1956
J. Phys. Chem. 1991, 95, 1956-1963
energy conformations consist of stretches of all-trans chain attached to the pyrrolidine ring. The tetrahedral geometry of the C-C linkages in the fatty acid chain dictates that the only allowed orientations of a methylene chain segment relative to the all-trans chain axis are 0,60, and 90°.22 The orientations of the nitroxyl z axis of 5-(2’,2’)FASL, 3-(2’,4’)FASL, 4-(2’,4’)FASL, 5(2’,4’)FASL, and 8 4 3’,2’)FASL correspond to these values, suggesting that the diffusion axis is collinear with the axis of the chain joining the pyrrolidine ring to the carboxyl group for these fatty acids. With the exception of 5-(2’,2‘)FASL, this would require one or more gauche conformations in the chain section immediately following the ring, in order to preserve an approximate cylindrical symmetry consistent with bilayer chain packing and axial rotation (cf. Figure 8). In the case of 4-(2’,5’)FASL, the value of 0 = 40° implies that the rotational diffusion axis does not lie parallel to the trans-chain axis but is tilted at an angle of (21) (a) Program MMP2 Rev. 6.0, Molecular Design Ltd., San Leandro, CA. (b) Allinger, N. L.; Burkert, V. Molecular Mechanics; American Chemical Sccietv: Washington. 1982. (22) Moser, M.; Marsh,-D.; Meier, P.; Wassmer, K.-H.; Kothe, G. Biophys. J . 1989, 55, 1 1 1-123.
approximately 20’ to this axis, possibly as a result of the slightly noncylindrical shape of the molecule (cf. Figure 8). Conclusion The methods employed here have allowed the determination of the orientation the nitroxyl group for six different pyrrolidine-based fatty acid spin labels in fluid lipid model membranes, in a manner consistent with the segmental flexibility of the chain. The orientations so derived indicate that the ESR spectra of all these nitroxyl fatty acids will be sensitive, to different extents, to rotation about the chain axis, in addition to trans-gauche isomerization, when they are incorporated as spin label probes in biological membranes. This accounts for the previously demonstrated sensitivity of the ESR spectra to lipid-protein interactions and provides the data necessary for a detailed characterization of the chain motional modes that are inhibited on interaction with integral membrane proteins. Registry No. DMPC, 13699-48-4; 5-(2’,2’)FASL, 601 13-93-1; 3(2’,4’)FASL, 107643-05-0; 4-(2’,4’)FASL, 130905-30-5; 5-(2’,4’)FASL, 130905-31-6; 4-(2’,5’)FASL, 130905-32-7; 8-(3’,2’)FASL, 130905-33-8; 4-SASL, 35545-52-9; 5-SASL, 29545-48-0; 7-SASL, 4095 1-82-4; 8SASL, 35375-98-5; IO-SASL, 5061 3-98-4; cholesterol, 57-88-5.
Femtosecond-Picosecond Laser Photolysis Studies on the Dynamics of Excited Charge-Transfer Complexes: Aromatic Hydrocarbon-Acid Anhydride, -Tetracyanoethyiene, and -Tetracyanoquinodimethane Systems in Acetonitrile Solutions Tsuyoshi Asahi and Noboru Mataga* Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan (Received: June 13, 1990; In Final Form: August 27, 1990)
Formation processes of contact ion pairs (CIP) from the excited Franck-Condon (FC) state of charge-transfer (CT) complexes of aromatic hydrocarbons with acid anhydride as well as cyano compound acceptors in acetonitrile solution and charge recombination (CR) rates ( k g r ) of produced CIP states have been investigated by femtosecond and picosecond laser photolysis and time-resolved absorption spectral measurements covering a wide range of free energy apAGoi, between the ion pair and the ground state. It has been confirmed that the CIP formation becomes faster and k@ of the produced CIP increases with increase of the strengths of the electron donor (D) and acceptor (A) in the complex, Le., with decrease of the -AGOi, value. This peculiar energy gap dependence of kEkP, quite different from the bell-shaped one observed in the case of the solvent-separated ion pairs (SSIP) or loose ion pairs (LIP) formed by encounter between fluorescer and quencher in the fluorescence quenching reaction, has been interpreted by assuming the change of electronic and geometrical structures of CIP depending on the strengths of D and A.
Introduction It is generally believed’ that the rate of the photoinduced C S (charge separation) and that of the CR (charge recombination) of the produced C T (charge transfer) or geminate IP (ion pair) state are regulated by the magnitude of the electronic interaction responsible for the ET (electron transfer) between D (electron donor) and A (electron acceptor) groups, the FC (Franck-Condon) factor which is related to the energy gap for the ET reaction, the reorganization energies of D and A as well as the surrounding solvent, and various solute-solvent interactions including the solvent orientation dynamics2 and solvent-induced electronic (1) See for example: (a) Marcus, R. A.; Sutin, N. Blochlm. Bfophys.Acta
1% 811,265. (b) Matam, N.In Photochemlcal EnrrgV Cower,don;Norria, J. R., Meisel, D., Eds.; Elsevier: N e w York, 1988; 32. (2) (a) Sumi, H.;Marcus, R. A. J . Chem. Phys. l&, 84,4894. (b) R i p , 1.; Jortner, J. J . Chem. Phys. 1987,87,2090. (c) Sparpagllone, M.; Mukamel, S.J . Chem. Phys. 1988, 88, 3263.
structure change of strongly interacting D,A systems3in the course of ET or C T in polar solvent. On the other hand, experimental observations on the photoinduced CS between D and A molecules, and CR of produced CT or IP state in polar solutions, have been made in the following cascs:lb (i) CS at encounter between fluorescer and electron donating or accepting quencher in strongly polar solvent leading to thc formation of geminate IP which undzrgoes C R and dissociation into free ions (in this case, the electronic interaction between D and A responsible for ET may be relatively weak); (ii) intramolecular photoinduced CS and C R of the produced intra(3) (a) Been&H.; Weller, A. Chem. Phys. L e f t 1969.3.666. (b) Beens, H.; Weller, A. In Organic Molecular Photophysics; Birks. J. B.. Ed.; Wiley-lnterncience: London, 1975; Vol. 2. (c) Mataga, N.In The Exciplex; Gordon, M., Ware, W. R., Eds.; Academic: New York, 1975. (d) Mataga, N. In Molecular Inferacfions; Ratajczak, H.,Orville-Thomas, W. J., Eds.; Wilcy: New York, 1981; Vol. 2. (e) Kim,H. J.; Hyns, J. T. J. Phys. Chem. 1990, 94, 2736 and references therein.
