Understanding Anionic Hyperporphyrins: TDDFT Calculations on Peripherally Deprotonated meso-Tetrakis(4-hydroxyphenyl)porphyrin.

Presented herein is a DFT/TDDFT study of meso-tetrakis(4-hydroxyphenyl)porphyrin (H₂[THPP]) and its O-deprotonated tetraanionic form; the latter was modeled as both a free tetraanion and with various counterions. Based on our calculations, the experimentally observed hyperporphyrin spectra are attributed to an admixture of phenol/phenoxide character into the a_2u-type HOMO of tetraphenylporphyrin. The admixture results in an elevation of the orbital energy of the HOMO in relation to other frontier orbitals, which accounts for the observed spectral redshifts. The calculations underscore differences in the performance of different exchange-correlation functionals. Thus, while the popular hybrid functional B3LYP greatly exaggerates the redshift of the far-red hyperporphyrin band of O-deprotonated H₂[THPP], the range-separated functional CAMY-B3LYP predicts a more moderate redshift. The latter, however, fails to reproduce experimentally observed absorptions in the 550-600 nm range, potentially underscoring the still imperfect modeling of anionic hyperporphyrins.