fate is hydrogen abstraction from the solvent leading to regeneration of the parent molecule, phenol. The latter path is confirmed by the detection of phenol in the photolysis of anisole. The rate constant for the loss of the cation in methanol was determined to be 2.3 X lo5 s-I, and in hexane the same rate constant was measured as 0.8 X IO5 s-I. No significant change in these values was observed in the presence of oxygen, even with the use of oxygen as the purge gas during the course of the photolysis. Similar measurements for the decay of the phenoxy radical (spectral feature at 247 nm) were 0.6 X lo5 s-I and 1.0 X lo5 s-' in hexane and methanol, respectively. All values were for deoxygenated solutions. Measurements of rate constants via the transient absorption spectra are necessarily prone to error and these values should be taken as estimates of the relative rates of radical versus cation reactivity, rather than as absolute values. However, the approximate factor of 3 difference in the rate constants was reproducible and easily observed in the typical transient absorption spectra presented in Figure 2.To summarize, it has been shown that resonantly enhanced multiphoton pumping of aromatic molecules in solution cleanly produces cationic species, which may then be used as reagents in ion-molecule chemistry. The MPIC technique may be thought of as a more selective type of ionizing radiation and may prove to be a useful technique in probing the nature and rate constants of ion-molecule reactions in solution. Further experimental work will involve the comparison of these solution-phase reactions with gas-phase photochemistry in the presence of the same solvent molecules in order to better characterize the relationship of these gas-phase "solvated" reactant studies to actual studies in solution.We examined charge-transfer (CT) state participation in an excited-state organic molecule (M)-molecular oxygen complex using a variety of timeresolved spectroscopic probes. We showed earlier that singlet oxygen ('4 0,) is formed upon photolysis into the M-0, CT absorption band of molecules whose CT state energy is lower than that of the triplet state (3Ml). Our data were consistent with the production of 'A 0, upon dissociation of the photoexcited CT complex. We now examine a molecule whose CT state energy is substantialfy higher than that of 3MI. Relative 'Ag 0, and 'MI yields were determined upon (1) photolysis into the M-02 CT band of 1-methylnaphthalene and (2) 3Z; 0, quenching of triplet 1-methylnaphthalene, which was independently produced. In nonpolar solvents, '9 0, and 'MI yields were independent of the *(Ma,) production method, indicating that relaxation of the 'JCT states to the *3(3M1...3Z, 0,) states is very efficient. In a polar solvent where the CT state is more stable, the data indicate that direct coupling between the CT and ground-state surface [3('Mo,3ZC; O,] may increase. CT-mediated indirect coupling of the ' V~(~M~. . .~Z ; O,), and other M-0, excited states, to the ground-state surface should also inc...
Singlet molecular oxygen ( 'Ago2) has been created, by energy transfer, from a triplet-state photosensitizer in a variety of solid organic polymers. In independent time-resolved experiments, the phosphorescence of 'Ago2 and the absorbance of the sensitizer triplet state were monitored as a function of the sample temperature, matrix rigidity, and molecular composition. In more glassy samples, the time-dependent behavior of the ' Agoz phosphorescence signal is strikingly different from that observed in liquid analogues, exhibiting long decay times with non-first-order kinetics. As the polymer matrix is made leas glassy, however, either by an increase in temperature or by using copolymers or low molecular weight additives, the lAg02 phosphorescence signal appearance and disappearance rates increase, approaching rates observed in liquid solvent analogum. At this limit, the '$02 decay follows fmborder kinetics. The triplet sensitizer flash absorption data indicate that, in the glassy organic polymers, the time-dependent behavior of the 1$02 phosphorescence signal principally reflects that of its precursor. Deconvolution of the sensitizer decay from the experimentally observed (or manifest) 'Ago2 phosphorescence signal yields intrinsic 'Ago2 lifetimes which, to a first-order determination, are independent of temperature and matrix rigidity and are approximately equivalent in magnitude to those recorded in liquid-phase analogues. In this convolution integral, it is necessary to incorporate a model in which the 'Ago2 sensitizer exists in a distribution of nonequivalent sites in the polymer matrix.Data obtained from a perdeuterated polymer give an intrinsic 'Ago2. lifetime that is an order of magnitude longer compared with that obtained from a perprotiated analogue and indicate that a solid-phase matrix exerts control over the intrinsic rate of 'Ago2 deactivation in a way very similar to that in liquid-phase systems.
charges on the porphyrin periphery do not exert the same effect as a point charge or a uniformly distributed charge (see, e.g., ref 22a). Moreover, the reference self-exchange rates listed in Table III were calculated in most cases by disregarding the work terms. Because of all these reasons it was decided to present only the results obtained without accounting for work terms (Table III).From the rate of reduction of IrCl61 2" we calculate a very low value of fc(S02/S02'~), in disagreement with the other values.The reason for the difference may lie in the fact that Kn in this case is very high, so that the rate of electron transfer kn may be low due to the effect predicted by the "inverted region" of the Marcus relationship, i.e. when further increase in driving force results in slowing down the reaction.A previous estimate of the self-exchange rate of S02/S02*" of 3 X 102 M"1 11*s'1 was derived40 from rate constants with two cobalt complexes.9 On the other hand, from reactions with viologens13 there was an indication of a very high self-exchange rate for S02/S02'" as is found here with methylviologen and the porphyrins. Taking all these results into account, we found k-(S02/S02'~) to vary over many orders of magnitude. The variations in fc(S02/S02*~) are reminescent of the wide variations found in the self-exchange rates for 02/02'~.47 This is not surprising since the behavior of S02*~w as suggested to parallel that of 02-.30 Such variations may be a general characteristic of redox pairs composed of a small number of atoms and one form of which is neutral, so that solvation of the two species varies greatly.47 Similar but smaller variations were noticed also in the reactions of C102 with different types of compounds.48 Acknowledgment. The research described herein was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy. We thank Dr. P. Hambright for many helpful discussions.
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