Femtosecond pump-supercontinuum probe and transient lens spectroscopy of adonixanthin

The ultrafast internal conversion (IC) dynamics of adonixanthin in organic solvents were studied by pump-supercontinuum probe (PSCP) and transient lens (TL) spectroscopy after photoexcitation to the S₂ state. Transient PSCP spectra in the range 344-768 nm provided the spectral evolution of the S₀ → S₂ ground state bleach and S₁ → S_n excited state absorption. Time constants were τ₂ = 115 and 111 fs for the S₂ → S₁ IC and τ₁ = 6.4 and 5.8 ps for the S₁ → S₀ IC in acetone and methanol, respectively. There was only an insignificant polarity dependence of τ₁, underlining the negligible importance of intramolecular charge transfer (ICT) in the lowest-lying excited state of C₄₀ carotenoids with carbonyl substitution on the β-ionone ring. A blueshift and a spectral narrowing of the S₁ → S_n ESA band, likely due to solvation dynamics, and formation of the adonixanthin radial cation at high pump energies via resonant two-photon ionization were found.