Energy Levels of Oligothiophenes in the Higher Excited Triplet States

Energy levels of oligothiophenes (nT: dimer (2T), trimer (3T), tetramer (4T), and pentamer (5T)) in the higher triplet excited state (Tn, n g 2) were ...
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J. Phys. Chem. C 2007, 111, 1024-1028

Energy Levels of Oligothiophenes in the Higher Excited Triplet States Yosuke Oseki, Mamoru Fujitsuka, Masanori Sakamoto, Xichen Cai, and Tetsuro Majima* The Institute of Scientific and Industrial Research (SANKEN), Osaka UniVersity, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan ReceiVed: September 3, 2006; In Final Form: October 14, 2006

Energy levels of oligothiophenes (nT: dimer (2T), trimer (3T), tetramer (4T), and pentamer (5T)) in the higher triplet excited state (Tn, n g 2) were estimated from triplet energy transfer (TET) from nT in the Tn state (nT(Tn)) to energy quencher by using the nanosecond-nanosecond two-color two-laser flash photolysis, in which nT in the lowest triplet excited state (nT(T1)) generated with the first laser flash was excited to nT(Tn) during the second laser irradiation. The energy levels (E(Tn)) of 3T(Tn), 4T(Tn), and 5T(Tn) in solution were experimentally estimated to be 3.4 ( 0.1, 3.3 ( 0.1, and 3.2 ( 0.1 eV, respectively, and the estimated energies are assigned to those of the second triplet excited state (T2). The E(T2) was gradually decreased with increasing the number of thiophene rings, suggesting that the triplet exited state in nT is relatively localized even in the higher excited state as well as nT(T1). The TET from the Tm (m g 3) state was also observed.

Introduction It has been well known that organic molecules have many electronically higher excited states above the lowest singlet and triplet excited states (S1 and T1, respectively).1,2 In general, excitation of organic molecules to the higher excited states induces photoionization, energy and electron transfer, bond dissociation, and isomerization even when these processes are energetically impossible from the S1 and T1 states.2-35 The photophysical properties and reactions of the higher excited states have been investigated by the experimental and theoretical studies. For example, some compounds, such as porphyrin and azulene, show the fluorescence from the second singlet excited state (S2) which is a good probe for reaction processes of the S2 state.2-11 Namely, the information on the S2 state, such as lifetime, energy level, and reaction process, can be obtained from the S2-fluorescence. On the other hand, it has been difficult to estimate the properties of the higher triplet excited state (Tn, n g 2) because of their quite short lifetimes and no fluorescent characters.2 Therefore, the lifetimes of the Tn states have been estimated indirectly for several aromatic compounds from the kinetic analysis of the bimolecular triplet energy transfer (TET) process.12-30 Recently, the photoinduced processes of the Tn states have been investigated by the direct methods using short pulse lasers.31-35 Oligothiophenes (nT: n denotes the number of thiophene rings) and polythiophenes are very attractive compounds because they are very useful for the opto-electronic and organic semiconductor devices.36-42 Chemically well-defined oligomers allow the systematic studies on the photophysical properties,43-45 photoinduced reactions, and dynamics46-59 of nT in the lowest singlet and triplet excited states. In particular, the lifetimes43 and energy levels58,60-64 of nT are fundamental to elucidate their photophysical behaviors and photoinduced reactions. Previously, we studied the direct observation of the transient absorption spectrum of nT in the Tn state (nT(Tn)) by using the nanosecondpicosecond two-color two-laser flash photolysis (ns-ps 2-LFP).32 For example, the lifetime of 4T(Tn) was estimated to be 38 ps. * Corresponding author. E-mail: [email protected].

