We present a critical analysis of the Raman spectra of unsubstituted oligothiophenes and rediscuss the well-known Raman dispersion of conjugated systems explicitly considering intermolecular interactions. Temperature-dependent Raman spectra and DFT calculations for dimers of different chain lengths show that the effect of intermolecular interactions on the frequency and intensity of carbon-carbon (CC) stretching modes is non-negligible. This effect has not been considered in previous works and might explain many spectral features of this class of compounds which are not completely interpreted by the usual models. Both intensities and frequencies are significantly affected by intermolecular interactions showing that molecular self-organization should be taken into account in future spectroscopic studies of conjugated molecules. In particular, the interactions among molecules cause an upward shift of the frequency of the R mode (amplitude mode) which can explain the lack of frequency dispersion with conjugation length of oligothiophenes, as experimentally observed for solid-state samples at room temperature.
In
this work, we report a comparative analysis of the infrared
and Raman spectra of octa(3-hexylthiophene) (3HT)
8
, trideca(3-hexylthiophene) (3HT)
13
, and poly(3-hexylthiophene) P3HT recorded in various phases, namely, amorphous, semicrystalline,
polycrystalline and single crystal. We have based our analysis on
the spectra of the (3HT)
8
single
crystal (whose structure has been determined by selected area electron
diffraction) taken as reference and on the results of DFT calculations
and molecular vibrational dynamics. New and precise spectroscopic
markers of the molecular structures show the existence of three phases,
namely: hairy (phase 1), ordered (phase 2), and disordered/amorphous
(phase 3). Conceptually, the identified markers can be used for the
molecular structure analysis of other similar systems.
This study elucidates the complex morphology and the related spectroscopic response of poly(vinylidene fluoride-co-trifluoroethylene) copolymer, with 80% molar VDF content, namely P(VDF-TrFE) (80/20).
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