The Interplay of Strongly\nand Weakly Exchange-Coupled\nTriplet Pairs in Intramolecular Singlet Fission

Singlet fission (SF) and triplet–triplet annihilation\nupconversion\n(TTA-UC) nominally enable the interconversion of higher-energy singlet\nstates with two lower-energy triplet states and vice versa, with both\nprocesses having envisaged application for enhanced solar power devices.\nThe mechanism of SF/TTA-UC involves a complex array of different multiexcitonic\ntriplet-pair states that are coupled by the exchange interaction to\nvarying extents. In this work a family of bounded intramolecular SF\nmaterials, based upon the chromophore 1,6-diphenyl-1,3,5-hexatriene,\nwere designed and synthesized. Their SF behavior was characterized\nusing fluorescence lifetime, transient absorption, and magnetic field\ndependence studies. The capacity for the formation of weakly exchange-coupled\ntriplet pairs, and subsequent spin-evolution, is shown to be strongly\ndependent upon the combined factors of oligomer size and geometry.\nBy contextualizing these results with the wider SF literature, we\npresent a general schematic model for SF/TTA-UC of greater completeness\nthan portrayed elsewhere.