Effect of the Side‐Chain‐Distribution Density on the Single‐Conjugated‐Polymer‐Chain Conformation

Abstract

The spatial arrangement of the side chains of conjugated polymer backbones has critical effects on the morphology and electronic and photophysical properties of the corresponding bulk films. The effect of the side-chain-distribution density on the conformation at the isolated single-polymer-chain level was investigated with regiorandom (rra-) poly(3-hexylthiophene) (P3HT) and poly(3-hexyl-2,5-thienylene vinylene) (P3HTV). Although pure P3HTV films are known to have low fluorescence quantum efficiencies, we observed a considerable increase in fluorescence intensity by dispersing P3HTV in poly(methyl methacrylate) (PMMA), which enabled a single-molecule spectroscopy investigation. With single-molecule fluorescence excitation polarization spectroscopy, we found that rra-P3HTV single molecules form highly ordered conformations. In contrast, rra-P3HT single molecules, display a wide variety of different conformations from isotropic to highly ordered, were observed. The experimental results are supported by extensive molecular dynamics simulations, which reveal that the reduced side-chain-distribution density, that is, the spaced-out side-chain substitution pattern, in rra-P3HTV favors more ordered conformations compared to rra-P3HT. Our results demonstrate that the distribution of side chains strongly affects the polymer-chain conformation, even at the single-molecule level, an aspect that has important implications when interpreting the macroscopic interchain packing structure exhibited by bulk polymer films.

Direct observation of backbone planarization via side-chain alignment in single bulky-substituted polythiophenes

“…3A, Top. This behavior is in line with other work on P3HT, in which repulsive S•••H interactions between the sulfur atom and the hydrogen atoms of the closest methylene unit in the adjacent hexyl group were shown to lead to nonplanar optimal structures (27,32,49,55). Optical and electronic properties.…”

“…For P3HT, interchain π−π interactions play an important role and determine the transition energy, to a large extent (22). Particularly, this latter system was studied extensively by ensemble (16,(23)(24)(25) and single-molecule spectroscopy (26)(27)(28).…”

Impact of backbone fluorination on nanoscale morphology and excitonic coupling in polythiophenes

“…3B reveals that the F-P3EHT is decidedly nonplanar in solution with a broad dihedral angle θ-distribution and a maximum at 75°. In contrast, P3EHT exhibits a slightly skewed conformation as we observed for P3HT (26), although with a moderate preference for cis configurations.…”

“…To examine how fluorination affects the morphology of single-polymer chains and aggregates, fluorescence excitation polarization analysis was performed (see SI Materials and Methods for details). In these experiments, the fluorescence intensity traces were modulated with a rotating linearly polarized excitation light and then fitted with IðαÞ ∝ 1 + M cos2ðα − ϕÞ, where α is the excitation polarization angle and ϕ is the polarization angle at maximum absorption (2,26,27). The modulation depth, M, represents the anisotropy of the absorption (excitation) tensor projected on the x−y plane of the laboratory frame and is related to the morphological order of individual molecules or aggregates.…”

The Polymer Physics of Multiscale Charge Transport in Conjugated Systems

“…One of the starting points to understanding the underlying physics in conjugated polymers is to probe their chain conformations, that is, the spatial configurations of constitutive mers along a polymer chain, which provide the underlying molecular basis of almost all their physical properties . The electronic properties of conjugated polymers and thus the performance of devices comprising them are also strongly dependent on their chain conformations . Conjugated polymers are typically semiflexible.…”