Longwave luminescent porphyrin probes

A set of luminescent dyes, namely the porphyrin-ketones and their zinc(II), platinum(II) and palladium(II) complexes, are described. The probes are derived from a partially oxidized porphyrin ring and have been studied by absorption and luminescence spectroscopy including time-resolved measurements. The metal-free porphyrin-ketones display strong and pH-dependent fluorescence due to a protolytic equilibrium that exists between the free base and the dication. The zinc(II) complexes also give strong fluorescence along with a weak long-decay emission at room temperature which are attributed to delayed fluorescence and phosphorescence, respectively. Platinum(II) and palladium(II) complexes exhibit room-temperature phosphorescence without any detectable fluorescence. Compared to existing porphyrins, the new dyes display a considerably longwave-shifted luminescence along with substantially improved (photo)chemical stability. They are considered to be promising probes for very near-infrared luminescence studies, and to be particularly useful for applications such as labelling of biomolecules, pH-sensing, and detection of quenching species such as molecular oxygen.