Conducting Electrospun Nanofibres: Monitoring of Iodine Doping of P3HT through Infrared (IRAV) and Raman (RaAV) Polaron Spectroscopic Features

Abstract

Aligned polymer nanofibres are prepared by means of the electrospinning of a chlorobenzene solution containing regioregular poly(3-hexyltiophene -2,5-diyl), P3HT, and poly(ethylene oxide), PEO. The PEO scaffold is easily dissolved with acetonitrile, leaving pure P3HT fibres, which do not show structural modification. Polymer fibres, either with or without the PEO supporting polymer, are effectively doped by exposure to iodine vapours. Doping is monitored following the changes in the doping-induced vibrational bands (IRAVs) observed in the infrared spectra and by means of Raman spectroscopy. Molecular orientation inside the fibres has been assessed by means of IR experiments in polarised light, clearly demonstrating that electrospinning induces the orientation of the polymer chains along the fibre axis as well as of the defects introduced by doping. This work illustrates a case study that contributes to the fundamental knowledge of the vibrational properties of the doping-induced defects—charged polarons—of P3HT. Moreover, it provides experimental protocols for a thorough spectroscopic characterisation of the P3HT nanofibres, and of doped conjugated polymers in general, opening the way for the control of the material structure when the doped polymer is confined in a one-dimensional architecture.

Insights into the Structural and Vibrational Properties of Polaron in Doped Poly(3-alkyl-thiophene), P3HT

“…Based on several experimental evidence, as collected in our laboratory by means of spectroscopic investigation on doped P3HT samples in solutions, doped oligomers, and fibers, , we favor the reading of IRAV frequencies as related to the absorption maxima. We will adopt this viewpoint, while comparing experiments and theoretical predictions, which, on the other hand, support our choice.…”

“…Both the huge intensity and the peak position of IRAVs allow unequivocal evidence of the doping process. ,,,, The Raman pattern of doped P3HT, with its Raman active vibrations (RaAV) bands, is distinctive of the doped material, and it can be also used as a diagnostic tool of polarons formation. , However, the spectral changes with respect to the neutral species are rather smooth, namely a moderate shift (of about 20 cm –1 ) of the main Raman line is observed, together with an evident band broadening. These features suggest that the Raman spectrum of doped P3HT is dominated by RaAVs bands mainly involving vibrations of the polymer segments which are barely affected by the charge transfer process, so their vibrational dynamics is only slightly perturbed by the presence of polarons. , …”

“…Several authors reported the experimental determination of the IRAVs of doped P3HT, ,,,,,− sometimes providing a list of observed frequencies, which should exhibit a one-to-one correspondence with the vibrational normal modes of the doped polymer. However, the correct reading of the IRAV characteristic frequencies is still under debate, and analysis of the scientific literature highlights two different viewpoints.…”

“…The appearance of the IRAVs has been described in the past ,, as a phenomenon that escapes the theoretical framework of vibrational spectroscopy, being their intensities often comparable to the intensities of the electronic transitions. Both the huge intensity and the peak position of IRAVs allow unequivocal evidence of the doping process. ,,,, The Raman pattern of doped P3HT, with its Raman active vibrations (RaAV) bands, is distinctive of the doped material, and it can be also used as a diagnostic tool of polarons formation. , However, the spectral changes with respect to the neutral species are rather smooth, namely a moderate shift (of about 20 cm –1 ) of the main Raman line is observed, together with an evident band broadening. These features suggest that the Raman spectrum of doped P3HT is dominated by RaAVs bands mainly involving vibrations of the polymer segments which are barely affected by the charge transfer process, so their vibrational dynamics is only slightly perturbed by the presence of polarons. , …”

“…These features suggest that the Raman spectrum of doped P3HT is dominated by RaAVs bands mainly involving vibrations of the polymer segments which are barely affected by the charge transfer process, so their vibrational dynamics is only slightly perturbed by the presence of polarons. 45,48 The main goal of the present investigation is to discuss the remarkable effect of doping on the IR spectrum, aiming at illustrating the relationship between the Raman features of the neutral species (point (1)) and IRAVs (3). DFT predictions on molecular models allow a nice description of the experimental characteristics of the Raman and IR spectrum of neutral P3HT and of the IR spectrum of the charged polymer.…”