On the other hand, Rentsch et al. have reported the energy levels of 3T and 4T in the second triplet excited state (T2) to be 3.4 and 3.13 eV, respectively, in gas phase using photodetachment photoelectron spectroscopy (PD-PES).64 In this case, the T2 states of 3T and 4T are higher than the corresponding S1 states, whereas the recent theoretical studies have suggested that the energy levels of nT in the T2 state lie energetically below the S1 state.65-67 In the present work, we estimated the Tn state energy levels (E(Tn)) of nT (dimer (2T), trimer (3T), tetramer (4T), and pentamer (5T), Figure 1) in solution using the ns-ns 2-LFP. Previously, we estimated the E(Tn) of oligo(p-phenylenevinylene)s to be 2.9 ( 0.2 and 2.6 ( 0.2 eV for trimer and tetramer, respectively, based on the TET from oligo(p-phenylenevinylene)s(Tn) to a series of triplet energy quenchers (Q).32 These experimentally determined Tn energy levels will be useful information to design the reaction from the Tn states. Results and Discussion Energy Levels of nT(Tn). TET has been used to estimate the T1 energy level (E(T1)) of organic molecules.58,68 It is known that the bimolecular TET rate is exponentially decreased from the diffusion-limiting rate with decreasing the energy gap between the triplet energy donor (D) and Q when the energy gap is below about 0.1 eV.1 In principle, the TET from the Tn state also proceeds only when the E(T1) of Q is lower than the E(Tn) of D. For some aromatic hydrocarbons, the TET from the Tn state to Q has been reported using the 2-LFP method, and the TET was observed as the bleaching of the T1 state absorption upon the second laser irradiation.23-30 When the E(Tn) of D is close to the E(T1) of Q, the bleaching of the T1 state absorption becomes smaller or undetectable. Therefore, the E(Tn) can be estimated from the TET by changing the E(T1) of Q (Table 1).33 Figure 2A shows the time profiles of 4T in THF in the presence of 1,4-dimethoxybenzene (1.0 M) as Q during the nsns 2-LFP. This absorption can be assigned to 4T(T1) generated by the first 355 nm ns-laser flash. The bleaching of the absorption of 4T(T1) was clearly observed upon the second 532

10.1021/jp065714w CCC: $37.00 © 2007 American Chemical Society Published on Web 12/07/2006

Oligothiophenes in the Higher Triplet States

J. Phys. Chem. C, Vol. 111, No. 2, 2007 1025

Figure 1. Molecular structures of nT.

nm ns-laser irradiation at 1 µs after the first laser, indicating the TET from 4T(Tn) to 1,4-dimethoxybenzene. The reaction between 4T(Tn) and 1,4-dimethoxybenzene, or 4T and 1,4dimethoxybenzene(T1), can be ruled out because of no change in the ground state absorption of 4T after the repeated twolaser irradiation. When 1-methylnaphthalene was used as Q, not only the bleaching of 4T(T1) absorption but also the recovery was observed (Figure S1). The recovery of 4T(T1) shows the back TET from Q(T1) to 4T regenerating 4T(T1) because of the relatively long lifetime of 1-methylnaphthalene(T1) (25 µs).69 On the other hand, the TET to tetramethylpyrazine was not confirmed (Figure 2B). Therefore, the E(Tn) of 4T can be estimated to be 3.3 ( 0.1 eV. The E(Tn) of 3T and 5T were also estimated to be 3.4 ( 0.1 (Figure S2) and 3.2 ( 0.1 eV (Figure S3), respectively, according to the similar procedure. In the case of 2T, significant bleaching of 2T(T1) absorption at 390 nm was observed during the ns-ns 2-LFP (first: 308 nm; second: 355 nm) even in the absence of Q, indicating the reaction of 2T (Figure S4). Therefore, it was difficult to estimate the E(Tn) of 2T using the ns-ns 2-LFP. A possible deactivation process is the interchange of the R-β carbon atom of a thiophene ring.70 In the present case, ionization can be excluded since the absorption band due to 2T•+ (400 nm)45 was not observed and similar bleaching intensity was observed in cyclohexane, a typical nonpolar solvent unfavorable to ionization. Energetic Consideration of nT(Tn). The recent theoretical studies have suggested that the T2 state is energetically lower than the S1 state for nT.65-67 If there is such low lying T2 state below the Tn state, the bleaching intensity should be markedly affected by the E(T1) of Q, because the presence of the T2 state below the Tn state opens up another deactivation process, namely TET from the T2 state. When the E(T1) of Q is energetically below the low lying T2 state, the bleaching amplitude should be significantly increased. Hence, we performed the ns-ns 2-LFP of 3T, as a representative, with widely changing the E(T1) of Q, 1,4-dimethoxybenzene (E(T1) ) 3.25 eV), aniline (E(T1) TABLE 1: Triplet Energies (E(T1)) of Quenchers (Q)

a

Q

E(T1)a/ eV

toluene 4-methylanisole tetramethylpyrazine 1,4-dimethoxybenzene aniline benzo[b]thiophene 1-methylnaphthalene 1,3-cyclohexadiene

3.59 3.39 3.32 3.25 3.08 2.97 2.63 2.27

Data from ref 69.

