The Journal of
Physical Chemistry
0 Copyright, 1989, by the American Chemical Society
VOLUME 93, NUMBER 18 SEPTEMBER 7,1989
LETTERS Charge Recombination Process of Ion Pair State Produced by Excitation of Charge-Transfer Complex in Acetonitrile Solution. Essentially Different Character of Its Energy Gap Dependence from That of Geminate Ion Pair Formed by Encounter between Fluorescer and Quencher Tsuyoshi Asahi and Noboru Mataga* Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan (Received: May 12, 1989)
Charge recombination rates (kCR)of geminate ion pairs (IP) formed by excitation of various charge-transfer complexes in acetonitrile solution have been investigated by femtosecond and picosecond laser photolysis and time-resolved absorption spectral measurements covering a wide range of energy gap between IP and ground state. An essentially different energy gap dependence of kCRfrom the bell-shaped one obtained in the case of IP formed by charge separation at encounter in the fluorescence quenching reaction has been observed.
Introduction The main processes of photoinduced charge separation (CS) between electron donating and accepting molecules and charge re"bination (CR) Of produced charge transfer (CT) Or ion pair (IP) state in polar solutions are the following:' (i) CS at encounter between excited molecule and electron donating or accepting wencher lading to the formation of geminate IP which undergoes CR and dissociation into free ions (in this case, there is no appreciable CT interaction in the ground state); (ii) excitation of the ground-state C T complex, followed by relaxation from the Franck-Condon excited state, leading to the formation of an I P in which the CR and dissociation into free ions compete with each other. The behaviors of the transient I P state are of crucial importance for the photochemical and photobiological reaction mechanisms, and the energy gap dependence of its C R process in case i has been a subject of recent lively investigations.l-l0
A theoretically predicted bell-shaped energy gap dependence of the C R rate within the geminate radical pair formed by electron-transfer quenching of the phosphorescent ruthenium(I1) compound as well as the rhodium(II1) compound by amines was observed.4,5 The geminate I p formed by fluorescence quenching reaction of aromatic molecules, however, showed only a decrease of the C R rate with increase of the energy gap, Le., only the inverted region.z.3,8,gNevertheless,some experimental results which
( 1 ) Mataga, N. In Photochemical Energy Conuersion; Norris, J., Meisel, D., Eds.; Elsevier: Amsterdam, 1989; p 32.
(10) Mataga, N.; Asahi, T.; Kanda, Y.; Okada, T.; Kakitani, T. Chem. Phys. 1988, 127, 249.
(2) Wasielewski, M. R.; Niemczyk, M. P.; Svec, W. A.; Pewitt, E. B. J . Am. Chem. SOC.1985, 107, 1080. (3) Kakitani, T.; Mataga, N. J . Phys. Chem. 1986, 90, 993. (4) Ohno, T.;Yoshimura, A.; Mataga, N. J. Phys. Chem. 1986,90,3295. (5) Ohno, T.;Yoshimura, A.; Shioyama, H.; Mataga, N. J . Phys. Chem. 1987, 91, 4365. (6) Mataga, N.; Kanda, Y.; Okada, T.J . Phys. Chem. 1986, 90, 3880. (7) Mataga, N. Acta Phys. Polon. 1987, A71, 761. (8) Harrison, R. J.; Pearce, B.; Beddard. G. S.:Cowan, J. A.; Sanders, J. K. M. Chem. Phvs. 1987. 116. 429. (9) Gould, I. R.; Ege, D.; Mattes, S. L.; Farid, S.J. Am. Chem. SOC.1987, 109, 3794.
