Figure 1.

Aggregation of astaxanthin. (a) AXT chemical structure. (b) Normalized UV–vis absorption of five stable aggregates of AXT in 9:1 water:DMSO (I and II) or 4:1 water:acetone (III–V) as well as monomeric AXT in acetone (dashed). Vertical lines indicate pump photon energies used for TA measurements. (c) Model of exciton decay in monomeric AXT. Ultrafast internal conversion from 1B_u to 2A_g is followed by fast nonradiative decay to the ground state. (d) Model of singlet fission and triplet annihilation in carotenoid aggregates, as measured here. Upon aggregation, the 1B_u state no longer couples to 2A_g. Instead, it converts directly into triplet pairs via singlet fission. The 2A_g state is lower in energy than the triplet pair and enables efficient recombination back to the ground state.