Among seven possible nitrogen-in constitutional isomers of porphyrin only one, porphycene, has been obtained so far in the free, unsubstituted form. Herein, the synthesis of another isomer, parent hemiporphycene (HPc), and its thorough structural, spectral, photophysical, electrochemical, and theoretical characterization are reported. Most of the properties of HPc are intermediate between those of porphyrin and porphycene, as evidenced by the values of inner-cavity dimensions, orbital-energy splittings, absorption coefficients, magnetic circular dichroism parameters, NH-stretching frequencies, fluorescence quantum yields, tautomerization rates, and redox potentials. The largest differences arise with respect to tautomerism, due to the low symmetry of HPc and inequivalence of the four nitrogen atoms that define the inner cavity. Two trans tautomers are observed, separated in energy by about 1 kcal mol-1 . Tautomerization from the higher- to the lower-energy form is detected in the lowest-excited singlet state and occurs at a rate that is about four orders of magnitude lower than that observed for porphycene. Hemiporphycene is a very good model for the investigation of inequivalent intramolecular H-bonds present in one molecule; two such bonds in HPc reveal unusual characteristics, and the bond strength results from the interplay between the N⋅⋅⋅N distance and the N-H-N angle.
Keywords: hydrogen bonds; isomers; porphyrinoids; structure elucidation; tautomerism.
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