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) κ-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) lifetimes were observed upon going from CN to OMe. With the more electron-donating NMe group, we observed in the κ 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 to intraligand charge transfer (ILCT), 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 κ-NMe complex resulting in a stable and efficient photocatalytic system reaching TON values of over 2100, representing the first application of the ILCT state of a rhenium(I) carbonyl complex in a stable photocatalytic system.