SHG behavior of the solid isomorphic compounds Ni- and Cu-5,10,15,20-tetraphenylporphyrinate. An experimental and theoretical study

We prepared and investigated the SHG behavior in the solid state of the two isomorphic Nickel and Copper 5,10,15,20-tetraphenylporphyrinate, that crystallize in the same acentric I42d space group. The compounds were synthesized and characterized, and the Second Harmonic Generation (SHG) response of powdered samples was measured employing a 1.907 μm pulsed laser radiation. During laser irradiation, for both compounds, a gradual increase of the intensity of the SH signal was observed until, after a few minutes, it reached a plateau of about fifty times (for the Ni-TPP) or ten times (for the Cu-TPP) the initial value. We attempted to understand the origin of this phenomenon both through experimental analysis and theoretical calculations. Absorption and vibrational spectroscopies, in solution and solid state, combined with powder X-ray diffraction, both before and after irradiation, do not evidence any chemical degradation, phase transition or amorphization process. Theoretical calculations on little fragments of the Ni/Cu-TPP structure simulate appropriately the structural features of the complexes and their initial experimental SHG response. Magnetic balance measurement in the solid state before and after irradiation, in conjunction with DFT/B3LYP optimization of excited spin states and TDDFT calculations, highlight a change in the electronic and/or spin state due to the irradiation of the complexes.