Spectroscopic and Dynamic Properties of the Peridinin Lowest Singlet Excited States

Abstract

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.

Femtosecond time‐resolved spectroscopy on biological photoreceptor chromophores

“…It can be speculated that the cyclopentanone ring is involved in this localization step. This assumption is likely because other groups having observed a decay pattern very similar to the one reported here, made their observations in peridinin, a carotenoid highly substituted and possessing a cyclopentanone ring as well [99][100][101][102]. This assumption is further corroborated by experimental results obtained on magnesium octaethylporphyrin (MgOEP) (data not shown) as MgOEP, in contrast to the structurally similar PChla, which in addition has an attached cyclopentanone ring to the porphyrin macrocycle, does not exhibit any significant excited-state dynamics, when directly being promoted into the Q 00 band [22].…”

“…They show that in n-hexane the barrier is high enough to prevent the population of the S ICT state to take place and only the S 1 → S 0 internal conversion is observed. These authors report on a decreasing barrier in parallel with increasing solvent polarity resulting in a quenching of the S 1 state by means of the S 1 → S ICT relaxation [99]. The results obtained in the course of this work might be explained in analogy to the arguments given above; in the polar solvents acetonitrile and methanol the intramolecular charge-transfer state lies below the initially photopopulated S 1 state and is therefore accessible for population.…”

“…This strikingly different decaying behavior deserves some thorough considerations. The experimental findings discussed so far might be explained on the basis of a split S 1 state in analogy to arguments given by Zigmantas et al [99] and Bautista et al [100] to account for the observed excited-state dynamics of peridinin, a highly substituted carotenoid. The second state involved would most likely possess intramolecular charge-transfer (ICT) character and is therefore stabilized in more polar solvents and -in turn -destabilized in a nonpolar environment.…”

Carotenoid to chlorophyll energy transfer in the peridinin–chlorophyll- a –protein complex involves an intramolecular charge transfer state

“…It was also recently shown that the lowest excited state of peridinin exhibits rather unusual behavior, in that its lifetime strongly depends on solvent polarity, the lifetime decreasing with an increase of the medium polarity. Polarity dependence was attributed to an intramolecular charge transfer (ICT) state in the excited state manifold of peridinin (16,17). Dynamics of the peridinin ICT state in polar solvent were recently studied by near-infrared femtosecond spectroscopy where it was shown that the ICT state can be detected via its pronounced stimulated emission band at Ϸ950 nm (17).…”

Excited State Structural Dynamics of Carotenoids and ChargeTransfer Systems

“…4,19,[25][26][27][28][29][30][31][32][33] Previous investigations of DCM 19,23,34 considered the formation of a TICT state involving rotation about the C-N bond linking the dimethylamino group to the aromatic ring, or rotation of the dimethylaminobenzyl moiety, but these experiments lacked any direct probe of structural change. Another proposed state is one analogous to the P* state 26,30,35 of stilbenes in which the molecule adopts a perpendicular conformation about the central double bond as an intermediate between cis and trans configurations. Electronic structure calculations for DCM using CS INDO have been able to locate a singlet state with a dipole moment large enough match the experiment, and in this state DCM adopts a configuration in which the dimethylamino moiety is perpendicular to the aromatic ring.…”