On the Negligible Impact of Ruffling on the Electronic Spectra of Porphine, Tetramethylporphyrin, and Perfluoroalkylporphyrins

Abstract

In the first part of this paper, the syntheses, structural characterization, molecular modeling, and electronic spectra for planar and nonplanar perfluoroalkylated porphyrins, (R(f))(4)P'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 (R(f))(4)P's has a negligible impact on their electronic spectra. These data bolster the findings of our earlier work and demonstrate that nonplanarity of (R(f))(4)P'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.

Magnetic circular dichroism spectroscopy of cobalt tetraphenyltetraacenaphthoporphyrin

“…Shelnutt and coworkers have proposed, based on molecular mechanics and INDO/s calculations, that there is a direct correlation between the red shifts observed for the Q and B bands of sterically hindered porphyrins and the degree of ligand non-planarity [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. On the other hand, Di Magno and coworkers [19][20][21] have pointed to the fact that saddled free base and cobalt tetrakis(perfluoroalkyl)porphyrins do not show a red shift relative to the planar tetraphenylporphyrins and have postulated that red shifts are due to the impact of electron withdrawing or donating effects and in-plane nuclear reorganization induced by bulky substituents on the HOMO-LUMO band gap and the energies of the frontier p-MOs.…”

“…On the other hand, Di Magno and coworkers [19][20][21] have pointed to the fact that saddled free base and cobalt tetrakis(perfluoroalkyl)porphyrins do not show a red shift relative to the planar tetraphenylporphyrins and have postulated that red shifts are due to the impact of electron withdrawing or donating effects and in-plane nuclear reorganization induced by bulky substituents on the HOMO-LUMO band gap and the energies of the frontier p-MOs. DFT calculations were reported [19,31], which predicted that ligand deformation via ruffling and saddling need not necessarily result in a significant red shift.…”

“…On the other hand, Di Magno and coworkers [19][20][21] have pointed to the fact that saddled free base and cobalt tetrakis(perfluoroalkyl)porphyrins do not show a red shift relative to the planar tetraphenylporphyrins and have postulated that red shifts are due to the impact of electron withdrawing or donating effects and in-plane nuclear reorganization induced by bulky substituents on the HOMO-LUMO band gap and the energies of the frontier p-MOs. DFT calculations were reported [19,31], which predicted that ligand deformation via ruffling and saddling need not necessarily result in a significant red shift. Shelnutt and coworkers [7] subsequently pointed out that these calcula- tions involved artificially constraining the molecular geometry, and reported a series of INDO/s calculations for ruffled structures of tetraalkylporphyrins that clearly showed a direct correlation between red shifts of the Q and B bands and the degree of ligand folding.…”

“…There has been considerable recent controversy over the origin of significant red shifts observed in the major p ? p * bands of peripherally crowded dodecasubstituted porphyrins, which have been used to model the effects of ligand folding on the optical spectroscopy and electronic properties of metal porphyrinoids in proteins [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. Ligand non-planarity is one of the key factors in determining the redox properties, the basicity of the inner nitrogen atoms, and the axial ligand binding affinity of metal porphyrinoids and is, therefore, believed to play a role in controlling the function of heme proteins, and, in particular, methyl-coenzyme M reductase [6,7].…”

Conformational analysis of octa- and tetrabromo tetraphenylporphyrins and their Ni(II) and Tb(III) complexes

“…In view of the recent debate in the literature [40][41][42][43] on the origin of red shifts in the optical absorption bands of non-planar porphyrins, which seems to be resolved in the most recent study by Shelnutt and coworkers [44], it was tempting to look into the correlation between the calculated degree of porphyrin core puckering (both for chloro- [15] and for bromo-derivatives studied in this work) and the available UV-visible data [26]. The position of the B(0,0) (Soret) band, measured in CH 2 Cl 2 and reported by Coutsolelos and co-workers [26], is plotted against the total out-of-plane deformation, obtained from the NSD analysis of the energyminimized MM structures, in Fig.…”