Experimental Evidence of the Sx State and Fluorescence Emission from the Intramolecular Charge Transfer States in Fucoxanthin

Fucoxanthin, a typical carotenoid that absorbs in the blue end of visible light, whose detailed electronic structures remain to be clarified. It is well known that carotenoids harvest energy from sunlight and transfer it to chlorophylls (Chls) and/or bacteriochlorophylls (BChls) through its excited states as the intermediate states, however, some excited states still need evidence to be definitely confirmed. Through steady-state fluorescence emission spectroscopy and femtosecond time-resolved fluorescence up-conversion technique, we provide new evidence for the identification of the excited Sx state in fucoxanthin, a representative of carotenoids. The fluorescence emission from the intramolecular charge transfer (ICT) states was also observed and identified for first time according to our limited survey. Our findings suggest that fucoxanthin absorbs the blue light and transfer most of energy to BChls via S_x and ICT1 states for certain bacteria, but release them via ICT1 state to protect aginst light damage for algea.