Experimental and theoretical investigations of the effect of deprotonationon electronic spectra and reversible potentials of photovoltaic sensitizers: Deprotonation of cis-L2RuX2 (L=2,2 '-bipyridine-4,4 '-dicarboxylic acid;X = CN-, NCS-) by electrochemical reduction at platinum electrodes

Deprotonation of the photovoltaic dye sensitizers cis-(H-2-dcbpy)(2)RuX2 (L 2RuX2) (X= -CN-, -NCS-; H-2-dcbpy = L = 2,2'-bipyridine-4,4'-dicarboxylic a cid) can be achieved in dimethylformamide by reductive electrolysis at plat inum electrodes at 20 degrees C, which allows the thermodynamic and spectra l changes associated with deprotonation to be established. The overall reac tion that occurs when a potential of -2.0 V vs Fc/Fc(+) (Fc = ferrocene) is applied to a platinum electrode can be summarized as (H-2-dcbpy)(2)Ru(NCS) (2) + xe(-) --> [(H2->x/2-dcbpy(x/2-))(2)Ru(NCS)(2)](x-) + x/2H(2), where x is always slightly less than 4. Thus, under certain experimental condition s, [(H-dcbpy(-))(2)RuX2](2-) is believed to be the major product formed by bulk electrolysis, where H-dcbpy(-) is the singly deprotonated H-2-dcbpy li gand. The hydrogen gas formed in this electrochemically induced deprotonati on can be generated heterogeneously at the electrode surface or via homogen eous redox reactions between ligand-reduced forms of L2RuX2 and protons or water. Short time domains, reduced temperatures, and glassy carbon electrod es lead to detection of transiently stable ligand-reduced forms of L2RuX2. The reversible half-wave potentials for the Ligand-based reduction of elect rochemically generated deprotonated L2RuX2 are 0.65 V more negative than th eir protonated counterparts. In contrast, deprotonation leads to the metal- based oxidation process being shifted by only about 0.3 V. Interestingly, p rotonated and deprotonated forms of L2RuX2 do not coexist in a facile acid- base equilibrium state on the voltammetric time scale. Data obtained from e lectrogenerated deprotonated forms of the sensitizers are compared to those found for "salts" used in photovoltaic cells which are prepared by reactio n of L2RuX2 With tetrabutylammonium hydroxide. Molecular orbital calculatio ns were employed to provide theoretical insights into the effect of deproto nation on reversible potentials and electronic spectra, and results are in good agreement with experimentally obtained data. Electronic spectra, measu red in situ during the course of reduction in a spectroelectrochemical cell , reveal that all bands shift to higher energies and that the absorbance de creases as deprotonation occurs. implications of the importance of the find ings related to reduction potentials and electronic spectra to the operatio n of photovoltaic cells that utilize deprotonated forms of sensitizers are considered.