZnFn Pc $ C60 Im : ZnFn Pc

ð1Þ

C60 Py þ ZnFn Pc $ C60 Py : ZnFn Pc

ð2Þ

Unlike for ZnPc binding to axial coordinating ligands, for which spectral changes for the visible bands are marginal, both ZnF8Pc and ZnF16Pc revealed significant changes accompanied by one or more isosbestic points during axial coordination. This was true even for pristine pyridine or phenylimidazole binding to these macrocycles. During the course of these titrations, no evidence for ground-state electron transfer between ZnPc and C60 (or pristine pyridine or phenylimidazole) was observed; this suggests that the observed changes in Figure 4 are indeed due to axial coordination. Further, by using the absorbance titration data, Benesi–Hildebrand plots[19] were constructed for all of the dyads (Figure 4 b). The formation constant, K, for the C60Py:ZnF16Pc dyad was found to be 3.5 105 m1, whereas for the C60Im:ZnF16Pc dyad it was 5.8 105 m1. The K values for other studied dyads are given in Table 1 and the spectral data is given in the Supporting Infor-

Table 1. Formation constant, K, calculated from the Benesi–Hildebrand method and using the absorbance and fluorescence data, and binding energies (BEs) obtained from B3LYP/6-31G*-optimised structures. Donor

Acceptor

K [m1][a]

K [m1][b]

BE [kcal mol1][c]

ZnPc

C60Im C60Py C60Im C60Py C60Im C60Py

1.3 105 8.6 104 2.2 105 1.1 105 6.0 105 3.7 105

1.1 105 7.9 104 2.1 105 1.0 105 5.8 105 3.5 105

24.95 21.60 27.68 23.95 29.71 25.63

ZnF8Pc ZnF16Pc

[a] Formation constant based on fluorescence titration data. [b] Formation constant based on absorbance data. [c] From B3LYP/6-31G*-optimised structures.

mation. The K values followed the trend ZnF16Pc > ZnF8Pc > ZnPc for given C60 derivatives, whereas between C60Im and C60Py higher values were obtained for C60Im binding. These trends generally agreed with the earlier reported dyads for fluorinated ZnP derivatives with C60Im and C60Py.[18] In other words, electron-deficient ZnPcs form dyads of higher stability. Better spectral changes seen for axial binding of fluorinated ZnPcs could be ascribed to higher binding constants. 2.3. Steady-State Fluorescence Measurements Figure 4. Spectral changes observed during the titration of ZnF16Pc (0.78 104 m) upon increasing addition of C60Py (0.113 mm each addition) in DCB. Inset: Benesi–Hildebrand plot constructed to evaluate the binding constant. A0 and A represent the absorbance of the ZnF16Pc in the absence and presence, respectively, of added C60Py.


Figure 5 shows the fluorescence spectra during the formation of the C60Py:ZnF16Pc supramolecular dyad. Upon the addition of various aliquots of C60Py at the same concentrations as those used for the absorbance titration, fluorescence quenching of Pc was observed along with a blueshift of 7–8 nm of the ChemPhysChem 2014, 15, 2462 – 2472

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Figure 5. Fluorescence spectral changes observed during the titration of ZnF16Pc (0.78 104 m) upon increasing addition of C60Py (0.113 mm each addition) in DCB, excited at l = 730 nm. Inset: Benesi–Hildebrand plot constructed to evaluate the binding constant. I0 and I represent the fluorescence intensity of ZnF16Pc in the absence and presence, respectively, of added C60Py.

www.chemphyschem.org firms better axial coordination of C60Im with ZnPc than that of C60Py, as seen from our previous study.[22] With the introduction of fluorine substituents on the Pc ring, the BE of the supramolecular dyads is seen to increase relative to ZnPc-based dyads. In the case of ZnF8Pc, there was an increase in BE by 2.45 and 2.73 kcal mol1, respectively, for C60Py and C60Im axially coordinated moieties. As the number of electron-withdrawing fluorine substituents increased to 16 for ZnF16Pc, a further increase in BEs amounting to 1.68 and 2.03 kcal mol1 were calculated for C60Py and C60Im binding, respectively. The ZnN distance of the axial bond was about 2.1 for both structures. The centre-to-centre distances were 12.2 and 10.1 for C60Im:ZnPc and C60Py:ZnPc dyads, respectively, that is, the close proximity of C60 to the Pc ring in the case of C60Py:ZnPc, compared with that of C60Im:ZnPc, was evident from these calculations. The frontier orbitals, namely, the HOMO and LUMO, were generated along with the supramolecular electrostatic potential maps. As shown in Figure 6 a and b, the HOMO was

