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 1Bᵤ^- state plays an intermediate state of the relaxation from the initially photoexcited 1Bᵤ⁺ state to the 2Ag^- state. After the relaxation to the 2Ag^- state, the excess energy is held in a C=C stretching mode (ν₁) of the 2Ag^- state. The lifetime of the ν₁ 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 ν₁ mode during the relaxation kinetics.