For the fabrication of macroscopic devices exploiting the properties of organic chromophores, the availability of well-defined thin films of high optical quality is an important prerequisite. Optical absorption properties are often strongly affected by inter-chromophore interactions, and therefore periodic assemblies with low defect densities offer substantial advantages over amorphous compounds. In this context, crystalline reticular networks have recently attracted considerable interest. By appropriate functionalization, most organic chromophores can be converted to function as di- or higher-topic linkers, which can coordinate to metal or metal-oxo centers so as to yield stable, metal-organic frameworks, or MOFs. Of particular interest in this context are light-switchable MOFs. However, the conventional powder form of MOFs obtained using solvothermal approaches, exhibits substantial drawbacks with regard to optical applications, e.g. limited efficiency resulting from absorption and light scattering caused by the (micrometer-sized) powder particles. Providing MOFs in the form of thin films avoids these problems. We have developed a layer-by-layer (lbl) deposition method to produce well-defined, highly oriented and monolithic MOF thin films on a number of different substrates. The resulting films are referred to as SURMOFs [1]. The fabrication of hetero-multilayers containing well-defined hetero-interfaces is rather straightforward with this lbl method. A particularly attractive option it to exploit the porous nature of the MOFs in connection with the lbl method to integrate C60 into SURMOF-based devices, e.g. in the context of photoconductivity [2]. In the context on nonlinear optical properties, using sophisticated assembly strategies were recently used to fabricate non-centrosymmetric SURMOFs with high SHG activity [3].

References:

[1] R. Haldar, L. Heinke, Ch. Wöll, Advanced Materials (2020), DOI: 10.1002/adma.201905227

[2] X. Liu, M. Kozlowska, T. Okkali, D. Wagner, T. Higashino, G. Brenner-Weiß, S. M. Marschner, Z. Fu, Q. Zhang, H. Imahori, S. Bräse, W. Wenzel, C. Wöll, L. Heinke, Angew. Chem. Int. Ed. 2019, 58, 9590

[3] A. Nefedov, R. Haldar, Z. Xu, H. Kühner, D. Hofmann, D. Goll, B. Sapotta, S. Hecht, M. Krstić, C. Rockstuhl, W. Wenzel, S. Bräse, P. Tegeder, E. Zojer, Ch. Wöll, Adv. Mat. 2103287 (2021)