emission band. To verify that the shift was due to binding of C60Py with ZnF16Pc, fluorescence emission was recorded by exciting ZnF16Pc at l = 692 nm, corresponding to the shoulder band of ZnF16Pc. A blueshift under these conditions was also observed. A similar spectral trend was observed during the formation of C60Im:ZnF16Pc, as shown in Figure S7 in the Supporting Information. The formation constant for the supramolecular dyads, calculated by constructing Benesi–Hildebrand plots[19] of fluorescence quenching data, is listed in Table 1. For the given dyad, the value of K was comparable to the corresponding value obtained by absorption studies. The trend in K values with respect to the nature of the Pc macrocycle and C60 ligating functionality discussed earlier also were observed herein. 2.4. Computational Study To understand the geometry and electronic structures of the supramolecular dyads, computational studies were performed with density functional theory. The Becke three-parameter, Lee–Yang–Parr hybrid functional (B3LYP) was chosen for these calculations due to the well-documented success of this function in geometry optimisations and predicted electronic properties, including Pcs,[21a] although linear Hartree–Fock calculations to predict properties of Pcs are also well known.[21b,c] A Pople-style 3-21G* basis set was used for geometry optimisations and 6-31G* was used to calculate BEs and the HOMO/ LUMO energies. All calculations were performed by using the Gaussian 09 computational software package.[20] The optimised structures were not much different from that reported earlier for dyads formed from ZnPs or zinc naphthalocyanine (ZnNc).[21a] The BEs, calculated from the difference between the energy of the optimised structure minus the sum of individual energies of ZnPc and C60, are listed in Table 1. Interestingly, the trend in these values tracked that of the binding constants, that is, ZnPc upon axial ligation of C60Im and C60Py revealed BEs of 24.95 and 21.60 kcal mol1, respectively. This con 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 6. a) Frontier HOMO and b) LUMO of the B3LYP/6-31G*-optimised supramolecular C60Py:ZnF16Pc dyad. Electrostatic potential map for c) C60Im:ZnF16Pc and d) C60Py:ZnF16Pc. For a coloured image, please see Figure S15 in the Supporting Information.

fully localised on the ZnPc macrocycle, irrespective of the large number of fluorine substituents, whereas the LUMO was on the C60 entity (see Figure S11 in the Supporting Information for the HOMOs and LUMOs of ZnF8Pc-derived dyads). The electrostatic potential map was also indicative of the electron-rich nature of ZnPc relative to the electron-deficient nature of C60 (Figure 6 c and d). 2.5. Redox Potentials and Energy Level Diagram Electrochemical redox potentials of the various ZnPc and C60Py derivatives obtained by using cyclic voltammetry techniques were evaluated in DCB containing 0.1 m tetrabutylammonium perchlorate as a supporting electrolyte. Under the solution conditions, the perfluorinated Pcs were sparingly soluble, just enough to measure the redox potentials. As predicted, the perChemPhysChem 2014, 15, 2462 – 2472

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Table 2. Electrochemical redox potentials (versus the ferrocene/ferrocenium couple, Fc/Fc + ),[a] free energy change for charge separation (DGCS) and charge recombination (DGCR) for the investigated dyads in DCB. Donor

Acceptor

Eox[b]

Ered[c]

DGCS[d] [eV]

DGCR[e] [eV]

