J. Phys. Chem. 1992, 96, 1520-1523
1520
0.86,12 respectively. In the case of tetraarylhydrazines, the dissociation rate is too large to change the spin state. Therefore, the predissociation should occur in the singlet manifold. This is also confirmed by the time-resolved ESR study of TPH.' There are two possibilities of the predissociation mechanism. One is the SI So internal conversion, which is known for acetylene in the gas phase.I3 The other is the internal conversion from SI to the lower lying electronically excited state, which is not optically accessible from the ground state. The former may not be the case, because the Sl-So energy gap of tetraarylhydrazines (about 350 kJ/mol) should be too large for the rapid internal conversion (tens of picoseconds) of the typical aromatic molecules. We propose the following mechanism. The molecules in the vibrationally relaxed fluorescence state go to the lower lying electronic state and then dissociate immediately. If the potential curves of the fluorescence state and the dissociative state intersect each other, the large Franck-Condon factor should make the transition very fast as revealed in the case of the internal conversion of the higher excited singlet states of aromatic compounds. Therefore, the potential curves may not cross but just come close to each other. In this case the Franck-Condon factor will be small, which can result in the slow internal conversion (tens of picoseconds). Moreover, we have another example of the relatively slow internal conversion in the liquid phase.I4 Although both the upper and lower states are bound in diphenylacetylene, the upper fluorescent state, which is about 3.6 kJ/mol higher than SI,has the lifetime of 8 ps.I4 In the present case, however, the lower state may be repulsive. The wave function of repulsive potential has large value only around the classical turning point. Therefore, if the SI state does not have a crossing point with the repulsive state, the internal conversion will become slow leading to the slow predissociation as in the present case of the tetraarylhydrazine.
TABLE I: Formation Time of Radicals and the Fluorescence Lifetime of TPH. TDTH. and TmTH in Various Solvents
~
solvent n-hexane
viscosity, CP
7,ps
TPH
TDTH
-
TmTH
~
0.24
cyclohexane
1.02
n-dodecane trans-decalin n-hexadecane
1.35 2.13 3.34
a
22 f 3 20.3 f 2.4a 24 f 5 22.8 f l.9a 25 f 4 27 f 5 32 f 5
73 f 4 85 f 7 74.6 f 3.1a 83.5 f 3.8" 77 f 8 87 f I O
86 f 6
98 f 10
- -
Fluorescence lifetime.
the S, SI band of TpTH and the pDTA' band. The reference spectra of the S, SI and the radical bands are the measured spectra at 10 p and 2 ns, respectively. The rise and decay of these components follow a good exponential kinetics as shown in Figure 4b. We should notice here that Hyde and Beddard2 measured the transient absorbance at 759 nm and observed the decay. We could not reproduce their results. The reason why they observed such a rapid decay is not clear. In the longer delay times up to 6 ns, we confirmed that the spectral shape did not change and that no decay of the product radical was observed. If the product radicals recombine geminately to some extent, the recombination should be much faster than the formation of the radical pair. It must occur within a few picoseconds. The formation times of the radical were measured in several solvents and the results are listed in Table I. The lifetime of the SI state appears to increase slightly with increasing solvent viscosity. However, the lifetime in n-hexadecane is 32 ps, which is much shorter than that obtained by Hyde and Beddard (55 ps). We think the solvent effect is not so large as observed by Hyde and Beddard2 at 759 nm. The most important findings in the present work is the direct observation of the slow N-N bond rupture in the solution phase. The dissociation is the predissociation from the SI state because of the agreement of the fluorescence lifetime with the formation time of the product radical. Intersystem crossing of these class of molecules should occur in nanoseconds. The fluorescence lifetime and the triplet yield of diphenylamine are 1.4 nsl0 and
(12) Rahn, R.; Troe, S. J.; Grellmann, K. H. J . Phys. Chem. 1989, 93, 7841. ( 1 3 ) Stief, L. J.; DeCarlo, V. J.; Mataloni, R. J. J . Chem. Phys. 1965, 42, 3113. (14) Hirata, Y.;Okada, T.; Mataga, N.; Nomoto, T. Presented at the Vllth International Symposium on Ultrafast Processes in Spectroscopy, Bayreuth, Germany, 1991, and to be submitted for publication.
Unusual Chemically Induced Dynamic Electron Polarizatlon of Electrons Produced by Photoionization A. S. Jeevarajan and Richard W. Fewenden* Radiation Laboratory and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 (Received: October 14, 1991; In Final Form: December 26, 1991)
ESR polarizations of eaq-produced by photoionization of phenoxide and thiophenoxide were observed in laser photolysis experiments to be absorptive and emissive, respectively. In both cases, random encounters in radiolysis experiments lead to emission. These observationscan best be explained if the ionizing states are singlet and triplet, respectively, and the exchange interaction is positive (triplet lower) in encounters of the radical pairs. The anion radical of benzoate behaves like eaq-in reactions with phenoxy1 and phenylthiyl radicals while that from pacetylbenzoate behaves normally. A reason for the positive exchange interaction is suggested. Introduction The ESR spectrum of the hydrated electron, eaq-, in liquid solution was originally reported only in pulse radiolysis experim e n t ~ . ' - ~Recently, the photolysis of sulfite has allowed ESR ( I ) Avery, E. C.; Remko, J. R.; Smaller, B. J . Chem. Phys. 1968,49,951. (2) Verma, N. C.; Fessenden, R. W. J . Chem. Phys. 1976, 65, 2139. (3) Fessenden, R. W.; Verma, N. C . J . A m . Chem. SOC.1976, 98, 243.
0022-3654/92/2096-1520$03.00/0
observation of ea; in steady-state experiments5 Such an experiment is possible only with a limited range Of compounds which both photoionize and do not react rapidly with ea;. Several additional sources of eaq- are described. (4) Shiraishi, H.; Ishigure, K.; Morokuma,
K . J . Chem. Phys. 1988, 88,
4637. ( 5 ) Jeevarajan, A. S.; Fessenden, R. W. J . Phys. Chem. 1989, 93, 3 5 1 I .
0 1992 American Chemical Society
Letters The time-resolved experiments of Fessenden and Verma3 found several examples of CIDEP (chemically induced dynamic electron polarization) which were backward from the usual case in that the line at higher field (ea
