Tunable electrical conductivity in covalent organic frameworks by intercalation of electron acceptor molecules

Covalent organic frameworks (COFs) are crystalline organic materials with high porosity. The COFs are typically electrical insulators because of their wide bandgaps and closed-shell structures despite their π–π layer stacking motif suitable for carrier conduction. Here, we report a strategy for realizing tunable electrical conductivity in COFs by intercalating redox-active guest molecules. The approach is demonstrated using COF-5 with a boronate-ester-linked two-dimensional structure and one-dimensional nanopore channels infiltrated by 7,7,8,8-tetracyanoquinododimethane (TCNQ). Successful incorporation of guest TCNQ molecules into the cavities of the COF-5 host is confirmed by vibrational and optical spectroscopies and results in the appearance of guest–host charge-transfer transitions for TCNQ-intercalated COF-5. Overall electrical conductivities are enhanced by the carrier generation induced by the charge transfer between TCNQ and COF-5 with the retained layer-stacked structures in COF-5 also contributing to the conduction pathways. The tunable electronic properties of COFs realized by the host–guest strategies established here will further promote refinement of design principles of conductive COFs and deeper understanding of the conduction mechanism while simultaneously providing opportunities for using COFs in diverse applications.