314
J. Phys. Chem. 1987, 91, 314-317
the initial one. Only at quite large v values does the deviation from adiabatic behavior becomes appreciable. Nonreactive deactivation rates have a value of 2.9 X 6.0 X lO-I4, 1.4 X and 4.9 X cm3/(molecule-s) respectively at v = 9, 11, 13, and 15. The importance of deactivation mechanisms is reflected also by the shape of the plots of detailed rate constants when reported as a function of n. Apart from the sharp peak for the elastic (n = 0) rate constants not reported in the figure, these plots have no definite trend with n. This is clearly illustrated by the lower panel of Figure 3, where the nonreactive k(v,u’ = ~ n ) rate constant values are reported at fixed v values. Although we did not carry out a graphical analysis of individual trajectories, we expect that the dynamics of nonreactive vibrational deactivation is determined by those events that spend most of their life in the strong interaction region before finding their way out. In fact, the dynamical outcome of these trajectories is known to have a sensitive dependence on even small variations of the initial conditions and, therefore, to lead to a substantially random population of final states.15
4. Conclusions Rate constants calculated for the N + N, reaction using a classical trajectory approach show that the system is scarcely reactive for reactions thermalized at 1000 K even when the initial vibrational number is large. The nitrogen system is less reactive than the similar hydrogen system, although the vibrational energy dependence of the reactive rate constants was found to be similar to that for the H H2reaction. Nonreactive collisions were found to be strongly biased toward vibrationally adiabatic behavior, in contrast with the dynamics of reactive collisions.
+
Acknowledgment. We thank M. Capitelli for stimulating the systematic investigation of the kinetics of vibrationally excited nitrogen molecules colliding with nitrogen atoms and for useful discussions. Registry No. N2, 7727-37-9; N, 17778-88-0. (15) Lagan& A.; Hernandez, M. L.; Alvarifio, J. M. Chem. Phys. Lett. 1984, 106, 41 and references therein.
Dynamics of Charge Recombination Processes in the Singlet Electron-Transfer State of Pyrene-Pyromeilitic Dianhydride Systems in Various Solvents. Picosecond Laser Photolysis Studies Noboru Mataga,* Hiroshi Shioyama, and Yu Kanda Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan (Received: April 14, 1986; In Final Form: August 26, 1986)
In order to elucidate the underlying mechanisms showing that no dissociated ion radicals are produced even in acetonitrile solution when some complexes of the strong electron donors and acceptors with CT absorption bands in the visible region are photoexcited, we have made detailed time-resolved transient absorption spectral measurements and timeresolved fluorescence measurements upon the pyrenepyromellitic dianhydride (PMDA) system in various solvents with picosecond laser spectroscopy. A weakly fluorescent electron-transfer (ET) state with 400-pslifetime is formed by photoexcitation in benzene solution, while nonfluorescent geminate ion pairs with much shorter lifetimes due to the charge recombination (CR) deactivation are formed in more polar solvents. In all solutions examined, dissociation into free ions from the geminate pair cannot compete with the CR deactivation which becomes faster in more polar solvents due to the decrease of the energy gap between the ion pair and neutral ground state. Moreover, it has been demonstrated that ion pairs produced by encounter between excited pyrene and unexcited PMDA in acetonitrile have more loose structure and show a smaller CR rate constant than those produced by exciting the ground-state complex.
