Deuteration as a Means to Tune Crystallinity of Conducting Polymers

Abstract

The effects of deuterium isotope substitution on conjugated polymer chain stacking of poly(3-hexylthiophene) is studied experimentally by X-ray diffraction (XRD) in combination with gel permeation chromatography and theoretically using density functional theory and quantum molecular dynamics. 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. 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. The quantum molecular dynamics simulations confirm the interchain correlation of the proton-proton and deuteron-deuteron motions but not of the proton-deuteron motion. Thus, isotopic purity is an important factor affecting stability and properties of conjugated polymer crystals, which should be considered in the design of electronic and spintronic devices.

Theoretical assessment of the nuclear quantum effects on polymer crystallinity via perturbation theory and dynamics

“…The dipole moment variance in the region of appreciable probability density for the proton and deuteron (Figure 5 in Ref. ) is the most significant in the plane of the ring (the in‐plane bend). For the out‐of‐plane bending the main change in the dipole moment is its orientation.…”

“…Our preliminary studies suggest that the change in crystallinity may be attributed to the difference in the ZPEs of the deuterated and protonated species and, additionally, to the correlated motion of the protons or deuterons belonging to the neighboring P3HT chains was also recognized. In this work we explore the nature of the associated long range intermolecular forces by decomposing the H/D vibrational interaction energy into electrostatic, induction and dispersion contributions, similar to the analysis of the electronic energy .…”

“…The choice of the ES method has been discussed in Ref. . The density functional theory (DFT) calculations have been performed with QChem‐4.3 .…”

“…In this paper we (1) present a theoretical framework for including NQEs relevant to crystallinity of polymers and (2) use it to examine NQEs in isotopically substituted P3HT. As follows from experiments , selective substitution of the hydrogen with deuterium atoms at the thiophene ring reduces the crystallinity of this polymer. This is surprising because the electronic structure (ES) and, hence, the molecular geometry, do not depend on the isotope substitution (with the exception of the usually negligible hyperfine coupling), and the experimental evidence essentially rules out kinetic effects.…”

“…In general, the dipoles D A and D B fluctuate in time due to the vibrational motion of the quantum nuclei around their mean values, Since these fluctuations are random, the basic description includes their interaction in average sense: the mean dipole on A interacts with the mean dipole on B. Beyond this mean‐field interaction picture, the dynamical correlation between dipole fluctuation needs to be taken into account: fluctuations of one of the dipoles cause local changes in the field experienced by the second dipole, whose response leads to correlation of motion between two dipoles . Following the ES theory , the dynamical correlation energy can be decomposed into the induction and the dispersion terms associated with the permanent‐instantaneous and instantaneous‐instantaneous dipole interactions, respectively.…”