Exciton Migration and Amplified Quenching on Two-Dimensional Metal–Organic Layers

The dimensionality dependency of resonance energy transfer is of great interest due to its importance in understanding energy transfer on cell membranes and in low-dimension nanostructures. Light harvesting two-dimensional metal–organic layers (2D-MOLs) and three-dimensional metal–organic frameworks (3D-MOFs) provide comparative models to study such dimensionality dependence with molecular accuracy. Here we report the construction of 2D-MOLs and 3D-MOFs from a donor ligand 4,4′,4″-(benzene-1,3,5-triyl-tris­(ethyne-2,1-diyl))­tribenzoate (BTE) and a doped acceptor ligand 3,3′,3″-nitro-4,4′,4″-(benzene-1,3,5-triyl-tris­(ethyne-2,1-diyl))­tribenzoate (BTE-NO2). These 2D-MOLs and 3D-MOFs are connected by similar hafnium clusters, with key differences in the topology and dimensionality of the metal–ligand connection. Energy transfer from donors to acceptors through the 2D-MOL or 3D-MOF skeletons is revealed by measuring and modeling the fluorescence quenching of the donors. We found that energy transfer in 3D-MOFs is more efficient than that in 2D-MOLs, but excitons on 2D-MOLs are more accessible to external quenchers as compared with those in 3D-MOFs. These results not only provide support to theoretical analysis of energy transfer in low dimensions, but also present opportunities to use efficient exciton migration in 2D materials for light-harvesting and fluorescence sensing.共振能量转移（resonance energy transfer）的维度依赖性因其在阐释细胞膜与低维纳米结构内能量转移过程中的关键作用而广受关注。光捕获型二维金属有机层（2D-MOLs）与三维金属有机框架（3D-MOFs）为以分子精度研究此类维度依赖性提供了可对比的模型体系。 本文报道了以给体配体（donor ligand）4,4′,4″-(苯-1,3,5-三基-三(乙炔-2,1-二基))三苯甲酸(BTE)与掺杂受体配体（doped acceptor ligand）3,3′,3″-硝基-4,4′,4″-(苯-1,3,5-三基-三(乙炔-2,1-二基))三苯甲酸(BTE-NO2)构建2D-MOLs与3D-MOFs的方法。上述两类材料均通过相似的铪簇（hafnium clusters）连接，二者的核心差异在于金属-配体连接的拓扑结构（topology）与维度。 通过测量并拟合给体的荧光猝灭（fluorescence quenching）行为，我们揭示了经由2D-MOL或3D-MOF骨架实现的给体到受体的能量转移过程。研究发现，3D-MOFs中的能量转移效率高于2D-MOLs，但相较于3D-MOF中的激子（excitons），2D-MOL上的激子更易被外部猝灭剂（external quenchers）猝灭。 本研究结果不仅为低维体系内能量转移的理论分析提供了实验支撑，同时也为利用二维材料中高效的激子迁移开展光捕获与荧光传感研究提供了新的可行路径。