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.