J. Phys. Chem. A 2010, 114, 10789–10794
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Electron Transfer from Oligothiophenes in the Higher Triplet Excited States Mamoru Fujitsuka,* Takeshi Nakatani, Masanori Sakamoto, Akira Sugimoto, and Tetsuro Majima* Institute of Scientific and Industrial Research (SANKEN), Osaka UniVersity, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan ReceiVed: July 1, 2010; ReVised Manuscript ReceiVed: August 30, 2010
In the present paper, we have investigated the inter- and intramolecular electron transfer processes from the higher triplet excited state (Tn) of oligothiophenes (3T and 4T). In the case of the intermolecular systems, two-color two-laser flash photolysis using nanosecond lasers was applied to the solution including benzophenone, oligothiophene, and halogenated benzene as a photosensitizer, an electron donor, and an electron acceptor, respectively. The first laser light irradiation generated the lowest triplet excited state (T1) of oligothiophene via energy transfer from benzophenone. Upon the second laser light irradiation, the absorption band of the radical cation of oligothiophene appeared with the simultaneous bleaching of the absorption band of the T1 state, indicating the electron transfer from the T2 state of the oligothiophene to the electron acceptor. The observed electron transfer rate dependent on the free energy change was explained on the basis of the Marcus theory. The intramolecular electron transfer in the dyad molecule of oligothiophene and acceptor was investigated using the two-color two-laser flash photolysis employing femtosecond laser. Upon the second laser light irradiation, which generates the Tn state, the kinetic trace of the absorption band of T1 state showed the bleaching and recovery, the rate of which depends on the driving force for the charge separation from the T2 state of the oligothiophene. This observation suggests the existence of charge separation process from the T2 state, and the observation of the charge-separated state was difficult probably due to the low charge separation yield and fast charge recombination. Introduction For years, various scientists have investigated polythiophenes and oligothiophenes because of their excellent properties applicable to electric and optical materials.1 From the viewpoints of the application to the photofunctional materials, investigations on their excited state properties, such as excitation-relaxation processes and photoinduced reactions, are essential. Actually, their excited state properties have been clarified in detail by using various spectroscopic methods, including the laser flash photolysis.2 Recently, the polymeric and oligomeric thiophenes have been employed as a constituent of various photofunctional organic molecules.3 In spite of these studies, little has been known on their higher excited state properties. The higher excited state chemistry is attractive, because the reactions, which are energetically impossible for the lowest excited state, become possible from the higher excited state.4 From this viewpoint, in the previous papers, we studied the higher triplet excited state (Tn) properties of oligothiophenes by using the two-color two-laser flash photolysis method.5 From the transient absorption change attributable to the formation of Tn state during the picosecond two-color two-laser flash photolysis of oligothiophenes, the lifetime of the Tn state was directly estimated.5a Furthermore, we also investigated the energy transfer from the Tn-excited oligothiophene to energy acceptors.5b From the dependence on the triplet energy levels of acceptors, the energy level of T2 state of the oligothiophenes was determined. In spite of these investigations, the study on the electron transfer from the Tnexcited oligothiophenes has not been reported. It should be * To whom correspondence should be addressed. E-mail: fuji@sanken. osaka-u.ac.jp (M. F.),
[email protected] (T. M.).
Figure 1. Molecular structures of oligothiophenes (3T and 4T) and dyad molecules (4T-PhR) investigated in this study.
pointed out that the number of the reports on the electron transfer from the Tn state is also limited.6 In the present paper, we studied inter- and intramolecular electron transfer processes from the Tn state of oligothiophene by using two-color two-laser flash photolysis. As oligothiophenes, we have selected dihexyl-substituted terthiophene (3T) and tetrathiophene (4T) indicated in Figure 1. Because the selective excitation of T1 state for the generation of Tn state is possible by using the two-color two-laser flash photolysis employing 355 and 532 nm lasers as the first and second lasers, respectively. For the study on the intramolecular electron transfer, the dyad molecules of 4T and electron acceptor connected by an amide linker were prepared. For the investiga-
10.1021/jp106056e 2010 American Chemical Society Published on Web 09/20/2010
