Fluorescent decay rates, effect

The significantly faster PuF6(g) fluorescence decay rate found using 532 nm excitation is unlikely to be due to a thermally induced effect (e.g. pyrolysis). The optical absorption coefficients of PuF6(g) at 532 nm is at most twice as large as at 1064 nm (15) Assuming the 1064 nm absorption... 

Instruments of this type may also be used quite effectively to evaluate kinetics of time-dependent changes in foods, be they enzymatic or reactive changes of other types. The computerized data-acquisition capabilities of these instruments allow precise measurement of absorbance or fluorescence changes, often over very brief time periods ( milliseconds). This is particularly useful for analysis of fluorescence decay rates, and in measurement of enzymatic activity in situ. A number of enzyme substrates is available commercially which, although non-fluorescent initially, release fluorescent reaction products after hydrolysis by appropriate enzymes. This kinetic approach is a relatively underused capability of computerized microspectrophotometers, but one which has considerable capability for comparing activities in individual cells or cellular components. Fluorescein diacetate, for example, is a non-fluorescent compound which releases intensely fluorescent fluorescein on hydrolysis. This product is readily quantified in individual cells which have high levels of esterase [50]. Changes in surface or internal color of foods may also be evaluated over time by these methods. 

Table3.1 Sensitivity p) ofthe Stokes shift, fluorescence decay rate constant, fluorescence quantum yield, and radiative deactivation rate constant to intramolecular substituent effects for two different groups of trans-4,4 -disubstituted stilbenes [87].

A peculiar effect was observed in the decomposition of 19 a with anthracene as fluorescer when oxygen was carefully removed from the solutions an increase of the chemiluminescence decay rate and of the dioxetane cleavage resulted. It was suggested that this was due to a catalytic effect of triplet anthracene (formed by energy transfer from triplet formate) on the decomposition of the dioxetane. When oxygen is present, triplet anthracene is quenched. Whether such a catalytic effect of triplet anthracene or similar compounds on dioxetane cleavage actually exists has not yet been fully established positive effects were observed by M. M. Rauhut and coworkers 24> in oxalate chemiluminescence and by S. Mazur and C. S. Foote 80> in the chemiluminescent decomposition of tetramethoxy-dioxetane, where zinc tetraphenylporphy-rin seems to exert a catalytic effect. However, the decomposition of trimethyl dioxetane exhibits no fluorescer catalysis 78h... 

Comparison of the Experimental and Simulation Results. The preceding discussion has shown that both the experimental anthracene fluorescence profiles and the simulated anthracene concentration profiles decrease in a manner which closely follows an exponential decay. Therefore, the most convenient way to compare the simulation results to the experimental data is to define an effective overall photosensitization rate constant, kx or k2, as described above. Adoption of this lumped-parameter effective kinetic constant allows us to conveniently and efficiently compare the experimental data to the simulation results by contrasting the rate constant obtained from the steady-state fluorescence decay with the value obtained from the simulated decrease in the anthracene concentration. 

The geometry of the nanoscaled metals has an effect on the fluorescence enhancement. Theoretically, when the metal is introduced to the nanostructure, the total radiative decay rate will be written as T + rm, where Tm corresponds to the radiative decay rate close to the metal surface. So, (1) and (2) should be modified and the quantum yield and lifetime are represented as ... 

The long lifetime has important consequences on the decay rates. First, we consider what affects the nonradiative rates (knr) which change the yields of fluorescence and phosphorescence. The nonradiative decay rate is often enhanced in molecules which have flexible constituents (the so-called loose-bolt effect). Therefore, both fluorescence and phosphorescence yields are generally larger for rigid molecules than flexible molecules. For the same reason, a rigid environment will increase the emission yields hence both fluorescence and phosphorescence yields often increase with increasing viscosity. 

Selected entries from Methods in Enzymology [vol, page(s)] Analysis of GTP-binding/GTPase cycle of G protein, 237, 411-412 applications, 240, 216-217, 247 246, 301-302 [diffusion rates, 246, 303 distance of closest approach, 246, 303 DNA (Holliday junctions, 246, 325-326 hybridization, 246, 324 structure, 246, 322-324) dye development, 246, 303, 328 reaction kinetics, 246, 18, 302-303, 322] computer programs for testing, 240, 243-247 conformational distribution determination, 240, 247-253 decay evaluation [donor fluorescence decay, 240, 230-234, 249-250, 252 exponential approximation of exact theoretical decay, 240, 222-229 linked systems, 240, 234-237, 249-253 randomly distributed fluorophores, 240, 237-243] diffusion coefficient determination, 240, 248, 250-251 diffusion-enhanced FRET, 246, 326-328 distance measurement [accuracy, 246, 330 effect of dye orientation, 246, 305, 312-313 limitations, 246,... 

