In acetonitrile solution, the monomer's electronic absorption spectra was recorded. (Fig. 2). The absorption spectrum has exhibited two peaks at 300 and 445 nm. The first peak at 300nm is corresponding to the n-π* transition in EDTT unit, however, the peak at 445nm is likely to be the π-π* transition from the conjugation of thiophene units. The onset of the absorption edge at highest wavelength 525.2 nm gives an optical HOMO-LUMO gap of 2.3 eV.
The cyclic voltammogram experiments of monomer were performed in acetonitrile solution. In the monomer oxidation run (Fig. 3).There are two irreversible peaks; the first one is at 0.45V; this peak is likely to be the oxidation in EDTT unit and second one is at 1.1 V that will be the form of radical cation on thiophene groups. In the reduction run (Fig. 4), the irreversible peak (-2.09V) is due to formation of an anion radical species from thiophene group, this reaction.
The electrochemical band gap was calculated from the difference in the onsets of the first oxidation and reduction peaks. The HOMO-LUMO levels are calculated by subtracting the onsets from the HOMO of Ferrocene. The monomer has given really similar HOMO-LUMO gap value (2.27eV) when compared against the optical LUMO gap value
(2.3eV). The cyclic process of the monomer is summarised in next results on Table 1.
Table 1
Electrochemical data for the monomer
Onset of oxidation /V | HOMO /eV | Onset ofreduction /V | LUMO /eV | HOMO –LUMO /eV gap |
0.37 | -5.17 | -1.90 | -2.9 | 2.27 |
The electropolymerisation of this compound was achieved through repetitive cycling (about 180 segments) over the first oxidation peak of the monomer. The growth of the polymer was monitored through the increase in current with each cycle and the development of a new reversible peak at lower potential, as shown in (Figure. 5).
The polymer has subjected to the oxidation and reduction analysis and the electrochemical band gap of the polymer was determined once again by cyclic voltammetry from the full cycle for energy level determination. The difference between the HOMO and LUMO values of the polymer gave an electrochemical HOMO-LUMO gap is 1.81eV.
The data are shown in Table 2.
Table 2
Electrochemical data for the Polymer
Onset of oxidation /V | HOMO /eV | Onset ofreduction /V | LUMO /eV | HOMO –LUMO /eV gap |
+ 0.24 | -5.04 | -1.57 | -3.23 | 1.81 |