ARTICLE pubs.acs.org/JPCC
Annulation of Tetrathiafulvalene to the Bay Region of Perylenediimide: Fast Electron-Transfer Processes in Polar and Nonpolar Solvents Mohamed E. El-Khouly,*,†,‡ Michael Jaggi,§ Belinda Schmid,§ Carmen Blum,§ Shi-Xia Liu,*,§ Silvio Decurtins,§ Kei Ohkubo,† and Shunichi Fukuzumi*,† †
Department of Material and Life Science, Graduate School of Engineering, Osaka University, ALCA, JST, Suita, Osaka 565-0871, Japan, Department of Chemistry, Faculty of Science, Kafr ElSheikh University, Kafr ElSheikh 33516, Egypt, § Departement f€ur Chemie und Biochemie, Universit€at Bern, Freiestrasse 3, CH-3012 Bern, Switzerland. ‡
bS Supporting Information ABSTRACT: A tetrathiafulvalene donor has been annulated to the bay region of perylenediimide through a 1H-benzo[d]pyrrolo[1,2-a]imidazol-1-one spacer affording an extended π-conjugated molecular dyad (TTFPDI). To gain insight into its ground- and excited-state electronic properties, the reference compound Ph-PDI has been prepared via a direct Schiff-base condensation of N,N0 -bis(1-octylnonyl) benzoperylene-10 ,20 :3,4:9,10-hexacarboxylic-10 ,20 -anhydride3,4:9,10-bis(imide) with benzene-1,2-diamine. Both the experimental and the computational (DFT) results indicate that TTFPDI exhibits significant intramolecular electronic interactions giving rise to an efficient photoinduced charge-separation process. Free-energy calculations verify that the light-induced process from TTF to the singlet-excited state of PDI is exothermic in both polar and nonpolar solvents. Fast adiabatic electron-transfer processes of a compactly fused, π-conjugated TTFPDI dyad in benzonitrile, 2-methyltetrahydrofuran, anisole and toluene were observed by femtosecond transient absorption spectral measurements. The lifetimes of radical-ion pairs slightly increase with decreasing the solvent polarities, suggesting that the charge-recombination occurs in the Marcus inverted region. By utilizing the nanosecond transient absorption technique, the intermolecular electron-transfer process in a mixture of TTF-diamine/Ph-PDI has been observed via the triplet excited PDI for the first time.
’ INTRODUCTION Studies on photoinduced electron transfer in covalently linked donoracceptor systems have witnessed enormous growth in recent years, which mainly address the mechanistic details of electron transfer in chemistry and biology, facilitating the development of artificial photosynthetic systems for light energy conversions and organic molecular electronic devices.18 As is well-known, the incorporation of electron donor (D) and electron acceptor (A) dye molecules leads to valuable electronic and photonic materials, especially if one or both of the dye manifolds form an interpenetrating network in which photogenerated charge carriers exhibit high mobility. Perylenediimide (PDI) dyes continue to emerge as archetypal components in a variety of photofunctional materials. Due to their unique light-harvesting9 and redox properties,10 easy accessibility and high chemical persistency, thermal durability and photostability,11 perylene dyes have been regarded as potential candidates for numerous applications, spanning from optoelectronics to biological fields, such as organic light-emitting diodes r 2011 American Chemical Society
(OLEDs),12 optical switches,13 organic field-effect transistors (OFETs),14 xerographic materials,15 organic solar cells,16 laser dyes17 and biosensors,18 etc. Specifically, the core-extended PDIs have been sought in order to attain novel chromophores that function as excellent dyes in molecular (opto)electronics. As a consequence, the annulation of electron donors to the bay region of the PDI electron acceptor would result in electrochemically amphoteric systems with promising optical properties. Among electron donors, tetrathiafulvalene (TTF) and its derivatives have been explored as p-type semiconductors and also frequently used in a variety of DA ensembles, although they are usually linked to acceptor components via flexible or rigid σ-spacers.1922 With respect to DA ensembles, important variables, besides the nature of the donor and acceptor components, are their relative distance, orientation, and the degree of electronic coupling Received: January 25, 2011 Revised: March 2, 2011 Published: April 04, 2011 8325
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Figure 1. Molecular structures of TTFPDI and the reference compounds TTF-diamine and Ph-PDI.
