Visible-Light-Driven Photoreactions of [(bpy)₂Ru(II)L]Cl₂ in Aqueous Solutions (bpy = Bipyridine, L = 1,2-Bis(4-(4‘-methyl)-2,2‘-bipyridyl) Ethene)

Abstract

Visible-light-induced photoreactions of [(bpy)2Ru(II)L]Cl2 (bpy = bipyridine, L = trans-1,2-bis(4-(4'-methyl)-2,2'-bipyridyl) ethene) in aqueous solution are examined. From pH titrations, it is found that the Ru complex is a stronger base (pKa* = 6) in the excited state than in the ground state (pKa = 4). Photolysis of the [(bpy)2Ru(II)L] complex in solutions at pH 7 and 12 led to formation of species with increased emission quantum yields, approximately 55 nm blue-shift of the emission maximum to 625 nm, and disappearance of the absorption band at 330 nm, the latter arising from the olefinic bond of the L ligand. No spectral changes are observed in solutions at pH < or = 4. With the help of chromatography, mass spectroscopy, Raman spectroscopy, and NMR, photoproducts formed at neutral pH have been analyzed. It is found that the major product is a dimer of [(bpy)2Ru(II)L], dimerizing around the double bond. Photoreactions do not occur in the dark or in the aprotic solvent acetonitrile. We propose that a Ru(III) radical intermediate is formed by photoinduced excited-state electron and proton transfer, which initiates the dimerization. The radical intermediate can also undergo photochemical degradative reductions. Below pH 4, the emission quenching is proposed to arise via protonation of the monoprotonated [(bpy)2Ru(II)LH] followed by electron transfer to the viologen-type moiety created by protonation. The products of photodegradation at pH > 12 are different from those of pH 7, but the mechanism of the degradation at pH > 12 was not elucidated.

Ultrafast Electron Transfer Dynamics in Ruthenium Polypyridyl Complexes with a π-Conjugated Ligand

“…RuL and RuL DQ were synthesized as described previously. , Salts with PF 6 − counterions were prepared because of their good solubility in acetonitrile. Acetonitrile was obtained from Burdick and Jackson and used as received.…”

Proton-Coupled Electron Transfer

“…The radical induces cyclodimerization with a second complex followed by re-oxidation by the Ru(III) formed by excited state quenching. 177 As a model for sensitized electron transfer in Photosystem II, PCET reactions were also investigated in polypyridyl Ru(II) and Re(I) complexes with tyrosine-like ligands, Figure 55. Also, note Section 8.2.…”

Ligand-based photooxidations of dithiomaltolato complexes of Ru(ii) and Zn(ii): photolytic CH activation and evidence of singlet oxygen generation and quenching

“…Photolysis of [Ru(bpy) 2 (ttma)] + at room temperature in H 2 O but in the absence of an electron transfer agent at room temperature leads to displacement of the ttma ligand to form the [Ru(bpy) 2 (H 2 O)-(OH)] + complex, as previously reported for analogous complexes (ESI SM3 ‡). 24,25 Subsequently, all photolysis experiments using complex 1 were conducted at 5 °C in the absence of water, with addition of basic water afterwards.…”