The photoinduced electron transfer (ET) and charge separation (CS) in solution take place through the encounter between excitedand ground-state molecules or excitation of the ground-state EDA (electron donor-acceptor) complex.’ Fairly detailed studies on behaviors of the ET state have been made for the systems with a relatively large energy gap between the E T and ground state such as typical exciplexes and weak EDA complexes.’ However, such studies on the systems with stronger electron donors and acceptors or with considerably smaller energy gaps between the ET and ground state were scarce, mainly because of the very rapid nonradiative degradation of the E T state to the ground state. Investigations on the dynamics of such short-lived ET states have become possible by the picosecond laser photolysis method.Ic” (1) See for example: (a) Mataga, N.; Ottolenghi, M. In Molecular Assocation; Foster, R., Ed.; Academic: New York, 1979; Vol. 2, p 1. (b) Mataga, N. In Molecular Interactions; Ratajczak, H., Orville-Thomas, W. J., Eds.; Wiley: Chichester, 1981; Vol. 2, p 509. (c) Mataga, N. Pure Appl. Chem. 1984, 56, 1255. (2) Mataga, N.; Karen, A.; Okada,T.; Nishitani, S.; Kurata, N.; Sakata, Y.; Misumi, S . J. Am. Chem. Soc. 1984,106,2442. (b) Mataga, N.; Karen, A.; Okada, T.; Nishitani, S.; Sakata, Y.; Misumi, S. J. Phys. Chem. 1984, 88, 4650. (c) Mataga, N.; Karen, A,; Okada, T.; Nishitani, S.; Kurata, N.; Sakata, Y.; Misumi, S. J . Phys. Chem. 1984, 88, 5138. (d) Mataga, N. THEOCHEM. 1986, 135, 279. (e) Mataga, N.; Okada, T.; Kanda, Y.; Shioyama, H. Tetrahedron, in press.
0022-365418712091-0314$01.50/0
Nevertheless, systematic picosecond laser photolysis studies on various factors affecting the dynamic behaviors of such systems are rather scarce. In the previous papers, we have reported the detection of the short-lived exciplex state of porphyrin-quinone systems by means of the picosecond laser photolysis method in nonpolar solvent and its solvation-induced ultrafast deactivation in polar solvents.2a,b In contrast to the case of typical exciplexes like the pyrene-N,N-dimethylaniline (DMA) and pyrene-p-dicyanobenzene CpDCNB) systems or weak EDA complexes like the 1,2,4,5-tetracyanobenzene (TCNB)-toluene system which are fluorescent in nonpolar or less polar solvents and undergo the dissociated ion radical formation in strongly polar solvents, the porphyrinquinone exciplex is practically nonfluorescent even in nonpolar solvents and does not show any dissociated radical ion formation even in strongly polar solvents due to the overwhelming ultrafast charge recombination (CR) deactivation. Since the porphyrin-quinone systems have much smaller energy gaps between ET and ground state compared to the typical exciplexes and weak EDA complexes, the above result is a demonstration of the strong energy gap (3) (a) Hilinski, E. F.; Masnovi, J. M.; Amatore, C.; Kochi, J. K.; Rentzepis, P. M. J. Am. Chem. Soc. 1983, 105, 6167. (b) Hilinski, E. F.; Masnovi, J. M.; Kochi, J. K.; Rentzepis, P. M. J . Am. Chem. SOC.1984, 106, 8071.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 2, 1987 315
Dynamics of Charge Recombination Processes
I
3.oF
TABLE I: Lifetimes of Ion-Pair State Obtained by the Time-Resolved Transient Absorption Measurements ( 7 , ) and by the Fluorescence Decay Time Measurements (7J dielectric solvent const 7./DS 7rlDS benzene 2.3 410 400 ethyl acetate 6.0 60 THF 7.4 50 acetone 20.7 -15 acetonitrile 37.5 10 ~
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400
500
600 Wavelength I n m
Figure 1. CT absorption band of the PMDA-pyrene EDA complex in acetonitrile solution.
