Temperature Independent Ultrafast Photoinduced Charge Transfer in

It was observed that the formation rates of the exciplex and the charge separated states, as well as the charge recombination rate, were all independe...
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J. Phys. Chem. C 2009, 113, 11475–11483

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Temperature Independent Ultrafast Photoinduced Charge Transfer in Donor-Acceptor Pairs Forming Exciplexes† Helge Lemmetyinen,* Nikolai V. Tkachenko, Alexander Efimov, and Marja Niemi Department of Chemistry and Bioengineering, Tampere UniVersity of Technology, P.O. Box 541, 33101 Tampere, Finland ReceiVed: February 20, 2009; ReVised Manuscript ReceiVed: March 30, 2009

The temperature dependence of photoinduced electron transfer reactions of phthalalocyanine- and porphyrinfullerene dyads, in which donor and acceptor moieties are covalently linked to each other, was studied. The dyads form an intramolecular exciplex as a transient state before the formation of the charge separation state. It was observed that the formation rates of the exciplex and the charge separated states, as well as the charge recombination rate, were all independent of temperature and thus activationless processes. It seems to be a general rule that the aromatic π-π interaction between the donor and acceptor moieties is an important factor for exciplex formation as transient states in photoinduced electron transfer reactions and this causes barrierless charge separation and recombination reactions. Introduction The photoexcitation of molecules causes changes in their electronic and geometrical structures and simultaneously induce changes in their interactions with environmental molecules. In excited states the molecules produce also inter- and intramolecular dynamic processes and reactions, among which photoinduced charge transfer (CT) and electron transfer (ET) are the most important from fundamental point of view. On the basis of the photoinduced processes the role of exciplexes and excimers in formation of the CT and ET states have been extensively investigated.1-7 The work was started by pioneering studies during the period 1950-1970 and still continues.1-3,7-38 Over the past decades an increasing number of bridged donor-acceptor (D-A) systems has been synthesized and photoinduced electron transfer have been studied between donor and acceptor moieties, which are held at more or less controlled distances and relative orientations by linking donor and acceptor pairs covalently by a single bond, molecular chain or chains, or by a rigid bridge. Thus the influence of external factors such as solvent and temperature is changed and, in some cases, loosed their importance.22-25,27,35 An important question remains, whether the electron transfer still could be described as a thermally activated process as suggested by the classical Marcus theory.39,40 According to classical transition state theory the population at the transient state follows the Boltzmann distribution and the rate constant for ET is39,40

( )

kET ) κelνn exp

-EA kBT

where κel is the electronic transmission coefficient, νn is the frequency of passage through the transition state, kB is the Boltzmann constant, and T is the temperature. It was suggested by Marcus that the activation energy is †

Part of the “Hiroshi Masuhara Festschrift”. * Corresponding author. Phone: +358 40 [email protected].

5811347.

E-mail:

EA )

(∆G + λ)2 4λ

where -∆G is the Gibbs free energy of the reaction and λ is the total reorganization energy. This leads to the Marcus equation

(

kET ) κelνn exp

-(∆G + λ)2 4λkBT

)

According the classical Marcus theory, κel is usually estimated to be unity and νn is of the same order of magnitude as molecular vibrational motion (1013 s-1).39 The reorganization energy can be presented as a sum of two contributions: the solvent independent inner term, λin, and the outer term or the solvent reorganization energy, λs.39 The inner term arises from the structural differences of the molecule in the reactant and product states, and it can be expected to be fairly small especially for the big, rigid, and highly symmetrical molecules. The outer term corresponds to the differences in the orientation and polarization of the solvent molecules around the studied molecule in the reactant state and in the product state.39 During the past decade we have studied a series of photoinduced electron transfer reactions of pheophytin-, phthalocyanine-, and porphyrin-fullerene dyads, in which donor and acceptor moieties are covalently linked to each other.41-52 The common feature for all of these compounds is that a π-π interaction in the D-A pair could have an important role in the ET reaction. Practically in all studied molecules the donor-acceptor pairs form an intramolecular exciplex as a transient state (Scheme 1) before the formation of a charge separation (CS) state or a tight ion pair. When the center-tocenter (c-c) distance of the donor and acceptor pair is short (7-10 Å) both the exciplex formation and the primary electron transfer, following it, are extremely fast with rate constants of 7-23 × 1012 and 40-1400 × 109 s-1, respectively. Rates become slower when the distance and orientational fluctuation increase. In order to understand better the role of exciplex as the primary transient state in the photoinduced electron transfer reactions, a series of temperature dependence experiments were performed for

