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Structurally well-defined TIPS-acetylene substituted tetracene (TIPS-BT1') and pentacene (TIPS-BP1') dimers utilizing a [2.2.1] bicyclic norbornyl bridge have been studied-primarily using time-resolved spectroscopic methods-to uncover mechanistic details about primary steps in singlet fission leading to formation of the biexcitonic 1 TT state as well as decay pathways to the ground state. For TIPS-BP1' in room-temperature toluene, 1 TT formation is rapid and complete, occurring in 4.4 ps. Decay to the ground state in 100 ns is the primary loss pathway for 1 TT in this system. For TIPS-BT1', the 1 TT is also observed to form rapidly (with a time constant of 5 ps), but in this case it occurs in concert with establishment of an excited-state equilibrium ( K ∼ 1) with the singlet exciton state S 1 at an energy of 2.3 eV above the ground state. The equilibrated states survive for 36 ns and are lost to ground state through both radiative and nonradiative pathways via the S 1 and nonradiative pathways via the 1 TT. The rapidity of 1 TT formation in TIPS-BT1' is at first glance surprising. However, our analysis suggests that the few-parameter rate constant expression of Marcus theory explains both individual and comparative findings in the set of systems, thus establishing benchmarks for diabatic coupling and reorganization energy needed for efficient 1 TT formation. Finally, a comparison of TIPS-BT1' with previous results obtained for a close constitutional isomer (TIPS-BT1) differing in the placement of TIPS-acetylene side groups suggests that the magnitude of exchange interaction in the correlated triplet manifold plays a critical role dictating 1 TT yield in the tetracenic systems.

Using Structurally Well-Defined Norbornyl-Bridged Acene Dimers to Map a Mechanistic Landscape for Correlated Triplet Formation in Singlet Fission