dependence of the C R rate and strong effects of solvation on it. In the present paper, we show results of picosecond laser photolysis studies on the dynamic behaviors of the excited singlet state of the pyrene-pyromellitic dianhydride (PMDA) EDA complex which is an intermediate case between the above extremes. On the basis of the present results, effects of the solvent polarity upon the nature of the ion-pair state produced by excitation of the pyrene-PMDA complex and its C R deactivation process will be discussed. Moreover, we have examined also the behaviors of ion pairs produced by encounter between excited pyrene and unexcited PMDA in acetonitrile solution. Results of such measurements indicate that ion pairs produced in the encounter have more loose structure than those produced by directly exciting the ground-state complex in acetonitrile and show a smaller rate of CR deactivation. These results will be discussed in comparison with the previous results on the existence of plural kinds of ion pairs in the case of some typical exciplex systems in polar so1vents.lc Experimental Section
Picosecond transient absorption spectra were measured by means of a microcomputer-controlled, mode-locked Nd3+:YAG laser photolysis ~ y s t e m . The ~ second harmonic or third harmonic of a single picosecond pulse (25-ps duration) was used for excitation. The transient absorption spectra and their decay curves (in 100 ns to microsecond regime) were obtained by using a laboratory-constructed dye laser photolysis apparatus5 or an excimer laser ( X e F 351 nm, fwhm 14 ns) photolysis system.6 Ground-state absorption spectra were measured with a JASCO UVIDEC- 1 spectrophotometer or a Shimazu UV-260 spectrophotometer. Calibrated fluorescence spectra were obtained with an Aminco-Bowman spectrophotofluorometer or Hitachi 850 spectrophotofluorometer. G R grade PMDA was recrystallized from ethyl acetate and sublimated under vacuum. Pyrene, 1,2-benzanthracene, and N-ethylcarbazole were purified by recrystallization and subsequent sublimation under vacuum. N,N-Dimethylaniline (DMA) was refluxed with acetic anhydride, washed with water, dried over potassium hydroxide, and distilled under vacuum. Acetonitrile, acetone, tetrahydrofuran (THF), ethyl acetate, and benzene were spectrograde reagents and used without purification. Sample solutions for the spectral measurements were deaerated by flushing with nitrogen gas. The concentration of pyrene was ca. 5 X M and that of PMDA was ca. 0.2 M in the case of the excitation of the ground-state EDA complex at the C T absorption band in the visible region.
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(4) (a) Miyasaka, H.; Masuhara, H.; Mataga, N. Laser Chem. 1983, 1, 357. (b) Masuhara, H.; Ikeda, N.; Miyasaka, H.; Mataga, N. J . Spectrosc. SOC.Jpn. 1982, 31, 19. ( 5 ) Shioyama, H.; Masuhara, H.; Mataga, N. Chem. Phys. Lett. 1982,88, 161. (6) (a) Hirata, Y.;Mataga, N. J. Phys. Chem. 1985,89,4031. (b) Hirata, Y.;Mataga, N. J . Spectrosc. SOC.Jpn. 1985, 34, 104.
Results and Discussion Structures and C R Decay Processes of Geminate Ion Pairs Produced by Excitation of Ground-State EDA Complexes in Various Solvents of Different Polarity. The lowest energy CT absorption bands of PMDA complexes with pyrene, 1,2-benzanthracene, N-ethylcarbazole, and DMA appear around 500 nm: their peak positions are 475, 470, 490 (sh), and 510 nm, respectively, in acetonitrile. As an example, the spectrum of PMDA-pyrene in acetonitrile is indicated in Figure 1. We have examined above EDA complexes in acetonitrile solution by means of dye laser photolysis to detect the dissociated ion radicals in the microsecond regime. We excited the EDA complexes with the light pulse (35 mJ, 503 nm) from the flash lamp pumped dye laser and tried to observe characteristic absorption bands of the PMDA anion at 665 nm. However, no such absorption band was detectable in any one of the above systems. This result indicates strongly that the ion pairs produced by photoexcitation of these EDA complexes in acetontrile undergo C R deactivation which is much faster than the dissociation into free ions, just as in the case of porphyrin-quinone systems.2a,b In order to observe directly the formation and decay processes of ion pairs and to examine the solvent polarity effect upon the dynamics of ion pairs, we have investigated time-resolved transient absorption spectra of the excited PMDA-pyrene EDA complex in various solvents by means of the picosecond laser photolysis method. Observed results are shown in Figure 2. The transient absorption bands in these figures are rather similar to each other and can be assigned to the transition, (PMDA--Pyr+) (PMDA-*--Pyr+) (Pyr = pyrene), because the absorption peak coincides with that of the PMDA anion radical. The rise and decay of the absorbance have been simulated by taking into consideration the pulse width of the exciting laser (-25 ps) and of the picosecond continuum (-24 ps), from which the lifetimes of the ion-pair