Impact of Side‐Chain Hydrophilicity on Packing, Swelling, and Ion Interactions in Oxy‐Bithiophene Semiconductors

Abstract

the operation of a range of bioelectronic devices, [1][2][3] such as organic electrochemical transistors (OECTs), [4][5][6] batteries, [7] and supercapacitors. [6] They have inherent advantages over traditional organic semiconductors for these electrochemical applications, [4,[8][9][10] in particular their ability to operate with an aqueous electrolyte. [11,12] Exchanging a traditional alkyl-based side chain for a glycol-based hydrophilic side chain has been widely adopted as a strategy for increasing uptake of water and ions in OMIECs, and subsequently increasing their capacitance. [5,7,[13][14][15] Whilst polythiophenes bearing long alkyl side chains are often reported to pack with interdigitating side chains [16][17][18][19][20][21][22] and π-stacks that are either straight or lightly tilted [17,20,[22][23][24] (see Figure S1, Supporting Information for backbone packing motifs referred to in this study), little is known about glycolated OMIEC packing, despite the chain packing being critical to both electronic and ionic transport. [21,[25][26][27][28] Experimental studies have suggested that glycolated OMIECs adopt smaller π-stack distances than their alkylated counterparts. [5,29] As well as solid-state packing, structural characterization of OMIECs should account for their swelling behavior, Exchanging hydrophobic alkyl-based side chains to hydrophilic glycol-based side chains is a widely adopted method for improving mixed-transport device performance, despite the impact on solid-state packing and polymer-electrolyte interactions being poorly understood. Presented here is a molecular dynamics (MD) force field for modeling alkoxylated and glycolated polythiophenes. The force field is validated against known packing motifs for their monomer crystals. MD simulations, coupled with X-ray diffraction (XRD), show that alkoxylated polythiophenes will pack with a "tilted stack" and straight interdigitating side chains, whilst their glycolated counterpart will pack with a "deflected stack" and an s-bend side-chain configuration. MD simulations reveal water penetration pathways into the alkoxylated and glycolated crystals-through the π-stack and through the lamellar stack respectively. Finally, the two distinct ways triethylene glycol polymers can bind to cations are revealed, showing the formation of a metastable single bound state, or an energetically deep double bound state, both with a strong side-chain length dependence. The minimum energy pathways for the formation of the chelates are identified, showing the physical process through which cations can bind to one or two side chains of a glycolated polythiophene, with consequences for ion transport in bithiophene semiconductors.

The Effect of Glycol Side Chains on the Assembly and Microstructure of Conjugated Polymers

“…MD simulations were performed using a force field based on OPLS-AA, 49 with interthiophene dihedrals parametrized to fit with DFT at the B3LYP/6311G** level of theory. 50 Glycol side chain parameters are from work by Woods et al 51 and are based on OPLS-aa atom types, with other backbone bonded parameters from work by Moreno et al and Bhatta et al 52,53 The force field used is closely related to that presented in a recent MD simulation study of oxy-bithiophene oligomers by Siemons et al 54 All simulations were performed with 20-mers on a Au substrate, with Au parameters from work by Su et al 55 As a general procedure, all simulations begin with an in-vacuum energy minimization. Either single or multiple polymers are then placed 3 Å from the Au surface.…”

“…MD simulations were performed using a force field based on OPLS-AA, with interthiophene dihedrals parametrized to fit with DFT at the B3LYP/6311G** level of theory . Glycol side chain parameters are from work by Woods et al and are based on OPLS-aa atom types, with other backbone bonded parameters from work by Moreno et al and Bhatta et al. , The force field used is closely related to that presented in a recent MD simulation study of oxy-bithiophene oligomers by Siemons et al…”

“… 50 Glycol side chain parameters are from work by Woods et al 51 and are based on OPLS-aa atom types, with other backbone bonded parameters from work by Moreno et al and Bhatta et al. 52 , 53 The force field used is closely related to that presented in a recent MD simulation study of oxy-bithiophene oligomers by Siemons et al 54 …”

Enhanced Organic Electrochemical Transistor Performance of Donor–Acceptor Conjugated Polymers Modified with Hybrid Glycol/Ionic Side Chains by Postpolymerization Modification

“…We hypothesize cations in PgBT-(TriEG)2gTT films are chelated by the adjacent double trimeric ethylene glycol side chains positioned on each benzothiadiazole unit. 49 The differing EQCM behaviors between parent PgBT(F)-2gTT and derivatives PgBT(Ion)2gTT/PgBT(TriEG)2gTT contrast with their equivalent hydrophilicities, as determined through contact angle measurements (Figure S46, Table S2).…”

A guide for the characterization of organic electrochemical transistors and channel materials

“…155 Siemons et al constructed an MD model to investigate the gradual exchange of hydrophobic alkyl-based side chains with hydrophilic EG-based side chains on polythiophenes. 160 The authors demonstrated that the oxygen content of the side chains influenced the packing behavior of the resulting OMIEC, where EG-based OMIEC was found to pack more effectively for charge transport in the solid state. Water was found to penetrate the OMIEC bulk through the π-stack (backbone) for the alkylated polymers and the lamellar stack (side chains) for the glycolated polymers.…”