One- and two-photon absorption of highly conjugated multiporphyrin systems in the two-photon Soret transition region.

This study presents a detailed investigation of near-infrared one- and two-photon absorption (TPA) in a series of highly conjugated (porphinato)zinc(II) compounds. The chromophores interrogated include meso-to-meso ethyne-bridged (porphinato)zinc(II) oligomers (PZn(n) species), (porphinato)zinc(II)-spacer-(porphinato)zinc(II) (PZn-Sp-PZn) complexes, PZn(n) structures featuring terminal electron-releasing and -withdrawing substituents, related conjugated arrays in which electron-rich and -poor PZn units alternate, and benchmark PZn monomers. Broadband TPA cross-section measurements were performed ratiometrically using fluorescein as a reference. Superficially, the measurements indicate very large TPA cross-sections (up to approximately 10(4) GM; 1 GM = 1x10(-50) cm(4) s photon(-1)) in the two-photon Soret (or B-band) resonance region. However, a more careful analysis of fluorescence as a function of incident photon flux suggests that significant one-photon absorption is present in the same spectral region for all compounds in the series. TPA cross-sections are extracted for the first time for some of these compounds using a model that includes both one-photon absorption and TPA contributions. Resultant TPA cross-sections are approximately 10 GM. The findings suggest that large TPA cross-sections reported in the Soret resonance region of similar compounds might contain significant contributions from one-photon absorption processes.