Photochemical reactions in organized monolayer assemblies. I. Cis-trans isomerization of thioindigo dyes
Photochemical reactions in organized monolayer assemblies. I. Cis-trans isomerization of thioindigo dyes
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有组织的单层组件中的光化学反应。
DOI：
10.1021/ja00809a052
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发表时间：
1974
影响因子：
15
通讯作者：
D. Whitten
中科院分区：
文献类型：
--
作者：
D. Whitten
图1.6，6‘-DIN-己氧基硫代铟(1)与三棕榈胺的单层组装(20层)的光谱，混合比为1：TP，1：2：实线，顺式-L；虚线反式-1；虚线表示从顺式-L到反式光异构化的中间点。经顺式L照射后的反式-1的组装量与直接由纯反式-1制备的相同。纯c/S-1或c/S-1：TP混合物的扩散膜在水面上也发生快速光异构化为反式，而不是反式。虽然含有c/S-1的薄膜在黑暗中可以长时间稳定，但只需几秒钟的可见光照射就足以产生定量的反式转化。因此，在涂膜和单层组件中都观察到了烯烃1-3的高效光异构化，但与溶液光异构化相反，仅从顺式到反式。所观察到的现象可以归因于两个因素引起的协同效应，即膜状单层组件施加的分子环境限制以及反式和顺式异构体的不同大小。虽然已知会发生中等大分子的有限扩散和小分子通过类似的单分子层组件的快速扩散，但在这些组件中分子的运动受到一定的限制，如果在激发态的寿命期间不禁止大的体积膨胀，应该是困难的。对于几种烯烃，特别是二苯乙烯衍生物，平面的反式异构体比非平面的顺式异构体小。17对于二苯乙烯来说，在不发生反式-顺式转化的条件下，半刚性玻璃中发生顺式-反式异构化，证明其大小顺序为反式-扭曲异构化前驱&顺式。418·19因此，在膜状单分子层组装体中，顺式-反式过程可以导致松弛，或者至少它应该对抗很小的阻力，19而反式-顺式过程应该是扩张的，在半刚性环境中受到空间限制。支持这一点的证据来自对这两种异构体的表面压力和扩散膜面积的调查。在15dyn/cm的恒压下，顺式-L光化学转化为反式(与TP以1：2的比例混合)伴随着体积减小3.0±0.4%。
Figure 1. Spectra of monolayer assemblies (20 layers) of 6, 6'-din-hexyloxythioindigo (1) with tripalmitine, mixing ratio 1: TP, 1: 2: Solid line, cis-l; dotted line trans-1; dashed lines indicate intermediate points in the photoisomerization from cis-l to trans. of assemblies of trans-1 from irradiation of cis-l in the assemblies are identical with those prepareddirectly from pure trans-1. Rapid photoisomerization of c/s-1 to trans, but not the reverse, also occurs on irradiation of spread monolayers of pure c/s-1 or c/s-1: TP mix-tures on a water surface. Although films containing c/s-1 are stable for a long period in the dark, irradiation with visible light for only a few seconds is sufficient to produce quantitative conversion to trans. Thus in both spread films and monolayer assemblies, efficient photoisomerization of olefins 1-3 is observed but, in contrast to the solution photoisomerization, exclusively from cis to trans.The observed phenomena can be attributed to a co-operative effect caused by two factors, the molecular environmental restraints imposed by the membrane-like monolayer assembly and the differing sizes of trans and cis isomers. Although limited diffusion of moderately large molecules and rapid diffusion of smaller molecules through similar monolayer assemblies are known to occur, 9 molecular motion is somewhat re-stricted in these assemblies, and large volume expansions should be difficult if not prohibited during the lifetime of theexcited state. For several olefins, es-pecially stilbene derivatives, it has been shown that the planar trans isomer is smaller than the nonplanar cis isomer. 17 For stilbene, the occurrence of cis-trans isomerization in semirigid glasses under conditions where the trans-cis conversion does not occur has been taken as evidence that the order of size is trans< twisted isomerization precursor< cis. 418· 19 Thus in the mem-brane-like monolayer assembly, the cis-trans process could result in a relaxation, or at least it should en-counter little resistance, 19 while the trans—»-cis process should be an expansion, sterically restricted in the semirigid environment. Evidence supporting this comes from an investigation of the surface pressure and area for spread films of the two isomers. At a constant pressure of 15 dyn/cm, the photochemical conversion of cis-l to trans (in a 1: 2 mixture with TP) is accompanied by 3.0± 0.4% reduction in volume.