Rationalizing the effect of benzo-fusion at [a] and [b] positions of BODIPY on fluorescence yields.

Abstract

The origin of the large difference of fluorescence yields between benzo[a] and benzo[b] BODIPY derivatives was investigated. The benzo[a]-BODIPY chromophore shows high fluorescence yields while the totally quenched fluorescence of benzo[b]-BODIPYs is observed. Quantum calculations indicated that larger spin-orbit coupling (SOC) and smaller singlet-triplet energy gaps result in non-fluorescence for benzo[b]-BODIPY. Benzo[b]-fusion makes a partial contribution to the HOMO but a full contribution to the HOMO-1, and thus the S1 → S0 and T2 → S0 transitions, involved in HOMO-LUMO and HOMO-1-LUMO, have different characteristics, which leads to spin flipping for intersystem crossing (ISC) and increases the SOC to 1.70 cm-1. However, benzo[a] contributes to HOMO and HOMO-1 equally, and minimizes the SOC between S1 and T2, leading to slow ISC from S1, thus possessing strong fluorescence. These results are useful for the rational design of heavy-atom-free triplet organic chromophores.