A New Ruthenium(II) Photosensitizer With Bipyridines Containing Electron Donor/Withdrawing Groups

Ruthenium(II) polypyridyl complexes have interesting physicochemical properties that can be applied in energy conversion processes. Since the development of the Gratzel cell, considerable efforts have been made to find new materials for dye-sensitized solar cells (DSSCs) [1]. In this work, a new polypyridylruthenium(II) complex, of formula [Ru(DMO-bpy)2(Mebpy-CN)]2+, (1), where DMO-bpy = 4,4'-dimethoxy-2,2'-bipyridine and Mebpy-CN = 4-methyl-2,2'-bipyridine-4'-carbonitrile, has been synthesized as a PF6- salt and characterized by spectroscopic, photophysical and electrochemical techniques. Mebpy-CN ligand is an adequate anchoring entity to bind ruthenium photosensitizers to ZnO semiconductor surfaces in DSSCs [2]. The UV-Vis absorption spectrum of (1) in CH3CN shows two intense bands with maxima at 453 and 492 nm that can be assigned to MLCT transitions from dpi(Ru) orbitals to pi* orbitals of DMO-bpy and Mebpy-CN respectively. The emission spectrum shows a band at a maximum of 679 nm, with a quantum yield Φ = 3x10-3. The redox potential of the RuIII/RuII couple was determined by CV as E1/2 = 1.12 V (vs. SCE). The cathodic shift of E1/2 and the red shifts of both the UV-Vis absorption and emission maxima respect to the corresponding values of [Ru(bpy)3]2+ are the result of destabilization of dpi(Ru) HOMOs induced by two bpys with electron-donor groups [3]. This may prove useful to enhance the efficiency of DSSCs.