No abstract

The excited-state dynamics of J aggregates of PIC have been studied by means of picosecond and subpicosecond absorption spectroscopy as well as integrated fluorescence yield measurements. The results of these measurements show that both the lifetime and the fluorescence yield are strongly dependent on excitation pulse intensity. At relatively high light intensity (1014 photons cm−2 pulse−1) the S1 lifetime is essentially pulse limited (<1 ps) and the fluorescence yield is very low. Upon decreasing the light intensity a gradual increase of the excited-state lifetime and fluorescence yield is observed. At very low excitation intensity (1010 photons cm−2 pulse−1) a single exponential lifetime of 400 ps is observed. At intermediate intensities the excited-state decay is strongly nonexponential. The observed intensity dependence of the excited-state dynamics is attributed to efficient exciton–exciton annihilation between the highly mobile singlet excitons. By applying an expression for bimolecular exciton annihilation, derived and used for photosynthetic antenna systems [Paillotin et al., Biophys. J. 25, 513 (1979)], and treating the energy migration as a hopping motion, information about aggregate size and exciton hopping rate was obtained.

The solvent dependence of the steady-state and time-resolved fluorescence emission from the S2 (1Bu) state of all- trans-β-carotene was investigated in a range of polar and nonpolar solvents and in hexane/carbon disulfide mixtures. The steady-state absorption and fluorescence emission maxima in both polar and nonpolar solvents showed parallel shifts with increasing solvent polarizability, indicating that the emitting state is the S2 (11Bu) state. The lifetime of the S2 state was determined using the fluorescence upconversion technique, and lifetimes varying from 120 fs in quinoline to 177 fs in hexane were found. An intramolecular relaxation process occurring on a similar time scale was observed as a broadening in the reconstructed time-dependent emission spectra in hexane and as emission wavelength-dependent lifetimes in all the solvents studied. Correlations were observed between the S2 lifetimes, the π* solvatochromic parameter, and the ratio of the absorption fine structure. A correlation between the rate of internal conversion from the S2 state and the previously reported S1 (21Ag) state Raman shifts of the CC stretching mode supports the contention that the S2 and S1 states are vibronically coupled. The implications of these results for light harvesting in photosynthesis are also discussed.

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...