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tion of intramolecular process, two-color two-laser flash photolysis using a femtosecond laser was employed. Experimental Section Materials. Oligothiophenes (nT, 3T, and 4T) were synthesized according to the reported procedures.7 The dyad molecules (4T-PhR, R ) H, (CN)2, NO2) were synthesized as described in the previous paper using the corresponding aniline compounds.8 Other regents were of the best grades available. Apparatus. The two-color two-laser flash photolysis experiments have be carried out by using two systems, namely a nanosecond system and a sub-picosecond system. The nanosecond system was used to investigate the intermolecular processes, while the latter was for the investigation of the intramolecular processes. The nanosecond system employed two Nd3+:YAG nanosecond lasers. The first laser (Quantel, Brilliant, 355 nm) was used as an excitation source of the sample in the ground state, while the second laser light (Continuum, Surelite II, 532 nm) was used for the excitation of the T1 state to the Tn state. The delay time of the second laser light from the first laser light was controlled using digital delay/pulse generators (Stanford Research Systems, DG535). Two laser beams were arranged to overlap at the sample in a quartz cell. The probe light from a Xe lamp was detected with a photomultiplier (PMT, Hamamatsu Photonics, R928) equipped with a monocromator (Nikon, G250). The signal from the PMT was recorded with a digital oscilloscope (Tektronix, TDS 580D). The sub-picosecond transient absorption spectra were measured by the pump and probe method using a regeneratively amplified titanium sapphire laser system reported previously.9 In the present study, the output of the amplifier was divided in two by a beam splitter. A part of outputs was converted to the second harmonic generation (400 nm) using a type I BBO crystal to excite the sample in the ground state. Another part of the output was fed to an optical parametric amplifier to generate 650-nm laser pulse to excite the sample in the T1 state. Two laser beams were arranged to be collinear. The delay of the second laser from the first laser was adjusted to be 6 ns by an optical delay. Optimized structures and volume of the molecules were estimated by theoretical calculation at the B3LYP/6-31G(d) level using the Gaussian 03 package.10 For simplicity of the calculation, alkyl groups of the compounds were reduced to methyl groups. Results and Discussion Intermolecular Electron Transfer from Tn State of nT. In order to generate the Tn state of nT, we employed the twocolor two-laser flash photolysis method in this study. The first laser light (355 nm) was irradiated to the solution including nT (3T or 4T), benzophenone (BP), and electron acceptor. To generate the T1 state of nT efficiently, excess amount of BP, from which the triplet energy transfer to nT is expected, was included in the solution. The triplet sensitization is also useful to diminish a strong emission from nT, which makes the kinetic traces obscure. Figure 2a shows transient absorption spectra obtained by the laser flash photolysis of the solution including 4T, BP, and 1,2,4trichlorobenzene (TCB) as an acceptor. The transient absorption band attributable to the T1 state of 4T appeared at 610 and 570 nm after the first 355 nm laser light irradiation as the spectrum indicated by solid circles.2a The second laser light (532 nm) was irradiated at 980 ns after the first laser light irradiation. It
Figure 2. (a) Transient absorption spectra of 4T (0.05 mM) with BP (20 mM) and TCB (1.2 M) in Ar-saturated acetonitrile at 1000 ns after the first laser light excitation during two-color two-laser laser flash photolysis using 355 and 532 nm nanosecond lasers. The second laser light (532 nm, 50 mJ pulse-1) was irradiated at 980 ns after the first laser light (355 nm, 5 mJ pulse-1). The spectra indicated by closed and open circles were obtained in the absence and presence of the second laser light excitation, respectively. (b) The kinetic traces of ∆O.D. at 610 and 660 nm during the two-color two-laser flash photolysis. Gray curves indicate kinetic traces measured under the same condition without second laser light irradiation.
SCHEME 1: Schematic Energy Diagram for Electron Transfer from the Tn-Excited Oligothiophene (nT ) 3T or 4T) to Electron Acceptor (A)a
a
The estimated rate constants in the present and previous papers are also indicated.5,8
should be pointed out that only 4T(T1) exhibits the absorption at 532 nm in the present sample solution, that is, the Tn state of 4T should be selectively generated by the second laser light irradiation. As shown in Figure 2b, irradiation of the second laser light induced the bleaching in the kinetic trace of ∆O.D. at 610 nm and a simultaneous rise in that at 660 nm. The transient absorption spectrum at 20 ns after the second laser light irradiation (open circles in Figure 2a) shows that a clear increase around 650 nm and a decrease around 610 nm were induced by the second laser light irradiation, when compared to the spectrum obtained by the first laser light irradiation only (solid circles in Figure 2a). Since the radical cation of 4T shows an absorption band around 650 nm,11 the present results indicate the generation of 4T radical cation by electron transfer to TCB upon excitation of 4T(T1) to the Tn state as indicated in Scheme 1. Although the second 532 nm laser light irradiation can generate Tn (n g 3) state, the T2 state is responsible to the present electron transfer due to its longest lifetime among Tn (n g 2) states on the basis of the energy gap law, as in the case of the energy transfer from the Tn excited 4T.5 Thus, the electron transfer from the T2 state is the competitive process of the T2 - T1 internal conversion (kIC ) 2.6 × 1010 s-1, Scheme 1).5a Since the ratio of the absorption of generated radical cation to
Electron Transfer from Tn Excited Oligothiophenes
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Figure 3. Relation between concentration of acceptor ([TCB]) and ∆∆O.D.610.