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indicate the energy gap dependence of CR in the normal region have been obtained by examining the geminate I P formed by strongly exothermic fluorescence quenching r e a c t i ~ n . ~In. ~view of this result, we have undertaken a systematic study on the C R process of geminate IP formed by fluorescence quenching reaction by directly observing their dynamics with ultrafast laser spectroscopy covering a wide energy gap range and have obtained not only the results for the inverted region but also the results for the top region as well as the normal region, confirming the bell-shaped energy gap dependence of the CR process.I0 On the other hand, no such systematic study has been made for the CR process of the IP formed by excitation of the C T complexes in polar solutions (case ii). There are some comparative studies we made on the C R processes of I P produced by fluorescence quenching reaction and that formed by excitation of the ground-state C T complex between the same pair and in the same solvent, a ~ e t o n i t r i l e . 6 ~ ~This ~ ~ 'investigation ~'~ was made to examine a hypothesis13proposed to interpret the lack of inverted region in the observed results of the energy gap dependence of the rate of fluorescence quenching reaction due to electron transfer in acetonitrile s~lution.'~According to this,12 in the system pyrene (Py)-tetracyanoethylene (TCNE) in acetonitrile, the lifetime of the geminate Pys+-TCNEs- I P formed by encounter between Py*(SI) and T C N E was ca. 200 ps while that of the I P formed by excitation of the complex of the same pair was shorter than a few picosecond^.^^^*'* A similar result was also obtained for the Py-PMDA (pyromellitic dianhydride) system in acetonitrile." IP formed by enThe C R rate constant of the Pys'-TCNEscounter is in the normal region and much smaller compared with that at the top region in the bell-shaped energy gap dependence while the CR decay of the IP formed by the C T complex excitation seems to be much larger than the value at the top of the bellshaped relation. Accordingly, the energy gap dependence of the CR decay in the I P formed by C T complex excitation can be essentially different from that observed for the I P formed by C S at encounter in the fluorescence quenching reaction. In the present study, we have examined many C T complexes in acetonitrile solution and have measured CR decay rates of IP's covering a wide energy gap range by directly observing the decay of the absorbance of I P with picosecond and femtosecond laser photolysis and time-resolved transient absorption spectral measurements.
Experimental Section A picosecond laser photolysis system with a repetitive modelocked Nd3':YAG laser was used for transient absorption spectral measurements in the 10 ps to nanosecond region.6.'0 The third harmonic generation (THG, 355 nm), second harmonic generation (SHG, 532 nm), or Raman scattering light (397 nm) obtained by focusing T H G into cyclohexane liquid was used for exciting sample solutions. For the measurements of time-resolved transient absorption spectra in the shorter time region, a femtosecond laser photolysis system was used.15916Either fundamental (710 nm) or SHG pulse (355 nm) was used for exciting the sample. Py and perylene (Per) were chromatographed on alumina and silica gel, recrystallized from ethanol, and sublimed under vacuum. Aldrich G. R. grade anthracene (An) was used without purification. Chrysene (Chr), naphthalene (Naph), hexamethylbenzene (HMB), and durene (Du) were recrystallized several times from alcohol. Spectrograde (Merck Uvasol) benzene (Bz) and toluene (Tol) as well as Gr grade m-xylene (m-Xyl) and p-xylene (p-Xyl) (Wako) were passed through a silica gel column. Phthalic anhydride (PA) and PMDA were purified by the same method as ( 1 1 ) Mataga, N.; Shioyama, H.; Kanda, Y. J. Phys. Chem. 1987,91,314. (12) Mataga, N.; Kanda, Y.; Asahi, T.; Miyasaka, H.; Okada, T.; Kakitam, T. Chem. Phys. 1988, 127, 239. (13) Masuhara, H.; Mataga, N. Acc. Chem. Res. 1981, 14, 312. (14) Rehm, D.; Weller, A. Isr. J . Chem. 1970, 8, 259. ( 1 5 ) Mataga, N.; Miyasaka, H.; Asahi, T.; Ojima, S.; Okada, T. Ultrafast Phenomena VI; Springer-Verlag: Berlin, 1988; p 5 1 1 . (16) Miyasaka, H.; Ojima, S.; Mataga, N. J. Phys. Chem. 1989,93,3380.
