An experimental and theoretical study on the photophysical properties of methylene green

Methylene Green (MG) is a cationic phenothiazine dye that can be considered as a nitro derivative of methylene blue (MB). The photophysical and spectroscopical properties of MG were investigated in a wide variety of solvents, including protic and aprotic ones. The absorption ( ν ¯ A ) and fluorescence ( ν ¯ F ) maxima, fluorescence quantum yields and emission lifetimes were measured and correlated with the properties of the media by using the Bakhshiev’s model and the empirical solvent parameter E T N . Fairly good linear correlations were obtained in both cases indicating the absence of specific interaction between the dye and the solvents studied. The fluorescence quantum yields were found to be in the range 0.004–0.06. Quantum mechanical calculations at the level of Density Functional Theory (DFT) and its Time-Dependent treatment for excited states (TD-DFT) performed on the ground and singlet excited states of MG (and MB) allowed finding a reasonable interpretation of the solvent effects observed on the photophysical properties of the dyes. The triplet excited state properties of the dye were investigated by laser flash photolysis in both protic and non-protic media. In all cases, the triplet quantum yields ( Φ T ) measured for MG were very small (