ZnPc

C60Im C60Py C60Im C60Py C60Im C60Py

0.08 0.03 0.39 0.34 0.58 0.57

1.07 1.08 1.03 1.04 0.95 0.96

0.68 0.72 0.30 0.34 0.15 0.16

1.15 1.11 1.42 1.38 1.53 1.52

ZnF8Pc ZnF16Pc

[a] From differential pulse voltammetry in DCB, 0.1 m tetrabutylammonium perchlorate [(TBA)ClO4]. [b] ZnPc-centred oxidation. [c] C60-centred reduction. [d] DGCS = EoxEredE(0,0). [e] DGCR = EoxEred, in which E(0,0) for ZnPc = 1.83 eV, E(0,0) for ZnF8Pc = 1.72 eV and E(0,0) for ZnF16Pc = 1.68 eV based on absorbance and emission data in DCB; Eox and Ered are the first oxidation potential of the donor, ZnPc and the first reduction potential of the acceptor, C60 recorded in DCB containing 0.1 m (TBA)ClO4 obtained by differential pulse voltammetry. The solvation energy was neglected in the free energy calculations due to the rigid nature of the donor and acceptor entities (Ref. [25]). Figure 7. Energy level diagram showing photochemical events occurring in the C60Py:ZnFnPc and C60Im:ZnFnPc (n = 0, 8, 16) dyads, leading to charge separation and charge recombination.

fluorinated ZnPcs revealed facile reduction, but more difficult oxidations as a result of electron-withdrawing fluorine substituents on the macrocycle periphery.[23] Upon coordinating C60Py or C60Im, small changes in the potentials of both ZnPc and C60 were observed. Table 2 lists the first oxidation potential of the ZnPc and the first reduction potential of C60 needed to estimate free energy changes for charge separation (DGCS) and charge recombination (DGCR); the voltammograms are shown in Figures S8–S10 in the Supporting Information. The DGCS and DGCR values were evaluated by using the Rehm–Weller approach[24] and are also given in Table 2. In these calculations, the solvation energies were omitted due to the rigid nature of both C60 and Pc entities, for which highly delocalised p-cation and p-anion radicals were expected to form.[25] From the values listed in Table 2, it is clear that substitution of fluorine atoms on the Pc ring lowers DGCS in the order of ZnPc > ZnF8Pc > ZnF16Pc. Concurrently, the calculated DGCR values increase with an increasing number of fluorine substituents on the Pc macrocycle. Figure 7 shows the energy level diagram related to photoinduced electron transfer from singlet excited perfluorinated ZnPc to coordinated C60. Owing to the redshift in the absorption and emission of fluorinated Pcs (Figure 2), there was a decrease in the singlet–singlet energy from 1.83 eV for ZnPc to 1.72 eV for ZnF8Pc to 1.68 eV to ZnF16Pc. Even then, the calculated DGCS values suggest that electron transfer is indeed possible from their singlet excited states. Interestingly, owing to the more difficult oxidation of perfluorinated Pc, the energy of the charge-separated states gradually becomes higher. The generation of high-energy radical ion pairs implies maximum utilisation of light energy. The radical ion pairs thus formed could populate low-lying 3ZnPc* (ET 1.1 eV) and not 3C60* (ET = 1.56 eV)[25] due to the high energy of the latter relative to that of the radical ion pairs prior returning to the ground state. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

2.6. Femtosecond Transient Absorption Studies To establish the mechanistic details of the formation of chargeseparated states, and to evaluate kinetic information on charge separation and recombination processes in the supramolecular dyads, transient absorption studies were performed by using the femtosecond pump–probe technique with l = 400 nm laser excitation. Toluene, instead of DCB, was used as the solvent due to better solubility of the investigated compounds. C60 in toluene upon excitation showed the instantaneous formation of an intense 1C60* broad peak centred at l = 950 and 500 nm that decayed at a rate of 7.9 1010 s1, as shown in Figure S12 in the Supporting Information. The decay of the singlet 1C60* at l = 950 nm was accompanied by an increase of the 3C60* state at l = 760 nm. The lifetime of the 1C60* state was calculated to be 2.6 ns by using a mono-exponential decay fit. Figure 8 a shows the femtosecond transient absorption spectra recorded for ZnPc in toluene at different time intervals. Upon excitation, instantaneous singlet–singlet spectral features were observed with characteristic excited singlet peaks at l = 438, 478, 594, 634 and 840 nm accompanied by transient bleaching at l = 610 and 684 nm; this is opposite to the absorbance spectral features of ZnPc.[26] The singlet features decayed with a concomitant increase in the triplet features, both in the visible and NIR regions. The peak at l = 478 nm could be attributed to the 3ZnPc* state. Similar spectral features were also observed for perfluorinated ZnPcs. As shown in Figure 8 b, upon excitation, ZnF8Pc revealed transient features with bleaching at l = 630 and 710 nm, corresponding to the absorbance bands and positive peaks at l = 500, 550, 578, 745 and 1130 nm. The singlet peaks decayed to populate the triplet state in the l = 500–600 nm range with a lifetime of 2.8 ns. ZnF16Pc also revealed such singlet–singlet features that decayed with a lifetime of 2.4 ns. ChemPhysChem 2014, 15, 2462 – 2472

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Figure 8. Femtosecond transient spectra of a) ZnPc and b) ZnF8Pc at an excitation wavelength of l = 400 nm by using a 100 fs pulsed laser in nitrogensaturated toluene at the indicated time intervals.

