Structure and Vibrational Spectra of 2,5-Diiodothiophene: A Model for Polythiophene

The
structure and vibrational spectroscopy of 2,5-diiodothiophene
have been described for the first time. The structure is remarkable
in that, despite the presence of eight molecules in the unit cell,
there is almost no evidence for any interaction between them. This
is reflected in the spectra that show coincident modes in the infrared,
Raman, and inelastic neutron scattering (INS) spectra and almost resolution-limited
line widths in the INS spectrum, showing that both the factor group
splitting and the phonon dispersion are very small. This property
has led to the use of the compound as a calibrant in INS spectroscopy.
The deductions from the spectra are confirmed by a periodic density
functional theory (DFT) calculation that allows a complete spectral
assignment to be made. Comparison to the INS and Raman spectra of
polythiophene (for which 2,5-diiodothiophene may be used as a monomer)
shows a marked similarity for the former and a marked difference to
the latter. The INS spectrum of polythiophene is dominated by the
C–H bending modes, which are unchanged on polymerization. In
contrast, the Raman spectrum of polythiophene is dominated by the
changed electronic environment that results in strong electron–phonon
coupling and a significant upshift of the CC stretch modes.