Mapping Microstructural Dynamics Up To The Nanosecond Of The Conjugated Polymer P3ht In The Solid State

We present a detailed microscopic study of the structure dynamics relationship of both regioregular (RR) and regiorandom (RRa) poly(3-hexylthiophene) (P3HT) using synergistically different elastic, quasi-elastic, and inelastic neutron scattering techniques. The length scale and the time scale covered by the study is tens of angstroms and the femtosecond to nanosecond, respectively. Deuteration is employed to modulate the coherent and incoherent cross sections of the materials, beyond a contrast variation purpose, allowing particularly access to both self-motions and collective dynamics of the materials. The neutron scattering measurements are underpinned by extensive quantitative numerical simulations using large-scale classical molecular dynamics (MD) simulations, as well as molecular and periodic first principles quantum chemical (QC) calculations. MD simulations reproduced well the main structural features and slow motions. Further, MD results shed light on differences in collective dynamics between Q-values linked with the pi-pi stacking and the lamellar stacking of the polymer, with the crystalline phase being the most impacted. However, MD led to a limited description of molecular vibrations. In this context, first principles molecular QC calculations described well the high-energy vibrational features (>900 cm(-1)), while periodic QC allowed description of the low and midenergy vibrational range (200-900 cm(-1)). The midenergy range is predominantly associated with both intramolecular and intermolecular mode coupling, which encloses information about both the polymer conformation and the polymer packing at short range. We show that the presented combined approach of neutron-based measurements and multicomputational calculations allows the full mapping out the structural dynamics of conjugated polymers such as P3HT. One of the outcomes of this study is the validation of the common assumption made that RRa-P3HT is a good approximation for the amorphous phase of RR-P3HT at the macroscopic level, although some differences are shown at the molecular level. The present work helps to clarify unambiguously the latter point which has been largely overlooked in the literature. By comparing the neutron vibrational results with available Raman and IR data in the literature, we highlight the importance to complement such optical spectroscopy techniques with inelastic neutron scattering. The latter offers the advantage of being insensitive to the delocalized pi-electron system and, thus, enables relevant quantities such as conjugation lengths to be inferred.