Metalloporphyrins for Quadratic Nonlinear Optics

$Cu^{II}$- and $Zn^{II}$-based donor-acceptor porphyrins have been examined for second-order nonlinear optics using the hyper-Rayleigh scattering technique at 1064 nm. Introduction of the metal ions enhances the first hyperpolarizability $(\beta)$ of the free-base donor-acceptor porphyrins significantly. The open shell $Cu^{II} (d^9)$ has a greater influence on $_\beta$ of these porphyrin systems than the closed shell $Zn^{II} (d^{10})$. The high $_\beta$ values in the $Zn^{II}$ porphyrins are understood in terms of the change in dipole moments upon excitation within the context of the two-state model as well as two-photon resonance enhancement. In $Cu^{II}$ porphyrin complexes a large number of excited states seem to contribute to $_\beta$ with, perhaps, a little contribution from two-photon resonance at the excitation wavelength