Jacek Jakowski1 Sophya Gararshchuk2 Jingsong Huang1 Kunlun Hong1 Yingdong Luo1 Jong Keum1 Bobby Sumpter1

1, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
2, University of South Carolina, Columbia, South Carolina, United States

The attractive optoelectronic properties of conjugated polymers have led to intense research to understand their semiconducting properties and to explore their applications in various devices. [Nature.Comm.5, 3180 (2014)] It is increasingly recognized that properties of conducting polymers can be affected by nuclear quantum effects an related isotopic substitution. [J.Phys.Chem.Lett., 8, 4333 (2017)] Here, we report an experimental and theoretical study of the isotopic effects of deuterium substitution on the properties of poly(3-hexylthiophene) (P3HT). The effects of deuterium isotope substitution on P3HT chain stacking is studied experimentally by X-ray diffraction (XRD) in combination with gel permeation chromatography and theoretically by density functional theory, quantum molecular dynamics, and discrete variable representation of nuclear wave functions. For four P3HT materials with different levels of deuteration (pristine, main-chain deuterated, side-chain deuterated, and fully deuterated), the XRD measurements show that main-chain thiophene deuteration significantly reduces crystallinity, regardless of the side-chain deuteration. It is found that the P3HT crystal structure is characterized by an anti-ferroelectric packing motif of the thiophene backbone. The reduction of crystallinity due to the main-chain deuteration is a quantum nuclear effect resulting from a static zero-point vibrational energy combined with a dynamic correlation of the dipole fluctuations. Specifically, it is attributed to a smaller crystal-field stabilization, and to a lower dynamic polarizability of the C-D bond, compared to the C-H bond. The quantum molecular dynamics simulations of the protons and deuterons confirm the inter-chain correlation of the proton-proton and deuteron-deuteron motions, but not of the proton-deuteron motion. It suggests that isotopic purity is an important factor that affects the stability and properties of conjugated polymer crystals, which should be considered in the design of electronic and spintronic devices. The current findings have an important implication in both fundamental science and device engineering, and pave the way to rationally tune conjugated polymers towards desired crystallinity that would drive a particular property more effectively.