Pump−Deplete−Probe Spectroscopy and the Puzzle of Carotenoid Dark States

Abstract

Pump−deplete−probe and transient absorption spectroscopy are applied to carotenoids with N = 11 conjugated double bonds in solution to study the origin of recently observed transient features that have been previously assigned to new electronic states. The depletion pulse pumps the transient near-IR band, whose lifetime coincides with the fluorescence lifetime, and is hence attributed to the S2 state. The subsequent signal of any lower-lying dark excited-state populated by internal conversion from S2 should be affected by the depletion pulse. Correspondingly, the signal in the S1 deactivation channel is diminished by the depleted excited population. In contrast, the Ssol* signal, purportedly reflecting an intermediate state on a competing deactivation pathway, is not affected by the depletion pulse. When comparing our results with literature data for other carotenoids, we find that the Ssol* lifetime is constant at 6.2 ± 0.4 ps for any N ≥ 11 carotenoid; for shorter chain lengths, it is equal to the S1 lifetime. To explain this puzzle, Ssol* is identified as a vibrationally excited ground state (Ssol* = hot S0), populated by a combination of impulsive Raman scattering of the pump pulse and internal conversion (S1−S0), and decaying by vibrational relaxation. The Ssol* state is not identical to the ST* state, which appears in the same spectral region when the carotenoid is embedded in light-harvesting complexes.