Lowest-Energy Crystalline Polymorphs of P3HT

We systematically
study low-energy crystalline polymorphs of the
archetypal conjugated polymer, regioregular poly-3-hexylthiophene
(rr-P3HT) using the best available density functional theory methods
benchmarked against the ab initio coupled cluster method. A comprehensive
conformational search is performed for two-dimensional π-stacks
being the most rigid structural unit of bulk P3HT. We have identified
a number of nearly isoenergetic polymorphs below the energy level
of room-temperature amorphous structures and well below the energy
of optimized best-fit experimental models. Classical molecular dynamics
simulations show that these crystals retain their structure at least
at 200 K. At room temperature, although the conjugated backbone of
the π-stack remains ordered, aliphatic side chains are melted,
transforming from low-energy folded conformations to high-entropy
fully unfolded structures. Our study shows that P3HT is a statistically
frustrated system with multiple competing interactions, which complicates
fabrication of highly ordered bulk forms but gives structural flexibility
of glasses.