Carotenoids are a highly colored (red, orange, and yellow) group of fat-soluble pigments and occur in all organisms that rely on the sun for energy. Their anti-oxidant effects enable these compounds to play a crucial role in protecting organisms against damage during photosynthesis - the process of converting sunlight into chemical energy. The basic structure of a carotenoid is that of a polyene with alternating single and double bonds. The structure of a typical carotenoid is shown in Fig. 1. In photosynthetic systems carotenoids display a plethora of functions. They absorb solar photons in the region around 450-550 nm (Fig. 2) and transfer the electronic excitation, on an ultrafast time-scale,
to a neighboring chlorophyll, where it can be used to drive photosynthesis (Fig. 3). Maybe even more importantly, they efficiently remove harmful triplets (Fig. 3) that can be created by light on chlorophyll and otherwise would give rise to the formation of poisonous singlet oxygen.
The electronic states and the ensuing dynamics of carotenoids are highly complex, in particular within the first picoseconds after excitation. Biophysics studies the excited-state dynamics of carotenoids both in situ and in vitro using a multitude of ultrafast spectroscopic techniques employing short (~50 fs) laser pulses, in particular pump-probe absorbance difference spectroscopy, with
detection in the visible or infrared parts of the spectrum, and more advanced techniques like pump-dump-probe spectroscopy, in which a certain excited state is ‘dumped’ with a second laser pulse arriving shortly after the first. Results obtained in our laboratory have revealed the existence of new ultrafast states and processes in several types of carotenoids.