Luminescence of BODIPY and dipyrrin: an MCSCF comparison of excited states

Low lying electronic states of the highly fluorescent BODIPY (boron dipyrromethene, 1) and its nonemissive cousin dipyrrin (2) were investigated by state-of-the-art quantum chemical methods. The opposed luminescence of 1 and 2 is explained by discovering distinct structural and energetic features for the intersection of the ground and first excited singlet state potential energy surfaces, S(0) and S(1). In accessing the intersection region, a B-N σ-bond in 1 has to be broken-an energetically prohibitive change on the nonemissive decay channel. On the contrary, 2 is deactivated via an energetically accessible S(0)/S(1) intersection point. Details of S(0), S(1), S(2), and T(1) wave functions for various regions of the potential energy surfaces were described. Unnoted features for multidimensional vectors that represent S(0) → S(1) and S(0) → T(1) transitions are reported. These correlations regarding S(0) → S(1) and S(0) → T(1) multidimensional vectors were also shown to apply to two highly fluorescent molecules: indole and coumarin.