Investigating spectroscopy and optical responses in azo and non-azo polymeric compounds: a theoretical approach

Three polymeric compounds, designated as PA, PB, and PC, were investigated for their nonlinear optical (NLO) characteristics and spectral properties. These compounds, derived from azo and non-azo structures (namely, (E)-butyl 4-((4-bis(2-chloroethyl) amino) phenyl) diazenyl) benzoate, (2E,6E)-4-(4-butoxyphenyl)-2,6-bis(4-hydroxybenzylid) cyclohexanone, and 2-(2,6-bis(4-hydroxystyryl)-4H-pyran-4-ylidene) malononitrile), were evaluated for their linear polarization (α), first (β) and second (γ) hyperpolarizabilities using the finite field method. Spectroscopic characterizations, such as geometrical parameters, and vibrational and electronic absorption spectra, were conducted. The study employed the dispersion-corrected B3LYP-D3 method with a diffused and polarized 6–311 +  + G (d, p) basis set, revealing the superior stability of the three polymers compared to other methods. Electronic absorption spectra were computed using time-dependent density functional theory (TD-DFT) at the same level of theory, finding key parameters such as wavelength of electronic transition, oscillator strength, molecular orbital analysis, and electronic properties. The investigation also explored the dependence of NLO properties like α, β, β_HRS, and depolarization ratio on global parameters like ionization potential, electron affinity, electronegativity, chemical hardness, and electrophilicity index. PA shows greater sensitivity to the first hyperpolarizability. The obtained results show a high total first hyperpolarizability (β_tot) up to 22,894 a.u. and a low energy gap of 0.4 eV. Interestingly, the magnitudes of β obtained from the B3LYP method surpassed those obtained from other methods. These findings suggest that the studied polymeric compounds, especially PA, have significant potential for application in optoelectronic devices due to their superior NLO properties and stability.