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

We study the porphyrin S1→S0 fluorescence and the photosensitized singlet oxygen 1Δg→3Σg phosphorescence, both originating from absorption of photons with energy less than the porphyrin S0→S1 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 photosensitization via HBA in porphyrin.