Investigation of excited states of BODIPY derivatives and non-orthogonal dimers from 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 photophysical processes from the intramolecular singlet fission (iSF) perspective. We have calculated singlet, triplet and quintet states at the XMCQDPT(8,8)/cc-pVDZ level of theory and diabatic singlet states at the XMCQDPT(4,4)/cc-pVDZ level of theory. In all the systems studied, charge transfer states (¹(CA) and ¹(AC)) couple strongly with locally excited (¹(S₁S₀)) and multiexcitonic (¹(T₁T₁)) states. The rates of formation of the multiexcitonic state from the locally excited state are very low on account of large activation energy (E(¹(T₁T₁)) − E(¹(S₁S₀))). A relaxed scan along the torsional angle revealed contrasting results for axial and orthogonal conformers. We proposed a probable mechanism for contrasting photophysical properties of dimers B[3,3] and B[2,8]. We also found that substitution of CN, NH₂ and BH₂ at meso, β and α positions reduces the energy gap (Δ_SF = 2E(T₁) − E(S₁)) significantly, making iSF a competing process in triplet state generation. Intrigued by the success of the CN group at the meso position in reducing the energy gap, we also studied the azaBODIPY monomer and its derivatives using the same methodology. The iSF is slightly endoergic with Δ_SF ∼ 0.2 eV in these systems and iSF may play an important role in their photophysical responses.