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 n BuCl 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 π‐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 π‐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 .