The role of the synthetic chromophore of GFP in generating polymorphism-dependent on/off photoluminescence

Abstract

Polymorphism, i.e. the ability of a solid material to exist in more than one crystal form, gives unique opportunity to study the correlation between the solid-state molecular arrangement and the photoluminescence properties, and allows optimizing the performances of materials with reduced synthesis effort. Compound 1, which results from the combination of 2-phenylbenzoxazole (PBO) and benzylideneimidazolinone (BDI), was known to be weakly fluorescent in solution and in the solid state. Time-dependent density functional theory (TD-DFT) and spin-flip TD-DFT calculations were first used to revisit the behaviour of the isolated molecule in solution. It was shown that the relaxation pathway in the lowest singlet excited state leads to a thermally-accessible conical intersection (CI), which favours non-radiative deactivation. Then, crystallizing molecule 1 under precise experimental conditions allowed four distinct polymorphs and one solvate to be obtained. Remarkably, forms 1a and 1b exhibited strong photoluminescence (PL), while polymorphs 1c-e were virtually not emissive. Recrystallization allowed passing reversibly from one crystalline species to another. Mechanical stimuli, exhibition to solvent vapours and heating were not very effective, but a non-trivial effect of PL enhancement upon freezing was observed for 1a and 1b. The non-emissive polymorphs were characterized by the presence of a centrosymmetric dimer structured by hydrogen bonds, in which paired molecules vibrate with the same mode and are in turn subjected to maximum deformation, according to DFT calculations. Based on the whole set of results, possible deactivation mechanisms were proposed to account for the opposite PL behaviour of the two types of crystalline species. It is shown that the emergence of polymorphism and of a rare on/off PL mechanism may be attributed to the presence of the BDI unit in the molecule.