0022-3654/91/2095- 1956$02.50/0 Q I99 1 American Chemical Society
Excited Charge-Transfer Complexes
The Journal of Physical Chemistry, Vol. 95, No. 5, 1991 1957
molecular CT or IP state in the donor-acceptor system combined by spacer or directly by a single bond (in this case, various strengths of the electronic interaction between D and A are possible); (iii) excitation of the ground-state C T complex and relaxation from the FC excited state leading to the formation of relaxed CT state and then geminate IP which undergoes CR and dissociation in strongly polar solvent. In such investigations as described above, the behavior of the transient IP or CT state formed by the photoinduced CS is of crucial importance for understanding the photochemical and photobiological reaction mechanisms, since it determines the fate of the successive reaction processes. In this respect, the energy gap dependence of the CR process of the transient I P in cases i and ii for the weak D-A interaction has been a subject of recent lively investigations.lb,e" For the CR of IP produced by ET at encounter between neutral fluorescer and quencher molecules in acetonitrile solutions, we have given for the first time the experimental confirmation of the theoretically predicted bell-shaped energy gap dependence including the normal, top, and inverted Before that, only the results for the inverted region were available. On the other hand, we have made a comparative study on the C R processes of IP produced by CS at encounter between fluorescer and quencher (case i) and that produced by excitation of the grcund-state CT complex (case iii) between the same pair and in the same solvent acetonitrile, results of which indicated that the CR rate in the latter was larger compared with the former in genera1.6s7.'2-13 In view of this result, we have made more systematic studies on the CR decay of CT or IP states formed by excitation of various CT complexes by directly observing the decay of those states with picosecond and femtosecond laser spectroscopy. I4-l7 In the case of some TCNB (1,2,4,5-tetracyanobenzene) (A)-methyl-substituted benzene (D) systems in acetonitrile, we have observed clearly the formation of CIP (contact IP or compact IP without intervening solvent between D+ and A-) from the excited CT state (FC state) and the double-exponential decay of the IP state in accordance with the scheme of eq 1, leading to the (excited C T complex)
- - CIP
kmdv
IkEk'
SSIP
kd*
As-
+ Ds+
IkSS'P CR
dissociated ion formation via SSIP (solvent-separated IP) or LIP (loose IP) with intervening polar solvent between D+ and A- with femtosecond and picosecond laser s p e c t r o ~ c o p y , ' ~where - ~ ~ kEf has been confirmed to be larger than We have conducted similar and more extensive studies on the aromatic hydrocarbon (D)-acid anhydride, -tetracyanoethylene (TCNE), and -tetracyanoquinoldimethane (TCNQ) (A) CT complexcs in polar solvents, a preliminary report of which was already p~b1ished.l~
(4) Wasielewski, M. R.; Niemczyk, M. P.; Svec, W. A,; Pewitt, E. B. J . Am. Chem. Soc. 1985, 107, 1080. ( 5 ) Ohno, T.; Yoshimura, A.; Mataga, N . J . Phys. Chem. 1986.90. 3295. ( 6 ) Mataga, N.; Kanda, Y.; Okada, T.J . Phys. Chem. 1986, 90, 3880. (7) Mataga, N . Acta Phys. Pol. 1987. A71. 767. (8) OhnorT.; Yoshimura. A.; Shioyama, H.; Mataga, N. J . Phys. Chem. 1987, 91, 4365. (9) Harrison, R. J.; Pearce, B.; Beddard, G. S.;Cowan, J. A.; Sanders, J. K. M. Chem. Phys. 1987, 116. 429. (10) Gould, 1. R.; Ege, D.; Mattes, S. L.; Farid, S.J . Am. Chem. Soc. 1987, 109, 3794. ( 1 1) Mataga, N.; Asahi, T.; Kanda, Y.; Okada, T.; Kakitani, T.Chem. Phvs. 1988. 127. 249. (12) Mataga.".; Shiovama. H.:Kanda. Y. J . Phvs. Chem. 1987. 91. 314. (13) Mataga, N.; Kanda, Y.; Asahi, T.; Miyasaka, H.; Okada, T.;Kakitani, T. Chem. Phys. 1988, 127. 239. (14) Asahi, T.;-Mataga, N . J . Phys. Chem. 1989, 93, 6575. ( 1 5) Miyasaka, H.; Ojima, S.; Mataga, N . J . Phys. Chem. 1989,93,3380. (16) Ojima, S.; Miyasaka, H.; Mataga, N . J . Phys. Chem. 1990,91, 5834. (17) Ojima, S.; Miyasaka, H.; Mataga, N. J . Phys. Chem. 1990,947534,
One of the most remarkable results in the studies of these complexes as well as the TCNB-methyl-substituted benzene complexes is that the dependeize of kEl on the free energy gap -AGoi (between IP and ground state)14J7is quite different from the be8-shaped energy gap dependence of the CR of SSIP formed by fluorescence quenching reaction at encounter between similar donor and acceptor molecules in polar solvent^,^^" indicating a different mechanism of CR in the strongly interacting IP formed by the CT complex excitation from that in the weakly interacting SSIP which can be interpreted by conventional ET theories. In the following, we demonstrate the results of our detailed investigations on this difference between the above two cases of the CR processes of IPS, which will be of crucial importance for the elucidation of the photochemical reaction mechanisms in solutions.
