Solid-state temperature-dependent luminescence of <i>C</i>,<i>C</i>′-diaryl-<i>o</i>-carboranes based on restriction of excited-state structural relaxation

Here, we show mechanistic analyses of solid-state temperature-dependent luminescence of <i>C</i>,<i>C</i>'-diaryl-<i>o</i>-carborane derivatives. Solid-state stimuli-responsiveness is a promising property to visualize environmental changes on the molecular scale. Thus, mechanistic study of responsiveness is meaningful for broadening the scope of materials. We previously reported solid-state temperature-dependent luminochromism of <i>C</i>-anthryl-<i>o</i>-carborane derivatives. Among them, we re-focused on <i>C</i>-anthryl-<i>C</i>'-phenyl-<i>o</i>-carborane as a prototypical compound and synthesized derivatives with electronically different substituents on aryl units. Based on extensive analyses with crystallography, variable temperature optical measurements and theoretical calculation, we show that luminochromic behaviors can be explained by partially restricted excited-state elongation of the carbon-carbon bond in the cluster. Our results indicate that thermal expansion and contraction of the crystalline lattice should play a key role in modulating the degree of excited-state structural relaxation including the bond elongation in the cluster.