Mechano-responsive luminescent emissions of an organic molecular crystal: Effects of aromatic stacking interactions and solid state packing
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 em 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 most stable. More interestingly, we found that successive application of mechanical and chemical stimuli triggered reversible phase transitions between 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-luminescent transformation of C-M and C-T forms to the bright green phase resulted from reduction in the aromatic stacking interactions, and disruption of non-radiative vibrational decay pathways through the formation of stronger hydrogen bonds.