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

Abstract

Abstract 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.