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Ethyne-linked push-pull chromophores: Implications of crystal structure and molecular electronics on the quadric nonlinear activity Paul Kautny, Helene Kriegner, Dorian Bader, Michal Dusek, Georg Reider, Johannes Fröhlich, and Berthold Stöger Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00404 • Publication Date (Web): 21 Jun 2017 Downloaded from http://pubs.acs.org on June 26, 2017
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Crystal Growth & Design
Ethyne-linked push-pull chromophores: Implications of crystal structure and molecular electronics on the quadric nonlinear activity Paul Kautny,†,* Helene Kriegner,† Dorian Bader,† Michal Dušek,‡ Georg A. Reider,§ Johannes Fröhlich,† Berthold Stögerǁ †
Institute of Applied Synthetic Chemistry, TU Wien, Getreidemarkt 9/163, A-1060 Vienna,
Austria ‡
Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 18221 Prague 8, Czech
Republic §
ǁ
Photonics Institute, TU Wien, Gußhausstraße 27-29, A-1040 Vienna, Austria
X-Ray Centre, TU Wien, Getreidemarkt 9, A-1060 Vienna, Austria
Three ethyne-linked push-pull materials consisting of a dimethylaniline donor and acceptors of increasing electron withdrawing strength were prepared as nonlinear optical chromophores. Despite a high similarity of the molecular structures, all three materials featured unrelated noncentrosymmetric crystallization behavior. One of the compounds exhibits a remarkable packing with Z´ = 16 molecules in the asymmetric unit forming two interpenetrating subsystems. Inspection of the nonlinear optical activity of single crystalline powders revealed an
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extraordinarily efficient second harmonic generation of one of the materials with a 1200 fold increased second harmonic response compared to potassium dihydrogen phosphate (KDP). The second harmonic generation efficiency of the materials under investigation has been related to both the intrinsic molecular properties, as well as the alignment of the individual chromophores within the crystal packing, highlighting the importance of a multidisciplinary approach to understand the properties of nonlinear optical materials.
INTRODUCTION π-conjugated organic donor-acceptor materials have been extensively investigated in the last couple of years due to manifold technologically relevant applications. Various push-pull derivatives consisting of an electron rich (donor) and an electron withdrawing (acceptor) subunit have been prepared for organic photovoltaics (OPVs),1, 2 nonlinear optical (NLO) materials,3-8 imaging9,
10
and organic light emitting diodes (OLEDs)11-13 to name a few. Nonlinear optical
materials are of particular importance for the telecommunication technology and quantum electronics. Nonlinear effects result in various phenomena, such as second harmonic generation (SHG), optical three-wave mixing, parametric amplification, the electro-optic effect and downconversion.14 The donor-acceptor architecture of bipolar organic materials provides the potential for a high macroscopic second-order susceptibility due to the high polarizability of the delocalized π-electrons on the molecular scale.7,
15, 16
Beside the molecular properties, the
macroscopic SHG of organic crystals is dependent on the condition that crystallization occurs in one of the non-centrosymmetric crystal classes (except 432).17 Unfortunately, the strong dipoledipole interaction between single bipolar molecules often leads to undesired antiparallel packing resulting in the occurrence of an inversion center and thus centrosymmetric crystallization or at least in almost complete cancellation of the individual polarization.18, 19
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Recently, we reported a set of 1,2,3-triazole and benzene-linked donor-acceptor materials and found significant intramolecular charge transfer upon photoexcitation in this class of compounds. Furthermore, we investigated the NLO properties of selected non-centrosymmetric crystalline powders of these materials and observed high SHG efficiencies up to 80 times the value of KDP.20 In this work we present a related series of three push-pull materials based on an N,Ndimethylaniline donor and different acceptors with increasing electron withdrawing properties (benzene
