Framework-Topology-Controlled\nSinglet Fission in Metal–Organic\nFrameworks

Abstract

Singlet fission (SF) has been explored as a viable route\nto improve\nphotovoltaic performance by producing more excitons. Efficient SF\nis achieved through a high degree of interchromophoric coupling that\nfacilitates electron superexchange to generate triplet pairs. However,\nstrongly coupled chromophores often form excimers that can serve as\nan SF intermediate or a low-energy trap site. The succeeding decoherence\nprocess, however, requires an optimum electronic coupling to facilitate\nthe isolation of triplet production from the initially prepared correlated\ntriplet pair. Conformational flexibility and dielectric modulation\ncan provide a means to tune the SF mechanism and efficiency by modulating\nthe interchromophoric electronic interaction. Such a strategy cannot\nbe easily adopted in densely stacked traditional organic solids. Here,\nwe show that the assembly of the SF-active chromophores around well-defined\npores of solution-stable metal–organic frameworks (MOFs) can\nbe a great platform for a modular SF process. A series of three new\nMOFs, built out from 9,10-bis(ethynylenephenyl)anthracene-derived\nstruts, show a topology-defined packing density and conformational\nflexibility of the anthracene core to dictate the SF mechanism. Various\nsteady-state and transient spectroscopic data suggest that the initially\nprepared singlet population can prefer either an excimer-mediated\nSF or a direct SF (both through a virtual charge-transfer (CT) state).\nThese solution-stable frameworks offer the tunability of the dielectric\nenvironment to facilitate the SF process by stabilizing the CT state.\nGiven that MOFs are a great platform for various photophysical and\nphotochemical developments, generating a large population of long-lived\ntriplets can expand their utilities in various photon energy conversion\nschemes.