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We demonstrate rational designs of excitation energies and electronic couplings using doubly-bridged chromophores for exciton down- and up-conversions, the former and latter of which are known as singlet fission and triplet–triplet annihilation, respectively. We deduce energetic conditions suitable for these two conversion processes based on quantum interferences within a bridge as well as between two bridges. The idea is at first proposed at the Huckel approximation level of theory in a theoretical model, and then, is realized for molecular systems of polyyne bridges with several lengths of ethynyl units as well as with different linked sites by ab initio quantum chemistry calculation. The result is analyzed in detail by decomposing the electronic couplings into direct-overlap and bridge-mediated couplings from each bridge, which definitely confirms the quantum interference between the bridges. Further analysis using perturbation theory clarifies this effect on the energetics concerning singlet fission and triplet–triplet annihilation. Estimation of the singlet fission time constants for the molecules designed to have exothermic singlet fission gave 102–104 ps, which is much faster than for most tetracene dimers reported previously. The present study provides a widely applicable molecular design guideline for tuning the energetic conditions by selective control of the electronic coupling matrix elements, which can be systematically achieved by considering the relative phases and distributions of the π-orbitals in chromophores and bridges.

Rational design of doubly-bridged chromophores for singlet fission and triplet–triplet annihilation