J. Phys. Chem. 1989,93, 3383-3386
3383
Charge-Transfer Rates in Symmetric and Symmetry-Disturbed Derivatives of 9,9’-Bianthryi N. Mataga,* H. Yao, T. Okada, Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan
and W. Rettig* iwan-N-Stranski institute, Technische Universitat Berlin, Strasse des 17. Juni 1 12, D- 1000 Berlin 12, FRG (Received: December 21, 1988)
A comparative study on the solvation-induced intramolecular charge transfer (CT) in the excited state has been made for 9,9’-bianthryl (BA), lO-chloro-9,9’-bianthryl (BACI), and N-(9-anthryl)carbazole(C9A) by means of picosecond time-resolved absorption and fluorescence measurements in order to elucidate the mechanism of the solvation-inducedsymmetry breaking process in BA. It has been confirmed that the CT rate is larger for the symmetry-disturbed compounds, which indicates that the light absorption of those compounds projects the ground-state equilibrium distribution onto a nonzero gradient of the excited-state potential or the slightly presolvated state for those symmetry-disturbed compounds will facilitate the CT process.
Introduction Bacterial photosynthetic reaction centers (RC’s), the structure of which has recently been determined by X-ray analysis,’ consist of an array of porphyrin-derived chromophores held rigidly in a 2-fold symmetric arrangement by the protein backbone. The primary light-induced charge separation occurs in the center of this array, where two bacteriochlorophyll chromophores overlap weakly to form the so-called special pair (SP). The charge separation process is thus coupled with symmetry breaking, leading from a pair of symmetric neutral chromophores to a radical cation/anion 9,9’-Bianthryl (BA), which belongs to the large class of TICT-forming molecules$-5 undergoes a similar light-induced charge separation and symmetry breaking6-7and can be viewed as a model compound for photosynthesis. In particular, the comparison with symmetry-disturbed bianthryl derivatives should be able to shed new light on the role of the symmetric arrangement in RC‘s. From the existence of an active L and a passive M branch in the RC,’ and from recent time-resolved fluorescence measurements,* it can be concluded that, in spite of the symmetric chromophore arrangement, some structural effects, possibly linked with the protein backbone, lead to differing electron-transfer rates in the two possible directions starting from SP toward the ends of the chromophore array. The kinetics of charge transfer (CT) in TICT compounds has been the subject of several recent investigations. In particular, of diphenyl sulderivatives of dimethylamin~benzonitrile,~-~~ (1) Deisenhofer, J.; Epp, 0.;Miki, K.; Huber, R.; Michel, H. J . Mol. Biol. 1984, 180, 385. Michel, H.; Deisenhofer, J. Encycl. Plant Physiol. New Ser. 1986. 19. 371.
(2) Boxer, S. G.; Gottfried, D.; Lockhart, D.; Middendorf, T. J . Chem. Phys. 1987,86, 2439. (3) Lockhart, D. J.; Boxer, S. G. Biochemistry 1987, 26, 664. (4) Grabowski, Z. R.; Rotkiewicz, K.; Siemiarczuk, A.; Cowley, D. J.; Baumann, W. N o w . J. Chim. 1979, 3,443. (5) Rettig, W. Angew. Chem., Int. Ed. Engl. 1986, 25, 971. (6) Nakashima, N.; Murakawa, M.; Mataga, N. Bull. Chem. SOC.Jpn. 1976, 49, 854. (7) Rettig, W.; Zander, M. Ber. Bunsen-Ges. Phys. Chem. 1983,87, 1143. (8) Horber, J. K. H.; Gobel, W.; Ogrodnik, A.; Michel-Beyerle, M. E.; Knaoo. E. W. In Antennas and Reaction Centers of Photosvnthetic Bacteria Srruciure, Interactions and Dynamics; Michel-Beykrle, M. E., Ed.; Springer: Berlin. 