Theoretical study by DFT and TD–DFT of NLO-active push–pull molecules composed of conjugated bridges based on cyclic rings: Titanol, Ferrol, Nickelol and Zinkol

Context

This research is based on the theoretical study of seven push–pull molecules composed of conjugated bridges based on two different organometallic rings, these bridges are linked at their ends by acceptor groups (–NO₂) and donor groups (–N(CH₃)₂) on the α position of the rings mentioned above. The location of the donor and acceptor groups revealed that the addition of the acceptor groups near the rings (Titanol, Ferrol and Nickelol) improves the NLO response in comparison with the grafting of these groups on the Zinkol ring and also influences the positioning of the π electrons at the level of the chromophores studied. The molecule 2B gave the highest values ​​of static first hyperpolarisabilitiy (β_tot) and static second hyperpolarisabilitiy (γ_av), knowing that: β_tot (2B) = 135.79 * 10^–30 esu and γ_av (2B) = 135.79 * 10^–35 esu. The highest values ​​of dynamic first \(\beta_\parallel^\lambda\left(-2\omega;\omega,\;\omega\right)\) and second \(\gamma_\parallel^\lambda\left(-2\omega;\omega,\;\omega,\;0\right)\) hyperpolarisabilities are assigned to the molecule 1C with the following values: \(\gamma_\parallel^\lambda\left(-2\omega;\omega,\;\omega,\;0\right)\) =1,218,310.00 * 10^–30 esu and \(\gamma_\parallel^\lambda\left(-2\omega;\omega,\;\omega,\;0\right)\)=1,324,520,000 * 10^–35 esu. The metal Zn is considered as an acceptor group and the remaining metals (Ti, Fe and Ni) are considered as donor groups. The specific solvents for the seven molecules are water, ethanol and acetonitrile. The maximum wavelengths recorded for all molecules in combination with all solvents are in the range of 421.39 to 765.28 nm. λ

Method

The calculations were performed using Gaussian 16 software to perform DFT calculations with B3LYP functional. The LanL2DZ basis–set was used for transition metals, while the 6–31 +  + G(d,p) basis–set was used for nonmetal atoms. The functionals used are: CAM–B3LYP, LC–wPBE, LC–BLYP, M11, wB97X, M08–HX, M06–2X, MN12SX, MN15, and M06HF. The basis–sets used are: 6–31G(d,p), 6–31 +  + G(d,p), cc–pVDZ, aug–cc–pVDZ, 6–311G(d,p), 6–311 +  + G(d,p), cc–pVTZ, and aug–cc–pVTZ. The Natural Bond Orbital (NBO) calculations are performed by the NBO program incorporated by default in the Gaussian 16 program. The solvation models studied are the CPCM model (conductor polarizable continuum model) and the SMD model (Solvation Model Density). Excited states calculations are calculated by the time-dependent DFT method (TD–DFT).