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2006, 110, 9368-9370 Published on Web 04/20/2006
Intramolecular Electron Transfer from Axial Ligand to S2-Excited Sb-Tetraphenylporphyrin Mamoru Fujitsuka,† Dae Won Cho,† Tsutomu Shiragami,‡ Masahide Yasuda,‡ and Tetsuro Majima*,† The Institute of Scientific and Industrial Research (SANKEN), Osaka UniVersity, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan, and Department of Applied Chemistry, Faculty of Engineering, UniVersity of Miyazaki, Gakuen-Kibanadai, Miyazaki 889-2192, Japan ReceiVed: April 1, 2006; In Final Form: April 11, 2006
The S2 state properties of Sb-tetraphenylporphyrin (SbTPP) derivatives were investigated using subpicosecond spectroscopic methods. The S2 fluorescence of various SbTPP derivatives was observed for the first time. It was revealed that the S2 fluorescence lifetime changed depending on the donor-ability of the ligand because of the contribution of the charge separation to the S2 excited SbTPP, which was confirmed by transient absorption spectroscopy.
Introduction It is well known that in the natural photosynthesis system excitation energy captured by the light-harvesting complex is transferred to the reaction center after an energy migration process.1 In the natural system, higher excitation energy corresponding to the higher excited states of dyes such as chlorophylls and carotenes is also utilized, despite its very short lifetime.2 Inspired by the natural system, artificial energy transfer systems, in which energy transfer of higher excited-state energy is realized, have been demonstrated by various researchers. 3 On the other hand, the examples of the charge separation (CS) via the higher excited states are rather limited, because of quite short lifetime of these intermediates. Intermolecular electron transfer from Zn-tetraphenylporphyrin (ZnTPP) to solvent has been reported by Chosrowjan et al.4a Some intramolecular CS systems using ZnTPP have been also reported.4b-g Recently, Mataga et al. have reported the systematic studies on the intramolecular CS process of S2-excited Zn-porphyrin derivatives, in which an electron acceptor is substituted at the meso position of the porphyrin unit.4d-f These results indicate that the higher excited state is an important intermediate in various photoinduced processes, including CS. In the case of Zn-porphyrin, S2-excited Zn-porphyrin acts as an electron donor. It is well known that the electron donating and accepting character of porphyrins changes according to the central metal. Porphyrin including SbV as a central metal exhibits an electron acceptor ability. Actually, some of the present authors and their collaborators have shown the intermolecular electron transfer to the S1 and T1 excited Sb-tetraphenylporphyrin (SbTPP) derivative from the electron donor.5 Thus, SbTPP derivatives are good candidates, which exhibit electron acceptor ability in the higher excited state, although a higher excited-state property of SbTPP is not known. * Corresponding author. E-mail:
[email protected]. † The Institute of Scientific and Industrial Research. ‡ University of Miyazaki.
10.1021/jp062023r CCC: $33.50
Figure 1. Molecular structures of SbTPP derivatives.
In the present study, we investigated the S2 state properties of SbTPP derivatives (Figure 1) by using subpicosecond spectroscopic methods. The S2 fluorescence from SbTPP was observed for the first time. Furthermore, CS from the ligand to the S2-excited SbTPP was confirmed. Experimental Section Materials. SbTPP derivatives (1-5) were synthesized as described in the Supporting Information. Derivative 6 was synthesized according to a previously reported procedure.6 All other chemicals were of the best commercial grades available. Apparatus. Details of the subpicosecond and steady-state spectroscopy and electrochemistry are described in the Supporting Information. In the present study, the samples were excited by the 400 nm laser pulse. Results and Discussion S2 Fluorescence of SbTPP. Figure 2 shows an absorption spectrum of 5 in acetonitrile. The peak attributable to the B band was observed at 420 nm, while the Q-bands were at 553 and 594 nm. Upon excitation of the Q-band, fluorescence due to the S1fS0 transition appeared at 597 and 649 nm. On the other hand, upon excitation of the B band at 405 nm, fluorescence peak attributable to the S2fS0 transition appeared at 425 nm with a shoulder at 446 nm. The S2 fluorescence band decayed according to the single-exponential function, from which the lifetime was estimated to be 1.8 ps (inset of Figure 2). The S2 fluorescence was observed for all the compounds © 2006 American Chemical Society
Letters
J. Phys. Chem. B, Vol. 110, No. 19, 2006 9369
Figure 2. Absorption (solid line) and S2 fluorescence spectra (dashed line) of 5 in acetonitrile. The asterisk indicates a Raman peak of the solvent. Inset: fluorescence decay profile at 425 nm upon excitation at 400 nm (fwhm 80 fs). Dashed line is irf.
