J . Phys. Chem. 1990, 94, 1443-1447 turn, modulates the relative energies of VOUroP charge transfer and d-d states. On the basis of the transient Raman data presented in this study and the theoretical work of Zerner et al.,'* the photochemically generated bottleneck state of the monomer is the 2E,, the 2Aluror the 2TI-4Tlthermal equilibrium. The 2Alu state, however, is favored by the Raman data of this study.
1443
Acknowledgment. This work was performed at the University of New Mexico and supported by the NIH (GM33330) and the Associated Western Universities, and at Sandia National Laboratories supported by US.Department of Energy Contract DEAC04-76DP-00789 and Gas Research Institute Contract 508260-0767.
Femtosecond-Picosecond Laser Photolysis Studies on the Photoinduced Charge Separation and Charge Recombination of a Produced Ion Pair State of Some Typical Intramolecular Exciplex Compounds in Alkanenitrlle Solvents Noboru Mataga,* Shinya Nishikawa, Tsuyoshi Asahi, and Tadashi Okada Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan (Received: May 30, 1989)
Femtosecond and picosecond laser photolysis and time-resolved absorption spectral studies have been made to observe directly the photoinduced charge separation (CS) and charge recombination (CR) of a produced CS state of aromatic amine (D) and aromatic hydrocarbon (A) combined systems: P-(CH,)~N-P~-(CH,),,-( 1-pyrenyl) (P,,,n = 1, 2, 3) and p-(CH3),Ns the photoinduced CS of Ph-(CH2),,-(9-anthryl) (A,,, n = 0, 1, 2, 3), in alkanenitrile solvents. The time constants 7 ~ of these systems are considerably longer than the solvent dielectric relaxation time 7L as well as the solvation time 7s (determined from the time-dependent Stokes shift of fluorescent probe) except that TCS of AI is close to 7 s . In the case of A,, where D and A groups are more strongly interacting than in A,, the photoinduced CS is much slower than that of A,, which cannot be interpreted simply by usual electron-transfer theories. The time constants TCR of the CR decay of the produced CS state have been confirmed to be more than 2-3 orders of magnitude longer than 7csand to become shorter with increase of chain number n contrary to the case of 7 ~ s .These results are discussed on the basis of the solvent dynamics and/or solvation, the magnitude of the D-A electronic interaction,structural rearrangements,and the energy gap dependence of electron transfer.
Introduction It is believed in general that the rate of the photoinduced C S and that of the CR of the produced ion pair (IP) state are regulated by the magnitude of the electronic interaction responsible for the electron transfer (ET) between D and A groups, the Franck-Condon (FC) factor which is related to the energy gap for the ET reaction, and the reorganization energies of D and A as well as the surrounding solvent and solutesolvent interactions and the solvent orientation dynamics in the course of ET in polar solvent. I When the electronic interaction between D and A is very weak, the ET process is considered to be nonadiabatic (a). When the interaction becomes fairly strong, the reaction will become adiabatic (b). If the electronic interaction becomes sufficiently strong in case b and the energy gap relations are also favorable, the ET process will become barrierless (c). In such a case, it is believed that the ET process is governed mainly by the orientational motions of polar solvent molecules or polar groups in the environment surrounding D and A, and the longitudinal dielectric relaxation time 7L will be important as a factor controlling the ET rate.* In a limit of strong interaction between D and A groups combined by rigid spacer or single bond, its excited singlet state can be regarded as a very polar single molecule and we can observe a large fluorescence Stokes shift due to the solvation in polar solvents (d), for which the first theoretical formula was given by ( I ) (a) Marcus, R. A. Annu. Reu. Phys. Chem. 1964, 15, 155. (b) Marcus, R. A.; Sutin, N. Eiochim. Eiophys. Acra 1985,811, 265. (c) Mataga, N. In Photochemical Energy Conuersion; Norris, J. R.,Meisel, D. Eds.; Elsevier: New York, 1988; p 32. (d) Mataga, N. Pure Appl. Chem. 1984.56, 1255. (2) (a) Sumi, H.; Marcus, R. J . Chem. Phys. 1986,84,4894. (b) Rips, 1.; Jortner, J. J . Chem. Phys. 1987,87, 2090. (c) Sparpaglione, M.; Mukamel, S.J. Chem. Phys. 1988, 88, 3263. (d) Rips, 1.; Klafter, J.; Jortner, J. In Photochemical Energy Conuersion; Norris, J. R., Meisel, D., Eds.; Elsevier: New York, 1988; p I .
