An MS-CASPT2 Calculation of the Excited Electronic States of an Axial Difluoroborondipyrromethene (BODIPY) Dimer.

The previously reported ( Duman et al., J. Org. Chem . 2012 , 77 , 4516 ) calculated state energies of monomeric difluoroborondipyrromethene (BODIPY) and its axial dimer would suggest that these dyes are promising candidates for singlet fission, and the dimer was computed to have an unusual low-lying doubly excited state. We find that these results were affected by the use of an imbalanced active space in multireference calculations and are not correct. Multistate complete-active-space second-order perturbation theory (MS-CASPT2/cc-pVDZ) calculations using an [8,8] (8 electrons in 8 orbitals) active space for the monomer and a [16,16] active space for the dimer reproduce quite well the observed excitation energies of the S1 states of both, and yield T1 excitation energies well in excess of half of the S1 excitation energies. We conclude that neither BODIPY monomer nor its axial dimer would permit exothermic singlet fission and are not worthy of investigation as potentially useful candidates, and that the unusual low-energy doubly excited states of the dimer were artifacts.