Mechanistic Insights into Oxygen Quenching and Singlet Oxygen Production by Osmium(II) Phenanthroline Derivatives: The Roles of Partial Charge Transfer and Reorganization Energy

Abstract

This study investigates the photophysical properties and oxygen-quenching dynamics of a series of osmium(II) 1,10-phenanthroline derivatives. The complexes demonstrate broad metal-to-ligand charge-transfer (MLCT) absorption and emit from the triplet state (3MLCT) in acetonitrile with lifetimes ranging from 44.2 to 180.7 ns and luminescence quantum yield, ΦL, in the range 0.9 × 10–3 to 3.75 × 10–2. Oxygen quenching of the 3MLCT state occurs with rate constants, kq, in the range 2.2–9.4 × 109 M–1 s–1 in accordance with the spin statistical factor being <4/9th the rate of diffusion, kd, producing singlet oxygen with quantum yields, ΦΔ, between 0.02 and 0.45 and overall sensitization efficiencies, fΔT, from 0.04 to 0.85. Transient absorption spectroscopy at different oxygen concentrations confirmed the absence of long-lived charge-separated intermediates. The quenching mechanism was analyzed within the spin-statistical model, which quantifies the charge-transfer contribution via the parameter pCT (0.1–0.9 for this series). Using a mean reorganization energy, λ, of 64 kJ mol–1 and an exciplex charge-transfer character, δ, of 0.48 derived from the data, the activation energies for exciplex formation were calculated to be 10.24–18.89 kJ mol–1. Furthermore, individual reorganization energies were correlated with the charge-transfer quenching rate constants, revealing the sensitivity to specific ligand modifications.