Directionality of Ultrafast Electron Transfer in a Hydrogen Evolving Ru–Pd-Based Photocatalyst

Abstract

Directionality of electron transfer and long-lived charge separation are of key importance for efficient photocatalytic water splitting. Knowledge of the processes that follow photoexcitation is essential for the optimization of supramolecular assembly designs in order to improve the efficiency of photocatalytic hydrogen generation. Photoinduced intramolecular electron transfer processes within the hydrogen-evolving photocatalyst [Ru(bpy)2(tpy)Pd(CH3CN)Cl]2+ (RuPd; bpy = bipyridine, tpy = 2,2′:5′,2″-terpyridine) have been studied by resonance Raman, femtosecond transient absorption, and time-resolved photoluminescence spectroscopies. Comparison of the photophysical properties of RuPd with those of the mononuclear precursor [(bpy)2Ru(tpy)]2+ (Ru) enables establishment of a photophysical model ranging from the femtosecond to the submicrosecond domain. Optical excitation of Ru and RuPd populates both bpy- and tpy-based 1MLCT (metal-to-ligand charge transfer) singlet states, from where intersystem crossing (ISC) into corresponding 3MLCT triplet states occurs. Electron density localized on the peripheral bpy ligands can subsequently flow to the tpy bridging ligand by interligand electron transfer, which process occurs with a time constant of 32.5 (±1.5) ps for RuPd. Not all electron density undergoes this process, most likely due to a competing loss channel on the bpy ligand caused by vibrational relaxation occurring at a time scale of 9.1 (±0.4) ps. The relaxed 3MLCTbpy and 3MLCTtpy states have excited state lifetimes of 400 (±1) ns and 88 (±1) ns, respectively. Electron transfer from the tpy ligand to Pd may take place on a ∼100 ns time scale, but it is also possible that the final relaxed excited state is delocalized over the tpy ligand and the Pd center. The insight that optical excitation populates both the peripheral bpy ligands and the bridging tpy ligand, and that part of the electron density subsequently flows from the former to the latter, is important for the realization of efficient photocatalytic hydrogen generation. The next step is to make the interligand electron transfer process faster, by functionalizing the peripheral ligands with electron-donating moieties, and adapting the nature of the bridging ligand and the catalytic metal center.