state in various solvents have been obtained as indicated in Table I. Some examples of the comparison of the simulated curve with observed values are indicated in Figure 3. One can see from these results that the lifetime of the ion pair in benzene is extraordinarily long compared to those in other solvents. This result suggests that the nature of the transient ion pair in benzene is somewhat different from those in other solvents. In accordance with this suggestion, we have observed weak C T fluorescence around 690 nm in benzene solution as indicated in Figure 4, while it was not possible to detect C T fluorescence in other solvents. We have determined the fluorescence lifetime 7f in benzene to be 400 ps by means of a streak camera. This 7f value agrees satisfactorily with the lifetime of the ion-pair state determined by the transient absorption measurement. Therefore, the weakly fluorescent contact ion pair seems to be formed by photoexcitation in benzene solution. Contrary to this, the solvent-shared ion pair or geminate pair of the solvated radical ions which is practically nonfluorescent seems to be formed in other solvents. We have determined an approximate value ( lo4- lo-' in its order of magnitude) of fluorescence quantum yield in benzene solution by using Ru(bpy)2+ solution as a reference. From this value of fluorescence yield and q,the radiative transition probability, kf,of the PMDA-pyrene complex in benzene is estimated to be 2.5 X 105-2.5 X lo4 s-I, which is a little smaller than that ( - 5 X lo6s-') of the typical exciplex of pyrene-DMA in nonpolar solvents. In polar solvents, the fluorescence yield should be smaller than 10"- lo-'. Accordingly, by use of the 7, values in Table
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Mataga et al.
316 The Journal of Physical Chemistry, Vol. 91, No. 2, 1987
l e
I
OPS
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IC
-lops
66 ps
loops
u)
n
a
300 ps
A
600
500 ps
800
700
900
600
700
Wavelength I nm
150 ps
t
900
600
800
700
Wavelength I nm
800 900 Wavelength I nm
Figure 2. Picosecond time-resolved transient absorption spectra of the PMDA-pyrene EDA complex in various solvents of different polarity. Solvents: (A) benzene, (B) ethyl acetate, and ( C ) acetonitrile.
I
than 1 ns in a ~ e t o n i t r i l e . ' ~ Although ~~ the C R decay of the geminate pair becomes slower in less polar solvents, the dissociation will become also slower due to the higher activation energies necessary for dissociation. It should be noted here that the CR decay rates of the present system in polar solvents provide an example of the energy gap dependence of the C R process, when compared with other systems with larger as well as smaller free-energy gaps, AGip, between the ion pair and ground states in polar solvents. The values of AGip of the present system in acetonitrile and acetone, for example, are estimated to be -1.70 and -1.74 eV, respectively, by using eq 1, -AGip = E(D/D+) - E(A-/A) - ( e 2 / t R ) AG, (1)
+
I
IC
L 7'61
0
loo
t I p s 200
Figure 3. Comparison of simulated rise and decay curves of the transient absorbance with observed values. Solvents: (A) benzene, (B) ethyl acetate, and ( C ) acetonitrile. (-) Simulated curves assuming T~ = 410, 60, and 10 ps for A, B, and C , respectively. ( 0 ) Observed values of absorbance. n l
where E(D/D+) and E(A-/A) are respectively the oxidation potential of pyrene and the reduction potential of PMDA in acetonitrile (vs. SCE), R is the center-to-center distance between D+ and A- assumed to be ca. 7 A, AG, is the correction term for the solvation energy of D+ and A- with radii R+ and R- respectively, in a solvent with dielectric constant t . The CR rate constants, kCR,in these solvents are given by 1/ T ~ , Le., kCR 1 x 10" s-l in acetonitrile and kCR 6.7 x 1O'O s-l in acetone. Compared with these CR rate constants of the PMDA-pyrene system in strongly polar solvents, kCR of the 5-ethyletioporphyrin-toluquinonesystem in acetonelc,k,bis > 10" s-I for AGip -1.46 eV, that of the TCNB-toluene system in acetonitrileIc**is 1.8 X lo9 s-l for AGiP -2.66 eV, and that of is -3 X lo7 s-l for thepDCNB-pyrene system in aceton~trile'~*~ AGi, -2.8 eV. These results are an experimental demonstration for the energy gap dependence of the CR reaction of the ion pairs produced by photoinduced electron transfer in the "inverted" region,I0 in that the electron-transfer rate constant decreases with an increase of the free-energy gap between the initial and final states. Actually, it has been demonstrated recently that the above energy gap dependence of kCRcan be reproduced by theoretical calculations. I Geminate Ion Pairs with Loose Structure Formed by Encounter between Excited Pyrene and Unexcited PMDA in Acetontrile. It should be noted here that, in the case of electron-transfer reactions
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10 Wavenumber/ 103cm-'
Figure 4. Charge-transfer fluorescence spectrum of the PMDA-pyrene system in benzene solution.
