Preparation and photoelectrochemical characterization of a red sensitive osmium complex containing 4,4′,4′′-tricarboxy-2,2′:6′,2′′-terpyridine and cyanide ligands

The complex ion [OsII(H3tcterpy)(CN)3]− (H3tcterpy=4,4′,4″-tricarboxy-2,2′:6′,2″-terpyridine) has been prepared by an easy methodology and characterized. The UV-Vis absorption in CH3OH shows a series of MLCT bands with distinct maxima spanning the whole visible spectrum and a remarkably intense band (ε≈1500 M−1 cm−1) at 811 nm associated to a spin-forbidden singlet–triplet MLCT transition allowed by spin–orbit coupling. Cyclic voltammetry of the complex in CH3OH showed a reversible OsII→OsIII oxidation process with E1/2=663 mV. Controlled shifting of MLCT absorption and Os oxidation potential is accomplished by simply changing the degree of protonation of the carboxylic groups. The behavior of the complex as sensitizer in dye sensitized solar cells (DSSCs) has been tested giving satisfactory IPCE values and showing contribution to the photoaction spectrum from the singlet to triplet transition. The photochemical stability was qualitatively investigated and proved to be better than that of the best Ru-based sensitizer known up to now.