Comparison of transient grating signals from spheroidene in an organic solvent and in pigment-protein complexes fromRhodobacter sphaeroides2.4.1

Abstract

The concurrent dynamics of the electronic excitation and vibronic oscillations of spheroidene have been investigated by means of the transient grating ͑TG͒ spectroscopy. The third-order optical responses of spheroidene in an organic solvent, in the LH2 light-harvesting antenna complexes, and in chromatophores have been compared in order to investigate the influence of the environment surrounding this photosynthetic pigment. Vibronic coherent oscillations with a period of several tens of femtosecond have been clearly observed superimposed on a slowly varying background, which reflects the electronic dynamics. The dynamics of the coherent oscillations have been analyzed by means of the wavelet analysis. Within our experimental accuracy, the decay times of the C v C and C u C stretching modes and C u CH 3 rocking mode of each specimen are very close. The experimental results have also been analyzed using a Brownian oscillator model. For these numerical calculations, the spectral density for the underdamped modes has been determined from the Raman spectrum of spheroidene. It was found that the low-frequency modes that reflect the influence of the protein environment can be approximated by the overdamped Brownian oscillator. The experimentally observed linear absorption spectra as well as the third-order optical responses, i.e., TG curves, are reproduced very well by these calculations. The close agreement between the experiments and calculations indicates that the Feynmandiagrammatic approach can be applied to express not only the internal conversion but also the intermolecular excitation energy-transfer processes. The vibronic decay rates of spheroidene in LH2 complexes and chromatophores are evaluated to be about 20% larger than in the organic solvent.

Vibronic coupling explains the ultrafast carotenoid-to-bacteriochlorophyll energy transfer in natural and artificial light harvesters

“…TG probes the sum of the absorptive and dispersive part of the induced third order signal, while pump-probe selects only the absorptive part, meaning that the retrieved time constants cannot be directly compared to pump-probe measurements. 48,49 Vibrational dynamics will however yield similar frequencies between the two techniques (see, e.g., vibrational dynamics in β-carotene investigated by pump-probe 18,50 and by TG 51,52 ). The vibrational response manifests as an oscillatory signal, superimposed on a slowly varying background.…”

Ultrafast time-resolved vibrational spectroscopies of carotenoids in photosynthesis

“…FWM signals in carotenoids are reported for β-carotene and its homologues, lycopene, astaxanthin, and spheroidene [47,[57][58][59][60][61][62][63][64][65][66][67][68]. As an example, the results of β-carotene are shown here.…”

Control of coherent vibronic oscillations in β‐carotene by ultrashort laser pulses

“…As discussed in our previous papers, it is necessary to introduce the S x state between S 1 and S 2 in considering the ultrafast phenomena of carotenoids [12,18,19,22,23]:…”