Lowest-Energy Crystalline Polymorphs of P3HT

A Zhugayevych and O Mazaleva and A Naumov and S Tretiak, JOURNAL OF PHYSICAL CHEMISTRY C, 122, 9141-9151 (2018).

DOI: 10.1021/acs.jpcc.7b11271

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 pi- 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 pi- 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.

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