TABLE 1: S2-State Properties (λS2 and τS2), Oxidation and Reduction Potentials, Free Energy Change for Charge Separation, Charge Separation Rates, Quantum Yields for Charge Separation, and Charge Recombination Rates of SbTPP Derivatives λS2 τS2 Eox (nm) (ps) (V)a 1 2 3 4 5
429 427 425 427 425
0.45 0.37 0.40 0.89 1.8
0.76 0.78 1.03 1.23 1.23
Ered (V)a -0.57 -0.56 -0.55 -0.42 -0.48
-∆GCS kCR kCS (eV) (1012 s-1)b ΦCS b (1012 s-1) 1.56 1.56 1.34 1.25 1.21
1.7 2.2 2.0 0.62 0.06
0.77 0.81 0.80 0.55 0.11
6.8 5.8 6.1 -c -c
a vs. SCE. b The values were estimated based on the S2 fluorescence lifetime. The lifetime of 6 in acetonitrile (2.0 ps, λS2 ) 425 nm) was used as a standard. c Charge separated state was not observed.
investigated in this study (1 - 6). The S2-fluorescence summarized in Table 1 exhibits the peak at positions similar to each other. As seen in Table 1, the S2 fluorescence lifetime (τS2) largely depends on the ligand of SbTPP, indicating the additional deactivation pathway to the radiative and nonradiative processes as discussed in the next section. It has been shown that the excited SbTPP works as an electron acceptor.5 Although 1-5 have a ligand with electron donating character due to N-phenyl group, 6 does not. Thus, the τS2 value of 6 (2.0 ps) can be regarded as the intrinsic lifetime of SbTPP, i.e., τS2 (2.0 ps) ) 1/(krS2 + knrS2), where krS2 and knrS2 are the rate constants of the radiative and nonradiative processes of the S2 state, respectively. Although the S2 fluorescence has been reported for Zn, Cd, Al, and rare earth metal coordinated TPP,7 this is the first observation of the S2 fluorescence of SbTPP. The lifetime is slightly shorter than that reported for ZnTPP (3.5 ps in acetonitrile).4a Electron Transfer to S2-Excited SbTPP. As indicated in the previous section, the lifetime of the S2 fluorescence largely depends on the ligand of SbTPP, i.e., the lifetimes are in the order of 1 ≈ 2 ≈ 3 < 4 < 5. In Table 1, estimated oxidation (Eox) and reduction (Ered) potentials of 1-5 are listed. Table 1 also summarized the free energy change of CS (∆GCS), which was estimated by assuming CS from the axial ligand to SbTPP via the S2 state using eq 1,
∆GCS ) Eox - Ered - ES2
(1)
where ES2 is the S2-state energy estimated from the peak position of the S2 fluorescence. Therefore, the measured lifetimes are in good relation with the ∆GCS values. That is, the fluorescence lifetime became shorter with an increase in the -∆GCS values. The presence of the CS process during the deactivation of the S2 state was examined by subpicosecond transient absorption
Figure 3. Transient absorption spectra (a) and kinetic trace at 710 nm (b) during the laser flash photolysis of 2 in acetonitrile upon excitation with 400 nm femtosecond laser.
