Semiconductive Porphyrin-Based Covalent Organic Frameworks for Sensitive Near-Infrared Detection

A porphyrin-based two-dimensional (2D) covalent organic framework (COF) was developed by a C₄ + C₄ topological diagram. It was constructed by the condensation of zinc 5,10,15,20-tetra(4-aminophenyl)porphyrin (TAPP) and zinc 5,10,15,20-tetra(4-formylphenyl)porphyrin (TFPP) under typical solvothermal conditions, leading to the formation of a porphyrin-based TAPP-TFPP-COF with tetragonal micropores at a size of 1.8 nm. The resultant crystalline framework exhibited high crystallinity, excellent stability, and good porosity. Resulting from the specific π-unit stacking columnar structure and excellent organic semiconducting property of porphyrins, the TAPP-TFPP-COF shows many promising applications in optoelectronics. Notably, after doping with iodine, the conductivity of this TAPP-TFPP-COF can be greatly enhanced from 1.12 × 10^-10 to 1.46 × 10^-7 S cm^-1. Furthermore, the nanometer-thick TAPP-TFPP-COF films were obtained using a liquid-air interface growth strategy. A spectroscopic detection device was constructed using COF thin films which displayed highly selective sensitivity toward the near infrared irradiation at 700 nm with an on-off ratio of up to 2.8 × 10⁴. This value ranks as the highest among other COF-based and metal-organic-framework-based semiconducting materials under similar conditions. These results illustrated the enormous potential of 2D porphyrin COFs for future applications in optoelectronic devices and constituted an important step toward the development of new types of functional crystalline materials.