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)-ortho-carborane were determined by X-ray crystallography.