measurements. To generate the S2 state of SbTPP, the sample in acetonitrile was excited with a 400 nm femtosecond laser. As shown in Figure 3, an absorption band appeared around 710 nm immediately after the laser excitation of 2 as well as the negative absorption bands at 553 and 594 nm due to the Q-bands of 2. The absorption band at 710 nm decayed within 1 ps (Figure 3). The absorption spectra after 1 ps can be attributed to the S1 state of SbTPP. On the other hand, the absorption band at 710 nm can be attributed to the reduced SbTPP, i.e., SbIVTPP• from the comparison with the previous intermolecular electron transfer study on SbTPP and amine.5 Therefore, the appearance of SbIVTPP• indicates intramolecular CS from the ligand to SbTPP via the S2 excited state according to eq 2,
[SbVTPP(OCH3)]+(S2)-NHC6H4CH3 f [SbIVTPP(OCH3)]•-NH•+C6H4CH3 (2) The rate constant of CS (kCS) was estimated according to eq 3,
kCS ) 1/τS2,sample - 1/τS2,6
(3)
where τS2,sample and τS2,6 are the S2 fluorescence lifetimes of the sample and 6, respectively, based on the assumption that the krS2 and knrS2 values of 1-5 are the same as those of 6. Furthermore, quantum yield for CS (ΦCS) was estimated by eq 4,
ΦCS ) (1/τS2,sample - 1/τS2,6)/(1/τS2,sample)
(4)
The estimated kCS and ΦCS values are listed in Table 1. The formation of the CS state was also confirmed for 1 and 3 in acetonitrile. The large ΦCS values (∼0.8 for 1-3) indicate the efficient CS, despite the short lifetime of the S2 state. On the other hand, the absorption band due to the CS state was not observed for 4 and 5 because of the smaller ΦCS values resulting from their smaller -∆GCS values. The estimated kCS values are in the order of 1 ≈ 2 ≈ 3 > 4 > 5, while the -∆GCS values are 1 ≈ 2 > 3 > 4 > 5. From the -∆GCS dependence of the kCS values, it seems that the CS processes in 1-5 are in the Marcus “normal” and “top” regions.8 As shown in Figure 3b, the CS state disappeared within 1 ps after the formation of the CS state. Because the CS states of
9370 J. Phys. Chem. B, Vol. 110, No. 19, 2006
Letters the contribution of the CS process, which was confirmed by the transient absorption spectroscopy. These results are a new example of the CS via higher excited state. 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.
Figure 4. Schematic energy diagram for the charge separation and recombination processes of 2.
1-5 (1.33-1.71 eV relative to the ground state) are located at lower energies compared with the S1 state of SbTPP (2.08 eV), the charge recombination (CR) does not generate the S1 state but generates the ground state as indicated in Figure 4. Based on this energetic consideration, the kinetic trace of 2 was fitted with 5.8 × 1012 and 2.2 × 1012 s-1 of rate constants for the appearance and disappearance of the CS state, respectively. The agreement between the kCS and the disappearance rate of the CS state indicates that the CR rate (kCR) is larger than the kCS.9 The kinetic trace of the CS product can be expressed as eq 5,
