Mechanistic Insights: Correspondence on "Tuning Co-Operative Energy Transfer in Copper(I) Complexes Using Two-Photon Absorbing Diimine-Based Ligand Sensitizers"

In a recent communication, Collins and coworkers presented a Cu(I) complex with photocatalytic activity under red light LED conditions, mainly for singlet oxygen-driven reactions. Guided by steady-state emission measurements with 800 nm excitation, the authors suggested that the underlying mechanism for the generation of the photoexcited key species is a simultaneous two-photon absorption via a virtual state. However, such a mechanism requires pulsed laser excitation and cannot compete when a conventional one-photon excitation is also feasible with the selected excitation wavelength range. Using several spectroscopic techniques and reactivity assays under different light color and intensity conditions, we unambiguously demonstrate that a conventional one-photon excitation followed by rather inefficient singlet oxygen generation (quantum yield <5%) is responsible for the observed photoreactivity of the Cu(I) complex. In addition, we briefly summarize general mechanistic considerations, estimate typical photon densities required for a variety of two-photon mechanisms, highlight the importance of optical filters and impurities to avoid artifacts in the emission spectra, and present some guidelines for the differentiation between one- and two-photon mechanisms.