Unveiling the electronic properties of metal-free and undoped covalent organic framework as a semiconductor

Abstract

Covalent organic frameworks (COFs) are an emerging category of semiconducting material. In the present investigation, a conjugated covalent organic framework (PPDA–TFPT–COF) composed of p-phenylenediamine (PPDA) and 2,4,6-tris-(p-formylphenoxy)1,3,5-triazine (TFPT) was designed and developed via Schiff-base reaction under solvothermal conditions. The morphology and electronic properties of PPDA–TFPT–COF were analyzed through various characterization techniques. The XPS studies reveal that imine linkages (CN) are formed during the reaction consisting of N-rich species possessing the property of reversibility. The semiconducting nature of PPDA–TFPT–COF is verified via the measurement of electrical conductivity (10⁻⁶–10⁻⁵ S cm⁻¹) by the four-probe measurement method. The band gap of the developed polymer is in the range of 3.2–3.4 eV, measured by UV spectroscopy, ultraviolet photoelectron spectroscopy (UPS) and cyclic voltammetry (CV), and theoretically validated by density functional theory (DFT) calculations of the ring monomer unit. Also, DFT calculation revealed that PPDA–TFPT–COF possesses π–π conjugation throughout the polymeric ring. This study brings out the fact that by controlling the moieties, it is possible to develop a semiconducting covalent organic framework within a short duration time-period without metal or charge doping.