Experimental Section A microcomputer-controlled femtosecond laser photolysis system was used for the measurement of time-resolved transient absorption spectra in the 100-fs to 10-ps time r e g i ~ n . ~ ~ J ~ * I ~ Pyridine-1 dye laser was frequency-doubled, and either the fundamental (710 nm) pulse or SHG (355 nm) pulse was used for exciting the sample. The rest of the fundamental pulse was used for the generation of the white light probe pulse by focusing into D20. When necessary, the observed spectra were corrected for the chirping of the monitoring white light pulse. The pulse generation procedures and the method of spectral measurements in the case of rhodamine 6G dye laser (590 nm) were essentially the same as those employed in the case of the pyridine-] dye laser. A microcomputer-controlled picosecond laser photolysis system with a mode-locked Nd3+:YAG laser was used for the measurement of time-resolved transient absorption spectra in the 10-ps to nanosecond time region. The THG (355 nm) pulse, SHG (532 nm) pulse, and Raman scattering light pulse obtained by focusing the THG pulse into cyclohexane liquid (397 nm) were used for exciting sample solutions. The samples of pyrene (Py), perylene (Per), anthracene (An), chrysene (Chr), naphthalene (Naph), toluene (Tol), hexamethylbenzene (HMB), phthalic anhydride (PA), pyromellitic dianhydride (PMDA), TCNE, and TCNQ were the same as described before.I4 Acetonitrile was spectrograde and used without purification. The method of the preparation of solutions for the measurements was the same as described before.14 Results and Discussion In the present study, we examine the C T complexes of PA, PMDA, TCNE, and TCNQ acceptors with various aromatic hydrocarbon donors. 1 . Formation Process of CIP from the Excited State of C T Complex. In our previous studies on the TCNB-methyl-substituted benzene complexes with femtosecond-picosecond laser spectroscopy,l53l6 it has been demonstrated that, for the CIP formation from the excited electronic state of the CT complex, some structural change including intracomplex configuration and surrounding solvent is necessary even in acetonitrile solution. The rate of such a process in the excited CT complex seems to depend on the donor-acceptor geometries of the complex in the ground and excited CT state as well as the CIP state. On the other hand, the donor-acceptor geometries in the complex may be determined by the strengths and geometrical structures of the donor and acceptor. In the case of the TCNB complex with the relatively weak donor, toluene, in acetonitrile, it has been demonstrated that the rcorientation of the polar solvent in the course of the relaxation from the excited FC (Franck-Condon) state can induce CS to a considerable extent within I ps but not c~mpletely.l~*'~ We have confirmed that it takes ca. 20 ps for the CIP formation by further CS accompanied by some intracomplex structural change. However, the CIP formation in the excited state of the TCNB (18) Mataga, N.;. Miyasaka, H.; Asahi, T.; Ojima, S.;Okada, T. Ulrrojarr Phenomena VI; Springer-Verlag: Berlin, 1988; p 51 I .
1958 The Journal of Physical Chemistry, Vol. 95, No. 5, 1991 I
Asahi and Mataga
$$q 120 ps
15ps
Figure 1. ( A ) Time-resolved transient absorption spectra of the PMDA-toluene CT complex in acetonitrilesolution measured with femtosecond laser photolysis system (excited at 350 nm). [PMDA] = 0.1 M, [Toll = 0.3 M. (B)Time profile of PMDA- absorbance of PMDAtoluene IP 0 , observed value: -, simulation taking into account the exciting pulse width and assuming the instantaneous formation of the CIP immediately after excitation.
complex with HMB, a much stronger donor than toluene, takes place within 5-6 ps. Compared with TCNB, PMDA is a little stronger electron acceptor (with higher reduction potential), which will lead to faster CIP formation from the excited FC state. Figure 1A shows time-resolved transient absorption spectra of PMDA-toluene complex in acetonitrile measured with the femtosecond laser photolysis system. We can see clearly that the absorption band at 0.5-psdelay time is considerably broader and a little red-shifted compared with that at 15 ps. This result indicates that, as in the case of the TCNB complexes, it takes several picoseconds to tens of picoseconds for the formation of CIP from the excited state of the C T complex also in the case of the PMDA complex. The time profile of the absorbance at the peak position is indicated in Figure I B together with the simulation of the rise and decay curve taking into account the exciting pulse width and assuming the instantaneous formation of the CIP immediately after excitation. By comparing the simulation with the observed time profile, the rise time of the CIP state was estimated to be ca. 7 ps in addition to its lifetime of ca. 30 ps and dissociation yield of 0.05. Similar analysis on the results of PMDA-HMB complex in acetonitrile indicated a few picoseconds as the rise time of the CIP in addition to its lifetime of ca. 5.2 ps and zero dissociation yield. When we use stronger electron acceptors such as TCNE and TCNQ, the CIP formation from the excited FC state of the CT complex will take place very rapidly. In a preliminary report,I4 we showed some results of the femtosecond laser photolysis studies on the TCNE complexes. More detailed results of the measurements on these and related strong CT complexes will also be discussed later in this paper. Actually, the rise of the 1P state of these systems is very rapid. Moreover, the decay times of the CIP state of these strong C T complexes in acetonitrile solution are very short in general, 500 and 300 fs for the Py-TCNE and Per-TCNE complexes, re~pectively.'~ The observed time profiles of the transient absorbance of the CIP state can bc reproduced by convolution of the exciting femtosecond laser pulse and the decay curve of CJP with short decay time, neglecting the dissociation of CIP and without taking into account the finite rise time of CIP state. The above results indicate strongly that the extent of thc geometrical structure change in the course of the relaxation from the FC state to the C I P in the excited state of those strong C T complexes is very small. That is, the "ion pair" state formed immediately after excitation of these C T complexes is very close to the excited CT state of the complex itself. In accordance with this reasoning, the transient absorption spectrum of the "ion pair" formed by excitation of the Py-TCNE complex in acctonitrilc, for example, is much broader compared with the supcrposition of the absorption bands of respective ion^.'^.'^
time /ns Figure 2. (A) Picosecond time-resolved absorption spectra of the Py-PA complex excited at 397 nm in acetonitrile solution. [Py] = 2 X IO-* M, [PA] = 0.4 M. (B) (a) Time profile of Py+ absorbance at 445 nm of Py-PA 1P formed by exciting the CT complex in acetonitrile ( 0 ) . [Py] =2X M, [PA] = 0.4 M. (b) That of the geminate IP of the Py-PA system observed by exciting the acetonitrile solution at 355 nm (0). The decay curve is mainly due to those I P S formed by encounter between 'Py* and PA, and only a small part showing a fast decay in the early stage of the decay is due to the IP formed by excitation of the CT complex. The highest value of the absorbance is normalized to that of (a). [Py] = 2 X IO-' M, [PA] = 0.4 M. (C) (a) Time profile of An* absorbance around 710 nm of An-PA IP formed by exciting the CT complex at 397 nm in acetonitrile ( 0 ) . [An] = 2 X IO-' M, [PA] = 0.5 M. (b) That of the geminate IP of the An-PA system formed by encounter between IAn* produced by excitation at 355 nm and PA (0). The highest value of the absorbance is normalized to that of (a). [An] = 2 X lo-) M, [PA] = 0.5 M.
