Investigation of Femtosecond Chemical Reactivity by Means of Fluorescence Up-Conversion

Abstract

Fluorescence spectroscopy is a powerful tool for the study of the reactivity of chemical and biological systems. The fluorescence spectrum, the fluorescence quantum yield, and the fluorescence lifetime of a chromophore depend on its microenvironment and the solute-solvent interactions. Fluorescence lifetimes also depend on the kinetics of possible chemical reactions. Fluorescence measurements in the time domain thus provide detailed information about intraand inter-molecular photophysical and photochemical processes. During the past 30 years the development of laser sources and ultrafast techniques has opened new ways for the study of the fluorescent excited states of molecules in the nano-, pico-, and femtosecond time range. When analyzing the rise and the decay (often multi-exponential) of their fluorescence intensity as a function of time, the pre-exponential factors and the fluorescence time constants have to be determined accurately after a careful deconvolution using a computer-based algorithm. Several techniques are used: phase modulation spectroscopy with a modulated laser pump source on a time scale comparable to the decay times of interest, time-correlated single photon counting under picosecond laser excitation (alternatively nanosecond flashlamp excitation), ultra-fast streak cameras designed for the conversion of time dependent light emission into distance dependent electron impact on a phosphor screen. Because a sufficient resolution is not accessible with those techniques for the measurement of fast decays in the 50 fs-100 ps range, fluorescence up-conversion in nonlinear optical crystals is the technique of choice. Nanosecond and Picosecond Time-Resolved Fluorescence TechniquesThe fluorescence lifetime TF of a molecule is the reciprocal of the rate TF of depopulation of the first excited singlet state following optical excitation from the ground state. In the case of a monoexponential decay TF is given by

Relaxation Dynamics of the Triazene Compound Berenil in DNA-Minor-Groove Confinement after Photoexcitation

“…The torsion can lead to large amplitude motions which are hindered in (bio)molecular confinements and viscous solvents so that in these environments the relaxation processes are significantly delayed relative to non-viscous solvents or the gas phase. 1 In particular, for chains with several conjugated bonds, there are, besides simple rotations, also different volume-conserving mechanisms with spatially localized nuclear motions conceivable, which are less sensitive to confinement and solvent viscosity. Known examples exhibiting mechanisms beyond rotation include azobenzenes, [2][3][4][5][6][7][8][9] green fluorescent proteins, 10 the photoactive yellow protein, 11,12 rhodopsin 13 and mutants, 14 and others.…”

“…These findings corroborate conclusions drawn from time-resolved fluorescence and transient absorption spectroscopy of berenil in solvents of different viscosity and bound to the DNA minor groove. [24][25][26] To get further insight into the interplay between the nuclear relaxation on the excited- 1 Berenil exhibits three molecular conformers with respect to the triazene NHN− −N functional group.…”

Solvatationsdynamik an biologischen Grenzschichten

“…Die Umwandlungseffektivität η q für die Summenfrequenzerzeugung läßt sich für diesen Fall unter Anwendung einiger Näherungen angeben als: [206,207,208], sie reduziert sich zu:…”

“…Dabei bezeichnet I die jeweiligen eingestrahlten Intensitäten, d eff den effektiven nichtlinearen Koeffizienten des Kristalls, L die Dicke des Kristalls, c die Lichtgeschwindigkeit und ε 0 die absolute Dielektrizitätskonstante des Vakuums[206].Akzeptanzwinkel und spektrale BandbreiteDie obigen Ableitungen gelten streng nur für die Phasenanpassung ∆ k = k S − k Fl − k Gate = 0. Sie ist jedoch nicht immer exakt erfüllt.…”

“…Ein Fluoreszenzaufkonvertierungsexperiment stellt prinzipiell eine Kreuzkorrelation zwischen Fluoreszenzlicht und einem intensiven Laserpuls dar[204,205,206,207,208,209,210]. Die zu untersuchende Probe wird mit einem Laserpuls der Frequenz ω Pump elektronisch angeregt und die dabei entstehende inkohärente Fluoreszenz der Frequenz ω Fl wird gesammelt und zusammen mit einem intensiven Laserpuls, dem sogenannten Gatepuls ( " Torpuls") der Frequenz ω Gate , in einen optisch-nichtlinearen Kristall fokussiert.…”