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 $S_2(1{}^{1}B_u^{+})\to S_1(2{}^{1}A_g^{-})$ internal conversion in a series of carotenoids with a number of conjugated double bonds ($N$) ranging from 5 to 15. For the longer carotenoids ($N\ge 9$), the measurements reveal the existence of an additional intermediate excited state lying between the optically allowed $S_2$ state and the lower-lying forbidden $S_1$ state. This state enables us to explain the nonmonotonic dependence of the $S_2\to S_1$ conversion rate on $N$ and is expected to play an important role in photosynthetic light harvesting.