Walter S. Struve

Picosecond kinetics of p‐dimethylaminobenzonitrile

The nanosecond and picosecond resolved dual fluorescences of p‐dimethylaminobenzonitrile (DAB), in various solvents and glasses excited by 266 nm 20 ps FWHM laser pulses, have been investigated. Pulse‐limited rise times are exhibited by the b*‐state emission whose decay in turn feeds directly the risetime of a*‐state emission at 440–600 nm in most solvents studied. The a*‐state emission was monitored at 520–600 nm in order to eliminate contribution from the b*‐state. Within the experimental resolution, the b*‐state fluorescence decay times vary approximately linearly with solvent viscosity. The a*‐state fluorescence decay times vary with both solvent and temperature, and may reflect either thermally assisted intersystem crossing from the solvated singlet a*‐state (presumably of twisted internal charge transfer character) to a corresponding solvated triplet of slightly higher energy, or a thermally activated internal conversion of the 1TICT to the ground state.

Oscillating anisotropies in a bacteriochlorophyll protein: Evidence for quantum beating between exciton levels

Abstract Optical anisotropies of bacteriochlorophyll a Qy electronic transitions in Fenna-Matthews-Olson trimers from the green photosynthetic bacterium Chlorobium tepidum eshibit strong oscillations with ∼ 220 fs period for certain wavelengths in one-color absorption difference experiments. The oscillations in the polarized pump-probe signals Δ A∥(t), Δ A⊥] (t) have opposite phase, and they nearly cancel in the isotropic signals. This combined with the fact that they only appear when the laserpulse spectrum overlaps both the lowest-energy groups of excitation levels clustered near 815 and 825 nm, suggests that the oscillations stem from quantum beating between exciton levels, rather than coherent from nuclear motion. In some cases, the anisotropy r(t) exhibits values > 0.4 at times well beyond the laser autocorrelation function.

TIME-RESOLVED FLUORESCENCE OF NITROBENZOXADIAZOLE-AMINOHEXANOIC ACID: EFFECT OF INTERMOLECULAR HYDROGEN-BONDING ON NON-RADIATIVE DECAY

The fluorescence kinetics of the nitrobenzoxadiazole (NBD) chromophore were studied at low concentrations in solvents with varying polarity and hydrogen‐bonding donor strength. The emission decay was essentially single exponential in all solvents studied. While the absorption and fluorescence solvatochromism is determined largely by the solvent polarity, the S, state decay kinetics are strongly modulated by the solvent H‐bonding capacity. The NBD emission lifetime, generally –7–10ns in the aprotic solvents, is reduced to 0.933 ns in water. The solvent deuterium isotope effect on the fluorescence decay is substantial in D20 and in methanol‐d4, but is insignificant in DMSO‐d6. These results are consistent with acceleration of S1S11 internal conversion through an accepting vibrational mode created by intermolecular hydrogen‐bonding of the NBD chromophore to an H atom‐donating solvent. This work bears on the practicality of using NBD as a fluorophore in assays for estrogen and progesterone receptors.

Real time speech formant analyzer and display

A speech analyzer for interpretation of sound includes a sound input which converts the sound into a signal representing the sound. The signal is passed through a plurality of frequency pass filters to derive a plurality of frequency formants. These formants are converted to voltage signals by frequency-to-voltage converters and then are prepared for visual display in continuous real time. Parameters from the inputted sound are also derived and displayed. The display may then be interpreted by the user. The preferred embodiment includes a microprocessor which is interfaced with a television set for displaying of the sound formants. The microprocessor software enables the sound analyzer to present a variety of display modes for interpretive and therapeutic used by the user.

