Research Institute for Electronic Science, Hokkaido University, N20, W10, Sapporo, Hokkaido 001-0020, Japan.
Chemistry. 2011 Jun 20;17(26):7304-12. doi: 10.1002/chem.201003526. Epub 2011 May 12.
New phototriggered molecular machines based on cyclic azobenzene were synthesized in which a 2,5-dimethoxy, 2,5-dimethyl, 2,5-difluorine or unsubstituted-1,4-dioxybenzene rotating unit and a photoisomerizable 3,3'-dioxyazobenzene moiety are bridged together by fixed bismethylene spacers. Depending upon substitution on the benzene moiety and on the E/Z conformation of the azobenzene unit, these molecules suffer various degrees of restriction on the free rotation of the benzene rotor. The rotation of the substituted benzene rotor within the cyclic azobenzene cavity imparts planar chirality to the molecules. Cyclic azobenzene 1, with methoxy groups at both the 2- and 5-positions of the benzene rotor, was so conformationally restricted that free rotation of the rotor was prevented in both the E and Z isomers and the respective planar chiral enantiomers were resolved. In contrast, compound 2, with 2,5-dimethylbenzene as the rotor, demonstrated the property of a light-controlled molecular brake, whereby rotation of the 2,5-dimethylbenzene moiety is completely stopped in the E isomer (brake ON, rotation OFF), while the rotation is allowed in the Z isomer (brake OFF, rotation ON). The cyclic azobenzene 3, with fluorine substitution on the benzene rotor, was in the brake OFF state regardless of E/Z photoisomerization of the azobenzene moiety. More interestingly, for the first time, we demonstrated the induction of molecular chirality in a simple monocyclic azobenzene by circular-polarized light. The key characteristics of cyclic azobenzene 2, that is, stability of the chiral structure in the E isomer, fast racemization in the Z isomer, and the circular dichroism of enantiomers of both E and Z isomers, resulted in a simple reversible enantio-differentiating photoisomerization directly between the E enantiomers. Upon exposure to r- or l-circularly polarized light at 488 nm, partial enrichment of the (S)- or (R)-enantiomers of 2 was observed.
基于环状偶氮苯的新型光触发分子机器被合成出来，其中一个 2,5-二甲氧基、2,5-二甲基、2,5-二氟或未取代的 1,4-二氧苯旋转单元和一个光致异构的 3,3'-二氧偶氮苯部分通过固定的双亚甲基间隔物桥接在一起。根据苯环上的取代基和偶氮苯单元的 E/Z 构象，这些分子在苯环转子的自由旋转上受到不同程度的限制。取代苯环转子在环状偶氮苯腔内的旋转赋予分子平面手性。带有甲氧基的环状偶氮苯 1，在苯环转子的 2-和 5-位，由于构象受限，在 E 和 Z 异构体中，转子的自由旋转都受到阻碍，相应的平面手性对映体被拆分出来。相比之下，带有 2,5-二甲基苯环作为转子的化合物 2 表现出光控分子刹车的性质，即 2,5-二甲基苯环部分的旋转在 E 异构体中完全停止（刹车 ON，旋转 OFF），而在 Z 异构体中允许旋转（刹车 OFF，旋转 ON）。带有苯环转子上氟取代基的环状偶氮苯 3 无论偶氮苯部分的 E/Z 光异构化如何，都处于刹车 OFF 状态。更有趣的是，我们首次通过圆偏振光在简单的单环偶氮苯中诱导了分子手性。环状偶氮苯 2 的关键特性，即 E 异构体中手性结构的稳定性、Z 异构体中快速外消旋化，以及 E 和 Z 异构体对映体的圆二色性，导致了简单的可逆对映体区分光异构化，直接在 E 对映体之间进行。在 488nm 处用 r-或 l-圆偏振光照射时，观察到 2 的（S）-或（R）-对映体的部分富集。
