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

Abstract

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 μs and 3.03 μs) and high triplet state energy levels (1.5 eV) as compared to a Pc complex bearing electron donating groups (triplet lifetimes <3.0 μs; triplet energy levels ≈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 T₁ 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.