The transient absorption spectra of C60Py:ZnPc and C60Im:ZnPc dyads at different delay times in toluene are shown in Figure 9 a and b, respectively. The instantaneously formed singlet decayed with the generation of new transient bands at l = 848 and 1020 nm. The former transient band is attributed to the formation of ZnPcC + , whereas the latter is attributed to the formation of C60C ;[27, 28] thus providing evidence for the formation of C60CPy:ZnPcC + and C60CIm:ZnPcC + radical ion pairs. These results demonstrate that 1ZnPc* formed undergoes photoinduced electron transfer instead of intersystem crossing to populate the triplet excited state of either ZnP or C60. The kinetics of charge separation, kCS, and charge recombination, kCR, were monitored by following the time profiles of the radical cation and radical anion bands, as shown in Figure 9 c and d, respectively, for the C60CPy:ZnPcC + and C60CIm:ZnPcC + radical ion pairs. The rise time was found to be smaller for C60Py:ZnPc (75 ps) than that of C60Im:ZnPc (200 ps) due to an increased distance between the donor and acceptor entities in the latter dyad. The rise time obtained here compared well with the rise time reported earlier for a C60Im:ZnNc dyad from picosecond transient studies.[28] The kCS values calculated from the rise time were 13.3 109 s1 for C60CPy:ZnPcC + formation and 5.0 109 s1 for C60.Im:ZnPcC + formation. The decay of both the radical cation and radical anion bands followed monoexponential decay and persisted for a little over 3 ns; the time window of our instrumental setup. The time constants estimated from the decay curve at Ao/e of the monoexponential decay curve were 2500 and 4800 ps, respectively, for the C60CPy:ZnPcC + and C60.Im:ZnPcC + radical ion pairs. The kCR value calculated 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemphyschem.org from these data were 4.0 108 and 2.1 108 s1 for the corresponding radical ion pairs; this indicated a slower charge recombination process in the case of the C60CIm:ZnPcC + radical ion pair due to increased donor–acceptor separation than that in the former dyad. Figure 10 shows the femtosecond transient absorption spectra of C60Py:ZnF8Pc and C60Im:ZnF8Pc dyads at different delay times. Upon photoexcitation of the dyads, slightly different spectral features were observed compared with dyads composed of ZnPc. This could be attributed to fluorine substituents on the Pc ring. The instantaneous formation of singlet spectral features was observed with a characteristic excited singlet peak corresponding to 1ZnF8Pc* in the l = 480–500 nm region. However, the decay of these peaks, including recovery of the absorption band in the l = 700 nm region, was much faster due to the occurrence of an intramolecular process accompanied by characteristic peaks at l = 893 and 1020 nm. These peaks could be assigned to the ZnF8PcC + cation and C60C anion radicals, respectively, resulting in the formation of C60CPy:ZnF8PcC + and C60.Im:ZnF8PcC + radical ion pairs. The identity of the ZnF8PcC + cation radical at l = 893 nm was confirmed by performing chemical oxidation of ZnF8Pc by using nitrosonium hexafluoroborate as an oxidising agent (see Figure S13 in the Supporting Information for the spectrum of the ZnF8PcC + cation radical). The nearly 50 nm redshift of ZnF8PcC + relative to ZnPcC + is consistent with the previously discussed absorption spectra of neutral Pc derivatives (Figure 2). The evolution of the transient bands was much faster than that observed for the ZnPc sensitiser. The time constants were 20 and 52 ps for the formation of C60CPy:ZnF8PcC + and C60.Im:ZnF8PcC + , respectively, which resulted in kCS values of 5.0 1010 and 1.9 1010 s1, respectively, for radical ion pair formation. These values are four to five times larger than those observed for ZnPc-derived dyads. The decay of the radical ion pairs was also faster: whereas the decay of C60CPy:ZnF8PcC + was over by 3 ns, the decay of C60CIm:ZnF8PcC + persisted for little over 3 ns. The measured kCR values were 7.9 108 and 6.4 109 s1, respectively, for the C60CPy:ZnF8PcC + and C60CIm:ZnF8PcC + radical ion pairs. Figure 11 a and b shows the transient spectra of C60Py:ZnF16Pc and C60Im:ZnF16Pc dyads at different delay times in toluene. Due to limited solubility of the dyads, it was difficult to obtain the transient spectra with a better signal/noise ratio. However, in the NIR region, the transient bands at l = 907 and 1020 nm were clearly observed. These peaks could be assigned to the ZnF16PcC + radical cation and C60C radical anion, resulting in the formation of C60CPy:ZnF16PcC + and C60CIm:ZnF16PcC + radical ion pairs. Similar to previously discussed ZnF8Pc-based dyads, the rise time of charge separation was smaller: 17 and 21 ps for C60Py- and C60Im-based dyads, respectively. The determined kCS values were 5.9 1010 and 4.7 1010 s1, respectively, for C60CPy:ZnF16PcC + and C60CIm:ZnF16PcC + radical ion pair formation. The decay of the radical ion pairs was also fast relative to the previously discussed dyads. The measured kCR values were 12.1 108 and 10.2 109 s1, respectively, for C60CPy:ZnF16PcC + and C60CIm:ZnF16PcC + radical ion pairs. ChemPhysChem 2014, 15, 2462 – 2472

