Planarity is one of the essential requirements for fluorescence in Red Fluorescent Protein chromophore analogs

•Synthesis and characterization of four RFP chromophore derivatives.•The crystal packing, non-covalent, and strong π-π stacking interactions were observed.•Twisted molecules are non-fluorescent, and planar molecules are fluorescent.•Hirshfeld surface analysis and DFT studies were analyzed. Synthesis and photophysical properties of four red fluorescent protein (RFP) chromophore analogs are reported. The analogs in crystals are held by different non-covalent intermolecular interactions such as C–H...O hydrogen bonding, C–H...π, π...π, and halogen bonding interactions, stabilizing the molecular stacks. The carbonyl group in the imidazol-5(4H)-one ring generates diverse packing systems in the compound via intermolecular C–H...O hydrogen bonds. Compounds 1 and 2 adopt a non-planar geometry, while 3 and 4 are slightly bent. Compound 3 shows a donor-acceptor type of π-π interactions resulting in a lateral offset stacking that is stabilized by strong π-π stacking between the imidazolin-5-one ring and methoxy substituted benzene, along with halogen...halogen bonding interactions. Compound 4 displays a strong π-π stacking between two imidazol-5(4H)-one rings. The Hirshfeld surfaces of two-dimensional fingerprint plots were used to analyze the strength of intermolecular interactions in the structures. The photoluminescence is also tuned in these molecules by appropriate substitution. Thus, the azine compounds are completely non-fluorescent compared to the other two. [Display omitted]