Singlet fission in carotenoid aggregates: insights from transient absorption spectroscopy

The excited-state dynamics of three types of zeaxanthin aggregates are probed with transient absorption spectroscopy on the femtosecond-to-microsecond timescale. Triplet excited states form via singlet fission in all three aggregates within several hundred femtoseconds. The transient absorption spectra are consistent with an S₂, but not S₁, parent state for singlet fission. The quantum yield of triplet states in one of the weakly-coupled aggregates is at least 60-80% immediately following photoexcitation. The same aggregate has a 10-30% yield of S1 excited states, which have a dominant decay time of ~8 ps. For the strongly-coupled H-aggregate, a new transient absorption band with maximum 400−420 nm is found. The band is assigned to a triplet state with T₁→T_n transition that is strongly exciton-coupled to either the S0→S2 transition of surrounding ground-state chromophores, or a T₁→T_n transition of a nearby triplet excited state. The yield of triplet states could be 180% or more in the strongly coupled aggregate, as inferred from the absence of S₁ signal. Fast annihilation depletes most of the triplet population in the aggregates on the picosecond timescale, however a measurable fraction persists beyond 1 μs.