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 ETN. 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 (ΦT)(ΦT) measured for MG were very small (<0.01). Triplet–triplet sensitization by duroquinone was required for the spectroscopic characterization of the triplet excited state spectrum of MG and the determination of the ΦTΦT in the different media. The triplet state shows important absorptions at 410, 488, and 865 nm. The molar absorption coefficient at 865 nm in acetonitrile, estimated using the depletion method, is ∼30,000 M−1 cm−1. Comparing the calculated values of the deactivation rate constants for the singlet excited states of MG and MB, it is concluded that the incorporation of the nitro group in the thiazine chromophore produces a large (∼30 times) decrease of the intersystem crossing rate constant. With the aid of quantum mechanical calculations it is concluded that the intersystem crossing mechanism in both dyes differ significantly.

Research highlights
► Methylene Green shows very low fluorescence and triplet quantum yields.
► The singlet excited state decays mainly through an internal conversion pathway.
► Comparing with Methylene Blue, the presence of the nitro group in the phenothiazine chromophore produces significant changes in the electronic structure of the excited states and in their photophysical and spectroscopical properties.
► This interpretation is supported by quantum mechanical calculations.