Volatile Aromatics Trigger Vapochromic and Vapoluminescent Responses in Cleft-Shaped 1,4-Bis(imidazolyl)benzene: Role of Charge-Transfer Interactions in Vapor-Induced Host–Guest Complexes

Advancements in vapochromic and vapoluminescent materials have garnered significant attention due to their potential applications in chemical sensing, environmental monitoring, and industrial processes. The increasing demand for efficient, lightweight, and easily synthesized chemical sensors has underscored the importance of developing small-molecule-based vapor-responsive materials to meet the needs of modern sensing technologies. In this study, the organic compound 1,4-bis(4,5-bis(4-bromophenyl)-imidazolyl)benzene (1) was investigated for its vapochromic and vapoluminescent properties. The cleft-like structure of compound 1, coupled with its ability to form hydrogen bonds and π-stacking interactions, enables efficient trapping of aromatic vapors. 1H NMR, TGA, and PXRD studies reveal the formation of stable host–guest complexes (1·guests), along with phase transformations in 1 upon vapor exposure. Notably, single crystals of select host–guest complexes grown via slow evaporation exhibit host–guest ratios and PXRD patterns identical with those of their vapor-exposed counterparts. Distinct color and luminescence changes validated by diffuse reflectance spectroscopy (DRS) and photoluminescence are attributed to charge-transfer interactions of the host with guest molecules. Computational studies indicate that aromatic guests with lower LUMO values exhibit stronger interactions, leading to significant luminescence quenching, as further confirmed by the reduction in the fluorescence quantum yield. The sensitivity, selectivity, and charge-transfer properties of compound 1 establish its potential as a robust candidate for small molecule gas sensors.