10.1021/jp901555m CCC: $40.75  2009 American Chemical Society Published on Web 04/16/2009

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SCHEME 1

few studied porphyrin-fullerene and phthalocyanine-fullerene dyads and, for comparison, for a porphyrin-quinone dyad. The compounds are shown in Figure 1. As can be seen, in compounds TBD6e and H2Pc-F the donor and acceptor units are covalently linked to each other by two linkers, but offering for the latter compound more freedom in distance and geometry compared to compound TBD6e. The compound H2P-S-F has an average c-c distance about the same as H2Pc-F, but seemingly less geometrical freedom for D-A interaction. The reference compounds, P-BQ, have a short c-c distance, but evidently cannot form a sandwichlike structure and thus an exciplex as a transient state is less probable. These qualitative observations explain also the observed differences in the rate constants of different reaction steps. Unfortunately, quite a few results have been published on the temperature dependence of ultrafast electron-transfer reactions, especially for systems containing a D-A pair bound with covalent linkers, which offer the freedom for optimal geometry, but simultaneously control it and the distance, as well. Thus comparisons for the charge transfer reactions should mainly be done with bichromophoric systems, in which the charge separation is controlled by solvent polarity in geminate ion pairs,26,28,53,54 by linking the D-A pair by a single bond,55-60 by using high concentrations of excited acceptors or donors,26,28,51,52 and by forming ground-state charge transfer complexes.61-65 Most of the covalently linked bichromophoric systems studied, consist of a porphyrin or phthalocyanine donor and a quinone or fullerene acceptor. In their excellent review Mataga et al. consider fundamental problems of exciplex chemistry in ultrafast charge transfer reactions in excited states.66 When some phenyl-anthryl and phenyl-pyrenyl systems, linked with flexible methylene chains, where studied by means of fast and ultrafast laser spectroscopic measurements, it was found that the time constants for the exciplex formation depend on the solvent polarity but are longer than the solvent reorientation times, which means that the photoinduced CS processes in these systems are not controlled by solvent dynamics, but by the magnitude of the electronic interaction responsible for ET (tunneling matrix element) between A* and D. In addition, the processes are not barrierless, having a small activation energy arising from the Franck-Condon factor of the reaction. Furthermore, when the solvent polarity decreases, -∆G decreases, whereas λV is independent of the solvent polarity, leading to increases in the activation energy and the exciplex formation time constant.66 These observations support the proposal done by Mataga et al., that formations of CT states of various electronic and geometrical structures are possible and that there may arise an ensemble of exciplexes and ion-pairs distributed over such states in solution.67-70 For bichromophoric systems linked directly by a single bond the geometrical freedom of the D-A pair is more restricted

