7534
J . Phys. Chem. 1990, 94,1534-7539
Femtosecond-Picosecond Laser Photolysis Studies on the Dynamics of Excited Charge-Transfer Complexes in Solution. 3. Dissociation into Free Ions and Charge Recombination Decay from the Ion-Pair State Formed by Charge Separation in the Excited State of li2,4,5-Tetracyanobenzene-Aromatic Hydrocarbon Complexes in Polar Solvents Seishi Ojima, Hiroshi Miyasaka, and Noboru Mataga* Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka. Osaka 560, Japan (Received: March 16, 1990)
Picosecond laser photolysis studies have been made on the ionic dissociation and charge recombination (CR) processes of geminate ion pairs (IPS) formed by exciting 1,2,4,5-tetracyanobenzene(TCNB)-aromatic hydrocarbon complexes, for which results of our recent femtosecond-picosecond laser photolysis investigations concerning the relaxation from the excited Franck-Condon state leading to the compact IP or contact IP (CIP) formation were accumulated. We have demonstrated various different cases of ionic dissociation and CR decay of IPS depending on the molecular nature and strength of aromatic hydrocarbon donors. The obtained results, including those indicating strongly the existence of loose IP (LIP) or solvent-separated IP (SSIP) in the course of the ionic dissociation from the CIP, those showing rapid CR decay of CIP without ionic dissociation, and also some other cases, have been discussed on the basis of some theoretical considerations.
Introduction In previous papers,'-3 we reported results of femtosecond-picosecond laser photolysis and time-resolved spectral studies on the charge-separation (CS) process of excited C T complexes in the case of TCNB (1,2,4,5-tetracyanobenzene)in benzene and methyl-substituted benzene solutions as well as TCNB-aromatic hydrocarbon complexes in polar solvents. It has been shown for the TCNB in benzene solutions that, immediately after excitation, a slight structural change within a 1:1 complex accompanied with an increase of the extent of CS takes place with time constant of ca. 1 ps, in the course of the relaxation from the Franck-Condon (FC) excited state. This structural change, however, does not lead to the complete CS, but further structural change including the formation of the 1:2 complex, I(A-D2+)*, has been concluded to be of crucial importance for the complete CS.'s2 It has been confirmed that, in the case of the TCNB-aromatic hydrocarbon complex in polar solvents such as acetonitrile, solvent reorientation can induce CS with a time constant shorter than 1 ps to a considerable extent but not completely, and for the complete CS to the geminate ion pair (IP) formation, further intracomplex structural change and solvation that take place with a time constant of several to 30 ps depending on the nature of electron-donating aromatic hydrocarbon and the solvent polarity are necessary.'J On the other hand, it is well-known that the fluorescence yield of the TCNB-aromatic hydrocarbon complex decreases with an increase of the solvent polarity and becomes practically zero in strongly polar solvents, which has been ascribed to a decrease of the fluorescence radiative transition probability and an increase of the rates of the nonradiative processes with an increase of the solvent p ~ I a r i t y . ~ J The nonradiative processes in polar solvents have been concluded to include the dissociation into free ions and charge-recombination (CR) deactivation to the ground state from the CT or IP state.@ ( I ) Miyasaka, H.; Ojima, S.; Mataga, N. J . Pfiys. Cfiem. 1989, 93, 3380. (2) Ojima, S.; Miyasaka, H.; Mataga, N. J. Pfiys. Cfiem. 1990.94, 4147. ( 3 ) OJima, S.; Miyasaka, H.; Mataga, N. J . Pfiys. Cfiem., in press. (4) (a) Mataga, N.; Murata, Y. J . Am. Cfiem. SOC.1969, 91, 3144. (b) Egawa, K.; Nakashima, N.; Mataga, N. Bull. Cfiem. Soc. Jpn. 1971,44,3238. (5) Kobayashi, T.; Yoshihara, K.; Nagakura, S. Bull. Chem. SOC.Jpn. 1971, 44, 2603. (6) (a) Masuhara, H.; Shimada, M.; Mataga, N. Bull. Cfiem. SOC.Jpn. 1970, 43, 1048. (b) Masuhara, H.; Shimada, M.; Tsujino, N.; Mataga, N. Ibid. 1971, 44, 3310. (c) Shimada, M.; Masuhara, H.; Mataga, N. Cfiem. Pfiys. Lett. 1W2,IS,364. (d) Shimada, M.; Masuhara, H.; Mataga, N. Bull. Cfiem. SOC.Jpn. 1973, 46. 1903.
