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 .