Photophysical characterization of hydroxy and ethoxy phenalenone derivatives

Phenalenone has been extensively studied due to its photophysical behavior. However, fewer are the articles referring to the phenalenone derivatives as singlet oxygen generators. The incorporation of substituents on the phenalenone ring rearranges the molecular electronic states changing its photophysical behavior. In order to rationalize the effect of the presence of electron-donor substituents on the phenalenone ring, we studied two ethoxy derivatives and their corresponding hydroxyl precursors. All derivatives prepared present smaller singlet oxygen quantum yield values than phenalenone. These lower quantum yields can be rationalized by considering non-radiative decay of singlet excited state absent in unsubstituted phenalenone. The hydroxy and alkoxy derivatives substituted in position 6 of phenalenone have larger fluorescence quantum yields than the ones substituted in position 3. Interestingly, 3-hydroxy-phenalenone shows a low emission quantum yield with two emission bands, which can be related with equilibria between diketo and enol tautomers in the first excited singlet state. Flash photolysis spectra for all derivatives were measured and the extinction coefficients of triplet–triplet absorption were evaluated near 500 nm.