Decay profiles

Morishima et al. [75, 76] have shown a remarkable effect of the polyelectrolyte surface potential on photoinduced ET in the laser photolysis of APh-x (8) and QPh-x (12) with viologens as electron acceptors. Decay profiles for the SPV (14) radical anion (SPV- ) generated by the photoinduced ET following a 347.1-nm laser excitation were monitored at 602 nm (Fig. 13) [75], For APh-9, the SPV- transient absorption persisted for several hundred microseconds after the laser pulse. The second-order rate constant (kb) for the back ET from SPV- to the oxidized Phen residue (Phen+) was estimated to be 8.7 x 107 M 1 s-1 for the APh-9-SPV system. For the monomer model system (AM(15)-SPV), on the other hand, kb was 2.8 x 109 M-1 s-1. This marked retardation of the back ET in the APh-9-SPV system is attributed to the electrostatic repulsion of SPV- by the electric field on the molecular surface of APh-9. The addition of NaCl decreases the electrostatic interaction. In fact, it increased the back ET rate. For example, at NaCl concentrations of 0.025 and 0.2 M, the value of kb increased to 2.5 x 108 and... 

For QPh-x, the decay of SPV- was faster than that for the APh-x-SPV system, and obeyed first-order kinetics [76]. The addition of NaCl (0.2 M) caused the decay profile to change i.e. the back ET was considerably slowed and the decay kinetics changed from first-order to second-order, with a reaction rate constant of kb = 1.8 x 109 M-1 s-1. These findings suggest the impossibility of escape of... 

Delaire et al. [124] have reported that laser photolysis of an acidic solution (pH 2.8) containing PMAvDPA (23) and MV2 + allows the formation of surprisingly long-lived MV + - and DPA cation radicals with a very high charge escape quantum yield. The content of the DPA chromophores in PMAvDPA is as low as less than 1/1000 in the molar ratio DPA/MAA. Figure 20 shows a decay profile of the transient absorption due to MV + monitored at 610 nm [124]. The absorption persists for several milliseconds. As depicted in Fig. 20, the decay obeys second-order kinetics, which yields kb = 3.5 x 10s M 1 s. From the initial optical density measured at 610 nm, the quantum yield for charge escape was estimated to be 0.72 at 0.2 M MV2 +. ... 

Fig. 21.8 Room-temperature fluorescent decay profile for Ba3BP30- 2.

Figure 21.23 exhibits the room-temperature fluorescence decay profiles of Ba3BP30i2 Eu powders. The experimental decay curve can be fitted by an equation with two exponential terms corresponding to two decay times of 20 ns (98.97%) and 522 ns (1.03%), respectively. 

MFEs on the dynamics of the radical pair in CtoN" clusters (C oN " ) -MePH system were examined in TH F-H2O (2 1) mixed solvent. M FEs on the decay profiles of the transient absorption at 5 20 nm due to the phenothiazine cation radical (P H " ) are shown in Eigure 15.9b. The decay was retarded in the presence of the magnetic field. In addition, the absorbance at 10 (is after laser excitation increased with increasing magnetic field. The result indicated that the yield of the escaped PH increased with the increase in magnetic field. Therefore, the MFEs on the decay profile were clearly observed. 

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. 

Fig. 4.8. Fluorescence lifetime of a stained section of Convallaria resolved with respect to lifetime, excitation and emission wavelength (A) intensity image integrated over the time-resolved excitation-emission matrix (EEM) (B, D) time-integrated EEM from areas A and B respectively in (A) (C) fluorescence decay profile for /ex 490 nm and Aem 700 nm corresponding to area A (E) fluorescence decay profile for Aex 460 nm and /em 570 nm corresponding to area B.

There is significant debate about the relative merits of frequency and time domain. In principle, they are related via the Fourier transformation and have been experimentally verified to be equivalent [9], For some applications, frequency domain instrumentation is easier to implement since ultrashort light pulses are not required, nor is deconvolution of the instrument response function, however, signal to noise ratio has recently been shown to be theoretically higher for time domain. The key advantage of time domain is that multiple decay components can, at least in principle, be extracted with ease from the decay profile by fitting with a multiexponential function, using relatively simple mathematical methods. 

The technique described in Section 11.5.2 above can be employed to create biological signaling units with protein biosensors [62, 63], changing shape upon binding of a ligand—for example, calcium or chloride [64]—and thereby changing relative conformation and FRET efficiency [65], Alternatively, synthetic dyes, which change decay profile in a predictable manner dependent on environment, can be employed. 

