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Title: Nature of the dark $S*$ state in carotenoids
Plain: 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: 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.
Settled by: Multireference excited-state dynamics on full-length carotenoids compared with two-dimensional electronic and vibrational spectra in solution and in protein.
Kind: phenomenon
Answer type: identification
References: (empty)
Literature status: contested
Status note: (empty)
Posed since: (empty)