Room temperature synthesis and substituent effect study of fluorene-based covalent triazine-based frameworks

Abstract

Efficient synthesis and the introduction of functional group are the focus of current research on microporous organic polymers (MOPs). In this report, a series of new covalent triazine-based framework polymers (CTFs) based on fluorene with different substituents (FCTF1~FCTF3) has been synthesized using trifluoromethanesulfonic acid (TFMS) catalyzed cyclotrimerization reactions at room temperature. The chemical structures of the polymers were confirmed by FTIR and elemental analysis. In the FTIR spectra, the nearly absence of peaks at around 2220 cm-1 along with the emergence of strong triazine absorption bands around 1500, 1360 and 800 cm-1 indicated qualitatively a high degree of polymerization. Thermogravimetric analysis (TGA) under nitrogen atmosphere revealed a high thermal stability with 5% weight loss at temperature up to 364 (FCTF1), 452 (FCTF2) and 238℃ (FCTF3). The solid UV-Vis spectra showed that the polymers could all absorb light from UV to visible light region. In the photoluminescence measurement, FCTF1~FCTF3 exhibited bright blue fluorescence with maximum emission wavelengths at 437 nm, 455 nm and 439 nm respectively. The specific surface areas of the polymers changed dramatically according to the substituent attached to the fluorine unit, with BET surface areas changing from nearly nil (FCTF3) to 621 m2/g (FCTF2) when the substituent changed from butyl to ethyl. Pore size distributions were calculated using nonlocal density functional theory (NL-DFT) and porous polymers FCTF1 and FCTF2 showed main pore sizes in the micropore region. CO2 adsorption capacities of the polymers were also measured and FCTF1 and FCTF2 showed high CO2 uptake of 1.7 and 1.8 mmol/g respectively at 273 K/1.1 bar. The isosteric heats of adsorption were calculated from the CO2 isotherms measured at 273 and 298 K. FCTF1 and FCTF2 showed adsorption heats of 26.4 and 22.7 kJ/mol respectively at the zero coverage, indicative strong binding affinity of the polymers with CO2. To the best of our knowledge, this is the first report on the substituent effect of fluorene-based CTFs and this research can probably enhance the understanding of the structure-property relationship of porous organic polymer materials.