Photophysical studies on meta- and para-carboranes containing 1-naphthyl groups

Four new carboranes, C-(1-naphthyl)-meta-, C,C'-bis(1-naphthyl)-meta-, C-(1-naphthyl)-para- and C,C'-bis(1-naphthyl)-para-carborane, reveal high-energy emissions at 332-343 nm with quantum yields of 21-38% and long lifetimes of 26-31 ns in degassed cyclohexane solutions. The experimental absorption and emission spectra of the mononaphthyl carboranes are successfully simulated by hybrid DFT vibronic calculations. The computed lowest energy excited state (S₁) geometries contain non-planar naphthyl groups due to shorter carborane C-aromatic C bonds and steric effects between the bulky naphthyl and carboranyl units. The high energy emissions are also reproduced in polymethylmethacrylate (PMMA) films and as solids. The lower energy emissions in 99 : 1 water : THF suspensions for these four compounds at 385-421 nm show quantum yields of 50-64% and long lifetimes of 28-40 ns. The findings indicate that meta- and para-carborane derivatives can generate different solid-state emissions as reported elsewhere for the intensively researched ortho-carborane analogues.