ja003137y_si_001.pdf (459.48 kB)
On the Negligible Impact of Ruffling on the Electronic Spectra of Porphine, Tetramethylporphyrin, and Perfluoroalkylporphyrins
journal contribution
posted on 2001-04-06, 00:00 authored by Alan K. Wertsching, Andrew S. Koch, Stephen G. DiMagnoIn the first part of this paper, the syntheses, structural characterization, molecular modeling, and
electronic spectra for planar and nonplanar perfluoroalkylated porphyrins, (Rf)4P's, are reported. These studies
demonstrate that the intrinsic substituent effect of the perfluoroalkyl group on the long-wavelength electronic
spectrum of porphyrins is substantial, and similar (in magnitude) to that of a phenyl ring. Moreover, it is
shown that out-of-plane distortion of (Rf)4P's has a negligible impact on their electronic spectra. These data
bolster the findings of our earlier work and demonstrate that nonplanarity of (Rf)4P's does not result in a
red-shift in their optical spectra. In the second part of this paper, time-dependent density functional spectral
calculations (B3LYP/6-311G*/TD) for porphine, 5,10,15,20-tetrakis(trifluoromethyl)porphyrin, and 5,10,15,20-tetramethylporphyrin in a variety of ruffled conformations are reported. The results of these studies indicate
that (1) substantial ruffling of porphyrins has a negligible effect upon their electronic spectra, (2) similarly
small effects upon electronic spectra are predicted if electron-withdrawing or electron-releasing groups decorate
the porphyrin periphery, (3) for sterically encumbered porphyrins, ruffling can actually result in hypsochromic
shifts in various absorption bands, and (4) the bulk of the red-shift commonly thought to be due to nonplanar
distortion actually arises from other substituent effects. These results pose serious challenges to previous
theoretical and empirical studies that have sought to find a cause-and-effect relationship between nonplanarity
and electronic spectra in porphyrins.