one of the present authors3 and Lippert4 and has been extended recently by Bagchi et aL5 and others to take into account its dynamical aspects. For the elucidation of the above mechanisms, especially the interactions of D and A with polar solvent including its dynamical effects on the E T process, which are believed to be controlling the ET reaction, systematic femtosecond-picosecond laser photolysis studies on various combined D,A systems with different degrees of electronic interaction between them are of crucial importance. However, results of such experimental investigations on the D,A combined systems which seem to be appropriate for such purpose are very few. The discrimination among the above cases a-d does not seem to be very clear in some systems examined until now'.6 Although these problems have been investigated mainly by means of time-resolved fluorescence measurements, time-resolved transient absorption spectral measurements are also very important for the elucidation of the ET mechanisms. The latter method gives direct information on the electronic structures of the system undergoing ET, which is extremely helpful for discriminating various cases of ET mechanisms. From the above viewpoints, we have examined the following systems with different degrees of electronic interactions between combined D,A groups by femtosecond and picosecond laser photolysis and time-resolved transient absorption spectral mea(P,,,n = l , 2, 3) surements: p-(CH3),N-Ph-(CH2),-(1-pyrenyl) and p-(CH3)2N-Ph-(CH2),,-(9-anthryl) (A,,, n = 1, 2, 3) in alkanenitrile solvents. Some results of picosecond laser photolysis (3) (a) Mataga, N.; Kaifu, Y.; Koizumi, M. Bull. Chem. Soc. Jpn. 1955, 28. 690. (b) [bid. 1956. 29. 465. (4) (a)'Gppert, E. Z.'Naiurforsch. 1955, 100, 541. (b) Eer. Bunsen-Ges. Phys. Chem. 1957, 61, 962. ( 5 ) Bagchi, B.; Oxtoby, D. W.; Fleming, G . Chem. Phys. 1984, 86, 257. (6) Kosower, E. M.; Huppert, D. Annu. Reu. Phys. Chem. 1986,37, 127.
0022-3654/9Q/2Q94-1443$02.50~Q 0 1990 American Chemical Society
1444
.~
The Journal of Physical Chemistry, Vol. 94, No. 4, 1990
-
.
..
~~
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Mataga et al.
I:?
a-
1
, .
!?I 5 3 0
t ,
l a 3
I
,
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603
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Figure 1. Time-resolved transient absorption spectra of P, (a), P2(b), and P3 (c) in ACN measured with the femtosecond laser photolysis system. [P,] = (5-10) X M.
(7) (a) Masaki, S.; Okada, T.; Mataga, N.; Sakata, Y . ;Misumi, S . Bull. Cfiem.SOC.Jpn. 1976, 49, 1277. (b) Oakda, T.; Fujita. T.; Mataga, N. Z . Pfiys. Cfiem.N . F.1976, I01,57. ( c ) Okada, T.; Mataga, N.;Baumann, W.; Siemiarczuk, A. J . Phys. Cfiem. 1987, 91, 4490. (8) Miyasaka, H.;Ojima, S.; Mataga, N. J . Pfiys. Cfiem. 1989.93, 3380. (9) Mataga, N.; Miyasaka, H.; Asahi, T.; Ojima, S.; Okada, T. (Iltrafast Phenomena Vk Springer Verlag: Berlin, 1988; p 5 1 I . (IO) Miyasaka, H.;Masuhara, H.; Mataga, N. Laser Cfiem. 1983, I , 357. ( 1 1) (a) Mataga, N.; Migita, M.; Nishimura, T.J. Mol. Srruct. 1978, 47, 199. (b) Okada, T.; Migita, M.; Mataga, N.; Sakata, Y . ;Misumi, S. J . Am. Cfiem.Soc. 1981,103, 4715. ( c ) Migita, M.; Okada, T.; Mataga, N.; Sakata, Y.; Misumi, S.;Nakashima, N.; Yoshihara, K. Bull. Cfiem.SOC.Jpn. 1981, 54,3304. (d) Nakatani, K.; Okada, T.; De Schryver, F. C.; van der Auweraer, M. Chem. Phys. Lerr. 1988, 145, 81.
