Switching the interior hydrophobicity of a self-assembled spherical complex through the photoisomerization of confined azobenzene chromophores
Switching the interior hydrophobicity of a self-assembled spherical complex through the photoisomerization of confined azobenzene chromophores
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DOI：
10.1002/anie.200700793
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发表时间：
2007-01-01
影响因子：
16.6
通讯作者：
Fujita, Makoto
中科院分区：
文献类型：
--
作者：
Murase, Takashi;Sato, Sota;Fujita, Makoto
随着局部结构和性质的改变，外界刺激对有限纳米空间的控制已经引起了人们的广泛关注在分子和超分子体系中，光是一种特别有用和有效的刺激，可以引起可逆的结构变化。偶氮苯是一种众所周知的生色团，它对光有响应并经历顺反异构化，导致其大小和极性发生很大变化例如，含偶氮苯共聚物的胶束化被光可逆地改变，因为发色团的反式到顺式异构化增加了偶极矩，极大地改变了核心的亲疏水平衡虽然在树状大分子外围具有偶氮苯官能团的树状大分子也得到了很好的研究，但在树状大分子内部组装许多偶氮苯基团[5,6]在合成上是很麻烦的。因此，将有限数量的偶氮苯发色团轻松地限制在纳米分子的明确核心区域是一项有趣的任务。这种结构使得通过光异构化可逆地改变内部环境的性质成为可能。我们之前已经证明，在与12个金属离子络合后，24个吡啶基弯曲桥接配体在溶液中自发组装成具有精确化学结构和均匀直径的球形配合物（方案1）将一个官能团附着在每个配体的凸面[7a]或凹面[7b-d]上，分别导致球体的24倍表面或内嵌功能化。我们在这里描述了一种高阳离子的球形配合物，它含有向内的偶氮苯基该复合物的核心是由24个偶氮苯组成的密集阵列。这种球形配合物可以看作是偶氮官能化的“逆枝状大分子”[9]，其分支在空间的收敛区域。我们发现偶氮苯基团的光异构化可以改变配合物的疏水性。以3,5 -二溴-4-羟基苄基醇为原料，经5步反应制备了收率较高的配体1a: 1) 4-（溴乙基）偶氮苯与苯酚o-烷基化；2)与4-乙基吡啶的Sonogashira偶联；3) CBr4/PPh3溴化；4)季铵盐生成，三甲胺大量过量；5)反阴离子交换成三氟离子（见支持信息）。非栓系配体1b也以类似的方法制备。配体1a （17.7 μmol）用Pd (OSO2CF3) 2[10]（8.9 μmol）在CD3CN （1.8 mL）中508C处理4 h，经1H NMR定量得到含偶氮苯的M12L24配合物2a。复合体2b也以与2a相似的方式制备。配合物2a和2b的冷喷雾电离质谱（CSI-MS）[11]清楚地证实了M12L24的组成，分子量分别为21987和17612 Da。
Control of restricted nanosized space by external stimuli that accompany changes of local structures and properties has attracted a great deal of attention.[1] Light is a particularly useful and efficient stimulus to cause reversible structural changes in molecular and supramolecular systems. Azobenzene is a well-known chromophore that responds to light and undergoes cis–trans isomerization, resulting in large changes in its size and polarity.[2] The micellization of azobenzenecontaining copolymers has been, for example, reversibly switched by light because the trans-to-cis isomerization of the chromophore increases the dipole moment, dramatically changing the hydrophilic/hydrophobic balance of the core.[3] Although dendrimers with azobenzene functionalities around the peripheries [4] have also been well studied, the assembly of many azobenzene moieties at the interior [5, 6] of dendrimers is synthetically troublesome. Therefore, it is an intriguing task to confine a restricted number of azobenzene chromophores in the well-defined core regions of nanosized molecules with ease. Such structures make it possible to reversibly change the properties of the interior environment by photoisomerization. We have previously shown that upon complexation with 12 metal ions, 24 pyridine-based bent bridging ligands spontaneously assemble in solution into a spherical complex with a precise chemical structure and uniform diameter (Scheme 1).[7] Attaching a functional group to the convex [7a] or concave [7b–d] side of each ligand leads to the 24-fold surface or endohedral functionalization of the sphere, respectively. We herein describe a highly cationic spherical complex bearing azobenzene groups that face inwards.[8] The core of the complex features the dense array of 24 azobenzenes. This spherical complex can be regarded as an azobenzenefunctionalized “inverse dendrimer”[9] with branches in the converging region of the space. We show that hydrophobicity in the complex can be switched by the photoisomerization of the azobenzene moieties.Ligand 1a was prepared in a relatively high yield from 3, 5-dibromo-4-hydroxybenzyl alcohol in five steps: 1) phenolic O-alkylation with 4-(bromomethyl) azobenzene; 2) the Sonogashira coupling with 4-ethynylpyridine; 3) CBr4/PPh3 bromination, 4) quaternary ammonium salt formation with a large excess of trimethylamine; and 5) counteranion exchange to triflate ions (see the Supporting Information). Nontethered ligand 1b was also prepared in a similar way. When ligand 1a (17.7 μmol) was treated with Pd (OSO2CF3) 2[10](8.9 μmol) in CD3CN (1.8 mL) for 4 h at 508C, the azobenzene-containing M12L24 complex 2a was quantitatively obtained, as indicated by 1H NMR spectroscopy. Complex 2b was also prepared in a similar way to 2a. Cold-spray ionization mass spectrometry (CSI-MS)[11] of the complexes 2a and 2b clearly confirmed an M12L24 composition with molecular weights of 21987 and 17612 Da, respectively.