Multidecker Sandwich Cluster VnBen+1 (n=1, 2, 3,4) as a Polarizable Bridge for Designing 1D Second-Order NLO Chromophore: Metal-pi Sandwich Multilayer Structure as a Particular Charge-Transfer Axis for Constructing Multidimensional NLO Molecules

摘要

Multidecker sandwich pi-coordination complexes with Multiple metal atoms and cyclic pi-ligands can be considered as building blocks for designing NLO molecules. In a previous work, we found that multidecker sandwich complexes of the form VnBn+1 (n = 1, 2, 3) can be considered as stronger electron donors than metallocene Fe(eta(5)-C5H5)(2) for the design of second-order NLO molecules, and a layer-number dependence of the first hyperpolarizability of V(n)Bz(n+1)-(C2H2)(3)-NO2 (n = 1, 2, 3) and the two-dimensional NLO character was observed from our calculation results. In the present article, by analogy with traditional donor-acceptor substituted conjugated polyenes, we took the multidecker sandwich tomplexes VnBn+1 (n = 1, 2, 3, 4) as the polarizable bridge instead of the traditional pi-electron conjugated bridge and designed several novel push pull,Systems of the form donor-(VnBn+1)-acceptor (n = 1, 2, 3, 4) by introducing electron-donating and electron-withdrawing groups at the terminal benzene ring of VnBn+1. The NLO properties of these new species were investigated in detail. It was found that the first hyperpolarizability beta(0) of the donor-(VnBn+1)-acceptor complexes increases with the number of layers n for n = 1-3. However, when the number of layers was increased to 4, there was an apparent decrease in beta(0), which is different from the behavior of traditional push-pull pi-systems. To understand the optical transition and NLO response, time-dependent density functional theory (TD-DFT) calculations were carried but for these 1D chromophores. By analyzing the excited state that is crucial to the NLO response, we found that metal (d orbital) to pi-ligand (antibonding, pi*-orbital) charge-transfer (MLCT) transitions indeed occur in the metal-pi sandwich multilayer structure and that small VnBn+1 clusters (n = 2 or 3) could play a role in the polarizable bridge or CT axis. The findings from this quantum-chemical study should provide hints to scientists designing new organometallic multidimengional NLO molecules with metal-pi sandwich multilayer structures as new charge-transfer axes.