Extremely strong solvent dependence of the S1 → S0 internal conversion lifetime of 12′-apo-β-caroten-12′-al

Abstract

The ultrafast internal conversion (IC) dynamics of the carbonyl carotenoid 12′-apo-β-caroten-12′-al has been investigated in solvents of varying polarity using time-resolved femtosecond transient absorption spectroscopy. The molecules were excited to the S₂ state by a pump beam of either 390 or 470 nm. The subsequent intramolecular dynamics were detected at several probe wavelengths covering the S₀ → S₂ and S₁ → S_n bands. For the S₁ → S₀ internal conversion process, a remarkably strong acceleration with increasing polarity was found, e.g., lifetimes of τ₁ = 220 ps (n-hexane), 91 ps (tetrahydrofuran) and 8.0 ps (methanol) after excitation at 390 nm. The observation can be rationalized by the formation of a combined S₁/ICT (intramolecular charge transfer) state in the more polar solvents. The effect is even stronger than the strongest one reported so far in the literature for peridinin. Addition of lithium salts to a solution of 12′-apo-β-caroten-12′-al in ethanol leads only to small changes of the IC time constant τ₁. In addition, we estimate an upper limit for the time constant τ₂ of the S₂ → S₁ internal conversion process of 300 fs in all solvents.