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 HOMOLUMO 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 UVvis spectra (in CH₂Cl₂), negative shift of the E _1/2 (in 0.1M n-Bu₄NPF₆THF), and also supported by DFT calculations. In aqueous THF, an irreversible oxidation of H ₂TPrPc-OH and [Ni(TPrPc-OH)] appears to be pH-dependent, with a negative E_pa shift upon increasing the solution pH (in the region from 3.0 to 10.5) with slopes of -62 and -63mV per pH unit for H ₂TPrPc-OH and [Ni(TPrPc-OH)], respectively, indicating the occurrence of {1H⁺/1e} proton-coupled electron transfer.