Nature of the dark $S*$ state in carotenoids
In plain words
Carotenoids, the orange pigments in carrots and plant leaves, have excited states that do not absorb light directly but appear in fast laser experiments. One of them, called S*, has been assigned to at least four different origins.
Precise statement
In ultrafast transient absorption of carotenoids (beta-carotene, lycopene, spirilloxanthin) in solution and in light-harvesting complexes, the $S*$ signal has been assigned to a vibrationally hot $S1$ ($2\mathrm{Ag}-$) state, a distinct electronic state ($1\mathrm{Bu}-$ or triplet-pair character), a twisted conformer, or ground-state population. Identify the electronic and vibrational character of $S*$, its formation path from $S2$ ($1\mathrm{Bu}+$), and its role in carotenoid-to-chlorophyll energy transfer. An answer is an assignment consistent with transient absorption and 2D spectra and with multireference calculations on polyenes of 9 to 13 conjugated double bonds.
What would settle it
Multireference excited-state dynamics on full-length carotenoids compared with two-dimensional electronic and vibrational spectra in solution and in protein.