between them. Quite often, D-σ-A ensembles show redox and photophysical properties that are just given by the sum of the properties of the corresponding building blocks due to the lack of strong electronic interactions between the D and A moieties. In contrast, sterically controlled and compactly fused D-π-A systems exhibit significant intramolecular interactions potentially giving rise to efficient photoinduced charge-separation processes.23 Moreover, such π-bridged systems allow the combination of a high-lying HOMO localized on D with a low-lying LUMO on A, resulting in conjugated systems bearing small HOMOLUMO gaps. It is no wonder that fused and π-extended heteroarenes prove to be the most promising molecular scaffolds in the search for high-performance organic semiconductors for molecular devices.24 However, the photodynamics of such π-bridged systems composed of TTF and PDI has yet to be scrutinized. As a continuation of our ongoing project, we describe herein a fast adiabatic electron-transfer process of a compactly fused, πconjugated TTFPDI (Figure 1), the synthesis of which was previously reported.21h As the focal point of this work, we explore the electronic absorption, photoinduced intramolecular electron transfer and electrochemical behavior of TTFPDI and Ph-PDI, in detail. The excited state photochemical events are monitored by femtosecond and nanosecond transient absorption techniques. In addition, the photoinduced intermolecular electron transfer from TTF to the triplet excited PDI has also been evidenced in a mixture of TTF-diamine and Ph-PDI in polar benzonitrile (PhCN). This observation is rarely found in the literature due to the inefficient formation of the triplet excited PDI.
’ EXPERIMENTAL SECTION Materials. The examined TTFPDI dyad,21h N,N0 -bis(1-octyl-
nonyl)benzoperylene-10 ,20 :3,4:9,10-hexacarboxylic-10 ,20 -anhydride3,4:9,10-bis(imide)25 and 5,6-diamino-2-(4,5-bis(propylthio)1,3-dithio-2-ylidene)benzo[d][1,3]-dithiole21d were prepared according to the literature procedures. All other chemicals and solvents were purchased from commercial sources and used without further purification. Preparation of Ph-PDI. N,N0 -Bis(1-octylnonyl)benzoperylene0 0 1 ,2 :3,4:9,10-hexacarboxylic-10 ,20 -anhydride-3,4:9,10-bis(imide) (55.6 mg, 57.8 μmol), o-phenylenediamine (18.5 mg, 171 μmol) and propionic acid (3.5 mL) were united in a 25 mL two-necked round-bottom flask fitted with a condenser. Upon flushing with Ar, the suspension was heated to 120 °C and kept at this temperature for 90 min. After cooling down to room temperature, the mixture was poured onto water (60 mL) and the resulting orange precipitate was collected through a bed of Celite
and washed with water (120 mL). The crude product was subjected to flash chromatography on silica gel using CH2Cl2 as eluant to afford analytically pure Ph-PDI (37.3 mg, 65%). 1H NMR (CDCl3): δ = 10.067 (s, 1H), 10.011 (s, 1H), 9.167 (d, J = 8.48 Hz, 2H), 9.024 (m, 2H), 7.728 (m, 2H), 7.300 (m, 2H), 5.255 (br, 2H), 2.328 (br, 4H), 1.939 (m, 4H), 1.18 (m, 48H), 0.745 (m, 12H). Selected IR data (cm1, KBr pellet): 3435 (br), 2952(w), 2924 (s), 2853 (s), 1746 (w), 1707 (m), 1663 (s), 1623 (w), 1594 (w), 1527 (w), 1463 (w), 1440 (w), 1414 (w), 1368 (s), 1346 (w), 1319 (m), 1262 (w), 1231 (w), 1164 (w), 1124 (w), 810 (m), 760 (w), 745 (w). MS (MALDI-TOF, negative mode) calcd for C68H80N4O5: 1032.61, found 1032.60. Measurements. The 1H NMR spectrum was acquired at 300 MHz. Chemical shifts δ were calibrated against TMS as the internal standard. FT-IR data were collected on a Perkin-Elmer Spectrum One spectrometer. UVvis spectra were recorded on a Perkin-Elmer Lambda 900 spectrometer. The mass spectrum was recorded on a FTMS 4.7T BioAPEX II using a MALDI method. Electrochemical measurements were performed on an ALS630B electrochemical analyzer in deaerated PhCN containing tetra-n-butylammonium hexafluorophosphate (TBAPF6; 0.10 M) as supporting electrolyte at 298 K. A conventional three-electrode cell was used with a platinum working electrode (surface area of 0.3 mm2) and