1985:. D 292. r ~ (9)’Wang, Y.; McAuliffe, M. J.; Novak, F.; Eisenthal, K. B. J . Phys. Chem. 1981, 85, 3736. (IO) Wang, Y.; Eisenthal, K. B. J . Chem. Phys. 1982, 77, 6076. (1 1) Hicks, J.; Vandersall, M.; Babarogic, Z.; Eisenthal, K. B. Chem. Phys. Leu. 1985, 116, 18. (12) Hicks, J. M.; Vandersall, M.; Sitzmann, E. V.; Eisenthal, K. B. Chem. Phys. L e f f .1987, 135, 413. (13) Huppert, D.; Rand, S. D.; Rentzepis, P. M.; Barbara, P. F.; Stuve, W. S.; Grabowski, Z. R. J . Chem. Phys. 1981, 75, 5714. (14) Heisel, F.; Mieht, J. A. Chem. Phys. Lett. 1983, 100, 183. (15) Heisel, F.; Mieht, J. A. Chem. Phys. 1985, 98, 233. ~
~
0022-3654/89/2093-3383$01.50/0
fone,27-33and of anilinonaphthalenesulf~nates~~~~ have been measured, and the’rate constants of charge transfer kcT, Le., of formation of the “twisted intramolecular charge transfer” (TICT) state, have been compared to solvent longitudinal relaxation times 7L. While for some cases, kCT seems to be close to T ~ - for~ other cases much faster C T rates have been found.24-26.30 Moreover, km depends on the size and ground-state twist angle of the rotating group^.'^*^' The reason is understandable in terms of the necessary twisting motion leading toward the TICT state in these cases, in addition to the charge-transfer step and solvent relaxation. 9,9’-Bianthryl has a perpendicular configuration in the ground thus the twisting relaxation to reach TICT geometry
(16) Heisel, F.; Mieht, J. A.; Martinho, J. M. G. Chem. Phys. 1985, 98, 243. (17) Meech, S. R.; Phillips, D. Chem. Phys. Leu. 1985, 116, 262. (18) Meech, S. R.; Phillips, D. J . Chem. SOC.Faraday Trans. 2 1987,83, 1941. (19) Rettig, W. J . Phys. Chem. 1982, 86, 1970. (20) Rettig, W.; Werrnuth, G. J . Photochem. 1985, 28, 351. (21) Rettig, W.; Gleiter, R. J . Phys. Chem. 1985, 89, 4676. (22) Rettig, W.; Vogel, M.; Lippert, E.; Otto, H. Chem. Phys. 1986, 103, 381. (23) Lippert, E.; Rettig, W.; BonaEiE-Kouteckg, V.; Heisel, F.; Mieht, J. A. Adv. Chem. Phys. 1987,68, 1. (24) Su,S. G.; Simon, J. D. J. Chem. Phys. 1988,89, 908. (25) Su, S. G.; Simon, J. D. Proc. SPIE-Int. Soc. Opt. Eng. 1988,910, 155. (26) Su, S. G.; Simon, J. D. J. Phys. Chem. 1989, 93, 753. (27) Su, S. G.; Simon, J. D. J. Phys. Chem. 1986, 90, 6475. (28) Su, S. G.; Simon, J. D. Chem. Phys. Lett. 1986, 132, 345. (29) Simon, J. D.; Su,S. G. Proc. SPIE-Int. Soc. Opt. Eng. 1987, 742,96. (30) Simon, J. D.; Su,S. G. J. Chem. Phys. 1987,87, 7016. (31) Su, S. G.; Simon, J. D. J . Phys. Chem. 1987, 91, 2693. (32) Simon, J. D.; Su,S. G. J . Phys. Chem. 1988, 92, 2395. (33) Simon, J. D. Acc. Chem. Res. 1988, 21, 128. (34) Kosower, E. M. Acc. Chem. Res. 1982, 15, 259. (35) Huppert, D.; Kanety, H.; Kosower, E. M. Faraday Discuss. Chem. SOC.1982, 74, 161. (36) Kosower, E. M.; Huppert, D. Chem. Phys. Let?. 1983, 96, 433. (37) Kosower, E. M. J . Am. Chem. SOC.1985, 107, 1114. (38) Kosower, E. M.; Huppert, D. Annu. Rev. Phys. Chem. 1986,37, 127. (39) Huppert, D.; Ittah, V.; Kosower, E. M. Chem. Phys. Lett. 1988, 144, 15. (40) Huppert, D.; Ittah, V.; Masad, A.; Kosower, E. M. Chem. Phys. Lett. 1988, 150, 349. (41) Yamasaki, K.; Arita, K.; Kajimoto, 0.;Hara, K. Chem. Phys. Left. 1986, 123, 277. (42) Khundkar, L. R.; Zewail, A. H. J . Chem. Phys. 1986, 84, 1302.