In view of the above results on the behaviors of CIP's obtained by exciting various C T complexes, it should be noted here that the character of the CIP state changes depending on the nature of the electron donor and acceptor. The electronic and geometrical structures of C I P will become closer to those of the excited CT state of the complex itself with increase of the strengths of the electron donor and acceptor (with decrease of the oxidation potential of D and increase of the reduction potential of A). Since the energy of the ET configuration becomes lower in the case of such strong D and A, the mixing (interaction) of the ET configuration with the ground configuration will become appreciable. As it will be discussed later in this paper, the above-described changes of the energy and the structure of the CIP state seem to profoundly affect the energy gap dependence of its C R rate. 2. Femtosecond-Picosecond Laser Spectroscopy of C R Decay and Dissociation of Transient IP. As discussed in the introduction, the CR decay and dissociation of IP's formed by excitation of some TCNB-methyl-substituted benzene complexes in acetonitrile solutions can be reproduced approximately by the reaction scheme of eq 1, In most of the systems examined in the present study, kEkp is predominant leading to negligible or very small dissociation yicld. Therefore, most of the observed results may be reproduced by the reaction scheme of eq 2, as it is actually confirmed in the following. (excited C T complex)
-. --. kdk
CIP ik$g
As-
+ Ds+
(2)
However, in the case of complexes between aromatic hydrocarbon and PA, which is the weakest one among acceptors used in the prcscnt invcstigation, a considerable amount of the dissociated ion radicals can be observed in acetonitrile solution. Therefore, wc givc a more or less detailed consideration on PA complexes at first. 2.A. PA-Aromatic Hydrocarbon Systems. The absorption bands in Figure 2A are due to the IP produced by exciting the Py-PA CT complex. The ion bands immediately after excitation arc slightly red-shifted and broader compared with those of free ions. Thc absorption band shape gradually changes in the course of thc relaxation of CIP, leading to the formation of free ions as
Excited Charge-Transfer Complexes
The Journal of Physical Chemistry, Vol. 95, No. 5, 1991
1959
delay t imp(ps )
0.5
450
X/nm Figure 3. Time-resolved transient absorption spectra of Py-PA IP observed by exciting the CT complex (the same as in Figure 2, A and B(a)).
500
The spectra are normalized around the peak position. indicated in Figure 3, where the absorbance is normalized around the peak and the spectrum at 500-ps delay time is fairly close to that of free ion. The decay curve b in Figure 2B is composed of a slow component with a lifetime of ca. 400 ps and a fast component which is due to the IP formed by excitation of the CT complex. Curve a shows much faster decay compared with curve b, which leads to a much smaller amount of the dissociated ions in the former case. The different behavior of the IP formed by excitation of the CT compltx from that due to the electron-transfer quenching of fluorescence at encounter for the same D,A pair in the same solvent strongly indicates that the structure of the IP including the surrounding solvent molecules is different depending on the mode of its formation. That is, CIP will be formed in case a in Figure 2B,C while SSIP seems to be formed in (b), and their structures are maintained at least during several hundreds of picoseconds even in acetonitrile solution. As we have discussed in the Introduction, we have already observed, in the case of some TCNB-methyl-substituted benzene complexes in acetonitrile, the ionic dissociation process which can be reproduced approximately by the reaction scheme of eq l,I5-l7 although results of other TCNB-aromatic hydrocarbon complexes can be well reproduced by the scheme of eq 2.I5-l7 Here, we examine the above results in Figure 2 on the basis of both reaction schemes. We assume that the SSIP in the scheme of eq 1 is the same as the one produced by the electron-transfer quenching of fluorescence at encounter for the same D,A pair. In this case, rate constants kwlvand kgk' are given by kolv
=
(91/92)Tl-1
(3)
k $ r = T ] - ' - k,,, = (1 - ( ~ I / @ ~ ) } T ~ - ' (4) where 9,is the ionic dissociation yield obtained by the CT complex excitation and a2is that from the SSIP formed by encounter in the fluorescence quenching reaction, that is, a2= k d l S , ( k ~ ~+' kdiss)-l = kdissT2 and @ I = ksolv(kgkp + ksolv)-1@2 = ksolvTI9.2. In the case of the Py-PA system, T~ was obtained to be 400 ps from the decay curve of SSIP formed by fluorescence quenching reaction. By subtracting the constant value due to the dissociated ions from the absorbance decay curve observed by exciting the C T complex, we have obtained a decay curve with the main component of fast decay and a very small tail of slow decay. Determination of the accurate decay time for the slow component was difficult owing to the rather large scatters of the observed values. Therefore, by assuming the two-component decay according to the scheme of eq 1 and taking the decay time of slow component to be 400 ps, we have obtained rl = 90 ps. We have = 0.16 and a2= 0.44 from the absorbance decay obtained curves. From these values, we have obtained kgk' = 7.1 X IO9 s-I and ksoIv= 4.0 X IO9 s-!. On the other hand, assuming approximately single-exponential decay of 1P according to the scheme of eq 2, T (=( ksk' + kdls)-l) was estimated to be 120 ps. By using this T value and = 0.16, we have obtained k$kP = 7.0 X IO9 s-I and kdiss = 1.3 X lo9 s-l, This kEkp is the same as that obtained on the basis of the scheme of eq I , and the kdlssvalue is rather close to the rate constant of
1I
t i me / ns
4
Figure 4. Simulations of the observed result in Figure 2B(a) on the basis of the reaction schemes of eqs 1 and 2: (---) with k g f = 7.1 X IO9 s-', kwJy= 3.7 X IO9 s-l; (-) with k z f = 7.0 X IO9 s-', kdua = 1.3 x IO9 s-l; (0)observed value.
