Conformational Properties of Nickel(II) meso-Tetraphenylporphyrin in Solution. Raman Dispersion Spectroscopy Reveals the Symmetry of Distortions for a Nonplanar Conformer

Abstract

We have measured the Raman spectra of nickel(II) tetraphenylporphyrin (NiTPP) in CS2 with excitation wavelengths covering the region of the Q0, Qν, B0, and Bν absorption bands. The spectra were subjected to a global fit which provides reliable spectral parameters even in the case of strongly overlapping bands. The band shapes of ν8, Φ8, ν1, ν11, ν19, ν2, and ν10 are clearly asymmetric and can be decomposed into two sub-bands, the intensity ratios of which depend on the excitation wavelength. The Raman excitation profiles (REPs) of the low-frequency sub-bands of the core size marker bands ν11, ν19, ν2, and ν10 are red shifted with respect to the REPs of the corresponding high-frequency sub-bands. The REPs of the ν8 sub-bands also show different resonance positions, but in this case the REP of the high-frequency sub-band is the red-shifted one. The sub-bands exhibiting the red-shifted REPs result from a nonplanar conformer, whereas the other sub-bands correspond to a planar form. On the contrary, the REPs of the ν1 and Φ8 sub-bands can be scaled onto each other. Their heterogeneity results either from Fermi resonance or from additional conformations of the NiTPP molecules which may differ in the orientation of their phenyl substituents. Information about the modes of distortion giving rise to the nonplanar structure are obtained from the wavelength dependence of the depolarization ratios. The depolarized B1g and B2g bands show no dispersion. On the contrary, some A1g bands show a slight and a few A2g bands even a strong DPR dispersion resulting from of an electronic A2g-type perturbation. This indicates that the nonplanar conformer exhibits two types of symmetry-lowering distortions, namely, ruffling (B1u) and saddling (B2u). This parallels recent findings on NiTPP crystals. Thus evidence is provided that the nonplanar species of NiTPP in solution is structurally similar to the conformation observed in the crystallized phase.