Moreover, it has been demonstrated that both rates of the ionic dissociation and C R deactivation increase with increase of the solvent polarity, and the CR rate becomes larger for the stronger donor (with lower oxidation Nevertheless, the precise mechanism leading to the formation of the dissociated ions from the excited state of the C T complexes was not clear. As stated above, we have made direct observation of the CS process leading to the IP formation from the excited FC state of TCNB complexes by means of femtosecond-picosecond spectro~copy.l.~ This geminate IP will undergo dissociation into free ions in strongly polar solvents. However, whether there is any intervening solvent between the cation and anion in this IP (solvent-separated IP (SSIP) or loose IP (LIP)) or not (contact IP or compact IP (CIP)) was not clear. On the other hand, we have confirmed in some donor-acceptor systems in acetonitrile solution that the CR decay of the IP formed by excitation of the C T complex is much faster compared with that formed by C S at encounter in the fluorescence quenching reaction between the same donor and acceptor by directly observing the behaviors of IPS with femtosecond-picosecond laser photolysis method.l*I5 These results indicate strongly the different structures of IPS depending on the mode of their formation. The IP formed by excitation of the CT complex is probably the CIP, while that produced by CS at encounter in the fluorescence quenching reaction in acetonitrile solution may be mainly SSIP. The dissociation and CR processes of the geminate IP formed by a fluorescence quenching reaction in acetonitrile solutions have been studied for a large number of systems by picosecond transient absorption meas~rementsl~ and also the dissociation process of some systems by picosecond laser-induced photocurrent measurements.16 In view of the above results, it is an important problem whether we can demonstrate the existence of both kinds of IP by directly (7) (a) Hinatsu, J.; Yoshida, F.;Masuhara, H.; Mataga, N. Cfiem. Pfiys. Lett. 1978, 59, 80. (b) Masuhara, H.; Mataga, N. Acc. Cfiem. Res. 1981, 14, 312. (8) Uemiya, T. Master's Thesis, Osaka University, 1984. (9) Mataga, N. Pure Appl. Cfiem. 1984. 56, 1255. (IO) Mataga, N.; Kanda, Y.; Okada, T. J . Pfiys. Cfiem. 1986,90, 3880. ( 1 I ) Mataga, N. Acta Pfiys. Pol. 1987, A71, 767. (12) Mataga, N.; Shioyama, H.; Kanda, Y. J . Pfiys. Cfiem. 1987.91, 314. ( 1 3) Mataga, N.; Kanda, Y. Asahi, T.; Miyasaka, H.; Okada, T.; Kakitani, T. Cfiem. Pfiys. 1988, 127, 239. (14) Mataga, N.; Asahi, T.; Kanda, Y.; Okada, T.;Kakitani, T. Cfiem. Pfiys. 1988, 127, 249. (15) Asahi, T.; Mataga, N. J . Pfiys. Cfiem. 1989, 93, 6575. (16) Hirata, Y.; Kanda, Y.; Mataga, N. J . Pfiys. Chem. 1983,87, 1659.
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 19. 1990 7535
Charge-Transfer Complexes in Solution detecting the intermediate SSIP in the course of the relaxation of the excited C T complex leading to the formation of dissociated ions in strongly polar solutions. For this purpose, it seems most suitable to examine the TCNB-aromatic hydrocarbon complexes, for which the results of the previous nanosecond laser photolysis studiese7 and our recent femtoswnd-picosecond laser photolysis studies on the relaxation from the excited FC state and the IP f ~ r m a t i o n l -are ~ accumulated. In the following, we discuss mainly the above problems of the ionic dissociation of TCNB complexes in polar solutions.
