Third-Order Nonlinear Optical Properties and Structures of (E)-N-(4-Nitrobenzylidene)-2,6-Dimethylaniline and (E)-N-(4-Nitrobenzylidene)-2,3-Dimethylaniline

Abstract

(E)-N-(4-Nitrobenzylidene)-2,6-dimethylaniline (1) and (E)-N-(4-nitrobenzylidene)-2,3-dimethylaniline (2) have been synthesized. The crystal structures of both compounds were determined by X-ray diffraction analysis. Quantum mechanical computations using a configuration interaction (CI) method estimate the maximum one-photon absorption (OPA) wavelengths to be in the UV region (shorter than 450 nm), indicating good optical transparency to visible light. To gain insight into the microscopic third-order nonlinear optical (NLO) properties, both dispersion-free (static) and frequency-dependent (dynamic) linear polarizabilities (α) and second hyperpolarizabilities (γ) at λ = 825–1125 nm and 1050–1600 nm were computed using the time-dependent Hartree-Fock (TDHF) method. The ab initio calculation results show that the title molecules exhibit non-zero second hyperpolarizabilities, implying microscopic third-order NLO behavior.

1. Introduction

Quadratic nonlinear optical (NLO) behavior has attracted significant interest due to its applications in optical communication and processing. Materials with large second-order NLO response are sought after for use in telecommunications, optical computing, and optical signal processing. The third-order response, governed by the second hyperpolarizability, offers even richer behavior due to the higher dimensionality of frequency space.

A variety of materials have been synthesized and their NLO properties explored using techniques such as degenerate four-wave mixing, Z-scan, and third-harmonic generation (THG). Schiff bases, in particular, have gained attention due to their relatively large molecular hyperpolarizabilities, attributed to π-electron delocalization. The effect of electron donor/acceptor substituents on the second hyperpolarizabilities of Schiff base molecules has been a focus of recent research.

This study aims to:

Characterize newly synthesized Schiff bases with donor/acceptor substituents using spectroscopic (UV-vis) and crystallographic (X-ray diffraction) techniques.Investigate third-order NLO behavior using ab initio TDHF calculations on both static and dynamic linear polarizabilities and second hyperpolarizabilities.

2. Experiment
2.1. Reagents and Techniques

4-Nitrobenzaldehyde, 2,6-dimethylaniline, 2,3-dimethylaniline, and ethanol were purchased from Merck (Germany). Elemental analyses were performed on a LECO CHNS932 analyzer. Infrared absorption spectra were recorded using a PerkinElmer BX II spectrometer with KBr discs.

2.2. Preparation of Compounds 1 and 2

Compound 1 was prepared by condensing 4-nitrobenzaldehyde (0.01 mol) with 2,3-methylaniline (0.01 mol) in 150 mL ethanol. The mixture was stirred for 4 hours and then placed in a freezer for 5 hours. The resulting yellow precipitate was filtered and washed with cold ethanol.

2.3. X-ray Structure Determination

Data collection was performed on an Enraf-Nonius diffractometer using graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å). Data reduction and corrections for absorption and crystal decomposition were applied. Structures were solved by SHELXS-97 and refined with SHELXL-97. Hydrogen atom positions were calculated and included using a riding model.

3. Theoretical Calculations

The geometries were optimized at the restricted closed-shell Hartree-Fock level. Linear polarizabilities and third-order hyperpolarizabilities were computed at various frequencies using the 6-311+G(d,p) basis set. Frequency-dependent (hyper)polarizabilities were determined using the time-dependent Hartree-Fock (TDHF) method as implemented in the GAMESS program.

Average linear polarizability (⟨α⟩) and third-order hyperpolarizability (⟨γ⟩) were calculated as:

⟨α⟩ = (α_xx + α_yy + α_zz)/3

⟨γ⟩ = [γ_xxxx + γ_yyyy + γ_zzzz + 2(γ_xxyy + γ_xxzz + γ_yyzz)]/5

Values were converted to electrostatic units (esu) for comparison.

The maximum one-photon absorption (OPA) wavelengths (λ_max) and oscillator strengths (f) were calculated using the CI method in GAUSSIAN98W.

4. Results and Discussion
4.1. Crystal Structure Description

The Schiff base molecules 1 and 2 are non-planar. For compound 1, the two Schiff base moieties are inclined at an angle of 88.3(1)°. For compound 2, the inclination is 21.69(2)°. Bond lengths indicate an extended series of π bonds throughout the molecules, with some bonds showing partial double-bond character, supporting conjugation.

4.2. Computational Results

UV-Vis Absorption

Both compounds display three OPA peaks in their spectra, with absorption maxima between 259–414 nm for 1 and 249–411 nm for 2, all in the UV region (λ < 450 nm), indicating good optical transparency in the visible region.

Linear Polarizability (α) and Second Hyperpolarizability (γ)

The calculated ⟨α⟩ values for both compounds are very similar.Both static and dynamic ⟨γ⟩ values are highly dependent on the position of methyl substituents.Compound 2 (with methyl groups at ortho and meta positions) has higher ⟨γ⟩ values than compound 1 (with ortho methyl groups).

5. Summary

The title compounds were synthesized and their third-order optical nonlinearities investigated. Structural characterization was performed by X-ray diffraction. Theoretical calculations indicate that both compounds exhibit non-zero (hyper)polarizability values, implying microscopic third-order NLO behavior.EN4 The position of substituents significantly affects the NLO properties.