Experimental and computational investigations of new indole derivatives: A combined spectroscopic, SC-XRD, DFT/TD-DFT and QTAIM analysis

Abstract Indole base derivatives have always been attractive to chemists due to diverse biological, industrial and optical applications. Novel indole based imines 6a-d [N-substitutedphenyl (4,5,6-trimethoxy-2,3-diphenyl-1H-indol-7-yl)methanimines] and indole based amines 7a-d [7-(phenylaminomethyl)-4,5,6-trimethoxy-2,3-diphenyl-1H-indoles] have been synthesized and characterized by nuclear magnetic resonance (1H- &13C NMR) techniques to determine the structures of the molecules. For additional spectroscopic support, fourier transform IR and UV–Vis techniques were used for the characterization of synthesized molecules (6a-d and 7a-d). A single crystal X-ray diffraction (SC‒XRD) study on 6c and 7d was carried out to determine the specific space groups, unit cells, bond angles and bond lengths. The same two synthesized compounds (6c and 7d) were further subjected to density functional theory (DFT) at B3LYP/6-311G(d,p) level to explore the optimized geometry, FT-IR vibrational, frontier molecular orbital (FMO), molecular electrostatic potential (MEP) and natural bonding orbital (NBO) investigations. A time dependent density functional theory (TD-DFT) at the B3LYP/6-311 + G(d,p) level was used to perform UV–Vis spectral analysis. A bunch of hyper conjugative interactions have been assessed by NBO analysis, which might be a reason for the stability of the title crystals (6c and 7d). In addition to hyper conjugative interactions, the investigated crystals have been stabilized by non-covalent interactions (NCIs) and intramolecular H-bonding network. The stability of the chemical structures of the title crystals have also been evaluated by Hirshfeld and QTAIM analyses. The QTAIM analysis showed that the 6c and 7d molecules are stabilized by hydrogen bonds and non-covalent interactions, which it further highlighted in their corresponding dimers 6c-d and 7d-d. The findings of the Hirshfeld and QTAIM analyses were also obtained in consistency with the NBO and solid-state results. Moreover, the spectroscopic and SC-XRD data of 6c and 7d were obtained in a reasonable agreement with the computational findings. The global reactivity parameters have been calculated utilizing the FMO energies of the studied compounds. The first order non-linear optical (NLO) properties of 6c and 7d have also been reported.