Experimental and Theoretical Investigations of the Effect of Deprotonation on 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

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Abstract

Deprotonation of the photovoltaic dye sensitizers cis-(H2-dcbpy)2RuX2 (L2RuX2) (X= −CN-, −NCS-; H2-dcbpy = L = 2,2‘-bipyridine-4,4‘-dicarboxylic acid) can be achieved in dimethylformamide by reductive electrolysis at platinum electrodes at 20 °C, which allows the thermodynamic and spectral changes associated with deprotonation to be established. The overall reaction that occurs when a potential of −2.0 V vs Fc/Fc+ (Fc = ferrocene) is applied to a platinum electrode can be summarized as (H2-dcbpy)2Ru(NCS)2 + xe- → [(H2-x/2-dcbpyx/2-)2Ru(NCS)2]x- + x/2H2, where x is always slightly less than 4. Thus, under certain experimental conditions, [(H-dcbpy-)2RuX2]2- is believed to be the major product formed by bulk electrolysis, where H-dcbpy- is the singly deprotonated H2-dcbpy ligand. The hydrogen gas formed in this electrochemically induced deprotonation can be generated heterogeneously at the electrode surface or via homogeneous redox reactions between ligand-reduced forms of L2RuX2 and protons or water. Short time domains, reduced temperatures, and glassy carbon electrodes lead to detection of transiently stable ligand-reduced forms of L2RuX2. The reversible half-wave potentials for the ligand-based reduction of electrochemically generated deprotonated L2RuX2 are 0.65 V more negative than their protonated counterparts. In contrast, deprotonation leads to the metal-based oxidation process being shifted by only about 0.3 V. Interestingly, protonated and deprotonated forms of L2RuX2 do not coexist in a facile acid−base equilibrium state on the voltammetric time scale. Data obtained from electrogenerated deprotonated forms of the sensitizers are compared to those found for “salts” used in photovoltaic cells which are prepared by reaction of L2RuX2 with tetrabutylammonium hydroxide. Molecular orbital calculations were employed to provide theoretical insights into the effect of deprotonation on reversible potentials and electronic spectra, and results are in good agreement with experimentally obtained data. Electronic spectra, measured in situ during the course of reduction in a spectroelectrochemical cell, reveal that all bands shift to higher energies and that the absorbance decreases as deprotonation occurs. Implications of the importance of the findings related to reduction potentials and electronic spectra to the operation of photovoltaic cells that utilize deprotonated forms of sensitizers are considered.

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