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Tetraceno[2,3-b]thiophene is regarded as a strongcandidate for singlet fission-based solar cell applications due to itsmixed characteristics of tetracene and pentacene that balanceexothermicity and triplet energy. An electronically weakly coupledtetraceno[2,3-b]thiophene dimer (Et2Si(TIPSTT)2) with a singlesilicon atom bridge has been synthesized, providing a new platformto investigate the singlet fission mechanism involving the two acenechromophores. We study the excited state dynamics of Et2Si-(TIPSTT)2 by monitoring the evolution of multiexciton coupledtriplet states, 1TT to 5TT to 3TT to T1 + S0, upon photoexcitationwith transient absorption, temperature-dependent transient ab-sorption, and transient/pulsed electron paramagnetic resonancespectroscopies. We find that the photoexcited singlet lifetime is 107ps, with 90% evolving to form the TT state, and the complicated evolution between the multiexciton states is unraveled, which canbe an important reference for future efforts toward tetraceno[2,3-b]thiophene-based singlet fission solar cells. 

Important applications of photon upconversion through triplet–triplet annihilation require conversion of near-IR photons to visible light. Generally, however, efficiencies in this spectral region lag behind bluer analogues. Herein we consider potential benefits from a conformationally well-defined covalent dimer annihilator TIPS-BTX in studies that systematically compare function to a related monomer model TIPStetracene (TIPS-Tc). TIPS-BTX exhibits weak electronic coupling between chromophores juxtaposed about a polycyclic bridge. We report an upconversion yield fUC for TIPS-BTX that is more than 20× larger than TIPS-Tc under comparable conditions (0.16%). While the dimer fUC is low compared to bluer champion systems, this yield is amongst the largest so-far reported for a tetracenic dimer system and is achieved under unoptimized conditions suggesting a significantly higher ceiling. Further investigation shows the fUC enhancement for the dimer is due exclusively to the TTA process with an effective yield more that 30× larger for TIPS-BTX compared to TIPS-Tc. The fTTA enhancement for TIPS-BTX relative to TIPS-Tc is indicative of participation by intramolecular multiexciton states with evidence presented in spin statistical arguments that the 5TT is involved in productive channels. For TIPS-BTX we report a spin statistical factor f = 0.42 that matches or exceeds values found in champion annihilator systems such as DPA. At the same time, the poor relative efficiency of TIPS-Tc suggests involvement of non-productive bimolecular channels and excimeric states are suspected. Broadly these studies indicate that funneling of photogenerated electronic states into productive pathways, and avoiding parasitic ones, remains central to the development of champion upconversion systems. 

The multiexciton quintet state,⁵TT, generated as a singlet fission intermediate in pairs of molecular chromophores, is a promising candidate as a qubit or qudit in future quantum information science schemes.