Klok Martin, Walko Martin, Geertsema Edzard M, Ruangsupapichat Nopporn, Kistemaker Jos C M, Meetsma Auke, Feringa Ben L
Laboratory of Organic Chemistry, Stratingh Institute for Chemistry and Zernike Institute for Advanced Materials, Faculty of Mathematics and Natural Sciences, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, Netherlands.
Chemistry. 2008;14(35):11183-93. doi: 10.1002/chem.200800969.
The introduction of dibenzocyclohepten-5-ylidene as part of a unidirectional light-driven molecular motor allows a more complete picture of the pathway of thermal helix inversion to be developed. The most stable conformation is similar to that found in related motors in that it has, overall, an anti-folded structure with the substituent at the stereogenic centre adopting an axial orientation. Photochemical cis/trans isomerisation at -40 degrees C results in the formation of an isomer in a syn-folded conformation with the methyl group in an axial orientation. This contrasts with previous studies on related molecular rotary motors. The conformation of the higher energy intermediate typically observed for this class of compound is the anti-folded conformation, in which the methyl group is in an equatorial orientation. This conformation is available through an energetically uphill upper half ring inversion of the observed photochemical product. However, this pathway competes with a second process that leads to the more stable anti-folded conformation in which the methyl group is oriented axially. It has been shown that the conformations and pathways available for second-generation molecular motors can be described by using similar overall geometries. Differences in the metastable high-energy species are attributable to the relative energy and position on the reaction coordinate of the transition states. Kinetic studies on these new molecular motors thus provide important insights into the conformational dynamics of the rotation cycle.
引入二苯并环庚烯-5-亚基作为单向光驱动分子马达的一部分，有助于更全面地了解热螺旋反转的途径。最稳定的构象与相关马达中的构象相似，总体上具有反折叠结构，手性中心的取代基呈轴向取向。在-40℃下进行光化学顺/反异构化，会形成一种异构体，其构象为顺折叠，甲基呈轴向取向。这与之前对相关分子旋转马达的研究形成对比。这类化合物通常观察到的高能中间体的构象是反折叠构象，其中甲基处于赤道取向。这种构象可通过观察到的光化学产物进行能量上向上的上半环反转获得。然而，该途径与另一个过程相互竞争，后者会导致甲基轴向取向的更稳定反折叠构象。研究表明，通过使用相似的整体几何结构，可以描述第二代分子马达的构象和途径。亚稳态高能物种的差异归因于过渡态在反应坐标上的相对能量和位置。因此，对这些新型分子马达的动力学研究为旋转循环的构象动力学提供了重要见解。
