Room-Temperature Phosphorescence of Crystalline Metal-Free Organoboron Complex

The photoluminescence (PL) properties of a metal-free organoboron complex, bis(4-iodobenzoyl) methanatoboron difluoride (1BF(2)), were elucidated. At room temperature, 1BF(2) emits blue fluorescence (FL) in nBuCl upon photoexcitation. In contrast, crystals of 1BF(2) emit green PL comprised of FL and phosphorescence (PH). The room-temperature PH of crystalline 1BF(2) is a consequence of 1) suppression of thermal deactivation of the S-1 and T-1 excited states and 2) enhancement of intersystem crossing (ISC) from the S-1 to T-2 or T-1. The results of X-ray crystallographic and theoretical studies supported the proposal that the former (1) is a result of intermolecular interactions caused by p-stacking in the rigid crystal packing structure of 1BF(2). The latter (2) is an effect of not only the heavy-atom effect of iodine, but also the continuous p-stacking alignment of 1BF(2) molecules in crystals, which leads to a forbidden S-1 -> S-0 transition and a small energy gap between the S-1 and T-2 or T-1.