含偶氮苯主-客体复合物的光致异构化反应对结合能与几何构象的影响（英文）

Influence of Photoisomerization on Binding Energy and Conformation of Azobenzene-Containing Host-Guest Complex

在分子尺度上构建光驱动的人工分子机器是超分子化学研究的一个热点。偶氮苯是一类具有双稳态的光致开关分子,能够完成高效、可逆的反式(E)→顺式(Z)的光致异构化过程,因而可以作为人工分子机器的功能单元。本文采用密度泛函理论(DFT)和反应分子动力学(RMD)模拟,研究了含偶氮苯封端基团的互锁型超分子体系中冠醚主体与二烷基铵客体间结合强度,模拟了偶氮苯Z→E异构化反应的动态过程,讨论了异构化反应对主客体分子构象的影响。在偶氮苯封端基团通过发生Z→E异构化实现体系单向可控运动时,较强的主-客体间结合能力是保证互锁型超分子体系稳定的必要前提。顺式客体与主体大环氢键相互作用比反式客体更强,因此顺式复合异构体具有比反式异构体更大的结合强度。偶氮苯基团发生E→Z光致异构化引入位阻效应,使得顺式复合物只能从环戊基准封端处进行脱环。主客体复合过程对偶氮苯基团的几何结构没有明显影响。偶氮苯光致异构化发生的速度快于客体脱环的速度是实现单向运动的动力学上的必要条件。在异构化反应后的500 ps内,大环会经历一个明显的结构驰豫过程。冠醚大环主体的柔性构象有助于实现在偶氮苯光致异构化发生过程中主客体间持续稳定的结合。各种超分子体系中,尽管客体组成各不相同,但是包含相似的主客体识别位点的超分子体系具有相似的结合能,显示了机械互锁型复合体系中各种功能性构建单元间主客体相互作用具有正交性。引入双稳态的偶氮苯功能基团对客体其他部分的几何结构影响很小。理论计算结果有助于理性设计更复杂的刺激响应性人工分子机器。

The construction of a photo-controllable artificial molecular machine capable of realizing the light-driven motion on a molecular scale and of performing a specific function is a fascinating topic in supramolecular chemistry.The bistable switchable molecule,azobenzene(AZO),has been introduced into the supramolecular architecture as a key building block,owing to its efficient and reversible trans(E)-cis(Z)photoisomerization.The binding strength of the dibenzo[24]crown-8(DB24C8)host and dialkylammonium-based rod-like guest consisting of an AZO moiety and the Z→E photoisomerization process in an interlocked host-guest complex have been investigated by the density functional theory(DFT)calculations and the reactive molecular dynamics(RMD)simulations by considering both torsion and inversion paths.The strong host-guest binding strength provides a necessary premise to stabilize the complex during the E-Z photoisomerization of the AZO unit,which is a terminal stopper to control the directional motion of the guest.A stronger binding strength for the Z isomer can be induced by the stronger hydrogen-bonding interaction.The steric effect is introduced into the Z isomer to force the ring slipping exclusively over the cyclopentyl terminal(pseudostopper).The host-guest complexation has a slight effect on the conformation of the AZO functional subunit for the two isomers.The faster Z→E photoisomerization process within the picosecond timescale is kinetically more favored than the dethreading of the ring through the pseudostopper subunit of the rod.After isomerization,a structure relaxation is observed for the crown ether ring within 500 ps.The flexible backbone of the crown ether ring is helpful in realizing steady and stable host-guest recognition during photoisomerization.Moreover,the orthogonality of the site-specific binding interaction is revealed by the similar binding energies obtained at similar hydrogen bonding recognition sites for various interlocked host-guest supramolecular systems although the constituents of the guests are different from each other.The introduction of two stereoisomers of the AZO subunit has little influence on the other conformations of guest subunits.These results are useful for the rational design of more sophisticated stimuli-controlled artificial molecular machines.