Mechano-responsive luminescent emissions of an organic molecular crystal: effects of aromatic stacking interactions and solid state packing

Abstract

Three polymorphic forms, with different solid state luminescent emissions, were obtained from 2-hydroxynaphthaldehyde semicarbazone (2-NOH-S). In the pristine form, 2-NOH-S exhibited bright green emissions, while the C-M form obtained from methanol was non-emissive and the C-T form obtained from tetrahydrofuran exhibited weak yellow emissions. Our studies revealed that gentle mechanical grinding of either C-M or C-T forms produced the bright green emissive form with λ_em of 492 nm and 527 nm. Slurry conversion experiments revealed a monotropic relationship between the three polymorphic forms of 2-NOH-S, with the pristine form being the most stable. More interestingly, we found that successive application of mechanical and chemical stimuli triggered reversible phase transitions between the C-M and C-T forms and the green emissive phase. Based on spectroscopic, thermal and powder X-ray diffraction studies, we suggested that the mechano-luminescence transformation of the C-M and C-T forms to the bright green phase resulted from a reduction in the aromatic stacking interactions and disruption of non-radiative vibrational decay pathways through the formation of stronger hydrogen bonds.