(Invited) Metal-Organic Frameworks As Prospective Materials in Photonic Applications
Authors: Fischer Roland A 
Published: February 2020

Integrated photonics, using photons as information carriers on the macro -or nanosize region, has a tremendous potential to overcome the fundamental speed limitation in electronics and therefore can create new techniques, important for telecommunication, information processing, data storage or photonic computing. A key factor for the development of advanced photonic applications is the manipulation and conversion of photons. This, however, necessitates sophisticated solid-state materials which combine molecular tailoring and flexibility with low-cost solution processable thin films for device integration at the one side and extending photo-physical properties at the other side. Multi-photon absorption (MPA), amplified stimulated emission (STE), wave-guiding are fundamental processes in the specific modulation of light and therefore of immense importance for the fabrication of various photonic devices at different sizes.
In particular, these processes have been studied in a number of material classes including single molecules, inorganic (nano)particles or organic polymers. Nevertheless, limiting factors include critical issues such as of concentration quenching, wavelength tunability, photo-stability, transparency or defect induced losses, if not even synthetic access and toxicity. In this regard, solid-state hybrid materials, namely metal-organic frameworks (MOFs) and coordination polymers (CPs), appear to be highly promising novel photonic materials, which could overcome such challenges.
The coordination of an optically active linker to a metal-node in a CP often increases the photo-physical response induced by the weakening of radiation-less decay mechanisms or by cooperative enhancement effects between adjacent chromophores or metal nodes. The restrictive conformation of the linker between metal centers impacts the polarizability of the latter, which plays also a significant role in the photo-physical response. The building-block concept of CPs offers a huge space for rational control over such parameters. This highlights the uniqueness of MOFs compared to other photoactive materials and therefore, MOFs are supposed to be among the most promising material class exploiting photonic properties in the solid-state by a thorough choice of organic linkers and metal nodes. Furthermore, the possibility to process MOFs as thin films makes them a perfect candidate for device integration, which is a prerequisite for materials to be used in technological applications.
We recently demonstrated exceptionally high two-photon absorption (2PA) action cross-section values alongside with intrinsic STE with low threshold energy from electron rich tetrakis-phenylethylene based MOFs. In a joined theoretical and experimental venture, we further derived and probed a quantitative model to predict the MPA properties of these materials, including important parameters such as charge polarization, length of π-system, fluorescence quantum yield, alignment and packing density as well as the conformation of the chromophores inside the MOF structure. We also showed that the 2PA response of MOFs can either be tuned by linker functionalization using acceptor groups or by an intermolecular approach through the interaction of different linkers via space. Moreover, we quantified below bandgap absorption on micrometer-sized MOF 2D-plates acting as low-loss optical waveguides, where the light is passed from linker to linker in a radiation-less energy transfer process.
The design of exceptionally high performing and optimized optically active MOFs in photonic applications needs a thorough understanding of the structure-property relationship to guide the required MOF crystal engineering. We are successively expanding the number of MPA active linkers and MOFs at the moment to deepen our understanding of this (non-)linear optically active material class both on an experimentally and theoretically level. Our results impressively demonstrate that MOFs are a highly interesting material class in photonic research and therefore can constitute to future techniques such as quantum computing, high density data storage or ultra-fast data transmission and processing.

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