Room-Temperature Phosphorescence of Crystalline Metal-Free Organoboron Complex.

The photoluminescence (PL) properties of a metal-free organoboron complex, bis(4-iodobenzoyl)methanatoboron difluoride (1BF2 ), were elucidated. At room temperature, 1BF2 emits blue fluorescence (FL) in nBuCl upon photoexcitation. In contrast, crystals of 1BF2 emit green PL comprised of FL and phosphorescence (PH). The room-temperature PH of crystalline 1BF2 is a consequence of 1) suppression of thermal deactivation of the S1 and T1 excited states and 2) enhancement of intersystem crossing (ISC) from the S1 to T2 or T1 . The results of X-ray crystallographic and theoretical studies supported the proposal that the former (1) is a result of intermolecular interactions caused by π-stacking in the rigid crystal packing structure of 1BF2 . The latter (2) is an effect of not only the heavy-atom effect of iodine, but also the continuous π-stacking alignment of 1BF2 molecules in crystals, which leads to a forbidden S1 →S0 transition and a small energy gap between the S1 and T2 or T1 .