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 γEFISH and γTHG, measured working with a non-resonant incident wavelength of 1.907 μm, 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 βvec from γEFISH being the contribution of the cubic term γ0(−2ω; ω, ω, 0) to γEFISH 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 γEFISH of the porphyrins and their metal complexes is increased by resonance enhancement when working with an incident wavelength of 1.34 μm, due to the presence of very strong absorption bands in the region 0.400–0.650 μm and that the γEFISH response is not significantly affected by 3dn (n=7–10) metal coordination. On the contrary γTHG decreases by metal coordination due to the high resonance enhancement of γTHG of free porphyrins. In fact the third harmonic 3ω (0.636 μm) in free porphyrins is quite close to some strong Q absorption bands above 0.615 μm, 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 γ0 third order NLO response was successfully applied for the first time to a push–pull porphyrinic system.