发光MOFs检测甲醛过程中的激发态氢键作用及其动力学机理研究

Porous metal-organic frameworks (MOFs) can be a highly sensitive colorimetric sensor to successively detect formaldehyde in a single-crystal-to-single-crystal fashion because of the hydrogen bonding between MOFs and formaldehyde. Hydrogen bonding in the electronically excited state is very important to understand the switchable properties of luminescent MOFs which can be used as chemical sensors. In this project, the hydrogen-bonded MOFs-formaldehyde complex is constructed based on the representative cluster clipped from the crystal structure. Time-dependent functional theory (TDDFT) is used to investigate the behavior of hydrogen bonding in the excited state as well as the relationship between hydrogen bonding and luminescent properties of MOFs. Upon electronically excitation, the dynamics of various normal and abnormal hydrogen bonds of MOFs-formaldehyde complex are explored by both computational and experimental methods. To understand the nature of excited-state hydrogen bonding, the similarity and difference of hydrogen bonding between ground state and excited state are investigated in detail. In the electronically excited state, the effects of hydrogen bonding on the electronic transition energy, charge transfer, proton transfer, energy transfer, and the competition between radiative and non-radiative transition are explored to obtain the relationship between hydrogen bonding and the luminescent properties of MOFs. The photo-physical and photo-chemical behavior of MOFs can be adjusted and controlled by understanding the sensing mechanism of MOFs. The final goal of this project is to provide theoretical insights and directions to design and develop luminescent functional MOFs which are more effective and sensitive to detect formaldehyde.

发光MOFs是一类具有较大比表面积的周期性多孔材料，能够与甲醛通过氢键作用形成复合物，进而改变其发光性质，在甲醛检测方面有很大的应用前景。研究激发态下氢键的动力学行为是理解发光MOFs对甲醛分子的识别和化学传感机制的关键。本项目根据已有的单晶结构，截取代表性结构基元，构建发光MOFs-甲醛氢键复合物，采用含时密度泛函理论研究激发态下氢键行为对MOFs发光性能的影响。考察MOFs-甲醛复合物中各种常规和非常规氢键在激发态下的行为，研究不同电子态下氢键行为的异同和内在联系，揭示激发态下氢键的本质；探索激发态的能量转移、电荷转移、质子转移以及辐射跃迁与非辐射跃迁竞争，阐述氢键作用与MOFs发光性能的关系；深入理解发光MOFs的分子识别和化学传感机制，掌握通过氢键作用来调整和控制MOFs体系光物理和光化学行为的方法，设计开发可用于甲醛检测的新型发光MOFs材料。

氢键的动力学行为是影响荧光探针分子光化学和光物理性质的关键因素。本项目针对七种不同类型的荧光探针分子：(1) 两种镉 (Cd-II)配位金属有机骨架化合物 (MOFs)，(2) 黄酮类化合物NO-和N2H4探针 3-HF，(3) CN-离子探针 TIC 和三种香豆素类化合物，在密度泛函和含时密度泛函理论框架下，以揭示氢键的动力学性质为基础，研究激发态下氢键的本质，探索激发态的能量转移、电荷转移、质子转移，理解氢键作用对荧光探针发光性能的影响，阐明氢键作用下荧光探针对于分子、离子的识别与化学传感机制，为试验工作中设计开发不同用途的荧光探针提供理论依据。.研究结果表明：(1) MOFs-Cd (II)的发光机理为配体-配体电荷转移。分子间氢键的形成对于MOFs-Cd (II)的发光机理没有影响，但会使其荧光性质发生明显改变。在设计应用于分子识别与传感的发光MOFs-Cd (II) 时应注意到溶剂化效应对其检测效果的影响。(2) 经由NO-和N2H4的亲核/亲电进攻后，3-HF的分子内氢键作用在激发态增强，诱导其在S1 态形成 ESIPT异构体，通过内转换作用产生较强的双荧光发射现象。(3) 大部分TIC分子在电子激发作用下经π-π*跃迁到达S1态，发生扭曲分子内电荷转移。另有部分TIC分子经π-π*跃迁到S2态，由于分子内氢键作用增强发生ESIPT反应而生成酮式结构，再经内转换过程跃迁到S1态。因此TIC分子表现出明显的双荧光性质。(4) 电荷密度分布、反应能垒对于香豆素探针检测CN-离子的灵敏度起着至关重要的影响。