Experimental Section Ultrafast laser photolysis systems were the same as used before.’-* Some of the picosecond transient absorption spectra were measured by using a dye laser photolysis system pumped by the second harmonics of a mode-locked Nd3+:YAG laser (Quantel, P i c o ~ h r o m e ) . ~ ~The J * combination of dyes cresyl violet (OK)/ LDS698 (amp) gives the picosecond pulse at 682 nm, and its second harmonics was used for excitation. For other picosecond transient absorption spectral measurements, the third harmonics or the second harmonics of a mode-locked Nd3+:YAG laser was used for excitation.’V2 TCNB, benzene, and methyl-substituted benzenes were the same as used before.’-3 Aromatic hydrocarbons used as donors were chromatographed on alumina and silica gel, recrystallized, and sublimated in a vacuum. Acetonitrile was spectrograde and used as received. Propionitrile and butyronitrile were dried over CaH, and distilled several times. Hexanenitrile was distilled several times before use. Sample solutions for the measurements were deoxygenated by freeze-pumpthaw cycles.
OL./ v
400
500
600
700
Wavelength / nm
L.-!-
0
1
2 Time/ns
3
5
Results and Discussion A. TCNB-Toluene Complex in Acetonitrile Solution. In our previous reports,’~~ we demonstrated that the photoinduced CS of the TCNB-toluene complex in acetonitrile consists of two steps: the very fast one taking place within 1 ps, followed by a slower one with a time constant of ca. 20 ps, leading to IP, as indicated in eq 1.
hv
< I ps
~ ( T c N B ~ . T o ~ * + ) ~ ~(TcNB~’-.ToI~’+)[~ solvation20 P
‘(TCNB6”-’To’d”+)s~ stmct change and further solvation. (TCNB-*Tol+)y (1) In the course of the relaxation from the Franck-Condon (FC) excited state, solvent reorientation seems to induce a fairly large extent of CS. For the complete CS, however, further solvation and intracomplex structural change taking place with a time constant of 20 ps is necessary. During these relaxation processes leading to IP, we have observed clearly the sharpening of the TCNB anion band with an increase of the absorbance at the peak position. In this paper, we are concerned with the CR deactivation and dissociation processes from the produced IP of eq 1. As an example, we show in Figure 1 time-resolved absorption spectra and time profiles of absorbance at the peak of TCNB anion band (462 nm) observed by picosecond laser photolysis and transient absorption spectral measurements of the TCNB-toluene complex in acetonitrile solution at 22 OC in the case of [toluene] = 0.47 M. The time-resolved absorption spectra in Figure l a show a rapid rise nearly equal to the time resolution of the picosecond laser photolysis apparatus in accordance with the reaction process determined by femtosecond laser photolysis method as indicated in eq 1 After the rapid rise, the absorbance of the TCNB anion band shows a decay and converges to a constant value due to the CR and dissociation of the IP state, as shown in Figure lb. After subtracting the constant value of the absorbance due to the dissociated ions from the decay curve, we have made semilogarithmic (17) Hirata, Y.; Mataga, N. J . Phys. Chem. 1989, 93, 7539. (18) Hirata. Y.; Ichikawa, M.; Mataga, N. J. Phys. Chem. 1990,94,3872. (19) (a) Pysh, E. S.;Yang, N. C . J. Am. Chem. SOC.1963,85, 2124. (b) Psover, N. E. Trans.Foraday Soc. 1966,62,3535. (c) &homburg, H. Ph.D. Thesis, Gottingen, 1975.
0
0.5
1 Time / n s
15
Figure 1. (a) Picosecond time-resolved absorption spectra of the TCNB-toluene complex in acetonitrile; [toluene] = 0.47 M. (b) Time profile of the T C N B anion band at 462 nm. (c) Semilogarithmic plot of the absorbance obtained by subtracting the absorbance due to dissociated ions from the observed decay curve of (b). The insert shows the fast component obtained by subtracting the slow one from the decay curve in (c).
plot of the remaining absorbance as indicated in Figure IC. This decay curve contains fast and slow components, and the former component obtained by subtracting the slow one from the decay curve is shown in the insert of Figure IC. By such procedure, we have obtained the lifetime of the fast component, rf = 100 ps, and that of the slow component, rs = 370 ps. We have obtained very similar results also in the case of TCNB-benzene, -m-xylene and -p-xylene complexes in acetonitrile solutions where an analogous mechanism seems to be controlling the ionic dissociation processes of the excited C T complexes. To elucidate the ionic dissociation mechanism in the above TCNB complexes with relatively weak donors, we have investigated in detail the TCNB-toluene complex, examining various factors affecting the dissociation processes. That is, the fact that the decay curve of the IP cannot be represented by a single exponential but is composed of fast and slow parts may be interpreted, for example, (a) by assuming the existence of different kinds of IPS undergoing C R and dissociation in parallel with each other or (b) by assuming the dissociation processes proceeding via intermediate states. In case a, the different kinds of IPS might be 1:l and 1:2 IPs.I*~In case b, the intermediate states in the course of the dissociation may be IPS with solvent molecules
Ojima et al.
