A new study indicates that porphycene-based photosensitizers have better optical properties than porphyrin-based photosensitizers for two-photon photodynamic therapy (PDT). Porphycenes were identified as candidate photosensitizers because their chemical structures strongly resemble those of well-studied porphyrins.In the study, researchers from the University of Århus, Denmark, Universitat Ramon Llull in Barcelona, Spain, and Heriot-Watt University in Edinburgh, UK, evaluated the two-photon PDT efficacy of 2,7,12,17-tetraphenylporphycene, with and without palladium, and used tetraphenylporphyrin as a control. Both physical experiments and computer modeling were employed. In the physical experiments, the researchers excited the photosensitizers across the near-infrared range (~700 to 850 nm) with femtosecond pulses from a Ti:sapphire laser and with an optical parametric amplifier pumped by a regenerative amplifier, all from Spectra-Physics of Mountain View, Calif. The phosphorescence signal from singlet oxygen was detected with a cooled Hamamatsu infrared photomultiplier tube in a photon-counting experiment. The pre-amplifier and photon counter were from Stanford Research Systems Inc. of Sunnyvale, Calif.As reported in the April 25 issue of the Journal of the American Chemical Society, the researchers found that tetraphenylporphycene and palladium-tetraphenylporphycene have a much greater probability of absorbing two photons than tetraphenylporphyrin in the near-infrared region of the spectrum, as indicated by two-photon cross sections. This occurs because the two-photon transition in the porphycene is almost resonant with the one-photon transition. Computational analyses agreed with the experimental results. The researchers concluded that the comparatively large two-photon cross sections in the near-infrared range will make tetraphenyl-substituted porphycenes useful photosensitizers for two-photon PDT.