ionic dissociation in acetonitrile solution obtained for many systems bef0re.l We have obtained similar rate constants also be examining the results for the An-PA system as follows: kc" I .3 X IO'O s-I and k,,,, = 3.1 X IO9 s-I (scheme of eq I ) , k!$'i 1.6 X 1Olo s-I and kd,,, = 1.0 X lo9 s-I (scheme of eq 2). In this case, k:! is a little larger than that for the Py-PA system, which seems to make more difficult the observation of the two-component decay of the IP. The above results suggest that it is difficult to determine which one of the two reaction schemes can better reproduce the observed results. Actually, as shown in Figure 4, the result in Figure 2B(a) obtained by excitation of the Py-PA CT complex can be reproduced equally well by both simulations based on the reaction schemes of eqs 1 and 2 and employing the respective rate constants obtained above. It seems to be generally believed that, when the C T complex is excited in strongly polar solution, the ionic dissociation takes place according to the scheme of eq 1, assuming the existence of SSIP as reaction intermediate. Actually, however, the existence of such an intermediate state in the course of ionic dissociation is not evident in the above examples of Py-PA and An-PA systems. Moreover, the results in Figure 3 indicate strongly that the structure of the Py-PA IP including the surrounding acetonitrile changes gradually from the CIP to SSIP or LIP through multiple intermediate states. Euen the dissociation of SSIP may be described as a process where the number of intervening solvent molecules between ions in the pair is gradually increasing. Although we have observed rather clearly the double-exponential decay of the IP suggesting the reaction scheme with SSIP as an intermediate in the case of some TCNB-methyl-substituted benzene complexes in acetonitrile, such examples are quite limited. Many other systems do not show such behavior, where also the above model without any definite SSIP intermediate state, that is, the existence of multiple kinds of I P S in the course of CR and dissociation, may be more probable. 2.B. PMDA-Aromatic Hydrocarbon Systems. We have examined PMDA complexes with Per, Py, Chr, and Naph. These PMDA complexes show faster CR decay compared with PA complexcs leading to the negligible ionic dissociation yield in acctonitrile solution except for the Naph complex which shows ionic dissociation with a yield of 0.03. As examples, time-resolved absorption spectra of complexes with Chr and Py are shown in Figure 5 , where the band with peak around 665 nm is due to PMDA- and that around 450 nm is due to Py+. The time profiles of the absorbance in these spectra and their simulations taking into account the exciting and probing pulse width are indicated in Figure 6. From these analyses, we have evaluated the lifetimes rCIP of the CIP's to be 15 ps (Chr complex) and IO ps (Py complex). S 5 ps. For Similarly, for the Per complex, we have obtained rCIp the Naph complex, we have evaluated rCIP= 25 ps by assuming the reaction scheme of eq 2. 2.C. TCNQ- and TCNE-Aromatic Hydrocarbon Systems. As we have discussed already in a preliminary report,14 the CIP's produccd by exciting the C T complexes of such aromatic hy1 ~ 1 3 9 1 7
-
1960 The Journal of Physical Chemistry, Vol. 95, No. 5, 1991
Asahi and Mataga
- 0.5 ps 0.2 5 ps
d
500
600
700 h/nm
I
500
I
I
600
I
I
700
hlnm
Figure 5. Time-resolved transient absorption spectra of the PMDA-Chr complex (A) and the PMDA-Py complex (B) in acetonitrile excited by the SHG pulse of a picosecond YAG laser. ( A ) [PMDA] = 0.1 M, M. [Chr] = 2 X IO-.' M. (B) [PMDA] = 0.1 M, [Py] = 1 X
600 Amm time/ ps Figure 7. (A) Time-resolved transient absorption spectra of the PyTCNQ CT complex in acetonitrile measured by the femtosecond laser M, [TCNQ] = photolysis system (excited at 710 nm). [Py] = 2 X 3 X 10-3 M. (B), (C) Time profiles of transient absorbance of the Py-TCNQ complex excited by 710-nm femtosecond laser pulse in acetonitrile: (-) simulation by taking into account the exciting and probing pulse width, (0)observed value; (B) Py+ band observed at 460 nm; (C) broad band observed at 620 nm.
L
0
bl
n
0
-1 I
O
' time/ 2
ps I
Figure 6. Time profiles of absorbance of CIP. (-) Simulation by taking
into consideration the exciting and probing pulse widths. (A) PMDAobChr system, (0)observed at 670 nm. (B) PMDA-Py system, (0) served at 452 nm, (A)observed at 670 nm. The shift of the time profile depending on the wavelength of observation is due to the wavelengthdependent distribution of the arrival time of probing white pulse at sample position. drocarbons as Py and Per with these strong electron acceptors in acetonitrile undergo ultrafast CR deactivation to the ground state. We show several examples of time-resolved absorption spectra and time profiles of the transient absorbance in Figures 7 and 8. The acetonitrile solution of the Py-TCNQ complex in the ground state shows a broad C T absorption around 700 nm. Time-resolved transient absorption spectra were measured by exciting the solution at 710 nm with the femtosecond pyridine-I dye laser photolysis systems as shown in Figure 7A. The absorption band around 450 nm can be assigned to the Py+ band in the CIP. The TCNQ- band appears around 850 nm. In addition to the Py+ band, the spectra in Figure 7A show broad absorption in the 500-650-nm region. The time profiles of the Py+ band at 460 nm and the broad band at 620 nm are indicated in Figure 7B. The results of a simulation of these time profiles show that the lifetime of the broad band is much shorter (7 = 500 fs) than that of Py+ band ( T = 2 ps). Although the nature of the excited state responsible for this broad band is not clear at the present stage of investigation, the excitation at 710 nm seems to populate not Gnly the CIP state but also a very short-lived excited state with different electronic structure from the CIP. We have also examined the Naph-TCNQ complex in acetonitrile, where we have observed clearly the TCNQ- band due to CIP around 850 nm, from which the l i f e h e of the CIP state has been estimated to be ca. 5 ps. In Figure 8, we show the transient absorption spectra of PyTCN E and Per-TCNE complexes immediately after excitation with femtosecond laser pulse at 710 nm in acetonitrile solution and time profiles of their transient absorbance. The ground-state
L
I
500
I
I
h/nm
600
I
Figure 8. Transient absorption spectra of Py-TCNE (A) and Per-TCNE (B) complexes in acetonitrile immediately after excitation at 710 nm by M, [TCNE] = 0.53 M (A); femtosecond laser pulse. [Py] = 2.0 X [Per] = 3 X M, [TCNE] = 1.2 M (B). In (A), the spectrum is a superposition of the Py+ band and TCNE- band. Time profiles of the transient absorbance for the Py-TCNE system (observed at 450 nm) (C) and Per-TCNE system (observed at 540 nm) (D), where (0)the observed value and (-) the simulation curves were calculated by taking into account the exciting and probing pulse width. from which the lifetime of CIP have been evaluated to be 500 fs (C) and 300 fs (D), respectively.
