Abstract The irreversible photobleaching of eosine in the aqueous solution and in alcohol-water mixtures was studied and it was found that the rate of photobleaching depends only on and is proportional to the quantity of ab-sorbed light when the dye concentration is 10−5 mol./1. The overall unimolecular rate constant k or the quantum yield, when the quantity of dissolved oxygen is varied, does not change over a wide region but begins to decrease below a certain critical point. The values of k in water-alcohol mixtures were found to be expressed by the following equation in the low concentration region of alcohol. (Remark: Graphics omitted.) where k° is the rate constant of the pure aqueous solution and K is a certain constant. The possible schemes were given particularly on the bases of the rate formula (6) and of the effect of the quantity of dissolved oxygen. It is most probable that the irreversible photobleaching of eosine by oxygen is a successive reaction as represented by the following scheme Ib) D\xrightarrowhνD*→Dt\xrightarrowO2Dt···O2\xrightarrowH2O··· Product. In this case, the probability with which Dt···O2 is deactivated by alcohol is about 100 times greater than that of Dt···O2 being attacked by water. There is some possibility that the reaction may proceed in the following way. IIb) D\xrightarrowhνD*→Dt\xrightarrowH2ODt···H2O\xrightarrowO2··· Product Two cases corresponding to this scheme are possible. When a moderate fraction of Dt contributes to the reaction, the intermediate Dt···H2O must be deactivated by alcohol far more frequently than it reacts with oxygen, while when a very small fraction of Dt participates in the reaction, the probability with which Dt deactivated by alcohol is much larger than that of the reaction with water. The former case is rather improbable, while, the latter one seems to conform to a newly discovered fact that eosine in alcoholic solution is photobleached quite easily under evacuated conditions. The kinetic studies of the vacuum-state-photobleaching of eosine in alcohol, the comparison of the effects of various alcohols upon the rate of oxygen-water-bleaching and the measurement of the rate of the possible vacuum-bleaching in various alcohols, all these investigations now in progress will give a more definite conclusion.

Imamura, M., & Koizumi, M. (1955). Irreversible Photobleaching of the Solution of Fluorescent Dyes. I. Kinetic Studies on the Primary Process. Bulletin of the Chemical Society of Japan, 28(2), 117–124.