Nonplanar Heme Deformations and Excited State Displacements in Nickel Porphyrins Detected by Raman Spectroscopy at Soret Excitation

Abstract

We have correlated the Raman intensities of out-of-plane modes of nickel porphyrins with the nonplanar deformations of specific symmetries, i.e., static normal coordinate deformations (SNCDs) expressed in terms of irreducible representations of the unperturbed D(4h) point group. The model porphyrins Ni(II) octaethyltetraphenylporphyrin (NiOETPP), Ni(II) tetra(isopropyl)porphyrin (NiT((i)Pr)P), Ni(II) tetra(tert-butyl)porphyrin (NiT((t)Bu)P), and Ni(II) meso-tetraphenylporphyrin (NiTPP) were chosen because they exhibit significant out-of-plane deformations of different symmetries. At B-band excitation, the Raman scattering of out-of-plane modes becomes activated mostly via the Franck-Condon mechanism. Some characteristic bands from out-of-plane modes in the spectra were identified as reliable predictors of the type and magnitude of out-of-plane deformation. The gamma(10)-gamma(13) bands are indicators of ruffling (B(1u)) deformations for porphyrins, as confirmed by data for NiTPP, NiT((i)Pr)P, and NiT((t)Bu)P, where the Raman intensity increases with the magnitude of the ruffling deformation. The gamma(15)-gamma(17) bands are indicators of saddling (B(2u)) deformations, as shown by data for NiOETPP, which is highly saddled. By comparing the relative intensities of these prominent Raman bands we estimated the vibronic coupling parameters using a self-consistent analysis, and showed that they reproduce the respective B-band absorption profiles. We also identified the deformations along the lowest wavenumber normal coordinates as the predominant reason for the Raman activity of out-of-plane modes. Our results suggest that some of the normal coordinates (gamma(10) and gamma(13)) may be used as tools to quantitatively probe the nonplanar deformations of metalloporphyrins with alkyl substituents at the meso-positions. Out-of-plane deformations also increase the vibronic coupling strength of some low frequency in-plane Raman modes, namely, nu(7) and nu(8). Generally, the Raman data suggest that the excited B-state is substantially more nonplanar than the ground state. The overall larger vibronic coupling of ruffled porphyrins yields substantially larger dipole strengths for the vibronic sidebands associated with the B-state transition, so that the relative absorptivity of the B(v) band can be used as a convenient tool to probe the nonplanarity of the porphyrin macrocycle.