An order of magnitude modulation of singlet fission rates in NDI cyclophanes by tuning inter-chromophoric electronic coupling

Abstract

Singlet fission (SF) is a process of multi-exciton generation in molecular aggregates where two independent triplet states are formed via an intermediate correlated triplet–triplet ¹(TT) pair state. SF has been shown to occur within a minimal dimeric unit with chromophores having monomer triplet energies that are half the allowed bright singlet state energy. However, optimal singlet fission rates can only be derived if structural rules are formulated for tunable interchromophoric coupling between the two monomer units, which has remained a significant challenge in the community. In order to uncover the electronic nature of the dynamic ¹(TT) stabilization in generic supramolecular dimeric constructs, herein we have synthesized five new napthalene-diimde (NDI) cyclophanes by altering the distance and angle between the NDI units. Using a combination of transient absorption spectroscopy supported by high-level electronic structure calculations, we find that SF is tolerant in these rigid dimeric structures up to distances of 4.5 Å while the rise of the ¹(TT)-state gradually slows down from 400 fs to 4.6 ps concomitant with the decrease in the inter-chromophoric electronic coupling. Our work therefore demonstrates that the thermodynamic criterion of singlet exciton fission isn't in the singlet–triplet energetics of the constitutive monomer but rather in the stabilization of the multiexcitonic ¹(TT) pair state in its minimal dimeric unit.