0 1989 American Chemical Society
,
~
~
3384
Letters
The Journal of Physical Chemistry, Vol. 93, No. 9, 1989
a)
0 :A
I
CT
CT
- -
a symmetry coordinate solvent relaxation 400
350
450
550
500
hlnm
Figure 2. Corrected fluorescence spectra of BA (- - -), BACl in 1-pentanolat room temperature.
(-a
600
-),
and
C9A (-)
'
I
c
Ihv
-
solvent relaxation
Figure 1. Schematic representation of ground- and excited-state hypersurfaces for a solute undergoing solvent-induced symmetry breaking. (a) Symmetric compound: light absorption from the ground state leads to a region of the excited-statehypersurface with vanishing driving force and equal symmetry reduction rates toward the energy minima for left-hand and right-hand charge-transfer states. (b) Symmetry-disturbed compound: after light absorption, the gradient (driving force) in the excited state toward one of the two energy minima is nonzero and the corresponding charge-transfer direction is favored.
is not necessary. Then, given sufficient electronic coupling between the two anthracene rings for the charge-transfer step to be very fast, the overall charge-transfer process should be determined by the dynamics of solvent relaxation.43" In fact, recent picosecond measurements by Barbara et al.4s-48show that the kcr values for BA in a variety of aprotic polar solvents are quite similar to the rate constant of the solvent relaxation dynamics as measured by the decay time T~ of the solvent correlation function C ( t ) determined from time-dependent fluorescence red shifts, whereas the corresponding T~ values for these solvents are much shorter than 7s.
Anthon and Clarke,49on the other hand, showed that BA in alcohol solvents exhibited a complicated behavior which could be fitted using three rate constants. The fastest, 30-60 ps, was ascribed to C T because it involved major spectral changes. The second, about 200 ps in pentanol, went along with a strong diminuition of the overall spectral density, which is related to the spectrally integrated fluorescence intensity as given by the product of fluorescence rate constant kf and normalized spectral shape function.4s This time is very close to T~ (174 ps for pentanol at 25 0C29). Eventually, the equilibrated TICT state decays on a comparatively slow time scale of 20 000-30 000 ps in degassed solutions. The experiments reported in this letter are designed to shed new light onto these questions. Especially the use of time-resolved absorption spectra yields additional information because, unlike (43) Surni, H.; Marcus, R. A. J . Chem. Phys. 1986,84, 4894. (44) Nadler, W.; Marcus, R. A. J . Chem. Phys. 1987, 86, 3906. (45) Nagarajan, V.; Brearley, A. M.; Kang, T. J.; Barbara, P. F. J . Chem. Phys. 1987, 86, 3183. (46) Kahlow, M. A,; Kang, T. J.; Barbara, P. F. J . Phys. Chem. 1987, 91, 6452. (47) Barbara, P. F.; Jarzeba, W. Acc. Chem. Res. 1988, 21, 195. (48) Kang, T. J.; Kahlow, M. A,; Giser, D.; Swallen, S.; Nagarajan, V.; Jarzeba, W.; Barbara, P. F. J . Phys. Chem. 1988, 92, 6800. (49) Anthon, D. W.; Clarke, J. H. J . Phys. Chem. 1987, 91, 3530.
