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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. 

Benzo[ghi]perylene monoimides (BPIs) have recently been employed as organic photocatalysts for challenging reductions. In probing their function, we identify a thermal degradation product involving imide ring opening, and this in turn motivates the development and synthesis of a high-symmetry model systema benzo[ghi]perylene diester (BPDE)whose structural simplicity is useful for mechanistic exploration relevant to the broader photocatalyst class. Using electrochemical and spectroscopic tools, we probe both the singly and doubly reduced states of BPDE and report the generation of [BP-H]−, a twoelectron, one-proton activated closed-shell super-reductant. This catalytically active species, after visible photon absorption, operates from its singlet excited state, where the motions of the added proton are coupled to an electron transfer event, which enables direct reduction of inert substrates like benzene and fluorobenzene. Traditional Birch chemistry on benzene has been previously realized only by solvated electrons or electrochemistry. The function of this model system uncovered in these mechanistic explorations suggests modes of operation for this photocatalyst class that will enable future optimizations.