Mesogenic, Luminescence, and Nonlinear Optical Properties of New Bipyrimidine-Based Multifunctional Octupoles

Abstract

International audienceA series of new bipyrimidine-based chromophores have been prepared presenting alkoxystyryl donor groups carrying aliphatic chains in the 3,4, 3,5 or 3,4,5 positions, connected to electron-accepting 2,2′-bipyrimidine cores. Their linear and nonlinear optical properties were investigated as well as their mesomorphic properties by various techniques (light-transmission measurements, polarized-light optical microscopy, and differential scanning calorimetry measurements). Only two derivatives, BPM-3,4-C12 and BPM-3,4-C16, were found to exhibit liquid-crystalline behavior with the formation of lamella-columnar phases and/or hexagonal columnar phases over large temperature ranges. Small-angle X-ray scattering analysis allowed proposing a stacking model inside the mesophase in which the molecules are interdigitated alternatively along their long axis and their short axis to form columns. Dielectric measurements were performed as a function of the temperature, showing the centrosymmetric nature of the mesophases. Large quadratic hyperpolarizabilities have been measured for the individual mesogens in solution by using hyper-Rayleigh scattering. These chromophores exhibit also cubic nonlinear optical properties, revealing relatively large two-photon absorption cross sections. The nonlinear optical properties in the liquid crystalline state of compounds BPM-3,4-C12 and BPM-3,4-C16 have been studied by wide-field second-harmonic generation and two-photon fluorescence microscopy, confirming centrosymmetry for these achiral mesogens and the excellent third-order nonlinearity for multiphoton imaging

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

“…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. [13] An elegant way to break centrosymmetry is the introduction of chirality.S ince chiral materials lack inversion symmetry,t he bulk NCS order is guaranteed by an efficient chirality transfer from the molecular to the supramolecular level. [11] Thef ormation of NCS liquid crystals containing optically active cores is apromising way to achieve efficient organic thin films for SHG.…”

“…Thea bsorption spectra display one main absorption maximum around 380 nm, which can be assigned to multiple p-p*e lectronic transitions,i mplying electronic delocalization along the stilbenyl p-linkers with ac harge transfer from the alkoxy electron-donating groups to the BPM electron-deficient core.E xcitation at 380 nm leads to astrong emission at 485 nm with quantum yields around 25 %. [13] Thef irst hyperpolarizability of the 3D octupolar compounds was determined by hyper-Rayleigh scattering (HRS). Theemission in the solid state is slightly red-shifted (Dl % 15 nm) relative to that in solution.…”

“…Thetwo compounds provide small-angle X-ray scattering (SAXS) patterns unambiguously characteristic of aw ellorganized hexagonal Col mesophase (Col h )b etween 40 8 8C [13] 500 AE 80 38 AE 8 BPM-3,4-C16 [13] 485 AE 25 42 AE 7…”

“…[13] and 100 8 8C ( Figure 1b;S upporting Information, Figure S7). Values for the achiral analogous compounds.…”

“…In conclusion, the grafting of ramified chiral alkyl chains on 3D octupolar BPM cores has allowed the emergence of NCS crystalline and liquid-crystalline phases.C Da nd SAXS measurements have revealed that these 3D octupolar (D 2 ) chromophores are organized into right-or left-handed twisted Col supramolecular aggregates depending on the chirality of the side chains.The compounds are easily processed into selforiented large area flexible thin films.Incontrast to structurally analogous compounds with achiral sidechains which we explored in earlier work, [13] this strategy allows translation of the nonlinear optical response from the molecular level (determined by HRS measurements) to the bulk level (determined by SHG measurements). Alongside strong fluorescence and two-photon absorption properties in the solid state,t hese materials display am arked SHG response under 1000 nm laser irradiation, on the order of inorganic crystals such as KDP.T he SHG response can moreover be tuned by temperature in the crystalline and the liquid crystalline phase and switches off completely in the isotropic phase.T his simple rational design strategy,that is,grafting of chiral fragments,a llows the NCS organization of various 2D and 3D octupoles into soft, malleable and processable materials and has great potential for the development of flexible and large area materials for second-order NLO applications.Moreover,these materials can be combined with photochromic fragments,l ight-induced excited spin-state trapping (LIESST) materials,o rp hotoresponsive single molecular magnets,which are appealing for the development of thin films for 3D storage of information.…”