Poly(3-hexylthiophene)
(P3HT), being a prototypic conjugated polymer,
bears a high charge mobility that is sensitive to its packing configuration
in the condensed phase. Despite its extensive experimental study with
X-ray diffraction, its specified packing structure still remains stymied.
This study searched for possible structures of crystalline P3HT and
identified the one that holds a simulated Raman spectrum most approximate
to the experimental one of ordered P3HT aggregates in the frozen solvent.
The spectral correspondence shows that the Raman-active C–C
stretch peak exhibits a red shift in frequency, while the CC
stretch peak displays a blue shift as the layer planarity of P3HT
is relaxed. Moreover, the CC peak splits into two when adjacent
thiophene rings in the P3HT chain hold a dihedral angle of 22°
with respect to each other. This study demonstrates that Raman spectroscopy
plus first-principles simulations can serve as a powerful tool to
resolve fine structures of molecular crystals