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 well-known as a photophysical process that converts low-energy light into higher-energy light. This process has drawn attention for its potential in various fields including light-emitting device, power genera-tion, and medical applications. To promote these societal applications, it is desirable to develop TTA-UC materials that exhibit high ener-gy conversion efficiency. For this, TTA emitters with large TTA rate constants (kTTA) are required. However, molecular design for accel-erating the bimolecular rate constant of kTTA has not been considered. We present a strategy to increase kTTA by assembling multiple chromophores linked with a boron in a rotationally symmetric manner causing a symmetry breaking charge-transfer type triplet-exciton in the TTA emitter. We examined tri(9-anthryl)borane, which consists of three anthracenes linked via boron as the TTA emitter. Time-resolved luminescence measurements confirmed that kTTA is improved compared to the conventional TTA-UC system employing DPA, an anthracene-based monomer. Time-resolved electron paramagnetic resonance measurements demonstrated that the improvement in kTTA is due to fast intramolecular triplet exciton hopping coupled with vibrational motions in the trimer molecule, expanding reactivity at collision distance between the excitons through the pseudo-rotational motions. The present TTA emitter molecular designs which im-prove the TTA reactivity is expected to contribute to the future development of TTA-UC materials.