Dynamics of Excited States of the Carotenoid Peridinin in Polar Solvents:  Dependence on Excitation Wavelength, Viscosity, and Temperature

Abstract

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.

Stark absorption spectroscopy of peridinin and allene-modified analogues

“…In these regards, it might be rational to assign the origin of band 1 to the S 0 → S 2-ICT transition. This assignment is consistent with the previous reports [7,9,25] and supports the idea by Zigmantas et al who suggested the presence of 'red' peridinin that has enhanced CT character [11]. A number of previous studies on peridinin showed importance of conformational disorder in explanation of excited-state properties.…”

“…They believe this is one of the origins of efficient energy-transfer from the S 2 state of Per to chlorophyll a. Linden et al carried out experimental [10] and theoretical [6] studies and found the S 2-ICT state close in energy to the one-photon allowed 1 1 B u + state. Zigmantas et al showed that excitation of peridinin to the very red edge of its S 0 → S 2 absorption generates excited-state dynamics different from that produced after excitation at the absorption maximum [11]. This effect was assigned to excitation of the S 2 state of a specific 'red' peridinin having a CT character.…”

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].…”

“…Therefore, focusing on the dynamics occurring within the first 12.1 ps after photoexcitation, PChlamethanol exhibits a rise component in addition to the primary decay, while the kinetics of PChla-glycerol can be accounted for by the 12.1-ps decay only. From the characteristic time scales of PChla-glycerol being significantly longer than the corresponding time constants obtained for PChla-methanol, it can be inferred that the increased viscosity affects the rate of the corresponding relaxation processes [19,[102][103][104][105][106][107]. As the nuclear motion of the vibrating molecule is slowed down in the highly viscose solvent the rate of energy dissipation associated with the shortest time scale decreases.…”

Photoprotection in a purple phototrophic bacterium mediated by oxygen-dependent alteration of carotenoid excited-state properties

“…2B). Although it is possible to separate the S 1 -like and ICTlike transitions in transient absorption spectra, the S 1 and ICT states of carbonyl carotenoids are strongly coupled in the S 1 ∕ICT state (32,35), and the intensity of the ICT-like transition is a measure of a degree of charge-transfer character of the S 1 ∕ICT state. It was recently shown that the intensity of the ICTlike transition significantly increases if a single conjugated carbonyl group of a carotenoid is positioned in an s-trans configuration with respect to the main conjugated backbone (36); however, this is not the case for spheroidenone in solution where the carbonyl group is in an s-cis orientation as evidenced by molecular modeling showing that s-cis configuration is far more stable (Fig.…”