7536 The Journal of Physical Chemistry, Vol. 94, No. 19, 1990
0
0.5
1.5
1
Figure 2. (a) Time profile of TCNB anion band at 462 nm observed for the TCNB-toluene complex in acetonitrile; [toluene] = 3.8 M. (b) The same as in Figure IC.
between positive and negative ions (SSIPor LIP),in contrast to the IP indicated in eq I (CIP). B. Effect of Toluene Concentration on the Decay of the IP. We have examined the acetonitrile solutions of the TCNB-toluene system with [toluene] = 0.094, 0.47, 0.94, 1.9, and 3.8 M. We have confirmed that, in all of these solutions, the decay curve of IP cannot be reproduced by a single exponential, but deviations from it are observed clearly, just as in the case of Figure lb,c. The decay time becomes longer in solutions with a high toluene concentration. For example, Tf = 180 ps and T , = 700 ps for [toluene] = 3.8 M. The decay curves of the IP for the solution with [toluene] = 3.8 M are shown in Figure 2a,b. The longer decay time of IP in the solution with the higher toluene concentration may be ascribed mainly to the decrease of solvent polarity due to the high concentration of toluene. With a decrease of the solvent polarity, the rate constant of dissociation will decrease.I6 Moreover, the C R rate constant of the IP will also decrease with a decrease of the solvent polarity because the C R rate of the TCNB-toluene system decreases with an increase of the free energy gap, which increases with a decrease of the solvent polarity just as we show later in this article (subsections C, E, and F). In solutions with a high toluene concentration ([toluene] = 0.94, 1.9, 3.8 M), we can observe a broad absorption of the toluene dimer cation due to the 1:2 IP in the wavelength region longer than 650 nm. However, the dimer cation formation decreases drastically in the solution with [toluene] = 0.47 M as indicated in Figure la. Nevertheless, we can observe similar deviation from exponential decay of the IP in both of these solutions. Moreover, although we cannot recognize the dimer cation formation in the solution with [toluene] = 0.094 M, we have observed a similar deviation from the simple exponential decay of the IP. In addition, it should be noted here that the decay profile of the dimer cation absorbance in those solutions with high toluene concentration has been confirmed to be approximately the same as that of the TCNB anion band. That is, the decay profile of the 1:2 IP due to CR and dissociation is practically the same as that of the 1 : 1 IP,which means that the 1 :1 and 1 :2 IP in those solutions are approximately in equilibrium. The above results reject the possibility that the existence of 1:1 and 1 :2 IPSis the reason for the deviation from simple exponential decay of the IPSin acetonitrile solution. C. Effect of Solvent Polarity on the Decay of the IP. The solvent polarity effect on the decay and dissociation of the IP has been examined by changing the solvent from acetonitrile to propionitrile, butyronitrile, and hexanenitrile. With a decrease of the solvent polarity, the absorbance decay becomes slower as a whole, and the dissociation yield decreases. In all of these nitrile solutions, the decay of the IP absorbance could not be reproduced by a single exponential. The absorbance decay curves at 462 nm
3
2
1
0
Time / ns
Time I ns
Figure 3. (a) Time profile of TCNB anion band at 462 nm observed for the TCNB-toluene complex in hexanenitrile solution; [toluene] = 0.47 M. (b) Semilogarithmicplot of the absorbanceobtained by subtracting the absorbance due to dissociated ions from the observed decay curve of (a).
