Interplay between Through-Space and Through-Bond Electronic Coupling in Singlet Fission

Singlet fission (SF) is a spin-allowed photophysical process that generates two triplet excited states for one absorbed photon. It therefore has the potential to boost solar cell efficiencies beyond the 33% detailed balance limit. A better understanding of through-space and through-bond electronic coupling in SF, and their interplay, is essential for practical applications of SF materials. We have therefore designed three structurally complex pentacene dimers, two of which contain additional subphthalocyanines (SubPcs). These dimers, on the one hand, provide a continuum of electronic coupling and, on the other hand, mimic the complex situation present in the solid state, where a myriad of different packing interactions exist. Quantum chemical calculations and molecular dynamics simulations helped to shed light on the structure-property relationships of these dimers. We employed both steady-state absorption and emission spectroscopy and transient absorption pump-probe experiments to unravel the excited-state dynamics. The SubPcs complement the characteristics of pentacenes and act as light-harvesting antennae that funnel quantitatively the excitation energy via intramolecular Förster resonance energy transfer to the pentacene dimers to realize panchromatic absorption. The latter are subsequently subject to intramolecular SF (i-SF). The formation of the correlated triplet-pair ¹(T₁T₁) intermediate occurs with rates and yields that are directly proportional to the interpentacene electronic coupling. In contrast, the yield of uncorrelated triplet excited states (T₁ + T₁) is indirectly proportional to the interpentacene electronic coupling. Thus, the three dimers demonstrate the decisive role of electronic coupling in i-SF and how to control it. They serve as textbook examples for the opposing dependencies of formation and decoherence of ¹(T₁T₁) on the electronic coupling.