Effect of the phenyl group on the non-linear optical (NLO) and aggregation induced emission (AIE) properties of ferrocene conjugated linear D–π–A/D–π–A–π–A chromophores

Abstract

New and effective D–π–A and D–π–A–π–A systems of chromophores 1–4 based on the ferrocene core have been synthesized using Knoevenagel condensation reactions and the characterization of chromophores 1–4 was done by modern spectroscopic methods. Moreover, 2 and 4 structures were confirmed using the single crystal X-ray diffraction (SC-XRD) technique. Crystal packing revealed that the chromophores are centrosymmetric and show non-covalent interactions. The observed positive solvatochromism reveals that the stabilization of molecules is greater in excited states than in the ground states. The fluorescence spectra of all the chromophores display weak emission in the solution state, and enhanced emission was observed in a THF:H₂O mixture, due to the restriction of intramolecular rotation (RIR), and it is known as aggregation-induced emission (AIE). The SHG (second harmonic generation) measurements were done by the Kurtz–Perry powder method with potassium dihydrogen phosphate (KDP) as a reference material. The presence of hydrogen bonding interactions in the molecules blocks the anti-parallel arrangements and shows second order NLO properties in the bulk, in spite of the fact that chromophores 2 and 4 crystallize in the centrosymmetric space group. The first hyperpolarizabilities (β) [β = 56.5 × 10⁻³⁰ esu (2); β = 81.5 × 10⁻³⁰ esu (4)] were measured using the hyper Rayleigh scattering (HRS) technique. Chromophore 4 shows enhanced second-order NLO response compared to the other chromophores, due to an extra phenyl group in the molecule creating more planarity and enhancing the conduction of charges, which happens within a molecule (ICT process), between the ferrocene and acceptor groups (–CN and –NO₂). The experimental results were corroborated using DFT calculations that comprise functionals like LC-BLYP, CAM-B3LYP and B3LYP and the 6-31+G** level of theory.