A Critical Evaluation of EFISH and THG non Linear Optical Responses of Asymmetrically Substituted Meso-Tetraphenyl Porphyrins and their Metal Complexes.

The experimental second and third order non-linear optical (NLO) responses \u3b3EFISH and \u3b3THG, measured working with a non-resonant incident wavelength of 1.907 \u3bcm, of some push-pull meso-tetraphenyl porphyrins and their metal complexes, substituted in the para position of one phenyl ring with a nitro group and with hydrogen or methyl or methoxy groups in the para position of the other three phenyl rings, are critically discussed, also with the support of semiempirical MNDO-TDHF calculations. Care must be taken in evaluating the quadratic hyperpolarizabilities \u3b2vec from \u3b3EFISH being the contribution of the cubic term \u3b30(-2\u3c9; \u3c9, \u3c9, 0) to \u3b3EFISH not negligible at least when the second order NLO response is relatively low, as in porphyrins and their metal complexes investigated in this work. In addition experimental evidence has been produced that the value of \u3b3EFISH of the porphyrins and their metal complexes is increased by resonance enhancement when working with an incident wavelength of 1.34 \u3bcm, due to the presence of very strong absorption bands in the region 0.400-0.650 \u3bcm and that the \u3b3EFISH response is not significantly affected by 3dn (n=7-10) metal coordination. On the contrary \u3b3THG decreases by metal coordination due to the high resonance enhancement of \u3b3THG of free porphyrins. In fact the third harmonic 3\u3c9 (0.636 \u3bcm) in free porphyrins is quite close to some strong Q absorption bands above 0.615 \u3bcm, which are lacking in the absorption spectra of their 3dn metal complexes. A vibrational method for the evaluation of the order of magnitude of the static cubic \u3b30 third order NLO response was successfully applied for the first time to a push-pull porphyrinic system