Tuning Structural and Optical Properties of Porphyrin‐based Hydrogen‐Bonded Organic Frameworks by Metal Insertion

Abstract Herein, a simple way of tuning the optical and structural properties of porphyrin‐based hydrogen‐bonded organic frameworks (HOFs) is reported. By inserting transition metal ions into the porphyrin cores of GTUB‐5 (p‐H₈‐TPPA (5,10,15,20‐Tetrakis[p‐phenylphosphonic acid] HOF), the authors show that it is possible to generate HOFs with different band gaps, photoluminescence (PL) life times, and textural properties. The band gaps of the resulting HOFs (viz., Cu‐, Ni‐, Pd‐, and Zn‐GTUB‐5) are measured by diffuse reflectance and PL spectroscopy, as well as calculated via DFT, and the PL lifetimes are measured. Across the series, the band gaps vary over a narrow range from 1.37 to 1.62 eV, while the PL lifetimes vary over a wide range from 2.3 to 83 ns. These differences ultimately arise from metal‐induced structural changes, viz., changes in the metal‐to‐nitrogen distances, number of hydrogen bonds, and pore volumes. DFT reveals that the band gaps of Cu‐, Zn‐, and Pd‐ GTUB‐5 are governed by highest occupied/lowest unoccupied crystal orbitals (HOCO/LUCO) composed of π‐ orbitals on the porphyrin linkers, while that of Ni‐GTUB‐5 is governed by a HOCO and LUCO composed of Ni dorbitals. Overall, our findings show that metal‐insertion can be used to optimize HOFs for optoelectronics and small‐molecule capture applications.