J. Phys. Chem. 1986, 90. 5506-5508
5506
however, was analyzed by assuming that two transients, a triplet state of HBO and the keto ground-state tautomer, 'K, revert independently in two first-order reactions to 'E. The transient absorbance in these experiments was about 0.15 at time zero and about 0.02 at the end of the kinetic analysis (cf. Figure 2 of ref 3). We submit that our results with varying flash intensities provide convincing evidence that under the above conditions TTA is the predominant decay process, as ranges from about 98% at the beginning to about 89% at the end of the kinetic analysis. Most of the information concerning the reaction pathways of the HBO triplet states is hidden in the end of such decay curves, which were apparently not analyzed. It should also be mentioned that TTA can compete effectively with oxygen quenching as long as the triplet-state concentration is high. Toward the end of the decay, a t low transient concentrations, oxygen quenching becomes predominant. Therefore the results of such quenching experiments may lead to the erroneous conclusion3 that laser flash excitation resulted in the formation of a rapidly decaying, oxygen-insensitive transient together with a second one that can be quenched. According to our results, the decay of the observed transient changes from predominantly second order to predominantly first order with decreasing flash intensity, whereas the spectrum of the transient does not change. Hence, there is no experimental evidence for the existence of a second, independently decaying species, and one cannot assume that 'K accumulates in detectable amounts after the excitation of HBO. The proton back-transfer reaction 'K -+ 'E within the singlet manifold (rate constant k,:) is presumably very rapid and the formation of the keto tautomer 'K) will be rate determining for'the decay of 'K. For (IK* this reason it will be extremely diffcult to detect the transient absorption of IK and in order to study the kinetics of the reaction 'E it would be more promising to monitor the bleaching 'K and the recovery of the enol tautomer absorption. Experiments of this kind have been performed with a number of compounds similar to HBO"' with the result that proton back-transfer in
-.
-+
the ground state is a very rapid process that occurs in the subnanosecond time region. Our conclusion that HBO is not exceptional in this respect may be of some practical importance because proton-transfer systems of this kind are of increasing interest in the field of applied laser physics.I2J3 presumably, the energy difference between 'K* and 3K* is small (> kk. Furthermore, based on energetic considerations, it is anticipated that k, would be of the order of lo9 to 1O'O s-I.l1 We therefore conclude that the rate-limiting step for formation of 'TS from SCRIP is intersystem crossing. The rate is -1.5 X lo7 s-l and solvent independent. Consequently, the effect of solvent on the lifetime of SCRIP is due to only the solvent-dependent processes, k,,, and ket.
Laser excitation (355 nm, 0.2 mJ, 25 ps) of the TS/FN complex in acetonitrile generates the SCRIP within the laser pulse and whose transient absorption spectrum, with A,, = 480 nm, is characteristic of the radical cation of trans-stilbene TS'. The dynamics of the TS+ are subsequently monitored from 25 ps to 125 ns. For the details of the experimental and kinetic analysis to give both k,, and kipssee ref 5 . Both k i p and k,, for TS/FN complex were measured in acetonitrile as a function of temperature, ranging from 8 to 72 "C. The resulting temperature dependence of kip is shown in Figure 1. An Arrenhius analysis of the kinetic data for kip gives kips = 1012~6+o.2 exp(-4200 f 1000 (cal/mol)/RT) s-I This corresponds to = -2.9 o,9 K-l mol-^ and iw*= 4.7 f 1.0 kcal/mol. Unfortunately, it is unclear how the enthalpic and entropic components of the barrier for ion pair separation are to between the ion pair electrostatic interaction and ion ;air-solvent interactions. N o temperature dependence on k,, was observed in acetonitrile, k,, (7.6 f 1.9) X lo9 s-l (8-72 " C ) . Thus the net effect of increasing temperature on the dynamics of SCRIP is to increase the concentration of SSRIP. Within experimental error, the rate
-
(IO) (a) Marcus, R. A. J . Chem. Phys. 1984, 81,4494. (b) Miller, J. R.; Calcateria, L. T.; Closs, G. L. J . Am. Chem. SOC.19884, 106, 3047. (11) Rehm, D.; Weller, A. Isr. J . Chem. 1970, 8, 259.
J . Phys. Chem. 1986,90, 5508-5518
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of back-electron transfer was also independent of temperature in both dimethoxyethane, k,, = (7.1 f 1.8) X lo* s-l (25-74 “C), and ethanol, k,, = (1.6 f 0.4) X 1Olo (25-71 “C). The SCRIP does not undergo appreciable ion-pair separation in either dimethoxyethane or ethanol within the temperature range studied. Consequently, even at the elevated temperatures, k i p
