Structurally well-defined TIPS-acetylene substituted tetracene (TIPS-BT1') and pentacene (TIPS-BP1') dimers utilizing a [2.2.1] bicyclic norbornyl bridge have been studiedprimarily 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 non-radiative pathways via the S 1 and non-radiative 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 fewparameter 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.

We demonstrate that a structurally rigid, weakly coupled molecular dimer can replace traditional monomeric annihilators for triplet fusion upconversion (TUC) in solution by observing emitted photons (λ = 540 nm) from a norbornyl-bridged tetracene homodimer following excitation of a triplet sensitizer at λ = 730 nm. Intriguingly, steadystate spectroscopy, kinetic simulations, and Stern−Volmer quenching experiments show that the dimer exhibits qualitatively different photophysics than its parent monomer: it is less effective at diffusion-mediated triplet exciton transfer, but it fuses extracted triplets more efficiently. Our results support the development of composite triplet-fusion platforms that go beyond diffusion-mediated triplet extraction, ultimately circumventing the concentration dependence of solution-phase TUC.

An improved, modular synthesis of rigid, geometrically well-defined, alkyne-substituted tetracene (1) and pentacene (2) dimers is reported. The synthesis is rooted in sequential Diels-Alder reactions of a norbornyl tetraene with triisopropylsilylacetylene-substituted (TIPS-acetylene) quinone dienophiles. The incorporation of solubilizing and stabilizing TIPS-acetylene groups early in the synthesis affords a mild and reliable route, providing access, for the first time, to norbornyl-bridged pentacene dimers. A preliminary exploration of the excited state behavior of these molecules is also described.

We report the first synthesis of norbornyl-bridged acene dimers (2 and 3) with well-defined and controlled spatial relationships between the acene chromophore subunits. We employ a modular 2-D strategy wherein the central module, common to all our compounds, is a norbornyl moiety. The acenes are attached to this module using the Diels-Alder reaction, which also forms one of the acene rings. Manipulation of the Diels-Alder adducts provides the desired geometrically defined bis-acenes. The modular nature of this synthesis affords flexibility and allows for the preparation of a variety of acene dimers, including functionalized tetracene dimers.

No abstract