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Lutein helps plants harvest sunlight and protect themselves from excess light. Ultrafast laser spectroscopy at I2BC shows that the polarisability of its environment reshapes its electronic excited states and their lifetimes.
In plant leaves, lutein, a yellow carotenoid also abundant in spinach and egg yolk, is bound to the proteins that collect sunlight for photosynthesis. It helps capture light, stabilises these proteins and, under strong illumination, contributes to dissipating excess energy safely. All these functions rely on the electronic excited states that lutein reaches after absorbing a photon, which last only a few picoseconds (trillionths of a second). Inside these proteins, lutein is thought to sit in a highly polarisable environment, that is, one whose electron clouds readily deform in response to the pigment. How this polarisability affects its excited states was unknown.
I2BC researchers compared lutein in two solvents of very different polarisability: tetrahydrofuran (THF, low) and carbon disulfide (CS2, high). Combining femtosecond transient absorption with femtosecond stimulated resonance Raman spectroscopy (FSRRS), which selectively records the vibrational fingerprint of each electronic state, they identified the same set of excited states in both solvents: a vibrationally 'hot' S1 state, the relaxed S1 state, a weak charge-transfer contribution and a 'dark' state called S*. High polarisability, however, changes how these states behave. The S1 absorption shifts to lower energy and broadens, and the molecule barely reorganises after excitation. The S* state lives almost twice as long (about 29 instead of 16 picoseconds), and its vibrational frequency deviates from the trend expected from the length of the conjugated chain.
Polarisability therefore does not simply lower the energy of the excited states: it alters their nature and dynamics. These results provide a reference for understanding how the protein environment tunes the excited states of lutein, and thus its roles in light harvesting and photoprotection.
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