Lemmetyinen et al. compared to molecules linked by flexible methylene chains. The fs-ps time-resolved transient absorption spectra of 9,9 -bianthryl in polar solutions were studied.71 The rise times observed for the formation of the CT state indicated the existence of a rather slow component in addition to the fast rise (1012 s-1 in the inverted region, for LIP and CIP, respectively, (b) kCR(CIP) > kCR(LIP) for relatively weaker D-A systems, as well, and (c) the energy gap dependence of kCR(CIP) is given by an exponential form of kCR(CIP) ) R exp[-β|∆GCR |], where R and β are constants independent of ∆GCR. The big difference in the energy gap dependence of the CR rate between CIP and LIP systems may originate from a difference in the geometrical structures in the pair. The difference suggests strongly the dominant effect of the intramolecular high-frequency quantum modes or various intracomplex vibrational modes and the rather minor role of solvent reorganization in the CR process.66 It was also confirmed that the activation energy for CR is very small and especially smaller for CIPs with small -∆GCR.72 Masuhara and Mataga have shown73 that the CT or exciplex fluorescence Stokes shift of TCNB-aromatic hydrocarbon complexes in non polar or slightly polar solvents decreased with decrease of the energy gap for the CT transitions. Thus intracomplex reorganization energy associated with CR transition is smaller for a CIP with smaller -∆GCR and observation of the normal region in the CR of a CIP seems to be practically impossible as later confirmed.74-76 Electron transfer temperature dependence of a cofacial porphyrin-quinone cage molecule, with the c-c distance of 6.5 Å, was studied in four solvents over wide ranges of temperature, 80-300 K.22 The CR rate constants were on the order of 109 s-1, and activation energies varying between -1 and +2 kJ/

mol. The effects of both solvent and temperature were small and best explained by nonadiabatic electron tunneling. Asahi et al. have investigated the energy gap and the temperature dependence of the photoinduced CS and CR of the product IP state of a series of fixed-distance covalently linked porphyrin-quinone dyads.24 They concentrated in their studies on kcs vs -∆Gcs and kcr vs -∆Gcr relations by varying the solvent polarity and temperature. They demonstrated that, although kcs shows temperature dependence and its extent increases with a decrease of the reaction exothermicity -∆Gcs, kcr was almost temperature-independent and do not show such a systematic dependence on the free energy gap -∆Gcr as in the case of kcs. Thus the practically activationless ET in the CR reaction for all energy gap regions examined could no be interpreted by the classical treatment of Marcus theory. Investigation on the temperature dependence of kcs and kcr in toluene solutions revealed that the activation energy for CS increases with a decrease of -∆Gcs, whereas CR of IP is activationless at all -∆Gcr values examined. This indicates the dominant effect of the quantum mechanical tunneling due to the high frequency modes for ET in the inverted region leading to the small effect of the solvent reorganization in spite of the increase of solvent polarity.24 No evidence on exciplex formation were shown. Kroon et al. studied the temperature dependence of electron transfer over a wide range of driving forces with the aid of fully rigid bridged D-A systems (aromatic hydrocarbon-dicyanoethene system).25 The rate of electron transfer was found to be independent of temperature in the inverted region, where nuclear

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Figure 2. Component spectra and their lifetimes of transient states of TBD6e in THF at temperatures of (a) 305 and (b) 205 K.

tunneling becomes dominant, in contradiction to the classical Marcus treatment. In the normal region, the theory was capable of giving a qualitative description of the temperature dependence.24 Photoinduced electron transfer of rigid carotenoid-porphyrinfullerene molecular triad was studied in different solvents and temperatures.27 The CR rates were relatively insensitive to solvent and temperature. The fullerene radical anion was also more insensitive to solvent stabilization effects than are the radical anions of quinones and related acceptors, which may be the major reason that fullerene-based systems carry out photoinduced electron transfer at low temperatures, whereas similar quinone-based systems do not. Experimental Methods Tetrahydrofuran (THF) and toluene used were of analytical grade. THF was distilled before using. Synthesis of the studied molecules TBD6e, H2Pc-F, H2P-S-F, and P-BQ are described elsewhere.49,51,44,77 The transient absorption measurements were carried out using a femtosecond pump-probe instrument as described elsewhere.41 Briefly, the samples were excited by 60 fs pulses at the second harmonic of Ti:sapphire laser fundamental radiation. A white continuum generated by the fundamental radiation of the same laser was used as probe. The samples, solutions in 1 mm quartz cuvettes, were placed in cryostat (Optistat DN, Oxford Instruments) with liquid N2 cooling. The measurements were done in temperatures ranging from 175 K (THF melting point is ∼165 K) to 305 K. The excitation wavelength was 402 nm and the changes in absorption were monitored at two separate wavelength ranges: 560-780 nm and 860-1080 nm. The sample cuvette used inside the cryostat could not be rotated