a platinum wire as the counter electrode. The Pt working electrode was routinely polished with BAS polishing alumina suspension and rinsed with acetone before use. The measured potentials were recorded with respect to Ag/AgNO3 reference electrode. All electrochemical measurements were carried out under an atmospheric pressure of argon. Density-functional theory (DFT) calculations were performed on a COMPAQ DS20E computer. Geometry optimizations were carried out using the Becke3LYP functional and 6-311G basis set, with the unrestricted HartreeFock (UHF) formalism and as implemented in the Gaussian 09 programs Rev. A.02. Graphical outputs of the computational results were generated with the Gauss View software program (ver. 5) developed by Semichem, Inc. Ab initio quantum chemical calculations were also performed, using TURBOMOLE V5.1026 as well as the GAMESS27 program packages. The ground-state geometries of TTFPDI and Ph-PDI were optimized with density functional theory (DFT) using the B3LYP functional and the valence triple-ζ plus polarization (TZVP) basis set and were constrained to Cs symmetry. Steady-state absorption spectra were recorded on a Shimadzu UV-3100PC spectrometer or a Hewlett-Packard 8453 diode array spectrophotometer at room temperature. Fluorescence measurements were carried out on a Shimadzu spectrofluorophotometer 8326
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Scheme 1. Synthesis of the Reference Compound Ph-PDI
(RF-5300PC). Phosphorescence spectra were obtained by a SPEX fluorolog τ3 spectrophotometer. Emission spectra in the NIR region were detected by using a Hamamatsu Photonics R5509-72 photomultiplier. An argon-saturated 2-methyltetrahydrofuran (2-MeTHF) solution containing TTF and Ph-PDI at 77 K was excited at indicated wavelengths. Femtosecond transient absorption spectroscopy experiments were conducted using an ultrafast source (Integra-C, Quantronix Corp.), an optical parametric amplifier (TOPAS, Light Conversion Ltd.) and a commercially available optical detection system (Helios provided by Ultrafast Systems LLC). The sources for the pump and probe pulses were derived from the fundamental output of Integra-C (780 nm, 2 mJ/pulse and fwhm =130 fs) at a repetition rate of 1 kHz. 75% of the fundamental output of the laser was introduced into TOPAS, which has optical frequency mixers resulting in a tunable range from 285 to 1660 nm, while the rest of the output was used for white light generation. Typically, 2500 excitation pulses were averaged for 5 s to obtain the transient spectrum at a set delay time. Kinetic traces at appropriate wavelengths were assembled from the time-resolved spectral data. All measurements were conducted at 298 K. The transient spectra were recorded using fresh solutions in each laser excitation. For nanosecond transient absorption measurements, deaerated solutions of the compounds were excited by a Panther OPO equipped with a Nd:YAG laser (Continuum, SLII-10, 46 ns fwhm) with a power of 1015 mJ per pulse. The photochemical reactions were monitored by continuous exposure to a Xe lamp (150 W) as a probe light and a photomultiplier tube (Hamamatsu 2949) as a detector. Solutions were deoxygenated by Argon purging for 15 min prior to the measurements.
’ RESULTS AND DISCUSSION Synthesis and Characterization. In analogy to TTFPDI, the reference compound Ph-PDI was obtained via a Schiff-base condensation of N,N0 -bis(1-octylnonyl)benzoperylene-10 ,20 :3,4:9, 10-hexacarboxylic-10 ,20 -anhydride-3,4:9,10-bis(imide)25 with commercially available benzene-1,2-diamine (Scheme 1) in reasonable yield (65%). The 1H NMR spectrum of the compound Ph-PDI is consistent with the proposed structure with the correct proton integration ratios. The matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrum shows a singly charged molecular ion peak at 1032.60, in good agreement with the calculated value. TTFPDI and Ph-PDI are soluble in common organic solvents.
Figure 2. Steady-state absorption spectra of TTFPDI along with the reference compounds Ph-PDI and TTF-diamine in PhCN.
