Structural Investigations on Octaethylporphyrin Using Density Functional Theory and Polarization-Sensitive Resonance Coherent Anti-Stokes Raman Scattering Spectroscopy

Abstract

Probing the structure and dynamics of porphyrins by vibrational spectroscopy is of major interest because this cyclic tetrapyrrole system constitutes the chromophore in different very complex biological systems carrying out essential processes in nature such as oxygen transport and storage (hemoglobin and myoglobin, respectively), electron transfer (cytochrome c), and energy conversion (chlorophyll). Most of the biological porphyrins are β-substituted, i.e., substituted at the outer pyrrole carbon atoms. Investigations on the structure of octaethylporphyrin (OEP) using density functional theory (DFT) as well as linear and nonlinear Raman spectroscopic techniques have been performed. The optimized geometry of OEP reveals a centrosymmetric molecule with local D2h symmetry of the porpyhrin macrocycle showing an excellent agreement with the X-ray structure of OEP. The DFT-derived harmonic vibrational wavenumbers together with the corresponding eigenvectors of several low-wavenumber modes and prominent ag(D2h) and b1g(D2h) modes of OEP are presented. Resonance Raman (RR) and multiplex polarization-sensitive resonance coherent anti-Stokes Raman scattering (PS RCARS) spectroscopy have been applied to OEP in dichloromethane to obtain complementary vibrational spectroscopic information. The RR spectrum obtained by excitation within the B or Soret bands reveals mainly ag modes, whereas the corresponding Raman measurements in resonance with the Q bands have been foiled by excessive fluorescence. In the PS RCARS spectra acquired with Q-band excitation, only b1g modes are detected. The different enhancement pattern of this β-substituted free-base porphyrin in comparison with metalloporphyrins (MP) can reasonably be explained in terms of symmetry lowering (D4h → D2h) supporting the DFT-derived structure.