Photon energy upconversion in porphyrins: one-photon hot-band absorption versus two-photon absorption

We study the porphyrin S-1 --> S-0 fluorescence and the photosensitized singlet oxygen (1)Delta(g) --> (3)Sigma(g) phosphorescence, both originating from absorption of photons with energy less than the porphyrin S-0 --> S-1 transition energy. By measuring the excitation intensity dependence of fluorescence at lowered sample temperatures, we are able to discriminate between two parallel processes of one-photon hot-band absorption (HBA) and simultaneous two-photon absorption (TPA). When the HBA and TPA contributions are comparable in magnitude, we use this new method to determine absolute TPA cross-section. We also demonstrate for the first time a singlet oxygen photo sensitization via HBA in porphyrin. (C) 2003 Elsevier Science B.V. All rights reserved.