Steady-State Absorption and Fluorescence Measurements. As shown in Figure 2, the green TTFPDI shows
intense absorptions in the UVvis-NIR spectral region in PhCN. A broad absorption band appears between 870 nm (11500 cm1) and 530 nm (18900 cm1), followed by a series of absorptions that peak at 465 nm (21500 cm1), 435 nm (23000 cm1) and 405 nm (24700 cm1). Similar features are observed in toluene and 2-methyltetrahydrofuran (see the Supporting Information, Figure S1). By comparison with the spectra of compounds Ph-PDI and TTF-diamine, the new electronic transitions which can only be observed in the fused molecule TTFPDI are attributed to intramolecular ππ* charge transfer (ICT) transitions. Clearly, the typical PDI absorption pattern with its vibrational progression of about 1500 cm1 is still observed within TTFPDI and Ph-PDI, without any shifts. It can be deduced therefore that the incorporation of the TTF donor does not appear to have a substantial perturbation of electronic transition states centered on the PDI core. However, Ph-PDI exhibits an additional weaker absorption band at 520 nm (19200 cm1), which seems more pronounced than the corresponding one in TTFPDI, presumably dominated by the transitions from the frontier orbitals delocalized on the Ph-PDI framework. Photoinduced intramolecular events of TTFPDI and PhPDI were investigated, first, by using steady-state fluorescence spectroscopy upon photoexcitation of the PDI entities in toluene (TN). Upon the photoexcitation at 460 nm (21740 cm1), the reference compound Ph-PDI shows the emission of the 8327
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The Journal of Physical Chemistry C singlet-excited state of PDI (1PDI*) at 520 nm (19200 cm1) with a quantum yield of 0.10 (Figure 3). In contrast, the emission intensity of 1PDI* in TTFPDI is significantly quenched in toluene. For example, the fluorescence quantum yield of TTFPDI in toluene is estimated to be 0.001, which is 1/100 of that of Ph-PDI. Additionally, changing the solvent from toluene to a more polar PhCN increases the overall quenching efficiency further by 5%. The quenching process in TTFPDI may involve the electron transfer from TTF to the singlet-excited state of PDI and/or the energy transfer from the 1PDI* to the TTF unit. The fact that the energy level of the 1PDI* is found to be lower than that of 1TTF* suggests that the energy transfer from 1PDI* to TTF is thermodynamically unfavorable. These
Figure 3. Steady-state fluorescence spectra of the TTFPDI dyad in benzonitrile and toluene, and the reference compound Ph-PDI in toluene; λex = 460 nm. Inset: Fluorescence spectrum of the TTFPDI dyad in toluene; λex = 590 nm.
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observations together with the calculated negative ΔGCS values via 1PDI* (in a forthcoming section) manifest that the chargeseparation process occurs from the electron-donating TTF to 1 PDI*, generating TTF•þPDI• in both polar and nonpolar solvents. Upon the photoexcitation of the charge-transfer (CT) transition band at 590 nm in toluene, the TTFPDI dyad exhibits a broad weak emission in the range of 620800 nm with a maximum at 658 nm that can be assigned to the excited CT complex (exciplex). However, the emission of the exciplex in benzonitrile was too weak to be detected, that can be explained by exciplex formation with the photoinduced electron charge transfer reaction. In general, strongly polar exciplexes (which are formed between “dissimilar” reactants) usually display little emission.28 Computational Studies. To gain insight into the ground- and excited-state electronic properties of TTFPDI and Ph-PDI, ab initio quantum chemical calculations were performed. The ground-state geometries of TTFPDI and Ph-PDI were optimized with density functional theory (DFT) using ab initio B3LYP/6-311G methods. In the optimized structure of TTFPDI, the radii of the PDI and TTF moieties were found to be 5.70 and 5.69 Å, respectively. The center-to-center distance (Rcc), that is, the distance between the PDI core and the TTF unit, was computed to be 11.0 Å. The important frontier molecular orbitals of the TTFPDI dyad and Ph-PDI are given in Figure 4 and Figure S2 in the Supporting Information, respectively. In the TTFPDI dyad, the localization of the highest occupied frontier molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) on the TTF and PDI moieties, respectively, has clearly been demonstrated, together with the extensions of the HOMO1 and LUMOþ1 into the bridging area. The orbital energies of the HOMO and LUMO were found to be 5.47 and 3.78 eV, respectively. Similar results were obtained with the long-range
Figure 4. Frontier molecular orbitals of the TTFPDI dyad by using ab initio B3LYP/6-311G method. 8328
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Figure 6. Cyclic voltammogram of the TTFPDI dyad in deaerated PhCN. Scan rate = 50 mV s1.
