A. V. Sharkov

Femtosecond energy transfer between chromophores in allophycocyanin trimers

Ultrafast energy-transfer processes in allophycocyanin (APC) trimers from Mastigocladus laminosus have been examined by a femtosecond absorption technique. Isotropic absorption recovery kinetics with τ=440±30 fs were observed in APC trimers at 615 nm. In APC monomers such a fast process was not observed. The anisotropy in both samples was constant and close to 0.4 during the first few picoseconds. The results are consistent with a model of the APC trimer in which the two APC chromophores have different absorption spectra with maxima about 600 and 650 nm. The transfer of energy from the 600 nm chromophore to the 650 nm chromophore occurs in 440 fs and is dominated by the Forster dipole—dipole energy-transfer mechanism.

Förster energy transfer between neighbouring chromophores in C-phycocyanin trimers

The excitation-energy transfer in C-phycocyanin (C-PC) trimers and monomers isolated from phycobilisomes of Mastigocladus laminosus has been studied by polarization femtosecond laser spectroscopy. Excitation with 70-fs pulses at 615 nm gave rise to a 500-fs energy-transfer process that was observed only in trimeric preparations. The rate of the process is in agreement with earlier calculated Forster energy transfer rates between neighbouring α-84 and β-84 chromophores of different monomeric subunits. This process is most clearly seen in the anisotropy decay kinetics. As a result of femtosecond excitation-energy transfer, the anisotropy relaxes from 0.4 to 0.23. The final anisotropy value is in fair agreement with the results of calculations based on the crystal structure and spectroscopic data of C-PC trimers. Our results support the conclusion that Forster energy transfer can occur between excitonically coupled chromophores.

Femtosecond spectral and anisotropy study of excitation energy transfer between neighbouring α-80 and β-81 chromophores of allophycocyanin trimers

Polarization pump-probe femtosecond spectroscopy was used to investigate photoinduced optical density changes in allophycocyanin (APC) trimers at 635–690 nm after excitation with 230-fs pulses at 618 nm. The initial bleaching observed at λ < 645 nm is followed by subpicosecond absorption recovery corresponding to 430 ± 40 fs recovery kinetics measured at 615 nm with 70-fs pulses. Only the red part of the APC absorption band remains strongly bleached at 3 ps after excitation. The spectral and kinetic results can be described in terms of two different models of interaction between neighbouring α-80 and β-81 chromophores of APC trimers. According to the first one, the observed subpicosecond kinetics corresponds to relaxation between the levels of excitonically coupled, spectrally identical α-80 and β-81 chromophores. Excited state absorption to doubly excited excitonic state should in this case contribute to the measured difference spectra. According to the second one, the femtosecond excitation energy transfer in APC trimers takes place between a donor chromophore absorbing predominantly at 620 nm and an acceptor chromophore absorbing at 650 nm. The high anisotropy value observed at 615 nm during the first 1.2 ps is in good agreement with the donor-acceptor model. Anisotropy values calculated in the 635–675 nm spectral region at 3 ps after excitation are in the 0.1–0.25 range corresponding to an angle of 30°–45° between donor and acceptor transition dipole orientations. The high anisotropy obtained at 658 nm during the excitation is probably due to stimulated emission of the donor chromophore.

Dynamics of optically induced anisotropy in an ensemble of asymmetric top molecules in the gas phase

A complex theoretical and experimental investigation of the evolution of optically induced anisotropy in an ensemble of free asymmetric tops under collisionless conditions is performed. The dynamics of the orientational correlation functions is studied and the results of femtosecond measurements of the relaxation of the anisotropy of perylene fluorescence in the gas phase are reported. The results obtained are interpreted in terms of the theory developed. It is shown that, in contrast to steady-state anisotropy, the time kinetics of optically induced anisotropy yields more complete and typical data on the dynamics of vector correlations in an ensemble of molecules.

Femtosecond analysis of free molecular rotation in the gas phase

Abstract The time-resolved S 1 →S * n absorption anisotropy decay resulting from rotation of free gas phase POPOP molecules at 593 K and PPO molecules at 483 K was studied after S 0 →S * 1 excitation by femtosecond linearly polarized pulses at 308 nm. The time evolution of the anisotropy measured through a few picoseconds after excitation is in agreement with the results of calculations made for POPOP and PPO using orientational correlation functions developed for rigid asymmetric top molecules.

Excited state dynamics of PEC trimer

One‐color kinetic measurements made with PEC trimers using 75 fs light pulses of different wavelengths indicate donor‐acceptor energy transfer between neighboring α‐84 and β‐84 chromophores of different monomer subunits. Transient absorption difference spectra measured in the 400–800 nm spectral range after excitation with 300 fs pulses at 587 nm exhibit bleaching of the long wavelength part of the PEC trimer absorption band and strong excited state absorption at 660 nm, attributed to the excited chromophores of the β‐subunit. Dipole‐dipole interaction between neighboring α‐84 and β‐84 chromophores cannot explain the appearance of this excited state absorption band, but can influence the femtosecond difference spectra.

Time-Resolved Decay of Absorption Anisotropy due to Rotation of Free Molecules in the Gas Phase

A photoinduced orientational anisotropy of an ensemble of free molecules has a complex time dependence, and differs significantly from the relaxation in condensed media. We have studied the time-resolved S1 *→ Sn * absorption anisotropy decay resulting from rotation of free POPOP molecules in vapour at 600 K and PPO molecueles at 483 K after S0→ S1 * excitation by femtosecond linearly polarized pulses at 308 nm. The maximum absorption anisotropy r=0.4 recorded at the moment of excitation speaks in favour of parallel dipoles of S1 *→ Sn * and S0→ S1 * transitions of POPOP and PPO molecules. Later, value r decays approximately to zero, then increases and gets a stationary nonzero level due to nonchaotic rotation of free molecules.