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      Giant intrinsic circular dichroism of prolinol-derived squaraine thin films

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          Abstract

          Molecular chirality and the inherently connected differential absorption of circular polarized light (CD) combined with semiconducting properties offers great potential for chiral opto-electronics. Here we discuss the temperature-controlled assembly of enantiopure prolinol functionalized squaraines with opposite handedness into intrinsically circular dichroic, molecular J-aggregates in spincasted thin films. By Mueller matrix spectroscopy we accurately probe an extraordinary high excitonic circular dichroism, which is not amplified by mesoscopic ordering effects. At maximum, CD values of 1000 mdeg/nm are reached and, after accounting for reflection losses related to the thin film nature, we obtain a film thickness independent dissymmetry factor g = 0.75. The large oscillator strength of the corresponding absorption within the deep-red spectral range translates into a negative real part of the dielectric function in the spectral vicinity of the exciton resonance. Thereby, we provide a new small molecular benchmark material for the development of organic thin film based chiroptics.

          Abstract

          The use of chiral molecules for optoelectronics remains underexplored. Here, the authors perform Mueller matrix spectroscopy on thin films of enantiopure prolinol functionalized squaraines and report an extraordinarily high circular dichroism, thus providing a benchmark material for chiroptical applications.

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          J-aggregates: from serendipitous discovery to supramolecular engineering of functional dye materials.

          J-aggregates are of significant interest for organic materials conceived by supramolecular approaches. Their discovery in the 1930s represents one of the most important milestones in dye chemistry as well as the germination of supramolecular chemistry. The intriguing optical properties of J-aggregates (in particular, very narrow red-shifted absorption bands with respect to those of the monomer and their ability to delocalize and migrate excitons) as well as their prospect for applications have motivated scientists to become involved in this field, and numerous contributions have been published. This Review provides an overview on the J-aggregates of a broad variety of dyes (including cyanines, porphyrins, phthalocyanines, and perylene bisimides) created by using supramolecular construction principles, and discusses their optical and photophysical properties as well as their potential applications. Thus, this Review is intended to be of interest to the supramolecular, photochemistry, and materials science communities. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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            Supramolecular Chirality in Self-Assembled Systems.

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              The spectral signatures of Frenkel polarons in H- and J-aggregates.

              Electronic excitations in small aggregates, thin films, and crystals of conjugated organic molecules play a fundamental role in the operation of a wide array of organic-based devices including solar cells, transistors, and light-emitting diodes. Such excitations, or excitons, are generally spread out over several molecules: a balance between the delocalizing influence of resonant intermolecular coupling and the localizing influence of static and dynamic disorder determines the coherence range of the exciton. Because of the "soft" nature of organic materials, significant nuclear relaxation in the participating molecules also accompanies the electronic excitations. To properly understand energy or charge transport, one must treat intermolecular (excitonic) coupling, electron-vibrational coupling, and disorder on equal footing. In this Account, we review the key elements of a theoretical approach based on a multiparticle representation that describes electronic excitations in organic materials as vibronic excitations surrounded by a field of vibrational excitations. Such composite excitations are appropriately called Frenkel excitonic polarons. For many conjugated molecules, the bulk of the nuclear reorganization energy following electronic excitation arises from the elongation of a symmetric vinyl stretching mode with energy approximately 1400 cm(-1). To appreciate the impact of aggregation, we study how the vibronic progression of this mode, which dominates the isolated (solvated) molecule absorption and emission spectra, is distorted when molecules are close enough to interact with each other. As we demonstrate in this Account, the nature of the distortion provides a wealth of information about how the molecules are packed, the strength of the excitonic interactions between molecules, the number of molecules that are coherently coupled, and the nature of the disorder. We show that the aggregation-induced deviations from the Poissonian distribution of vibronic peak intensities take on two extremes identified with ideal H- and J-aggregates. The sign of the nearest neighbor electronic coupling, positive for H and negative for J, distinguishes the two basic aggregate forms. For several decades, researchers have known that H-aggregates exhibit blue-shifted absorption spectra and are subradiant while J-aggregates exhibit the opposite behavior (red-shifted absorption and superradiance). However, the exact inclusion of exciton-vibrational coupling reveals several more distinguishing traits between the two aggregate types: in H(J)-aggregates the ratio of the first two vibronic peak intensities in the absorption spectrum decreases (increases) with increasing excitonic coupling, while the ratio of the 0-0 to 0-1 emission intensities increases (decreases) with disorder and increases (decreases) with increasing temperature. These two extreme behaviors provide the framework for understanding absorption and emission in more complex morphologies, such as herringbone packing in oligo(phenylene vinylene)s, oligothiophenes and polyacene crystals, as well as the polymorphic packing arrangements observed in carotenoids.
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                Author and article information

                Contributors
                manuela.schiek@uni-oldenburg.de
                Journal
                Nat Commun
                Nat Commun
                Nature Communications
                Nature Publishing Group UK (London )
                2041-1723
                20 June 2018
                20 June 2018
                2018
                : 9
                : 2413
                Affiliations
                [1 ]ISNI 0000 0001 2240 3300, GRID grid.10388.32, Kekulé Insitute of Organic Chemistry and Biochemistry, , Rheinische-Friedrich-Wilhelms-University of Bonn, ; Gerhard-Domagk-Str. 1, D-53121 Bonn, Germany
                [2 ]ISNI 0000 0001 1009 3608, GRID grid.5560.6, Energy and Semiconductor Research Laboratory, Institute of Physics, , Carl-von-Ossietzky-University of Oldenburg, ; Carl-von-Ossietzky-Str. 9-11, D-26129 Oldenburg, Germany
                [3 ]ISNI 0000 0001 0728 0170, GRID grid.10825.3e, Mads Clausen Institute, University of Southern Denmark, ; Alsion 2, DK-6400 Sønderborg, Denmark
                [4 ]ISNI 0000 0004 1937 0247, GRID grid.5841.8, Department of Applied Physics and IN2UB, , University of Barcelona, ; Barcelona, 08028 Spain
                Author information
                http://orcid.org/0000-0002-6228-6839
                http://orcid.org/0000-0003-4429-0823
                http://orcid.org/0000-0002-0108-2998
                Article
                4811
                10.1038/s41467-018-04811-7
                6010436
                29925832
                82dc3f6f-b1f5-41c7-a185-fccf3e868c63
                © The Author(s) 2018

                Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

                History
                : 21 December 2017
                : 18 May 2018
                Funding
                Funded by: FundRef https://doi.org/10.13039/501100001659, Deutsche Forschungsgemeinschaft (German Research Foundation);
                Award ID: GRK1885
                Award Recipient :
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