3,5-Difluoro-4-hydroxybenzylidene imidazolinone
3,5-Difluoro-4-hydroxybenzylidene imidazolinone or DFHBI is an imidazolinone fluorophore used in various biochemical studies. The benzene ring of DFHBI can freely rotate around the single bond but when it is fixed in planar conformation, DFHBI fluoresces. It is a synthetic analog of the GFP fluorophore. References 

Imidazolinones
Fluoroarenes
Phenols