Toward photophysical characteristics of triplet-triplet annihilation photon upconversion: a promising protocol from the perspective of optimally tuned range-separated hybrids.

The photon upconversion (UC) process assisted by the triplet-triplet annihilation (TTA) mechanism has recently come into the spotlight. Given the rich collection of efforts in this area, theoretical explorations regarding TTA-UC are relatively limited and have proven to be challenging for its control in devices. In this contribution, the photophysical properties crucial for TTA-UC, such as triplet excited state energies and triplet-triplet energy transfer gaps of the essential ingredients involved in the process, namely sensitizers, annihilators and their pairs, have theoretically been investigated using optimally tuned range-separated hybrid functionals (OT-RSHs) and their screened exchange counterparts, OT-SRSHs. Taking a series of experimentally proven-to-work sensitizer/annihilator pairs as working models, we have constructed and validated several variants of OT-RSHs using both full time-dependent and Tamm-Dancoff formalisms for a reliable description of the TTA-UC photophysics. Given the bimolecular biphotonic nature of the TTA-UC process under study, particular attention is paid to the influence of the factors like the underlying density functional approximations and the tunable parameters such as short- and long-range exact-like exchanges as well as the range-separation parameter for both the sensitizers and annihilators separately. Dissecting all the aspects and relying on the appropriate choices from the tested models, we propose an OT-RSH with the correct asymptotic behavior as a cost-effective yet useful tool for this purpose. Not only against the standard RSHs but also in comparison to the conventional hybrids, the newly developed OT-RSH yields a more reliable description for the TTA-UC energetics in the gas phase and dielectric medium. Accountability of the proposed model has further been confirmed for several theoretically designed sensitizer/annihilator pairs prone to be used in the TTA-UC process. Summing up, in light of this study additional pieces of convincing evidence on the quality of OT-(S)RSHs for computational modeling and experimental verifications of the photophysics of the photon UC based on TTA and other possible technologies are showcased.