Study on the alignment control of liquid crystalline molecules by photoisomerization reaction

用拉曼光谱、紫外-可见和红外吸收光谱研究了4-(4‘-(甲氧基苯偶氮)苯氧基)十一烷-1-硫醇(AZO-11)自组装单分子膜在蒸发金膜上的光致异构化行为。与AZO11在溶液中的异构化相比，自组装的AZO11分子的反式顺式光异构化受到限制。此外，从顺式AZO11分子截面积大于反式AZO11分子截面积可知，顺式到顺式的异构化比顺式到反式的异构化受到更大的抑制。在此基础上，我们认为顺式样品总体上更适合于顺式反式光异构化反应，以控制液晶的分子取向。这确实在我们的实验中得到了证实，在顺式和反式AZO11表面沉积了向列相液晶(4-氰基-4‘-戊基联苯：5CB)的薄膜。红外反射吸收光谱显示，5CB分子倾向于与其长轴垂直于反式AZO11层的平面排列。然而，由于部分转化为顺式异构体，这种比对受到紫外线照射的影响。这一事实表明，液晶的取向可以通过改变衬底表面的分子形状来控制。在被吸附的AZO11和十六硫醇(C&lt；16&gt；H&lt；33&gt；SH)的混合物覆盖的金膜表面也进行了类似的红外光谱测量。在这种情况下，AZO11光异构化的限制被放宽，观察到液晶排列发生了较大的变化。同时也展示了红外透射率和拉曼散射测量在银岛薄膜上的应用。

The photoisomerization of self-assembled monolayer films of 4-(4'-(methoxyphenylazo)phenoxy)undecane- 1-thiol (AZO 11) on evaporated gold films was investigated by means of Raman scattering and UV-visible and IR absorption measurements. The trans * cis photoisomerization of the self-assembled AZO11 molecules were restricted in comparison to the isomerization of AZO11 in solutions. Moreover, the trans-to-cis isomerization was more suppressed than the cis-to-trans isomerization, as expected from the molecular cross section of cis AZO11 that is larger than trans AZO11. On this basis we propose that cis-form samples are on the whole more suitable for cis trans photoisomerization in order to control the molecular alignment of liquid crystals. This indeed was verified in our experiments where thin films of a nematic liquid crystal (4-cyano-4'-pentylbiphenyl : 5CB) were deposited on the cis and trans AZO11 surface layers. IR reflection absorption spectroscopy revealed that 5CB molecules tend to align with their long axes perpendicular to the plane of the trans AZO11 layer. However, this alignment was affected by UV irradiation due to partial conversion to cis isomer. This fact demonstrates that liquid crystal alignment can be controlled by changes in the molecular shape on the substrate surface. Similar IR measurements were performed on gold film surfaces covered with a mixture of adsorbed AZO11 and hexadecanethiol (C_<16>H_<33>SH). in this case the limitation of AZO11 photoisomerization was relaxed and a larger change in the liquid crystal alignment was observed. The utility of IR transmission and Raman scattering measurements on silver island films was also demonstrated.