Enlightening Materials with Photoswitches

REVIEW www.advmat.de Enlightening Materials with Photoswitches Alexis Goulet-Hanssens,* Fabian Eisenreich,* and Stefan Hecht* reversible conversion between their stable Incorporating molecular photoswitches into various materials provides and (meta)stable isomers triggered by unique opportunities for controlling their properties and functions with high light. The return to their native, thermo spatiotemporal resolution using remote optical stimuli. The great and largely dynamically most stable isomer is gener ally induced by using another wavelength still untapped potential of these photoresponsive systems has not yet been of light or simply by heat, depending on fully exploited due to the fundamental challenges in harnessing geometrical the thermal stability of the (meta)stable and electronic changes on the molecular level to modulate macroscopic and isomer. Retinal—the prototypical photo bulk material properties. Herein, progress made during the past decade in switch in biology—showcases impressively the field of photoswitchable materials is highlighted. After pointing to some how structural changes occurring during a lightinduced doublebond isomerization general design principles, materials with an increasing order of the integrated can lead to vision or allow microorganisms photoswitchable units are discussed, spanning the range from amorphous to convert photons into metabolic energy. settings over surfaces/interfaces and supramolecular ensembles, to liquid In analogy, over the years, scientists have crystalline and crystalline phases. Finally, some potential future directions are learned that the lightdriven isomeriza pointed out in the conclusion. In view of the exciting recent achievements tion of small molecules can be collected, in the field, the future emergence and further development of light-driven possibly amplified, and transformed into macroscopic property changes, such as and optically programmable (inter)active materials and systems are eagerly mechanical motion,[1] charge carrying anticipated. ability,[2] assembly and disassembly of macroscopic aggregates,[3] as well as switching of surfaces and their properties.[4] 1. Introduction Materials, which for the purpose of this review are consid ered to be solid objects or organized assemblies in solution, The incorporation of photoswitchable molecules into materials provide unique advantages to enhance the function of photo has been proven to be a blueprint to collectively, and thus effec switches and even create new functions when compared to tively, capitalize on their individual lightinduced (re)actions. their standalone molecular selves (for the primarily discussed Photoswitches are photochromic molecules, which undergo a photochromic molecules herein, see Figure 1). One must, how ever, consider the inherent challenges that may present them selves when working with photoswitches in close proximity to Dr. A. Goulet-Hanssens, Dr. F. Eisenreich,[+] Prof. S. Hecht one another. Intermolecular interactions, such as aggregation, Department of Chemistry & IRIS Adlershof excitation quenching, low freevolume, as well as the poten Humboldt-Universität zu Berlin Brook-Taylor-Str. 2, 12489 Berlin, Germany tially much higher optical density of materials can drastically E-mail: [email protected]; [email protected]; lower the overall photoresponse. Yet with proper macromo [email protected] lecular and supramolecular design, functions can be achieved Prof. S. Hecht through the incorporation of photoswitches in a material that DWI - Leibniz Institute for Interactive Materials simply cannot be obtained by an isolated molecule on its own. Forckenbeckstr. 50, 52056 Aachen, Germany Here, we highlight recent examples from the literature that have E-mail: [email protected] appeared during the past decade since our previous review in Prof. S. Hecht [5] Institute of Technical and Macromolecular Chemistry this journal, with particular focus on the order of the photos RWTH Aachen University witches in relation to their environment and its impact on mate Worringer Weg 2, 52074, Aachen, Germany rial properties and device performance. The ORCID identification number(s) for the author(s) of this article Conceptually, one can think about maximizing a photo can be found under https://doi.org/10.1002/adma.201905966. switching unit’s effect by giving it “order” with relation to its [+]Present address: Institute for Complex Molecular Systems, Laboratory surrounding, in particular to other switching units (Figure 2). of Macromolecular and Organic Chemistry, Eindhoven University The lowest possible order is undoubtedly demonstrated by dis of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands persing photochromic molecules in a bulk amorphous material. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, If aggregation of the photoswitches can be overcome, switching Weinheim. This is an open access article under the terms of the Creative units are placed in random orientation to each other giving Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. rise to a more or less isotropic material. Starting from this scenario of randomly distributed photochromic entities, one DOI: 10.1002/adma.201905966 can think about providing order in two dimensions by placing Adv. Mater. 2020, 1905966 1905966 (1 of 23) © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.advmat.de photoswitches on a surface, where their position is fixed with relation to the underlying substrate but not necessarily with Alexis Goulet-Hanssens respect to each other. Whether being a flat, patterned, or nano received his Ph.D. in 2014 particular surface a conformal coating with lightresponsive from McGill University moieties at an interface can be achieved. Introducing order on under the supervision a 2D surface by noncovalent or covalent anchoring provides of Prof. Christopher Barrett, physi and chemisorbed (mono)layers, respectively. Expanding with whom he worked on into three dimensions longrange order can be provided by developing novel materials organizing the switching entities in supramolecular systems, that use light to guide cell where noncovalent interactions and packing effects confine growth. He subsequently was them either in aggregates in solution, in liquid crystalline a postdoctoral fellow in the phases, or in the bulk. Finally, the highest degree of order is group of Prof. Stefan Hecht reached in crystalline materials, where a precise and longrange at Humboldt-Universität orientation of the photoswitches is present in all three dimen zu Berlin, where he worked on the electrochemistry of sions. This can be achieved by either crystallizing the photo azobenzene systems and light-responsive metal–organic switchable molecule itself or in the presence of a nonswitchable frameworks. matrix, which results either in porous or nonporous bulk (co) crystals. In the following, we will structure the discussion along this theme of increasing order of the photoswitchable units Fabian Eisenreich studied within the materials. chemistry at Humboldt- Universität zu Berlin, where he subsequently obtained his doctoral degree under 2. Photoswitches in an Amorphous Setting the guidance of Prof. Stefan Although the dispersion of photochromic molecules into an Hecht in 2019. During his amorphous environment is seemingly the most straightforward doctoral research, which method to create photoresponsive materials, it requires a profound was focused on controlling knowledge of the physical and/or chemical interplay between the polymerization processes photoswitching entity and its surroundings to obtain the desired with light, he also worked in effect upon illumination with light. Crucial hurdles such as phase the laboratory of Prof. Craig segregation, selfaggregation, or high optical density must be Hawker at the University of California, Santa Barbara. overcome to guarantee an efficient photochemical performance. Currently, he is a Feodor Lynen postdoctoral fellow Once these requirements are fulfilled, photoswitchable molecules of the Alexander von Humboldt-Foundation in the group can be utilized for a variety of applications, which range from of Prof. Bert Meijer at Eindhoven University of Technology, electronic and memory devices to healable plastics. working on photoredox-active polymeric nanoparticles. Stefan Hecht studied 2.1. Modulating Charge Transport chemistry at Humboldt- Universität zu Berlin and One of the bulk properties that can readily be controlled with obtained his Ph.D. from the molecular photoswitches is conductivity based on the material’s University of California at [2] charge carrying capacity. Small molecule dopants have exten Berkeley in 2001, working sively been used for years to affect the electronic structure of a under the guidance of given semiconductor by acting as traps for charges, thereby Prof. Jean M. J. Fréchet. After modulating the conductance of the material (Figure 3). Blending establishing his own research a photoswitchable molecule, such as a diarylethene (DAE) deriva group at Freie Universität tive with largely different HOMO and LUMO levels in its ring Berlin and a subsequent open and ringclosed isomers, respectively, into a film composed position as a group leader at of either ptype or ntype organic semiconductors permits gating the Max-Planck-Institut für Kohlenforschung

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