Femtosecond transient infrared and stimulated Raman spectroscopy shed light on the relaxation mechanisms of photo-excited peridinin

Abstract

By means of one- and two-dimensional transient infrared spectroscopy and femtosecond stimulated Raman spectroscopy, we investigated the excited state dynamics of peridinin, a carbonyl carotenoid occurring in natural light harvesting complexes. The presence of singly and doubly excited states, as well as of an intramolecular charge transfer (ICT) state, makes the behavior of carbonyl carotenoids in the excited state very complex. In this work, we investigated by time resolved spectroscopy the relaxation of photo-excited peridinin in solvents of different polarities and as a function of the excitation wavelength. Our experimental results show that a characteristic pattern of one- and two-dimensional infrared bands in the C=C stretching region allows monitoring the relaxation pathway. In polar solvents, moderate distortions of the molecular geometry cause a variation of the single/double carbon bond character, so that the partially ionic ICT state is largely stabilized by the solvent reorganization. After vertical photoexcitation at 400 nm of the S2 state, the off-equilibrium population moves to the S1 state with ca. 175 fs time constant; from there, in less than 5 ps, the non-Franck Condon ICT state is reached, and finally, the ground state is recovered in 70 ps. That the relevant excited state dynamics takes place far from the Franck Condon region is demonstrated by its noticeable dependence on the excitation wavelength.

Vibrational relaxation as the driving force for wavelength conversion in the peridinin–chlorophyll a-protein

“…There is no need for a population transfer between 1B u and 2A g as both states appear to be strongly absorbing when in the protein environment; we also find no crossings with 2A g along the 1B u relaxation path contrary to earlier calculations [12]. A very recent peridinin 2D-IR study shows that different transient IR relaxation profiles occur depending on the UV/vis excitation wave length [45].…”

Femtosecond Stimulated Raman Spectroscopy

“…For instance, the effective time resolution is not only determined by the cross‐correlation width of actinic pump and Raman probe but is also limited by group‐velocity mismatch in the sample . The most common way for mounting liquid samples is to use a 0.2 to 10 mm pathlength flow cell with quartz windows . Shorter pathlengths imply higher time‐resolution.…”

“…This compensates for the probe fluctuations on a shot‐to‐shot basis but requires the simultaneous detection of two pulses at a repetition rate of 1 kHz. Therefore, this scheme is most common in connection with photodiode array detectors but is also used with CCD cameras albeit at lower readout rates . Jin et al .…”

“…Diode arrays feature high readout rates in combination with a high dynamic range. In addition, dual‐array detectors can easily be integrated in a single spectrograph to allow the simultaneous acquisition of signal and reference spectra, which makes them attractive for FSRS experiments . In contrast to CCD detectors, diode arrays are also available for other spectral windows, like InGaAs arrays for the IR .…”

“…and Buckup et al . Other systems, where FSRS has been used to unravel internal conversion dynamics, include all‐ trans ‐neurosporene, diphenyloctatetraene, riboflavin and flavin adenine dinucleotide, metmyoglobin, and peridinin …”