As part of an ongoing study of the chemistry and properties of N-methylporphyrins and their metal complexes, we have observed that the excitation and emission spectra of N-methyltetraphenylporphyrin and N-methyltetra( p-sulfophenyl)porphyrin and their zinc complexes closely resemble corresponding spectra of chlorophyll a. We have characterized fluorescence spectra for these species in a variety of solvents commonly used for quantitative determination of chlorophylls. Quantum yields for the N-methylporphyrins are lower by approximately a factor of 10 than yields for chlorophyll a in such solvents as acetone, ethanol, and mixed aqueous/organic solvent systems. The yields are quite sufficient, however, for these species to be used as fluorescence standards. Solution of N-methylporphyrins are stable and the compounds can be inexpensively synthesized in high purity. The tetra( p-sulfophenyl) species are soluble in aqueous and highly polar organic solvents, whereas the tetraphenyl species are soluble in nonpolar organic solvents.
