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Effects of introducing a carbonyl group into the conjugation system of carotenoids were studied for four naturally occurring carotenoids: peridinin, fucoxanthin, siphonaxanthin and spheroidenone. The conjugated carbonyl group affects energetics and dynamics of all these carotenoids in a similar way, although the magnitude of the changes depends strongly on the carotenoid structure. Firstly, presence of a carbonyl group considerably narrows the S 1 /ICT-S 2 gap, and this effect does not depend on polarity. The S 1 /ICT energies of carotenoids were measured by recording S 1 /ICT-S 2 spectral profiles in the near-infrared region and the resulting energies were 16100 cm À1 for peridinin, 16520 cm À1 for fucoxanthin and 16610 cm À1 for siphonaxanthin. Narrowing of the S 1 /ICT-S 2 gap has important consequences for functionality of these carotenoids in light-harvesting systems of oceanic organisms, since while the S 2 state is red-shifted to capture green light, the S 1 /ICT state is still high enough to transfer energy to chlorophyll. The S 1 /ICT energy of spheroidenone was determined to be 13000 cm À1 . Secondly the carbonyl group introduces some polarity-dependent effects: (1) polarity-induced change of the S 1 /ICT lifetime. When changing from nonpolar to polar solvent, the S 1 /ICT lifetime is changed from 160 to 8.5 ps for peridinin, from 60 to 30 ps for fucoxanthin, from 60 to 20 ps for fucoxanthin, while for the longer carotenoid spheroidenone the S 1 /ICT lifetime remains 6 ps regardless of solvent polarity. This effect is explained in terms of stabilization of charge-transfer character of both ground and excited states. (2) stabilization of the charge-transfer character in polar solvents is also demonstrated by appearance of new polarity-induced bands in the transient absorption spectra. (3) polarity-induced changes of the ground state are manifested by asymmetric broadening of the absorption spectrum accompanied by a loss of vibrational structure.

The dynamics of the excited states of the carotenoid peridinin in polar solvents were studied using femtosecond transient absorption spectroscopy in the spectral range of 500−1900 nm. A broadening of the absorption spectrum in polar solvents is caused by a distribution of conformers having different ground-state properties. In addition, the dependence of the peridinin lifetime on the excitation wavelength reveals that two peridinin forms coexist in protic solvents, where a “red”-absorbing form is produced by hydrogen bonding via the carbonyl group. The observed dynamics show that the S1 and intramolecular charge transfer (ICT) states of peridinin are strongly coupled, forming a collective S1/ICT state whose lifetime is determined by the degree of ICT character. In nonpolar solvent, pure S1 character with a lifetime of ∼160 ps is observed, whereas in polar solvents an increase in the ICT character leads to a lifetime as short as 10 ps in methanol and 13 ps in ethylene glycol. In protic solvents, the ICT character of the S1/ICT state of the red peridinin form is further enhanced by hydrogen bonding, resulting in lifetimes shorter than 6 ps. A weak dependence of peridinin dynamics on viscosity shows that the ICT state is not formed via a twisted ICT mechanism. At 190 K in methanol, a significant increase in the S1/ICT lifetime is observed, suggesting that thermal coupling is involved in the S1/ICT state mixing. At 77 K in ethylene glycol glass, a multiexponential decay is revealed, indicating the presence of several conformers with different S1/ICT state properties.

Spectroscopic properties as well as excited state dynamics of the carotenoid peridinin in several solvents of different polarities were investigated by time-resolved fluorescence and transient absorption techniques. A strong dependence of the peridinin lowest excited states dynamics on solvent polarity was observed after excitation into the strongly allowed S 2 state. Peridinin relaxes to the ground state within 10 ps in the strongly polar solvent methanol, while in the nonpolar solvent n-hexane a 160 ps lifetime was observed, thus confirming the previous observations revealed by transient absorption spectroscopy in the visible region et al. J. Phys. Chem. B 1999, 103, 8751). In addition, the solvent dependence in the near-IR region is demonstrated by a strong negative feature in the transient absorption spectrum of peridinin in methanol, which is not present in n-hexane. This band, characterized by a 1 ps rise time, is ascribed to stimulated emission from an intramolecular charge-transfer (ICT) state. Time-resolved fluorescence data support assignment of this band to the emissive singlet state, whose dynamic characteristics depend strongly on the dielectric strength of the medium. On the basis of all our time-resolved measurements combined with simulations of the observed kinetics, we propose the following model: the initially populated S 2 state decays to the S 1 state within less than 100 fs for both solvents. Then, the population is transferred from the S 1 state to the S 0 and ICT states. The S 1 f ICT transfer is controlled by a solvent polarity dependent barrier. In n-hexane the barrier is high enough to prevent the S 1 f ICT transfer and only S 1 f S 0 relaxation characterized by a time constant of 160 ps is observed. An increase of solvent polarity leads to a significant decrease of the barrier, enabling a direct quenching of the S 1 state by means of the S 1 f ICT transfer, which is characterized by a time constant of 148 ps for tetrahydrofuran, 81 ps for 2-propanol, and 11 ps for the most polar solvent methanol. The ICT state is then rapidly depopulated to the ground state. This relaxation also exhibits solvent dependence, having a time constant of 1 ps in methanol, 2.5 ps in 2-propanol, and 3.5 ps in tetrahydrofuran.