Study On The Redox Mechanism Of 1,4-naphthoquinone By IR Spectroelectrochemistry

Quinone compounds widely exist in nature. The carbonyl group in the structure is the active center of the redox reaction, therefore quinones are good carrier of electron transfer with good electrochemical activity. Quinones play important roles in the metabolic process of living organisms. The electrochemical character of 1, 4-naphthoquinone (Q) in aprotic media and proton donors mixed solvent was explored by cyclic voltammetry (CV), in situ FT-IR spectroelectrochemistry, cyclic voltabsorptometry (CVA) and derivative cyclic voltabsorptometry (DCVA) techniques. In addition, attenuated-total-reflection surface-enhanced infrared spectroscopy electrochemistry (ATR-SEIRAS-EC) was explored preliminarily. The main works were summarized as follows:1. The electrochemical behavior of Q in dimethyl sulfoxide and acetonitrile was studied.(1) In the thin-layer cell, the cyclic voltammetry(CV) of Q shows two well-defined couples of redox peaks in DMSO. We assign the IR absorption peaks in the the corresponding 3D spectra.1664; 1510 and 1364,1272,1202 cm-1 can be used to trace the concentration of Q, Q- and Q2- respectively in the electrochemical process. It can be observed that the absorbance at 1510 cm-1 corresponding to Q·-increases and decreases periodically in both reduction and oxidation process. In addition, the absorbance at all IR peaks return to the initial value when the experiment is over. To illustrate the behavior of the absorption at different wavenumbers clearly, the rapid-scan IR spectra are conducted to CVA and DCVA. The results suggest that in DMSO, Q is reduced in normal two-step one-electron transfer, forming the anion radical (Q·-) in the first step and the dianion (Q2-) in the second step accordingly. The redox process is listed as follows:Q(?)Q·-(?)Q2-.(2) In the thin-layer cell, three couples of redox peaks and a weak cathodic peak C2 are observed in the CV. In the 3D spectra, a new positive-going band at 1541 cm-1 is observed, which is assigned to the dimer.1664; 1541; 1510 and 1364,1272, 1202 cm-1 can be used to trace the concentration of the species respectively. It can be observed that the absorbance at 1664 and 1541 cm-1 do not return to the initial value. Combined with the results of CVA and DCVA, we infer that Q undergoes the dimerization reaction along with the irreversible electrochemical process in CH3CN. The possible reaction mechanism is as follows: C1/A1:Q+e(?)Q·-Q+Q·-(?)Q2·-C3/A3:Q·-+e(?)Q2-Q2·-+e(?)Q22-C4/A4:Q22-+e(?) Q23-2. The electrochemical behavior of Q in aprotic-proton donors mixed solvent was studied.(1) in CH3CN-C2H5OH mixed media, all redox peaks shift positively with the concentration of C2H5OH increase. In addition, for given concentration of C2H5OH, the latter two couples of peaks at more negative potential shift larger than the one at more positive. Based on the results of 3D spectra, CVA and DCVA. We can infer that the dimerization to form the neutral-anion radical occurs along with the electrochemical reactions and hydrogen-bonding forms between Q2-(Q·-, Q2·-, Q22-, Q23-) and C2H5OH.(2) in DMSO-C2H5OH mixed media, addition of C2H5OH to DMSO results in both reduction steps of Q positive shift. The more C2H5OH is added, the larger the potential shifts. In addition, a new couple of redox peaks can be observed when certain concentration of C2H5OH is added. The new positive-going band at 1549 cm-1 is observed in the 3D spectra, which does not emerge in DMSO without C2H5OH. The 3D spectra is conducted to CVA and DCVA. It is found that the dimerization to form the neutral-anion radical occurs when certain concentration of C2H5OH is added to DMSO and hydrogen-bonding forms between C2H5OH and Q2-(Q·-, Q2·-, Q22-, Q23-).3. We choose silicon as the base material and Au nanofilms on silicon is prepared by electroless deposition method. The electrochemical measurement is performed in solution containing HClO4 and PNBA to test the properties of Au nanofilms. The absorption behavior of K3Fe(CN)6 and 1,4-naphthoquinone on the surface of Au nanofilms was studied by ATR-SEIRAS and electrochemical technique. Au nanofilms on Si is used as the working electrode and infrared window. The results of ATR-SEIRAS suggest that blue shift for IR peak of K3Fe(CN)6 and red shift for IR peak of K4Fe(CN)6 occur due to absorption on Au nanofilms. In addition, we infer that the absorption of K3Fe(CN)6 on the surface of Au nanofilms may be stronger than K4Fe(CN)6. The IR peaks of 1,4-naphthoquinone become complicated after absorbing on the surface of Au nanofilms and further research is needed.