Picosecond processes in chromatophores at various excitation intensities

Abstract

The aim of this paper is to review and discuss the results obtained by fluorescence and absorption spectroscopy of bacterial chromatophores excited with picosecond pulses of varying power and intensity. It was inferred that spectral and kinetic characteristics depend essentially on the intensity, the repetition rate of the picosecond excitation pulses as well as on the optical density of the samples used. Taking the different experimental conditions properly into account, most of the discrepancies between the fluorescence and absorption measurements can be solved. At high pulse repetition rate (>10(6) Hz), even at moderate excitation intensities (10(10)-10(11) photons/cm(2) per pulse), relatively long-lived triplet states start accumulating in the system. These are efficient (as compared to the reaction centers) quenchers of mobile singlet excitations due to singlet-triplet annihilation. The singlet-triplet annihilation rate constant in Rhodospirillum rubrum was determined to be equal to 10(-9) cm(3) s(-1). At fluences >10(12) photons/cm(2) per pulse singlet-singlet annihilation must be taken into account. Furthermore, in the case of high pulse repetition rates, triplet-triplet annihilation must be considered as well. From an analysis of experimental data it was inferred that the singlet-singlet annihilation process is probably migration-limited. If this is the case, one has to conclude that the rate of excitation decay in light-harvesting antenna at low pumping intensities is limited by the efficiency of excitation trapping by the reaction center. The influence of annihilation processes on spectral changes is also discussed as is the potential of a local heating caused by annihilation processes. The manifestation of spectral inhomogeneity of light-harvesting antenna in picosecond fluorescence and absorption kinetics is analyzed.

Ultrafast dynamics of singlet-singlet and singlet-triplet exciton annihilation in poly(3-2′-methoxy-5′octylphenyl)thiophene films

“…At higher laser pulse fluence exceeding 10 13 photons/ cm 2 , the exciton lifetime is also shortened as a result of an additional relaxation channel due to singletsinglet ͑S-S͒ annihilation [11][12][13][14][15] with some possible contribution of triplet-triplet ͑T-T͒ annihilation. 13 Manifestation of the nonlinear annihilation in -conjugated oligomers and polymers has been reported. [17][18][19][20][21][22][23] S-S annihilation has been invoked to account for the observed intensity dependence of the fluorescence lifetime at high excitation intensities.…”

“…[17][18][19][20][21][22][23] For pulse repetition rates exceeding 10 6 Hz, even moderate laser pulse fluence of 10 10 -10 11 photons/ cm 2 can already lead to accumulation of relatively long lived triplet states via intersystem crossing, as was demonstrated by picosecond time-resolved fluorescence studies of pigmentprotein complexes of a photosynthetic bacterium Rhodospirillum rubrum. 13 In this case, singlet-triplet ͑S-T͒ annihilation takes place and results in efficient quenching of photoluminescence. At higher laser pulse fluence exceeding 10 13 photons/ cm 2 , the exciton lifetime is also shortened as a result of an additional relaxation channel due to singletsinglet ͑S-S͒ annihilation [11][12][13][14][15] with some possible contribution of triplet-triplet ͑T-T͒ annihilation.…”

“…13 In this case, singlet-triplet ͑S-T͒ annihilation takes place and results in efficient quenching of photoluminescence. At higher laser pulse fluence exceeding 10 13 photons/ cm 2 , the exciton lifetime is also shortened as a result of an additional relaxation channel due to singletsinglet ͑S-S͒ annihilation [11][12][13][14][15] with some possible contribution of triplet-triplet ͑T-T͒ annihilation. 13 Manifestation of the nonlinear annihilation in -conjugated oligomers and polymers has been reported.…”

“…Exciton-exciton annihilation has been extensively studied in molecular crystals, 3-7 molecular aggregates, [8][9][10] biological systems ͑photosynthetic light harvesting antennae and separate light harvesting complexes͒, [11][12][13][14][15][16] and -conjugated polymers. [17][18][19][20][21][22][23] For pulse repetition rates exceeding 10 6 Hz, even moderate laser pulse fluence of 10 10 -10 11 photons/ cm 2 can already lead to accumulation of relatively long lived triplet states via intersystem crossing, as was demonstrated by picosecond time-resolved fluorescence studies of pigmentprotein complexes of a photosynthetic bacterium Rhodospirillum rubrum.…”

Energy Transfer and Connectivity in Chloroplasts: Competition between Trapping and Annihilation in Pulsed Fluorescence Induction Experiments

“…Therefore the "size" of the antenna can only be obtained from annihilation measurements. [17][18][19][20][21] Quenching by triplet states is generally much more efficient than by open RC's, both in photosynthetic bacteria 21,22 and in chloroplasts, 23,24 and is therefore close to migration limited. When modeling fluorescence induction and S-T annihilation simultaneously, finite antenna sizes have to be taken into account.…”