Development of a facile time-resolved spectroelectrochemical method: An application to determine the rate constant of protonation for anions of 9,10-diphenylanthracene, biphenyl, and p-quaterphenyl

Using a conventional three-electrode system and electrochemical cell, we developed a simple spectroelectrochemical method that can be used to determine the rate constant of protonation reaction for several reduced species. A glassy carbon (GC) disk electrode as a working electrode and halogen lamp as a light source were employed. The reflected light on the GC electrode surface through the diffusion layer of electrochemically generated species was recorded by an electron multiplying charge coupled device camera equipped with a spectrometer. With this system, the absorbance change by the generation of the radical anion of 9,10-diphenylanthracene (DPA), biphenyl, and p-quaterphenyl was monitored. The molar extinction coefficient (ε) of the radical ions was calculated with the absorbance and charge during potential steps. The absorption spectrum of electrochemically generated dianion of p-quaterphenyl was also monitored, and the ε value was determined. The comproportionation reaction between p-quaterphenyl²⁻ and neutral p-quaterphenyl was also clearly observed on the spectroelectrochemical data. The absorbance change during the protonation at different concentration of ethanol (c_EtOH) for DPA^•−, biphenyl^•−, p-quaterphenyl^•−, and p-quaterphenyl²⁻ and the ε values were used to determine the first-order rate constant of the protonation reaction. The first-order rate constant increased linearly against c_EtOH² for DPA^•−, p-quaterphenyl^•−, and p-quaterphenyl²⁻, and against c_EtOH³ for biphenyl^•−. From the slopes, the reaction rate was determined to be 0.45, 4.2×10², and 4.3×10⁴ M⁻² s⁻¹, for DPA^•−, p-quaterphenyl^•−, and p-quaterphenyl²⁻, respectively, and for biphenyl^•−, 6.8×10⁴ M⁻³ s⁻¹.