Electronic, structural and nonlinear optical investigation of manganese carbonyl complexes of isatin derivatives by DFT

Isatin-Schiff bases have wide applications in chemistry. The π conjugated electronic system and heterocylic structure of these materials make them valuable for use as photosensitized materials. The delocalization of π-electrons throughout the structure causes the UV–vis absorption spectra to shift to longer wavelengths. In this study, the isatin manganese carbonyl complex shifted the UV–vis absorption wavelength to a longer wavelength (vis. 400–700 nm) compared with our previous studies. The isatin derivatives (ISE (3[4-ethyl(phenyl)imino][indoline-2-one]), ISB (3[4-butly(phenyl)imino][indoline-2-one])) and their Mn carbonyl complexes MnISE ((ISE)Mn(CO)₃), and, MnISB ((ISB)Mn(CO)₃) were investigated via density functional theory (DFT) in different solvent media. The most stable complexes were found in medium polarity THF. The calculated HOMO–LUMO energy gap shows that the charge transfer occurs within the molecule. The HOMO–LUMO energy gap was increased with increasing solvent polarity for all investigated compounds. The smaller energy gap indicates that charge transfer occurs within the Mn(II) complex, in contrast to ISE and ISB, which exhibit larger energy gaps. As a result, the maximum absorption of Mn complexes shifts to the visible region. MnISB has the smallest HOMO–LUMO energy gap in THF. Additionally, the global reactivity parameters indicated that the MnISE complex has the highest electrophilicity index. DFT calculations have also been performed to investigate polarizability and first-order hyperpolarizability of these compounds. In water, the ISE had higher NLO values than the other structures did. These results indicate that all the studied molecules in different solvents could be good candidates for use in photosensitized and nonlinear optical materials.

Geometries were determined at the DFT level via the LANL2DZ basis set for Mn and cc-PVTZ for other atoms in the molecules with the B3LYP functional. The UV–vis absorption spectra and HOMO–LUMO energies of ISE, ISB, and their Mn complexes were calculated by Time-dependent DFT (TDDFT) with CAM-B3LYP using the same basis sets. The UV–vis absorption spectra of ISE were also measured in acetonitrile and compared with the calculated spectra, which were consistent with the experimental results. All calculations were repeated in different solvents with the polarizable continuum model (PCM).