Substituent Effects on the Fluorescence Properties of ortho-Carboranes: Unusual Emission Behaviour in C-(2 '-Pyridyl)-ortho-carboranes

Seventeen compounds including the parent ortho-, meta- and para-carboranes and derivatives of ortho-carborane were investigated for luminescence in cyclohexane and dichloromethane solutions. Fifteen of these carboranes revealed very weak emissions in the 285-493 nm range. These carboranes may arguably be viewed as non-emissive in solutions at room temperatures. No emissions could be observed for 1,2-dimethyl-ortho-carborane and 2-methyl-1-phenyl-ortho-carborane. The carboranes with a 2'-pyridyl substituent at the cluster carbon atom gave unusual local and charge-transfer emissions indicating that different excited states are generated on photoexcitation. Of all the carboranes investigated, only 2-(diphenylphosphino)-1-phenyl-ortho-carborane, 1,2-diphenyl-ortho-carborane and 1-phenyl-2-(2'-pyridyl)-ortho-carborane are luminescent in the solid state with emissions at 476-612 nm and large Stokes shifts of 12000-13600 cm(-1). The solid-state structures of 1,2-bis(2'-pyridyl)-and 1-phenyl-2-(2'-pyridyl)-orthocarborane were determined by X-ray crystallography.