Figure 5. Electronic absorption spectrum of TTFPDI in dichloromethane together with the vertical LRC-TDBLYP/6-31G(2d) (μ = 0.15) calculated S0 f Sn transitions.
corrected TDDFT (LRC-TDDFT) method applying the B3LYP functional and the 6-31G(2d) basis set (Figure S3 in the Supporting Information). The frontier molecular orbitals of Ph-PDI show that the HOMO is delocalized over the whole Ph-PDI framework and the LUMO mainly over the PDI core with a small contribution of the bridging area (Figures S2 and S3 in the Supporting Information). Vertical electronic transitions were calculated with the longrange corrected TDDFT (LRC-TDDFT) method applying the BLYP functional and the 6-31G(2d) basis set. As illustrated in Figure 5, the S0 f S1 and S0 f S2 electronic excitations are calculated to be close-lying in-plane ππ* transitions at 15641 and 17330 cm1 with oscillator strengths fcalc = 0.13 and 0.09. They are polarized along the TTF long axis and dominated by the HOMO f LUMOþ1 and HOMO f LUMO one-electron contributions. Both transitions agree very well with the broad absorption band centered at 16670 cm1 (600 nm). These lowenergy electronic excitations clearly reflect the π-electron flow from the TTF donor to the fused PDI acceptor. These results suggest that the charge-separated state in electron-transfer reactions of TTFPDI is TTF•þPDI•. The small shoulder at 20000 cm1 (500 nm) for TTFPDI can be rationalized by the S0 f S3 electronic transition, predicted at 22 056 cm1 with fcalc = 0.10. The intense absorptions above 21 000 cm1 correspond to the S0 f S4 and S0 f S5 electronic excitations with calculated energies of 22 254 and 23 682 cm1 and oscillator strengths of 0.12 and 0.21, respectively. These transitions bear ICT as well as PDI-localized ππ* characteristics. Electrochemical Studies and ET Driving Force. An accurate determination of the driving force for the electron-transfer processes requires the redox potentials of the studied chromophores. In the present study, the measurements were carried out by cyclic voltammetry (CV) technique in deaerated PhCN solutions containing tetra-n-butylammonium hexafluorophosphate (TBAPF6; 0.1 M) as a supporting electrolyte (Figure 6 and Figure S5 in the Supporting Information). The cyclic voltammogram of TTFdiamine exhibits two oxidations located at E1/2 = 0.04 and 0.33 V vs Ag/AgNO3. The cyclic voltammogram of Ph-PDI reveals two reductions located at E1/2 = 0.86 and 1.20 V vs Ag/AgNO3. As depicted in Figure 6, the TTFPDI dyad shows two oxidation waves at 0.32 and 0.63 V vs Ag/AgNO3 corresponding to the
oxidation of the TTF moiety. Scanning to the negative potentials, the TTFPDI dyad shows two reversible waves at 0.86 and 1.23 V vs Ag/AgNO3 attributed to the reduction processes of the PDI entity. Obviously, the presence of the TTF unit seems to have a negligible influence on the reduction potentials of the PDI unit, in good agreement with the aforementioned optical properties. The thermodynamic driving forces for the charge-recombination (ΔGCR) and charge-separation (ΔGCS) processes were calculated using the following expressions:29 ΔGCR ¼ E1=2 ðTTF•þ =TTFÞ E1=2 ðPDI=PDI• Þ þ ΔGs ΔGCS ¼ ΔE00 ð ΔGCR Þ where E1/2(TTF•þ/TTF) is the first oxidation potential of TTF, E1/2(PDI/PDI•) is the first reduction potential of PDI, ΔE00 is the energy of the 00 transition energy gap between the lowest excited state and the ground state of PDI, and ΔGs refers to the static Coulomb energy, calculated by using the “dielectric continuum model”29 according to the following expression: ΔGS ¼ e2 =4πε0 ½ð1=ð2Rþ Þ þ 1=ð2R Þ ð1=RCC Þ=εS Þ ðð1=ð2Rþ Þ þ 1=ð2R ÞÞ=εR Þ where Rþ and R are the radii of the radical cation (5.69 Å) and radical anion (5.70 Å), respectively; RCC is the center-to-center distance between TTF and PDI, which was calculated for the optimized structure to be 11 Å. The symbols ε0, εs and εR represent