Deconstructing the Excited-State Dynamics of β-Carotene in Solution

The femtosecond to nanosecond dynamics of the all-trans beta-carotene carotenoid dissolved in 3-methylpentane is characterized and dissected with excitation-wavelength and temperature-dependent ultrafast dispersed transient absorption signals. The kinetics measured after red-edge (490 nm) and blue-edge (400 nm) excitation were contrasted under fluid solvent (298 K) and rigid glass (77 K) conditions. In all four measured data sets, the S* population kinetics was resolved prompting the development of a modified multicompartment model. The temperature-dependent and excitation wavelength-dependent S* quantum yield is ascribed to a competition of population surmounting a weak (55 cm(-1) energy barrier on the S-2 state to favor S-1 generation and rapid internal conversion that favors S* generation. When cooled from room temperature to 77 K the S* decay time scale shifted significantly from 30 to 400 ps, which is ascribed to small-scale structural relation with a 115 cm(-1) energy barrier. For the first time under low-energy excitation conditions, the triplet state is observed and confirmed to not originate from S* or S-1, but from S-2. The interconnectivity of the S* and S-1 populations is discussed, and no observed population flow is resolved between S* and S-1. Comparison of samples obtained from different laboratories with different purity levels demonstrates that sample contamination is not the primary origin of the S* state.