The Journal of
Physical Chemistry.
0 Copyright, 1992, by the American Chemical Sociery
VOLUME 96, NUMBER 2 JANUARY 23,1992
LETTERS Photoinduced Electron-Transfer Reactions of Porphyrin Heteroaggregates: Energy Gap Dependence of an Intradimer Charge Recombination Process Hiroshi Segawa, Chie Takehara, Kenichi Honda,lPTakeo Shimidzu,* Division of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto 606, Japan
Tsuyoshi Asahi,lb and Noboru Mataga Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka 560, Japan (Received: September 16, 1991; In Final Form: December 2, 1991) A direct measurement of the kinetics of intradimer photoinduced electron-transfer reactions of a porphyrin heteroaggregate composed of a water-solublegold porphyrin and a water-soluble zinc porphyrin has been made by picosecond time-resolved absorption studies in aqueous solution. By the photoexcitation of the heterodimer, the contact radical ion pair (CIP) of the heterodimer is formed through an extremely fast process within a few picoseconds. The CIP does not dissociate readily to solvated radicals but tends to decay through a nonradiative charge recombination process. The rate constant of the charge recombination of the CIP decreases as the exothermicity of the electron transfer increases.
Introduction Excited-state properties and photoinduced electron-transfer reactions of covalently linked porphyrin dimers2-I0 have been (1) (a) Present address: Tokyo Institute of Polytechnics, Honcho 2-5-9, Nakano-ku, Tokyo 164, Japan. (b) Present address: Microphotoconversion Project, ERATO, Research Development Corporation of Japan, Kyoto Research Park, Shimogyo-ku, Kyoto 600, Japan. (2) (a) Netzel, T. L.; Krogen, P.; Chang, C.-K.; Fujita, I.; Fajar, J. Chem. Phys. Leu. 1979,67,223-228. (b) Netzel, T. L.; Bergkamp, M. A,; Chang, C.-K. J . Am. Chem. SOC.1982, 104, 1952-1957. (c) Fujita, I.; Fajar, J.; Chang, C.-K.; Wang, C.-B.; Bergkamp, M. A,; Netzel, T. L. J . Phys. Chem. 1982.86.3754-3759. (d) Bergkamp, M. A.; Chang, C.-K.; Netzel, T. L. J . Phys. Chem. 1983,87,4441-4446. (3) (a) Overfield, R. E.; Scherz, A.; Kaufmann, K. J.; Wasielewski, M. R. J . Am. Chem. Soc. 1983,105,4256-4260. (b) Overfield, R. E.; Scherz, A.; Kaufmann, K. J.; Wasielewski, M. R. J. Am. Chem. SOC. 1983, 105, 5747-5752. (c) Johnson, D. G.; Svec, W. A.; Wasielewski, M. R. Isr. J. Chem. 1988,28, 193-203. (d) Johnson, S. G.; Small, G. J.; Johnson, D. G.; Svec, W. A.; Wasielewski, M. R. J . Phys. Chem. 1989,93, 5437-5444. (e) Wasielewski, M. R.; Johnson, D. G.; Niemczyk, M. P.; Gaines, G. L. 111; ONeil, M. P.; Svec, W. A. J . Am. Chem. SOC.1990, 112, 6482-6488.
