Aggregation induced emission (AIE)‐active N‐arylated ferrocenyl pyrazole‐based push–pull chromophores: Structural, photophysical, theoretical and effect of substitution on second‐order non‐linear optical properties

Abstract

In this study, we report the synthesis and structural characterization of N‐arylated ferrocenyl pyrazole [Fc‐Pz‐(CF3)‐C6H5‐R] (R = CH3, 1; OCH3 2), where Pz = pyrazole and Fc = ferrocenyl = Fe(η5‐C5H5)(η5‐C5H4) chromophores with D–π–A–π–D system. The solvatochromic behaviour of compounds 1 and 2 was investigated in various solvents from non‐polar to polar. When solvent polarity increases, the charge transfer process produces the more polarized excited state, and consequently, positive solvatochromism was observed. The emission performance of the compounds is weak luminescence in CH3CN solution but becomes high emissive in a mixture of water/CH3CN fraction at (fw) 90%, demonstrating aggregation‐induced emission (AIE) characteristics, and the mechanism of AIE has been attributed to restricted intramolecular rotation (RIR) process. The time‐resolved fluorescence lifetime studies have been demonstrated in water/CH3CN mixture at 90%, as it shows three relaxation pathways in their fluorescence decay. In addition, density functional theory calculations have been employed to optimize the geometry of the molecules, followed by computation of dipole moment (μ), first hyperpolarizability (β) using various functionals (B3LYP, long‐range corrected CAM‐B3LYP and LC‐BLYP) and frontier molecular orbital levels for understanding the charge transfer, electrochemical and non‐linear optical (NLO) properties. The second‐order non‐linear optical properties were studied by hyper‐Rayleigh scattering (HRS) technique using an incident wavelength at 1064 nm. The chromophores exhibited the first hyperpolarizability (βHRS = 38.6 × 10−30 e.s.u. for 1 and βHRS = 116.8 × 10−30 e.s.u. for 2) and were found to be 6.8 times greater than that of the reference material, p‐nitroaniline (pNA) (βHRS = 17.1 × 10−30 e.s.u.). Also, compound 2 shows higher NLO effect because the presence of –OCH3 group makes effective charge transfer process by mesomeric effect (+M).