I. V. Kryukov

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.

Time-resolved fluorescence anisotropy of styryl dye–cucurbituril complexes

It is known that photophysical properties of a dye molecule change significantly in supramolecular host–guest complexes of organic luminophores with cucurbit[n]urils (CBn) [1]. For example, the f luorescence intensity of a styryl dye–CB6 complex increases more than 12-fold in comparison with the free dye [2]. This phenomenon is relatively poorly studied and is very interesting property from both fundamental and applied points of view. In this communication, we present new experimental results on ultrafast f luorescence depolarization of a styryl dye, 4-[4-(dimethylamino)styryl-1-methylpyridinium] iodide (DSM), in the host-guest complexes with CB6, in ethylene glycol and water solutions. Ethylene glycol (Khimmed, analytical grade), water (Millipor Simplicity), cucurbit [6]uril (Sigma Aldrich), and DSM (Sigma Aldrich) were used without additional purification. The measurements were carried out at ambient temperature, with the DSM concentration being 5 × 10–6 mol/L in steady-state experiments or 2 × 10–4 mol/L in the study on picosecond timescale. The concentration of CB6 aqueous solution was 2.3 × 10–4 mol/L. Steady-state f luorescence spectra and fluorescence anisotropy were measured on a Fluorolog 3 Tau spectrofluorimeter. Fluorescence decay kinetics on nanosecond timescale was studied by correlated single-photon counting on a PicoQuant spectrofluorimeter. Femtosecond fluorescence depolarization was studied by the upconversion (frequency summation) method using a chromium-forsterite laser (pulse duration of 90 fs, first harmonics of 1280 nm). The detailed description of the experimental setup is given elsewhere [3]. The upconversion nonlinear BBO crystal is physically a polarizer (analyzer). The transmission direction of the crystal is determined by the direction of oscillations of the electric vector of a linearly polarized gate pulse. The time-resolved f luorescence anisotropy as an important characteristic of the f luorophore was determined experimentally as

A Fluorimeter on the Basis of a Femtosecond Cr+4: Forsterite Laser

An apparatus for measuring luminescence lifetimes using the femtosecond up-conversion method is presented. A laser system based on a chromium–forsterite crystal with a lasing wavelength of 1250 nm is used to generate ultrashort pulses. Luminescence is excited in a sample by optical harmonics that are generated in nonlinear optical crystals. The width of the instrument function of the system is 200 fs. Methods for optimizing the signal-to-noise ratio in measurements are discussed. The results of measurements of the fluorescence- decay dynamics and the anisotropy of aqueous solutions of an inclusion complex of the 4-DASPI styrene dye in cucurbit[6]uril on short (5 ps) and long (500 ps) time scales are presented.

Effect of Heavy Water on Ultrafast Dynamics of the Fluorescence Stokes Shift for a Styryl Dye and its Complexes with Cucurbiturils

It has been recently found [1] that upon excitation, host–guest complexes of styryl dye cations with cucurbit[n]urils (CBn), which are barrel-shaped hollow macrocyclic molecules consisting of n glycouril units (usually n = 5–8) linked with methylene groups, operate as a molecular machine with a function of switch [2]. An external stimulus (light in this case) causes high-amplitude translational motion of the guest molecule relative to the macrocycle. The physical origin of such molecular motion is Coulomb interaction between the negatively charged portal of cucurbituril and the charge of the dye molecular cation. Photoexcitation results in charge transfer from the dye pyridine moiety to the styryl group, thereby creating Coulomb forces that act to shift the electronically excited cation into the cavitand cavity. After relaxation of the excited state within 102 ps [1], the system returns to the initial equilibrium state.