Understanding the Influence of Side Chains on Molecular Packing and Mobility in Thienothiophene Based Conjugated Polymer Mixed Conductors

The operational mechanisms of organic mixed ionic-electronic conductor materials (OMEICS) are a relevant to a wide range of applications, such as sensing devices [1], electrochromics [2], light-emitting electrochemical devices, bioelectronics and energy storage [3]. All these applications are underpinned by charge carrier mobility, so understanding the factors that control mobility is fundamental to structural design. In this work, we explore the relationship between chemical structure, molecular packing and mobility in organic electrochemical transistor (OECT) structures, to understand the origin of high OECT mobility a new class of thienothiophene-bithiophene conjugated polymer. Structural analysis of monomer crystals indicates that glycol side chain length influences molecular packing motif, suggesting that a similar effect may apply to the polymers.
We will present a simulation study of the effect of side-chain structure on packing motifs and charge transport. We use molecular dynamics (MD) to simulate monomer crystal structures, validating our atomistic forcefield, and then to explore the impact of glycol side chain length on packing of oligomeric models of the polymers. We use these models to rationalise observed charge transport trends. We also explore the thermodynamics of inter-chain packing behaviour using enhanced sampling methods [3]. We will address the degree to which monomers’ packing behaviour helps to explain the behaviour of the polymers, and so attempt to extract design rules for high mobility systems.

[1] Dipanjan Giri, Shraman Kumar Saha, Nicholas Siemons, Iona Anderson, Hang Yu, Jenny Nelson, Ram Kumar Canjeevaram Balasubramanyam, and Satish Patil. Ion Size-Dependent Electrochromism in Air-Stable Napthalenediimide-Based Conjugated Polymers. ACS Applied Materials & Interfaces. 2023 .15 (14), 17767-17778
[2] Giovannitti, A., Sbircea, D. T., Inal, S., Nielsen, C. B., Bandiello, E., Hanifi, D. A., ... & Rivnay, J. (2016). Controlling the mode of operation of organic transistors through side-chain engineering. Proceedings of the National Academy of Sciences, 113(43), 12017-12022.
[3] Hazra S, Banerjee A, Nandi AK. Organic Mixed Ion-Electron Conductivity in Polymer Hybrid Systems. ACS Omega. 2022 Sep 8;7(37):32849-32862.
[4] Siemons, N., Pearce, D., Yu, H., Sheelamanthula, R., Hallani, R., McCulloch, I., Giovannitti, A., LeCroy, G., Frost, J., Nelson, J., Controlling Swelling In Mixed Transport Polymers Through Alkyl Side Chain Physical Crosslinking. Proc Nat Acad Sci, 2023.