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Figure 9. Femtosecond transient spectra of a) C60Py:ZnPc and b) C60Im:ZnPc dyads at the excitation wavelength of l = 400 nm using 100 fs pulsed laser in nitrogen saturated toluene at the indicated time intervals. The time profile of the C60C band at l = 1020 nm and ZnPcC + band at l = 840 nm of the C60CPy:ZnPcC + (c) and C60CIm:ZnPcC + (d) radical ion pairs are also shown.

Table 3 lists kinetic data for the various dyads discussed herein. Both kCS and kCR increased with an increasing number of fluorine substituents on the Pc ring periphery, and the rates were generally higher for C60Py-derived dyads than those of C60Im-derived ones. This trend agrees well with recently reported dyads based on halogenated P and C60 assembled using

Table 3. Rise time, rate of charge separation (kCS), decay time constant and rate of charge recombination (kCR) calculated for various supramolecular dyads of perfluorinated ZnPc–C60Py dyads in nitrogen-saturated toluene. Donor ZnPc ZnF8Pc ZnF16Pc

Acceptor C60Py C60Im C60Py C60Im C60Py C60Im

tCS [ps] 75 200 20 52 17 21

kCS [s1][a] 9

13.3 10 5.0 109 50 109 19 109 58 109 47 109

tCR [ps] 2507 4800 1262 1568 822 980

kCR [s1][b] 8

4.0 10 2.1 108 7.9 108 6.4 108 12.1 108 10.2 108

kCS/kCR 33 24 63 30 50 46

[a] kcs = 1/tCS. [b] kCR = 1/tCR; kCR calculated based on A0/e calculations with mono-exponential decay fit. The error in the measured kinetic values is 5.


the metal–ligand axial coordination approach.[18] Additionally, for a given dyad, the kCS value was one to two orders of magnitude higher than that of kCR ; a result that can be attributed to a low reorganisation energy demand of the donor and acceptor employed herein.[29] Interestingly, the kinetic results did not follow the expected trend in the free energy change relationship (Table 2), according to Marcus theory.[30] The geometry of the dyads and populating the phthalocyanine triplet state during charge recombination (Figure 7) are attributed to this cause. Furthermore, the extent of charge stabilisation was evaluated by taking the ratio of kCS/kCR (Table 3). These values for the C60Py-derived dyads were slightly higher than those of the C60Im-derived dyads. Higher values of kCS/kCR for perfluorinated Pc derived dyads were obtained; this implies better charge stabilisation.

