Large Two-Photon Absorption Cross Sections of Hemiporphyrazines in the Excited State: The Multiphoton Absorption Process of Hemiporphyrazines with Different Central Metals

A series of five hemiporphyrazines (Hps) with different coordinating central atoms (H<sub>2</sub>, GeCl<sub>2</sub>, InCl, Pt, Pb), and the acyclic derivative 1,3-bis-(6′-amino-4′-butoxy-2′-pyridylimino)-1,3-dihydroisoindoline have been synthesized and their multiphoton absorption properties examined at the second harmonic frequency of the Nd:YAG laser in the nanosecond time regime. Metal-free and platinum Hps display saturation of optical transmittance within incident fluence values of 6 J cm<sup>−2</sup>. Comparison with other similar molecular structures like phthalocyanines and related molecules shows that Hps are strong nonlinear absorbers. The experimental curves of nonlinear transmission at 532 nm have been fitted by means of a three-level model with the occurrence of simultaneous two-photon absorption from an excited state. In the sole case of the InCl complex we found that a five-level model is needed because of the participation of triplet states. Contrary to phthalocyanines, naphthalocyanines, and porphyrins, a heavy central atom does not improve the nonlinear absorption properties since a different excited states dynamic is involved. The large nonlinear absorption of Hps combined with the very small absorption in the visible spectral range makes these molecules a very interesting class of molecules for nonlinear optical applications.