Impact of Halogen Termination and Chain Length on π‑Electron Conjugation and Vibrational Properties of Halogen-Terminated Polyynes

We explored the optoelectronic and vibrational properties
of a
new class of halogen-terminated carbon atomic wires in the form of
polyynes using UV–vis, infrared absorption, Raman spectroscopy,
X-ray single-crystal diffraction, and DFT calculations. These polyynes
terminate on one side with a cyanophenyl group and on the other side,
with a halogen atom X (X = Cl, Br, I). We focus on the effect of different
halogen terminations and increasing lengths (i.e., 4, 6, and 8 sp-carbon
atoms) on the π-electron conjugation and the electronic structure
of these systems. The variation in the sp-carbon chain length is more
effective in tuning these features than changing the halogen end group,
which instead leads to a variety of solid-state architectures. Shifts
between the vibrational frequencies of samples in crystalline powders
and in solution reflect intermolecular interactions. In particular,
the presence of head-to-tail dimers in the crystals is responsible
for the modulation of the charge density associated with the π-electron
system, and this phenomenon is particularly important when strong
I··· N halogen bonds occur.