Chiral Side Groups Trigger Second Harmonic Generation Activity in 3D Octupolar Bipyrimidine‐Based Organic Liquid Crystals

Abstract

Abstract.Design of organic noncentrosymmetric materials remains the Holy Grail in the field of second-order nonlinear optics. Unlike inorganic crystals currently used in second-order nonlinear optical applications, organic materials represent an attractive alternative due to their fast electro-optical response and processability, but their alignment into noncentrosymmetric film still remains challenging. Here, symmetry breaking by judicious functionalization of 3D organic octupoles allows the emergence of multifunctional liquid crystalline chromophores which can easily be processed into large, flexible, thin and self-oriented films presenting second harmonic generation responses competitive to the prototypical inorganic KH 2 PO 4 crystals. Additionally, liquid crystalline nature of these chiral organic films permits the modulation of the nonlinear optical properties due to the sensitivity of the supramolecular organization to temperature leading to the development of tunable macroscopic material. The development of numerous nonlinear optical (NLO) inorganic crystals, such as BaB 2 O 4 (BBO), [2] LiB 3 O 5 (LBO), [3] KH 2 PO 4 (KDP), [4] KTiOPO 4 (KTP), [5] AgGaS 2 (AGS) [6] and AgGaGeS 4 (AGGS), [7] has led to significant advances in the application of laser devices from UV to IR spectral wavelengths.NCS crystals of organic molecules such as nitroaniline derivatives are efficient for second order applications as well. [8] However, crystalline materials lack processability and growing large high-quality crystals remains challenging. Alternatively, NLO activity can be achieved in polymeric films by using the "poling process", in which a strong electric field is applied to align dipolar organic chromophores inside a polymer matrix near its glass transition temperature (T g ). Unfortunately, after cooling, the obtained NLO-active polymeric materials lack long-term stability and reproducibility of the preparation process, while the strong electric fields used during the poling process can lead to degradation. The Langmuir-Blodgett film deposition technique has also been used to create NCS films using amphiphilic chromophores but the tedious monolayer-by-monolayer deposition method remains a main obstacle. [10] Liquid crystalline materials open another route for creating functional organic NLO materials.They offer numerous advantages such as their ease of processing into high-quality thin films, their self-healing capacity and the possibility of alignment by shear forces, surface treatment or electric or magnetic fields.[11]The formation of NCS liquid crystals containing optically active cores is a promising way to achieve efficient organic thin films for SHG.Engineering organic liquid crystalline materials for second-order nonlinear optical applications is challenging, since at least 3 requirements must be met: i) a large molecular NLO response (i.e. first hyperpolarizability β), requiring NCS on the molecular level, ii) liquid crystalline behavior and iii) a supramolecular NCS organization. Since this approach re...

The arylvinylpyrimidine scaffold: a tunable platform for luminescent and optical materials

“…30). 76,77 Compounds 75a and 75b with C 12 and C 16 chains exhibit mesomorphic as well as second-order NLO properties in solution (β 0 = 38-42 × 10 −30 esu, measured by hyper-Rayleigh scattering in CH 2 Cl 2 ). Nevertheless, they are SHG silent in their liquid crystalline phases due to the formation of columnar mesophases with hexagonal symmetry.…”

New Multifunctional Bipyrimidine‐Based Chromophores for NLO‐Active Thin‐Film Preparation**

“…Recently, we demonstrated that pseudo‐tetrahedral octupolar compounds presenting donor 3,4‐alkoxystyryl groups attached to 2,2′‐bipyrimidine give rise to liquid crystal and linear and nonlinear optical properties [13] . In another study, we described similar derivatives involving chiral chains at the 3,4 positions, thermodynamically stable liquid crystal phases presenting strong second harmonic generation (SHG) [14] …”

“…[13] In another study, we described similar derivatives involving chiral chains at the 3,4 positions, thermodynamically stable liquid crystal phases presenting strong second harmonic generation (SHG). [14] In this present contribution, we describe the synthesis and complete physicochemical characterization (including linear and nonlinear optical, mesomorphic, thermal, structural) of a series of octupolar four-branched compounds incorporating a bipyrimidine (BPM) group as an electron-deficient core (Scheme 1) and four divergent vinyl phenyl arms bearing alkoxy aliphatic chains on position 4. Initially, we described similar derivatives involving chiral chains at the 3,4 positions, thermodynamically stable liquid crystal phases presenting strong second harmonic generation.…”

Design Strategy of Highly Efficient Nonlinear Optical Orange‐Colored Crystals with Two Electron‐Withdrawing Groups

“…This nonlinear response of organic crystals can be applied for various nonlinear optical and electrooptic applications. [1][2][3][4][5][6][7][8][9][10] In the past few decades, many studies on organic nonlinear optical crystals have been carried out to enhance their macroscopic second-order optical nonlinearity toward improving the nonlinear optical performance. [10,11] However, enhancing the macroscopic optical nonlinearity of organic crystals is highly challenging.…”