Hydration of a Side-Chain-Free n-Type Semiconducting Ladder Polymer Driven by Electrochemical Doping

Abstract

We study the organic electrochemical transistor (OECT) performance of the ladder polymer poly-(benzimidazobenzophenanthroline) (BBL) in an attempt to better understand how an apparently hydrophobic side-chain-free polymer is able to operate as an OECT with favorable redox kinetics in an aqueous environment. We examine two BBLs of different molecular masses from different sources. Regardless of molecular mass, both BBLs show significant film swelling during the initial reduction step. By combining electrochemical quartz crystal microbalance gravimetry, in-operando atomic force microscopy, and both ex-situ and in-operando grazing incidence wideangle X-ray scattering (GIWAXS), we provide a detailed structural picture of the electrochemical charge injection process in BBL in the absence of any hydrophilic side-chains. Compared with ex-situ measurements, in-operando GIWAXS shows both more swelling upon electrochemical doping than has previously been recognized and less contraction upon dedoping. The data show that BBL films undergo an irreversible hydration driven by the initial electrochemical doping cycle with significant water retention and lamellar expansion that persists across subsequent oxidation/ reduction cycles. This swelling creates a hydrophilic environment that facilitates the subsequent fast hydrated ion transport in the absence of the hydrophilic side-chains used in many other polymer systems. Due to its rigid ladder backbone and absence of hydrophilic side-chains, the primary BBL water uptake does not significantly degrade the crystalline order, and the original dehydrated, unswelled state can be recovered after drying. The combination of doping induced hydrophilicity and robust crystalline order leads to efficient ionic transport and good stability.

Materials design and applications of n-type and ambipolar organic electrochemical transistors

“…Surprisingly, this hydration process creates a hydrophilic environment that facilitates rapid ion hydration, without affecting the crystallization of BBL despite its rigid backbone and the absence of hydrophilic side chains. 70 The findings suggest the promising potential of side chain-free polymers in OECTs.…”

Crystallinity Determines Ion Injection Kinetics and Local Ion Density in Organic Mixed Conductors

“…Giridharagopal et al used correlated electrochemical strain microscopy (ESM) and AM-FM stiffness mapping to demonstrate that local heterogeneity plays a crucial role in ion uptake in P3HT . Multiple groups have also demonstrated spatially heterogeneous swelling in poly­(benzimidazobenzophenanthroline) (BBL), a promising n-type OMIEC. , To enhance transport processes in OMIECs, some fabrication strategies, including solvent engineering, aim to control polymer morphologies …”

“…3 Multiple groups have also demonstrated s p a t i a l l y h e t e r o g e n e o u s s w e l l i n g i n p o l y -(benzimidazobenzophenanthroline) (BBL), a promising ntype OMIEC. 28,29 To enhance transport processes in OMIECs, some fabrication strategies, including solvent engineering, aim to control polymer morphologies. 30 Many studies have also focused on understanding the arrangement of ions and polarons in the polycrystallineconjugated polymer matrix.…”

p-Type Semiconducting Ladder Poly(pyrrolobenzothiazine)s: Effects of N-Alkyl Side Chains on the Chain Conformation, Electronic Structure, and Charge Transport Properties

“…It is well known that the backbone conformation of π-conjugated polymers directly impacts their electronic structure and properties including molecular orbital distribution, reorganizational energy during charge transfer process, intermolecular transfer integrals, , and the degree of electronic delocalization. In contrast to semiflexible polymers that show conformational torsions and rotations along the single bonds that link the polymer repeat units, π-conjugated ladder polymers feature a double-stranded architecture that is thought to limit conformational distortions and promote backbone rigidity. ,− The most notable member of the π-conjugated ladder polymers, poly­(benzimidazobenzophenanthroline) (BBL) (Chart ), first synthesized in 1966 for aerospace applications due to its extremely high thermal and chemical stability, has been successfully used in a variety of organic electronic and optoelectronic applications over the past 3 decades. ,− BBL and its derivatives are the only known examples of π-conjugated ladder polymers that have rigid-rod backbones in solution and solid state; − the scarcity is primarily because of the difficulty in the synthesis of π-conjugated ladder polymers in general. ,,− For example, in polymers such as BBL and ladder-type polyaniline derivatives, installing basic sites such as imine nitrogens in the backbone is a key design feature that enables either processing in protic acids or offers unique properties such as acid-doped conductivity. Beyond BBL, there is one other conjugated ladder polymer, called ladder poly­( p -phenylene) (LPPP), that has been subjected to rigorous studies of its backbone conformation and persistence length .…”