2-Phenylpyridine- and 2-(benzo[<i>b</i>]thiophen-2-yl)pyridine-based <i>o</i>-carboranyl compounds: impact of the structural formation of aromatic rings on photophysical properties

2-Phenylpyridine- and 2-(benzo[b]thiophen-2-yl)pyridine-based (ppy- and btp-based) o-carboranyl (Car1 and Car2) and their B(CH3)2-C∧N-chelated (Car1B and Car2B) compounds were prepared and fully characterised by multinuclear NMR spectroscopy and elemental analysis. The solid-state structure of Car2B was determined by single-crystal X-ray diffraction, which revealed a four-coordinated dimethylboryl centre. All compounds displayed major absorption bands that were assigned to π-π* transitions involving the ppy and btp moieties, as well as weak intramolecular charge-transfer (ICT) transitions between the o-carboranes and their aryl groups. Furthermore, the chelated compounds exhibited dominant low-energy absorption bands (λabs = 333 nm for Car1B and 383 nm for Car2B) resulting from the reinforcement of ICT transitions that correspond to the o-carborane moieties through the restriction of aromatic-ring free rotation. While Car1 and Car2 did not exhibit photoluminescence emissions in toluene at 298 K, Car1B and Car2B showed intense emissions, which are assignable to π-π* transitions associated with each chelated aryl group. However, Car1 and Car2 evidently emitted at around 450 nm in solution at 77 K, invoked by radiative ICT transitions between the carborane and the ppy or btp moiety, indicating that ICT-based radiative decay is only invigorated in the rigid state in the absence of structural variations, such as C-C bond fluctuations in the carborane cage and aromatic-ring free rotation. Interestingly, while Car1 in the film state exhibited a weak ICT-based emission spectrum, and Car1B and Car2B showed intense emissions originating from π-π* transitions associated with each chelated aryl group, Car2 showed significantly enhanced emissions in the same energy region as that exhibited in solution at 77 K, resulting in a much larger quantum efficiency over that in solution. DFT-optimised structures of Car1 and Car2 in their ground and the first-excited states clearly reveal that the enhanced emissive features of Car2 in the film state are strongly associated with the retained planarity of the btp moiety in both the ground and excited states. The photophysical results for these o-carboranyl compounds definitively reveal that the planarities of the aryl groups appended to the o-carborane decisively affect the efficiency of radiative decay based on ICT involving the o-carborane.