Jindal Swati, Anjum Gulshan, Maka Vijay Kumar, Moorthy Jarugu Narasimha
Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India.
Nanoscale. 2021 Jun 3;13(21):9668-9677. doi: 10.1039/d1nr00898f.
A water-stable In-MOF, constructed based on a conformationally-flexible tetraacid linker, i.e., 2,7-bis(3,5-dicarboxyphenyl)-9,9'-diphenyl-9H-fluorene, i.e., H4DPF, is shown to exhibit a significantly enhanced solid-state fluorescence quantum yield (φf) of 23% in comparison with that of the linker (φfca. 4%) as a consequence of rigidification of the latter by metalation. Application of external stimulus in the form of grinding of the In-MOF leads to a drastic enhancement by 29%, φf from 23 to 52%. Solid-state absorption and emission spectra show that the absorption in the region of 368-550 nm gets diminished with a concomitant change in the emission maximum with a blue shift upon grinding. Fluorescence enhancement with grinding is correlated with a gradual reduction in the size of the particles, as established by SEM analysis. MOF particle aggregation has been invoked to account for the observed fluorescence enhancement in addition to a subtle conformational change in the structure of the linker upon grinding. Intriguingly, the ground MOF particles exhibit aggregation behaviour in the DMF-water solvent system with the emission further increasing up to 75% for the increase in the water fraction (fw) from 0 to 60%; hydrophobic aggregation of particles evidently leads to a change in the conformation of the linker and particle aggregation-enhanced emission (AEE). De-aggregation of particles ensues for fw = 70-90%, as reflected by a gradual decrease in the emission intensity. It is shown that the suspension of ground In-MOF particles in water permits sensing of metal ions, in particular Al3+ ions, by fluorescence quenching with detection at a sub-ppb level. The observed results comprise first demonstration of both mechanoluminescence and AEE of MOF particles.
一种基于构象灵活的四酸连接体（即2,7-双（3,5-二羧基苯基）-9,9'-二苯基-9H-芴，即H4DPF）构建的水稳定铟基金属有机框架（In-MOF），由于连接体通过金属化作用而刚性化，其固态荧光量子产率（φf）相较于连接体（φf约为4%）显著提高至23%。对In-MOF进行研磨这种外部刺激后，荧光量子产率急剧提高29%，从23%提升至52%。固态吸收光谱和发射光谱表明，368 - 550 nm区域的吸收减弱，同时发射最大值发生变化，研磨后出现蓝移。如扫描电子显微镜（SEM）分析所证实，研磨导致的荧光增强与颗粒尺寸逐渐减小相关。除了研磨后连接体结构发生细微构象变化外，MOF颗粒聚集也被认为是观察到的荧光增强的原因。有趣的是，研磨后的MOF颗粒在N,N-二甲基甲酰胺（DMF）-水溶剂体系中表现出聚集行为，随着水含量（fw）从0增加到60%，发射进一步增强至75%；颗粒的疏水聚集显然导致连接体构象变化以及颗粒聚集增强发射（AEE）。当fw = 70 - 90%时，颗粒发生解聚，这表现为发射强度逐渐降低。结果表明，研磨后的In-MOF颗粒在水中的悬浮液能够通过荧光猝灭检测金属离子，特别是Al3+离子，检测限可达亚纳克/升水平。观察到的结果首次证明了MOF颗粒的机械发光和AEE现象。
