Ultrafast relaxation kinetics of excited states in a series of mini- and macro-β-carotenes

Relaxation kinetics in a series of all-trans-carotenes that have excited states with different energies has been investigated using femtosecond absorption and Raman spectroscopy. The dependence on the energies shows that the ${1B}_{\mathrm{u}}^{-}$ state plays an intermediate state of the relaxation from the initially photoexcited ${1B}_{\mathrm{u}}^{+}$ state to the ${2A}_{\mathrm{g}}^{-}$ state. After the relaxation to the ${2A}_{\mathrm{g}}^{-}$ state, the excess energy is held in a $\mathrm{C}=\mathrm{C}$ stretching mode $\left({\nu }_1\right)$ of the ${2A}_{\mathrm{g}}^{-}$ state. The lifetime of the ${\nu }_1$ mode is longer than several picoseconds in short (mini) carotenes, but it is shorter than 1 ps in long (macro) carotenes. Vibrational feature in carotenoids is of importance in energy transfer of photosynthesis, because the excess energy of the photoexcitation is stored in the ${\nu }_1$ mode during the relaxation kinetics.