extensively investigated in order to elucidate the functions of the organized chromophores in the photosynthetic reaction centers (RCs). In these studies, long-distance energy- or electron-transfer reactions in the dimers have been mainly discussed with respect (4) Mialocq, J. C.; Giannotti, C.; Maillard, P.; Momenteau, M. Chem. Phys. Lerr. 1984, 112, 87-93. ( 5 ) Regev, A.; Galili, T.; Levanon, H.; Harriman, A. Chem. Phys. Lett. 1986, 131, 140-146. (6) Gaspard, S.;Giannotti, C.; Maillard, P.; Schaeffer, C.; Tran-Thi, T.-H. J. Chem. Soc., Chem. Commun. 1986, 1239-1241. (7) (a) Heiler, D.; McLenden, G.; Rogalskyj, P. J . Am. Chem. Soc. 1987, 109,604-606. (b) Helms, A.; Heiler, D.; McLenden, G. J . Am. Chem. SOC. 1991, 113, 4325-4327. (8) (a) Osuka, A.; Maruyama, K. J . Am. Chem. SOC.1988, 110, 4454-4456. (b)'Osuka, A.; Maruyama, K.; Yamazaki, I.; Tamai, N. Chem. Phys. Lett. 1990, 165, 392-396. (c) Osuka, A.; Maruyama, K.; Mataga, N.; Asahi, T.; Yamazaki, I.; Tamai, N. J. Am. Chem. Soc. 1990,112,49584959. (9) Meier, H.; Kobuke, Y.; Kugimiya, S. J. Chem. Soc., Chem. Commun. 1989. 923-924. (1O)Chardon-Noblat, S.; Sauvage, J.-P.; Mathis, P. Angew. Chem., Int. Ed. Engl. 1989, 28, 593-595. ~
0022-3654/92/2096-503%03.00/00 1992 American Chemical Society c
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The Journal of Physical Chemistry, Vol. 96, No. 2, 1992
Letters 2.0I
CHART I R
R
M
R
1
M
p: 1.0 11
to their geometrical factors so far; short-distance phenomena in stacked dimers with u-u interactions have got to be completely defined for the elucidation of functions of the special pair in the RCs. In recent studies on bacterial RCS,"-'~it has been pointed out that the initial excited state of the special pair is quite different from that of a typical aromatic dimer and that the excited state may have some intradimer charge-transfer (CT) character. Although the postulated initial excited state in the wild-type RCs is not a complete CT state, the preparation and investigation of the complete CT state of porphyrin aggregates are useful for an understanding of the intradimer CT c h a r a ~ t e r . ' ~ The complete CT state is closely related to a contact ion pair (CIP) formed from direct excitation of a ground-state donoracceptor complex. The CIP is characterized by the anomalous energy gap dependence of the charge recombination rate as Asahi and Mataga have pointed out.I5 Therefore, the study of the energy gap dependence of the charge recombination rate of the CIP of a porphyrin aggregate has essential importance for the understanding of the excited state of the porphyrin aggregate. Segawa et al. have recently shown that electron-attracting gold porphyrins and electron-releasing zinc porphyrins form heteroaggregates with charge-transferinteractions in aqueous solution.I6 The heteroaggregates are suitable for the systematic investigation of not only the excited dimer with unsymmetrical electronic distribution but also the fundamental features of electron transfer in tightly stacked donor-acceptor complexes. In this paper, we wish to report the direct measurement of intradimer photoinduced electron transfer and energy gap dependence of the electron transfer kinetics in the metalloporphyrin heteroaggregates.
Experimental Methods Water-soluble gold(II1) porphyrins [ 1, gold meso-tetrakis(4carboxypheny1)porphyrin (AuTCPP); 2, gold meso-tetrakis(4sulfonatopheny1)porphyrin (AuTSPP); 3, gold meso-tetrakis(3N-methylpyridy1)porphyrin (AuTMPyP(3)); 4, gold meso-tetrakis(4-N-methylpyridy1)porphyrin (AuTMPyP(4))I and watersoluble zinc(I1) porphyrins [5, zinc meso-tetrakis(4-carbxypheny1)porphyrin (ZnTCPP); 6, zinc meso-tetrakis(4-sulfonate pheny1)porphyrin (ZnTSPP); 7, zinc meso-tetrakis(4-(trimethy1amino)phenyl)porphyrin (ZnTAPP)] were synthesized and purified according to the 1 i t e r a t ~ r e . l ~(See ~ ~ 'Chart ~ I.) Optical absorption spectra were recorded with a Shimadzu MPS-2000 spectrophotometer. Aggregation constants for dimerizations of the porphyrins were determined by absorption spectral changes
%
d a
300
400
600
500 Wavelength ( nm )
700
Figure 1. Absorption spectral changes of a mixed solution of AuTCPP, 1 (2 X lo4 mol dm-3), and ZnTCPP, 5 (2 X 10" mol dm-'), due to the heteroaggregation promoted by the addition of CH3COONa. A
Figure 2. A possible structure for the heterodimer composed of a gold porphyrin and a zinc porphyrin.
