An Order of Magnitude Modulation of Singlet Fission Rates in NDI Cyclophanes by Tuning Inter-chromophoric Electronic Coupling

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 (1TT) 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. Recently this idea was challenged by the first report of a cyclophane constructed from naphthalene-di-imide (NDI) units which do not satisfy the triplet energy criteria. In order to uncover electronic nature of the dynamic 1TT stabilization in such supramolecular constructs, herein we have synthesized five new 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 1TT-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 criteria of singlet exciton fission isn’t in the Singlet-Triplet energetics of the constitutive monomer rather in the stabilization of the multiexcitonic 1TT pair state in its minimal dimeric unit.