I, kfin these solvents should be smaller than lo4- lo3 s-' in its order of magnitude, which means that the ion pairs in these solvents have a loose structure due to strong solvation. The above results show that, although the geminate pairs of solvated radical ions are produced by photoexcitation of the complex in acetonitrile solutions, their C R decay (with a decay time of ca. 10 ps) seems to be considerably faster than the dissociation into free ions which takes place in a time slightly shorter
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(7) Hirata, Y.;Kanda, Y.;Mataga, N. J . Phys. Chem. 1983, 87, 1659. (8) Uemiya, T.; Miyasaka, H.; Mataga, N., unpublished results. (9) Kanda, Y.;Hirata, Y.;Okada, T.; Mataga, N., unpublished results. (10) (a) Marcus, R. A. J . Chem. Phys. 1956,24,966. (b) Marcus, R. A. Annu. Rev. Phys. Chem. 1964, IS, 155. (11) Kakitani, T.; Mataga, N. J . Phys. Chem. 1986, 90, 993.
J . Phys. Chem. 1987, 91, 317-322 through the encounter in polar solution, some typical exciplex systems such as pyrene-p-DCNB and pyrene-DMA show the formation of plural kinds of ion pairs undergoing C R deactivation and dissociation into free i o n ~ . l ~ ,Namely, ~ ~ , ~ , some ~ ion pairs undergo easily the dissociation into free ions, while others undergo only the CR deactivation without dissociation. Presumably, former ion pairs may have considerably more loose structure, and the latter will have more compact structure. In this respect, we have examined the behavior of pyrene-PMDA systems when ion pairs are formed by electron transfer at the encounter in the strongly polar solvent, acetontrile. We have made time-resolved transient absorption spectral measurements upon concentrated acetontrile solution with a large amount of the ground-state EDA complex ([pyrene] 5 X lo-’ M, [PMDA] -0.2 M) by exciting the solution with the third harmonic (355 nm) of the picosecond YAG laser. In this case, we excite mainly the ground-state EDA complex, and most of the produced ion pairs disappear within ca. 50 ps due to the rapid C R deactivation. However, we can recognize a small amount of absorbance due to ion radicals even in the few nanosecond to 100 ns regime, which means that a small amount of dissociated ions are produced in competition with CR deactivation of the loose ion pair. This loose ion pair may be formed by encounter between excited pyrene and ground-state PMDA, since a small amount of free pyrene is excited in this experiment. The rise time of the loose ion pair estimated by the SternVolmer equation 7 = so/(l kq70 [PMDA) is ca. 250 ps for the concentrated solution with [PMDA] 0.2 M (saturated). Since we have not detected any decay of the ion pairs in the 100-ps regime, the lifetime of the geminate ion pairs produced by encounter should be shorter than 250 ps. Therefore, it is difficult to detect directly the decay process of the loose geminate ion pairs in the concentrated solution. In order to obtain the approximate value of the lifetime of the of loose ion pairs, we have examined the quantum yield (aion) dissociated ion formation by using dilute solution ([PMDA] 0.02 M) where we excite almost exclusively the free pyrene by a 355-nm laser pulse. The concentration of the initially produced
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317
pyrene SI state has been determined from the absorbance observed immediately after excitation, and that of the pyrene cation has been determined from the absorbance obtained at 5 4 s delay time, by using the molar extinction coefficients of the respective species.I2 From these procedures, approximate values of ai,,,,have been estimated to be 0.1 -0.2. Assuming competition between the CR deactivation (ICCR) and dissociation ( k d )in the geminate ion pair, is given by kd/(kd 4- kCR). On the other hand, we can obtain both aiOn and qp= (kd kCR)-lseparately in the case of pyrene cation-tetracyanoethylene anion geminate ion pairs produced by encounter in acetontrile, from which the rate constants have been estimated to be kd = 2.6 x IO9 s-l and kCR= 2.5 x IO9 s-l. In the case of the TCNB anion-toluene cation ion pair in acetonitrile, we have obtained kd = 1.5 X lo9 s-l.lC3*In view of these kd values in acetontrile, we assume kd 2 X lo9 s-I, which gives, with aion i= 0.1-0.2, kCR 2 x 10”-8 x lo9 S-I. The above results clearly indicate that the lifetime of the loose geminate ion pair produced by encounter between excited pyrene and ground-state PMDA in acetontrile is considerably longer than that of the more “tight” geminate ion pair produced by excitation of the ground-state EDA complex due to the small CR deactivation rate constant in the former ion pair. Therefore, not only the energy gap dependence of the kCRof the geminate ion pairs but also the structure (“loose” or “tight”) of the geminate ion pairs, which depends on the way of its production are important factors determining the ionic dissociation yield of various electron donoracceptor systems including exciplexes as well as much stronger EDA complexes.
