Enhanced Triplet-Triplet Annihilation Upconversion Luminescence through Conformational Restriction Based on Donor-Acceptor-Heavy Atom Molecules

Designing optimized systems for triplet-triplet annihilation upconversion (TTA-UC) with high UC emission efficiency at low power densities remains challenging. To improve the lifetime of triplet photosensitizers for TTA-UC, donor-acceptor-heavy atoms were used to synthesize rotationally restricted PY-MBD-BR and unrestricted PY-BD-BR photosensitizers. The singlet oxygen quantum yields of both photosensitizers was approximate to 0.60 due to their structural similarity, except for the degree of rotation between the donor and acceptor. The triplet lifetime of PY-BD-BR and PY-MBD-BR was 352 mu s and 1,503 mu s, respectively. The latter may be among the highest reported for BODIPY photosensitizers. The prolonged triplet lifetime enabled excellent green-to-blue upconversion emission of PY-MBD-BR, with a high upconversion emission efficiency (eta(UC)) of 8.8 % and low threshold intensity (I-th) of 90 mW/cm(2) compared to PY-BD-Br (low eta(UC) of 4.0 % and high I-th of 381 mW/cm(2)). The rigid molecular design of triplet photosensitizers based on donor-acceptor-heavy atom systems has potential for efficient TTA-UC.