3. Conclusions High oxidation potential perfluorinated ZnPcs were employed as electron donors in donor–acceptor dyads with C60 as an electron acceptor. A self-assembly method using metal–ligand axial coordination was employed to form these dyads. The ChemPhysChem 2014, 15, 2462 – 2472

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Figure 10. Femtosecond transient spectra of a) C60Py:ZnF8Pc and b) C60Im:ZnF8Pc dyads at an excitation wavelength of l = 400 nm by using a 100 fs pulsed laser in nitrogen-saturated toluene at the indicated time intervals. The time profile of the C60C band at l = 1020 nm and the ZnF8PcC + band at l = 893 nm of the C60CPy:ZnF8PcC + (c) and C60CIm:ZnF8PcC + (d) radical ion pairs are also shown.

spectroscopically determined binding constants were higher than those observed for dyads formed by ZnPc lacking fluorine substituents. By using the B3LYP/6-31G* method, the geometries and electronic structures of the dyads were determined, and the frontier HOMO and LUMO levels and electrostatic potential maps were derived. The energy of the charge-separated states was estimated from electrochemical and spectral studies. The energy of the charge-separated state for the dyads composed of perfluorinated ZnPc was higher than that of the dyad assembled with ZnPc. By using femtosecond transient absorption studies in nonpolar toluene, evidence for charge separation and kinetics of charge separation and recombination was determined. The occurrence of ultrafast charge separation and charge recombination, resulting in the formation of high-energy charge-separated states in dyads composed of perfluorinated Pc and C60, was observed.

whereas bulk solvents utilised in the syntheses were from Fischer Chemicals (Plano, TX). Fulleropyrollidine derivatives, C60Im and C60Py, were synthesised according to literature procedures.[26]

Synthesis of ZnF8Pc 4,5-Difluorophthalonitrile (700 mg, 4.27 mmol) and ZnCl2 (2.32 g, 0.017 mol) were kept in a 100 mL round-bottomed flask under N2 for 30 min. Then 2-(dimethylamino)ethanol (DMAE; 8 mL) was added and the whole mixture was heated at 150 8C for 18 h. After cooling the mixture to room temperature, the solution was diluted with methanol and water (15:5 mL each) and centrifuged for 1.5 h. The obtained green-coloured residue was dissolved in a minimum of chloroform and purified by column chromatography on silica gel. The desired compound (120 mg, 25 %) was obtained as the second fraction eluted by chloroform/MeOH (85:15 v/v). MS (ESI): m/z: 722.67 [M + H] + .

Synthesis of ZnF16Pc

Experimental Section Chemicals Reagents, ZnPc and C60 were purchased from Aldrich Chemicals (Milwaukee, WI), TCI America and SES Research (Houston, TX), 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Tetrafluorophthalonitrile (500 mg, 2.5 mmol) and ZnCl2 (1362 mg, 10 mmol) were kept in a 100 mL round-bottomed flask under N2 for 30 min. Then, DMAE (4 mL) was added and the whole mixture was heated at 150 8C for 18 h. After cooling the mixture to room temperature, the solution was diluted with methanol and water (15:5 mL each) and centrifuged for 1.5 h. The obtained green-colChemPhysChem 2014, 15, 2462 – 2472

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Figure 11. Femtosecond transient spectra of a) C60Py:ZnF16Pc and b) C60Im:ZnF16Pc dyads at an excitation wavelength of l = 400 nm by using a 100 fs pulsed laser in nitrogen-saturated toluene at the indicated time intervals. The time profile of the C60C band at l = 1020 nm and ZnF16PcC + band at l = 907 nm of the C60CPy:ZnF16PcC + (c) and C60CIm:ZnF16PcC + (d) radical ion pairs are also shown.

oured residue was dissolved in a minimum of chloroform and purified by column chromatography on silica gel. The desired compound (140 mg, 28 %) was obtained as the second fraction eluted by chloroform/MeOH (75:25 v/v). MS (ESI): m/z: calcd. 863.93, found 863.9 (see Figure S14 in the Supporting Information).

Instrumentation The UV/Vis/NIR spectral measurements were performed with a Shimadzu 2550 UV/Vis spectrophotometer or a Jasco V-670 spectrophotometer. The steady-state fluorescence emission was monitored by using a Varian (Cary Eclipse) fluorescence spectrophotometer or a Horiba–Jobin–Yvon Nanolog UV/Vis/NIR spectrofluorometer equipped with PMT (for UV/Vis) and InGaAs (for NIR) detectors. Lifetimes were measured with the TCSPC lifetime option with a nanoLED excitation source (l = 674 nm) on the Nanolog. The time correlation factor for the system was 0.439 ns per channel. All solutions were purged prior to spectral measurements with nitrogen gas. Cyclic and differential pulse voltammograms were recorded on a Princeton Applied Research potentiostat/galvanostat Model 263A by using a three-electrode system. A platinum button electrode was used as the working electrode, whereas a platinum wire served as the counter electrode and an Ag/AgCl electrode was 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