Letters h
.-s'
'0
\
U
8
v
0, lJ
c
a
f0
t/ns
!?
400
500
600 Vnm Figure 1. (A) Picosecond time-resolved absorption spectra of Py-PA M, complex excited at 397 nm in acetonitrile solution. [Py] = 2 X [PA] = 0.43 M. (B) (a) Time profile of Py' absorbance of Py-PA IP formed by exciting the CT complex in acetonitrile (0). [Py] = 2 X M, [PA] = 0.43 M. (b) That of the geminate IP of the same system formed by encounter between IPy* and PA (0). The highest value of the absorbance is normalized to that of (a). [Py] = 2 X IO-' M, [PA] = 0.4 M.
1 2 ps I
5 500
600
700 h/nm
I
10
I
t /ps
15
Figure 2. Femtosecond time-resolved absorption spectra of PMDAHMB complex excited at 355 nm (A) in acetonitrile solution, and time profiles of absorbance at 665 nm observed for PMDA-HMB (B) and PMDA-Naph (C) systems. [PMDA] = 5.2 X M, [HMB] = 2.2 X lo-* M (A, B), [PMDA] = 2.3 X lo-* M, [Naph] = 2.8 X M (C).
used before.I0 T C N E was recrystallized twice from monochlorobenzene and once from dichloroethane. 7,7,8,8-Tetracyanoquinodimethane (TCNQ) was recrystallized several times from acetonitrile. All solutions for the measurements were deaerated by the freeze-pumpthaw method or deoxygenated by irrigating with nitrogen gas stream.
Results and Discussion In the case of the 1,2,4,5-tetracyanobenzene(TCNB)-toluene complex, it has been demonstrated that for the complete C S leading to the IP formation from the excited FC (Franck-Condon) state, some structural changes including intracomplex configuration and surrounding solvent which take place with a time constant of ca. 20 ps are necessary even in acetonitrile so1ution.l6 However, the rate of such CS in the excited state of the complex will depend on the donor (D)-acceptor (A) geometries of the complex in the ground, FC, as well as C S state. For example, when we use stronger D's such as HMB, the complete CS from the excited FC state of the TCNB complex in acetonitrile takes place very rapidly within a few picosecond^.'^ In most of the acid anhydride complexes as well as TCNE or TCNQ complexes in acetonitrile examined here, the CS from the excited FC state takes place very rapidly in general within a few picoseconds, and we are concerned here with the decay process of the produced IP state. (1 7 ) Ojima, S.; Miyasaka, H.; Mataga, N. To be submitted for publication.
The Journal of Physical Chemistry, Vol. 93, No. 18, 1989 6577
Letters
13c
h/nm
t/ps
t/ps
Figure 3. (A) Transient absorption spectrum Py-TCNE CIP in acetonitrile immediately after excitation at 710 nm by femtosecond laser pulse. [Py] = 2.0 X lo-* M, [TCNE] = 0.53 M. (B) Time profile of the absorbance at 450 nm for the transient absorption of the system in (A). The solid line represents the simulation curve calculated with T$' = 500 fs. (C) Time profile of the absorbance of Per-TCNE CIP in acetonitrile observed at 540 nm. The solid line represents the simulation curve calculated with T$'' = 300 fs. [Per] 3 X lo-) M, [TCNE] 1.2 M.
