Novel Push−Pull Phthalocyanines as Targets for Second-Order Nonlinear Applications

Abstract

The second harmonic generation (SHG) hyperpolarizabilities of a family of peripherally substituted push−pull phthalocyanines 1, 2, and 3, having an ethynediyl subunit in the linking bridge between the acceptor and donor moieties, have been measured. Electric field induced second harmonic (EFISH) generation experiments at 1.064 and 1.900 μm and hyper-Rayleigh scattering (HRS) experiments at 1.064 μm have been carried out. The quadratic hyperpolarizabilities values derived from these experiments are quite significant and superior to those found for similar push−pull compounds lacking of a triple C-bond in the donor−acceptor path. The main electronic parameters of the three compounds (electric dipole moments, orbital electronic distributions, and optical transition moments) have been calculated using the ZINDO/S method for molecular geometries that have been optimized through the PM3 method. Excellent agreement with spectroscopic data has been achieved. The SHG results have been analyzed through a 3-level model associated to the ground level and two split levels responsible for the Q-band, assuming either strict (for compound 2) or approximate (for compounds 1 and 3) C2v planar symmetry. The parameters used in the model were those obtained from the electronic calculations. For molecule 2, both the theoretical diagonal and off-diagonal elements of the beta tensor, as well as βEFISH and βHRS, have been determined. Good agreement with the experimental HRS values is obtained, whereas EFISH values considerably differ from those obtained from the simple expression for γEFISH ignoring the electronic contributions.