Functionality-Dependent Electrical Conductivity in Two-Dimensional Covalent Organic Frameworks

Two-dimensional (2D) covalent organic frameworks (COFs) represent a unique structural paradigm owing to their intriguing properties. The diverse chemical tunability and good charge carrier mobility make these materials suitable for electronics and optoelectronic applications. However, the poor conductivity and processability restrict their full potential in device fabrication. Several strategies were followed to improve the conductivity in 2D COFs, including redox-active linkers and doping. Herein, we report four 2D COFs, by modulating the substituents on one of the monomeric units. Interestingly, less-bulky groups (H and OH) yielded a Kagome lattice in ETTA-TA- and ETTA-DHTA-COFs, whereas square-grid networks were obtained for ETTA-An-ph- and ETTA-BPTA-COFs with bulky substituents (anthracene and propargylic moieties). Further, we investigated the effect of the topology and the functionalities of these COFs on electrical conductivity. The current–voltage (I–V) measurements reveal that ETTA-DHTA-COF with a Kagome lattice exhibits higher electrical conductivity of 1.3 × 10–4 S cm–1 compared to other congeners (square-grid and Kagome-type COFs) and many reported undoped COFs. The trade-off between the frameworks' topology (Kagome vs square-grid) and the functionalities (H, anthracene vs OH, oxy-propargyl) exhibits the synergetic role in stimulating the conductivities in the resulting COFs. This strategy of switching framework topology and the effect of functional groups on electrical conductivity unveil a unique approach to fine-tuning the electronic properties in the 2D COFs.