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Acknowledgment. This work was partly supported by a Grant-in-Aid (No. 58430003, 58045097) from the Japanese Ministry of Education, Science and Culture to N. M. Registry No. THF, 109-99-9; PMDA, 89-32-7; PMDA-pyreneEDA complex, 3470-21-1; pyrene, 129-00-0;benzene, 71-43-2;ethyl acetate, 141-78-6; acetone, 67-64-1; acetonitrile, 75-05-8. (12) Schomburg, H. Ph. D. Thesis, Gottingen, 1975. (13) Mataga, N.; Kanda, Y . ;Okada, T. J . Phys. Chem. 1986, 90, 3880.
Product Yields and Mechanism of the Excimer Laser Photolysis of Azomethane at 193 nm James E. Baggott,+ Mark Brouard,: Mary A. Coles, Andrew Davis,g Phillip D. Lightfoot: Martyn T. Macpherson,l and Michael J. Pilling* Physical Chemistry Laboratory, Oxford University, Oxford OX1 3Q2,England (Received: May 27, 1986) The photodecomposition of azomethane by single pulse excimer laser photolysis at 193 nm has been studied by using end product analysis and Lyman a resonance fluoresence. The major product (290%) was ethane, as expected, but methane, ethene, ethyne, and propane were also formed in small quantities. The methane and ethene yields were observed to increase with increasing pulse energy, while the propane yield remained constant. Lyman a resonance fluorescence shows that H atoms are produced directly by photolysis and that, at high pulse energies, they are also formed by a secondary reaction. Two minor H-producing photolysis channels are indicated, the first producing H + CH2N2CH, and the second involving secondary photolysis of vibrationally excited CH3 radicals and producing H + CH2; the fractional product yields from the second channel incrgase linearly with pulse energy. A rate constant of (3.4 A 0.2) X cm3 molecule-’ s-l is obtained for the reaction of H atoms with azomethane at 300 K.
Introduction Excimer laser flash photolysis is widely used in the production of reactive s p i e s for time-resolved kinetic measurements.’ Rate ‘Present address: Department of Chemistry, University of Reading, Whiteknights, Reading RG6 2AD. *Present address: Department of Chemistry, The University, Nottingham. 8 Present address: Smith, Kline and French Research Ltd., The Frythe, Welwyn, Hertfordshire. Present address: BP Research Center, Chertsey Road, Sunbury-onThames, Middlesex.
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data of high precision may be obtained2 provided the photolysis sytem is well-defined, i.e. competing fragmentation pathways are shown to be unimportant or can be incorporated quantitatively in the data analysis. Azomethane photolysis at 193 nm was used as the methyl radical source in a recent study of the pressure and (1) See, for example, Baggott, J. E.; Pilling, M. J. Annu. Rep. Prog. Chem. Sect. C. 1982, 80, 199.
(2) Tulloch, J. M.; Macpherson, M. T.; Morgan, C. A.; Pilling, M. J. J . Phys. Chem. 1982, 86, 3812.
0 1987 American Chemical Society