E I C
$ I C
r-
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~~
(a)
studies on these P, and A,, in viscous alcohol solutions were published already.ld$" Transient absorption spectra of these systems in the SI state show the simple CS from the state localized in the pyrenyl or anthryl part to the intramolecular IP state and the C R deactivation to the ground state. We have examined by the same method also A. where the 9-anthryl ( A ) and N,N-dimethylanilino (D) groups are directly combined by a single bond and, accordingly, the electronic interaction between two groups may be much stronger' compared with the case of the systems with intervening methylene chains. In the following, we give results of investigations on these D-A combined systems and discuss them on the basis of the models a-d given above. We discuss the results of photoinduced C S in the P, ( n = I , 2, 3) and A,, ( n = 1, 2, 3) systems by comparing with those of A. and related systems and also with those of some C T complexes which consist of more strongly interacting D,A pairsS8
system was used for the measurement of time-resolved absorption spectra in subpicosecond to 10 ps regions.*q9 Pyridine-1 dye laser (710 nm) pulse was frequency doubled (355 nm) and 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 D20.8*9When it is necessary, the observed spectra were corrected for the chirping of the monitoring white light pulse. A microcomputer-controlled picosecond laser photolysis system with a mode-locked Nd3+:YAG laserlo was used for the measurement of time-resolved spectra in 10 ps to nanosecond regions. THG (355 nm) was used for exciting the sample. P, ( n = 1, 2 , 3) and A, (n = 0, 1,2, 3) were the same samples as used before" and their purities were checked by absorption and fluorescence spectral measurements. Spectrograde acetonitrile (ACN) was used as received. Butyronitrile (BuCN) (Tokyo Kasei) was dried by contacting with molecular sieves and over CaH,, and distilled several times. Hexanenitrile (HexCN) (Tokyo
:::
Figure 2. Time-resolved transient absorption spectra of A, (a), A2 (b), and A, (c) in BuCN measured with the femtosecond laser photolysis system. [A,] = (5-10) X lo4 M. E I.
Experimental Section A microcomputer-controlled femtosecond laser photolysis
1
L
I
I
I
0
I
I
5
10
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tips
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Figure 3. Absorbance rise curves of A, and A2 observed at 480 nm in ACN. [A,,] = (5-10) X lo4 M.
Kasei) was distilled several times before use. Sample solutions for the measurements were 10-4-10-3 M depending on the path length of optical cell (10 mm or 2 mm) and deaerated by freeze-pump-thaw cycles. Results and Discussion 1 . Femtosecond-Picosecond Time-Resolved Absorption Spectra of P, and A,, in Alkanenitrile Solutions. In Figure 1, time-resolved absorption spectra of P, in ACN measured with the femtosecond laser photolysis system are indicated. We can see the rapid rise of the characteristic sharp absorption band at 500 nm due to the intramolecular I P state. We can recognize also the rapid decay of absorbance around 470 nm which can be assigned to the S,, S, transition. Very similar time-resolved absorption spectra of P, have been observed also in BuCN and HexCN solutions. In these cases, however, the rise of the sharp absorption a t 500 nm and the decay of absorbance at 470 nm become a little slower compared with ACN solution. Time-resolved absorption spectra of A,, in the BuCN solution measured with the femtosecond laser photolysis system are shown in Figure 2 . These compounds show the rapid rise of a characteristic absorption band around 480 nm which is ascribed to the DMA (N,N-dimethylaniline) cation of the intramolecular IP state of A,. A characteristic absorption band at 590 nm due to the S, SI transition localized in the anthracene part can be observed also at 4 ps in Figure 2b (A2) and 2c (A3). At 0-2 ps, it is difficult to observe this band due to the wavelength-dependent arrival time of the monitoring light at the sample position and,