Invariably, the energy of the light emitted is less than that originally taken on. By studying the amount and energy of the fluorescence radiation decay rates, depolarization effects, excimer stability, and structure can be determined. 

Conventional EPR techniques have been successfully used to measure the D and E values of matrix-isolated carbenes in the ground triplet state because the steady-state concentration of triplet species is sufficiently high in the system. The technique cannot be used, however, for excited species having triplet hfetimes of the order of 10-100 ns, since their steady-state concentration is too low. The D parameters are estimated from the external magnetic field effect on the T—T fluorescence decay in a hydrocarbon matrix at low temperamre. The method is based on the effect of the Zeeman mixing on the radiative and nonradiative decay rates of the T -Tq transition in the presence of a weak field. The D values are estimated by fitting the decay curve with that calculated for different D values. The D T ) values estimated for nonplanar DPC (ci symmetry) is 0.20... 

This effective dye relaxation time rp is the spontaneous fluorescence decay time shortened by stimulated emission which is more severe the higher the excitation and therefore the higher the population density w j. The dependence of fluorescence decay time on excitation intensity was shown in 34 35>. Thus, fluorescence decay times measured with high intensity laser excitation 3e>37> are often not the true molecular constants of the spontaneous emission rate which can only be measured under low excitation conditions. At the short time scale of modelocking the reorientation of the solvent cage after absorption has occurred plays a certain role 8 > as well as the rotational reorientation of the dye molecules 3M°)... 

A similar but smaller intramolecular quenching effect was seen by Phillips and co-workers 44,4S) for 1-vinylnaphthalene copolymers incapable of excimer fluorescence. The monomer fluorescence lifetime of the 1-naphthyl group in the methyl methacrylate copolymer 44) was 20% less than the lifetime of 1-methylnaphthalene in the same solvent, tetrahydrofuran. However, no difference in lifetimes was observed between the 1-vinylnaphthalene/methyl acrylate copolymer 45) and 1-methylnaphthalene. To summarize, the nonradiative decay rate of excited singlet monomer in polymers, koM + k1M, may not be identical to that of a monochromophoric model compound, especially when the polymer contains quenching moieties and the solvent is fluid enough to allow rapid intramolecular quenching to occur. 

As the effect of the natural fluorescence decay has been removed from eqn. (263), the effective second-order quenching rate coefficient is k p). The rate of fluorescence quenching is kpM, where p is the quencher density. Because a steady state exists, this rate of quenching balances the... 

At T = 77 K in MTHF, the kinetics of fluorescence decay of P-L-Q with a bridge containing one bicyclo[2.2.2]octyl is of a non-exponential character. This effect can be explained by the coexistence in the frozen solution of several rotational conformations of the P-L-Q molecule (rotation of the porphyrin fragment around the a bond in its meso position is meant here). The characteristic time of the fluorescence decay for the predominant portion of the P-L-Q particles at 77 K, r 1.1 x 10 1°s, virtually coincides with the value of r = l/k(e1 at 298 K, i.e. the rate of tunneling from P to Q is independent of temperature. The exponential character of the fluorescence decay curve at 298 K indicates that, at this temperature, the rate of rotation exceeds k(e1. ... 

Level crossing spectroscopy has been used by Fredriksson and Svanberg44 to measure the fine structure intervals of several alkali atoms. Level crossing spectroscopy, the Hanle effect, and quantum beat spectroscopy are intimately related. In the above description of quantum beat spectroscopy we implicitly assumed the beat frequency to be high compared to the radiative decay rate T. We show schematically in Fig. 16.11(a) the fluorescent beat signals obtained by... 

The first systems without anomalous fluorescence discussed in the context of TICT states were coumarine laser dyes. Jones and coworkers208210 demonstrated that dialkylaminocoumarines like 7C showed an increase of the nonradiative decay rate in strongly polar solvents. This effect is dramatically increased if the acceptor properties of the coumarine skeleton are stronger, for example, in F7C. On the other... 

Marginal fluorescence quantum yields (1%) are generally observed though 25 and 33 fluorescence with 8% and 14% yields, respectively. Such low quantum yields are indicative of the effective competition of radiationless processes such as the Si —> Tj ISC and fast internal conversion (Si —> S0). The rate constants for radiative decay of Si (kF) range from 8 x 106 to 1.3 x 108 s-1, and the nonradiative decay rate constants (fcNR) range from 1.9 x 108 to 3.5 x 109 s / The nonradiative deactivation pathway is thus six times faster than the radiative one for 33 (anti) and about 110 times faster for 32 (syn). 

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