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

Abstract

The design of efficient noncentrosymmetric materials remains the ultimate goal in the field of organic secondorder nonlinear optics.Unlike inorganic crystals currently used in second-order nonlinear optical applications,o rganic materials are an attractive alternative owing to their fast electrooptical response and processability,b ut their alignment into noncentrosymmetric film remains challenging. Here,symmetry breaking by judicious functionalization of 3D organic octupoles allows the emergence of multifunctional liquid crystalline chromophores whichc an easily be processed into large, flexible,t hin, and self-oriented films with second harmonic generation responses competitive to the prototypical inorganic KH 2 PO 4 crystals.T he liquid-crystalline nature of these chiral organic films also permits the modulation of the nonlinear optical properties owingtothe sensitivity of the supramolecular organization to temperature,l eading to the development of tunable macroscopic materials.Noncentrosymmetric (NCS) compounds are of current interest in materials chemistry because of their technologically important second-order nonlinear optical properties, with applications such as second harmonic generation (SHG), electro-optical switching,t elecommunications,o ptical computing, laser technologies,a nd bioimaging. [1] Thed evelopment 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.N CS crystals of organic molecules such as nitroaniline derivatives are efficient for second-order applications as well. [8] However, crystalline materials lack of processability,and growing large high-quality crystals remains challenging.Alternatively,NLO activity can be achieved in polymeric films by using the poling process,i nw hich as trong electric field is applied to align dipolar organic chromophores inside ap olymer matrix near its glass transition temperature (T g ). [9] Unfortunately,a fter 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. TheLangmuir-Blodgett film deposition technique has also been used to create NCS films using amphiphilic chromophores,but the tedious monolayerby-monolayer deposition method remains amain obstacle. [10] Liquid-crystalline materials open another route for creating functional organic NLO materials.T hey offer numerous advantages,such as their ease of processing into high-quality thin films,t heir self-healing capacity,a nd the possibility of alignment by shear forces,s urface treatment, or electric or magnetic fields. [11] Thef ormation of NCS liquid crystals containing optically active cores is apromising way to achieve efficient organic thin films for SHG.Engineering organic liquid-crystalline mat...

Conformational studies of bipyrimidine-based mesogens by combination of DFT calculations and temperature-dependent infrared studies

“…In this packing model, the bipyrimidine core adopt pseudo-tetrahedral octupolar symmetry and the bipyrimidines aromatic cycles have a non-zero dihedral angle, giving the molecules a twisted cross conformation. Such molecular packing was also proposed more recently for the stacking of bipyrimidine molecules, carrying chiral carbon chains, into flexible and noncentrosymmetric organic liquid crystalline thin films [11]. In order to get more insight about the molecular conformation of the bipyrimidine core inside the columnar liquid crystalline phases and to confirm that the molecule do not adopt a flat conformation, DFT calculations and temperature-dependent infrared spectroscopy experiments were performed on the BPM-3,4-Cn (n = 12, 16) compounds.…”