Twisted intramolecular charge transfer in a model green fluorescent protein luminophore analog

► (4Z)-4-(4-N,N-dimethylaminobenzylidene)-1-methyl-2-phenyl-1,4-dihydro-5H-imidazolin-5-one (DMPI), an analog of the green fluorescent protein (GFP) luminophore shows a gradual increase of Stokes shift with increase in solvent polarities. ► In non-polar solvents and in polar viscous solvents fluorescence intensity of DMPI is high and the molecule demonstrates a strong viscosity dependent increase in quantum yield. ► These experiments suggest that DMPI behaves like a molecular rotor. ► Time dependent density functional theory (TDDFT) calculations reveal substantial charge transfer from the highest occupied molecular orbital to the lowest unoccupied molecular orbital. ► The results imply that the protein environment around the GFP luminophore could be playing a similar role by increasing the viscosity around the luminophore leading to the enhanced quantum yield of GFP.