A search for mechanoluminophors capable of pressure-induced thermal population of excited states

During the process of deforming a crystal, a high pressure is developed near the tip of mobile cracks, which may in turn produce a new ground state by thermal electron transfer. Upon sudden release of pressure, the electron can either relax to one atmosphere ground state or remain in the excited state potential well long enough to relax to one atmosphere and radiatively transfer back to the ground state. For analysing the pressure induced thermal population of the excited state, the mechanoluminescence(ML) and high pressure photoluminescence(PL) of several organic and inorganic crystals were measured. The study indicated that usual pressure coefficient of energy shift of the order of 50–100 cm−1/kbar and the stress at the crack-tip of the order of 5–10 kbar, are not sufficient to cause the thermal population of the excited state. If by any means the product of pressure coefficient and stress at the mobile crack-tip can be increased by 50 to 100 times, then the thermal population of the excited states may take place. Using the pressure coefficient of energy shift and the difference in ML and PL spectra, and using independently the change in relative intensities of the vibronic peaks, the pressure at the emitting mechanoluminescent crystal sites is evaluated and it is found to be of the order of several kbar which varies from crystal to crystal.