Experimental and Computational studies on Intramolecular charge transfer, Terahertz and Two photon absorption of 3-[(4-Nitrophenyl Azo)]-9H-Carbazole-9-Ethanol (NPACE) from their Vibrational spectra for Optical limiting and NLO applications

[Display omitted] •Vibrational spectral investigation has been carried out on 3-[(4-Nitrophenyl Azo)]-9H-Carbazole-9-Ethanol (NPACE) with the aid of B3LYP/6–311++G (d, p).•Vibrational contribution to the linear electro-optic effect has been examined.•Intramolecular charge transfer mechanism is discussed based on the simultaneous occurrence of a few vibrational modes from their vibrational spectra.•Second Harmonic Generation experiment is performed to validate the intramolecular charge transfer.•Terahertz spectral study is carried out to explore the low wavenumber vibrations.•Two-photon absorption coefficient and optical limiting threshold are estimated to be 0.9 × 10-11 mW−1 and 1.52 × 1013 Wm−2. Non-linear optical (NLO) features of 3-[(4-Nitrophenyl Azo)]-9H-Carbazole-9-Ethanol (NPACE) chromophore were investigated by FT-IR, FT-Raman, and UV–visible spectra aided by Density Functional Theory (DFT) using the B3LYP/6–311++G(d,p) basis set of Gaussian 16 W package. It is observed from the DFT calculation that the slight increase in the endocyclic angle of C13 -C14 -C15 and the reduction in exocyclic angle of N40 –N39 -C14 and C15 -C14 –N39 ascertained by experimental XRD values indicating the intramolecular charge-transfer interaction between the carbazole and nitrophenyl group through the diazo bridge. The vibrational contribution to the linear electro-optic effect is 15% of the total hyperpolarizability being calculated at B3LYP/6–311++G (d, p) for the NPACE molecule. It is also observed that the 8a, 19a, and 19b modes of the carbazole ring and the 8a, 8b, and 19b modes of the phenyl ring are found to be simultaneously and intensely active in IR and Raman spectra explaining the charge transfer interactions throughout the molecule. The low value of the HOMO- LUMO energy gap (2.5843 eV) and the deviation between the measured absorption wavelength (3.36 eV) from the computed (3.87 eV), both these facts substantiate the intramolecular charge transfer. The polarizability and first-order hyperpolarizability were calculated as 6.48 × 10-24 and 3.8 × 10-29 esu, respectively. The second harmonic generation (SHG) measurement experiment of NPACE was carried out using the powder method. The SHG efficiency is measured in comparison with the urea standard. The calculated torsional mode at 20 cm−1 is in excellent quantitative agreement with the experimentally determined terahertz absorption peak. The two-photon absorption coefficient of NPACE was estimated to be 0.9 × 10