Figure 2. Time profiles of ∆OD at 600 nm of 4T in THF in the presence of 1,4-dimethoxybenzene (A) and tetramethylpyrazine (B) (1.0 M) during the ns-ns 2-LFP (first laser: 355 nm, 5 ns fwhm, 2 mJ pulse-1; second laser: 532 nm, 5 ns fwhm, 10 mJ pulse-1). The second laser was irradiated at 1 µs after the first laser. The broken and solid lines show the usual one-laser flash photolysis and 2-LFP, respectively. The spike signal induced by the second laser flash is due to the laser scattering.

Figure 3. Plots of ∆∆OD at 470 nm of 3T(T1) vs [1,4-dimethoxybenzene] (open square), [aniline] (open circle), and [1-methylnaphthalene] (open triangle).

) 3.08 eV), and 1-methylnaphthalene (E(T1) ) 2.63 eV) (Table 1). As shown in Figure 3, the bleaching amplitude of 3T(T1), ∆∆OD470 ) ∆OD470(one laser) - ∆OD470(two lasers), was not affected by the E(T1) of Q, indicating the absence of the low lying T2 state below the Tn state estimated in the former section. Therefore, it is supposed that the estimated Tn energy levels are assigned to those of the T2 state. On the basis of the energy gap law, the internal conversion rate from the Tn to T1 state (kIC) tends to increase as the Tn-T1 energy gap decreases. For example, the kIC from the T2 to T1 states of naphthalene, anthracene, and chrysene were reported to be 2.2 × 1011, 9.1 × 1010, and 1.7 × 1010 s-1, respectively, while corresponding energy gaps between the T2 and T1 states (∆E(T2-T1)) are 1.17, 1.39, and 1.75 eV, respectively.13,17,22-24 On the other hand, the ∆E(T2-T1) values of nT were estimated to be 1.5 ( 0.1 eV for all nT (Table 2). Previously, we have directly observed nT(Tn) using the ns-ps 2-LFP.32 The generation of nT(Tn) was confirmed by the bleaching of the nT(T1) absorption band with the generation of new absorption band due to nT(Tn). From the fit to the recovery of nT(T1) and decay of nT(Tn), the kIC values of nT(Tn) to nT(T1) were directly estimated to be (2.6 ( 0.8) × 1010, (2.6 ( 0.8) × 1010, and (3.2 ( 0.6) × 1010 s-1 for 3T, 4T, and 5T, respectively. No

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Oseki et al.

TABLE 2: Excitation Energies (E) and Energy Gaps (∆E) for nT in the Excited States nT

E(T2)/ eV

2T 3T 4T 5T

3.7b 3.4 ( 0.1 3.3 ( 0.1 3.2 ( 0.1

E(S1)a/ ∆E(T2-S1)/ E(T1)a/ ∆E(T2-T1)/ ∆E(S1-T1)/ eV eV eV eV eV 3.67 3.04 2.75 2.57

0 0.4 ( 0.1 0.6 ( 0.1 0.6 ( 0.1

2.23 1.93 1.81 1.72

1.5 1.5 ( 0.1 1.5 ( 0.1 1.5 ( 0.1

1.44 1.11 0.94 0.85

a Data from ref 61. b Estimated from the fitted line for nT(T2) in Figure 4.