vacuum permittivity and dielectric constant of solvent used for photochemical and electrochemical studies, respectively. Based on the CV data of TTFPDI, the ΔG CR values were calculated as 1.23 eV (PhCN), 1.25 eV (2-methyltetrahydrofuran), 1.30 eV (anisole) and 1.60 eV (toluene). Together with the energy of 1PDI* (2.43 eV), the driving forces for the charge-separation process (ΔGCS) were calculated as 1.20 eV (PhCN), 1.18 eV (2-methyltetrahydrofuran), 1.13 eV (anisole), and 0.83 eV (toluene), suggesting that the electron transfer from TTF to 1 PDI* is thermodynamically feasible in the studied solvents. Femtosecond and Nanosecond Transient Absorption Measurements. Spectroscopic evidence for the electron transfer of the TTFPDI dyad was obtained from the femtosecond transient absorption measurements in benzonitrile (Figure 7) by using 430 nm laser light, with which the PDI moiety was selectively excited. In the time-resolved spectrum at 2 ps, the absorption bands at 640 and 850 nm were clearly observed that are ascribed to the formation of the radical-ion pair (TTF•þPDI•). 8329
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Figure 8. Decay profiles of TTF•þPDI• at 850 nm in solvents of different polarities. Figure 7. Femtosecond transient absorption spectra of the TTFPDI dyad in deaerated PhCN; λex = 430 nm. Inset: decay profile at 850 nm.
This assignment is confirmed as follows: the absorption band at 850 nm is assigned to the one-electron oxidized species TTF•þ by comparison with the spectra obtained by chemical oxidation of TTF-diamine with tris(4-bromophenyl)aminium hexachloroantimonate (Figure S6 in the Supporting Information). Similarly, the absorption band at 650 nm is assigned to the one-electron reduced species PDI• by comparison with the spectra of Ph-PDI in the presence of the reducing agent tetrakis(dimethylamino)ethylene (Figure S7 in the Supporting Information). An important finding is the similarity of the transient spectra in nonpolar toluene (εs = 2.38) (Figure S8 in the Supporting Information), anisole (εs = 4.28) and in 2-methyltetrahydrofuran (εs = 6.90) to that in PhCN (εs = 25.70) (Figure 7). An explanation for this analogy is based on the corresponding energy levels of (TTF•þPDI•) relative to 1PDI*, guaranteeing high driving forces for the associated charge separation and charge recombination processes in both polar and nonpolar solvents. The absorption band of TTF•þ was used to examine the kinetics of the charge-separation and charge-recombination processes (Figure 8). The time profiles of TTF•þ at 850 nm showed fast rises, from which the rates of the charge-separation process (kCS) via 1PDI* were calculated as 2.9 1012 (PhCN), 2.34 1012 (2-methyltetrahydrofuran), 2.34 1012 (anisole), 2.11 1012 (toluene) s1 . The finding that the kCS values are almost independent of solvent polarities can be understood if charge separation occurs near the apex of the Marcus parabolic dependence.3032 On the other hand, the decays of TTF•þ were found to follow the first-order kinetics suggesting that the radical pair keeps the singlet-spin character because of the charge separation via the singlet excited PDI. From the decay profiles of TTF•þ, the rate constants of the charge-recombination processes (kCR) and lifetimes of the radical pair (τRIP = 1/kCR) were evaluated to be 4.80 1011 s1 and 2.1 ps (PhCN), 2.6 1011 s1 and 3.30 ps (2-methyltetrahydrofuran), 1.90 1011 s1 and 5.26 ps (anisole), and 1.02 1011 s1 and 9.80 ps (toluene). The observation that the kCR values slightly increase with increasing solvent polarity (i.e., the lifetimes of the charge-separated states are longer in nonpolar solvents than in polar ones) suggests that the charge recombination occurs in the upper region of the Marcus inverted parabola.3032
Figure 9. Energy level diagram showing photochemical events of the TTFPDI dyad in the studied solvents.
