Band-edge Excitation of Carotenoids Removes S* Revealing Triplet-pair Contributions to the S1 Absorption Spectrum
The nature of the low-lying electronic states in carotenoids has been debated for decades. We use excitation-dependent transient absorption spectroscopy and comparison with published results on \b{eta}-carotene to demonstrate that the so-called S* feature in astaxanthin, echinenone and spheroidenone spectra is due to an impurity in the sample. Excitation at the absorption band-edge results in a transient absorption spectrum dominated by the pure S1 photo-induced absorption, with no contribution from impurities (S*). We find that this S1 absorption resembles the triplet excited-state absorption spectrum, shifted by ~200meV. To explain this, we suggest that the individual triplets that make up the dominant triplet-pair (TT) configuration of 2A$_g^-$ contribute strongly to the S1 absorption band. Comparison with recent literature on molecular polycrystalline films which undergo singlet fission suggests the shift could be related to the binding energy of the triplet pair state. These findings have implications for understanding triplet-pair states in organic semiconductors and the excitation-energy dependence of singlet fission in carotenoid aggregates and biological systems.