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

• Photo-activatable SHG observed in centrosymmetric zinc porphyrinates. • Electronic origins of photo-activatable SHG explored by DFT and TD-DFT. • Different laser sources produce distinct effects on photo-activatable SHG. • DFT and TD-DFT calculations suggest an LMCT phenomenon. 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 μm and 0.930 μm 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. TD-DFT 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.