10.1021/ja992402g.s001
Georg Wolfbauer
Georg
Wolfbauer
Alan M. Bond
Alan M.
Bond
Glen B. Deacon
Glen B.
Deacon
Douglas R. MacFarlane
Douglas R.
MacFarlane
Leone Spiccia
Leone
Spiccia
Experimental and Theoretical Investigations of the Effect of
Deprotonation on Electronic Spectra and Reversible Potentials of
Photovoltaic Sensitizers: Deprotonation of <i>cis</i>-L<sub>2</sub>RuX<sub>2</sub> (L =
2,2‘-Bipyridine-4,4‘-dicarboxylic Acid; X = CN<sup>-</sup>, NCS<sup>-</sup>) by
Electrochemical Reduction at Platinum Electrodes
American Chemical Society
1999
L 2 RuX 2
dcbpy
voltammetric time scale
photovoltaic cells
electrode
deprotonation
deprotonated L 2 RuX 2
H 2
2 RuX 2
CN
Platinum Electrodes Deprotonation
deprotonated H 2
electrogenerated deprotonated forms
Short time domains
NCS
deprotonated forms
1999-12-21 00:00:00
Journal contribution
https://acs.figshare.com/articles/journal_contribution/Experimental_and_Theoretical_Investigations_of_the_Effect_of_Deprotonation_on_Electronic_Spectra_and_Reversible_Potentials_of_Photovoltaic_Sensitizers_Deprotonation_of_i_cis_i_-L_sub_2_sub_RuX_sub_2_sub_L_2_2_-Bipyridine-4_4_-dicarboxylic_Acid_X_CN_sup_-_/3676479
Deprotonation of the photovoltaic dye sensitizers <i>cis</i>-(H<sub>2</sub>-dcbpy)<sub>2</sub>RuX<sub>2</sub> (L<sub>2</sub>RuX<sub>2</sub>) (X= −CN<sup>-</sup>,
−NCS<sup>-</sup>; H<sub>2</sub>-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<sup>+</sup> (Fc = ferrocene) is applied to a platinum electrode can be summarized as (H<sub>2</sub>-dcbpy)<sub>2</sub>Ru(NCS)<sub>2</sub> +
<i>x</i>e<sup>-</sup> → [(H<sub>2</sub><sub>-</sub><i><sub>x</sub></i><sub>/2</sub>-dcbpy<i><sup>x</sup></i><sup>/2-</sup>)<sub>2</sub>Ru(NCS)<sub>2</sub>]<i><sup>x</sup></i><sup>-</sup> + <sup>x</sup>/<sub>2</sub>H<sub>2</sub>, where <i>x</i> is always slightly less than 4. Thus, under certain
experimental conditions, [(H-dcbpy<sup>-</sup>)<sub>2</sub>RuX<sub>2</sub>]<sup>2-</sup> is believed to be the major product formed by bulk electrolysis,
where H-dcbpy<sup>-</sup> is the singly deprotonated H<sub>2</sub>-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 L<sub>2</sub>RuX<sub>2</sub> and protons or water. Short time domains, reduced
temperatures, and glassy carbon electrodes lead to detection of transiently stable ligand-reduced forms of L<sub>2</sub>RuX<sub>2</sub>. The reversible half-wave potentials for the ligand-based reduction of electrochemically generated
deprotonated L<sub>2</sub>RuX<sub>2</sub> 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 L<sub>2</sub>RuX<sub>2</sub> 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 L<sub>2</sub>RuX<sub>2</sub> 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.