Electrochemically- and Photo-Induced IR Absorption of Low Band-Gap Polydithienothiophenes: A Comparative Study
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"polythiophene-like" chain (sketched by bold bonds in figure 1(a)) are strongly affected by the presence of the other two fused rings (thienothiophene moiety) of the monomer. As shown in the inset of figure 1(a), the aromatic moiety fused to the ring that lies in the backbone forces "quinoidal" character along the polymer chain so that the increased inter-rings delocalization of the π-electrons reduces the polymer band-gap [5]. S
S
S
[
S
]
[
n
pDTT1 Eg= 1.15 eV
[
]
n
S
pDTT2 Eg= 1.12 eV S
S
a)
S
]n
pDTT3 Eg= 1.05 eV
Reference Electrode Electrolyte
Detector
S
S
S
Counter Electrode
S
S
S
S
b)
IR beam
Ge reflection Element
pDTT film
Figure 1: (a) Chemical structures and band gap values of pDTT1, pDTT2 and pDTT3; (b) Cell for the in-situ ATR-FTIR spectroelectrochemistry. According to this picture, pDTTs have band-gaps that decrease with increasing of the aromaticity of the thienothiophene fused moiety [6]. Data from in-situ ATR-FTIR spectroelectrochemistry give further evidence for the strong influence of the different fused moieties to the "polythiophene-like" pDTTs chain properties. In addition, we confirm these results by the study of photodoping processes and by recording Raman spectra of the neutral polymers, using several excitation wavelengths (both off- and resonance with the π-π* transition).
EXPERIMENTAL The polymer films were electrosynthesized using the three electrodes spectroelectrochemical cell shown in figure 1(b). The working electrode consisted of a germanium reflection element coated with a thin evaporated layer of platinum. As counter and reference electrode, a platinum foil and a Ag/AgCl wire electrode were used, respectively. The electrochemical equipment consisted of a potentiostat (Jaissle 1002TNC), a sweep generator (Prodis 1/14I) and an X-Y recorder (Rikadenki RY-PIA). For the polymerizations and for the in-situ spectroelectrochemical measurements, the electrolyte was 0.1 M tetrabutylammoniumhexafluorophosphate (TBAPF6) (dried at 180°C under vacuum prior the use) in dry acetonitrile. The electrolyte solutions were prepared and kept under argon flow to exclude moisture and oxygen during the electrochemical processes. The monomers, synthesized as reported in the literature [7], were dissolved into the electrolyte solution in a dry argon box using shielded glassware and then transferred to the spectroelectrochemical cell via syringe. After polymerization, the cell was carefully washed and filled with fresh electrolyte. All the experiments were done at room temperature. FTIR absorption spectra were recorded in situ during electrochemical
potential cycling measurements with a scan rate of 5 mV/s. To obtain specific spectral changes during the electrochemical doping a spectrum just before the considered process was chosen as reference. The subsequent spectra were related to this spectrum. The resulting spectra are plotted as ∆(-log(TATR)), where TATR is the transmission in ATR geometry. Each spectrum is calculated from a coaddition of 32 interferograms and covers
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