-
-
Charge Separation and Recombination in Alkanenitrile TABLE I: Rise Times of the Photoinduced IP State ( T ~ and ) CR Decay Times of the Intramolecular IP State ( T ~ of) P, and A, in Alkanenitrile Solutions 7CdnS
TCS/ PS
ACN
P, P2
P, A, A2 A3
1.7
6.1 11
0.65 2.1 2.7
BuCN 2.5 7.1 25 1.0 4-5 10
HexCN 4.5 15 1.4 6.2
ACN 7.0 3.3 1.1 4.0 1.0
0.7
BuCN 11 10 4.1 7.1 3.2 5.2
HexCN 11
8 .O
in the case of A, (Figure 2a), the photoinduced intramolecular C S is already completed at 4 ps, leading to the absence of the 590-nm band. We have confirmed in the case of A3 that the decay time of the 590-nm band is in satisfactory agreement with the rise time of the DMA cation band at 480 nm. In the HexCN solution, the photoinduced CS rate is rather close to and slightly slower than that of the BuCN solution. In the ACN solution, the photoinduced C S is much faster compared to the BuCN solution and the rise of the DMA cation band at 480 nm is accomplished within several picoseconds. The absorbance rise curves of A, and A2 observed at 480 nm in ACN are indicated in Figure 3, which converge respectively to constant values within several picoseconds and do not show any decrease. The rise curve of A, is similar to that of A2 except that the rise process is a little slower. Similar results have been obtained also in the cases of A, in BuCN and HexCN, and P, in these alkanenitrile solutions. The fact that no decrease of the IP absorbance is observed up to some 10 ps can be ascribed to the much slower CR decay of the IP state compared with the photoinduced C S process, as is discussed below. The rise curves of the IP state absorbance can be expressed approximately by an exponential function, from which the rise times of the IP state ( T ~ of) P, and A, in these alkanenitrile solvents have been obtained as indicated in Table I. In both P, and A,, systems, TCS becomes longer with increase of n and it is longer in higher alkanenitrile solutions than ACN solutions in general. Although the details of the geometrical structures of these P, and A, compounds in the ground state are not clear,'dJ' the increase of T~~ with increase of n indicates that the average distance between A and D groups increases with increase of n, which will affect the tunneling matrix element of electron transfer as discussed later. It should be noted here that, at ca. 1 ps delay times after the excitation of the aromatic molecules with the femtosecond light pulse, the intramolecular vibrational relaxation (cooling from the state with excess vibrational energy) will not be completed yet, although the intramolecular redistribution will take place within 100 fs.I2 Therefore, especially in the case of A, in acetonitrile, there is a possibility that the C S is taking place from the state with excess vibrational energy. We have confirmed that the time constant ( T c R ) of the CR decay of the IP state is more than 2 orders of magnitude longer than T= in both P, and A, systems and in all alkanenitrile solutions examined, as indicated in Table I. It should be noted here that, ~ becomes , shorter with increase of contrary to the case of T ~ TCR the intervening chain number n in general, although it is longer in the BuCN and HexCN than ACN solution just as in the case of TCS. An important conclusion derived from the results in Table I is that the T~~ values in the table are much longer in general than the longitudinal dielectric relaxation time of the solvent: TL(ACN) = 0.2 ps, and TL(BuCN) = 0.5 ps and TL(HexCN) 2 0.7 ps.I3 Therefore, the solvent orientation dynamics in these systems is not important as the mechanism controlling the photoinduced CS process. The C S process will not be barrierless but will be controlled mainly by the magnitude of the D-A electronic interaction (12) Laermer, F.; Elsaesser, T.; Kaiser, W. Chem. Phys. Lett. 1989, 156, 381. (13) Kahlow, M. A.; Kang, T. J.; Barbara, P. F. J . Phys. Chem. 1987, 9 / , 6452.