significant change in the kIC values of nT(Tn) can be attributed to the similar ∆E(T2-T1) values of these oligomers. The estimated energy levels, energy gaps between the T2 and S1 states (∆E(T2-S1)), and the ∆E(T2-T1) of nT are summarized in Table 2. Figure 4 shows the plots of the E(T2), E(S1), and E(T1) of oligomers against the reciprocal of the ring number (1/n). The E(T2) plot shows a linear relation against 1/n as well as E(S1) and E(T1).60-63 The E(T2) values of nT gradually decreased with increasing the number of thiophene rings, as did the E(T1) values; therefore, the ∆E(T2-T1) values of all nT were almost the same in contrast to those of ∆E(S1-T1). The localization of the T1 state, compared with the S1 state, in the π-conjugated molecules has been known.71,72 Similar slopes of nT(T2) and nT(T1) show that the delocalization of the triplet excited state of nT is relatively limited even in the higher excited state. Paa et al. have reported the contribution of the T2 state in the intersystem crossing (ISC) of 2T and 3T.57 For 2T, the decay rate of the S1 state was estimated to be 29.6 ps. The extremely fast decay of 2T(S1) has been ascribed to the ISC to the energetically closed T2 state. As shown before, a straight line can be drawn when the E(S1), E(T1), and E(T2) of nT are plotted against 1/n. According to the fitted line for the E(T2) of nT in Figure 4, the E(T2) of 2T was estimated to be 3.7 eV, which is almost the same as the E(S1). This result supports the fast ISC from 2T(S1) to energetically close 2T(T2). In the former section, the photochemical reaction of 2T was found during the ns-ns 2-LFP. However, the reaction in 2T(T2) can be ruled out because the quantum yield of 2T(T1) generation has been reported to be 0.99, indicating that almost all excited 2T deactivates via 2T(T2), while photochemical reaction followed by S1 excitation is not reported.43 It should be pointed out that the excitation energy of 2T achieved by the 2-LFP (E(T1) + 355 nm ) 5.7 eV) corresponds to the larger energy than the E(T2) of 2T (3.7 eV), indicating that the 355 nm second laser irradiation generates 2T(Tm) (m g 3) from 2T(T1). The reaction of 2T during the 2-LFP possibly occurs from 2T(Tm). For 3T, the ISC mechanism consisting of the fast (2.3 ( 0.8 ps) and slow (133 ( 0.7 ps) components has been reported.57 The fast ISC was clearly dependent on the excitation wavelength, and it was not observed when 3T was excited at the

Figure 4. Plots of the energies of nT in the S1 (open square), T1 (open circle), and T2 states (open triangle) vs 1/n. Solid lines show the leastsquares fits.

Figure 5. Transient absorption spectra of 4T in toluene (A), 4-methylanisole (B), 1-methylnaphthlalene (C), and 1,3-cyclohexadiene (D) at 200 ns after the second laser irradiation during the ns-ns 2-LFP (first laser: 355 nm, 5 ns fwhm, 2 mJ pulse-1; second laser: 532 nm, 5 ns fwhm, 20 mJ pulse-1). Broken and solid lines show the one-laser flash photolysis and 2-LFP, respectively. Inset: time profiles of ∆OD at 600 nm.

longer wavelength edge of the ground state absorption spectrum. From the excitation wavelength dependence of the triplet formation yield, they suggested the existence of the high lying T2 state above the S1 state, i.e., ISC from the Franck-Condon state to T2 state enhances the ISC yield. The energy level of the T2 state was estimated to be about 3.3 eV, which is essentially the same as the evaluated value in the present study (3.4 ( 0.1 eV) or slightly lower. Slight deviation perhaps can be explained on the basis of the conformational distribution of 3T. It has been supposed that the fast ISC mechanism includes the intermediate state with a non-relaxed conformation.57 It is known that the relaxed excited state of nT exhibits a quinoidlike planar structure.43 On the other hand, the ground state is a mixture of the planar and perpendicular conformers.73 The perpendicular conformer of 3T has the absorption band at shorter wavelength than the planar conformer.44,73 The excitation of perpendicular conformer of 3T with a higher photon energy will attain slightly larger excitation energy when compared with the excitation of planar conformer, inducing the fast ISC to the T2 state competing with the thermal deactivation to the relaxed S1 state (3.04 eV). Triplet Energy Transfer from 4T(Tm) (m g 3). The absorption maximum of 4T(T1) (620 nm) corresponds to excitation from the T1 state to higher state than the E(T2) of 4T, indicating that the second laser irradiation excites 4T(T1) to 4T(Tm) (m g 3) followed by the rapid internal conversion to 4T(Tn). The energy gap between 4T(Tm) and 4T(T2) is significantly smaller than that between 4T(T2) and 4T(T1) (1.5 ( 0.1 eV). Therefore, it is expected that the lifetime of 4T(Tm) is quite shorter than that of 4T(T2) (38 ps) according to the energy gap law.32 In fact, the TET from 4T(Tm) was not observed in the presence of 1.0 M of Q, as stated above, indicating the extremely fast internal conversion from the Tm state to the T2 state. However, it is possible to observe the TET from the Tm state by using a very high concentration of Q, for example by using it as a solvent. Figure 5 shows the transient absorption spectra of 4T in toluene (A), 4-methylanisole (B), 1-methylnaphthalene (C), and 1,3-cyclohexadiene (D) during the ns-ns 2-LFP. The bleaching of 4T(T1) absorption was confirmed even in toluene and 4-methylanisole in spite of no bleaching in a 1.0 M solution of tetramethylpyrazine (Figure 2B). The bleaching in toluene and 4-methylanisole is smaller than that in 1-methylnaphthalene and