absorption spectrum of the Py-TCNE system shows the longest wavelength broad C T band around 650 nm and that of the Per-TCNE system is observed around 800 nm. As we have pointed out in section 1, the absorption spectra in Figure 8A,B are much broader compared with the spectra of free ionsI3as well as those of IP's of Py+-A- and Per+-A- systems with weaker A's. This result suggest strongly that the structure of these short-lived C l P s is rather close to that of the excited CT state of the respective complex itself. We have also examined time-resolved transient absorption spectra of the Naph-TCNE complex excited by a rhodamine 6G femtosecond laser pulse at 590 nm in acetonitrile solution. The longest wavelength CT absorption band of this complex is observed in the 500-600-nm region. We have observed a quite broad transient absorption spectrum in the 400-500-nm region and estimated the lifetime of the CIP state to be 1 ps by analyzing the time profile of the absorbance at 450 nm. In the above-described studies, we have examined mainly C T complexes of several polycyclic aromatic hydrocarbon donors with
The Journal of Physical Chemistry, Vol. 95, No. 5, 1991 1961
Excited Charge-Transfer Complexes TABLE I: Lifetime ( T ) , CR Rate Constant (G'), Ionic Dissociation Yield ($,), and Free Energy Cap between the Ion Pair and Ground State (-AGOlp) of CIP Produced by Excitation of (TComplexes of Polvcvclic Aromatic Hvdrocarbon with Acceptors
I
13b
I
':i
4 .'
Y
2 3 4 5 6
7 8 9
IO II 12
Py Per Nph Py Nph Chr Py Per Py An
TCNE TCNE TCNQ TCNQ PMDA PMDA PMDA PMDA PA PA
Per
PA
0.5 0.3 55 2 25 15
IO 55 120 60 60
2 X IOi2 3 X loi2 Z 2 X 10" 5 X 10" 3.8 X loio 6.7 X IOio 1 x 10" >2 X 10" 7.0 X lo9 1.6 X IOio 1.6 X IOio
0 0 0 0 0.03 0 0 0 0.16 0.08 0
0.91 0.56 1.30 0.96 2.04 1.85 1.70 1.38 2.46 2.35 2.1 1
acceptors PA, PMDA, TCNQ, and TCNE in acetonitrile solution. We have also examined PMDA complexes with benzene and various methyl-substituted benzenes in acetonitrilei4and also in several other polar solvents. Moreover, we have examined some complexes of benzene and methyl-substituted benzene donors with TCNQ and TCNE acceptors in acetonitrile. It should be noted here that, in general, the kEr of the methyl-substituted benzene-acceptor system is a little larger than that of the polycyclic aromatic hydrocarbon-acceptor system when compared at the same value of the free energy gap -AGOip between the ion pair and ground state. This may be ascribed to the stronger electronic interaction responsible for CR in the case of methyl-substituted benzene due to its smaller dimension compared to the polycyclic aromatic hydrocarbon. However, we shall concentrate here on the complexes of polycyclic aromatic hydrocarbon donors with various acceptors. Details of our studies on various benzene complexes with acceptors will be published e1se~here.I~ 3. Energy Gap Dependence of C R Rate of CIP Produced by C T Complex Excitation in Acetonitrile ;Solution. The above results of femtosecond-picosecond laser photolysis studies on the C R process of CIP's formed by exciting C T complexes of polycyclic aromatic hydrocarbons with various acceptors in acetonitrile solution are collected in Table I. In Table I, the -AGO. values have been evaluated by wellknown eq 5 with ( e 2 / t R y = 0.5 eV, which is frequently used for -AGOi, = E,,(D+/D) - Er4(A/A-) - ( e 2 / t R ) ( 5 ) SSIP in acetonitrile solutions. Since we are concerned here with CIP, the structure of which is different from that of SSIP, the validity of eq 5 for the evaluation of -AGOi, may,be very doubtful. Actually, in a recent theoretical work to explain the different energy gap dependence of the CS rate constant in the fluorescence quenching reaction from that of the CR of the produced IP, it was necessary to use different functional forms of the D+,Ainteraction potential for CIP and SSIP.*O Probably there are several different ways to estimate -AGOi, values for CIP and SSIP. Here, however, we examine an empirical method to estimate the energy gap, by using experimental results of the fluorescent C T complex of tetrachlorophthalic anhydride (TCPA) and HMB.2' By extrapolating the results of the solvent polarity effect on the observed fluorescence Stokes shift,2i we have estimated the wavenumber of the CT fluorescence band peak (ijkax)of the TCPA-HMB complex in acetonitrile where this complex is practically nonfluorescent. From this sL,, and the wavenumber of the CT absorption band peak (piax), we can estimate the sum (AS) of the FC destabilization energies in the excited and ground state by (nkax= AS (6) (19) Asahi, T.; Ohkohchi, M.; Mataga, N. Manuscript in preparation. (20) Kakitani, T.; Yoshimori, A.; Mataga, N. In ,Electron Transfer in Inorganic, Organic, and Biological Systems; Bolton, J., Mataga, N., McLendon, G., Eds.; Advances in Chemistry Series 228; American Chemical Society: Washington, DC, 1991; Chapter 4. (21) Czckalla, J.; Meyer, K.-0. 2.Phys. Chem. (Munich) 1961, 27, 184.
790
0''
-,lo
-
0
gt
010
'?
O.10
ro
1
0 , '
O
"\\
'9 I 1.o
I
2 .o
-AG?&V
I
3.0
Figure 9. Ener gap dependence of k?: given in Table I in comparison with that of k$ taken from our previous investigation." The number in the figure corresponds to that in Table I.