The Journal of Physical Chemistry, Vol. 94, No. 4, I990 1445 responsible for ET (tunneling matrix element) and the FC factor (the energy gap) of the reaction. Even if we use the solvation time T s I 3 instead of T ~this , conclusion is not altered except in the case of A, where T~~ is rather close to T ~ suggesting , the possibility that the C S process is mainly governed by the solvation dynamics. Therefore, the mechanism of photoinduced C S of these compounds in alkanenitrile solutions may be interpreted as follows in general. The energy gap for CS, -AGOcs (-0.5 eV for both anthracene- and pyrene-DMA pairs), is a little larger for A, than P,. The solvation energy of the I P state in these alkanenitrile solvents will decrease in the order of ACN > BuCN > HexCN, owing to the decrease of the solvent dielectric constant (ts) in this order. This results in a slight decrease of -AGOcs in this order. Since the kcs ( = T ~ ~ - I ) vs -AGOcs relation is in the normal region at -AGOcs 0.5 eV, this decrease of the -AC0cs value will lead to a slight decrease of kcs with decrease of the solvent polarity. The -AGOcs value may be affected also by the intervening chain number n since the average distance between the D and A groups may increase with increase of n, which will result in a decrease of -AGOcs due to the decrease of Coulomb attraction energy between the ions in the pair. Of course, the change in the D,A distance will affect rather strongly the magnitude of the tunneling matrix element. Moreover, the kcs vs -AGOcs relation around -AGO, 0.5 eV is rather dull because it is rather near the top or plateau in the normal r e g i ~ n . ~ Therefore, ~ , * ~ ~ the difference of kcs among the compounds with different chain numbers may be ascribed not only to the small difference in -AC0cs but also to the difference in d,14 N
-
A = 2al(i1%'V)12/h2(w) where (i17f'V) is the tunneling matrix element and ( w ) is the average angular frequency of the intramolecular mode. A for A, will be larger than that of P, due to the larger amplitude of atomic orbital in the molecular orbital at the 9-position where DMA is attached. By taking d = 1012-1013s-,, theoretical calculation give kcs = 1011-10'2s-l l4 around - A G 0 a = 0.5 eV in agreement with the observed results, where kcs is the largest for A, and the smallest for P,. On the other hand, the much smaller rate of C R can be well understood as due to the overwhelming effect of the FC factor. The kCR( ~ 7 c R - l )vs -AGocR relation is in the inverted region 2.8 eV for both P, and at the large energy gap around - A G O , , A, and the energy gap for A,, is slightly smaller. On the basis of our e ~ p e r i m e n t a l and l ~ ~ theoretical studies,I4 by taking A = l o i 3s-I for n = 1 compounds, we obtain kCR = lo8 s-I in an approximate agreement with the observation. In n = 3 compounds, configurational change to the sandwich type can take place immediately after CS,'d*" which increases A and decreases -AG0cR leading to the faster C R with TCR 1 ns. Such configuration change of n = 3 compounds from the extended "loose" ion pair state to the sandwich type taking place with time constant of ca. 1 ns was observed before clearly in viscous alcohol and acetone solutionlla* and also in the case of P3 in acetonitrile solution.IId In n = 2 compounds, the freedom for such configuration change is smaller, giving the intermediate TCR values. In the case of BuCN solutions, higher viscosity ( ~ ( B u C N )= 0.624 CP(15 "C), 7(ACN) = 0.375 CP (1 5 "C)) seems to affect the rate of such configuration change. The larger TCR value of A3 than that of A2 in BuCN might be ascribed to such solvent viscosity effect on the more extensive configuration change of A, in the course of CR. 2. Comparison of the Results of P, and A, ( n = 1 , 2, 3 ) with Those of D-A Systems Directly Combined by Single Bond and the Excited State of CT Complexes. We have examined previously the photoinduced CS processes of the D-A system directly combined by a single bond in viscous alcohol solvents by means of picosecond time-resolved absorption spectral measurement^?*'^*'^ N
-
(14) (a) Kakitani, T.; Mataga, N. Chem. Phys. 1985,93,381. (b) J. Phys. Chem. 1985, 89, 8 . (c) Ibid. 1986, 90, 993. (d) Ibid. 1987, 91, 6211. ( ! 5 ) {a) Mataga, N.; Kanda, Y.; Asahi, T.; Miyasaka, H.; Okada, T.; Kakitani, T. Chem. Phys. 1988,127,239. (b) Mataga, N.; Asahi, T.; Kanda, Y.; Okada, T.; Kakitani, T. Ibid. 1988, 127,249.