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Figure 3. (a) Formation of exciplex and (b) decay of the CT state of TBD6e in THF at different temperatures. The lines correspond the fitting curves. Monitoring wavelength was 660 nm.

and thus sample degradation could occur more easily than in the regular pump-probe measurements. This was avoided by using low excitation intensity. Possible changes in the sample were controlled by measuring the absorption spectra before and after the pump-probe measurements in a certain temperature range. The time-resolved spectra were collected for 50-70 delay times and converted into decay curves for global data analysis.41 Results An electron D-A dyad, in which the porphyrin moiety is linked to the C60 moiety covalently with two linkers, forming a rigid dyad with a short and constant D-A distance (TBD6e in Figure 1), was synthesized and spectroscopically studied.45,48 The dyad performed, via an exciplex transient state, a fast intramolecular photoinduced electron transfer, in benzonitrile solution as shown in Scheme 1. In toluene the exciplex relaxed directly to the ground state. At room temperature, the rate constants for the formation of the exciplex were 8.3 × 1012 s-1, that for the transformation to CS state 1.5 × 1011 s-1, and the rate constants for the charge recombination 2.2 × 109 s-1. The rate constants were in toluene 6.2 × 1012 and 0.31 × 109 s-1 and in cyclohexane 10 × 1012 and 0.36 × 109 s-1 for the exciplex formation and its direct decay to the ground state, respectively.45 In the present work, the kinetics of TBD6e was studied in tetrahydrofuran (THF) in the temperature range from 185 to 305 K. Figure 2 shows the component spectra of the compound obtained by a three exponential fitting at temperatures of 205 and 305 K. The spectra are practically identical having the formation times of 1.3-1.8 and 60-70 ps for the exciplex and CS state, respectively, and for the recombination of the CS state 0.9-1.3 ns. Taking into account the differences in solvent

Charge Transfer in Donor-Acceptor Pairs

Figure 4. Component spectra and their lifetimes of transient states of H2Pc-F in THF at temperatures of (a) 305 and (b) 175 K.

properties, the formation times correspond well to the values measured in polar benzonitrile, less polar toluene, and nonpolar cyclohexane. The most important result of these temperature dependent measurements is, however, that both the formation of the exciplex and its recombination are both practically independent of the temperature. This can clearly be seen from the formation and decay curves of the exciplex and the CR state in Figure 3, panels a and b, respectively. A very small increase, if any, in the formation rate of the exciplex (Figure 3a) corresponds to the activation energy of about 0.01 eV, which is negligible compared to the accuracy of the measurements. In Figure 3b, the decay curves of the CS state at different temperatures are presented. Here the fastest decay is observed at the lowest temperature, indicating actually very small negative activation energy. The difference in rates is, however, so small that it can be explained by changes in the solvent properties as the temperature changes. Similar exciplex mediated ET, as described for the previous porphyrin-fullerene compounds, was obtained also for the phthalocyanine-fullerene compound H2Pc-F. Excitation of the phthalocyanine moiety of the dyad results in a rapid ET from phthalocyanine to fullerene via an exciplex state in both polar and nonpolar solvents.51,52 At room temperature, the rate constants for the formation of the exciplex was 1.8 × 1012 s-1, that for the transformation to the CS state 2.9 × 1011 s-1, and the rate constants for the charge recombination 14 × 109 s-1. In the present work, the kinetics was measured for compound H2Pc-F in THF. Figure 4 shows the component spectra of the compound obtained by a four exponential fitting at temperatures of 175 and 305 K. The spectra are very similar having the formation times 0.5-0.6 ps for the exciplex, 7 ps for the CS

