Photo-activatable second harmonic generation in centrosymmetric zinc porphyrinates: Experimental and theoretical study

We investigated the Second Harmonic Generation (SHG) behavior in the solid state of zinc 5,10,15,20-tetraphenylporphyrinate, which crystallizes in the centrosymmetric I4/m space group, and its two new centrosymmetric derivatives. The new compounds were synthesized and characterized by single crystal X-ray diffraction, and the SHG response of each powdered sample was measured using 1.907 mu m and 0.930 mu m pulsed laser radiation. During laser irradiation, the three compounds showed a photoactivation phenomenon that is stable over time and dependent on the laser irradiation setup. We investigated the origin of this SHG behavior through both experimental analysis and theoretical calculations. Vibrational spectroscopy in the solid state, before and after irradiation, did not show any chemical degradation. DFT calculations on small fragments of the structures appropriately simulated the structural features of the complexes and their initial experimental SHG response. TDDFT calculations better simulated the SHG responses after irradiation, leading us to hypothesize a permanent change in the electronic state due to the high intensity of the lasers.