OsO₄-Mediated Dihydroxylation of <i>meso</i>-Tetraphenylporphyrin <i>N</i>-Oxide and Transformation of the Resulting Diolchlorin <i>N</i>-Oxide Regioisomers

The OsO4-mediated dihydroxylation of meso-tetraphenylporphyrin N-oxide yields two regioisomeric chlorin N-oxides. These chlorin N-oxides can be manipulated to provide pairs of regioisomers of pyrrole-modified porphyrin N-oxides. The UV−vis absorption and fluorescence emission spectra of the neutral and protonated regioisomers are distinct from each other, and generally different from the parent chromophore. The outcome of diol oxidation reactions of some N-oxide diolchlorins varies from the corresponding reactions of the parent diolchlorins. The crystal structure of a free base porpholactone N-oxide carrying the N-oxide on the oxazolone moiety is reported.