(Invited) Engineering Molecular Solids and Monolithic Thin Films from Chromophoric Building Blocks: Model Systems and Integration into Devices
Authors: Woell Christof 
Published: September 2023

Functioning devices exploiting the properties of organic chromophores, e.g. in the context of photovoltaics, organic electronics, light-emitting devices, and photodetectors, often rely on the assembly of the molecules into appropriate solids and their interfacing to electrodes. In this context, recently the MOF-based approach [1] is receiving increasing attention. In this case, the organic chromophores (anthracene, pentacene, naphthalene-diimides, perylene-diimides, porphyrins, phthalocyanines, ...) are equipped with at least two coupling units so as to yield ditopic linkers suited for the coupling to metal- or metal-oxo nodes, thus yielding metal-organic frameworks (MOFs). Although the standard form of MOFs, powders, are not well suited for the integration of the arrayed chromophores into devices, in the past decade a number of methods able to yield high quality, monolithic MOF thin films deposited on a variety of substrates have been developed. Sophisticated fabrication methods, including layer-by-layer approaches yield films of impressive optical quality.[2]
Optical absorption properties of condensed chromophores are often strongly affected by inter-molecular interactions. The periodic structure of MOFs provides the basis for a rational design of such assemblies, and e.g. J-aggregates have been first simulated and then realized experimentally.[3] In addition, band-structure effects like indirect band gaps can be realized in such periodic arrangements.[4] Finally, the implementation of chiral linkers allows to realize chiroptical phenomena, including the emission of circular polarized light and helicity-sensitive photodetectors.[5]
Lbl architectures also allow the fabrication of heterostructures with built-in interfaces,[6] with interesting application e.g. in the context of photon up-conversion. A particularly attractive option is to exploit the porous nature of the MOFs in connection with the lbl method. An interesting example is the loading of C60 into the pores of SURMOFs and the subsequent integration into devices, e.g. in the context of photoconductivity [7] or organic diodes.[8] In the context of nonlinear optical properties, using sophisticated assembly strategies were recently used to fabricate non-centrosymmetric SURMOFs with built-in electric fields and large SHG activity.[9]
In this presentation we well present selected successful examples of integrating organic chromophores into SURMOFs and the subsequent fabrication of functioning devices. We will in particular highlight the close interaction with theory. Such an advanced in-silico conquering of chemical spaces is mandatory, since the number of known MOFs (> 100.000) is so large that experimental trial-and-error approaches are highly inefficient.

References:

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[2] L. Heinke, Ch. Wöll, Adv. Mater., 31, 1970184 (2019)
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