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Fluorophores with high quantum yields are desired for a variety of applications. Optimization of promising chromophores requires an understanding of the non-radiative decay channels that compete with the emission of photons. We synthesized a new derivative of the famous laser dye 4-dicyanomethylen-2-methyl-6- p -dimethylaminostyryl-4 H -pyran ( DCM ), i.e. , merocyanine 4-(dicyanomethylene)-2- tert -butyl-6-[3-(3-butyl-benzothiazol-2-ylidene)1-propenyl]-4 H -pyran ( DCBT ). We measured fluorescence lifetimes and quantum yields in a variety of solvents and found a trend opposite to the energy gap law. This motivated a theoretical investigation into the possible non-radiative decay channels. We propose that a barrier to a conical intersection exists that is very sensitive to the solvent polarity. The conical intersection is characterized by a twisted geometry which allows a subsequent photoisomerization. Transient absorption measurements confirmed the formation of a photoisomer in unpolar solvents, while the measurements of fluorescence quantum yields at low temperature demonstrated the existence of an activation energy barrier.

The origin of the solvent dependence of fluorescence quantum yields in dipolar merocyanine dyes