We study the enhancement of two-photon absorption (TPA) in a series of porphyrins and tetraazaporphyrins by measuring the absolute TPA cross sections with 100-fs-duration pulses in two ranges of laser wavelengths, from 1100 to 1500 and from 700 to 800 nm. The cross section in the Q transition region is 2 ϳ 1 -10 GM (where 1 GM ϭ 10 Ϫ50 cm 4 s Ϫ1 photons Ϫ1 ), a value that is explained by partial lifting of the prohibition that is due to a parity selection rule. In the Soret transition region we find 2 enhancement by ϳ1 order of magnitude owing to the Q transition, which acts as a near-resonance intermediate state, and also owing to the presence of gerade energy levels, which we identify in this spectral region. In tetraazaporphyrins symmetrically substituted with strong electron acceptors, we find further enhancement (up to 2 ϳ 1600 GM). As a possible application, we demonstrate for the first time to our knowledge the photosensitization of singlet-oxygen luminescence by TPA in porphyrin.

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