N,N′-Unsubstituted Naphthodithiophene Diimide: Synthesis and Derivatization via N-Alkylation and -Arylation

Enhancing the Backbone Coplanarity of n-Type Copolymers for Higher Electron Mobility and Stability in Organic Electrochemical Transistors

Electron-transporting (n-type) conjugated polymers have recently been applied in numerous electrochemical applications, where both ion and electron transport are required. Despite continuous efforts to improve their performance and stability, n-type conjugated polymers with mixed conduction still lag behind their hole-transporting (p-type) counterparts, limiting the functions of electrochemical devices. In this work, we investigate the effect of enhanced backbone coplanarity on the electrochemical activity and mixed ionic-electronic conduction properties of n-type polymers during operation in aqueous media. Through substitution of the widely employed electron-deficient naphthalene diimide (NDI) unit for the core-extended naphthodithiophene diimide (NDTI) units, the resulting polymer shows a more planar backbone with closer packing, leading to an increase in the electron mobility in organic electrochemical transistors (OECTs) by more than two orders of magnitude. The NDTI-based polymer shows a deep-lying lowest unoccupied molecular orbital level, enabling operation of the OECT closer to 0 V vs Ag/AgCl, where fewer parasitic reactions with molecular oxygen occur. Enhancing the backbone coplanarity also leads to a lower affinity toward water uptake during cycling, resulting in improved stability during continuous electrochemical charging and ON-OFF switching relative to the NDI derivative. Furthermore, the NDTI-based polymer also demonstrates near-perfect shelf-life stability over a month-long test, exhibiting a negligible decrease in both the maximum on-current and transconductance. Our results highlight the importance of polymer backbone design for developing stable, high-performing n-type materials with mixed ionic-electronic conduction in aqueous media.

Synthesis of Heterocyclic Core-Expanded Bis-Naphthalene Tetracarboxylic Diimides

A highly reactive bis-naphthalene tetracarboxylic diimide (bis-NDI) intermediate, TBrDNDI, was designed and synthesized for core-expanded NDIs. Based on this intermediate, we achieved 9- and 11-membered core-expanded bis-NDI derivatives. Through expanding the NDI core and introducing electron-donor or electron-acceptor groups, the frontier energy orbitals, optical and electrical properties of these bis-NDIs can be finely tuned to obtain air-stable ambipolar or n-type materials.

Theoretical investigations into the charge transfer properties of thiophene α-substituted naphthodithiophene diimides: excellent n-channel and ambipolar organic semiconductors

A theoretical study was carried out to investigate the electronic structures and the charge transport properties of a series of naphthodithiophene diimide (NDTI) thiophene α-substituted derivatives NDTI-X using density functional theory and classical Marcus charge transfer theory. This study deeply revealed the structure-property relationships by analyzing the intermolecular interactions in crystal structures of C8-NDTI and C8-NDTI-Cl thoroughly by using the Hirshfeld surface, QTAIM theories and symmetry-adapted perturbation theory (SAPT). Our results suggested that a 2-D brick-like π-stacking structure makes C8-NDTI-Cl a more excellent n-type semiconducting material with μ_max-e of 2.554 cm² V^-1 s^-1 than C8-NDTI with a herringbone-like slipped π-stacking motif. In addition, the calculated results showed that by modifying the thiophene α-positions of NDTI with electron-withdrawing substituents, -F, -Cl and -CN, low-lying LUMO energy levels and a high adiabatic electron affinity EA(a) can be obtained; while introducing electron-donating groups, benzene (-B), thiophene (-T), benzo[b]thiophene (-BT) and naphtha[2,3-b]thiophene (-NT), expanded the molecular π-conjugated backbone, and narrow band gaps, high EA(a) and small reorganization energies can be obtained. Theoretical simulations predict that NDTI-CN is an excellent air-stable n-type organic semiconducting material with an average electron mobility μ_e of up to 1.743 cm² V^-1 s^-1. Owing to their high EA(a), moderate adiabatic ionization potential IP(a) as well as small hole and electron reorganization energies, NDTI-BT and NDTI-NT are two well-balanced air-stable ambipolar semiconducting materials. The theoretical average hole/electron mobilities are as high as 2.708/3.739 cm² V^-1 s^-1 for C8-NDTI-NT and 1.597/2.350 cm² V^-1 s^-1 for C8-NDTI-BT, respectively.

Tuning the electronic properties of thiophene-annulated NDIs: the influence of the lateral fusion position

We report a new asymmetric thiophene-annulated naphthalenediimide with high electron mobilities.

Fluoroalkyl-modified naphthodithiophene diimides

Two kinds of fluoroalkyl-modified naphthodithiophene diimides (NDTI) were designed and synthesized. α-Modified NDTI could form favorable slipped one-dimensional (1D) stacking and N-modified NDTI shows a torsion cofacial stacking. Single-crystal transistors confirm that both fluoroalkyl-modified NDTI possess good electron transport ability with electron mobilities of 0.065 cm² V^-1 s^-1 and 1.59 cm² V^-1 s^-1, respectively.

Control of Major Carriers in an Ambipolar Polymer Semiconductor by Self-Assembled Monolayers

Selective unipolarization of an ambipolar polymer semiconductor, PNDTI-BT, by using different self-assembled monolayers, is demonstrated. For p-unipolarization, 1H,1H,2H,2H-perfluorodecyltriethoxysilane is most effective, whereas for n-unipolarization, 3-(N, N'-dimethylamino)propyltriethoxysilane is the best. Using these selective unipolarization effects, the complementary inverters based on the ambipolar polymer fabricated by a simple solution process show greatly improved switching behaviors with low power consumption.