that of decreased 4T(T1) is almost the same as the ratio of extinction coefficients of the corresponding species,2a,12 the other bimolecular processes such as energy transfer from the Tn state can be ruled out from the deactivation process of 4T(Tn). As seen in the kinetic trace of ∆O.D. (Figure 2b), the absorption band due to the generated radical cation kept its intensity during the time window of the present experimental condition (several microseconds), indicating that the back electron transfer generating the 4T(T1) or 4T(S0) is inhibited. Because the decomposition of radical anion of chlorobenzene to chloride and corresponding radical is reported to occur with the rate of 1.8 × 109 f 2 × 1010 s-1,13 the fast decomposition of the radical anion makes the back electron transfer an impossible process. The amount of the bleaching at 610 nm and the simultaneous rise at 650 nm upon the second laser light irradiation increased with the concentration of TCB, the electron acceptor. The change in ∆O.D. caused by the bimolecular electron transfer (∆∆O.D.) of the short-lived intermediate, which was generated by the second laser light irradiation, can be represented as a function of the concentration of the electron acceptor using the following equation (eq 1),14
(
1 1 )β1+ ∆∆O.D. kETτ[A]
)
(1)
where β is a constant dependent on the reaction system, kET is a rate constant of the electron transfer, τ is the lifetime of the exited state, and [A] is the concentration of the electron acceptor. The (∆∆O.D.610)-1 value was plotted against [TCB]-1 in Figure 3, in which a linear relation between (∆∆O.D.610)-1 and [TCB]-1 was confirmed as expected from eq 1. From the ratio of the slope and intercept of the linear line of Figure 3, the kETτ value was estimated. In the previous paper, we determined the lifetime of 4T(T2) state to be 38 ps from the transient absorption measurement during picosecond two-color two-laser pump and probe method.5a Employing the τ value, the kET value was estimated to be 4.4 × 1010 M-1s-1. We have also investigated the electron transfer from the Tn state of 3T. Figure 4a shows the transient absorption spectra obtained during the two-color two-laser flash photolysis of the solution containing 3T, BP, and TCB. The transient absorption band due to 3T(T1) with a peak at 470 nm as well as a shoulder at 550 nm (solid circles of Figure 4a) indicates the generation of 3T(T1) by the triplet energy transfer from BP.2a In the present case, only 3T(T1) has an absorption at 532 nm, which is the wavelength of the second laser light. Upon the second laser light irradiation, the transient absorption band showed bleaching at all wavelengths examined, whereas the amount of the bleaching depends on the wavelength. That is, the bleaching at 470 nm was apparent, whereas that at 560 nm was little. In the
Figure 4. (a) Transient absorption spectra of 3T (0.05 mM) with BP (20 mM) and TCB (1.2 M) in Ar-saturated acetonitrile at 1000 ns after the first laser light excitation during two-color two-laser laser flash photolysis using 355 and 532 nm nanosecond lasers. The second laser light (532 nm, 50 mJ pulse-1) was irradiated at 980 ns after the first laser light (355 nm, 5 mJ pulse-1). The spectra indicated by closed and open circles were obtained in the absence and presence of the second laser light excitation, respectively. Inset: transient absorption spectra normalized at the peak top. (b) The kinetic traces of ∆O.D. at 470 and 560 nm during the two-color two-laser flash photolysis. Gray curves indicate kinetic traces measured under the same condition without second laser light irradiation.
normalized spectra (inset of Figure 4a), it is clear that the second laser light irradiation generates a product with an absorption band with a peak at 550 nm, which is the same wavelength as the peak position of the radical cation of 3T,11 indicating the electron transfer from the Tn state of 3T to TCB generating the radical cation of 3T. The absence of rise at 550 nm upon the second laser light excitation can be explained on the basis of a smaller extinction coefficient of 3T radical cation than that of 3T(T1).2a The absorption change induced by the second laser light irradiation was maintained during the time window of the present experimental condition, indicating that the decomposition of radical anion inhibited the back electron transfer. In the case of 3T, the T2 state is also responsible to the electron transfer as in the case of 4T. The kET value was estimated to be 2.2 × 1010 M-1s-1 by using eq 1 and the T2 state lifetime of 3T.5a In addition to TCB, chlorobenzene (CB), bromobenzene (BrB), 1,2-dichlorobezene (oDCB), and 1,4-dichlorobenzene (pDCB) have been examined as an electron acceptor for the Tn-excited 3T or 4T in the present study. In the case of CB as an electron acceptor, the bleaching and rise upon the second laser light irradiation were quite small both for 3T and 4T, even when the excess amount of CB was added to the sample solution. For other acceptors, electron transfer from the Tn state was confirmed and the kET values were estimated as summarized in Table 1. The driving force (∆G) for the present electron transfer from the T2 state was estimated using the following relation (eq 2),15
∆G ) EOX - ERED - E(T2) - wp
(2)
where EOX, ERED, E(T2), and wp are oxidation potential of the donor, reduction potential of the acceptor,12,16 T2 state energy of the donor, and Coulombic term, respectively. The T2 state
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TABLE 1: Driving Force (∆G) and Estimated Rate Constant (kET) of the Bimolecular Electron Transfer from nT(Tn) to Acceptor donor
acceptor
-∆G (eV)a
kET (M-1s-1)
4T(T2)
CB oDCB pDCB BrB TCB CB oDCB pDCB BrB TCB
-0.03 0.19 0.21 0.27 0.41 -0.05 0.17 0.19 0.25 0.39