1446 The Journal of Physical Chemistry, Vol. 94, No. 4, 1990
Mataga et al. second laser photolysis system. The spectral band shapes are corrected for the chirping of the monitoring white light pulse. The transient absorption spectra of A. in BuCN as well as HexCN show more complex time-dependent changes compared with the case of A, ( n = 1, 2, 3) in alkanenitrile solvents. At early stages, the spectra are rather similar to the absorption band due to the S, SI transition localized in anthracene part of A, ( n = 1, 2 , 3), although they are very broad. This means that the SI state of A, immediately after excitation maintains the character of anthracene SI state though it is perturbed a little by chargetransfer interaction with the DMA groups7 With increase of the delay time, the spectra change gradually toward those indicated at the bottom panel of each figure (1 8.0 ps in BuCN and 40.0ps in HexCN)). I t is difficult to reproduce the time-resolved spectra at intermediate delay times by the superposition of the spectra immediately after excitation and those at the bottom panel. This result is analogous to the case of the picosecond laser photolysis study of the time-resolved spectra of A,, in viscous alcohol solvent.7c We must invoke multiple intermediate states in the course of the solvent-induced change of the electronic and geometrical structure of A. in the SI state also in alkanenitrile solvents. We can estimate the approximate decay time of the initial nonrelaxed state immediately after excitation or the approximate rise time of the equilibrium excited state by plotting the absorbance at a wavelength where its time-dependent change is large. From the approximately exponential decay of absorbance around 600 nm, we have obtained the rise time of the charge-transfer state of to be T = 5.0 ps in HexCN and T = 2.7 ps in BuCN. These values of the formation time of charge-transfer state are much longer compared with the sa values of A, ( I .4ps in HexCN and 1.0 ps in BuCN). These results on A, indicate clearly that, for such a system as A, with strong D-A interaction, the photoinduced charge transfer is not simply determined by the solvent dynamics and also cannot be interpreted simply by the usual electron-transfer theories assuming weak interaction but proceeds via intermediate states with different electronic structures. The time constant of photoinduced charge transfer in A, is much longer than solvent T~ and solvation time T ~ and , also longer than the rCsvalues of A, where the D-A electronic interaction will be much weaker than in A@ These results may be ascribed to the fact that, for the CS in such strongly interacting D-A systems, more extensive solvations than in weakly interacting D-A systems and some geometrical rearrangements are necessary in order to prevent the electronic delocalization interaction in the CS state. Finally, we compare the present results with an extreme case of C S by the strong D-A interaction in the course of relaxation from the excited FC state of the C T complex with the partial C T character to an IP state. The mechanism and dynamics of the CS process of those more strongly interacting systems are not clear compared with the case of the ET by much weaker interaction examined by the usual ET theory. According to our results of the femtosecond laser photolysis and time-resolved absorption spectral studies on 1,2,4,5-tetracyanobenzene-toluene (TCNBTol) complex in the ACN solution,s it takes ca. 20 ps for the complete C S from the excited FC state. Solvent reorientation and a slight intracomplex structural change induces CS within ca. 1 ps to a considerable extent but not completely, and further solvation and intracomplex structural change taking place with the time constant of 20 ps are necessary.8 That is, the rate of CS depends strongly on the intracomplex and the surrounding solvent structures in the ground, the excited FC, and the IP states, and rather extensive rearrangements are necessary in the case of TCNB-To1 complex even in the ACN solution to cut off the electronic delocalization interaction between TCNB- and TOP. This is analogous to the case of photoinduced CS of A,. The mechanism of CS by the gradual electronic structure change via multiple states as concluded above for the A, and TCNB-To1 complex will be common to such systems with the strong D-A electronic interactions. Although such a mechanism has not been taken into account explicitly in the electron-transfer + -
I 400
500 600 $YAMLEICTH n m
400
500
600
Figure 4. Time-resolved transient absorption spectra of & in BuCN (a) and A. in HexCN (b) measured with the femtosecond laser photolysis system. [A,] = (5-10) X IO4 M. The spectra were corrected for the chirping of the monitoring white light pulse.
