This work reports the degenerate two-photon absorption spectrum for a series of Salen complexes possessing two azo dye units. The observed spectra peak at 1000 nm, twice the wavelength of the linear absorption peak, and increase monotonically to the blue due to resonant enhancement of the non linearity. The two-photon absorption properties of these molecules were found to be an additive effect of chromophores and independent of the metal ions. The two-photon absorption spectra were described with a sum-over-states model, from which the transition dipole moment and dipole moment change were determined and compared with quantum chemistry calculation. (c) 2007 Elsevier B.V. All rights reserved.