Assuming this AS value to be typical of the CIP state produced by exciting a relatively strong CT complex in acetonitrile, rough values of -AGOi, for the CIP state formed by exciting the nonfluorescent CT complexes examined in the present investigation may be given by -AGip
-
hc{%ax - (1 /2)AS]
(7)
The -AGOip values estimated by eq 7 are shifted about 0.2 eV to the higher energy side as a whole compared with the values obtained by eq 5 . One of the main purposes of the present study is the comparison of the energy gap dependence of the C R rate of IP (SSIP) formed by diffusion encounter in the fluorescence quenching reaction with that of the IP (CIP) formed by CT complex excitation for the similar D,A pairs in the same solvent acetonitrile. Since the -AGOipvalues seem to shift as a whole even if we employ a quite different method for estimating those values as described above, the functional form of the energy gap dependence of k:: may not seriously be affected by the method for evaluating -AGOi,. In Figure 9, the values of kEr given in Table I are plotted against -AGOi together with the values of kg" obtained by observing the d;namics of SSIP produced by CS at an encounter in the fluorescence quenching reaction," where -AGOip is evaluated with eq 5 in both cases. The most conspicuous feature in Figure 9 is the quite different energy gap dependence of kEr from the bell-shaped one of k;?'. Especially, in the case of such strong D-A systems as Per-TCNE, Py-TCNE, Per-PMDA, etc., their k:?' values are in the normal region of the bell-shaped energy gap dependence showing decrease with decrease of the -AGOi, value, while their k $ f values show the increase only, when the energy gap is decreased. For example, k:?' = 6.1 X IO8 and 2.6 X lo9 s-l while k : r = 3 X loi2and 2 X I O i 2 s-I for Per-TCNE (-AGOi, = 0.56 eV) and Py-TCNE (-AGOi, = 0.91 eV) systems, respectively. The observed results for CIP can be represented approximately by the relation
klF
-
CY
exp[-P)AGoipl]
(8)
where a and 0 are constants independent of AGOi This remarkable difference in the energy gap dependence of 6 R between the two kinds of IP may originate from the difference in the structures including D+ and A- configurations as well as solvation. However, it is difficult to give a reasonable interpretation for the relation of eq 8 covering such a wide energy gap range on the basis of the usual ET theories.Ia It is interesting to see that the energy gap dependence of eq 8 is qualitatively analogous to that of the radiationless transition probability in the so-called "weak coupling" limit.22 Nevertheless, it seems to be difficult to interpret quantitatively the relation of eq 8 covering such a wide energy gap range with a quite mild slope, (22) Englman, R.;Jortner, J. Mol. Phys. 1970, 18, 145.
1962 The Journal of Physical Chemistry, Vol. 95, No. 5, 1991
on the basis of such radiationless transition theory. 4. Interpretation of the Energy Gap Dependence of kEkP in Terms of Multiple IP States Model. In sections 1 and 2 of Results and Discussion, we have discussed the difference of the structure of CIP from that of SSIP and also the change of electronic and geometrical structures of C I P depending on the strengths of the electron donor and acceptor in the pair. That is, with increase of the strength of the donor and acceptor (with increase of the reduction potential of A and decrease of the oxidation potential of D), the formation of CIP from the excited FC state of the CT complex becomes faster and the absorption spectrum of the transient CIP state becomes much broader compared with that of free ions as well as that of the SSIP in acetonitrile solutions. This result indicates strongly that the extent of the electronic and geometrical structure change including the surrounding solvent in the course of the FC state CIP process in the excited state of strong CT complexes is small, and the structure of the CIP state may be very close to that of the excited CT state of the complex itself. These reasonings suggest that the stronger the complex, the closer the position of the potential minimum of the CIP state on the reaction coordinnte to that of the CT complex itself, which makes dgflcult the observation of normal region in the energy gap depencence of k$kPas suggested in Figure IOA. Moreover, the energy of the ET configuration becomes lower in the case of stronger D and A leading to the increase of the mixing of the ET configuration with the ground configuration, which might facilitate the ultrafast nonradiative CR deactivation of CIP. As discussed already in section 3, it is difficult to interpret quantitatively the energy gap dependence of kEkP covering wide energy gap range on the basis of the radiationless transition theory for the "weak coupling" limit. Nevertheless, the observed results of k $ r 2 10l2s"' for -AGOip 0.5-1.0 eV in the case of the strong D,A systems, Py-TCNE and Per-TCNE, may be reasonably well fitted to the prediction by such radiationless transition theory. On the other hand, the position of the potential minimum of the SSlP state on the reaction coordinate may be much different from that of the ground state of the complex. Therefore, for the strong D and A, i.e. for the small energy gap between the IP and ground state, k : f p is in the normal region as indicated in Figure IOB. Since CIP undergoes ultrafast CR deactivation in such strong D,A systems, it is not possible to observe the transformation, CIP SSIP, in such systems. However, it is possible to observe such transformation and also formation of free ions from SSIP as indicated in Figure IOC, in the case of weak complexes, owing to their longer lifetimes. 5. Concluding Remarks. By examining behaviors of excited CT complexes of aromatic hydrocarbon with PA, PMDA, TCNQ, and TCNE in acetonitrile solution with femtosecond-picosecond laser photolysis and timc-resolved absorption spectral measurements, some new aspects of the dynamics of transient ion pairs produced from the excited FC state of the complex have been elucidated. (a) We have observed directly the formation process of CIP from the excited CT complex and have demonstrated that the formation becomes faster for stronger D,A systems. This result means that the extent of the electronic and geometrical structural changes including the surrounding solvent configurations in the course of CIP formation from the FC excited state of the complex is smaller for the stronger D,A systems. In othcr words, thc electronic and geometrical structures of CIP including the solvation state change gradually depending on the strengths of D and A in the complex. (b) The structural changes described in (a) affect the position of the potential minimum of the CIP state on the reaction coordinate, and it becomes closer to that of the C T complex itself with increase of the strengths of D and A, which secms to result in the peculiar energy gap dependence of the CR dccay rate of the CIP state, that is, a monotonous (exponential) incrcasc of the CR rate with decrease of the free energy gap - A G O i , , betwccn the CJP and the ground state.
Asahi and Mataga
-
I
Reaction Coordinate
I
B
I
-
;1 I
Reaction Coordinate C
w
LI
-
I
Reaction Coordinate
Figure 10. Free energy curves for the ion pair states and the ground state (G.S.) of D,A systems against reaction coordinate. (A) Change of the position of the potential minimum of the CIP depending on the change of the -AGO,, value illustrating that the CR reaction of CIP is in the inverted region for all -AGO,, values. (B) Relation between the free energy curves between the ground state and the SSlP state corresponding to the small -AGO, value, where the position of the potential minimum of SSlP IS greatly s h e d against the minimum of the ground state, which brings the CR rate of SSlP in this case of small -AGoi, to the normal region. (C) The case of the relatively weak D,A system which can undergo the CIP SSlP transformation within the inverted region.
-
I n othcr words, the above peculiar energy gap dependence of thc CH ratc of CIP in the polar solvent can be interpreted only
by assuming the gradual change of the electronic and geometrical structure o f CIP or the existence of multiple ion pair states, depending on the strengths of D and A, and the solvation state. This modcl is closely connected with the original idea proposed many ycars ago by the prescnt a ~ t h o r ~ ~ *that * ' * *the ~ electronic and geometrical structures of the excited CT complex23and the c x c i ~ l c xcan ~ ~change gradually depending on the strength of D,A electronic interaction as wcll as the solvation. It should be notcd here that more detailed information con(23) Mataga, N . ; Murata, Y. J . Am. Chem. SOC.1969, 9 / , 3144. ( 2 4 ) Mataga. N . ; Okeda, T.; Yamamoto, N . Chem. Phys. Len. 1967, 1. 119.