-
-
I . Femtosecond-Picosecond Laser Spectroscopy of CR Decay and Dissociation of Transient IP. Some examples of time-resolved absorption spectra observed by exciting the C T complex in acetonitrile solution are indicated in Figures 1-3. In all cases examined here, the time-resolved spectra were observed by exciting exclusively at the C T absorption band. The spectra in Figure 1A show ion bands due to the IP immediately after excitation, although these ion bands are a little broader and slightly red-shifted compared with those of free ions. In Figure 1B, the absorbance at the peak position of the ion band is plotted against the delay time (a) together with that observed in the case of the IP formed by encounter between excited Py*(S,) and PA (b).Io The decay curve b in Figure 1B 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 C T complex. The curve a shows much faster decay compared with curve b, which leads to the much smaller amount of the dissociated ions in the former case. By subtracting the constant absorbance due to the dissociated ions from the observed decay curve a, the remaining absorbance decay has been approximately reproduced by a single-exponential function with a decay time of ca. 120 ps. The different behavior of the I P formed by excitation of the C T complex 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 I P including the surrounding polar solvent molecules is different depending on the mode of its formation. In the case of (a) io Figure lB, compact IP (CIP) probably with no intervening solvent molecule between A- and D+ (contact IP) will be formed while more loose I P (LIP) with intervening solvent molecules between A- and D+ (solvent-separated IP, SSIP) seems to be formed in (b), and their structures are maintained at least during several hundred picoseconds in acetonitrile solution. From the observed decay time = 120 ps) of C I P and its dissociation yield (@:E = 0.16) obtained by taking the ratio of the absorbance of the dissociated ion and the initial value estimated by extrapolating the decay curve a to t = 0, we have evaluated k::! = 1.3 X lo9 s-l and kEf = 7 X lo9 s-l for Py-PA in acetonitrile.
(TF
&A
ground state oi complex
It may be argued that the dissociation from CIP will take place via a definite LIP or SSIP state. CIP
- ksd
LIP
by:
As-
I
.
+ Ds'
(2)
However, the observed absorbance decay can be reproduced approximately by a single-exponential function. Similar results have also been obtained for other systems. Moreover, in almost all cases
\ I
0
1.o
1
2.0 -A
-
I
3.0
Gyp /ev
Figure 4. Energy gap dependence of CR rate constant of CIP (0,A) compared with that of LIP (0)in acetonitrile. (Results for LIP are taken from ref 10). 1, Pyt-PA-; 2, Ant-PA-; 3, Pert-PA-; 4, Napht-PMDA-; 5, Chr+-PMDA-; 6, Py+-PMDA-; 7, Pert-PMDA-; 8, Naph+-TCNQ-; 9, Pyt-TCNE-; 10, PeP-TCNE-; 11, Bzt-PMDA-; 12, Tol+-PMDA-;
13, m-Xylt-PMDA-; 14, p-Xylt-F'MDA-; 15, Dut-PMDA-; H MB+-PM DA-.
16,
examined here, kgF is predominant leading to very small or negligible dissociation yield. Therefore, we have evaluated kgf by the reaction scheme of eq 1. In Figure 2, time-resolved absorption spectra and time profiles of ion absorbance of PMDA-HMB and PMDA-Naph systems measured by femtosecond laser photolysis method are indicated. The ion absorbances due to PMDA- rise up within the time resolution of the apparatus in both cases. From the time profile in Figure 2C as well as the results of picosecond laser hotolysis measurements on the same system, we have obtained & ? a n d to be 25 ps and 0.03, respectively, from which k$ has been evaluated to be 3.8 X lo9 s-I for PMDA-Naph. Similarly from = 0 for PMDA-HMB, which Figure 2B, .$Ip = 5.2 ps and gives kEf = 1.9 X 10" s-l. In the case of the aromatic hydrocarbon-TCNE and -TCNQ complexes in acetonitrile, the C I P state is very short-lived due to the large k g f values, inhibiting dissociation of C I P into free ions. Although the spectral band shape distortion due to the chirping of the monitoring light is larger in these systems compared to other systems described above, corrected time-resolved spectra are clearly the superposition of A- and D+ bands. As an example, the spectrum of the Pj-TCNE complex in acetonitrile immediately after the rise of absorbance is shown in Figure 3A, which is similar = 445 nm) and TCNE- (A,, to the superposition of Py+ (A,, = 425 nm) bands. The spectrum we have obtained in the same way for the Per-TCNE complex in acetonitrile shows an absorption peak at 540 nm which can be assigned to Per'. The time profiles of absorbance of these systems are indicated in Fi ure 3B,C for Py-TCNE and Per-TCNE systems, which give T~~ dP = 500 and 300 fs, respectively, by simulation. 