Intense fluorescence of 1-aryl-2,3,4,5-tetraphenylphosphole oxides in the crystalline state

A series of 2,3,4,5-tetraphenylphosphole oxides with various aryl groups on the phosphorus atoms were synthesized and their fluorescence properties in the crystalline state were investigated. Some of these compounds showed an intense fluorescence with the quantum yields of 0.75-0.91. These intense emissions are attributable not only to the restricted rotation of the peripheral phenyl rings, but also to the long intermolecular distances between the adjacent molecules in the crystal packing.