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Figure 5. (a) Decay of the CT state and (b) formation of the exciplex and CT states of H2Pc-F in THF at temperatures of 175 and 305 K. The lines correspond the fitting curves. Monitoring wavelength was 690 nm.

state, and for the recombination of the CS state 64 and 150 ps, at temperatures of 175 and 305 K, respectively. The formation and recombination times correspond well to those measured earlier in benzonitrile and toluene.52 The long-living components (>0.5 and >1.6 ns) with low intensity have a lifetime on the upper limit of the instrument and evidently have a much longer lifetime than measured at room temperature in benzonitrile and toluene.51,52 It is most probably the CT triplet state, which has been observed earlier in toluene.52 It is important to notice that the recombination is faster at low rather than high temperature (Figure 5, panels a and b) indicating negative activation energy. The dyads H2P-S-F (Figure 1) has a moderate D-A c-c distance (close to 1 nm), which is short enough to promote the CS state, but long enough to make weak electronic coupling between the porphyrin and fullerene moieties.43 An emissive component, which was attributed to an exciplex, was resolved for the dyad in toluene. The exciplex preceded the CS state in polar benzonitrile and the excited singlet state of fullerene in non polar toluene. In benzonitrile at room temperature the fluorescence lifetime of the porphyrin chromophore was 27 ps, which correlates well with the formation time constant (24 ps) of the CS state. The rate constants for the formation of the exciplex was thus 0.037 × 1012 s-1 and the formation of the CS state from the exciplex 1.5 × 1010 s-1. For the recombination of the CS state, a rate constant of 1.1 × 109 s-1 was measured.43,44 The lifetimes in toluene were 30 ps, 230 ps, and 1.4 ns for the locally excited porphyrin, exciplex, and locally excited fullerene, respectively.43 In the present work, the kinetics was measured for compound H2P-S-F in THF at the temperature range from 185 to 305 K. Figure 6 shows the component spectra of the compound obtained

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Figure 6. Component spectra and their lifetimes of the transient states of H2P-S-F obtained by a four exponential fitting at temperatures of (a) 305 and (b) 185 K.

by a four exponential fitting at temperatures of 185 and 305 K. The spectra are very similar having the formation times close to 10 ps for the exciplex, 90 and 40 ps for the CS state, and 0.8 and 1.2 ns for the recombination of the CT state, at temperatures of 185 and 305 K, respectively. The fast components, 0.2 and 0.4 ps, in panels a and b in Figure 6, respectively, correspond to the formation of exciplex from the second singlet excited state.44 In Figure 7 the formation and decay curves of the CS state H2P-S-F are presented at different temperatures. Here the fastest formation is observed at the highest, but the fastest decay at the lowest temperature, the same phenomenon as was observed for TBD6e and H2Pc-F above. For all chromophore-fullerene dyads of this study, the formation of the CT state was demonstrated also in the nearinfrared (NIR) region, 860-1070 nm, of the spectrum, where the fullerene anion radical absorbs.43-45,52 The transient absorption signals in the NIR region are less intense compared to those at the visible region and the signal-to-noise ratios are typically less than 10. Therefore, the kinetics was not possible to perform as accurate as at shorter wavelength area and biexponential fittings were used to analyze the measurements. The component spectra of H2Pc-F in benzonitrile at room temperature in the near-infrared region (860-1070 nm) were presented earlier,52 and those at temperatures of 175 and 295 K in THF are presented in panels a and b in Figure 8, respectively. In both cases the characteristic absorption band of the phthalocyanine cation appears at about 890 nm. The spectrum of disubstituted fullerene anion is broad and featureless in the infrared region.45 The lifetimes of the both components are almost independent of temperature, which also can be seen from the recombination curves of the CT state (Figure 8c) at the temperatures of 175

Lemmetyinen et al.

