Lifetime measurement time-resolved

The effect of oxygen on cyclic 1,3-diradicals shows that conformation can affect the triplet state lifetime ST Time resolved resonance Raman spectroscopy has been used to examine triplet states produced from different isomers of p-carotene. A theoretical study has also been reported on the a-cleavage of the triplet states of symmetric and non-symmetric ketones S mechanism for triplet state relaxation of aromatic molecules has been used to explain experimental data for substituted benzenes. The decay kinetics of triplet-triplet fluorescence in the mesitylene biradical (two sub-levels) have been measured between 10 and 77K in Shpolski matrices triplet state of dimesityl... 

Pulsed sources, including both lamps and lasers, are used for special apphca-tions such as dynamic measurements of luminescence lifetimes and time-resolved... 

These features allow both lifetime and time resolved polarization measurements to be extended into the subnanosecond region and the high repetition frequency greatly facilitates rapid accumulation of data. Time resolved polarization measurements are also made easier by the intrinsic polarization of the light source. 

What all FLIM instruments have in common is that 1) the excitation light is intensity-modulated or pulsed, and 2) that the emitted fluorescence light is measured time-resolved. Since the lifetimes that have to be resolved are typically in the nanosecond range, this means that both the modulation of the excitation light and the detection need to be performed at extremely high speeds. Consequently, most of the conventional instrumentation used for steady-state fluorescence microscopy cannot be used. In the next section several modes of implementation of FLIM are discussed. 

Conceptually, time-domain lifetime measurement is easier to understand than frequency-domain lifetime measurement. In time-domain lifetime measurement, a short (relative to the fluorescence lifetime) pulse of excitation light is given, after which the emitted fluorescence is measured time resolved [15], resulting directly in decay curves like that described in Eq. (1) (see also Fig. 2). Due to the requirement of short light pulses and fast detection, time-domain measurements became possible only about 40 years later than frequency-do-main measurements using a flashlamp as excitation source [16]. 

Fig. 2 Principle of time-domain lifetime measurement (see text). Fluorochromes are excited using a short pulse of light, after which the emitted fluorescence is measured time-resolved. Usually, fluorescence is recorded in two or more discreet time intervals...

Measurements of the lifetimes of NIESST states by time-resolved MES [51] and of LIESST states by time-resolved optical spectroscopy [52] on the very same system (a single crystal of [FC , Mni , (bpy)3] (bpy = bipyridine)) gave similar results. This supports the suggestion that the mechanisms for LIESST and NIESST relaxation are very similar, at least for the low-energy regime. The NIESST effect was also studied in Co(ll) SCO compounds, viz. [ Co/Co(terpy)2]X2 H20 (X = [CIOJ, n = V2,X = Cl , n = 5), where terpy is the tridentate ligand terpyridine... 

In the mechanism of an interfacial catalysis, the structure and reactivity of the interfacial complex is very important, as well as those of the ligand itself. Recently, a powerful technique to measure the dynamic property of the interfacial complex was developed time resolved total reflection fluorometry. This technique was applied for the detection of the interfacial complex of Eu(lII), which was formed at the evanescent region of the interface when bathophenanthroline sulfate (bps) was added to the Eu(lII) with 2-thenoyl-trifuluoroacetone (Htta) extraction system [11]. The experimental observation of the double component luminescence decay profile showed the presence of dinuclear complex at the interface as illustrated in Scheme 5. The lifetime (31 /as) of the dinuclear complex was much shorter than the lifetime (98 /as) for an aqua-Eu(III) ion which has nine co-ordinating water molecules, because of a charge transfer deactivation. 

The simplest fluorescence measurement is that of intensity of emission, and most on-line detectors are restricted to this capability. Fluorescence, however, has been used to measure a number of molecular properties. Shifts in the fluorescence spectrum may indicate changes in the hydrophobicity of the fluorophore environment. The lifetime of a fluorescent state is often related to the mobility of the fluorophore. If a polarized light source is used, the emitted light may retain some degree of polarization. If the molecular rotation is far faster than the lifetime of the excited state, all polarization will be lost. If rotation is slow, however, some polarization may be retained. The polarization can be related to the rate of macromolecular tumbling, which, in turn, is related to the molecular size. Time-resolved and polarized fluorescence detectors require special excitation systems and highly sensitive detection systems and have not been commonly adapted for on-line use. 

Fig. 22 (a) Comparison of fluorescence lifetime (blue triangles), calculated from (13), and measured by time-resolved fluorescence red circles) as a function of solvent polarity for G19. (b) Fluorescence quantum yield blue squares) and peak ground state absorption wavelength red circles) as a function of solvent polarity given by the percentage of toluene (T) in toluene-ACN mixtures for G19... 

In (8), the solvent-independent constants kr, kQnr, and Ax can be combined into a common dye-dependent constant C, which leads directly to (5). The radiative decay rate xr can be determined when rotational reorientation is almost completely inhibited, that is, by embedding the molecular rotor molecules in a glass-like polymer and performing time-resolved spectroscopy measurements at 77 K. In one study [33], the radiative decay rate was found to be kr = 2.78 x 108 s-1, which leads to the natural lifetime t0 = 3.6 ns. Two related studies where similar fluorophores were examined yielded values of t0 = 3.3 ns [25] and t0 = 3.6 ns [29]. It is likely that values between 3 and 4 ns for t0 are typical for molecular rotors. 

The most sophisticated techniques require time-resolved measurements (lifetime, anisotropy, spectra) either in the time or frequency domain ([6-10] for a focused journal issue on the subject see [11]). Thus, the significance of new, versatile, commercially available light... 

For fluorescence decay curves of the J-aggregate LB films of [CI-MC] mixed with various matrix agents, measured with a picosecond time-resolved single photon counting system, three components of the the lifetimes fitting to exponential terms in the following equation ... 

Attractive for the use of QDs are their long lifetimes (typically 5 ns to hundreds of nanoseconds), compared to organic dyes, that are typically insensitive to the presence of oxygen. In conjunction with time-gated measurements, this provides the basis for enhanced sensitivity [69]. This property can be also favorable for time-resolved applications of FRET. The complicated size-, surface-, and wavelength-dependent, bi- or multi-exponential QD decay behavior (Fig. 2) can complicate... 

Dr can be determined by time-resolved fluorescence polarization measurements, either by pulse fluorometry from the recorded decays of the polarized components I l and 11, or by phase fluorometry from the variations in the phase shift between J and I as a function of frequency (see Chapter 6). If the excited-state lifetime is unique and determined separately, steady-state anisotropy measurements allow us to determine Dr from the following equation, which results from Eqs (5.10) and (5.41) ... 

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