Crystal structure and triboluminescence .2. 9-anthracenecarboxylic acid and its esters

Triboluminescence activity is usually assumed to be a consequence of crystal dissymmetry; however, the multitude of reports of centrosymmetric triboluminescent materials cast doubt on that assumption. We examine here the connection between triboluminescence and crystal symmetry without prejudice, by synthesizing 12 esters of 9-anthracenecarboxylic acid and correlating their triboluminescence activity and that of the acid with crystal structure, purity, and photoluminescence. In this series, a noncentrosymmetric crystal structure is necessary but not sufficient for triboluminescence in relatively pure materials; impurities are necessary for triboluminescence activity in all centrosymmetric and one noncentrosymmetric materials. Crystallization motif and other structure factors were only weakly correlated with triboluminescence. The results provide evidence in support of a charge separation and recombination mechanism for the excitation of the triboluminescence.