Carotenoids

Carotenoids are organic pigments found in plants and animals. They provide much of the colour we see in nature (flowers, lobsters, carrots...) due to their polyene-type structure.


Carotenoid aggregate photophysics

Carotenoids like to aggregate causing interaction between pi-electrons on neighbouring molecules. Once they aggregate, their physical and chemical properties change dramatically. For example, their linear absorption spectrum changes and depending on the aggregate structure either blue or red-shifts (see figure). We can make a range of carotenoid aggregate structures with absorption spectra spanning the entire visible range.

The photophysical processes that occur upon absorption of light also change. In isolated carotenoids, absorption of light forms a singlet exciton which decays to the lowest-lying excited state within a few hundred femtoseconds. From there, the excitation decays to the ground-state, losing energy as heat, within a few picoseconds.

We have recently demonstrated that in carotenoid aggregates, however, a process known as 'Singlet Exciton Fission' occurs, producing two triplet excitons within 100 femtoseconds of light absorption. These triplet excitons can be long-lived.

Linear absorption spectra of astaxanthin monomer (dashed line) and five aggregates with different structures.

Linear absorption spectra of astaxanthin monomer (dashed line) and five aggregates with different structures.

Singlet exciton fission is a quantum mechanical process whereby a spin-0 (singlet) excited state spontaneously splits into two spin-1 (triplet) excited states within a picosecond. In the carotenoid aggregates, singlet fission is very efficient, occurring with a time-constant of less than 100 femtoseconds.

This process was discovered in the 1950s but has recently become a very hot topic due to the demonstration of singlet exciton fission solar cells. In current solar cells, much of the UV and blue light absorbed is converted into waste heat as the absorbed photon energy is much higher than the collected electronic energy. One mechanism for harvesting all of the absorbed photon energy is to use singlet exciton fission, where the first excited state splits into two lower energy states which can both be harvested. This could dramatically improve solar cell efficiency.

The mechanism of singlet exciton fission in polyene-type materials such as carotenoids is not understood. We are working to develop a new theory and to better understand singlet exciton fission to develop new materials for solar cells.


Carotenoid aggregates in photoprotection

One common function of carotenoids in plants and animals is their role in photoprotection:

Carotenoids accumulate in human skin, for example, where they are likely to protect against skin cancer. In the eye, they prevent macular degeneration. Plants accumulate carotenoids when they are put under stress (in drought, for example).

The unicellular algae Haematococcus pluvialis live in salty ponds by the sea. To protect themselves when the ponds dry out, the algae synthesize the carotenoid astaxanthin. They produce astaxanthin in such large quantities that we grow them to feed fish!

Given the high concentration of carotenoids present in all of the photoprotective systems mentioned above and the tendency of carotenoids to aggregate, it seems likely that carotenoid aggregation is in some way involved in photoprotection. Aggregation, where the electrons on neighbouring molecules interact, dramatically alters the carotenoid's properties. However, surprisingly little work has been done to study the biological relevance or the photophysics of carotenoid aggregates.

Given the likely ubiquitousness of carotenoid aggregates in natural photoprotection, this is clearly a topic that needs addressing further. We study the excited state photophysics of carotenoid aggregates in vivo in model biological systems (unicellular algae) and compare them with carotenoid aggregates in vitro to determine and potentially re-evaluate the roles of carotenoids in photoprotection.

 

[1] The Nature of Singlet Exciton Fission in Carotenoid Aggregates A. J. Musser, M. Mauri, D. Brida, G. Cerullo, R. H. Friend and J. Clark Journal of the American Chemical Society, 137, 5130 (2015)

 

[2] Activated Singlet Exciton Fission in a Semiconducting Polymer, A.J. Musser, M. Al-Hashimi, M. Mauri, D. Brida, M. Heeney, G. Cerullo, R.H. Friend and J. Clark, J. Am. Chem. Soc., 135, 12747 (2013)