An experimental and theoretical study on the photophysical properties of Methylene Green

C. A. GLUSKO; C. M. PREVITALI; D.M.A. VERA; C.A. CHESTA; H. A. MONTEJANO

DYES AND PIGMENTS

ELSEVIER SCI LTD

Año: 2011 vol. 90 p. 28 - 35

0143-7208

Methylene Green (MG) is a cationic phenothiazine dye that can be considered as a nitro derivative ofmethylene blue (MB). The photophysical and spectroscopical properties of MG were investigated in a widevariety of solvents, including protic and aprotic ones. The absorption ðnAÞ and fluorescence ðnF Þ maxima,fluorescence quantumyields andemission lifetimesweremeasured and correlated with the properties of themedia by using the Bakhshiev’s model and the empirical solvent parameter ENT . Fairly good linear correlationswere obtained in both cases indicating the absence of specific interaction between the dye and the solventsstudied. The fluorescence quantum yields were found to be in the range 0.004e0.06. Quantum mechanicalcalculations at the level of Density Functional Theory (DFT) and its Time-Dependent treatment for excitedstates (TD-DFT) performed on the ground and singlet excited states of MG (and MB) allowed findinga 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 proticand non-protic media. In all cases, the triplet quantum yields ðFT Þ measured for MG were very small(<0.01). Tripletetriplet sensitization by duroquinone was required for the spectroscopic characterizationof the triplet excited state spectrum of MG and the determination of the FT in the different media.The triplet state shows important absorptions at 410, 488, and 865 nm. The molar absorption coefficientat 865 nm in acetonitrile, estimated using the depletion method, is w30,000 M1 cm1. Comparing thecalculated values of the deactivation rate constants for the singlet excited states of MG and MB, it isconcluded that the incorporation of the nitro group in the thiazine chromophore produces a large (w30times) decrease of the intersystem crossing rate constant. With the aid of quantum mechanicalcalculations it is concluded that the intersystem crossing mechanism in both dyes differ significantly.