Ion Size-Dependent Electrochromism in Air-Stable Napthalenediimide-Based Conjugated Polymers

Abstract

Conjugated polymers (CPs) that show stable and reversible cation insertion/deinsertion under ambient conditions hold great potential for optoelectronic and energy storage devices. However, n-doped CPs are prone to parasitic reactions upon exposure to moisture or oxygen. This study reports a new family of napthalenediimide (NDI) based conjugated polymers capable of undergoing electrochemical n-type doping in ambient air. By functionalizing the NDI-NDI repeating unit with alternating triethylene glycol and octadecyl side chains, the polymer backbone shows stable electrochemical doping at ambient conditions. We systematically investigate the extent of volumetric doping involving monovalent cations of varying size (Li + , Na + , tetraethylammonium (TEA + )) with electrochemical methods, including cyclic voltammetry, differential pulse voltammetry, spectroelectrochemistry, and electrochemical impedance spectroscopy. We observed that introducing hydrophilic side chains on the polymer backbone improves the local dielectric environment of the backbones and lowers the energetic barrier for ion insertion. Surprisingly, when using Na + electrolyte, the polymer films exhibit higher volumetric doping efficiency, faster-switching kinetics, higher optical contrast, and selective multielectrochromism when compared to Li + or TEA + electrolytes. Using well-tempered metadynamics, we characterize the free energetics of side chain−ion interactions to find that Li + binds more tightly to the glycolated NDI moieties than Na + , hindering Li + ion transport, switching kinetics, and limiting the films' doping efficiency.

Enhanced Optical Contrast and Switching in Near‐Infrared Electrochromic Devices by Optimizing Conjugated Polymer Oligo(Ethylene Glycol) Sidechain Content and Gel Electrolyte Composition

“…This trend demonstrates the ability of the EG n sidechains to increase ion uptake and exchange through the polymer‐electrolyte interactions, such as the cation‐sidechain binding detailed in Figure 1A. [ 66,67 ] Additionally, changing the cation identity from Bu 4 N + to K + or Li + increases C for DPP‐E (12.7 →15.8 F g −1 ) and DPP‐g2T (24.0 →37.0 F g −1 ) indicating increased ion insertion and efficient exchange within the polymer. [ 66 ] Note, the potential scan range for gDPP‐E and gDPP‐g2T in the K + PF 6 − and Li + PF 6 − electrolytes is narrowed, −0.80 V to +0.80 V, due to visible film delamination during measurement, which can explain the low capacitances for these polymer‐electrolyte combinations.…”

“…These differences in E ox onset can be attributed to improved electrolyte ion exchange for the smaller cations within the polymer films thereby facilitating electrochemical oxidation. [ 41,66 ]…”

Ambipolar Doping in π-Conjugated Polymers

“…The molecular structure of the polymer, n- and p-dopants, selected in this work is illustrated in Figure a. Typically, the ambient stability of the doped polymer is correlated with the EA of the pristine polymer, particularly if it falls within the electrochemical stability window. , The energies of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) are estimated from cyclic voltammetry (CV) measurements …”

Efficient Iodine Uptake of Ultra Thermally Stable Conjugated Copolymers Bearing Biaceanthrylenyl Moieties and Contorted Aromatic Units Using a [3 + 2] Palladium-Catalyzed Cyclopolymerization Reaction

“…Conjugated polymers emerged as a fascinating family of materials which have revolutionized various fields of science and technology − because of their exceptional optical and electronic properties bestowed with their versatile processability, thus allowing for their use across a myriad of applications, notably, optoelectronic devices and energy storage and conversion. − These applications encompass organic photodetectors and photovoltaics (OPDs and OPVs), , light-emitting diodes (OLEDs), and field-effect transistors (OFETs). , …”