TABLE I: Gromnd-State Absorption Data for Gold Porphyrins, Zinc Porphyrins, and Their Heterodimers in Aqueolrp Solution Xlnm
no. 1 2
3 4 5 6
7 1-5 2-5 3-5
4-5 1-6
2 4 3-6
4-6 (11) Meech, S. R.;Hoff, A. J.; Weirsma, D. A. Chem. Phys. Lett. 1985, 121, 287-292. (12) (a) Boxer, S.G.; Mittendorf, T. R.; Lockhart, D. J. Chem. Phys. Lett. 1985,123,476-482.(b) Lockhart, D. J.; Boxer, S. G. Chem. Phys. Lett. 1988, 144, 243-250. (13)Boxer, S. G.;Goldstein, R. A.; Lockhart, D. J.; Mittendorf, T. R.; Takiff, L. J. Phys. Chem. 1989,93,8280-8294. (14) For example, in a study of the mutant of Rb. capsulatus in which the BCh12donor is replaced by a BCI-BPh heterodimer, several important results have been obtained where the initial step actually includes the complete CT state in the heterodimer: (a) Kirmaier, C.; Holten, D.; Bylina, E. J.; Youvan, D. C. Proc. Natl. Acad. Sci. U.S.A. 1988,85,7562-7566.(b) McDowell, L. M.; Kirmaier, C.; Holten, D. J. Phys. Chem. 1991, 95,3379-3383. (15) (a) Asahi, T.;Mataga, N. J. Phys. Chem. 1989,93,6575-6578.(b) Asahi, T.;Mataga, N. J . Phys. Chem. 1991, 95, 1956-1963. (16) Segawa, H.; Nishino, H.; Kamikawa, T.; Honda, K.; Shimidzu, T. Chem. Lett. 1989, 1917-1921. (17) (a) Kalyanasundaram, K.; Neumann-Spallart, M. J. Phys. Chem. 1982,86,5163-5169.(b) Neumann-Spallart, M.; Kalyanasundaram, K. Z. Naturforsch. 1981, 36B, 596-660. (18)Shimidzu, T.; Segawa, H.; Iyoda, T.; Honda, K. J. Chem. SOC., Faraday Trans. 2 1987,83,2191-2200.
0.5
1-7 2-7
porphyrin AuTCPP AuTSPP AuTMPyP(3) AuTMPyP(4) ZnTCPP ZnTSPP ZnTAPP AuTCPP-ZnTCPP AuTSPP-ZnTCPP AuTMPyP( 3)ZnTCPP AuTMPyP(4)ZnTCPP AuTCPP-ZnTSPP AuTSPP-ZnTSPP AuTMPyP(3)-ZnTSPP AuTMPyP(4)-ZnTSPP AuTCPP-ZnTAPP AuTSPP-ZnTAPP
Soret band 406 404 402 404 422 421 420 404,439 406,439 402,441
Q band A&-,,"-: 522 520 520, 554 522, 556 557,597 556, 595 555,593 529, 564, 606 9 524, 563, 606 9 521, 557, 606 9
404, 439
524, 560, 606
9
405,440 405,438 402, 436 403, 438 403, 430 (sh) 403,433
530, 561,602
7
526, 562,603 523, 559,603 526, 563,604 532, 558, 598 527, 560,600
8 8 9 5 7
Ah(,,, = [Xp0,, of porphyrin heterodimer] - [h,,,,,, of Zn porphyrin
monomer".
arising from the dimeri~ati0ns.l~A microcomputer-controlled picosecond laser photolysis system with a mode-locked Nd3+:YAG laser (fwhm = 24ps) was used for the measurement of the timeresolved transient absorption ~pectra.~The second harmonic generation (532 nm) pulse was used for the excitation of sample solutions. (19)(a) Pasternack, R. F.; Huber, P. R.; Boyd, P.;Engasser, G.; Francesconi, L.; Gibbs, E.; Fasella, P.; Venturo, G. c.;Hinds, L. de c. J . Am. Chem. SOC.1972, 94, 4511-4517. (b) Krishnamurthy, M.; Sutter, J. R.; Hambright, P. J . Chem. Soc., Chem. Commun. 1975, 13-14. (c) Hofstra, U.;Koehorst, R. B. M.; Schaafsma, T.J. Chem. Phys. Lett. 1986, 130, 555-559. (20)Miyasaka, H.; Masuhara, H.; Mataga, N. Loser Chem. 1983,I , 357.
The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 505
Letters
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"
J
-~
-98 300ps
-.-
2 -
-
-
10 -
looops
I
I
I
0.8
1.0
1.2
1.4
1.6
-AGOI eV F m 4. Energy gap dependence of the intradimer charge recombination rate constant (kCR)of the radical ion pair formed by the photoexcitation of the porphyrin heteroaggregate in aqueous solution.
