Promoting Spontaneous Second Harmonic Generation through Organogelation

Abstract

-An organogelator based on the Disperse Red NLO-phore was synthesized according to a simple and efficient three-step procedure. The supramolecular gel organization leads to xerogels which display a spontaneous second harmonic generation (SHG) response without any need for pre-processing and this SHG activity appears stable over several months. These findings, based on an intrinsic structural approach are supported by favorable intermolecular supramolecular interactions, which promote a locally noncentrosymmetric NLO-active organization. This is in sharp contrast with most materials designed for SHG purposes, which generally require the use of expensive or heavy-tohandle external techniques for managing the dipoles alignment.Tremendous efforts are continuously dedicated to the development of better organic and SHG active materials for various applications in the fields of photonics, 1 optoelectronics, 2,3 or anti-counterfeiting. 4 Two major requirements have to be fulfilled in order to prepare these materials. Firstly, the corresponding molecules have to display strong hyperpolarizabilities. In this direction, hundreds of publications report the synthesis and characterizations of organic push-pull conjugated NLO-phores. 5,6 Secondly, the latter have to be organized in a non-centrosymmetric fashion in order to prevent destructive interferences between second harmonicgenerated photons. As a consequence, various approaches have been successfully considered in order to prepare SHGactive materials. These include the utilization of chiral compounds, 7,8 Langmuir-Blodgett films, 9 self-assembled layers, 10,11 elaborated matrices, 12 or corona poling. 13 Though efficient, these strategies require expensive conditions (i.e. chiral NLO-phores) or heavy-to-handle pre-treatments (e.g. corona poling) that impede their transfer at larger scales. Therefore, it appears of utmost interest to develop new methodologies to process SHG-active materials, by designing derivatives with specific functions able to promote the required organization for instance. The present work falls within this approach with the utilization of an organogelator. Gel-based materials have already shown their strong potential in the context of optoelectronics and photonics, 14-17 notably thanks to their high degree of organization at the supramolecular scale and their user-friendly processing. Various external methodologies have been developed in the past to promote the alignment of the micro-and nanofibers responsible for the gel state, be that through application of electric or magnetic fields along their formation or for instance shear stress.18-21 Conversely, our approach consisted in constraining SHG-active units in the desired non-centrosymmetric headto-head arrangement without any additional external mediation. This could be accomplished thanks to the use of urea moieties, well-known for both their narcissistic character in supramolecular chemistry and their ability to promote gelation.22 This proof-of-concept study was led on the Disperse Re...

Supramolecular control over the structural organization of a second-order NLO-active organogelator

“…devoid of auxiliary organizing substituents, drop-casted from a toluene solution. 11 Even though no gelation could be observed with compound 2, this observation illustrates the prominent role of the tris(alkoxy)benzamide and tris(alkoxy)benzoester units in directing the selfassembly process.…”

“…The experimental setup for recording the SHG images was used as described previously. 11 Both samples prepared from compounds 1 and 2 showed SHG responses in accordance with a local non-centrosymmetric organization and whose distribution is reminiscent of the anisotropic organization of the respective corresponding materials (Fig. 4b, d and 5; Fig.…”

“…10 In this context, we recently reported a strategy based on the self-assembly of a urea-based organogelator. 11 The latter allows for a spontaneous noncentrosymmetric arrangement of the chromophores, based on a favorable balance of supramolecular forces, without the need of any additional external mediation. However, dipolar chromophores exhibit a strong propensity to self-assemble in a centrosymmetric fashion because of antiparallel donoracceptor intermolecular interactions.…”

(Super)gelators derived from push–pull chromophores: synthesis, gelling properties and second harmonic generation

“…19 Still in the context of photonics, we recently described gel-based materials derived from A, B and C, which display a Second Harmonic Generation (SHG) response. 20,21 To generate a second harmonic with such chromophores, it is generally necessary to process the materials with sophisticated techniques to reach a non-centrosymmetric state, which is requisite to observe the phenomenon. 22 In previous studies based on Disperse Red 1 (DR1) derivatives, we demonstrated that such heavy processes can be avoided by designing suitable organogelators, which were able to spontaneously self-assemble into SHG-active xerogels after a simple drop casting and evaporation sequence.…”

“…Compound A, which includes a push-pull azobenzene chromophore derived from DR1 and a urea-dodecyl gelling fragment, allowed us to deliver a proof of principle. 20 This strategy was subsequently extended to DR1 derivatives B and C, which include other gelling moieties. 21 The results presented herein show that the scope of our approach can be extended to other push-pull systems displaying a non- character (1 and 2) appeared a as particularly relevant choice since previous studies showed that quinoid spacers promote efficient intramolecular charge transfers, bathochromically shifted absorption in the UV-visible region and higher nonlinear optical responses.…”