Electronic absorption and resonance Raman signatures of hyperporphyrins and nonplanar porphyrins

We have carried out a broad survey of tetraphenylporphyrin derivatives in relation to their possible hyperporphyrin character. The majority of the free-base tetraphenylporphyrins studied, i.e., TArPH2; Ar = P-X-C6H4, where X = CH3, H, F, CF3, and NO2, when dissolved in trifluoroacetic acid (i.e. when centrally diprotonated), exhibit red-shifted "hyperporphyrin" spectra. The "hyper" features are attributable to phenyl-to-porphyrin charge-transfer transitions. However, certain free-base tetraphenylporphyrins with extremely electron-deficient phenyl groups, such as TPFPPH2, do not exhibit hyperporphyrin spectra in trifluoroacetic acid solution. Certain anionic tetraphenylporphyrin derivatives such as T(p-OH-P)PH2 or Ni[T(p-OH-P)P] dissolved in methanolic Bu4NOH also qualify as hyperporphyrins. The hyper transitions in these cases involve charge transfer from anionic phenolate substituents to the neutral porphyrin core. This study also presents a first systematic resonance Raman spectroscopic exploration of hyperporphyrins. Comparison of the Soret-resonant Raman spectra of various normal, hyper-, and hypso- tetraphenylporphyrin derivatives indicates that the former two categories generally exhibit a more intense v, band, which is the fully symmetric C-meso-C-phenyl stretching vibration, relative to hypsoporphyrins such as square-planar nickel tetraarylporphyrins. We have also reinvestigated recent reports of large red shifts observed for the electronic spectra of saddled porphyrins in polar solvents, an effect attributed to increased N-H...solvent hydrogen bonding in polar solvents. Interestingly, we find that such solvent-induced red shifts are observed for the relatively electron-deficient porphyrin Br8TPPH2, Cl8TPPH2, and OETNPH2 but not for the relatively electron-rich OETPPH2. Resonance Raman spectra of these saddled porphyrins in different solvents reveal little shift in the high-frequency marker bands, which is consistent with little change in macrocycle conformation with solvent polarity. The observed solvent-induced red shifts in the electronic spectra therefore appear to reflect a largely electronic (as opposed to conformational) effect of N-H...solvent hydrogen bonding in polar solvents. Finally, we also present a chronological summary of the controversial question as to whether nonplanar deformations are actually responsible for the red-shifted electronic spectra of the majority of nonplanar porphyrins.