共价有机框架COF66/COF366的电子结构: 从单体到二维平面聚合物

doi:10.6023/A23040134

摘要/Abstract

二维共价有机骨架(COFs)是由有机小分子结构单元通过共价键连接形成的一类新型的、具有周期性多孔网络框架结构的结晶性聚合物, 在未来的光电功能材料领域有着优异的应用前景. 因此, 研究COFs结构与性质之间的内在关系, 并以此反馈分子设计, 将对此类光电功能材料的发展有着至关重要的意义. 以共价有机框架66 (COF66)和共价有机框架366 (COF366)为例, 分析了有机构建单元、COFs片段分子和周期性二维平面聚合物前线轨道能级/分布以及电子性质, 阐述了有机构建单元与COFs电子结构之间的内在关系. 计算结果表明, 有机构建单元小分子的几何构型会影响其二维周期性平面聚合物的结构. 另外, 有机构建单元自身的前线轨道能级的相对位置会直接影响到COFs片段分子前线轨道的能级和分布, 也决定了片段分子的电离势和电子亲和能. COFs周期性结构的电子性质与片段分子相似. 因此, 有机构建单元的前线轨道能级以及匹配情况决定了COFs周期性结构的电子性质. 除此之外, 中心分子和连接分子之间的共轭相互作用越大, 电子耦合作用也相应较大, 价带顶和导带底则会相对比较分散, 会更有助于载流子在二维平面结构内的迁移.

关键词: 有机构建单元, 二维共价有机骨架, 电子结构, 密度泛函理论

Two-dimensional covalent organic frameworks (COFs) are an emerging important class of porous materials with periodically ordered structures consisting of core and linker units held together by strong covalent bonds. Their well-designed architecture, tunable functional properties and good thermal stabilities provide great potential for applications in photoenergy conversion and semiconductors. For the aim to explore the relationship between the structure and properties of COFs, which should be very useful for the design of new type of such attractive photo functional materials, the energy character of organic building units, fragment molecules and periodic two-dimensional planar structures are analyzed for COF66 and COF366. The results show that the structure of periodic planar frameworks will be influenced by the geometry of organic building units. Moreover, the relative energy level of the frontier orbitals of the organic building blocks will affect the ionization potential and electron affinity. And the electronic properties of the COFs are also in the same case. Therefore, the electronic property of organic building units is a key factor for determining the electronic structure of periodic structure of COFs. In addition, the electronic coupling of COFs will be larger if the conjugation of the organic building blocks is better. Then, the valence band maximum and conduction band minimum will be more dispersive, and the charge carrier mobility will get larger.

Key words: organic units available, two-dimensional covalent organic framework, electronic structure, density functional theory

引用此文

梁雪峰, 荆剑, 冯昕, 赵勇泽, 唐新员, 何燕, 张立胜, 李慧芳. 共价有机框架COF66/COF366的电子结构: 从单体到二维平面聚合物[J]. 化学学报, 2023, 81(7): 717-724.

Xuefeng Liang, Jian Jing, Xin Feng, Yongze Zhao, Xinyuan Tang, Yan He, Lisheng Zhang, Huifang Li. Electronic Structure of Covalent Organic Frameworks COF66 and COF366: from Monomers to Two-Dimensional Framework[J]. Acta Chimica Sinica, 2023, 81(7): 717-724.

