DFT and TD–DFT study of push–pull molecules with extended conjugated bridges: theoretical insight into organometallic rings for the enhanced Nonlinear Optical (NLO) properties

Context

This work presents a theoretical investigation of nine push–pull molecules featuring conjugated bridges composed of three distinct organometallic rings: Titanol, Chromol, Ferrol, Nickelol, and Zinkol. These bridges are connected at their ends to acceptor (NO₂) and donor (N(CH₃)₂) groups positioned on the α-sites of the aforementioned rings. In systems incorporating cyclic groups, it was found that placing Titanol and Nickelol rings between two Zinkol units enhances the nonlinear optical (NLO) response. A similar enhancement is observed when Chromol and Ferrol rings are positioned at the termini of the Zinkol units. Frontier orbital analysis reveals that the Zinkol rings exhibit electron-accepting characteristics, whereas the other rings act as electron donors. The two molecular systems conform to the architectures D–A–D–A–A (Titanol–Nichelol–Zinkol) and D–D–A–A–A (Chromol–Ferrol–Zinkol), respectively. Computed static β_tot values range from 142.71 to 512.07 × 10^–30 esu, while static |γav| values fall within 1.88 to 245.05 × 10⁻³⁵ esu. The study also includes an analysis of dynamic \({\beta }_{||}^{\lambda }(-2\omega ;\omega ,\omega )\) values, which lie between 2478.43 and 1,036,410.00 × 10^–30 esu, and dynamic \({\gamma }_{||}^{\lambda }(-2\omega ;\omega ,\omega ,0)\) values ranging from 26,017.87 to 193,013,000 × 10⁻³⁵ esu. Nonlinear refractive indices (n₂) were also evaluated, with values spanning from 5.11 × 10⁻¹⁵ cm²·W⁻¹ to 2.91 × 10⁻¹⁰ cm²·W⁻¹. Additionally, most of the investigated molecules exhibit absorption within the 450–900 nm range, both in vacuum and in various solvents.

Method

All calculations were performed using Gaussian 16 program. The methods used are DFT and TD–DFT.Several functionals were treated: CAM–B3LYP, LC–ωPBE, LC–BLYP, M11, ωB97X, M08HX, M062X, MN12SX, MN15, M06HF.Several basis–sets was studied: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, AUG–cc–pVTZ and LanL2DZ. Implicit Solvation Model used are CPCM and SMD. Finally, NBO method is used also.