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 sigma-bond in I 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.