Light-driven electron transfer in a modular assembly of a ruthenium(II) polypyridine sensitiser and a manganese(II) terpyridine unit separated by a redox active linkage. DFT analysis

Abstract

A series of ruthenium polypyridine-based complexes covalently bound to a terpyridine coordinating site for Mn II ion coordination has been developed. A redox active unit separates the photoactive unit and the manganese complex. Introducing ester groups on the bipyridine skeleton allows modulation of redox properties of the chromophore. Intramolecular electron transfer from the Mn II to the photogenerated Ru III was studied by timeresolved transient absorption and EPR. Photophysical studies support the participation of the imidazole unit in the electron transfer process from the Mn(II) complex and Ru(III) in the case of ester containing chromophores. DFT calculations were performed and used to rationalize the photophysical behavior of the complexes, in particular the effect of coordination of the Mn II ion to the terpyridine cavity as well as the influence of the electron withdrawing groups on the Ru chromophore.

Proton-controlled Action of an Imidazole as Electron Relay in a Photoredox Triad

“…We have pursued our effort to unravel the implication of a molecular electron relay in photosensitizer-redox-active-manganese complexes. In a previous study, we found that decorating the bipyridine ligands with ester functions acting as electron-withdrawing groups and inducing an anodic shift by ca.160 mV of the Ru III/II redox couple led to the distinction of the imidazole redox activity in contrast with our previous models [18]. However, its participation in the oxidation of the Mn(II) center could not be established.…”

“…The presence of the divalent ion in the terpyridine also affects this band implicating a certain degree of interaction. Mirroring the red-shift observed on the electronic absorption spectra, the emission maximum for the complexes occurs at 645-650 nm instead of 610 nm (Table S1) [18]. Compared to Ru(bpy) 3 , the emission lifetime is increased from 0.8 to 1-1.6 µs (Fig.…”

“…The coordination of Mn(II) in the terpyridine cavity leads to a decreased emission yield which is attributed to partial fast energy transfer from the Ru II * excited state to the terpyMn II unit. Due to the presence of the ester substituents and the phenyl spacer this energy transfer quenching is considerably attenuated compared to complexes without these features [18]. No emission quenching is observed in presence of the Zn(II) ion in the terpyridine cavity.…”

“…The synthesis of the mononuclear Ru compound 1 was performed as previously described [17,19]. Compounds 2 and 3 were obtained by metalation with MnCl 2 or ZnCl 2 , respectively, as previously described [17,18]. Our synthetic strategy was to use a [Ru II (bpyCO 2 Et) 2 (phendione)] 2+ complex as convenient synthon for the incorporation of a photoactive unit as part of more complex systems [17].…”

“…Light-induced Mn oxidation in this kind of complexes has been demonstrated by EPR measurements in presence of irreversible electron acceptors [17,18]. However, in these studies Mn III formation is not time-resolved and therefore the detection of Mn oxidation is not a proof for the occurrence of intramolecular ET.Photochemical & Photobiological Sciences…”