Structural investigations on octaethylporphyrin using density functional theory and polarization-sensitive resonance coherent anti-Stokes Raman scattering spectroscopy

Probing the structure and dynamics of porphyrins by vibrational spectroscop y is of major interest because this cyclic tetrapyrrole system constitutes the chromophore in different very complex biological systems carrying out e ssential processes in nature such as oxygen transport and storage (hemoglob in and myoglobin, respectively), electron transfer (cytochrome c), and ener gy conversion (chlorophyll). Most of the biological porphyrins are beta -su bstituted, i.e., substituted at the outer pyrrole carbon atoms. Investigati ons 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 centrosymmetr ic molecule with local D-2h symmetry of the porpyhrin macrocycle showing an excellent agreement with the X-ray structure of OEP. The DFT-derived harmo nic vibrational wavenumbers together with the corresponding eigenvectors of several low-wavenumber modes and prominent a(g)(D-2h) and b(1g)(D-2h) mode s of OEP are presented. Resonance Raman (RR) and multiplex polarization-sen sitive resonance coherent anti-Stokes Raman scattering (PS RCARS) spectrosc opy have been applied to OEP in dichloromethane to obtain complementary vib rational spectroscopic information. The RR spectrum obtained by excitation within the B or Soret bands reveals mainly a(g) modes, whereas the correspo nding Raman measurements in resonance with the Q bands have been foiled by excessive fluorescence. In the, PS RCARS spectra acquired with Q-band excit ation, only b(1g) modes are detected. The different enhancement pattern of this beta -substituted free-base porphyrin in comparison with metalloporphy rins (MP) can reasonably be explained in terms of symmetry lowering (D-4h - -> D-2h) supporting the DFT-derived structure.