A(t) ) C[kCS/(kCS - kCR)][exp(-kCR t) - exp(-kCS t)] (5) where A(t) and C are absorbance of the CS product and a constant, respectively. When kCR is larger than kCS, the appearance rate of the CS product corresponds to kCR; i.e., kCR ) 5.8 × 1012 s-1 for 2. Similarly, the kCR values of 1 and 3 were also larger than the corresponding kCS values as summarized in Table 1. Similar kCR values of 1, 2, and 3 resulted from similar -∆GCR values of these compounds. Consequently, this is the first example indicating that the S2 excited Sb-porphyrin acts as an electron acceptor. Conclusion In the present study, the S2 state properties of SbTPP derivatives were investigated. It became clear that SbTPP shows the S2 fluorescence, of which intrinsic lifetime was 2.0 ps. Furthermore, it was revealed that the S2 fluorescence lifetime changed according to the donor ability of the ligand because of
Supporting Information Available: Synthetic methods of SbTPP derivatives and details of the subpicosecond and steadystate spectroscopy and electrochemistry. These materials are available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) McDermott, G.; Prince, S. M.; Freer, A. A.; HawthornthwaiteLawless, A. M.; Papiz, M. Z.; Cogdell, R. J.; Isaacs, N. W. Nature 1995, 374, 517. (b) Koepke, J.; Hu, X.; Muenke, C.; Schulten, K.; Michel, H. Structure 1996, 4, 582. (c) McLuskey, K.; Prince, S. M.; Cogdell, R. J.; Isaacs, N. W. Biochemistry 2001, 40, 8783. (2) (a) Shrev, A. P.; Trautman, J. K.; Frank, H. A.; Owens, T. G.; Albrecht, A. C. Biochim. Biophys. Acta 1991, 1058, 280. (b) Nagae, H.; Kakitani, T.; Katoh, T.; Mimuro, M. J. Chem. Phys. 1993, 98, 8012. (3) (a) Nakano, A.; Yasuda, Y.; Yamazaki, T.; Akimoto, S.; Yamazaki, I.; Miyasaka, H.; Itaya, A.; Murakami, M.; Osuka, A. J. Phys. Chem. A 2001, 105, 4822. (b)Yeow, E. K. L.; Steer, R. P. Phys. Chem. Chem. Phys. 2003, 5, 97. (4) (a) Chosrowjan, H.; Taniguchi, S.; Okada, T.; Takagi, S.; Arai, T.; Tokumaru, K. Chem. Phys. Lett. 1995, 242, 644. (b) LeGourre´rec, D.; Andersson, M.; Davidsson, J.; Mukhtar, E.; Sun, L.; Hammarstro¨m, L. J. Phys. Chem. A 1999, 103, 558. (c) Andersson, M.; Davidsson, J.; Hammarstro¨m, L.; Korppi-Tommola, J.; Peltola, T. J. Phys. Chem. B 1999, 103, 3258. (d) Mataga, N.; Chosrowjan, H.; Shibata, Y.; Yoshida, N.; Osuka, A.; Kikuzawa, T.; Okada, T. J. Am. Chem. Soc. 2001, 123, 12422. (e) Mataga, N.; Chosrowjan, H.; Taniguchi, S.; Shibata, Y.; Yoshida, N.; Osuka, A.; Kikuzawa, T.; Okada, T. J. Phys. Chem. A 2002, 106, 12191. (f) Mataga, N.; Taniguchi, S.; Chosrowjan, H.; Osuka, A.; Yoshida, N. Chem. Phys. 2003, 295, 215. (g) Hayes, R. T.; Walsh, C. J.; Wasielewski, M. R. J. Phys. Chem. A 2004, 108, 2375. (5) Shiragami, T.; Tanaka, K.; Andou, Y.; Tsunami, S.; Matsumoto, J.; Luo, H.; Araki, Y.; Ito, O.; Inoue, H.; Yasuda, M. J. Photochem. Photobio. A. Chem. 2005, 170, 287. (6) Shiragami, T.; Matsumoto, J.; Inoue, H.; Yasuda, M. J. Photochem. Photobiol. C. ReV. 2005, 6, 227. (7) As a review, Tokumaru, K. J. Porph. Phthalo. 2001, 5, 77. (8) (a) Marcus, R. A. Annu. ReV. Phys. Chem. 1964, 15, 144. (b) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265. (c) Marcus, R. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 1111. (9) Ozeki, H.; Nomoto, A.; Ogawa, K.; Kobuke, Y.; Murakami, M.; Hosoda, K.; Ohtani, M.; Nakashima, S.; Miyasaka, H.; Okada, T. Chem. Eur. J. 2004, 10, 6393.
