Maximizing Room-Temperature Red Phosphorescence in Contorted Hexabenzocoronene Derivatives

Organic molecules exhibiting second-scale room-temperature phosphorescence (RTP) in the red/near-infrared are particularly rare because low-energy excited states that are characteristic of these chromophores are susceptible to nonradiative deactivation. Here, we observe second-scale red RTP from contorted hexabenzocoronene (cHBC) embedded in a rigid polymer. This RTP is uniquely efficient as approximately 23% of the steady-state photoluminescence originates from triplets. We propose that this efficient triplet generation stems from intersystem crossing (ISC) that outcompetes symmetry-forbidden fluorescence. Density functional theory and time-dependent density functional theory calculations suggest that ISC occurs from the lowest energy singlet state of cHBC into a nearly resonant triplet state. Perdeuterating cHBC substitutes C-H stretches with lower-energy C-D stretches, which further suppresses nonradiative recombination and prolongs red RTP. The phosphorescence lifetime of perdeuterated cHBC-polymer composites exceeds 5 s, and has a steady-state phosphorescence fraction of 44%.