Reducing the excessive exoergicity through a helically locked tether-driven approach for high-efficiency singlet fission chromophores

Abstract

Singlet fission (SF) is a process in which the absorption of a single photon results in the generation of a pair of triplet excited states, showing potential for enhancing solar conversion efficiency. The thermodynamic driving force behind SF is determined by the energy difference between the first singlet excited state and the first triplet excited state, denoted as ΔE₁ = E(S₁) − 2E(T₁). In general, an excessively large ΔE₁ value (i.e., excessive exoergicity) can facilitate alternative relaxation pathways for excitons, thereby diminishing SF efficiency. Consequently, when designing high-efficiency SF chromophores, optimization of ΔE₁ becomes crucial. Herein, we introduce a helically locked tethering strategy to optimize ΔE₁ for low-efficiency SF chromophores. Specifically, different dihedral angles are induced by tethering tethers of different lengths (Cn = –(CH₂)_n–, n = 1–6) to tetraazaacenes, allowing us to systematically monitor the variational characteristic as a function of the dihedral angle. Tethered products show strong chirality with a high energy barrier to twist back and forth. A tunable ΔE₁ has been realized by adjusting the tether length, allowing us to identify the optimal ΔE₁ of 0.29, 0.26, and 0.11 eV at tether lengths of n = 3 or 2. Our results suggest that this strategy could be applied to existing low-efficiency SF databases that are not typically considered for future application in the SF field, thereby creating novel high-efficiency and stable SF chromophores. This strategy not only makes full use of the existing resources but also greatly expands the SF arsenal.