Figure 7. (a) Formation of the exciplex and the CT state and (b) formation and decay of the CT state of H2P-S-F in THF at temperatures of 225 and 305 K. The lines correspond the fitting curves. Monitoring wavelength was 620 nm.

and 295 K. The recombination is a bit faster at lower than at higher temperatures, similar as at the visible region (Figure 5a). Finally, for comparison, the temperature measurements were done for a porphyrin-benzoquinone compound, P-BQ, in which the quinone moiety is directly bound by a single bond to the porphyrin moiety (Figure 1). In this compound the freedom for movement of the quinone unit is restricted to rotation over the C-C bond and even a partial sandwich structure, and thus the exciplex formation, is impossible. Only two components were observed by pump-probe measurements: the singlet excited-state of porphyrin and the CS state, with the lifetimes of 1.6 and 43 ps, respectively, when measured in toluene at temperature 190 K (Figure 9). The temperature dependent decay curves show clear dependence on the temperature at the temperature range from 190 to 305 K. At higher temperatures, the decays are faster. Spectroscopic behavior of P-BQ dyad is evidently different than that of the compounds TBD6e, H2Pc-F, and H2P-S-F, in which the existence of exciplex was observed in polar benzonitrile, nonpolar toluene, and in the present study in moderately polar THF. Discussion From the data and literature review presented above, it is clear that the CS and CR dynamics of the D-A dyads covalently linked to each other deviates in many aspects from classical Marcus treatment, although the Marcus electron transfer theory was applied to some dyads42,43,48,52 of the present study. The most important difference is the independence of the temperature of both the CS and CR processes, which is in contradiction to the classical Marcus treatment. This property is rather connected

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Figure 9. (a) Component spectra and their lifetimes of transient states of P-BQ in toluene at temperature of 190 K and (b) decays of the CT state (monitored at 700 nm) of P-BQ at different temperatures. In panel b, the lines correspond the fitting curves.

Figure 8. Component spectra and their lifetimes of transient states of H2Pc-F in THF at temperatures of (a) 175 and (b) 295 K in the NIR region (860 - 1070 nm), and (c) decay of the CT state of the same compound at temperatures of 175 and 295 K, when monitored at 880 nm. In panel c, the lines correspond the fitting curves.

to observation of formation of the exciplex transient before the final CS state. Because of this, comparisons should be done with the ET systems, in which the charge separation takes place in geminate ion pairs controlling the process by solvent polarity, by linking the D-A pair by a flexible chain or by a single bond, by using high concentrations of excited acceptors or donors, and by forming ground-state charge transfer complexes, as was discussed above. In their studies67-70 of ultrafast charge transfer reactions in excited states Mataga et al. came to the conclusions that formations of CT states of various electronic and geometrical structures are possible and that there may arise an ensemble of exciplexes and ion pairs distributed over such states in solution. A similar conclusion can be drawn based on the results of the present and preceding studies.41-52 Although the chemical

structures of the donors (pheophytin, porphyrin, or phthalocyanine) and the acceptor (fullerene) are the same, big differences were observed in the rate constants of exciplex and CS state formations, 10-0.1 × 1012 s-1 and 290-1.1 × 109 s-1, respectively, and in the charge recombination, 1.1-14 × 109 s-1. The rate constants depend on the c-c distance and mutual orientations of the donor and acceptor, which are determined by linker structures. The rates depended on solvent polarity, being slower in nonpolar than in polar environments, but the absolute orders, determined by the spacer of the dyad, remained the same. At high concentrations when geminate D-A pairs were created, no good agreements between experimental results and theoretical predictions on the energy gap dependence of the ET rate were observed.28,66 In the inverted regime, a small temperature dependence of kCR was observed, but in the normal region, the CR was a thermally activated process, with the barrier height increasing with diminishing exergonicity.28 In the present work, the rates of exciplex and CS state formations are very high and do not show any inverted region behavior. Both the CS and CR processes were clearly independent of temperature in all reactions, in which the exciplex formations were observed. The differences observed in experiments at high concentrations can be explained with the fact, that diffusion plays always a role in systems in which the donor-acceptor pair is not covalently linked and with the difference in the geometrical structures in the pair. For a porphyrin-quinone pair, with the c-c distance of 6.5 Å almost temperature independent CR process was observed22 with the activation energy of -1 to +2 kJ/mol, but increased to about 4 kJ/mol, when the distance was increased. The effects of both

