Hydroxy-Functionalized Porphycenes: Structure, Spectroscopy, and Electrochemistry

A series of porphycenes bearing acetoxy and hydroxy groups at the meso-positions have been prepared and fully characterized: H2TPrPc-OH, [Ni(TPrPc-OH)], H2TPrPc-OAc, and [Ni(TPrPc-OAc)], where TPrPc-OH: 9-hydroxy-2,7,12,17-tetrapropylporphycenato dianion and TPrPc-OAc: 9-acetoxy-2,7,12,17-tetrapropylporphycenato dianion. H2TPrPc-OH is structurally determined by single-crystal X-ray diffraction, showing the enol-form in the solid state. The introduction of the electron-donating hydroxy group to the meso-position decreases the HOMO–LUMO gap by ca. 0.2 eV relative to the unsubstituted parent porphycene due to the remarkable destabilization of the HOMO level. This is experimentally confirmed by red shift of the Q band in the UV–vis spectra (in CH2Cl2), negative shift of the E1/2 (in 0.1 M n-Bu4NPF6–THF), and also supported by DFT calculations. In aqueous THF, an irreversible oxidation of H2TPrPc-OH and [Ni(TPrPc-OH)] appears to be pH-dependent, with a negative Epa shift upon increasing the solution pH (in the region from 3.0 to 10.5) with slopes of −62 and −63 mV per pH unit for H2TPrPc-OH and [Ni(TPrPc-OH)], respectively, indicating the occurrence of {1H+/1e} proton-coupled electron transfer.