Electrochemical, spectroelectrochemical and theoretical studies on the reduction and deprotonation of the photovoltaic sensitizer [(H-3-tctpy)Ru-II(NCS)(3)](-) (H-3-tctpy=2,2 ': 6 ',2 ''-terpyridine-4,4 ',4 ''-tricarboxylic acid)

The electrochemical reduction of the black dye photosensitizer [(H-3-tctpy) Ru-II(NCS)(3)](-) (H-3-tctpy = 2,2':6',2"-terpyridine-4,4',4"-tricarboxylic acid) used in photovoltaic cells has been found to be a complex process wh en studied in dimethylformamide. At low temperatures, fast scan rates and a t a glassy carbon electrode, the chemically reversible ligand based one-ele ctron reduction process [(H-3-tctpy)Ru(NCS)(3)](-) + e(-) reversible arrow [(H-3-tctpy(...))Ru(NCS)(3)](2-) is detected. This process has a reversible half-wave potential (E-1/2(r)) of - 1585 +/- 20 mV versus Fc/Fc(+) at 25 d egreesC. Under other conditions, a deprotonation reaction occurs upon reduc tion, which produces [(H3-x-tctpy(x-))Ru(NCS)(3)]((1+x)-) and hydrogen gas. Mechanistic pathways giving rise to the final products are discussed. The E-1/2(r)-value for the ligand based reductions of the deprotonated complex is 0.70 V more negative than for [(H-3-tctpy)Ru(NCS)(3)](-). Consequently, data obtained from molecular orbital calculations are consistent with the r eaction [(H-3-tctpy)Ru(NCS)(3)] +e(-) --> [(H-2-tctpy(-))Ru(NCS)(3)](2) - 1/2H(2) yielding the monodeprotonated complex as the major product obtaine d after electrochemical reduction of [(H-2-tctpy)Ru(NCS)(3)](-). The E-1/2( r)-values for the metal based Ru-II/III process differ by 0.30 V when data obtained for the protonated and deprotonated forms of the black dye are com pared. Electronic spectra obtained during the course of experiments in an o ptically transparent thin layer electrolysis configuration are consistent w ith the overall reaction scheme proposed on the basis of voltammetric measu rements and molecular orbital calculations. Reduction studies on the free l igand, H-3-tcpy, are consistent with results obtained with [(H-3-tctpy)Ru(N CS)(3)](-). (C) 2000 Elsevier Science S.A. All rights reserved.