Ultrafast energy transfer in light-harvesting chlorosomes from the green sulfur bacterium Chlorobium tepidum

Two independent pump-probe techniques were used to study the antenna energy transfer kinetics of intact chlorosomes from the green sulfur bacterium Chlorobium tepidum with femtosecond resolution. The isotropic kinetics revealed by one-color experiments in the BChl c antenna were inhomogeneous with respect to wavelength. Multiexponential analyses of the photobleaching/stimulated emission (PB/SE) decay profiles typically yielded (apart from a approximately 10 fs component that may stem from the initial coherent oscillation) components with lifetimes 1-2 ps and several tens of ps. The largest amplitudes for the latter component occur at 810 nm, the longest wavelength studied. Analyses of most two-color pump-probe profiles with the probe wavelength red-shifted from the pump wavelength yielded no PB/SE rise components. PB/SE components with approximately 1 ps risetime were found in 790 --> 810 and 790 --> 820 nm profiles, in which the probe wavelength is situated well into the BChl a absorption region. A 760 --> 740 nm uphill two-color experiment yielded a PB/SE component with 4-6 ps risetime. Broadband absorption difference spectra of chlorosomes excited at 720 nm (in the blue edge of the 746 nm BChl c Qy band) exhibit approximately 15 nm red-shifting of the PB/SE peak wavelength during the first several hundred fs. Analogous spectra excited at 760 nm (at the red edge) show little dynamic spectral shifting. Our results suggest that inhomogeneous broadening and spectral equilibration play a larger role in the early BChl c antenna kinetics in chlorosomes from C. tepidum than in those from C. aurantiacus, a system studied previously. As in C. aurantiacus, the initial one-color anisotropies r(0) for most BChl c wavelengths are close to 0.4. The corresponding residual anisotropies r(infinity) are typically 0.19-0.25, which is much lower than found in C. aurantiacus (> or = 0.35); the transition moment organization is appreciably less collinear in the BChl c antenna of C. tepidum. However, the final one-color anisotropies at 789 and 801 nm are approximately 0 and 0.09 respectively, and the final anisotropy in time 780 --> 800 nm experiment is approximately -0.1. These facts indicate that the BChI a transition moments themselves exhibit some order, and are directed at an angle > 54.7 degrees on the average from the BChl c moments. The one-color profiles exhibit coherent oscillations at most wavelengths, including 800 nm; Fourier analyses of these oscillations frequently yield components with frequencies 70-80 and 130-140 cm-1.

Ultrafast energy transfer in LHC-II trimers from the Chl a b light-harvesting antenna of Photosystem II

Time-resolved absorption difference profiles were obtained for LHC-II trimers, isolated from Photosystem II in spinach with n-dodecyl β-d-maltoside, using one-color and two-color pump-probe techniques. The one-color isotropic signals are predominantly excited state absorption at 640 nm, and a combination of photobleaching and stimulated emission at wavelengths ≥ 665nm. At intermediate wavelengths, dynamic red-shifting due to downhill energy transfer among the chlorophyll (Chl) spectral forms produces a bipolar signal, in which prompt photo-bleaching/stimulated emission is superseded at later times by excited state absorption. Triexponential analyses of these profiles yield the lifetime components 2–6 ps (associated with the spectral shifting), 14–36 ps (possibly due to energy transfer between LHC-II monomers), and several hundred picoseconds. The one-color anisotropy decays are resolvable at 665–675 nm, with lifetimes of 4.3 to 6.3 ps. They are unresolvably fast (i.e., exhibit subpicosecond lifetimes) at 640–650 nm. The two-color isotropic absorption difference signals show clear spectral evolution arising from equilibration among the LHC-II spectral components for excitation wavelengths shorter than 670 nm. However, most of this spectral evolution occurs within less than 2.5 ps. No resolvable anisotropy decay was observed in the two-color experiments. Taken together, the one-color and two-color experiments indicate that both picosecond and subpicosecond energy transfer steps occur in this antenna. The faster processes appear to dominate the spectral equilibration; slower processes occur in isoenergetic energy transfers among the longer-wavelength Chl a spectral forms that absorb between 665 and 675 nm. The values of the long-time anisotropic r(x), measured in the one-color and two-color experiments, are qualitatively consistent with static linear dichroism spectra of these preparations.

