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Quantum interference (QI)—the constructive or destructive interference of conduction pathways through molecular orbitals—plays a fundamental role in enhancing or suppressing charge and spin transport in organic molecular electronics. Graphical models were developed to predict constructive versus destructive interference in polyaromatic hydrocarbons and have successfully estimated the large conductivity differences observed in single-molecule transport measurements. A major challenge lies in extending these models to excitonic (photoexcited) processes, which typically involve distinct orbitals with different symmetries. Here we investigate how QI models can be applied as bridging moieties in intramolecular singlet-fission compounds to predict relative rates of triplet pair formation. In a series of bridged intramolecular singlet-fission dimers, we found that destructive QI always leads to a slower triplet pair formation across different bridge lengths and geometries. A combined experimental and theoretical approach reveals the critical considerations of bridge topology and frontier molecular orbital energies in applying QI conductance principles to predict rates of multiexciton generation. 

We herein describe a new design principle to achieve B/N-doped cyclophane where an electron-donor block of three triarylamines (Ar3N) and an acceptor block of three triarylboranes (Ar3B) are spatially separated on opposite sides of the π-extended ring system. DFT computations revealed the distinct electronic structure of the block-type macrocycle MC-b-B3N3 with a greatly enhanced dipole moment and reduced HOMO–LUMO energy gap in comparison to its analogue with alternating B and N sites, MC-alt- B3N3. The unique arrangement of borane acceptor Ar3B and amine donor Ar3N components in MC-b-B3N3 induces exceptionally strong intramolecular charge transfer in the excited state, which is reflected in a largely red-shifted luminescence at 612 nm in solution. The respective linear open-chain oligomer L-b-B3N3 was also synthesized for comparison. Our new approach to donor–acceptor macrocycles offers important fundamental insights and opens up a new avenue to unique optoelectronic materials. 

Conjugated polymers composed of tricoordinate boron and π-conjugated units possess extended conjugation with relatively low-lying LUMOs arising from p B –π interactions. However, donor–acceptor (D–A) polymers that feature triorganoboranes alternating with highly electron-rich donors remain scarce. We present here a new class of hybrid D–A polymers that combine electron-rich dithienosiloles or dithienogermoles with highly robust tricoordinate borane acceptors. Polymers of modest to high molecular weight are readily prepared by Pd-catalyzed Stille coupling reaction of bis(halothienyl)boranes and distannyldithienosiloles or -germoles. The polymers are obtained as dark red solids that are stable in air and soluble in common organic solvents. Long wavelength UV-vis absorptions at ca. 500–550 nm indicate effective π-conjugation and pronounced D–A interactions along the backbone. The emission maxima occur at wavelengths longer than 600 nm in solution and experience further shifts to lower energy with increasing solvent polarity, indicative of strong intramolecular charge transfer (ICT) character of the excited state. The powerful acceptor character of the borane comonomer units in the polymer structures is also evident from cyclic voltammetry (CV) analyses that reveal relatively low-lying LUMO levels of the polymers, enhancing the D–A interaction. Density functional theory (DFT) calculations on model oligomers further support these experimental observations.