Excited-State Dynamics of Monomeric and Aggregated Carotenoid 8′-Apo-β-carotenal

Excited-state properties of monomeric and aggregated carbonyl carotenoid 8′-apo-β-carotenal were studied by means of femtosecond transient absorption spectroscopy. For monomers, the polarity-dependent behavior characteristic of carotenoids with conjugated carbonyl group was observed. In <i>n</i>-hexane the S₁ lifetime is 25 ps, but it is shortened to 8 ps in methanol. This shortening is accompanied by the appearance of new spectral bands in transient absorption spectrum. On the basis of analysis of the transient absorption spectra of monomeric 8′-apo-β-carotenal in <i>n</i>-hexane and methanol, we propose that the polarity-induced spectral bands are due to the S₁(A_g^–)–S₃(A_g⁺) transition, which is enhanced upon breaking the symmetry of the molecule. This symmetry breaking is caused by the conjugated carbonyl group; it is much stronger in polar solvents where the S₁ state gains significant charge-transfer character. Upon addition of water to methanol solution of 8′-apo-β-carotenal we observed formation of aggregates characterized by either blue-shifted (H-aggregate) or red-shifted (J-aggregate) absorption spectrum. Both aggregate types exhibit excited-state dynamics significantly different from those of monomeric 8′-apo-β-carotenal. The lifetime of the relaxed S₁ state is 20 and 40 ps for the H- and J-aggregates, respectively. In contrast to monomers, aggregation promotes formation of triplet state, most likely by homofission occurring between tightly packed molecules within the aggregate

Role of Carotenoids in Light-Harvesting Processes in an Antenna Protein from the Chromophyte <i>Xanthonema debile</i>

Chromophytes are an important group of microorganisms that contribute significantly to the carbon cycle on Earth. Their photosynthetic capacity depends on efficiency of the light-harvesting system that differs in pigment composition from that of green plants and other groups of algae. Here we employ femtosecond transient absorption spectroscopy to study energy transfer pathways in the main light-harvesting complex of <i>Xanthonema debile</i>, denoted XLH, which contains four carotenoidsdiadinoxanthin, heteroxanthin, diatoxanthin, and vaucheriaxanthinand Chl-<i>a</i>. Overall carotenoid-to-chlorophyll energy transfer efficiency is about 60%, but energy transfer pathways are excitation wavelength dependent. Energy transfer from the carotenoid S₂ state is active after excitation at both 490 nm (maximum of carotenoid absorption) and 510 nm (red edge of carotenoid absorption), but this channel is significantly more efficient after 510 nm excitation. Concerning the energy transfer pathway from the S₁ state, XLH contains two groups of carotenoids: those that have the S₁ route active (∼25%) and those having the S₁ pathway silent. For a fraction of carotenoids that transfer energy via the S₁ channel, energy transfer is observed after both excitation wavelengths, though energy transfer times are different, yielding 3.4 ps (490 nm excitation) and 1.5 ps (510 nm excitation). This corresponds to efficiencies of the S₁ channel of ∼85% that is rather unusual for a donor–acceptor pair consisting of a noncarbonyl carotenoid and Chl-<i>a</i>. Moreover, major carotenoids in XLH, diadinoxanthin and diatoxanthin, have their S₁ energies in solution lower than the energy of the acceptor state, Q_<i>y</i> state of Chl-<i>a</i>. Thus, binding of these carotenoids to XLH must tune their S₁ energy to allow for efficient energy transfer. Besides the light-harvesting function, carotenoids in XLH also have photoprotective role; they quench Chl-<i>a</i> triplets via triplet–triplet energy transfer from Chl-<i>a</i> to carotenoid

Equilibration Dependence of Fucoxanthin S₁ and ICT Signatures on Polarity, Proticity, and Temperature by Multipulse Femtosecond Absorption Spectroscopy

To demonstrate the value of the multipulse method in revealing the nature of coupling between excited states and explore the environmental dependencies of lowest excited singlet state (S₁) and intramolecular charge transfer (ICT) state equilibration, we performed ultrafast transient absorption pump–dump–probe and pump–repump–probe spectroscopies on fucoxanthin in various solvent conditions. The effects of polarity, proticity, and temperature were tested in solvents methanol at 293 and 190 K, acetonitrile, and isopropanol. We show that manipulation of the kinetic traces can produce one trace reflecting the equilibration kinetics of the states, which reveals that lower polarity, proticity, and temperature delay S₁/ICT equilibration. On the basis of a two-state model representing the S₁ and ICT states on the same S₁/ICT potential energy surface, we were able to show that the kinetics are strictly dependent on the initial relative populations of the states as well as the decay of the ICT state to the ground state. Informed by global analysis, a systematic method for target analysis based on this model allowed us to quantify the population transfer rates throughout the life of the S₁/ICT state as well as separate the S₁ and ICT spectral signatures. The results are consistent with the concept that the S₁ and ICT states are part of one potential energy surface

Different Response of Carbonyl Carotenoids to Solvent Proticity Helps To Estimate Structure of the Unknown Carotenoid from <i>Chromera velia</i>

In order to estimate the possible structure of the unknown carbonyl carotenoid related to isofucoxanthin from <i>Chromera velia</i> denoted as isofucoxanthin-like carotenoid (Ifx-l), we employed steady-state and ultrafast time-resolved spectroscopic techniques to investigate spectroscopic properties of Ifx-l in various solvents. The results were compared with those measured for related carotenoids with known structure: fucoxanthin (Fx) and isofucoxanthin (Ifx). The experimental data were complemented by quantum chemistry calculations and molecular modeling. The data show that Ifx-l must have longer effective conjugation length than Ifx. Yet, the magnitude of polarity-dependent changes in Ifx-l is larger than for Ifx, suggesting significant differences in structure of these two carotenoids. The most interesting spectroscopic feature of Ifx-l is its response to solvent proticity. The transient absorption data show that (1) the magnitude of the ICT-like band of Ifx-l in acetonitrile is larger than in methanol and (2) the S₁/ICT lifetime of Ifx-l in acetonitrile, 4 ps, is markedly shorter than in methanol (10 ps). This is opposite behavior than for Fx and Ifx whose S₁/ICT lifetimes are always shorter in protic solvent methanol (20 and 13 ps) than in aprotic acetonitrile (30 and 17 ps). Comparison with other carbonyl carotenoids reported earlier showed that proticity response of Ifx-l is consistent with presence of a conjugated lactone ring. Combining the experimental data and quantum chemistry calculations, we estimated a possible structure of Ifx-l