J. Phys. Chem. 1984, 88, 4937-4943
is not known, but it appears to be shorter than 0.01 ns.4 Therefore, only Br- and I- can interact with IAQS* in the region where +AQsrises with concentration. However, if such interaction occurs, it should probably interfere with the production of AQS- since back electron transfer is usually more effective for singlet than for triplet esciplexes. The view that singlets do not play a major role in the formation of AQS- is also supported by experinents on mixed anions. The fact that even 0.05 M of added anion is sufficient to appreciably decrease the yield of AQS- (Figure 9) indicates that the reduction of AQS by C1- involves species much longer lived than IAQS*. In fact, these results can be explained by a simple scheme involving competition between C1- and the added anion for triplet AQS, converting it to the corresponding exciplexes: 3AQS*
+ C1-
3AQS*
+ X-
kqCT
kqT __*
3(AQS-.Cl)
(9)
3(AQS-*X)
(10)
We also assume that each exciplex reacts further as if the other anion was not present in the system; i.e., it interacts only with the anion already present in the exciplex. With this assumption we ignore cross-reactions such as e.g. 3(AQS-.Cl) with I- or CIwith 3(AQS-.I). Such a simple scheme leads to the following linear correlation:
4937
where D, Dcl, and D , are the initial (extrapolated) transient absorbances produced at constant X with both anions present, with C1- alone, and with X- alone, respectively, at the given concentrations. Figure 9 shows the validity of eq 8' for Br-, I-, and N3-, and the values of kqx-/kqc' are included in Table I. The agreement with the ratios obtained from direct measurement of the quenching constants or from anion inhibition of photohydroxylation is very good for I- and also for N3- if we consider the behavior at [N3-] < 0.15 M. With Br- the value obtained (kqB'/kqC' = 2.6) is much lower than that measured directly (7.6). This, and the upward curvature of the plot for N,- (Figure 9), indicates that our approximations are crude, e.g. that N< does quench the chloro exciplex. It should also be borne in mind that at high ionic concentrations the ratios of rate constants may be appreciably different from these measured at moderate concentrations. (At 1 M X-, the average distance between X- and the organic molecule is -7 8, and a change in the solvation layer should also occur.) In this respect the agreement is surprisingly good. Altogether, we believe that these results strongly support the conclusion that the photoreduction of AQS by these anions proceeds through the triplets.
Acknowledgment. We much appreciate support of this work by grants from the US-Israel Binational Science Foundation (Research Grant No. 2657/81) and the U S . Department of Energy, Division of Basic Sciences (Contract No. DE-AC0276ER03117). Registry No. AQS, 84-48-0;AQS-, 38821-96-4;CI-, 16887-00-6;N