Equilibrium constants (Klmo1-l dm3) of the heterodimerizations at p = 1.0 were quite high values [log K = 7.3 (H), 6.3 (H), 7.8 (1-7), 7.1 (2-5), 6.1 (2-6), 7.3 (2-7), 6.6 (3-5), 5.5 (3-6), 7.3 (H), and 6.4 (H)] compared to those of self-aggregation and oligomer formati~n.~'Therefore, the heterodimers can be investigated without any interference from the other complicated aggregations in this condition.28 CIP Forumlion by the Direct Excitationof the Metalloporphyria Heteroaggregntes. The fluorescence originating from the zinc porphyrin was strongly quenched by the aggregation with the gold porphyrin in aqueous solution. The fluorescence quenching was proved to be due to only static quenching, because the fluoresctnce yield decreased linearly with the heteroaggregation ratio, and the lifetime of the residual fluorescence did not change regardless of the concentration of the porphyrins. For the static quenching, one of the probable mechanisms is intradimer electron transfer. Figure 3 shows the picosecond time-resolved absorption spectra of AuTCPP-ZnTCPP heterodimer (1-5) in aqueous solution containing NaN03.28 At 26 ps after the pulse excitation, the transient absorption spectrum in the wavelength region longer than 450 nm is quite different from the excited state absorptions of the monomeric ZnTCPP (5) (S-Sand T-T absorption) and the but it is very close monomeric AuTCPP (1) (T-T ab~orption)?~ to the superposition of the absorption spectrum of a radical cation of and that of a radical anion of 1.18 The transient absorption spectrum implies the formation of a radical ion pair of the heterodimer. Additionally, the bleaching observed around 439 nm, (25) The wavelength shift of Q(o,o)band of a face-to-face porphyrin dimer relative to that of a corresponding monomeric porphyrin is strongly affected by the distance between two porphyrin components. A dimer with shorter linkages than that with longer ones exhibit a larger shift of the Q band as reported: Collman, J. P.; et al. J . Am. Chem. Soc. 1981, 103, 516. k i n e the wavelength shift is a result of an excitonic interaction and a direct electronic interaction between the porphyrin rings, the wavelength shift of the QItoSband of a porphyrin dimer should be also affected by the conformationa c ange between two components. Anyhow, if the distance and conformation between two components in the heterodimers are not so different, the shift values of Q(o,o,bands of a series of the heterodimers composed of similar porphyrin monomers should be almost constant. (26) Cationic gold porphyrins (3 and 4) and cationic zinc porphyrin (7) did not form the heterodimer. This is due to a strong electrostatic repulsion of meso substituents. (27) Aggregation constants of the self-aggregationsof the gold porphyrins are less than lo4 mol-' dm3 and those of the zinc porphyrins are less than mol-' dm3 (refs 16, 18, 19a, and 19b). (28) In low ionic strength ( p 0) solution, aggregation constants of the heterodimerization of the cationic porphyrins and the anionic porphyrins are quite high values [log K = 10.3 (1-7), 8.6 (2-7), 9.9 ( 3 5 ) , 7.8 (3-6), 11.0 (4-5), and 9.1 (&)I, but those of the anionic porphyrins are not so high 4.4 , (2-5) and 3.7 (26)], because of the values [log K = 4.4 (I+), 3.5 (Id) electrostatic repulsion of meso substituents as mentioned in ref 26. In the latter case, the presence of monomer interferes with the measurement of the transient absorption spectra of the heterodimer. Therefore, transient absorption spectra of the heterodimers were measured under high ionic strength condition ( p = 2) where all the aggregation constants of the hetcrodimerizations are high enough to form the heterodimers predominantly. In the present experiments, the kinetics of the intradimer electron transfer were independent of the ionic strength. (29) S-S absorption spectrum of the gold porphyrin was not measured because the lifetime of the S1 state was very short ( < I ps).
