NLO and opto-optic switching of guest-loaded SURMOF-layers: Second-order optical effects

在过去的几个月里，我们已经准备了各种基于SURMOF的主客体系统，并在非线性光学中对它们进行了评估。采用不同的气相和液相负载技术，将不同的光学活性分子作为客体引入到不同的SURMOF孔中，在不同的载体上制备了纳米复合材料。采用溶剂热合成法、喷涂法、浸渍法和界面合成法制备了厚度为1µm的SURMOF层。然而，这些准备工作散落在光芒中。为了得到<lt；1微米的光学透明无散射SURMOF层，我们开发了“控制注入技术”。在负载了光学活性客体分子后，我们测量了产生二次谐波(SHG)、和频(SFG)、差频(DFG)等非线性光学效应的二阶非线性极化率X_i(2)(通过Pockels效应)。令人惊讶的是，我们发现了巨大的气(2)数据。在FTO玻璃上的亚微米Cu-MOF-74SURMOF膜的一维孔道中测得对硝基苯胺(PNA)的最高值为337 pm/V。到目前为止，这种巨大的chi(2)值是未知的；最高实验值及其理论解释要小一个数量级。很可能，这些巨值只能通过PNA在Cu-MOF-74的一维孔中的一种特殊的吸收机制来解释：每个PNA分子必须以一定的取向进入孔中，从而在一维孔内形成稳定的偶极链。然而，理论估计表明，这种平行偶极链的几何形状只能给出一个高的chi(2)值，大约比测量的值小一个数量级。这意味着，在一维Cu-MOF-74中，PNA的定向偶极链结构域的存在不能单独解释巨大的X(2)值。因此，必须有额外的机制来解释巨大的CHI(2)数据，例如(I)完全H偶联的链，或(Ii)与PNA分子的TO偶极场相反的MOF晶格中的反偶极场。因此，该项目的目的将是从分子上了解定向吸收和由此产生的巨大CHI(2)值，并开发一个工具箱，用于结合合适的载体(FTO、ITO、硅晶片)、SURMOF结构、制备技术和染料负载(气相或液态)的高CHI(2)材料。所得到的非线性光学材料将在(I)普克尔斯效应、(Ii)克尔效应、(Iii)光-光开关中进行测试。

During the last few months, we have prepared a variety of SURMOF-based host guest systems and evaluated them in non-linear optics. The nano-composites were prepared on different supports by bringing different optically active molecules as guests into the pores of different SURMOFs by using different loading techniques from gas or liquid phase. The SURMOF layers of < 1 µm thickness were prepared by solvothermal synthesis, spraying, dipping and interfacial synthesis. However, these preparations scattered light. To get optically transparent non-scattering SURMOF layers of < 1 µm, we developed the „controlled injection technique“. After loading with optically active guest molecules, we measured the 2nd-order non-linear susceptibility chi(2) (via the Pockels effect) which is responsible for such non-linear optical effects like Second Harmonic Generation (SHG), Sum Frequency Generation (SFG), Difference Frequency Generation (DFG). Surprisingly, we found giant chi(2) data. The highest value of 337 pm/V was measured for p-nitroaniline (pNA) in the 1D pores of a sub-µm Cu-MOF-74 SURMOF layer on FTO glass. Such giant chi(2) values are so far unknown; highest experimental values and their theoretical explanation are one order of magnitude smaller. Very likely, the giant values can be only explained by a special uptake mechanism of the pNA into the 1D pores of Cu-MOF-74: Every pNA molecule must enter the pore in a certain orientation thus forming stable dipole chains inside the 1D pores. However, a theoretical estimate shows that this geometry of parallel dipole chains can give only a high chi(2) value which is about one order of magnitude smaller than the measured one. This means, the existence of domains of oriented dipole chains of pNA in the 1D Cu-MOF-74 alone cannot explain the giant chi(2) values. Therefore, there must be additional mechanisms to explain the giant chi(2) data such as (i) perfectly H-coupled chains, or (ii) counter dipole fields in the MOF lattice opposite the to dipole field of the pNA molecules. It will be, therefore, the aim of the project to give a molecular understanding of the oriented uptake and the resulting giant chi(2) values, and to develop a Tool Box for high chi(2) materials combining suitable supports (FTO, ITO, Si wafer), SURMOF structures, preparation techniques, and dye loading (from gas or liquid phase). The resulting non-linear optical materials will be tested in (i) Pockels effect, (ii) Kerr effect, (iii) opto-optical switching.