The complementary nanosecond transient absorption spectra of Ph-PDI in deaerated toluene showed a distinct absorption band at 530 nm corresponding to the triplet excited PDI (3PDI*) (Figure S9 in the Supporting Information). It should be noted here that the formation of 3PDI* is frequently too weak to be detected in the nanosecond time region because of the high fluorescence quantum yield.33 When turning to TTFPDI, the nanosecond absorption spectra showed quite different behavior from those of Ph-PDI. Upon photoexcitation of TTFPDI in a deaerated toluene solution at 460 nm, no characteristic absorption bands for 3PDI* and/or the radical pair were observed (Figure S10 in the Supporting Information). This observation suggests that (a) the electron transfer via 3PDI* is not favorable due to the energetic considerations, and (b) the charge recombination of the radical-ion pair leads to population of the ground state, but not the triplet state of PDI. As depicted in Figure 9, the CS process from the TTF moiety to 1PDI* takes place quite efficiently, leading to the formation of the radical-ion pair (TTF•þPDI•) which decays to populate the ground state with lifetimes of 210 ps in the studied solvents, as confirmed by the femtosecond laser measurements. Intermolecular Electron Transfer from TTF to the Triplet Excited PDI. Photoinduced electron transfer in the supramolecular 8330
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Figure 10. Nanosecond transient spectra of Ph-PDI (0.08 mM) in the presence of TTF-diamine (0.12 mM) in deaerated PhCN; λex = 460 nm.
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Figure 11. Dependence of the rates of formation of Ph-PDI• at 650 nm on the concentration of TTF-diamine [TTF] in deaerated PhCN solution.
Scheme 2. Electron-Transfer Process of TTF-diamine/ Ph-PDI Mixture System via the Triplet Excited PDI in PhCN Solution
association of a TTF electron donor to the singlet or triplet excited state of various acceptors, but not to 3PDI*, has been reported.34 By the use of nanosecond laser pulses, the electron transfer process via 3PDI* can be observed in real time. Upon photoexcitation of Ph-PDI (0.08 mM) in PhCN using 460 nm laser photolysis, the transient absorption spectrum, immediately after the laser pulse, exhibits only an absorption band at 530 nm, which is assigned to 3PDI*. Upon photoexcitation of Ph-PDI (0.08 mM) in the presence of TTF-diamine (00.18 mM) in argon-saturated PhCN using 460 nm laser photolysis, the transient spectra show the characteristic band of 3PDI* at 530 nm. With the decay of 3PDI*, the concomitant rises of the radical ions TTF•þ and PDI• were observed (Figure 10). These observations demonstrate clearly the electron transfer from TTF to 3PDI* as depicted in Scheme 2. In oxygen-saturated solutions, a fast intermolecular energy transfer from 3PDI* to oxygen emerges, suppressing the electron transfer event between TTF and 3PDI*. A more detailed picture of the kinetic event is shown in Figure 11 and Figure S11 in the Supporting Information, where the rate constant of the electron-transfer process (ket) was evaluated by monitoring the decays of 3PDI* or the rises of PDI• as a function of the concentration of TTF-diamine. The decaytime profiles of 3PDI* obey first-order kinetics; each rate constant is referred to k1st. The linear concentration-dependence of the observed k1st values gives the ket value, calculated as 5.07 109 M1 s1. This value is very close to the diffusion-controlled limit (kdiff) in PhCN.35 The electron-transfer process via 3PDI* was supported from the viewpoint of thermodynamics of
Figure 12. Decay of PDI• on long time scale produced under the same conditions as described in Figure 8. Inset: second-order plot.
