Photophysical properties of palladium/platinum tetrasulfonyl phthalocyanines and their application in triplet-triplet annihilation upconversion

Triplet photosensitizers showing strong absorption in the red/deep red spectral region, high intersystem crossing, a long-lived triplet state and a high triplet state energy level are crucial for triplet-triplet annihilation upconversion. Herein we selected two tetrasulfonyl-substituted phthalocyanine (Pc) Pt(ii) and Pd(ii) complexes (Pd-Pc and Pt-Pc) likely to meet the above criteria. The complexes showed prolonged triplet state lifetimes (15.9 s and 3.03 s) and high triplet state energy levels (1.5 eV) as compared to a Pc complex bearing electron donating groups (triplet lifetimes <3.0 s; triplet energy levels approximate to 1.2 eV). Weak fluorescence was observed for Pd-Pc, whereas a fluorescence/phosphorescence dual emission feature was observed for Pt-Pc. Based on the phosphorescence emission, intermolecular triplet-triplet-energy-transfer (TTET) experiments and TD-DFT computations, the T-1 state energy levels of Pd-Pc and Pt-Pc were determined as 1.25 eV and 1.5 eV, respectively. The two complexes were used for triplet-triplet-annihilation upconversion, with deep red excitation at 658 nm, upconversion with a quantum yield of 0.63% and an anti-Stokes shift of 3996 cm(-1) was achieved.