Pump-Dump-Probe Spectroscopy of Fuxocanthin in Solution and in FCP Complex

Marcel Fucimana, Robert Westa, Valentyna Kuznetsovaa, Tomáš Polívkaa
aFaculty of Science, University of South Bohemia, Branišovská 1760, Ceské Budejovice 370 05, Czech Republic

The pump-dump-probe and pump-repump-probe technique was used to thoroughly explore excited states of fucoxanthin. In the multipulse transient absorption the initial excitation (pump) pulse is tuned to the strongly allowed S0 - S2 absorption transition. A second pulse either dumps or re-excites a selected transition enabling us to study carotenoid excited states, which are not directly accessible by standard pump-probe spectroscopy. A third, spectrally broad pulse probes the sample’s absorption spectrum. To determine whether the intramolecular charge transfer (ICT) state of the carotenoid fucoxanthin is coupled to the S1 state, the population of the ICT state was  selectively depleted (at 1.7 ps) by tuning the dump pulse to the transition corresponding to ICT stimulated emission (950 nm).

Observing the transient absorption spectra monitoring the S1-Sn and ICT-SN transitions after dumping, it is clear that part of the population of fucoxanthin’s ICT state was removed by dumping at 950 nm. It was found that the ICT state population may be affected independently from the S1 population. However, repopulation of the ICT state from the S1 state is observed afterwards, revealing equilibration to the original S1-Sn/ICT-SN ESA ratio. The rate of the equilibration process strongly depends on the environment (acetonitrile, methanol). Furthermore, higher levels of fucoxanthin and their relaxation dynamics were studied by repumping either the S1-Sn or the ICT-SN transitions by tuning the repump beam to their excited state absorption wavelengths (i.e. 540 nm or 640 nm, respectively). The pump-dump-probe spectroscopy with dumping at 950 nm was also applied to fucoxanthin bound to FCP complex.