1963
J . Phys. Chem. 1991, 95, 1963-1969
cerning the free energy surface of CIP and its intersection with ground-state surface as well as the mechanism of C R decay of the CIP might be obtained by examining the temperature effects on k$kP of various D,A systems, which are now going on in this laboratory. (c) We have also examined the dissociation processes of the IP’s into frcc ions for several weak D,A systems. In the case of the excitation of the C T complex with strong D,A systems, the CIP state undergoes very fast CR deactivation leading to practically zero dissociation yield. Our results of the direct observation of the dissociation process did not give any positive support for the conventional mechanism assuming SSIP as a definite intermediate state in the dissociation from CIP to free ions. From the
present results as well as our previous r e s ~ l t s ~ of ~ Jthe ’ direct observation on the ionic dissociation processes from the excited CT complexes, it can be concluded that the example of the photoinduced ionic dissociation of CT complexes by the two-step mechanism including single type SSIP, CIP SSIP free ions, is very limited, and actually, many systems seem to undergo gradual change of structure from the excited CT complex to CIP and then to free ions probably via several intermediate states of loose structure SSIP’s (LIP). +
-
Acknowledgment. N.M. acknowledges the support by a Grant-in-Aid (No. 62065006) from the Ministry of Education, Science and Culture, Japan.
Fourier Transform Fluorescence and Phosphorescence of Porphine in Rare Gas Matrices Juliusz G. Radziszewski,* Jacek Waluk,IP MiloIi NepraS,Ib and Josef Michl* Center for Structure and Reactivity, Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712-1 167 (Received: June 13, 1990: I n Final Form: September 17, 1990)
Matrix-isolated free-base porphine and its deuterated analogues show highly resolved intense fluorescence in neon, with evidence of strong b,, vibronic activity, and highly resolved intense phosphorescence in xenon, dominated by totally symmetric vibrations. Triplet lifetimes were determined from T-T absorption decay, which revealed a new band at 310 nm. It was concluded that the heavy-atom enhancement of the T, So radiative rate is dramatic. Both emissions exhibit site structure, and single-site spectra were obtained upon narrow-bandwidth excitation. The TI So emission site pattern is identical with that of SI So emission and S, +- So absorption and totally different from that of S2 So absorption, suggesting that T, and SI are both Q, and have a common orbital origin, contrary to the currently accepted assignment. The implications of the results for the mechanism of the photoinduced double proton transfer in free-base porphine are considered. All the results are compatible with the proposal that the isomerization occurs in TI in a stepwise fashion through the “cis” isomer. The advantages of interferometric recording of highly resolved visible and near-IR emission spectra are pointed out.
-
-
-
+-
Although the porphyrins have long been a fashionable object to study in organic spectroscopy, some important issues still remain to be clarified for the parent compound of the series, free-base porphine (1). For example, (i) no highly resolved phosphorescence has been reported so far, and the intensity of the reported lowresolution phosphorescence ~ p e c t r a was ~ . ~ too weak to permit an analysis of vibronic structure, (ii) the assignment of the symmetry of the lowest triplet state has only been made indirectly, on the basis of comparison of ESR spectra with the results of simple MO calculations$ and (iii) the mechanism of the photoinduced double proton transfer observed for porphine and its has not been clucidated. It is not clear whether the protons move stepwise or synchronously, whether this process occurs in the excited singlet or triplet state or both, or whether it is a hot ground-state reaction. In the present work, we report highly resolved fluorescence and phosphorescence of free-base porphine ( l-do)and its deuterated
CHART I
( I ) (a) Permanent address: Institute of Physical Chemistry, Polish Academy of Sciences, 01-224, Warsaw, Kasprzaka 44, Poland. (b) Permanent address: Research Institute of Organic Syntheses, 532 18 Pardubice-Rybitvi, Rybalkova 1361, Czechoslovakia. (2) Gouterman, M.; Khalil, G.-E. J . Mol. Spectrosc. 1974, 53, 88. (3) Tsvirko, M. P.; Solovyov, K. N.; Gradyushko, A. T.; Dvornikov, S. S. Opt. Spectrosc. (Engl. Transl.) 1975, 38, 400. (4) Van Dorp. W. G.; Soma, M.; Kooter, J. A.; van der Waals, J. H . Mol. Phys. 1974, 28, 155 I . ( 5 ) Zalesski, I. E.; Kotlo, V. N.; Sevchenko, A. N.; Solov’ev, K. N.; Shkirman, S. F. Sou. Phys.-Dokl. (Engl. Transl.) 1973, 17, 1183. (6) Solov’ev, K. N.; Zalesski, I. E.; Kotlo, V. N.; Shkirman, S. F. Phys. Letf. 1973, 17, 332. (7) Korotaev. 0. N.; Personov, R. 1. Opt. Spectrosc. (Engl. Transl.) 1972, 32. 479. (8) Volker. S . ; van der Waals, J. H. Mol. Phys. 1976, 32, 1703. (9) Voelkcr, S . ; Macfarlane, R. M.; Genack, A. 2.; Trommsdorff, H. P.; van der Waals. J H. J Chem Phys. 1977, 67, 1759.
1-d,
0022-3654/9l/2095-1963$02.50/0
Y b
1-do
1-d,
O
D
0
D
1-d,
deriv,atives l-dz, l-d4, l-ds, and l - d 1 2(Chart I) in noble gas matrices. In previous papers, we discussed the merits of using rigid rare gas solvents for the determination of transition polarizations in the IR’O and visible” absorption regions. This was accomplished by Fourier transform (FT) measurements on phot o ~ r i e n t e d lsamples. ~ ~ ~ ~ Presently, we use the FT technique to ~~
~~
(IO) Radziszewski, J. G.; Waluk, J.; Michl, J. Chem. Phys. 1989, 136, 165. ( 1 1 ) Radziszewski, J. G.; Waluk, J.; Michl, J. J . Mol. Spectrosc. 1990, 140, 373. In this paper, the graphics of Figures 6 and I O were interchanged in the publication process, and the graphics of Figure 4 are not shown at all (this figure contains a repeat of the graphics of Figure 9). The captions of all the figures refer correctly to the originally intended graphics.
0 1991 American Chemical Society
