Yu Jierui, Park JaeHong, Van Wyk Andrea, Rumbles Garry, Deria Pravas
Department of Chemistry and Biochemistry , Southern Illinois University , 1245 Lincoln Drive , Carbondale , Illinois 62901 , United States.
Department of Molecular Engineering , Kyoto University , Katsura, Nishikyo-ku , Kyoto 615-8510 , Japan.
J Am Chem Soc. 2018 Aug 22;140(33):10488-10496. doi: 10.1021/jacs.8b04980. Epub 2018 Aug 8.
Molecular assemblies in metal-organic frameworks (MOFs) are reminiscent of natural light-harvesting (LH) systems and considered as emerging materials for energy conversion. Such applications require understanding the correlation between their excited-state properties and underlying topological net. Two chemically identical but topologically different tetraphenylpyrene (1,3,6,8-tetrakis( p-benzoicacid)pyrene; HTBAPy)-based Zr MOFs, NU-901 ( scu) and NU-1000 ( csq), are chosen to computationally and spectroscopically interrogate the impact of topological difference on their excited-state electronic structures. Time-dependent density functional theory-computed transition density matrices for selected model compounds reveal that the optically relevant S, S, and S states are delocalized over more than four TBAPy linkers with a maximum exciton size of ∼1.7 nm (i.e., two neighboring TBAPy linkers). Computational data further suggests the evolution of polar excitons (hole and electron residing in two different linkers); their oscillator strengths vary with the extent of interchromophoric interaction depending on their topological network. Femtosecond transient absorption (fs-TA) spectroscopic data of NU-901 highlight instantaneous spectral evolution of an intense S → S transition at 750 nm, which diminishes with the emergence of a broad (580-1100 nm) induced absorption originating from a fast excimer formation. Although these ultrafast spectroscopic data reveal the first direct spectral observation of fast excimer formation (τ = 2 ps) in MOFs, the fs-TA features seen in NU-901 are clearly absent in NU-1000 and the free HTBAPy linker. Furthermore, transient and steady-state fluorescence data collected as a function of solvent dielectrics reveal that the emissive states in both MOF samples are electronically nonpolar; however, low-lying polar excited states may get involved in the excited-state decay processes in polar solvents. The present work shows that the topological arrangement of the linkers critically controls the excited-state electronic structures.
金属有机框架（MOF）中的分子组装让人联想到天然的光捕获（LH）系统，被视为用于能量转换的新兴材料。此类应用需要了解其激发态性质与底层拓扑网络之间的相关性。我们选择了两种化学性质相同但拓扑结构不同的基于四苯基芘（1,3,6,8 - 四（对苯甲酸）芘；HTBAPy）的Zr MOF，即NU - 901（scu）和NU - 1000（csq），通过计算和光谱研究拓扑差异对其激发态电子结构的影响。针对选定模型化合物的含时密度泛函理论计算的跃迁密度矩阵表明，光学相关的S₁、S₂和S₃态在四个以上的TBAPy连接体上离域，最大激子尺寸约为1.7 nm（即两个相邻的TBAPy连接体）。计算数据进一步表明了极性激子的演化（空穴和电子位于两个不同的连接体中）；它们的振子强度随发色团间相互作用的程度而变化，这取决于它们的拓扑网络。NU - 901的飞秒瞬态吸收（fs - TA）光谱数据突出了在750 nm处强烈的S₁→S₂跃迁的瞬时光谱演化，随着源自快速激基缔合物形成的宽（580 - 1100 nm）诱导吸收的出现，该演化逐渐减弱。尽管这些超快光谱数据首次直接光谱观测到了MOF中快速激基缔合物的形成（τ = 2 ps），但在NU - 1000和游离的HTBAPy连接体中明显没有观察到NU - 901中出现的fs - TA特征。此外，作为溶剂介电常数函数收集的瞬态和稳态荧光数据表明，两个MOF样品中的发射态在电子上是非极性的；然而，低能极性激发态可能参与极性溶剂中的激发态衰变过程。目前的工作表明，连接体 的拓扑排列对激发态电子结构起着关键的控制作用。
