Crystal structures of four green fluorescent protein (GFP) chromophore analogues with different packing interactions could be tuned by appropriate substitutions around the imidazolin-5-one ring are reported here. Compound 1 was crystallized from tetrahydrofuran at room temperature while compounds 2-4 have been crystallized from a mixture of methanol and dichloromethane in 3:1 ratio. Molecule 1, 2 and 3 crystallized in monoclinic lattice while molecule 4 preferred to crystallize in a triclinic crystal system. The crystal packing of these molecules was stabilized by C-H$\ldots \pi$ stacking and C-H$\ldots $O type of supramolecular interactions. The results reveal that packing diversity can be easily accomplished in these molecules by tuning the substituents around the imidazolin-5-one ring. Photophysical studies also reveal that all have good quantum yield and fluoresce typically in red region due to presence of electron donating groups around the imidazolin-5-one ring.

Crystal structures of four red kaede fluorescence protein chromophore analogues are reported here. Molecules I-III adopt a non-planar geometry stabilized by 𝜋 …𝜋 stacking and hydrogen bonding. Introduction of an electron push-pull system induces molecule IV to be planar and a C = O…𝜋 supramolecular interaction is observed as well. Strong electron withdrawing and donating groups also ensure formation of a higher order two and three dimensional supramolecular architecture through hydrogen bonds in molecules I and IV. All the analogues exhibit good photoluminescence properties and emit in the red region with excellent quantum yields.

The imino nitrogen of p-methoxybenzylideneimidazolinone (pMBDI) was protonated using aqueous HClO4 and HCl acid and crystallized in monoclinic crystal system with P2₁/n space group. Interestingly, the protonated compounds are fluorescent in polar solvents but non-fluorescent in non-polar solvent as well as in the solid state. These results point to the existence of a non-radiative pathway involving the imidazole nitrogen in the quenching of excited states in these compounds.