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

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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.