Experimental evidence of the Sx state and fluorescence emission from the intramolecular charge transfer states in fucoxanthin

Fucoxanthin is a typical carotenoid that absorbs light in the blue region of the visible spectrum, and its 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 the first time according to our limited survey. Our findings suggest that fucoxanthin absorbs the blue light and transfers most of the energy to BChls via Sx and ICT1 states for certain bacteria, while releasing them via the ICT1 state to protect against light-induced damage for algae. © 2024 The Royal Society of Chemistry.