Papers

Nonradiative decay through conical intersections limits the photoluminescence quantum yield of organic emitters. We derive, from group theory and the standard vibronic-coupling Hamiltonian, a symmetry selection rule predicting which substitution patterns on triangulene-type polycyclic frameworks make the lowest conical intersection symmetry-forbidden. The rule depends only on the irreducible representations of the frontier orbitals and the available vibrational modes, requiring no system-specific fitting. We work out the predictions for a family of substituted triangulenes and identify substituents that should raise the nonradiative barrier. We propose density-functional and multireference calculations to test the predicted ordering, and state the approximations under which the rule holds.