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.