GFP is widely used as a biological marker because of its superb fluorescence properties. We have synthesized GFP chromophore and its derivatives, and their photochemistry and photophysics were investigated using various steady-state and time-resolved techniques. We first compared their optical properties between solution and solid state. We found some O-alkyl GFP chromophore derivatives exhibit large fluorescent enhancement in the solid state. Cis/trans isomerization is thought to be responsible for blinking behavior in fluorescent protein; however, the mechanism of the thermal reverse isomerization is more problematic. Using Hammett plot studies, we conceived, for the first time, a novel nucleophilic addition/elimination mechanism. Finally, a hydroxyl substituent at the meta position shows enhanced charge transfer and greater acidity in the excited state. As a result, we have demonstrated that the fast quenching of the excited state by internal conversion to the ground state is much slower in meta- than in para-HOBDI derivatives. This allows studies of this ultrafast intermolecular ESPT that competes with isomerization.