Critical transition to a highly conductive state through PEDOT oligomer percolation in redox-active COFs

Abstract

Covalent organic frameworks (COFs) have garnered significant attention in recent years, but they generally suffer from low electrical conductivity. In our previous studies, we observed that the nano-hybrid material PEDOT@AQ-COF (PEDOT = poly(3,4-ethylenedioxythiophene) and AQ-COF = ketoenamine-linked COF with anthraquinone (AQ) moieties), formed by polymerizing the organic conductive polymer PEDOT within the hollow pores of a COF, demonstrates outstanding electrical conductivity and energy storage properties. In this study, we systematically synthesized samples with various ratios of PEDOT to AQ-COF by adjusting the concentration of the precursor molecule in AQ-COF. Elemental analysis results indicated that PEDOT exists as oligomers with a degree of polymerization between 3.4 and 5.6, and there is a saturation point for the amount of PEDOT. This saturation state suggests that the PEDOT chains are densely packed within the cavity of AQ-COF, forming molecular contacts between PEDOT and AQ-COF. We assessed the electrical conductivity, electrochemical properties, and electron paramagnetic resonance (EPR) of the PEDOT@AQ-COF series. We observed that, upon reaching the saturation point of PEDOT, a critical transition occurs to a highly conductive state. In this state, the cyclic voltammetry curves exhibit redox reactions of AQ-COF, assisted by the PEDOT guest molecules. Compared to the low-conductivity sample, the high-conductivity samples in EPR displayed a broader linewidth component with nearly temperature-independent spin susceptibility.