Modulation of the cis- and trans-Conformations in Bis-o-carborane Substituted Benzodithiophenes and Emission Enhancement Effect on Luminescent Efficiency by Solidification

Bis-carborane-substituted benzo[1,2-b:4,5-b]dithiophenes (DCB-R, where R = H, tBu) were synthesized and characterized. Their three-dimensional conformations were tuned by introducing the tert-butyl substituent at the para-positions of the phenyl rings. Both molecules showed emission enhancement behavior, especially in the solid state. The emission quantum efficiencies were over 0.90 in the crystalline state. Moreover, it was shown that the efficiency of DCB-tBu was over 0.70 in the amorphous state. From structural analyses and mechanistic investigations, it was proposed that the tert-butyl substituents play a critical role in the formation of the trans-conformation followed by suppression of aggregation-caused quenching because of the o-carborane units located at each plane of the benzodithiophene ring.