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506 The Journal of Physical Chemistry, Vol. 96, No. 2, 1992
which coincides with the characteristic ground-state absorption of the heterodimer (1-5),I6suggests that the radical ion pair was directly formed by the photoexcitation of the heterodimer (1-5) through an extremely fast process within a few picoseconds (reaction 1). In this case, the electron transfer in the heterodimer
hu
[ A u ~ ~ ~ P ] - [ Z ~ ~ *[Au~~~P'-]-[Z~~IP'+] P] (1)
occurs on both porphyrin rings, because the central metals in both porphyrins are inert for the redox reaction. During the electron transfer, the apparent charge of the gold porphyrin moiety changes from +1 to 0 whereas that of the zinc porphyrin moiety changes from 0 to +1. Therefore, the electron-transfer reaction is regarded as the pseudo charge shift reaction. The time profile of the transient absorption decay assigned to the charge recombination process is quite simple without any shift of the absorption maxima (Figure 3) and in accordance with the single-exponential component. Furthermore, the decay time constant of the radical absorption and the recovery time constant of the bleaching around 439 nm are in perfect agreement. These results suggest that the conversion from the CIP to solvent-separated radical ion pair (SSIP) or dissociated free radical (FR) is negligible. Since a dynamic quenching process cannot be observed as mentioned above, the formation of the SSIP by this route is also negligible. Consequently, the charge recombination rate of reaction 2 can
kcR
[ A u ~ ~ I P ' - ] - [ Z ~ ~ ~ P ' + ][ A u ~ ~ ' P ] - [ Z ~ ~ ~ P (2) ] be directly measured by the decay of the transient absorption. While almost identical transient absorption spectra assigned to the CIPs were obtained in other heterodimers (1-6,l-7,2-5,2-6, 2-7,3-6,4-5, and 4-6),30the charge recombination rate (ICCR) is strongly affected by the charge recombination energy (-AGO) of the CIP of the heterodimer as described below. Energy Gap Dependence of the Intradimer Charge Recombination Rate of the MetaUoporpbyrin Heteroaggregates. A series of the CIPs of the heterodimers with different -AGO are easily obtained by the photoexcitation of the various heterodimers of the gold porphyrins and the zinc porphyrins with different meso substituents. The -AGO values were estimated from the differences of oxidation potentials of the zinc porphyrins and reduction potentials of the gold p0rphyrins.2~9~~ In the CIPs, the overestimation (30)Absorption maxima (nm) of the transient absorption spectra of the heterodimers are as follows: 460,635(1-5); 460,635 (16); 455,680 (1-7); 460,630 (2-5); 460, 630 ( 2 6 ) ; 460, 660 (2-7); 460,630 (3-6); 460,630 (4-5); 460,630 (4-6). All the absorption spectrum were assigned to those of CIPs of the heterodimers. (31) In order to obtain the exact charge recombination energy (-AGO) of the heterodimer, we should take into account some other perturbation terms such as the charge-transfer stabilization term in the ground states. However, the estimation of such terms is very difficult and may cause confusion for the interpretation. Furthermore, it is considered that the perturbation terms are not so large and are almost kept constant in the series of heteroaggregates. Therefore, we used the -AGO data obtained directly from the difference of oxidation potentials of zinc porphyrins and reduction potentials of gold porphyrins.
Letters of the -AGO based on the Coulombic stabilization term of the radicals can be avoided, since the charge density is spreaded over the large A systems of the porphyrins and the charge recombination is also a pseudo charge shift type reaction. Figure 4 shows the energy gap dependence of the intradimer charge recombination rate (kCR)of the CIP of the porphyrin heteroaggregate in aqueous solution. Interestingly, the kCR decreases with increasing exothermicity of the charge recombination as if it is in the "inverted region", even for the small -AGO values of about 1.0 eV. This energy gap dependence seems difficult to interpret by traditional electron-transfer theories, because a very low solvent reorganization energy must be applied for a good fit in spite of the reaction in highly polar water. The observed results for the CIPs of the heteroaggregates can be represented approximately by the simple relation (3)
where a and 8 are constants independent of AGO. It is interesting to see that the energy gap dependence of eq 3 is qualitatively analogous to that of the radiationless transition probability in the so-called "weak coupling" limite3* In these strongly stacked systems, it may be better to interpret the charge recombination as internal conversion with Born-Oppenheimer breakdown. Although this relation seems to be difficult to interpret quantitatively by the radiationless transition theory because of the much milder slopes of the ICcR -AGO relation, it should be noted that the present result shows slightly steeper slope (0 = 5.5 eV-') compared to the previous ones.15 As pointed out in the previous literature,I5 a small deviation of a structure of a charge-transfer complex depending upon the strength of donor-acceptor interactions could lead to a small change in the difference of the potential minimum positions between a CIP and a ground state of the complex. In general, the change in the difference of the potential minimum is considered to cause the deviation of the slope of the kCR -AGO relation. In the present porphyrin heteroaggregate systems, the heterodimer are considered to be more rigidly held compared with the charge-transfercomplexes studied previo~sly,'~ because the conversion from the CIP to the SSIP or the FR is not observed. Therefore, one of the possible explanations for the steeper slope in the kCR N -AGO relation of the heterodimer can be offered by the rigidity of the toothed wheels structure of the heterodimer. Anyway, further experimental and theoretical investigations of the rigidity of the complexes is also required for a more strict explanation of the electron-transfer kinetics of the complexes.
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Acknowledgment. This work was partially supported by grants from the Ministry of Education, Science, and Culture of the Japanese Government. This work was also funded in part by Photo Science and Technology Foundation. (32)Englman, R.;Jortner, J. Mol. Phys. 1970, 18, 145-164.
