Irreversible Photobleaching of the Solution of Fluorescent Dyes. II. Photobleaching of the Alcoholic Solution of Eosine in Vacuo

Photobleaching of eosine in methanol, ethanol, isopropanol and n-butanol in vacuo was investigated and it was concluded that the primary process is the dehydrogenation of alcohols by the metastable excited eosine molecule. In the presence of oxygen, the photobleaching is suppressed completely, but the sensitized photoöxidation of alcohol takes place. When a small amount of oxygen is added to the evacuated alcoholic solution of eosine, the induction period appears, after which the photobleaching proceeds with almost the same rate as that in the evacuated state. Both the quantum yield for the consumption of oxygen during the induction period (γ) and that of the photobleaching in vacuo (k) were determined and the ratio γ/k was found to be ca. 3. This result and the reaction products detected (aldehyde in vacuo, acid and hydrogen peroxide in the presence of oxygen) were satisfactorily explained by the mechanism proposed by Bolland and Cooper. Further results somewhat important from the general standpoint of photobleaching of dye are : (1) The lifetime of the intermediate to be attacked by oxygen (which is most probably DH·) is not less than 10−2-10−3 sec. (2) The quantum yield for the primary process of the photobleaching D→D*→Dt(+RH)→DH·(+R·) in pure ethanol is ca. 10×10−4 at 20°C. (3) In pure ethanolic solution, the decomposition of dye by the interaction between DH· and R· practically does not occur.