Concerning the dual emission of porphyrazines employed in biomedical imaging

Previous reports of unusual dual fluorescence in several porphyrazines proposed for use in photodynamic therapy and biomedical imaging have been re-examined. The blue-violet emissions previously assigned to S2-S0 fluorescence of these porphyrazines are shown to exhibit fluorescence excitation spectra that do not coincide with their absorption spectra, and the photophysical properties of the emission calculated from measured quantum yields and decay parameters do not match those calculated from the absorption spectra. In addition, the blue-violet emission intensities increase on exposure of aerated solutions of the porphyrazines to a broad spectrum of UV-visible light. The blue-violet emissions previously assigned to S2-S0 fluorescence of these porphyrazines themselves are therefore re-assigned to S1-S0 fluorescence of small amounts of photolysis products strongly absorbing in the same near UV spectral regions.