used as the reference electrode. The Fc/Fc + redox couple was used as an internal standard. All solutions were purged with nitrogen gas prior to electrochemical and spectral measurements. Femtosecond transient absorption spectroscopy experiments were performed by using an Ultrafast Femtosecond Laser Source (Libra) by Coherent incorporating a diode-pumped, mode locked Ti:sapphire laser (Vitesse) and diode-pumped intracavity doubled Nd:YLF laser (Evolution) to generate a compressed laser output of 1.45 W. For optical detection, a Helios transient absorption spectrometer coupled with a femtosecond harmonics generator both provided by Ultrafast Systems LLC was used. The source for the pump and probe pulses were derived from the fundamental output of Libra (compressed output 1.45 W, pulse width 91 fs) at a repetition rate of 1 kHz; 95 % of the fundamental output of the laser was introduced into a harmonic generator, which produced second and third harmonics of l = 400 and 267 nm in addition to the fundamental l = 800 nm for excitation, whereas the rest of the output was used to generate a white-light continuum. Herein, the second harmonic l = 400 nm excitation pump was used in all experiments. Kinetic traces at appropriate wavelengths were assembled from the time-resolved spectral data. Data analysis was performed by using Surface Xplorer software supplied by Ultrafast Systems. All measurements were conducted at 298 K as solutions in degassed N2. ChemPhysChem 2014, 15, 2462 – 2472

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CHEMPHYSCHEM ARTICLES Acknowledgements This work was supported by National Science Foundation (grant no. 1110942 to F.D.). We also thank the National Science Foundation for equipment support via NSF CRIF (CHE-0741936). Additional computing resources were provided by the Academic Computing Services at the University of North Texas.

Keywords: donor–acceptor systems · electron transfer · fullerenes · phthalocyanines · zinc

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tungsten-alkylidyne dyads: generation of reactive porphyrin and metallo radical states.

Carborane dyads for photoinduced electron transfer: photophysical studies on carbazole and phenyl-o-carborane molecular assemblies.

Model dyads for 2PA uncaging of a protecting group via photoinduced electron transfer.

Photoinduced reduction of surface states in Fe:ZnO.

Photoinduced states in a Mott insulator.

Direct observation of hole shift and characterization of spin states in radical ion pairs generated from photoinduced electron transfer of (phenothiazine)(n)-anthraquinone (n = 1, 3) dyads.

Photoinduced mixed valency in zinc porphyrin dimer of triruthenium cluster dyads.

Effect of microtubule polymerization on photoinduced hydrogen generation.

Effect of mutual position of electron donor and acceptor on photoinduced electron transfer in supramolecular chlorophyll-fullerene dyads.

Photoinduced electron detachment and proton transfer: the proposal for alternative path of formation of triplet states of guanine (G) and cytosine (C) pair.

Ultrafast Photoinduced Electron Transfer and Charge Stabilization in Donor-Acceptor Dyads Capable of Harvesting Near-Infrared Light.

Exploring excited states of Pt(II) diimine catecholates for photoinduced charge separation.

Bids for joint attention by parent-child dyads and by dyads of young peers in interaction.

Ultrafast time resolved spectroscopic studies on the generation of the ketyl-sugar biradical by intramolecular hydrogen abstraction among ketoprofen and purine nucleoside dyads.

Generation and Characterization of Indoor Fungal Aerosols for Inhalation Studies.

On the photo-induced charge-carrier generation within monolayers of self-assembled organic donor-acceptor dyads.

On-chip generation of photon-triplet states.

Sensitive colorimetric visualization of perfluorinated compounds using poly(ethylene glycol) and perfluorinated thiols modified gold nanoparticles.

Sociophysiology of relationships in squirrel monkeys. I. Formation of female dyads.

Photoswitchable Spiropyran Dyads for Biological Imaging.

Cooperative dyads for two-photon uncaging.

New insights into perfluorinated adsorbents for analytical and bioanalytical applications.

Time-resolved fluorescence study of exciplex formation in diastereomeric naproxen-pyrrolidine dyads.

Stationary distribution of self-organized states and biological information generation.