The ground- and excited-state properties of six rhenium(I) κ² N -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 (³ MLCT) lifetimes were observed upon going from CN to OMe. With the more electron-donating NMe₂ group, we observed in the κ² N 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 ³ MLCT to intraligand charge transfer (³ 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 κ² N -NMe₂ complex resulting in a stable and efficient photocatalytic system reaching TON_Re values of over 2100, representing the first application of the ³ ILCT state of a rhenium(I) carbonyl complex in a stable photocatalytic system.