Ultrafast pump-probe spectroscopy of bacteriochlorophyll c antennae in bacteriochlorophyll a-containing chlorosomes from the green photosynthetic bacterium Chloroflexus aurantiacus

Time-dependent isotropic and anisotropic absorption difference profiles have been obtained with 2 ps resolution for the (BChl) bacteriochlorophyll c antenna in BChl a -containing chlorosomes from the green photosynthetic bacterium Chloroflexus aurantiacus . The isotropic absorption difference spectra are bipolar; they are dominated by excited state absorption at wavelengths 735 nm. The entire isotropic spectrum exhibits a dynamic blue shift over approx. 4 nm with a time constant of approx. 7 ps. All of these phenomena are rationalized in terms of a linear exciton model that resembles the theory of J -aggregates. In this theory, the occurrence of the dynamic blue shift corresponds to relaxation between Q y excitation components; it cannot readily be explained using a model that assumes that the excitations are localized on single chromophores within 1 ps. The anisotropic decay times, resolved here for the first time with our 2 ps fwhm instrument function, are approx. 7 ± 1 ps at 720 nm and 4 ± 1 ps at 740 nm. The residual anisotropy r (∞) is 0.32 ± 0.02 at 740 nm, which is in good agreement with fluorescence and linear dichroism measurements. These anisotropy functions initialize to 0.4 within error, indicating that subpicosecond depolarization is not prevalent.

Emission from the 1(n, π*) state of azobenzene: Spectrum and ultrashort decay time

Abstract Picosecond lifetime and depolarization studies of 1 (n, π * ) emission from trans-azobenzene indicate that fluorescence contributes substantially- to the emission decay pattern. The 1 (n, π * ) states decay time is ≈25 ps, in contrast to the nanosecond lifetimes exhibited by fluorescing singlet states in stilbenes and styrenes.

Steady-state and time-resolved polarized light spectroscopy of the green plant light-harvesting complex II

Abstract The major chlorophyll a/b light-harvesting complex from spinach thylakoid membranes was analyzed by steady-state polarized light spectroscopy at 4 K and by one-color and two-color pump-probe spectroscopy at room temperature. Steady-state absorption, linear dichroism and circular dichroism spectra indicate that the Chl Q y (0−0) absorption region is characterized by at least six transitions with significant differences in absorption, orientation and rotational strength. Steady-state low-temperature fluorescence spectra suggest that the fluorescence arises for a large part from several energetically similar species that form a circularly degenerate oscillator in the plane constituted by the two long axes of the particle. The possible presence of special red-absorbing pigments at low temperature is discussed. The time-resolved data suggest that the kinetics of chlorophyllb → a excitation energy transfer, as well as those of downhill excitation transfer among chlorophylla spectral forms, are heterogeneous with both sub-picosecond and picosecond lifetime components.

Excitation transport in solution: A quantitative comparison between GAF theory and time‐resolved fluorescence profiles

Time‐correlated single photon counting was used to monitor fluorescence concentration depolarization for DODCI in glycerol. For DODCI concentrations below ∼10−3 M, the present experiments have minimized self‐absorption and excitation trapping artifacts to the extent where they contribute negligibly to the observed differences between experimental fluorescence profiles and profiles computed from the Gochanour–Andersen–Fayer three‐body theory for excitation transport in solution. The three‐body theory accurately describes fluorescence depolarization at the lower dye concentrations. At higher concentrations, the measured decay in Gs(t), the probability that the excitation resides on the laser‐excited molecule, is perceptibly slower than predicted by the three‐body theory. This deviation may arise from nonrandom dye distributions in solution, rather than from errors in the three‐body theory. The experimental decay is equally well described at all concentrations by an earlier analytic theory which was develope...