Efficient crystallization-induced emission in fluorenyl-tethered carboranes

Abstract

Fluorenyl-tethered o-carborane derivatives (Fl-Cb) as both mono- and di-aryl Cb assemblies were synthesized from the corresponding alkyne and B₁₀H₁₂(Et₂S)₂ in ca. 60% yields. Crystallographic studies of di-aryl examples show rigid head-to-tail or tail-to-tail packing, while for the mono-aryl examples, only loose packing was observed. In solution, the Fl-Cb compounds exhibit strong quenching of their emission, while aggregation in a mixed solvent results in an enhanced, but still weak, emission [0.11 quantum efficiency (Φ)]. Crystallization-induced emission was observed with the di-aryl examples [Φ up to 0.95]. The excitation spectra are broadened, consistent with considerable orbital degeneracy within the crystals. Theoretical calculations suggest that the inherent orbital degeneracy and the highly-ordered crystalline aggregates contribute to the excellent crystallization-induced emission in these Fl-Cb compounds.