Does Halogen Bonding Promote Intersystem Crossing and Phosphorescence in Benzaldehyde? C

In the past decade, halogen bonding (XB) has been utilized extensively in the design of novel materials and new drugs. One of the emerging applications of XB is in devising organic room-temperature phosphorescent materials. Several reports showed that the use of benzaldehyde-based phosphors with appropriate XB donors results in high phosphorescence quantum yields because of enhanced intersystem crossing (kISC) and phosphorescence (kPH) rates. It is often advocated that a combination of factors, namely, rigidification, heavy-atom effect, and reduction of quenching by triplet dioxygen, is responsible for the enhancement. However, to what extent each factor contributes to the enhancement is unknown. In this study, we performed ab initio excited-state calculations on two XB complexes, benzaldehyde···XF (X = Br and I), with varying XB distance to elucidate the effect of XB on kISC and kPH. Our results show that XB reduces the kISC of benzaldehyde and changes the character of T₁ from which phosphorescence takes place. Hence, the generally accepted assumption that XB enhances spin–orbit coupling and kISC is oversimplified.