TM-A, (4-(9-anthryl)-N,N,2,3,5,6-hexamethylaniline) in l-butanol showed clearly the change from the LE (locally excited state of anthracene part) to the CS state. Our preliminary measurement on the same molecule in ACN with the femtosecond laser photolysis method indicates that, for the rise of CS state, it takes ca. 6 ps, which is much longer than T~ and TS of ACN.ls Therefore, in this case too, the C S takes place by the weak interaction mechanism similar to that discussed in subsection I , because the benzene ring is perpendicular to both the anthracene plane and the dimethylamino group, and the "through bond" interaction does not seem to be effective. In the case of 9,9'-bianthryl (BA) in 1-pentanol, we have confirmed that the rise time of the solvent-induced broken symmetry state, res, is close to 7 L . 1 6 , 1 7However, even in this case, if we slightly perturb the symmetry by C1 substitution at the 10-position ( 1 0-CI-BA), the C S process becomes considerably faster than TL,which indicates that the intramolecular ET process is not simply determined by the solvent orientation dynamics. It should be noted here that the C S state of BA and IO-C1-BA is not the complete CS state but has some admixture of the LE state due to rather strong electronic interaction between two groups combined directly by the single bond. Our preliminary time-resolved absorption spectral study on BA in ACN with femtosecond laser photolysis method indicates that C S takes place within 1 ps in accordance with the result of the fluorescence dynamics measurement.I3 In the case of A,, where the D-A electronic interaction is stronger due to the structure being more close to the coplanar one, we have reported previously7c that its time-resolved absorption spectra in I -butanol cannot be simulated by the simple scheme of the LE ET state but we must assume that the electronic structure gradually changes over the delay times longer than 200 ps via many states with different degrees of intramolecular C T and solvation and also different degrees of intramolecular structural rearrangements. In Figure 4, we show time-resolved transient absorption spectra of A. in BuCN (a) and HexCN (b) measured with the femto-
-
(16) Mataga, N.; Yao, H.; Okada, T.; Rettig, W. J . Phys. Chem. 1989, 93, 3383. (17) Mataga, N.: Yao, H.: Okada, T.; Rettig, W., to be submitted for publication. (18). Mataga, N.; Kanaji, K.; Hagihara, M.; Okada, T.; Baumann, W., unpublished work
J . Phys. Chem. 1990, 94, 1447-1452 theories, it may be important in the actual photoinduced electron transfer reactions in solution and also in amorphous solids. It should be noted here that such a mechanism of photoinduced C S process was proposed for the first time by the present authorIg many years ago in relation to the electronic and geometrical (19) Mataga, N.; Okada, T.; Yamamoto, N. Chem. Phys. Lett. 1968, 1, 119.
1447
structure change of an exciplex depending on solvent polarity.
Acknowledgment. We are grateful to Prof. S. Misumi and Prof. y. Sakata for the samples of pnand A". N.M. acknowledges the support by Grant-in-Aid (No. 62065006) from the Ministry of Education, Science and Culture, Japan. Registry No. P,,38801-65-9; P2,38764-40-8; P,, 38764-41-9; A , , 38532-94-4; Ai, 38474-10-1; A, 38474-1 1-2.
Kinetic Analysis of Free-Radical Reactions in the Low-Temperature Autoxidation of Triglycerides Jingmin Zhu and Michael D. Sevilla* Department of Chemistry, Oakland University, Rochester, Michigan 48309-4401 (Received: July 3, 1989; In Final Form: September 6, 1989)
The kinetics of the low-temperatureautoxidation of triglycerides has been investigated by electron spin resonance spectroscopy. After initial radical production, four reaction stages are found in the overall autoxidation of unsaturated lipids: (1) formation of peroxyl radicals by addition of molecular oxygen to the initial carbon radicals, (2) consumption of oxygen in the autoxidation cycle, (3) decay of the lipid peroxyl radical into allylic and pentadienyl radicals, and (4) recombination of the carbon-centered radicals. Kinetic analyses reveal that peroxidation of the initial carbon-centered radicals (stage 1) is controlled by O2migration which has an apparent activation energy of 24 kJ/mol in unsaturated lipids. The autoxidation cycle (stage 2) is dependent on the nature of the parent lipid matrices. Our analysis gives the activation energies of 9 i 2, 34 8, and 88 f 11 kJ/mol for the decay of the peroxyl radical into allylic and pentadienyl radicals (stage 3) in trilinolenin, trilinolein, and triolein, respectively. The very low activation energy found for radical decay of the trilinolenin peroxyl radical suggests this process proceeds easily through intra- or interchain hydrogen abstraction and cyclization at low temperatures (
