Pump-deplete-probe spectroscopy and the puzzle of carotenoid dark states

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 S_sol 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 S_sol 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, S_sol is identified as a vibrationally excited ground state (S_sol = 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 S_sol state is not identical to the S_T state, which appears in the same spectral region when the carotenoid is embedded in light-harvesting complexes.