Impact of Homocyclic and Heterocyclic Rings of Chalcones on Charge Transfer Behaviour: A Nonlinear Optical Study

This paper presents the effects of incorporating thienyl and phenyl rings as the terminal ring in the cinnamoyl counterparts of chalcones with donor-acceptor-donor (D-A-D) push-pull setting, on their photophysical responses. Two fluoro-methoxy substituted chalcone derivatives with distinct cinnamoyl systems, 4-methylthiophene-2-carbaldehyde (2F4MA48) and 2,4-dimethylbenzaldehyde (2F4MA96), were synthesized. Characterization of these chalcone derivatives was performed using nuclear magnetic resonance (NMR), Fourier-transform infrared (FTIR) and single-crystal X-ray diffraction (SCXRD). Comparative studies on the linear and nonlinear optical responses were carried out through both experimental analyses and theoretical density functional theory (DFT) calculations. The electronic absorption spectra computed using DFT were compared with the UV-Vis spectra to elucidate the intramolecular charge transfer mechanism via the hole-electron distribution and interfragmentary charge transfer analyses. Molecular properties were examined through electrostatic potential (ESP) maps, π-electron localization function (ELF-π), and frontier molecular orbitals (FMOs). The incorporation of thienyl ring is found to give a stronger push-pull effect and greater electron delocalization. Experimental nonlinear optical responses were investigated using Z-scan technique with a continuous-wave incident laser at a wavelength of 532 nm. Both compounds showed two-photon-induced reverse saturation absorption (RSA) and self-defocusing responses. It was observed that the thienyl-based chalcone, exhibited greater optical nonlinearity in terms of nonlinear absorption (NLA), nonlinear refraction (NLR), and optical limiting. The experimental third-order nonlinear optical susceptibility (χ(3)) was determined to be 8.04 × 10−8 esu for 2F4MA48 and 1.08 × 10−8 esu for 2F4MA96. The onset limiting threshold (FS) was found to be 2.63 kJ cm−2 for 2F4MA48 and 5.23 kJ cm−2 for 2F4MA96. The steep optical limiting traces of 2F4MA48 show superior performance compared to 2F4MA96.