Enhancing the Statistical Probability Factor in Triplet Fusion Upconversion via TIPS Functionalization.

We investigated the effect of triisopropylsilyl (TIPS) functionalization of annihilators in triplet-triplet annihilation photon upconversion on their spin statistical probability factor. A new green annihilator 3,9-bis((triisopropylsilyl)ethynyl)perylene (TIPS-PY) displaying record red-to-green TTA-UC quantum yield of 13.7% (50% theoretical maximum) is synthesized. This remarkable efficiency was achieved due to the following features of the TIPS functionalization of PY: 1) high fluorescence quantum yield of 95%, 2) reduced triplet energy to 1.29 eV enabling efficient triplet energy transfer (~100%) from the sensitizer, PdTPBP (T1= 1.5 eV), and 3) a high efficiency of singlet generation after triplet coupling, indicated by the statistical probability factor, f = 39.2% ± 2.4%. Notably, the f value of TIPS-PY surpasses other annihilators in the 470-540 nm emission range by ≥1.5-fold. DFT and TDDFT calculations revealed that TIPS-functionalized chromophores (TIPS-PY, TIPS-naphthalene, and TIPS-anthracene), exhibit singlet-like character in their triplet dimer state which can enhance the coupling of the triplet-pair state with the excited singlet-state, thereby increasing the efficiency of singlet generation a phenomenon undisclosed before. Furthermore, the suitable T1 of TIPS-PY enables upconversion of 730 nm light when sensitized with Os(m-peptpy)2(TFSI)2 (T1 = 1.63 eV), demonstrating the broad upconversion range of TIPS-PY in the phototherapeutic window desired for biological applications