Stimuli responsive and reversible crystalline-amorphous transformation in a molecular solid: Fluorescence switching and enhanced phosphorescence in the amorphous state

The relatively weak non-covalent intermolecular interactions and the impact of the resulting molecular assemblies on the optical attributes, should enable sensitive responses to be triggered in molecular materials, by various external stimuli. A small molecule (diaminodicyanoquinodimethane) based material exhibiting enhanced fluorescence emission in the solid state (compared to the solution state) is shown to undergo reversible crystalline-amorphous transformation upon mechanical grinding and solvent fuming; amorphization is accompanied by a decrease in the fluorescence intensity and a clear red shift. Significantly, delayed fluorescence and phosphorescence are observed in the material, with an unusual enhancement of the latter in the amorphous state, in sharp contrast to the decrease in fluorescence. Detailed structural and photophysical investigations suggest that the contrasting luminescence variations accompanying the crystalline-to-amorphous transformation are likely to arise from the increased excited state geometry relaxation leading to significant non-radiative decay of the S₁ state, and the destruction of the ordered lattice structure diminishing the impact of triplet diffusion and the non-radiative decay of the T₁ state.