Mechanochemical synthesis of π-conjugated naphthotetrathiophene-based covalent organic frameworks and their post-modification via the Doebner reaction

Abstract

We report that mechanochemistry enables the synthesis of new π-conjugated covalent organic frameworks (COFs) based on naphthotetrathiophenetetracarbaldehydes 2TTN and 3TTN, expanding the scope of COF structures with two new isomeric tetradentate nodes. While recent studies have explored COF isomerism by varying bidentate linkers to tune electronic properties, the impact of node isomerism remains largely unexplored. Here, we overcome these limitations using mechanochemistry to access COFs based on insoluble fused polycyclic heteroaromatic monomers. These COFs exhibit broad absorption across the visible spectrum with 3TTN COFs showing a red-shifted absorption compared to their 2TTN-counterparts. Microwave-assisted post-modification of COFs via the Doebner reaction converts imine into quinoline linkages, enhancing the chemical stability of the COF. We explore the effect of the node structure (2TTNvs.3TTN) and the network connectivity (phenylenediamine, biphenylenediamine and biquinoline) on optical band-gaps of the COFs. All COFs have shown a marked increase in electrical conductivity, by 6 orders of magnitude, upon p-doping with iodine. Quinoline-linked COFs showed proton conductivity up to 1.2 × 10⁻³ S cm⁻¹ at 60 °C and a relative humidity of 75%. This work suggests new possibilities for electronic modulation in COFs through node isomerism and offers a sustainable route to robust, tunable, heteroaromatic frameworks.