2. Energy Gap Dependence of C R Rate of CIP Produced by CT Complex Excitation in Acetonitrile Solution. By means of the femtosecond-picosecond laser photo1 sis measurements as outlined in Section 1, we have obtained k& values for many C T complexes composed of various A and D molecules with different strength, covering a wide range of free energy gap -AGip (from 0.5 to 2.9 eV) between the ion pair and ground state. Those kEf values (0 and A) are plotted against -AGOip values in Figure 4, where the energy gap dependence of the CR rate of LIP'S (0) produced by C S at encounter between the same or similar pairs in fluorescence quenching reactions in the same solvent is also indicated. The free energy gap -AGOip for CIP in Figure 4 is evaluated by the same procedure as used for the L I P S produced by C S at encounter.I0 Since the structure of C I P will be different from that of LIP, there is no reason to use the same -AGO,, value for C I P and LIP even though they are composed of the same A-, D+ pair. Nevertheless, for the purpose of comparison between these two cases and because there is no reliable conventional way for evaluating -AGOip of C I P in acetonitrile, we use simply the same -AGOip value. Roughly speaking, even if we use different -AGOip values for CIP taking into consideration its compact structure, -AGOi, values for various pairs will shift
@E:
J . Phys. Chem. 1989, 93, 6578-6581
6578
as a whole to higher or lower energy, since the oxidation potential of D and the reduction potential of A are usually proportional to their gas-phase ionization potential and electron affinity, respectively, though Coulomb attraction between A- and D+ may be somewhat different from system to system.I8 Therefore, the functional form of the energy gap dependence of k:kP will be similar to that indicated in Figure 4. Anyhow, the energy gap dependence of k:: does not follow the bell-shaped one observed for the C S at encounter in the fluorescence quenching reaction but can be given approximately by k$E cy exp[-PACoip] (3)
-
where a and p are constants independent of AGOiF The cy value for the alkylbenzene-PMDA series is a little larger than that of the polycyclic aromatic hydrocarbon-acceptor series. This remarkable difference between the energy gap dependence of CR processes of CIP and LIP in acetonitrile should be ascribed to the difference in their structures including A-, D+ configurations as well as solvation, although the theoretical mechanism causing such difference is not clear at the present stage of investigation. It is not possible to give a reasonable interpretation for the relation (18) (a) Foster, R. Organic Charge Transfer Complexes; Academic Press: London, 1969. (b) Mataga, N.; Kubota, T. Molecular Interactions and Electronic Spectra; Marcel Dekker: New York, 1970
of eq 3 covering such a wide energy gap range on the basis of the conventional electron-transfer theories.19 It should be noted here also that the nature of the solvent seems to affect significantly k::, since we have confirmedI2 that the T? value of the PyTCNE system becomes much longer than 500 fs in the less polar solvent chloroform. Investigations on the solvent polarity effects upon the energy gap dependence of k:: are going on in this laboratory, and details of the present results including studies on such solvent effect will be reported shortly elsewhere with some theoretical considerations. In this paper, we have demonstrated the essential difference of the energy gap dependence of k g p from that of in acetonitrile, which cannot be interpreted by the usual electron-transfer theories assuming relatively weak electronic interaction between D and A. One must be very careful in discussing electron-transfer mechanisms concerning the C S in the formation of CIP and LIP as well as their CR processes and should not mix up these quite different behaviors. Acknowledgment. The authors are grateful to Dr. T. Okada, Dr. Y . Hirata, and Dr. H. Miyasaka for their helpful advice in the experimental measurements. N.M. acknowledges the support by a Grant-in-Aid (No. 62065006) from the Ministry of Education, Science and Culture, Japan. (19) Marcus, R. A,; Sutin, N. Biochem. Biophys. Acta 1985, 811, 265.