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图/表 11

图1. COF66和COF366的化学结构示意图

Figure 1. Illustration of the chemical structures for COF66 and COF366

图2. 在DFT/PBE-D3水平上优化得到的TBPP和TAPP有机单元分子, THAn-TBPP和TPAL-TAPP二聚体以及TBPP-THAn-TBPP和TAPP-TPAL-TAPP三聚体的几何结构(H原子用白色的小球表示, B原子用粉色的小球表示, C原子用灰色的小球表示, N和O原子分别用蓝色和红色的小球表示)

Figure 2. Optimized geometry structures of TBPP and TAPP monomers, THAn-TBPP and TPAL-TAPP dimers, as well as TBPP-THAn-TBPP and TAPP-TPAL-TAPP trimers obtained at the DFT/PBE-D3 level (H is represented in white, B in pink, C in gray, N in blue and O in red)

	COF66				COF366
	THAn	TBPP	Dimer	Trimer	TPAL	TAPP	Dimer	Trimer
LUMO＋1	－1.37	－2.06	－2.67	－2.79	－1.20	－1.64	－3.04	－2.96
LUMO	－2.57	－3.09	－3.20	－3.23	－2.21	－2.89	－3.65	－3.37
HOMO	－4.47	－4.46	－4.52	－4.57	－3.97	－4.12	－4.31	－4.13
HOMO－1	－5.07	－4.83	－4.93	－4.96	－6.18	－4.64	－4.79	－4.69

表1. 在DFT/PBE-D3水平上得到的COF有机构建单元和片段分子的前线轨道能级a

Table 1. HOMO and LUMO energies for COF monomers and fragments, obtained at the DFT/PBE-D3 level

图3. 在DFT/PBE-D3水平上得到的COF66有机构建单元和片段分子的前线轨道能级以及分布

Figure 3. Contour plots of frontier orbitals and their energy gaps of the COF66 monomers and fragment obtained at the DFT/PBE-D3 level

图4. 在DFT/PBE-D3水平上得到的COF366有机构建单元和片段分子的前线轨道能级以及分布

Figure 4. Contour plots of frontier orbitals and their energy gaps of the COF366 monomers and fragment obtained at the DFT/PBE-D3 level

Porphyrin	TBPP	Dimer	Trimer	COF66	TAPP	Dimer	Trimer	COF366
NBO	－0.04	－0.03	－0.04	—	－0.09	－0.07	－0.09	—
Hirshfeld-I	－0.19	－0.18	－0.19	－0.16	－0.25	－0.22	－0.26	－0.20

表2. Porphyrin环在有机单元分子以及聚合物中的电荷分布情况(in |e|)

Table 2. Charge distribution of porphyrin ring included in core molecules and frameworks (in |e|)

图5. 单层COF66和单层COF-366聚合物1×1晶胞的优化结构

Figure 5. 1×1 unit cell of the optimized structure of a free-standing COF66/COF366 monolayer

图6. 单层COF66/COF366聚合物的分态密度和能带结构(红色曲线代表TBPP/TAPP中心的分态密度, 而蓝色曲线代表THAn/TPAL连接分子的分态密度)

Figure 6. Projected density of states and band structure of a freestanding COF66/COF366 monolayer (The red curve represents the PDOS of TBPP/TAPP core whereas the blue curve represents the PDOS of THAn/TPAL linker)

图7. COF66两个近简并价带顶(VBM-1和VBM)和两个近简并导带底(CBM和CBM＋1)的电荷密度分布图(等值面为0.0002 e/nm)

Figure 7. Partial charge density plot of two nearly degenerate states at the valence-band maximum and two nearly degenerate states at the conduction-band minimum for COF66 (the isosurface is 0.0002 e/nm)

	Hole		Electron
	M→X	M→X′	M→X	M→X′
COF-66	9.11	9.21	2.04	2.63
COF-366	0.68	0.68	0.46	0.74

表3. 计算二维COFs的有效质量(单位为m0)及其方向

Table 3. Calculated effective masses (in m0) and their directions for the 2D COFs

图8. COF366两个近简并价带顶(VBM-1和VBM)和两个近简并导带底(CBM和CBM＋1)的电荷密度分布图(等值面为0.0002 e/nm)

Figure 8. Partial charge density plot of two nearly degenerate states at the valence-band maximum and two nearly degenerate states at the conduction-band minimum for COF366 (the isosurface is 0.0002 e/nm)

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