Department of Chemistry and Biochemistry, University of South Carolina, Columbia, 29208, United States.
J Am Chem Soc. 2011 Sep 7;133(35):13975-83. doi: 10.1021/ja203807h. Epub 2011 Aug 17.
The stability and bulk properties of two-dimensional boronate ester-linked covalent organic frameworks (COFs) were investigated upon exposure to aqueous environments. Enhanced stability was observed for frameworks with alkylation in the pores of the COF compared to nonalkylated, bare-pore frameworks. COF-18Å and COF-5 were analyzed as "bare-pore" COFs, while COF-16Å (methyl), COF-14Å (ethyl), and COF-11Å (propyl) were evaluated as "alkylated-pore" materials. Upon submersion in aqueous media, the porosity of alkylated COFs decreased ∼25%, while the nonalkylated COFs were almost completely hydrolyzed, virtually losing all porosity. Similar trends were observed for the degree of crystallinity for these materials, with ∼40% decrease for alkylated COFs and 95% decrease for nonalkylated COFs. SEM was used to probe the particle size and morphology for these hydrolyzed materials. Stability tests, using absorbance spectroscopy and (1)H NMR, monitored the release of monomers as the COF degraded. While nonalkylated COFs were stable in organic solvent, hydrolysis was rapid in aqueous environments, more so in basic compared to neutral or acidic aqueous media (minutes to hours, respectively). Notably, alkylation in the pores of COFs slows hydrolysis, exhibiting up to a 50-fold enhancement in stability for COF-11Å over COF-18Å.
研究了两种二维硼酸酯键合共价有机框架（COFs）在暴露于水相环境时的稳定性和体相性质。与非烷基化的、无孔 COF 相比，具有孔内烷基化的 COF 表现出增强的稳定性。COF-18Å 和 COF-5 被分析为“无孔”COF，而 COF-16Å（甲基）、COF-14Å（乙基）和 COF-11Å（丙基）则被评估为“孔内烷基化”材料。在浸入水相介质后，烷基化 COF 的孔隙率下降了约 25%，而非烷基化 COF 几乎完全水解，几乎完全失去了所有的孔隙率。这些材料的结晶度也表现出相似的趋势，烷基化 COF 下降了约 40%，而非烷基化 COF 下降了 95%。SEM 用于探测这些水解材料的粒径和形貌。稳定性测试采用吸收光谱和（1）H NMR 监测 COF 降解过程中单体的释放。非烷基化 COF 在有机溶剂中稳定，但在水相环境中水解迅速，在碱性条件下比在中性或酸性水相介质中更快（分别为几分钟到几小时）。值得注意的是，COF 孔内的烷基化减缓了水解，COF-11Å 的稳定性比 COF-18Å 提高了 50 倍。
