Q_y-Excitation Resonance Raman Spectra of Chlorophyllaand Bacteriochlorophyllc/dAggregates. Effects of Peripheral Substituents on the Low-Frequency Vibrational Characteristics

Abstract

Low-frequency (80−700 cm-1) Q y -excitation resonance Raman (RR) spectra are reported for thin-solid-film aggregates of several chlorophyll (Chl) a and bacteriochlorophyll (BChl) c/d pigments. The pigments include Chl a, pyrochlorophyll a (PChl a), methylpyrochloropyllide a (MPChl a), methylbacteriochlorophyllide d (MBChl d), [E,M] BChl c S, [E,E] BChl c F, and [P,E] BChl c F. The BChl c/d's are the principal constituents of the chlorosomal light-harvesting apparatus of green photosynthetic bacteria. Together, the various Chl a's and BChl c/d's represent a series in which the peripheral substituent groups on the chlorin macrocycle are varied in a systematic fashion. All of the Chl a and BChl c/d aggregates exhibit rich low-frequency vibrational patterns. In the case of the BChl c/d's, certain modes in the very low-frequency region (100−200 cm-1) experience exceptionally strong Raman intensity enhancements. The frequencies of these modes are qualitatively similar to those of oscillations observed in femtosecond optical experiments on chlorosomes. The RR data indicate that the low-frequency vibrations are best characterized as intramolecular out-of-plane deformations of the chlorin macrocycle rather than intermolecular modes. The coupling of the out-of-plane modes in turn implies that the Q y electronic transition(s) of the aggregate have out-of-plane character. The RR spectra of the BChl c/d's also reveal that the nature of the alkyl substituents at the 8 and 12 positions of the macrocycle plays an important role in determining the detailed features of the low-frequency vibrational patterns. The frequencies of the modes are particularly sensitive to larger substituent groups whose conformations may be more easily perturbed in the tightly packed aggregates. These factors also make aggregates of pigments containing larger substituents more susceptible to structural, electronic, and vibrational inhomogeneities. Collectively, the RR studies of the various pigments delineate the factors which influence the low-frequency vibrational characteristics of chlorosomal aggregates.

Unraveling the nature of coherent beatings in chlorosomes

“…Since the frequencies of the observed coherent oscillations in the 2D spectra coincide well with the group of lines observed in the resonance Raman experiments (within the resolution of 18 cm −1 , given by the t 2 scan), 16,17 the oscillations can be attributed to the coherent vibrational wavepacket motion. For the possible assignment of the modes we follow the detailed work of Czarnecki et al 28 on the reaction centers from purple bacteria containing BChl a.…”

“…Moreover, only the modes around 50-170 cm −1 are observed as coherent beatings in the femtosecond experiments, whereas the other modes of comparable and even higher intensities in the resonance Raman experiments are not observed in time-resolved studies. 12,14,15 In contrast to the chlorosome data reported by Cherepy et al, 15 the resonance Raman spectra of the BChl c thin-solid-film aggregates 17 are dominated by two distinctive peaks at frequencies of ∼115 cm −1 and ∼145 cm −1 . These modes represent a close match to the Fourier transforms frequencies of the oscillatory components observed in the ultrafast experiments.…”

“…Some Raman spectral features in the reaction centers are similar to the features observed in the BChl c aggregates. 17 Particularly, the group of modes around 137 cm −1 is amplified in the resonance Raman spectrum of a strongly coupled special pair, as compared to the identical modes of weakly coupled accessory BChl a pigments. Based on ab initio calculation and the isotope labeling, these modes were assigned to the macrocycle doming mode, out-of-plane deformations of the pyrrole rings and inplane deformation of the acetyl group at C3 (according to the IUPAC nomenclature); all effective at modulating the electronic structure of the dimer.…”

“…tepidum two major frequencies of 70-90 cm −1 and 130-160 cm −1 are usually resolved, 12,15 which approximately coincide with the group of low-frequency modes observed in the resonance Raman scattering data on chlorosomes 16 or artificial thin-solid-film BChl c aggregates. 17 The observed oscillations in transient spectra were thus assigned to the coherent beatings between vibrational levels of these modes. 5,[12][13][14][15] Cherepy et al 16 speculated that these coherences are mostly of excited state origin, however, Ma et al 14 convincingly showed the presence of ground state vibrational coherences by the means of two-color transient absorption measurements on chlorosomes from Chlorobium phaeobacteroides, where the Q y transition was probed after Soret band excitation.…”

Excitonic Relaxation and Coherent Vibrational Dynamics in Zinc Chlorin Aggregates for Artificial Photosynthetic Systems

“…Meanwhile, the FT amplitude spectra at eight wavelengths are shown in Figure 7 64, 97, 146, 268, 321, 448, 519, 596, 701, 876, 945, 1015, 1152, 1259, 1339, 1418, 1554, and 1662 cm −1 , symmetric vibrational structures exhibit clear spectral features of the first derivative of the ground-state absorption spectrum. Most of the intense frequency signals higher than 1000 cm −1 are mainly assigned to the CH 3 bending, CH bending, and CO-, CC-, and CN-stretching vibrations, 37−40 while, the low-frequency (100−200 cm −1 ) modes, as reported by RR spectra in BChl aggregates, 22 are a consequence of intramolecular out-of-plane deformations of the chlorin macrocycle rather than intermolecular modes. Those coupling of the out-of-plane modes in turn implies that the Q-exciton transition of the aggregates have out-of-plane character.…”