Photoisomerization of a 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran analog dye: a combined photophysical and theoretical investigation

A combination of experimental and theoretical investigations of a photoisomerizable analog of 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) dye molecule is presented. We provide evidence that the 4 main isomers and conformers of DCM contribute to its photochemical and photophysical processes. The absorption and emission spectra, as well as time-resolved fluorescence experimental results, are discussed and compared to DFT/TDDFT calculations. The E <-> Z isomerization is induced photochemically, whereas the s-cis <-> s-trans conformational interconversion is a thermal process which may also happen during irradiation. The photoreaction pathways from the first excited state down to the ground state are shown to be mediated by two conical intersections, as revealed using spin-flip TDDFT calculations. The rationalization of these isomerization schemes provides important insights into the photophysical properties of DCM, responsible for its photoswitchable fluorescence.