Symmetry-Specific Densities of Rovibrational Energy Levels for Nonseparable Systems Beatriz M. Toselli* and John R. Barker‘ Department of Atmospheric, Oceanic and Spaces Sciences, Space Physics Research Laboratory, The University of Michigan, Ann Arbor, Michigan 48109-2143 (Received: May 30, 1989)
Symmetry-specificdensities of states, Ni(E) = ( i = symmetry species) for nonseparable vibrational and rovibrational coupled systems, are calculated from spectroscopic data by a relatively simple and efficient Monte Carlo technique, which employs Tisza’s rules for the symmetry assignments of individual states. Results are presented for NOz and CH20. The calculations show the effects of the intermode couplings on Ni(E) and the rate of convergence of Ni(E) toward the group theoretical “regular representation” at high energy.
1. Introduction
In a recent paper,] a technique for calculating densities of states N ( E ) for nonseparable degrees of freedom (DOF) was described that gives results in very good agreement with those obtained from multidimensional Monte Carlo sums of states.2 In the present paper, the technique of ref 1 is extended to calculate N ( E ) for the specific symmetry species in nonseparable vibrational and rovibrational coupled systems. In the past few years there has been considerable interest in the calculation of symmetry-specific densities and sums of states. Some time ago, Quack3 discussed a general method for obtaining detailed symmetry selection rules in reactive collisions and applied them to evaluate symmetry corrections in statistical theories of scattering. More recently, he presented4 a detailed and general group theory discussion related to the decomposition of N ( E ) according to the regular representation. Ledermann, Runnels, and Marcuss presented a simple statistical formula expressing relative densities for vibrational states of a given symmetry of nonlinear molecules. Later, Ledermann and Marcus6 extended the method to linear molecules with inclusion of the effects of angular momentum. Sinha and Kinsey’ presented a fast com-
* Address correspondence to this author. ‘And Department of Chemistry. 0022-3654/89/2093-6578$01 SO10
putational technique that makes use of the Stein-Rabinovitch8 method to evaluate symmetry-specific sums of states for separable systems. Pechukasg has given a general rederivation of the statistical limit based on group theory, but neglecting angular momentum. Knowledge of the number of states for each symmetry is of importance in spectroscopy at low energies,*Oand theoretical calculations” predict that the specific rate constants k ( E ) depend on symmetry in some unimolecular reactions. Each of the statistical methods mentioned above applies in the limit of high densities of states. Calculations reported in ref 3-7 confirm the validity of the regular representation for the density of states, but all of these calculations made use of the BeyerSwinehartI2 algorithm or its extention by Stein and Rabinovitch.8 (1) Toselli, B. M.; Barker, J. R. Chem. Phys. Lett., in press. (2) Barker, J. R. J . Phys. Chem. 1987, 92, 3849. (3) Quack, M. Mol. Phys. 1977, 34, 21. (4) Quack, M. J . Chem. Phys. 1985, 82, 3277. (5) Ledermann, S. M.; Runnels, J. H.; Marcus, R. A. J . Phys. Chem. 1983, 87, 4364. (6) Ledermann, S. M.; Marcus, R. A. J . Chem. Phys. 1984, 81, 5601. (7) Sinha, A,; Kinsey, J. L. J. Chem. Phys. 1984, 80, 2029. (8) Stein, S. E.; Rabinovitch, B. S. J . Chem. Phys. 1973, 58, 2438. (9) Pechukas, P. J . Phys. Chem. 1984, 88, 828. (10) Diibal, H. R.;Quack, M. Chem. Phys. Lett. 1981, 80. 439. ( 1 1) Waite, B. A.; Gray, S.K.; Miller, W. H. J . Chem. Phys. 1983, 78, 259.
0 1989 American Chemical Societv
