Multidecker Sandwich Complexes V_nBen_n+1 (n = 1, 2, 3) as Stronger Electron Donor Relative to Ferrocene for Designing High-Performance Organometallic Second-Order NLO Chromophores: Evident Layer Effect on the First Hyperpolarizability and Two-Dimensional NLO Character

During the past 3 decades, significant progress has been made in multidecker sandwich complexes. Replacing metallocene in the metallocene-based second-order nonlinear optical chromophores with multidecker sandwich complexes will create a variety of new second-order NLO organometallic complexes. To explore the possibility of improving NLO properties of organometallic complexes by using multidecker sandwich complexes, we theoretically design five second-order NLO moleculars V_nBz_n+1-(C₂H₂)₃-NO₂ (n = 1, 2, 3) with widely investigated V_nBz_n+1 (n = 1, 2, 3) by changing the layer number n or position of conjugation π–A system in the D−π–A structure. The NLO properties of these five complexes are investigated in detail. By comparison with the β₀ of ferrocenyl derivative Fe­(η⁵-C₅H₅)₂-(C₂H₂)₃-NO₂, the larger NLO response (β₀, 10215–36917 au) is found in V_nBz_n+1-(C₂H₂)₃-NO₂ (n = 1, 2, 3). The dependence of the first hyperpolarizabilities β₀ on the layer number n and the position of conjugation π–A system is discussed in the article. The evident layer effect on the first hyperpolarizabilities β₀ is shown. And, the enlargement of the first hyperpolarizability β₀ by the layer number n can be rationalized by considering the enhancement of parallel-x-MLCT (from more than one metal center to acceptor, −NO₂), perpendicular-x-MLCT (from multiple metal centers to multiple C₆H₆ ligands) transition and two-dimensional NLO character with the two charge transfer axes. By comparison with ferrocene or V₁Bz₂, the multidecker sandwich complexes V_nBz_n+1 (n = 2, 3) can be considered as stronger donor group for designing NLO moleculars. We hope, this groundbreaking work may evoke one’s attention to design new, highly efficient second-order NLO molecular materials with excellent building blocks: multidecker sandwich complexes.