Vibronic Trimer Design Enhancing Intramolecular Triplet-Exciton Hopping to Accelerate Triplet-Triplet Annihilation for Photon Upconversion

Photon upconversion via triplet-triplet annihilation (TTA-UC) is a well-known process that converts low-energy light into higher-energy light. This process has attracted attention for its potential in various fields, including light-emitting devices, power generation and medical applications. It is desirable to develop TTA-UC materials of TTA emitters with large TTA rate constants (kTTA). However, molecular design to accelerate the bimolecular rate constant of kTTA has not been considered. We present a strategy to manipulate kTTA by assembling multiple chromophores linked to a boron in a rotationally symmetric manner, causing asymmetric motions by a localized triplet exciton. We have studied tri(9-anthryl)borane, which consists of three anthracenes linked via boron, as a TTA emitter. Time-resolved luminescence measurements confirmed that kTTA is improved compared to the conventional TTA-UC system using DPA, an anthracene-based monomer. Time-resolved electron paramagnetic resonance measurements showed that the improvement in kTTA is due to fast intramolecular triplet exciton hopping coupled with vibrational motions in the trimer molecule, which extends the reactivity at the collision distance between the excitons through the pseudo-rotational motions. This molecular design that enhances TTA reactivity is expected to contribute to the future development of TTA-UC materials for sensing fluid environment.