for the hexanenitrile solution in the case of [toluene] = 0.47 M are shown in Figure 3. D. Temperature Effect on the Decay of the IP. We have measured the absorbance decay of the IP for the solution with [toluene] = 0.47 M, at -20 and 54 OC. At both temperatures, the observed absorbance decay could not be reproduced by a single exponential. The fast part in the decay curve increased compared with the slow part, and the decay as a whole became slower by lowering the temperature. Both the solvent polarity and temperature will affect the formation process of the intermediate state from the compact IP and also the dissociation process of the intermediate state. In the following, we examine the observed results on the TCNB complexes with relatively weak donors by assuming the dissociation mechanism of the IP via the intermediate state and discuss the above results on the basis of the results obtained by this analysis. E . Analysis of the Dissociation Process of IP Assuming Intermediate State. We assume the reaction mechanism of eq 2. k
A2Bkd-C lkn
(2)
1kr
According to the discussion given at the end of subsection A, A is the CIP (the last product in eq l), B will be the SSIP or LIP, and C is the dissociation ions. We assume here that the extinction coefficient t of the TCNB anion band at 462 nm is approximately the same in A, B, and C in eq 2. This seems to be a reasonable assumption since the transient absorption band shape is approximately the same in the time regions we are concerned with here. In this case, we obtain eq 3 for the transient absorbance O D ( [ )a t 462 nm, O W t ) = e([A(t)l + [B(t)l + IC(t)l) = 4A(O)l[a exp(-(k, + k,)tl + P e x p W ,
+ kd)d + rl
(3)
where (Y = k n T f - kskrT?Ts(Ts - T$', P = kskrTfT?(Ts - T f ) - ' , 7 = k , k d T f T , , T f = ( k , ks)-l,and 7, = (k,+ kd)-'. Evidently, (Y + P y = 1 and T , should be longer than T P Some results of the analysis of the observed decay curves of IP by using eq 3 are collected in Table I, which shows clearly that the rate of the formation of the intermediate state, (k, + ks),is considerably larger than the rate of its decay, ( k , + kd), and also k, is not much different from k,. These results are conditions necessary for the observation of the intermediate state. Moreover, by comparing the rate constant k, and k, for benzene, toluene (0.47 M), m-xylene, and p-xylene donors in acetonitrile, we can see that both of these rate constants increase in this order.
+
+
Charge-Transfer Complexes in Solution
The Journal of Physical Chemistry, Vol. 94, No. 19, 1990 7537
TABLE I: Results of the Analysis of the Time Profiles of IPS of Some TCNB-Alkylbenzene Systems on the Basis of Eq 3 (at 22 "C) solvent D PI/M a Y k,/s-I k,/& k,/s-I kd1S-l acetonitrile toluene 0.094 0.34 0.24 9.9 x 109 1.4x 1010 2.4 x 109 1.6 x 109 0.47 0.51 0.26 5.7 x 109 4.3 x 109 1.1 x 109 1.6 x 109 2.7 x 109 2.9 x 109 3.8 0.40 0.28 6.6 X lo8 7.5 X IO8 butyronitrile toluene 0.47 0.46 0.19 3.6 x 109 3.1 x 109 8.4 X IO8 5.8 X IO8 hexanenitrile 0.47 0.52 0.16 1.9 x 109 1.2 x 109 5.2 X IO8 3.8 X lo8 acetonitrile benzene 1.1 0.31 0.42 2.7 x 109 4.5 x 109 6.8 X IO8 1.5 X IO9 m-xylene 0.41 0.79 0.091 1.9 X 1OIo 4.6 X lo9 2.2 x 109 1.9 x 109 p-xylene 0.41 0.79 0.057 2.2 x 1010 4.6 x 109 5.1 x 109 2.6 x 109
It should be noted here that the free energy gap between the IP and the ground state decreases in this order. We can see also that k, is larger than k , in all cases examined here. This is similar to the general tendency observed when we compare the C R decay of the IP formed by the excitation of the CT complex with that of the IP formed by the C S a t encounter between the fluorescer and quencher of the same or similar electron-donating and -accepting molecules in the same polar solvent acet~nitrile."'