Singlet fission in carotenoid aggregates: insights from transient absorption spectroscopy

Abstract

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→Tn transition that is strongly exciton-coupled to either the S0→S2 transition of surrounding ground-state chromophores, or a T1→Tn transition of a nearby triplet excited state. The yield of triplet states could be 180% or more in the stronglycoupled 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.