Non-Covalent Interactions and Helical Packing in Thiophene-Phenylene Copolymers: Tuning Solid-State Ordering and Charge Transport for Organic Field-Effect Transistors

Abstract

In this study, we introduce two thiophene-phenylene-thiophene (TPT) polymers designed to leverage noncovalent intramolecular interactions to regulate main-chain conformation and enhance solid-state ordering. By incorporating unsubstituted thiophene (T) or bithiophene (2T) units, we reveal striking divergence in the thermal, morphological, and optoelectronic properties of the resulting films, facilitated by these noncovalent interactions. Using a combination of computational and experimental approaches, we show that annealing yields remarkably different polymer conformations and, consequently, charge transport properties. TPT-T undergoes a significant structural transformation, adopting a more planar backbone conformation and a highly crystalline, edge-on molecular orientation. In contrast, the introduction of a single additional thiophene unit in TPT-2T leads to a more isotropic molecular orientation with a slight preference for face-on alignment, resulting in a heterogeneous film structure that hinders charge transport despite achieving tighter molecular packing. Remarkably, despite being composed of achiral components, TPT-2T develops chirality upon annealing, indicating the formation of a helical conformation. Organic field-effect transistor measurements reveal that the well-ordered alignment in annealed TPT-T films results in higher charge carrier mobility and a narrower distribution of mobility values than in TPT-2T. These findings provide critical insights into the structure-property relationships of conjugated polymers, offering guidance for optimizing molecular design and processing strategies for high-performance organic electronic materials.

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(a) Schematic representation of the backbone dihedrals: phenylene-thienylene (P-T*), thienylene-thienylene (T-T*) and thienylene-thienylene (T-T) in TPT-T and TPT-2T (where an asterisk denotes a thienylene ring within the TPT unit). Torsional potential energies of P-T*, T-T* and T-T dihedrals in (b) TPT-T and (c) TPT-2T repeat units, as calculated at the DFT M06–2X/6-31G** level of theory. Schematic representation of the effect of syn and anti-configurations in (d) TPT-T and (e) TPT-2T, where the top structures incorporate the computationally predicted syn-conformations between thienylene (T*) and the thienylene (T) rings and the bottom structures have all thienylene rings in an all-anti-conformation. Molecular dynamics simulated torsional populations for (f) P-T, (g) T-T*, and (h) T-T dihedrals in the bulk polymers.

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UV–vis spectra of 0.1 mg/mL solutions of (a) TPT–T and (b) TPT-2T in chlorobenzene (orange trace), chloroform (purple trace), and toluene (green trace) at room temperature, as well as their corresponding thin-film spectra (blue trace, spun-coated from 5 mg/mL chlorobenzene solutions). (c) Differential pulse voltammograms of as-cast TPT-T (dashed lines) and TPT-2T (solid lines) films on ITO/glass in 0.1 M TBAPF₆/ACN. The voltammograms were obtained on films that had not undergone any electrochemical cycling to ensure that redox properties reflect the behavior of as-cast film morphologies.

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DSC of the first cooling and second heating thermograms of (a) TPT-T and (b) TPT-2T powders. The thermograms were recorded from 0 to 250 °C under nitrogen atmosphere at ramp rate of 10 °C/min. In situ CPOM images of first heating and following cooling of as-spun (c) TPT-T and (d) TPT-2T films. The temperatures at which images were taken correspond to the dot markers on the DSC scans, with the films being equilibrated at each temperature for 10 min.

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GIXD patterns and corresponding CPOM images of (a) TPT-T and (b) TPT-2T thin films before and after thermal annealing. For TPT-T, thermal annealing results in numerous sharp GIXD peaks and enhanced birefringence under CPOM, indicating increased crystallinity. In TPT-2T films, annealing produces two tilted π–π stacking peaks alongside CD and Schilieren-like textures in CPOM, suggesting chiral liquid crystal-mediated structural ordering. CD spectra and structural illustrations for (c) TPT-T and (d) TPT-2T in as-spun and annealed states. TPT-T shows zero ellipticity, confirming its achiral nature, while TPT-2T exhibits nonzero ellipticity due to the formation of chiral mesophases during annealing.

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(a) Schematic representations of the backbone dihedrals for four proposed conformers of TPT-T. Conformer A and B share similar backbone dihedrals, while Conformer C was derived by a T*-T syn-to-anti transformation and Conformer D, by flipping S–O to S–H for P-T* dihedral. (b) The energetics each of the four proposed conformers. (c) Geometrical structures of the four investigated conformers of TPT-T with (d) the corresponding simulated GIXD scattering patterns. The predesigned structures were optimized using DFT calculations within the generalized gradient approximation (GGA) framework, employing the Perdew–Burke–Ernzerhof (PBE) functional as implemented in the Vienna Ab Initio Simulation Package (VASP). − The GIXD patterns were simulated using SimDiffraction. More details can be found in the Supporting Information.

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(a) Top-gate, bottom contact OFET device geometry with PFBT SAM-treated contacts. (b) Transfer curve in the saturation regime for an OFET based on annealed TPT-T film. (c) Output characteristics obtained in the same film. The length (L) and width (W) values are 90 and 800 μm, respectively.

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Mobility histograms of OFETs based on TPT-T (top) and TPT-2T (bottom) as cast and annealed films. The average mobility value is included in the inset.