Modification of the Green Fluorescent Protein Chromophore with Large Aromatic Moieties: Photophysical Study and Solid-State Emission

Green fluorescent protein (GFP)-like chromophores 1-5, in which the p-hydroxylphenyl group is substituted with large aromatic moieties, were synthesized and their crystal structures were determined. Compared with that of p-hydroxybenzylideneimidazolinone (GFP chromophore), the fluorescence spectra of 15 are red-shifted because of elongated of p-conjugation. However, the incorporation of large aromatic moieties cannot inhibit the intramolecular torsional motions and, thus, 1-5 are generally weakly fluorescent in solution. The results manifest that the solid-state emission of these GFP-like chromophores is significantly influenced by the intermolecular packing. The emission spectra of 1-5 are further red-shifted in the solid-state, and 5 in the crystalline state emits in at red wavelengths with an emission maximum at 624 nm. 1, 3, and 4 are weakly emissive in both crystalline and powder forms. Interestingly, one crystalline state of 2 (2B) emits relatively strongly, whereas another crystalline state 2A emits rather weakly.