Designed Synthesis and Characterization of Novel Germanium Centered Porous Aromatic Frameworks (Ge-PAFs)

Abstract

A novel series of germanium centered porous aromatic frameworks (Ge-PAFs) based on tetrakis(4-bromophenyl)germane as building units and 1,4-benzenediboronic acid or 4,4'-biphenyldiboronic acid as linkers were synthesized. These materials were characterized by Fourier transform infrared spectroscopy (FT-IR), C solid-state NMR, thermogravimetric analysis (TGA), powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (TEM), and N2 gas sorption. FTIR spectroscopy is useful in probing the structure of Ge-PAFs networks. From the disappearance of the intense B—OH bands (at 3370 cm) and the C—Br bands (at 482 cm) in the FT-IR spectra, the formation of the product can be preliminarily confirmed. The solid-state C CP/MAS NMR spectra of Ge-PAFs indicate that a group of signals with a chemical shift in the range of δ 120—145 could be observed. They are related to aromatic carbon atoms in the framework-building phenylene groups. Both the FTIR and C CP/MAS NMR analyses show no obvious signals assigned to the halogen or boronic acid end groups, testifying that an almost complete coupling reaction has taken place. PXRD was carried out to investigate the crystallinity of Ge-PAFs, which indicate the amorphous texture. No long-range frameworks could be due to distortion and interpenetration of the phenyl rings. SEM images showed that Ge-PAF-1 and Ge-PAF-2 afforded a spherical morphology. Simultaneously, TEM revealed that both of Ge-PAFs possess a worm-like texture. The thermal stability of Ge-PAF-1 and Ge-PAF-2 was analyzed by TG analysis under air. These materials were stable up to 420 °C in air corresponding to 5% weight loss, which suggest that the Ge-PAFs exhibit high thermal stability. Moreover, Ge-PAF-1 and Ge-PAF-2 also show high chemical stability. It is also insoluble in common organic solvents such as methanol, ethanol, acetone, THF, CHCl3 and DMF. The N2 sorption isotherms were measured at 77 K to characterize the porosity of the Ge-PAFs networks. All networks give rise to type I nitrogen gas sorption isotherms with a distinct hysteresis for the whole range of relative pressure. This result confirms the mesoporous nature in networks which was also observed by previous reports, especially for non-ordered porous polymers. It is attributed to elastic deformations during the course of N2 sorption; that is, the network is swelling. This is in accordance with the low surface areas of Ge-PAFs. When the Brunauer-Emmett-Teller (BET) model of the adsorption is adopted, the apparent surface areas of Ge-PAF-1 and Ge-PAF-2 are 49 and 65 m•g respectively, which are lower when compared to other reported POFs. The pore size distribution calculated from non-local density functional theory (NL-DFT), indicated that the Ge-PAFs have a widespread pore size distribution between 0.5 and 5 nm, which also confirms the existence of mesopores in networks. What’s more, the introduction of germanium in Ge-PAFs might provide possibilities for applications in organic semiconductors.