the TICT fluorescence, the transient absorption spectra of intraand intermolecular C T states of many systems do not show an appreciable polar solvent-induced spectral ~ h i f t , ~although * ~ ~ there are some exception^.^^ This is due to the fact that the lower and upper states relevant to the absorption spectra of those C T or TICT states have similar electronic structure with high polarity, while the ground state responsible for the fluorescence transition from these highly polar states has nearly nonpolar or only weakly polar electronic structure. A more detailed discussion of this problem with respect to BA will be found in a parallel transient absorption study of BA in the highly polar and viscous solvent glycerol triacetate at high pressures3 as well as in nonpolar and polar solvents at various temperature^.^^ The comparison of BA with symmetry-disturbed derivatives is shown to yield information on the influence of the symmetry breaking process on the rates. It can be expected that a slight asymmetry will shift ground and excited states in such a way that light excitation projects the ground-state equilibrium distribution onto a nonzero gradient of the excited-state potential. This is schematically shown in Figure 1. As a consequence, increased charge-transfer rates are expected for the asymmetric case. Experimental Section Picosecond time-resolved transient absorption spectra, fluorescence rise, and decay curves were measured by using a microcomputer-controlled picosecond laser photolysis system with a repetitive mode-locked Nd3+:YAG laser as the excitation source. The third harmonic was used for exciting the BA and its derivatives. The details of this laser photolysis system are given elsewhere.55 BA was the same sample as used beforeS6 The symmetry-disturbed compounds BACl and C9A were the same as used in a previous study.' Wako GR grade 1-pentanol and
0A
C9A BACl
1-propanol were dried over MgS04 and distilled under reduced
pressure. Corrected steady-state fluorescence spectra were measured on a Perkin-Elmer 650-60 spectrometer. (50) Mataga, N. Pure. Appl. Chem. 1984, 56, 1255. (51) Mataga, N.; Shioyama, H.; Kanda, Y .J . Phys. Chem. 1987,91, 314. (52) Okada, T.; Mataga, N.; Baumann, W.; Sierniarczuk, A. J . Phys. Chem. 1987, 91, 4490. (53) Luck, H.; Windsor, M. W.; Rettig, W. Manuscript in preparation. (54) Okada, T.; Yao, H.; Mataga, N. Manuscript in preparation. ( 5 5 ) Miyasaka, H.; Masuhara, H.; Mataga, N. Laser Chem. 1983, I , 357.
The Journal of Physical Chemistry, Vol. 93, No. 9, 1989 3385
Letters
1
I
a
b
lCC0s
- L
300Ds
i
1
g ,’
I
BA
Cl
BACI
n -pentanol
n -pentanol
, 1$
-t
Figure 3. Transient absorption spectra of BA (a) and BACl (b) in n-hexane and acetonitrile (lowest panels; equilibrium excited state in each solvent) and at various delay times in 1-pentanolat 23 OC (upper panels).
Results and Discussion Figure 2 shows the steady-state fluorescence spectra of the investigated compounds. The vibrational structure for BA and BACl on the blue side of the spectrum is due to the “locally excited (LE)” anthracene-type fluorescence which is emitted from an excited state in equilibrium with the TICT stateS6v7 The LE fluorescence and C T fluorescence bands overlap considerably, but for BaCI, the equilibrium is shifted toward the TICT state.’ Judging from the structureless blue edge of C9A, the excited-state equilibrium is shifted even further for this compound. Figure 3 shows the picosecond time-resolved absorption spectra for BA and BAC1. The lowest panels show the spectra in n-hexane and acetonitrile. The spectrum of BA in n-hexane is a little diffuse and slightly blue shifted compared with that of anthracene or 9-alkylanthracene in n-hexane, which indicates that the SI state is a little delocalized over two anthracene rings though it is nonpolar.s4 Similarly, the spectrum of BACl in n-hexane is a little broad and slightly blue shifted compared with that of 9-chloroanthracene in n-hexane. This result also indicates that the almost nonpolar SI state of BACl is a little delocalized over two rings in n-hexanees6 In acetonitrile, the observed spectra are not the same as the superposition of absorption bands of acceptor anion and donor cation radicals of the component groups indicating some admixture of the LE state. In solvents of intermediate polarity, the transient absorption spectra can be reproduced approximately as linear combination of the spectra in hexane and acetonitrile.S3~54~56 The same applies to the time evolution of the spectra in 1-pentanol, as shown in the upper panels of Figure 3. The time-dependent changes of the spectra converge to an equilibrium. The spectra for C9A are more complicated and will be discussed ~eparateIy.5~ From a kinetic analysis of these transient absorption spectra, approximate decay times T,(LE) of the LE state and rise times of the C T state r,(CT) have been estimated and are collected in Table I, together with literature values. Table I1 shows the results obtained by analyzing streak camera results of the fluorescence both in the red edge (fluorescence rise time T,(CT)) and in the ( 5 6 ) Mataga, N.; Yao, H.; Okada, T.; Rettig, W. To be published.
