The recent results of stationary-state and time-resolved absorption, fluorescence and Raman spectroscopies of some typical carotenoids are summarized. Theoretical analyses of carotenoid singlet states and of carotenoidto-bacteriochlorophyll singlet-energy transfer are also included. On the bases of the energies, the lifetimes and other properties of singlet excited states of the carotenoids in solution and bound to the light-harvesting complexes, the energetics and the dynamics of the light-harvesting function in purple photosynthetic bacteria are discussed with emphasis on the 2A,-and B,+ states.

Articles you may be interested inThe photophysical behavior of 3-chloro-7-methoxy-4-methylcoumarin related to the energy separation of the two lowest-lying singlet excited states Lowest energy excited singlet states of isomers of alkyl substituted hexatrienes J. Chem. Phys. 94, 4691 (1991); 10.1063/1.460581Theoretical study of the force field of the lowest singlet electronic states of long polyenes J. Chem. Phys. 91, 6215 (1989); 10.1063/1.457388Polyene spectroscopy: The lowest energy excited singlet state of diphenyloctatetraene and other linear polyenes In this paper we explore the intramolecular relaxation processes within two long carotenoids, namely decapreno-␤-carotene ͑M15͒ and dodecapreno-␤-carotene ͑M19͒ with 15 and 19 conjugated double bonds ͑N͒, respectively. Amplified 200 fs pulses at 590 nm were used to excite the optically allowed S 0 →S 2 ͑1 1 A g →1 1 B u ͒ transition of the two carotenoids. The excited state dynamics were probed by continuum light between 400-890 nm in solvents with different polarizabilities. The transient absorption spectra consist of a bleaching region, due to loss of ground state absorption, and of an excited state absorption region at longer wavelengths, due to the S 1 →S n transition. The S n state was assigned to an n 1 B u state. The overall wavelength dependence of the measured kinetics could be well described by introducing three decay time constants. One reflects the S 1 lifetime ͑ 1 ͒ and was determined to 1.1 and 0.5 ps for M15 and M19, respectively. A second lifetime, between 5 and 15 ps, was attributed to vibrational cooling in the ground state. A third decay time was in the subpicosecond range, and was ascribed to the vibrational redistribution and relaxation of the S 1 potential surface after being populated by the subpicosecond S 2 -S 1 internal conversion. No significant change of the decay constants was observed for M15 embedded in a 77 K matrix. This shows that the relaxation rates are only influenced by intramolecular processes. The S 2 lifetime was shorter than the pulse duration and was estimated to be in the order of 100 fs. The S 0 →S 2 transition of M15 in the liquid phase exhibits a 0.39 anisotropy at short times, while the S 1 →S n transition has an initial value of only 0.31. This corresponds to an angle of 23°between the transition dipoles. The measured S 1 rate constants were analyzed, together with decay constants of shorter carotenes, in terms of the energy gap law. When going from the shortest ͑Nϭ5͒ to the longest ͑Nϭ19͒ polyene, 1 decreases about 6000 times, i.e., from 3 ns to 0.5 ps. By using an empirical form of the energy gap law the 0-0 transition of S 1 ͑2 1 A g ͒→S 0 was estimated to be located at 11 300 and 10 200Ϯ1 000 cm Ϫ1 for M15 and M19, respectively. By fitting the excitation energies of all carotenes in the series ͑3рNр19͒ with a truncated two or three term expansion of a power series in 1/N the long-chain limit values were extrapolated to be 11 000 and 3 500 cm Ϫ1 for the 1 1 B u and 2 1 A g state, respectively. The implication o...

Solvent and temperature effects on the dipole-allowed SZ -SO and the symmetry-forbidden SI -SO transitions of all-trans-carotenes, with 5 (m-5), 7 (m-7), 8 (m-8), 9 (m-9), 11 (all-trans-p-carotene), 15 (decapreno-/?-carotene), and 19 (dodecapreno-8-carotene) conjugated double bonds (N), have been investigated by steadystate and time-correlated single-photon counting (SPC) experiments. The measured fluorescence quantum yields of the SI -SO emission (@I) decrease from 7 x at room temperature when going from m-5 to p-carotene. For the longest compounds N = 15 and 19 only the S2 emission was observed, with fluorescence yields (@a) of about 5 x lo-*. The measured S1 fluorescence lifetime of m-5, m-7, m-8, and m-9 was found to decrease with decreasing energy gap between SI and SO (AE(SI-S0)), in accordance with the energy gap law (EGL). @a indicates that the SZ lifetime is on the order of 100 fs for all compounds.Fluorescence emission from the S I state of p-carotene in room temperature liquids was observed with the 0-0 transition located at 14 200 f 500 cm-l. The intensity ratio 12/11, where I 2 represents the integrated SZ -SO emission and 11 the S I -SO emission spectrum determined by time-resolved methods, depends on the AQSI-SO) in a similar way as @&&I (=kr2kl/(krlk21)). When N increases from 5 to 11, 12/11 (%@&&I) increases about 2000 times, while the rate of internal conversion between S I and SO (kl) increases by a factor of 300. Thus, the term kr2/(krlk21) is also affected by N , where kr2 and krl are the radiative rate constants of the SZ and SI state, respectively, and k21 is the rate of internal conversion between SZ and S I . The solvent polarizability (a) affects the dual emission pattern (@n/@fl), as was clearly observed for m-8 and m-9. This is mainly due to an enhancement of k21 at larger a, since the larger the a, the smaller is the s 2 -S~ energy gap (AE(Sz-Sl)). zfl is about 2-3 times longer for m-7, m-8, and m-9 in 77 K glasses than in room-temperature liquids. The weak temperature dependence indicates that no large-amplitude vibronic motion couples the S 1 and SO states. The steady-state anisotropy of the SI -SO transition of m-7 and m-8 in 77 K glasses is about 0.38 and 0.37, respectively. At room temperature the anisotropy is lower, as a result 6f rotational diffusion motion. Because of the short Sz lifetime, the fluorescence anisotropy of the S2 -SO transition is always close to 0.4, irrespective of the temperature.to about 4 x

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Abstract–Solvent induced absorption spectral shifts of the electronic transition from ground 1 Ag state to the excited 1Bu state in carotenoids have been studied. It is shown that the shift depends only on dispersion interactions in non‐polar solvents. In polar media there is just a small extra contribution to the red‐shift, due to other forms of interactions. The spectral shifts are well described by the theory, which expresses the shift relative to the gas phase value, as a function of solvent polarizability. The main conclusion is that the dominating mechanism behind the large red‐shifted absorbance of carotenoids in the proteinacous environment, in vivo, is the mutual polarizability interactions between the carotenoids and the surrounding medium. The solution‐phase values of the dipole moments of the lAg to 1Bu transitions and the differences of isotropic polarizability between 1Bu and lAg states of carotenoids in non‐polar solvents are calculated and found to be around 13 D and 360 Å3 respectively. From the great overlap of absorption spectra between carotenoids in quinoline and carotenoids in vivo in purple bacterial antenna complexes, it can be expected that the carotenoids are surrounded by several aromatic amino acids in vivo. Comparisons have been done between the exicted states in carotenoids and in linear conjugated polyenes.