COMMUNICATION 1701220 (1 of 7) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advmat.de Ink-Free Reversible Optical Writing in Monolayers by Polymerization of a Trifunctional Monomer: Toward Rewritable “Molecular Paper” Vivian Müller, Tim Hungerland, Milos Baljozovic, Thomas Jung, Nicholas D. Spencer, Hadi Eghlidi, Payam Payamyar,* and A. Dieter Schlüter* V. Müller, Dr. T. Hungerland, Dr. P. Payamyar, Prof. A. D. Schlüter Laboratory of Polymer Chemistry Swiss Federal Institute of Technology (ETH Zurich) Vladimir-Prelog-Weg 5, Zurich 8093, Switzerland E-mail: payamyar@g.harvard.edu; dieter.schluter@mat.ethz.ch M. Baljozovic, Prof. T. Jung Laboratory for Micro- and Nanotechnology Paul Scherrer Institute 5232 Villigen, Switzerland Prof. N. D. Spencer Laboratory for Surface Science and Technology ETH Zurich Vladimir-Prelog-Weg 5, Zurich 8093, Switzerland Dr. H. Eghlidi Laboratory of Thermodynamics in Emerging Technologies Department of Mechanical and Process Engineering ETH Zurich Sonneggstrasse 3, 8092 Zurich, Switzerland The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201701220. DOI: 10.1002/adma.201701220 great success so far, [1] this quest is still in its early phases for the world of optics. The challenge of reaching device sizes well below the diffraction limit of light has been reported in a handful of exam- ples. [2,3] Considering that in 2007, 28% of the global information storage [4] con- sisted of optical data, [5] it is crucial to aim at miniaturized optical devices for various integrated functionalities such as guiding, multiplexing, amplification, and recording of optical energy and signals. Photosensitive materials that combine extreme thinness and macroscopic lateral size with reversible writability offer great potential for integrated optical memories and recording components. [6] This could allow for implementation of such sensitive layers with other planar miniature optical components such as meta-lenses and waveguides. [2,3] In order to show writability, the sensitive layers should exhibit, among others, a measurable optical change upon exposure to light with a reasonable stability of the written pattern. Recently, we observed that compressed monolayers (MLs) of monomer 1 (Figure 1a) at an air/water interface lose their (excimer) fluorescence when exposed to light of an appro- priate wavelength. [7] This loss was combined with the absence of emission from unpaired anthracene units (A) and was thus taken as evidence for 2D polymerization, during which all the photosensitive A units in the ML are converted into the corre- sponding dimers by [4+4]-cycloaddition reactions. If this inter- pretation holds true, MLs of monomers such as 1 hold promise in connection with the above-mentioned goal, since not only do they exhibit a measurable optical change (the disappearance of fluorescence) but they can also be obtained in macroscopic lateral extensions and with molecular-level thickness. However, other issues are also of concern in reaching such goals. They include whether (a) the bleaching can only be performed at an air/water interface or, for very practical reasons, also on a solid substrate (in a device), (b) the bleaching can be confined to a predetermined area, (c) the optical change is durable, and (d) the fluorescence quenching is reversible. From a chemistry point of view, the issue of reversibility is particularly complex and requires looking into the chem- ical reaction that is responsible for the observed fluorescence quenching and whether this reaction itself is reversible. While A Langmuir–Blodgett film consisting of a dense array of trifunctional mono- mers bearing three 1,8-diazaanthracene units is polymerized at an air/water interface or after transfer on solid substrates. The transfer does not affect the excimer fluorescence of the film, indicating that the monomers’ packing with their diazaanthracene units stacked face-to-face is retained—a prerequisite for successful polymerization. The monomer film can be polymerized in con- fined areas on solid substrates by UV irradiation with a confocal microscope laser. The underlying chemistry of the polymerization, a [4+4]-cycloaddition of the diazaanthracene units, leads to disappearance of the fluorescence in the irradiated regions which enables writing into the monolayer on a μm scale— thus the term “molecular paper.” The reaction can be reversed by heating which leads to a recovery of the fluorescence and to erasing of the writing. Alternative pathways for this phenomenon are discussed and control experi- ments are conducted to rule them out. 2D Polymers Miniaturization of electronic and optical devices remains a high-priority task for the science and engineering communi- ties. Numerous advantages including energy saving, enhanced sensitivity, improved storage capacity as well as integrated func- tionalities become possible as a consequence of the ultracom- pact size of the components implemented into optoelectronic devices. While reaching nano sizes in electronics has had Adv. Mater. 2017, 1701220