One of the issues which have impeded the study of tissue AF is the complex decay profile that it exhibits. It is often assumed that fluorescence decay is necessarily a simple exponential. This assumption is not necessarily valid, as is explained in Chapter 4. 

This type of modeling, as well as wide field FLIM [84-86] and endoscopy [72] have tremendous potential for clinical diagnosis. Although still at the preclinical stage, various groups have looked at the differences in fluorescence spectrum and decay profiles in breast cancer [87, 88], the gastrointestinal tract [89], and skin cancer [90, 91] (see also Chapter 4). 

Figure 4. Luminescence decay profile of an oxygen indicator dye excited by a short flash of light, in (a) solution and (b) embedded into a gas-permeable film used to fabricate fiber-optic sensors for such species. The logarithmic scale of the Y-axis allows to compare the exponential emission decay in homogeneous solution and the strongly non-exponential profile of the photoexcited dye after immobilization in a polymer matrix.

When the fluorophore is immobilized on a solid support, the decay profile usually departs from the exponential kinetics predicted by equation 5 and verified in homogeneous media (e.g. solution, Figure 4). In this case, it is customary to fit the kinetic data to a sum of exponentials (equation 7) and mean lifetime values are used to characterize the return of the photoexcited molecule to the ground state28. If the so called pre-exponential weighted mean lifetime (tm) is used, equation 6 may still be used (equation 8) ... 

Measurements of the hydrocarbon fluorescence lifetimes provide important information which is useful in interpreting the Stern-Volmer plots. In cases where Equation 1 is valid, the hydrocarbon fluorescence decay profiles must be the same with and without DNA. In some cases, BP for example, this is not the case. For BP the observed decay profile changes significantly when DNA is added (72). 

However for several of the molecules shown in Figures 1 and 2, DNA has only a small effect on the observed fluorescence lifetime. These molecules include trans-7,8-dihydroxy-7,8-dihydro-BP (15,18,19), trans-4,5-dihydroxy-4,5-dihydro-BP (15,18), BPT (7,18), 1,2,3,4-tetrahydro-BA (12), 8,9,10,11-tetrahydro-BA (14), 5,6-dihydro-BA (12), anthracene (12) and DMA (14). Typical decay profiles obtained in fluorescence lifetime measurements of trans-7,8-dihydroxy-7,8-dihydro-BP and of 8,9,10,11-tetrahydro-BA are shown in Figure 6. The lifetimes extracted from the decay profiles shown here have been obtained by using a least-squares de-convolution procedure which corrects for the finite duration of the excitation lamp pulse (77). 

For 8,9,10,11-tetrahydro-BA the lifetimes measured with and without DNA are the same within experimental error ( 2 nsec). Without DNA the decay profile of trans-7,8-dihydroxy-7,8-dihydro-BP follows a single-exponential decay law. With DNA the decay profile has a small contribution from a short-lived component (x = 5 nsec) which arises from DNA complexes. This indicates that Equation 1 is not strictly valid. However, the analysis of the decay profile with DNA also indicates that the short lifetime component contributes less than 11% to the total emission observed at [POa ] 5 x 10 M. Under these conditions Equation 1 still yields a good approximate value to the association constant for intercalation. 

Figure 6. Fluorescence decay profiles of trans-7,8-dihydroxy-7,8-dihydro-BP and 8,9,10,11-tetrahydro-BA measured at 23 °C with and without native DNA. Taken from refs. 14 and 15. The upper left-hand corner contains an instrument response profile. Emission and excitation wavelengths, lifetimes, and values of x2 obtained from deconvolution of the lifetime data are also given.

Because the amount X is proportional to the concentration, a similar equation describes the time-decay profile of the drug concentrahon instead of the ammmt ... 

The spectrum of the assigned TCC transient is broad and featureless and, in this way, similar to transients assigned to the orthogonal X transient. It could be that in the present case, the observed transient is also similar to X, but even if this is the case, then the subsequent isomerization should occur through the TCC isomer. In Fig. 5, the rise and decay profiles for NOSI3 and At-isobutyl NOSI3 and the other compounds studied are shown. 

If the S2 fluorescence is mainly produced by the triplet-triplet annihilation (mechanism [A]), the decay profile must depend upon the decay rate constant (k ) of the first excited triplet state, and... 

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