Intermediates in the Photoreduction of Eosine as Revealed by a Flash-Photolysis Study

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Abstract
Besides the triplet state, semi-reduced eosine (Rd) and semi-oxidized eosine (Ox) were observed as transient intermediates in an aqueous solution; the former has a peak at 408 mμ, and the latter, one at 456 mμ. These two species are produced by the reaction DT+D→Rd+Ox or DT+DT→Rd+Ox. The sum of the rate constants for the reactions DT+D→Ox-+Rd and DT+D→2D is about 8.0×108 m−1 sec−1, approximately equal to the sum of the rate constants for DT+DT→2D and DT+DT→Ox+Rd. Triplet eosine, by reacting with ATU or EDTA, produces two kinds of half-reduced eosine, Rd′ and Rd″, which have absorption spectra similar to those of Rd but with different life-times. These are perhaps loose compounds of half-reduced dye and a half-oxidized reductant. The rate constants for DT+ATU→Rd′ and DT+EDTA→Rd″ are 3.4×105 m−1 sec−1 and 5×105 m−1 sec−1 respectively. Rd disappears bimolecularly by means of the Rd+Ox→2D reaction with the rate constant of 8.8×107 m−1 sec−1. Ox, by reacting with EDTA or ATU, regenerates the original dye with the rate constants of 1×106 and 8×105 m−1 sec−1 respectively. The results enable us to conclude that the D–D mechanism occurs under suitable conditions.