Ultrafast hydrogen-bonding interactions between a photoexcited Cu–anthraquinone donor–acceptor dyad and protic solvents

The rational design of solar energy catalysts requires a mechanistic understanding of the ultrafast interactions with the solvent environment. We have designed a new Cu(I)–anthraquinone framework (CuEthyneAnQ) to serve as a model for studying hydrogen-bonding effects in charge accumulating photocatalysts. Herein, we report the ground and excited-state characterization of CuEthyneAnQ by electrochemical and ultrafast optical transient absorption (OTA) spectroscopy measurements. Significant stabilization of the AnQ-centered reductions due to hydrogen-bonding was observed by electrochemical measurements in protic solvent mixtures. Analysis of the excited-state photophysics with OTA reveals electron transfer occurring in tens of picoseconds after metal-to-ligand charge transfer excitation, resulting in the charge-separated state of Cu(II)EthyneAnQ·–. Charge recombination occurs in 4 ns in aprotic solvent and extends to 19 ns in protic solvent. In order to examine the influence of hydrogen-bonding on the electron-transfer dynamics, we performed OTA measurements on CuEthyneAnQ in varying aprotic:protic solvent mixtures. We observe three effects that depend on the concentration of the protic solvent: (1) after charge separation, a diffusion-limited hydrogen-bond forms with the reduced AnQ·–; (2) the slowdown in charge recombination with protic solvent addition is due to hydrogen-bond stabilization in accordance with Marcus theory; and (3) a spectral shift occurs in the charge-separated state due to an increasing number of hydrogen-bond interactions. Our results are supported by time-dependent density functional theory calculations with explicit solvent hydrogen-bonding interactions. These insights underscore the potential of Cu-based donor–acceptor complexes and mixed-solvent systems to offer valuable guidelines for the design of more efficient photocatalytic systems.