-'~ That is, the C R decay of the former IP is faster than that of the latter in general.I*ls Presumably, the former IP is CIP,15 and the latter LIP. In addition, the C R decay rate constant, kCR,of the former IP shows exclusively a decrease upon increase of the free energy gap -AGOCR," while the kCRof the latter shows a bell-shaped dependence upon -AGoCR.I4 The bRof the LIP of TCNB-alkylbenzene systems (k,) in Table I appears to be in the inverted region. Summarizing the above results and discussions, k , and k, in Table I may be identified with kCRof the CIP and that of the LIP, respectively, and the small increase of k, and k, in the order benzene, toluene (0.47 M), m-xylene, and p-xylene may be partly ascribed to the decrease of the energy gap -AG°CR is this order. We have examined also whether the behaviors of those IPS are affected by the change of excitation wavelength within the same C T band by using the picosecond dye laser pulse at 341 nm for excitation compared with the 355-nm pulse of the Nd3+:YAG laser. The obtained results in both cases were practically the same, which means that the excess vibrational energy given to the excited complex in the case of the 341-nm excitation is not effectively used for changing the structure of the ions pairs to a looser one. This is consistent with the fact that the excess vibrational energy in the excited TCNB-toluene complex in toluene solution does not affect the intracomplex structural change from an asymmetrical donoracceptor configuration toward a more symmetrical overlapped one taking place immediately after excitation within ca. 1 P S . ~ F. Behaviors of IPS Produced by Excitation of TCNB Complexes with Stronger Donors. We have examined also aromatic hydrocarbon donors with lower oxidation potentials as well as larger condensed ring compounds. In the case of these donors, the double-exponential decay of the IP leading to the dissociated ions was hardly observed. For the TCNB-durene and TCNB-hexamethylbenzene complexes in acetonitrile solution, we have already shown our results of femtosecond laser photolysis studies on the time-resolved absorption ~ p e c t r a .The ~ decay of the CIP of these systems could be reproduced by a single-exponential function, and the decay time could be estimated to be 50 and 41 ps, respectively. The timeresolved absorption spectra of TCNB-durene in acetonitrile measured by the picosecond laser photolysis system are shown in Figure 4. We can see clearly a fairly rapid decay of absorbance. By taking into account the exciting laser pulse width, we have obtained the decay time of 1P by deconvolution to be 50 ps. Similarly, we have determined the decay time of the IP of TCNB-hexamethylbenzene in acetonitrile by a similar measurement and procedure to be 41 ps, in complete agreement with the results of femtosecond laser photolysis measurements. Owing to the short life of CIP, which may be due to the predominant C R decay, practically no dissociated ions can be observed in the case of TCNB-durene and TCNB-hexamethylbenzene systems. On the other hand, excited TCNB complexes with naphthalene, phenanthrene, and biphenyl in acetonitrile give a considerable
L--
400
..J.
_I_
.
-L
500
600 Wavelength / nm
I
.
700
Figure 4. Picosecond time-resolved absorption spectra of TCNB-durene complex in acetonitrile; [durene] = 0.10 M.
L
Time I ns
\
0
250
500 Time I ps
Figure 5. (a) Time profile of the TCNB anion band observed by exciting
the TCNB-phenanthrene complex in acetonitrile. Measurements were made by picosecond laser photolysis method; [phenanthrene]= 0.080 M. (b) Semilogarithmicplot of the absorbance obtained by subtracting the absorbance due to dissociated ions from the observed decay curve. amount of dissociated ions owing to the slower C R decay of IP, although their oxidation potentials are rather close to those of durene and hexamethylbenzene. As an example, we show the observed absorbance decay curve at the peak of the TCNB anion band for the TCNB-phenanthrene complex in Figure 5 . We have examined also larger condensed ring aromatic hydrocarbon donors such as pyrene and anthracene. Results obtained for the TCNB-pyrene complex in acetonitrile are indicated in Figure 6. The transient absorbance of the IP shows a rapid decay
7538 The Journal of Physical Chemistry, Vol. 94. No. 19, I990
Ojima et al.
TABLE 11: Decay Rate Constants and Dissociation Yields of CIP of Some TCNB Complexes in Acetonitrile Obtained by the Reaction Scbeme of Ea. 4 (at . 22 "C)
D durene hexamethylbenzene biphenyl naphthalene phenanthrene pyrene anthracene
-AGcRo/eVa 2.25 2.12 2.38 2.20 2.13 1.86 1.75
WI/M 0.10
