Octupolar molecules for nonlinear optics: from molecular design to crystals and films with large second-harmonic generation

Synthesis, photophysical, linear, and non-linear optical properties of 4-methoxyphenyl dicyanovinylene dyes: DFT and TD-DFT studies

Dyes were synthesized via Knoevenagel condensation using various aldehydes of julolidine, n-hexylcarbazole, n-hexylphenothiazine, triphenylamine, and 7-N, N-diethylaminocoumarin named as MB1, MB2, MB3, MB4, and MB5. The dyes were characterized using FT-IR, 1H NMR, 13C NMR, and HRMS, which provided detailed structural information. Significant absorption, emission, and Stokes shift properties were observed through spectroscopic analysis. Notably, MB2, MB3, and MB4 exhibited solid-state and aggregation-induced emission (AIE) properties. MB1, MB3, and MB4 showed high viscosity sensitivity in polar solvents. Dyes containing nitrogen donors (MB1, MB2, and MB3) demonstrated acidochromism, responding to trifluoroacetic acid, while MB4 and MB5 remained neutral. Solvatochromic analysis indicated enhanced hyperpolarizabilities (αCT, βCT and γCT) in polar solvents, with MB1 exhibiting the highest linear and second-order hyperpolarizability, making it ideal for non-linear optical (NLO) applications. MB5 also showed a notable solvent polarity response. Computational studies using B3LYP and CAM-B3LYP functionals confirmed increased hyperpolarizabilities in polar environments. All dyes displayed solvatochromism and promising NLO properties, with MB3 and MB5 showing lower band gaps and higher dipole moments in polar solvents, indicating their potential for photonics, bioimaging, and sensors.

Nanoscale ordering of planar octupolar molecules for nonlinear optics at higher temperatures

We develop scenarios for orientational ordering of an in-plane system of small flat octupolar molecules at the low-concentration limit, aiming towards nonlinear-optical (NLO) applications at room temperatures. The octupoles interact with external electric poling fields and intermolecular interactions are neglected. Simple statistical-mechanics models are used to analyze the orientational order in the very weak poling limit, sufficient for retrieving the NLO signals owing to the high sensitivity of NLO detectors and measurement chains. Two scenarios are discussed. Firstly, the octupolar poling field is imparted by a system of point charges; the setup is subject to cell-related constraints imposed by mechanical strength and dielectric breakdown limit. The very weak octupolar order of benchmarking TATB molecules is shown to emerge at Helium temperatures. The second scenario addresses the dipoling of octupolar molecules with a small admixture of electric dipolar component. It requires a strong field regime to become effective at Nitrogen temperature range. An estimation of the nonlinear susceptibility coefficient matrix for both scenarios is done in the high-temperature (weak interaction) limit formalism. We argue that moderate modifications of the system like, e.g., an increase of the size of the octupole, accompanied by dipole-assisted octupoling, can increase the poling temperature above Nitrogen temperatures.

NLOphoric Triphenylamine Derived Donor-π-Acceptor-π-Donor Based Colorants: Synthesis, Spectroscopic, Density Functional Theory and Z-scan Studies

Three Donor-π-Acceptor-π-Donor type styryl dyes (5a-c) with different secondary donors are synthesized and characterized to study their nonlinear and linear optical properties. The structure-property relationships of the dyes are described in the light of systematic photophysical and theoretical investigations. The photophysical characteristics of 5a-c are influenced by the polarity of the medium, with an appreciable bathochromic shift in emission (5b = 81 nm) and large Stoke shifts (5b = 104-173 nm) in polar solvents. 5a-c showed intramolecular charge transfer characteristics recognized with the help of emission solvatochromism, solvent polarity graphs, natural bond orbital analysis and HOMO-LUMO energy difference. The optimized geometry and frontier molecular orbitals reveal that the electron donation takes place from secondary donors and not from a fixed donor (triphenylamine) which is more twisted. The nonlinear optical properties obtained using solvent induced spectral shift and computational methods are found within the limiting values. Z-scan results reveal saturable kind of behavior for 5a, 5b and 5c, whereas 5a and 5b show reverse saturable kind of behavior in acetone and ethanol and hence give optical limiting values. The two-photon absorption cross section described by two-level approximation is highest for 5b (251-300 GM).