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solvent and temperature were small. The results were explained by nonadiabatic electron tunnelling. These results deviate from the results obtained in the present study, in which no temperature and thus no activation energy dependences were observed for donor-fullerene systems, but a clear dependence of the electron transfer rates on the c-c distance. When CS and CR processes were studied24 for fixed distances in porphyrin-quinone dyads, it was shown that kcs was temperature dependent but kcr was almost temperature-independent and did not show systematic dependence on the free energy gap, -∆Gcr. The practically activationless CR reaction for all energy gap regions examined could not be interpreted by the classical treatment of Marcus theory. This result, which is in accordance with the CR process of the present study, indicates the dominant effect of the quantum mechanical tunnelling due to the high frequency modes for ET in the inverted region.24 Exciplex formations were not reported in either of the abovementioned porphyrin-quinone systems.22,24 The fullerene radical anion is much less sensitive to environment stabilization effects than are the radical anions of quinones and related acceptors, which is very likely the major reason that fullerene-based systems carry out photoinduced electron transfer at low temperatures, whereas similar quinone-based systems do not.27 Conclusions The temperature dependence of the photoindced electron transfer reaction has been studied for a series of electron acceptor and donor dyads, where one or two covalent linkers between them control their orientational fluctuations, geometries, energies, and distances. For the short center-to-center distance (7-10 Å) of the donor and acceptor both the exciplex formation and primary electron transfer are extremely fast. Rates become slower when the distance and orientational fluctuation increases. The most important observations are that the formation of an intramolecular exciplex preceded the charge separated state in all studied molecular systems and that the CS and CR processes are temperature independent. Although it is not yet perfectly confirmed, it seems to be a general conclusion that the aromatic π-π interaction between the donor and acceptor moieties is an important factor for exciplex formation as transient states in photoinduced electron transfer reactions and this causes barrierless charge separation and recombination reactions. Acknowledgment. H.L. thanks Academy of Finland for supporting the research project NoVel Approach to the Mechanism of Excited-State Electron Transfer. References and Notes (1) Mataga, N.; Ottolenghi, M. In Molecular Association; Foster, R., Ed.; Academic Press: London, 1979; Vol. 2, p 1. (2) Mataga, N. In The Exciplex; Gordon, M., Ware, W. R., Eds.; Academic Press: New York, 1975; p 113. (3) Mataga, N. Pure Appl. Chem. 1984, 56, 1255. (4) Barbara, P. F.; Jarzeba, W. AdV. Photochem. 1990, 15, 1. (5) Verhoeven, J. W. Pure Appl. Chem. 1990, 62, 1585. (6) Maroncelli, M.; McInnes, J.; Fleming, G. R. Science 1989, 243, 1674. (7) Electron Transfer in Inorganic, Organic and Biological Systems, AdVances in Chemistry Series, 228; Bolton, J. R., Mataga, N., McLendon, G., Eds.; American Chemical Society: Washington, DC, 1991. (8) Dynamics and Mechanisms of Photoinduced Electron Transfer and Related Phenomena; Mataga, N., Okada, T., Masuhara, M., Eds.; Elsevier: Amsterdam, 1992. (9) Mataga, N.; Miyasaka, H. Prog. React. Kinet. 1994, 19, 317. (10) Kakitani, T.; Matsuda, N.; Yoshimori, A.; Mataga, N. Prog. React. Kinet. 1995, 20, 347. (11) Mataga, N.; Miyasaka, H. AdV. Chem. Phys. 1999, 107, 431.

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