electron-transfer processes. Based on the first oxidation potential of TTF-diamine, the first reduction potential of Ph-PDI and the triplet excited state energy of Ph-PDI, the free-energy change (ΔGetT) value via the triplet Ph-PDI was estimated as 0.26 eV (25.08 kJ mol1).36 The negative ΔGetT value via 3PDI* suggests that the quenching process should be close to the diffusioncontrolled limit (kdiff).37 In long time-scale, PDI• and TTF•þ begin to decay slowly after reaching the maximal absorbance (Figure 12). The decay time profile of PDI• was fitted with second-order kinetics, suggesting the bimolecular back electron-transfer process between TTF•þ and PDI•. The rate-constant of the back electrontransfer (kbet) was estimated as 4.9 109 M1 s1, which is close to the diffusion-controlled limit in PhCN. In contrast to the observed electron-transfer behavior in the polar solvent PhCN (Figure 10), in the nonpolar toluene the transient absorption spectra of a mixture of TTF-diamine and Ph-PDI show different features. The quenching of 3PDI* was observed without any evidence of the electron- and energytransfer processes (Figure 13 and Figure S12 in the Supporting Information). The quenching rate-constant of 3PDI* by TTF was found to be 5.5 109 M1 s1. The electron transfer from TTF to 3PDI* in polar PhCN, but not in toluene, can be rationalized as follows: once the radical ions are formed from neutral reactants, 8331
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’ ACKNOWLEDGMENT This work was supported by a Global COE program, “the Global Education and Research Center for Bio-Environmental Chemistry” from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and KOSEF/MEST through WCU project (R31-2008-000-10010-0) from Korea as well as by the Swiss National Science Foundation (Grant No. 200020130266/1). ’ REFERENCES
Figure 13. Nanosecond transient absorption spectra of Ph-PDI (0.08 mM) in the presence of TTF-diamine (0.09 mM) in deaerated toluene; λex = 460 nm. Inset: Dependence of the decay rates of the triplet Ph-PDI at 530 nm on the concentration of TTF-diamine.
polar solvent molecules rapidly surround each ion and screen the electrostatic interactions and consequently prevent the electron return. On the contrary, the radical ions formed in toluene have a tendency to stay in close proximity, favoring the electron return. Escape from the solvent cage is indeed a key feature of electron transfer in solution.38
’ CONCLUSIONS The electron-transfer reactions of the π-conjugated TTF PDI have been examined in various solvents with different polarities. The occurrence of fast and efficient charge separation processes from TTF to 1PDI*, which leads to the formation of the radical pair (TTF•þPDI•) in both polar and nonpolar solvents, has been demonstrated by femtosecond transient absorption spectral measurements. The lifetimes of the resulting radical-ion pair (210 ps at room temperature) decrease substantially in polar media, whereas the rates of charge-separation processes are much less solvent-dependent. This is qualitatively in line with the fact that the charge separation occurs near to the Marcus apex region, while charge recombination occurs in the upper Marcus inverted region. In addition, the electron transfer from TTF to 3PDI* in the mixture of TTF-diamine/Ph-PDI in PhCN was studied in detail. ’ ASSOCIATED CONTENT
bS
Supporting Information. Emission spectra, MO calculations of Ph-PDI, absorption spectra of the PDI radical anion and TTF radical cation and nanosecond transient absorption spectra of TTF-diamine/Ph-PDI mixture system in polar PhCN and nonpolar toluene. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Fax: (þ81)-6-6879-7370. E-mail:
[email protected] (M.E.K.),
[email protected] (S.-X.L.), and fukuzumi@ chem.eng.osaka-u.ac.jp (S.F.).
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(36) The feasibility of the electron-transfer process from the TTFdiamine to the triplet Ph-PDI is controlled by the free-energy change (ΔGetT), which can be expressed by the RehmWeller relation: ΔGetT = (Eox Ered) ET þ Ec, where Eox is the first oxidation potential of the TTF-diamine, Ered is the first reduction potential of PhPDI, ET is the triplet energy of Ph-PDI, and Ec is the Coulomb energy term (approximately 0.06 eV in the polar benzonitrile). (37) Handbook of Photochemistry; Murov, S. I., Ed.; Marcel Dekker: New York, 1985. (38) (a) Kavarnos, G. J.; Turro, N. G. Chem. Rev. 1986, 86, 401–449. (b) Fundamentals of photoinduced electron transfer; Kavarnos, G. J., Ed.; VCH Publisher: New York, 1993; Chapter 3, pp 103184. (c) Mataga, N.; Okada, T.; Yamamoto, N. Chem. Phys. Lett. 1967, 1, 119–121. (d) Gould, I. R.; Farid, S. Acc. Chem. Res. 1996, 29, 522–528.
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