A Symmetry Selection Rule for Suppressing Nonradiative Decay in Triangulene Derivatives
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.