TABLE I: Decay Times (Td(LE)) of the Population of LE and Rise ) ) CT States Derived from Transient Absorption Times ( T ~ ( C Tof Measurements in 1-Pentanol at 23 OC BA BACl
170
180 140
140
174
“The corresponding value for the solvent longitudinal relaxation time TL
=
TDC,,/f,.29
TABLE I 1 Decay Times (rd(LE)) and Rise Times (r,(CT)) Derived from Fluorescence Decay (375 nm I X I 415 nm) and Rise Curves (X 2 520 nm)
compd
Td(LE)/ps
BA BACl C9A
200 150
TACT)IPS 170
110 85
blue edge (Td(LE)) of the fluorescence spectrum. In all cases, the C T rates, equated to rd-’(LE) and T;I(CT), are faster for the symmetry-disturbed compounds, by a factor ranging between 1.2 and 2. 11 is noteworthy that Td(LE) and T,(CT) nearly coincide and are similar for both the transient absorption and time-resolved emission measurements. This is a direct indication that the electron-transfer step corresponds to the second component measured by Anthon and Clarke (200 ps for BA in pentanol at room temperature, very close to the values determined here) and ~ also not to the much shorter lifetime ( T ~N 40-60 P S ) . ~This shows that the C T is governed, for BA, mainly by solvent dynamics, because the T ~ ( L Eand ) T,(CT) values are similar to the ~ 174 p ~ * ~ ) . longitudinal relaxation time for pentanol at 25 OC ( T = The similarity of kcT and solvent relaxation as measured by 7;’ is also suggested by the data of Barbara et al. for BA in glycerol tria~etate.~~-~~ The larger C T rates for the symmetry-disturbed compounds can be explained either by the reasoning outlined for Figure 1 or by the assumption that solvation rates 7;’ are solute-dependent. For the solutes possessing a ground-state dipole moment like BACl and C9A, the solvent molecules in the first shell are already somewhat ordered toward the final arrangement around the re-
3386
J . Phys. Chem. 1989, 93, 3386-3392
laxed TICT state leading to increased solvation rates. A similar acceleration of the C T rate in the TICT formation process is found In this case, the for pretwisted dialkylaminobenzonitriles.20~21 acceleration is due mainly to the preexponential factor and not to the activation energy. The C T in BA and derivatives helps in understanding the primary charge separation in photosynthesis. The time-resolved measurements on reaction centers of photosynthetic bacteria show that ET occurs preferentially to one side (the L branch).8 Thus, nature has built in a similar symmetry-disturbing bias as occurs for BACl and C9A. The present experiments reveal that such a bias can accelerate charge-transfer rate constants. From an evolutionary aspect, it has been assumedl that earlier photosynthetic systems possessed a symmetry-undisturbed R C and that
the invention of a slightly different s u r r ~ u n d i n grepresents ~~,~~ a step forward on the ladder of evolution. This can thus be understood in terms of accelerated charge transfer. Acknowledgment. W.R. thanks the Deutsche Forschungsgemeinschaft for support through a Heisenberg fellowship. This work is part of Project No. 05 3 14 FA I5 of the Bundesministerium fur Forschung und Technologie. N.M. acknowledges the support by a grant-in-aid (6265006) from the Japanese Ministry of Education, Science and Culture. (57) Michel, H.; Deisenhofer, J. Chem. Scr. 1987, 278, 173. (58) Michel, H.; Deisenhofer, J. In Progress in Photosynthesis Research; Biggins, J., Ed.; Martinus Nijhoff Dordrecht, 1987; Vol. 1, p 1.4.353.
