Investigation of excited states of BODIPY derivatives and non-orthogonal dimers in the perspective of singlet fission

Abstract

We report state of the art electronic structure calculations RICC2 and XMCQDPT of BODIPY nonorthogonal dimers to understand the triplet state generation process from intramolecular singlet fission (iSF) perspective. We have calculated singlet, triplet and quintet states at SA8-XMCQDPT(8,8)/cc-pVDZ level of theory. iSF found to be highly endoergic (∆SF = 2E(T1 )−E(S1 ) ≈ 1.0eV) at the optimized geometry and is inefficient. Rotation along the torsional angle alters the energies of the states and make the iSF less endoergic. In all the systems studied, the diabatic couplings between charge transfer states with locally excited states and multiexcitonic state found to be high. The singlet fission rates are very low on the account of large ∆SF value. We found substitution of CN, NH2 and BH2 at meso, β and α positions reduce the energy gap ∆SF significantly allowing iSF an competing process in triplet state generation. Intrigued by the success of CN group at the meso position in reducing the energy gap, we also studied the azaBODIPY monomer and its derivatives using same methodology. The iSF is slightly endoergic ∆SF ≈ 0.3 eV in these systems and iSF may play an important role in their photo physical responses.