Oligothiophenes in the Higher Triplet States SCHEME 1: Formation, Decay, and Triplet Energy Transfer Processes of nT(Tn)a

J. Phys. Chem. C, Vol. 111, No. 2, 2007 1027 Acknowledgment. This work has been partly supported by a Grant-in-Aid for Scientific Research (Project 17105005, Priority Area (417), 21st Century COE Research, and others) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government. Supporting Information Available: Figures containing the ns-ns 2-LFP of 4T in the presence of 1-methylnaphthalene (S1); ns-ns 2-LFP of 3T in the presence of tetramethylpyrazine and 4-methylanisole (S2); ns-ns 2-LFP of 5T in the presence of aniline and 1,4-dimethoxybenzene (S3); and ns-ns 2-LFP of 2T in the absence of energy quencher (S4). This material is available free of charge via the Internet at http://pubs.acs.org.

a IS, IC, Fl, TET, BTET, and ISC denote intermediate state, internal conversion, fluorescence, triplet energy transfer, back triplet energy transfer, and intersystem crossing, respectively.

1,3-cyclohexadiene, of which E(T1) values are much lower than the E(T2) values of 4T. These results suggest that the bleaching in toluene and 4-methylanisole can be attributed to the TET from 4T(Tm) to toluene and 4-methylanisole, because the excitation energy by the 2-LFP (E(T1) + 532 nm ) 4.1 eV) is higher than the E(T1) of toluene and 4-methylanisole. The weak bleachings can be ascribed to the extremely fast internal conversion from 4T(Tm) to 4T(T2), of which rate is comparable with that of the TET to the surrounding solvent molecules.74 The relatively large bleaching in 4-methylanisole compared with that in toluene implies the existence of another electronic state lying between toluene(T1) and 4-methylanisole(T1), e.g. 4T(T3). These results also support the conclusion in the former section that estimated nT(Tn) can be assigned to nT(T2), because the existence of the low lying T2 state should arouse the extremely fast internal conversion from nT(Tn) to nT(T2), which is as fast as that from 4T(Tm) to 4T(T2) and results in quite low TET efficiency from nT(Tn) to Q. Formation, decay, and TET processes of nT(Tn) are summarized in Scheme 1.57 Conclusions In the present study, the excitation energies of oligothiophenes in the higher triplet excited state were estimated from the triplet energy transfer process. In contrast to the recent theoretical studies, the energy levels of oligothiophenes (trimer, tetramer, and pentamer) in the second triplet excited states were significantly higher than that of the lowest excited singlet states. The energy levels of oligothiophenes were gradually decreased with increasing the number of thiophene rings, suggesting the delocalization of the triplet excited state of nT is relatively limited even in the higher excited state. The TET from Tm (m g 3) was also observed. Therefore, detailed investigations of the TET during the 2-LFP give useful information of the Tn state, of which various ambiguous points still remain. Experimental Section nT (n ) 2, 3, 4, and 5) were synthesized according to known nickel catalyzed Grignard coupling reaction.75 Toluene, THF, and cyclohexane are spectral grade. Tetramethylpyrazine and 1,4-dimethoxybenzene are the best commercial grade. These were used as received. The other compounds were of the best commercial grade available and used after distillation or recrystallization. Sample solutions were freshly prepared in a rectangular Suprasil cell (1 × 0.5 × 3 cm3) and deoxygenated by bubbling with argon gas for 45 min before laser irradiation. The ns-ns 2-LFP set up has been reported in the previous paper.33

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