Ru(II) and Os(II) complexes (P) of [4'-(p-phenyl)]terpyridyl ligand (ptpy) derivatized with an electron acceptor (A) of the triphenylpyridinium (H3TP+) type have been recently proposed as functional models for electron-transfer (ET) processes in the context of artificial photosynthesis. These inorganic dyads, P-A, are expected to undergo intramolecular photoinduced ET to form a charge separated (CS) state of pivotal interest. To draw a complete picture of possible ET processes, the ground- and excited-state properties of these complexes, both in their native and monoreduced forms, have been studied by the means of density functional theory (DFT). A time-dependent-DFT approach (TDDFT) was used to interpret the electronic spectra, while additional spectroscopic measurements have been carried out in order to complete the available experimental information and to further confirm the theoretical issues. Besides the noticeable quantitative agreement between computed and experimental absorption spectra, our results allow us to clarify, by first principles, the actual nature and interplay of the electronic and geometrical coupling between the acceptor moiety and the photosensitizer. The possibility of a direct (optical) ET from the ground state to the targeted *[P+-A-] CS state is theoretically postulated and found to be consistent with available photophysical data (transient absorption spectroscopy). Concerning backward ET (from the CS state), the occurrence of a quinoidal-like electronic redistribution inherent to the photoreduced acceptor-ligand is proposed to favor efficient charge recombination.

Photosensitizers used in homogeneous photocatalytic systems for artificial photosynthesis, such as hydrogen production, are typically based on expensive transition metal complexes such as d 6 ruthenium(II) or iridium(III). In this work, we demonstrate efficient H 2 production in acidic water by using an organic dye derived from the triazatriangulenium (TATA + ) family as a visible-light-absorbing photosensitizer (PS). By associat ing the hydrosoluble t ris(ethoxyet hanol)triazatriangulenium with an efficient H 2 -evolving cobalt catalyst and ascorbic acid as sacrificial electron donor (SD), remarkable photocatalytic performances were reached in aqueous solution at pH 4.5, under visible-light irradiation, with up to 8950 catalytic cycles versus catalyst. The performances of this dye largely exceed those of the benchmark Ru tris-bipyridine in the same experimental conditions when low concentrations of catalyst are used. This higher efficiency has been clearly ascribed to the remarkable robustness of the reduced form of the organic dye, TATA • . Indeed, the combination of the planar structure of TATA + together with the presence of the three electron-donating nitrogen atoms promotes the stabilization of TATA • by delocalization of the radical, thereby preventing its degradation in the course of photocatalysis. By contrast, the reduced form of the Ru photosensitizer, [Ru II (bpy) 2 (bpy •− )] + ("Ru − "), is much less stable. Nanosecond transient absorption experiments confirm the formation of TATA • in the course of the photocatalytic process in accordance with the mechanism initiated by the reductive quenching of the singlet excited state of TATA + by ascorbate. The second electron transfer from TATA • to the catalyst has also been evidenced by this technique with the detection of the signature of the reduced Co(I) form of the catalyst. The present study establishes that certain organic dyes are to be considered as relevant alternatives to expensive metal-based PSs insofar as they can exhibit a high stability under prolonged irradiation, even in acidic water, thereby providing valuable insights for the development of robust molecular systems only based on earth-abundant elements for solar fuel generation.