Polarization-sensitive resonance CARS spectroscopy of magnesium octaethylporphine

The structure and Franck-Condon region of protochlorophyllide a, a precursor in the biosynthesis of chlorophyll and substrate of the light-regulated enzyme protochlorophyllide oxidoreductase (POR), were investigated by Raman and resonance Raman (RR) spectroscopy. The spectroscopic results are compared to the spectra of the structurally closely related porphyrin model compound magnesium octaethylporphyrin (MgOEP), and interpreted on the basis of density functional theory (DFT) calculations. It is shown that the electronic properties of the two porphyrin macrocycles are affected by different vibrational coupling modes, resulting in a higher absorption cross section of protochlorophyllide a in the visible spectral region. Furthermore, a comparison of the Fourier transform (FT)-Raman and RR spectra of protochlorophyllide a indicates the modes that are resonantly enhanced upon excitation. Based on vibrational normal mode calculations, these modes include C-C ring-breathing and C C stretching vibrations of the porphyrin macrocycle. In particular, the strong band at 1703 cm −1 can be attributed to the C O carbonyl vibration of the cyclopentanone ring, which is attached in conjugation to the π-electron path of the porphyrin ring system. The enhancement of that mode upon electronically resonant excitation is discussed in the light of the reaction model suggested for the photoreduction of protochlorophyllide a in the POR.

Section: Mgoepsupporting

confidence: 90%

Probing the structure and Franck–Condon region of protochlorophyllide a through analysis of the Raman and resonance Raman spectra

Hanf¹,

Tschierlei²,

Dietzek³

et al. 2009

“…Such a pattern of mode-selective resonance enhancement is consistent with that typically reported for metallo-octaethylporphyrins, which are excited in the Q 00 -Q 01 absorption range. 5,7,9,18,19,21,23 The most pronounced enhancement is undergone by the antisymmetric A 2g modes ( 19 , 21 and 22 ), which are seen to dominate all spectra except spectrum (d) in Fig. 3.…”

Section: Vibrational Assignment In Zn-oepmentioning

confidence: 95%

Vibrational bands of luminescent zinc(II)‐octaethylporphyrin using a polarization‐sensitive ‘microscopic’ multiplex CARS technique

Otto

Voroshilov

Kruglik

et al. 2001

Polarization-sensitive, multiplex coherent anti-Stokes Raman scattering (ps-MCARS) has been used to detect the vibrational bands of the highly luminescent zinc(II)-octaethylporphyrin (Zn-OEP). We show here that ps-MCARS can be used to measure the vibrational bands under resonant conditions. Polarization-sensitive CARS further helps to resolve composite and overlapping Raman bands on the basis of variations in vibrational symmetry. This paper also exemplifies the advantages of multiplex CARS (MCARS) spectroscopy carried out under tight focusing conditions as proposed earlier. We demonstrate that a 'microscopic' CARS focusing and collection system in combination with the multiplex technique allows a reduction in the amount of sample and a shortening of the measurement time. We analyse the relationship between the coherent vibrational phases and the symmetry of the different vibrational band of Zn-OEP skeletal modes. The values of the coherent vibrational phases are similar for vibrational bands with the same symmetry.

“…Recalling that the position of the peak along the CARS wavenumber axis is given approximately by the arithmetic mean wavenumber of two vibrational modes beating with each other and that the peak location along the FFT wavenumber axis corresponds to the wavenumber difference of the two modes, we can compare the peak position to the vibrational mode spectrum of MgOEP reported in the literature. 28 We thus attribute this peak to a quantum beating between the two modes 10 and 11 . Both of these modes possess b 1g symmetry and they are located at wavenumber positions of 1606 and 1552 cm 1 , respectively.…”

Section: Spectrally Dispersed Transient Cars Signalmentioning

confidence: 90%

“…The assignment of these peaks to vibrational modes was performed by a comparison of our results with data obtained from resonance Raman measurements and ns-CARS experiments. 28,30 To this end we took advantage of the fact that the FFT wavenumber position of the peaks in the PSD plots directly corresponds to the wavenumber difference Q j Q k of modes j and k . This information allowed us to derive spectral information on the relative peak positions of individual vibrational modes from our measurements.…”

Section: Spectrally Dispersed Cars Signals Of Mgoep For Different Excmentioning

confidence: 99%

Two‐dimensional probing of ground‐state vibrational dynamics in porphyrin molecules by fs‐CARS

Heid

Schlücker

Schmitt

et al. 2001

We report on spectrally dispersed femtosecond time-resolved coherent anti-Stokes Raman scattering (CARS) for the investigation of the electronic ground-state vibrational dynamics of porphyrin molecules in solution. The two-dimensional experimental data obtained in our measurements provide a detailed mapping of the large number of molecular vibrations in these systems, excited by broadband ultrashort laser pulses, and make a thorough interpretation of the complex signal structure possible. In particular, an analysis of the data by applying Fourier transform methods to the time domain signal allows a clear identification of the excited porphyrin normal modes when compared with results on the mode structure from spectrally resolved (cw) spectroscopy. Altogether the method is capable of yielding detailed information on the dephasing behavior and on the relative spectral positions of the excited vibrations at the same time. Three porphyrin systems, dissolved in dichloromethane, were investigated: magnesium octaethylporphyrin, magnesium tetraphenylporphyrin and the free-base porphyrin octaethylporphyrin. The measurements were performed in the spectral region of the ring vibrations of the porphyrin macrocycle. We also provide a discussion of the expected signals on a theoretical basis by deriving the CARS third-order non-linear polarization for these experiments.