Excited-State and Two-Photon Absorption in Salicylidene Molecules: The Role of Zn(II) Planarization

This paper report on the excited-state (ESA) and two-photon absorption (2PA) features of two novel salicylidene compounds: N,N'-bis(salicylidene)-1,2-phenylenediamine (salophen) and its Zn(II) coordination compound ([Zn(salophen) (OH2)]). The molecular structure of salophen is a tetradentated ligand with iminic and phenolic bonding sites, allowing the coordination of metal ions such as zinc(II) forming [Zn(salophen) (OH2)]. According to our data, the Zn(II) coordination complex modifies the ESA and 2PA spectrum of the salophen ligand, increasing significantly its hyperpolarizability and improving its optical properties due to the increase of the planarity and rigidity of the ligand framework In order to obtain more information about optical properties of these molecules, theoretical calculations were performed at the DFT/TD-DFT level using aug-cc-pVDZ/CAM-B3LYP. Our results suggest that the molecular geometries of the ligand, corresponding to S-0 and S-1 states, are quite similar. However, the coordination of the metal atom leads to an increment of the planarity and consequent increase of the electron delocalization of the ligand pi framework, leading to an increment in the hyperpolarizability of the complex.