Conjugation length dependence of internal conversion in carotenoids: Role of the intermediate state

We report on a sub-20-fs transient absorption study of the S2(1Bu+)-S1(2Ag-) internal conversion in a series of carotenoids with a number of conjugated double bonds (N) ranging from 5 to 15. For the longer carotenoids (N>9), the measurements reveal the existence of an additional intermediate excited state lying between the optically allowed S2 state and the lower-lying forbidden S1 state. This state enables us to explain the nonmonotonic dependence of the S2-S1 conversion rate on N and is expected to play an important role in photosynthetic light harvesting.