FEATURE ARTICLE Recent Developments in the Local Mode Theory of Overtone Spectra Lauri Halonen Department of Physical Chemistry, University of Helsinki, Meritullinkatu 1 C, SF-001 70 Helsinki, Finland (Received: November 14, 1988)
An extension of a simple local mode model for stretching vibrations of polyatomic molecules to include bending degrees of freedom is discussed. In this model kinetic and potential energy operators expressed in terms of curvilinear internal coordinates are expanded and operators describing local modes and Fermi resonances between bending and stretching vibrations are retained in the final Hamiltonian. An excellent agreement with previous anharmonic force field calculations can be obtained in the case of well-bent triatomic molecules. The rotational energy level structure of stretching vibrational states in small symmetrical molecules is discussed in the context of localized vibrations. Simple relations can be found to exist between different vibration-rotation parameters in the local mode limit.
Introduction The vibrational and rotational energy level structure of highly excited vibrational states in polyatomic molecules is of current theoretical and experimental interest. The intensive experimental work in this area is understandable in the light of rapid developments in interferometric and various laser techniques that have made it possible to probe excited The aim of the theoretical work is in the first place to construct preferably simple and physically clear models to describe these states. This is important when one wants to understand, for example, problems related to intramolecular energy transfer and multiphoton absorption pathways. The stretching vibrations are particularly relevant in this context because a bond must break in a chemical reaction. The customary way to model vibrational energy level structures of polyatomic molecules is to express the vibrational term values as a power series in normal mode vibrational quantum numbers (1) Long, M. E.; Swofford, R. L.; Albrecht, A. C. Science (Washington, D.C.)1976, 191, 183.
(2) Scherer, G. J.; Lehmann, K. K.; Klemperer, W. J . Chem. Phys. 1983, 78, 2817; Ibid. 1984, 81, 5319. (3) Wong, J. S.; Green, W. H.; Lawrence, W. D.; Moore, C. B. J . Chem. Phys. 1987, 86, 5994. (4) Douketis, C.; Anex, D.; Ewing, G.; Reilly, J. P. J . Phys. Chem. 1985, 89,4173. Page, R.H.; Shen, Y. R.; Lee, Y.T. J . Chem. Phys. 1988,88.4621. (5) Coy, S. L.; Lehmann, K. K. J . Chem. Phys. 1986, 84, 5239
0022-3654/89/2093-3386$01.50/0
involving both harmonic and anharmonic terms. In the case of stretching vibrations only, in bent XY2 molecules the vibrational term values are expressed as6
where w, is the harmonic wavenumber of the rth mode, the x constants describe the anharmonicity, and v I and v3 are vibrational quantum numbers for the symmetric and antisymmetric stretching vibrations, respectively. However, it has turned out that in H2X and D2X molecules (X = 0, S, or Se) this traditional approach gives a poor description of the energy levels due to a strong 2,2 resonance (Darling-Dennison resonance) between the normal modes.7 In water this resonance couples, for example, overtone and combination levels 2u1 with 2ujr 3u1 with u1 2u3, and 2ul + uj with 3u3, where u l and u3 are symmetric and antisymmetric stretching fundamentals, respectively. Although this modification of the standard treatment produces accurate results, this perspective does not provide the best possible physical picture because the resonance effects are so profound here that the resulting eigenvalues possess no obvious connection to the zereorder normal mode model. On the other hand, there is a beautiful alternative
+
(6) Herzberg, G. Infrared and Raman Spectra;Van Nostrand: New York, 1945.
(7) Darling, B. T.; Dennison, D. M. Phys. Reu. 1940, 57, 128
0 1989 American Chemical Society
