Group 4 metallocene derivatives as a new class of singlet oxygen photosensitizers

Photosensitizers, which produce singlet oxygen O2(1Δg) under light irradiation, have attracted much attention due to their application potential in medicine and material science. In this respect, we recently reported on cationic metallocene complexes with pendant N-donor moieties, which exhibited enhanced luminescence in the solid-state originating from their triplet states with luminescence quantum yields, ΦL, up to approximately 0.52, unprecedented for this class of compounds. This finding suggested that these complexes can be efficient singlet oxygen photosensitizers. Herein, we evaluate the photogeneration of O2(1Δg) by several Ti, Zr and Hf metallocenes in dichloromethane solution, including recently described cationic derivatives. After excitation by UV A light, cationic Zr and Hf metallocenes stabilized by N-donor imine and pyridine moieties exhibited green, yellow or orange luminescence from long-lived 3LMCT excited states (ΦL ∼ 0.02 – 0.40 in argon-saturated solutions) and efficient O2(1Δg) production (ΦΔ ∼ 0.29 – 0.77 in air-saturated solutions). In contrast, parent neutral Zr complexes and cationic Ti derivatives with enhanced absorption of visible light, exhibited lesser ability to produce O2(1Δg) (ΦΔ < 0.10), similar to Zr and Ti metallocene dihalogenides. The experimentally established spectroscopic and photophysical behavior was probed computationally at the TD-DFT level.