Lifetime of Fluorescence

The inhomogeneous broadening effect will be apparent in practically all cases, and the character of this broadening may be both stationary (in rigid solutions or when time of relaxation xr is less than lifetime of fluorescence x, or xr>t) and dynamic in nature. Inhomogeneous broadening affects all spectral characteristics of organic molecules in solutions. 

Dependencies of luminescence bands (both fluorescence and phosphorescence), anisotropy of emission, and its lifetime on a frequency of excitation, when fluorescence is excited at the red edge of absorption spectrum. Panel a of Fig. 5 shows the fluorescence spectra at different excitations for the solutes with the 0-0 transitions close to vI vn, and vra frequencies. Spectral location of all shown fluorescence bands is different and stable in time of experiment and during lifetime of fluorescence (panel b)... 

Encapsulation in a cucurbit[7]uril (CB7) macrocycle was suggested to help improve solubility, prevent undesirable aggregation, and thereby increase the quantum yields and fluorescence lifetimes of fluorescent dyes [46]. 

In general, the natural radiative lifetimes of fluorescence and phosphorescence should be independent of temperature. But the emission intensities may vary due to other temperature dependent and competitive rate constants. 

The lifetime of fluorescence (or phosphorescence), r0, is defined using the rate constant kf as r0 = 1 /k. By this definition, the residual intensity according to equation (12.1) is only 36.7% of the initial intensity at r0 63.3% of the initial species have relaxed to a non-emissive state. Because fluorescence lifetimes are only a few nanoseconds, fluorimeters require measurements to be made at the same time as... 

Fluorescence and phosphorescence are relatively rare. Molecules generally decay from the excited state by radiationless transitions. The lifetime of fluorescence is always very short (10-8 to 10-4 s). The lifetime of phosphorescence is much longer (10-4 to 102 s). Therefore, phosphorescence is even rarer than fluorescence, because a molecule in the T] state has a good chance of undergoing intersystem crossing to S0 before phosphorescence can occur. 

A more recent work by Paech ct al. (789) on the collision-free lifetimes of N02 excited by a tunable laser near 4880 and 5145 A states that although only a single level is excited, three different lifetimes of fluorescence, 3, 28, and 75 /rsec, have been observed. The results lead them to conclude that the initially formed 2B, state crosses over rapidly to another state, 2B2, with higher level density. The 2B2 state can have two different lifetimes (28 and 75 ftscc), depending on the extent of interaction with the ground state. The short life observed, 3 //sec, is determined primarily by the rate of internal conversion from the 2B, to 2B2 state. The results of some reported collision-free fluorescence lifetimes are given in Table VI-6 . 

B n near the bottom of the potential curve as indicated in Fig. V-21. The FI state must cross the B3n state again near i> = 25 (/. = 5500 A) where the dissociation quantum yield shows a subsidiary maximum. The measured lifetime of fluorescence changes accordingly. The lluorcscencc lifetime is shortest at v = 4 (0.53 / = 13, then decreases (852). This dependence of lifetime on the vibrational level may be... 

It would be desirable to insert a probe into the polymer to ascertain the local environmental conditions. In addition to having microscopic dimensions, the probe must act as a timing device which specifies the time-scale of the observation. Such a probe is a fluorescent molecule. Its dimensions are about the size of a monomer residue, namely of the order of 10 A, and the lifetime of fluorescence, r, varies between about 10-9— 10"7 sec., depending on the fluorescent compound and the medium (9). Still longer time-scales, namely, 10"4—10 sec., are achieved with organic molecules in the phosphorescent state (21). 

C5-C6 bond [10, 11] (or the pyramidalization of C5 [14, 22]), which is presumed to be the reaction coordinate for the subpicosecond internal conversion. It is therefore expected that both the intensity and lifetime of fluorescence will be dramatically greater for TMC and TMU compared to those for cytosine and uracil. 

In the electron-transfer process generalized in Eq. 1, one of the components of the reactant state may be fluorescent. This spin-allowed radiative process will thus be in competition with the nonradiative electron-transfer reaction and the two processes will contribute to the overall decay of the reactant state. The intrinsic lifetimes of fluorescent molecular states range typically from 10 to longer than 10 s. The occurrence of electron transfer involving the fluorescent state will shorten its lifetime and measurement of this quantity will therefore allow computation of the rate constant for electron transfer. 

VV Klimov, SI Allakhverdiev and VZ Pashchenko (1978) Measurement of the activation energy and lifetime of fluorescence of photosystem 2 chlorophyll. DokI Akad Nauk SSR 242 1204-1207... 

The rate constant of fluorescence kfl can likewise be calculated from the absorption spectra of fluorescent compounds67-69. The radiative lifetime of fluorescence, r0, i.e. the liefetime of the state Sj, when the latter vanishes due solely to emission of fluorescence, is equal to... 

Table 2. Spectral-Luminescence groups and types of organic molecules and some of their characteristics (Molar absorptivity of the longest wavelength band - emax Rate constants of fluorescence -kfl, phosphorescence - kph, intersystem crossing - kisc Lifetimes of fluorescence - zih phosphorescence - Tph)...

This emission can be classified as fluorescence, which has a very rapid decrease in intensity or phosphorescence, where emission decay is much slower. The difference between the two is characterized by the value of the constant k, which for fluorescence is much greater than for phosphorescence. The lifetime of fluorescence Tq is defined, using the rate constant k, by Tq = l/k. At the instant Tq, the intensity f will become, according to expression 11.1, 36.8 per cent of the initial intensity 7q. In other words a fluorescent compound corresponds, on the microscopic scale, to a population of individual species of which 63.2 per cent have relaxed to an non-emissive state after this brief period of time. 

A fluorescence decay time is a measurement, at fixed wavelength, of fluorescence signal as a function of time. Despite the fact that this time is very short, certain instruments can measure the lifetime of fluorescence. There exist several methods based, either upon the recording of the decrease curve of the light intensity... 

Pulse fluorometry has been favoured over phase techniques since hitherto no general method has been available for determining the proportions and lifetimes of fluorescence components in complex systems. Weberhas presented an exact solution of the problem using the values of the phase shifts and relative modulation of the overall fluorescence of as many light-modulation frequency as there are components. The simplicity and speed of the numerical methods involved... 

Although the short lifetimes of fluorescent probes limits their use for direct relocation studies, the excited state can be employed as a marker for the access of other molecules to the site where the probe resides. Thus, quenching studies can yield information on the mobility of the quencher through the supramolecular structure. This is an indirect method, as the excited probe is... 

Write a balanced nuclear equation for the reaction in which oxygen-15 undergoes positron emission. 7. Thorium-229 is used to increase the lifetime of fluorescent bulbs. What type of decay occurs when thorium-229 decays to form radium-225 8. Challenge The figure at right shows one way that bismuth-212 can decay, producing isotopes A and B. a. Write a balanced nuclear equation for this decay. b. Identify the isotopes A and B that are produced. V Beta particle Bismuth-212 Alpha particle J... 

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