Living Long and Prosperous: Productive Intraligand Charge-Transfer States from a Rhenium(I) Terpyridine Photosensitizer with Enhanced Light Absorption

The ground- and excited-state properties of six rhenium(I)kappa N-2-tricarbonyl complexes with 4'-(4-substituted-phenyl)-terpyridine ligands bearing substituents of different electron-donating abilities were evaluated. Significant modulation of the electrochemical potentials and a nearly 4-fold variation of the triplet metal-to-ligand charge-transfer ((MLCT)-M-3) lifetimes were observed upon going from CN to OMe. With the more electron-donating NMe2 group, we observed in the kappa N-2 complex the appearance of a very strong absorption band, red-shifted by ca. 100 nm with respect to the other complexes. This was accompanied by a dramatic enhancement of the excited-state lifetime (380 vs 1.5 ns), and a character change from (MLCT)-M-3 to intraligand charge transfer ((ILCT)-I-3), despite the remote location of the substituent. The dynamics and character of the excited states of all complexes were assigned by combining transient IR spectroscopy, IR spectroelectrochemistry, and (time-dependent) density functional theory calculations. Selected complexes were evaluated as photosensitizers for hydrogen production, with the kappa N-2-NMe2 complex resulting in a stable and efficient photocatalytic system reaching TONRe values of over 2100, representing the first application of the (ILCT)-I-3 state of a rhenium(I) carbonyl complex in a stable photocatalytic system.

Automated summary

The ground- and excited-state properties of six rhenium(I)kappa N-2-tricarbonyl complexes with 4'-(4-substituted-phenyl)-terpyridine ligands bearing substituents of different electron-donating abilities were evaluated. Significant changes in electrochemical potentials and triplet metal-to-ligand charge-transfer lifetimes were observed, with the most electron-donating group leading to the appearance of a strong absorption band and a dramatic enhancement of the excited-state lifetime. The dynamics and character of the excited states were assigned using a combination of techniques, and selected complexes showed promising results as photosensitizers for hydrogen production.