Cu-II- and Zn-II-based donor-acceptor porphyrins have been examined fo
r second-order nonlinear optics using the hyper-Rayleigh scattering te
chnique 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)). T
he high beta values in the Zn-II porphyrins are understood in terms of
the change in dipole moments upon excitation within the context of th
e two-state model as well as two-photon resonance enhancement. In Cu-I
I porphyrin complexes a large number of excited states seem to contrib
ute to beta with, perhaps, a little contribution from two-photon reson
ance at the excitation wavelength.