Synthesis and fluorescence properties of 4-diarylmethylene analogues of the green fluorescent protein chromophore

New green fluorescent protein (GFP) chromophore analogues, namely 4-(diarylmethylene)imidazolinones (DAINs), were readily synthesized under weakly acidic conditions using a novel condensation reaction between methyl imidate (or thioimidate) and ethyl N-(diarylmethylene)glycinate. DAINs showed notable fluorescence properties. Although they were nearly non-fluorescent in the solution, visible emissions were detected in most of their frozen solution states and crystalline powder states. Therefore, control of the molecular motions significantly affected emissions by DAINs. Comparison of the fluorescence properties of DAIN 5a with those of the corresponding GFP-chromophore analogues 8 revealed that 5a possessed superior solid-state fluorescence properties.