The peculiar SHG behaviour of Co-5,10,15,20-tetraphenilporphyrinate in the solid state. An experimental and theoretical study

We prepared and investigated the SHG behavior in the solid state of the Cobalt 5,10,15,20-tetraphenylporphyrinate that crystallizes in the acentric I4¯2d space group. The compound was synthesized and characterized, and the Second Harmonic Generation (SHG) response of a powdered sample was measured through a 1.907 μm pulsed laser radiation. During laser irradiation, 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 the initial value. We attempted to understand the origin of this peculiar SHG behavior, both through experimental analysis and theoretical calculations. Mass spectrometry, absorption, emission, and vibrational spectroscopies in solution combined with powder X-ray diffraction analysis in solid state, both before and after irradiation, do not evidence any chemical degradation, phase transition or amorphization process. Theoretical calculations on little fragments of the Co-TPP structure simulate appropriately the structural feature of the complex and its initial experimental SHG response. UV–VIS and EPR spectra collected in the solid state before and after irradiation, in conjunction with DFT/B3LYP optimization of excited spin states and TDDFT calculation, highlight a change in the electronic and/or spin state due to the irradiation of the complex.