Solvent-dependent ultrafast internal conversion dynamics of n′-apo-β-carotenoic-n′-acids (n = 8, 10, 12)

Abstract

The ultrafast internal conversion dynamics of 12′-apo-β-carotenoic-12′-acid (12′CA), 10′-apo-β-carotenoic-10′-acid (10′CA) and 8′-apo-β-carotenoic-8′-acid (8′CA) have been investigated by femtosecond pump–probe spectroscopy. The three apocarotenoic acids were excited to the S₂ state with different excess energies. Time constants τ₁ for the IC process S₁/ICT → S₀ were measured by probing the dynamics at 390 nm (S₀ → S₂), 575 nm (S₁/ICT → S_n), 850, 860 and 890 nm (S₂ → S_n and S₁/ICT → S₀). In nonpolar solvents, the observed reduction of the τ₁ values with increasing conjugation length of the acids is consistent with a reduction of the energy gap between the S₁/ICT and S₀ states. The values are in good agreement with those of the corresponding apocarotenals studied previously in our groups. In polar solvents, a pronounced reduction of τ₁ values was observed for 12′CA, however the behavior was different from that observed for the respective aldehyde 12′-apo-β-caroten-12′-al studied previously: First, the degree of τ₁reduction in methanol was milder for 12′CA (218 → 55 ps) than for 12′-apo-β-caroten-12′-al (220 → 8 ps). Secondly, for 12′CA the plateau of solvent independent τ₁ values extended further into the mid-polar range (up to 0.5 on the Δf scale) than previously observed for the 12′-aldehyde. For 10′CA the polarity effect on the τ₁ values was weaker (∼71 ps in n-hexane and 34 ps in methanol) and for 8′CA it disappeared completely (∼24 ps averaged over all solvents). The polarity-induced reduction of τ₁ is likely due to the stabilization of an intramolecular charge transfer state in polar solvents. This S₁/ICT state is also responsible for the stimulated emission in the near IR, which has been observed in this specific class of carotenoids with a terminal carboxyl group for the first time. The occurrence of stimulated emission in the near IR region is also consistent with the steady-state fluorescence spectra which are reported along with the absorption spectra of these species. Possible reasons for the different behavior of the apocarotenoic acids compared to the respective aldehydes are discussed.