Five Concomitant Polymorphs of a Green Fluorescent Protein Chromophore (GFPc) Analogue: Understanding Variations in Photoluminescence with P-Stacking Interactions

Abstract

The synthetically modified green fluorescent protein chromophore analogue 3,4,5-tri­meth­oxybenzyl­idene imidazolinone (1) yielded five polymorphs (I, II, III, IV, V) concomitantly irrespective of the solvent used for crystallization. The pentamorphic modification of 1 is solely due to the interplay of iso-energetic weak intermolecular interactions in molecular associations as well as the conformational flexibility offered by a C—C single bond, which connects the electron-deficient moiety imidazolinone with the electron-rich tri­meth­oxybenzyl­idene group. A common structural feature observed in all the polymorphs is the formation of a `zero-dimensional\\\’ centrosymmetric dimeric unit through a short and linear C—H…O hydrogen bond engaging phenyl C—H and imidazolinone carbonyl oxygen. However, the networking of these dimeric units showed a subtle difference in all the polymorphs. The 2D isostructurality was observed between polymorphs I, II and III, while the other two polymorphs IV and V revealed only `zero-dimensional\\\’ isostructurality. The different fluorescence emissions of Form I (blue) and Forms II to V (yellow) were attributed to the differences in π-stacking interactions. It shows that one can modulate the photophysical properties of these smart materials by slightly altering their crystal structure. Such an approach will aid in developing new multi-colour organic fluorescent materials of varying crystal structures for live-cell imaging and fluorescent sensing applications.