Emission intensities and lifetimes

Fluorescence resonance energy transfer (FRET) has also been used very often to design optical sensors. In this case, the sensitive layer contains the fluorophore and an analyte-sensitive dye, the absorption band of which overlaps significantly with the emission of the former. Reversible interaction of the absorber with the analyte species (e.g. the sample acidity, chloride, cations, anions,...) leads to a variation of the absorption band so that the efficiency of energy transfer from the fluorophore changes36 In this way, both emission intensity- and lifetime-based sensors may be fabricated. 

The authors believe that the decreases in decay times are associated primarily with changes in quantum yield. This may be inferred from the fact that both the emission intensities and lifetimes are falling off at about the same rate with temperature. One thus concludes that the luminescence of sulfuric acid solutions of terbium sulfate is subjected to much greater temperature quenching than the luminescence in aqueous solution of the same salt. The increasing probability of radiationless transitions is undoubtedly connected in some manner with greater interaction of the radiating ion with the solvent molecules. 

Previous studies on paraffins, rhodamine dyes, and l,3-bis(N-carbozoyl) propane excimers have concluded that there is a relationship between km and polymer viscosity and free volume [103-105], Indeed, this dependence has been investigated in the context of decreasing free volume during methyl methacrylate polymerization [83,84], It has been shown that the nonradiative decay processes follow an exponential relationship with polymer free volume (vf), in which kra reduces as free volume is decreased [see Eq. (5)]. Here, k. represents the intrinsic rate of molecular nonradiative relaxation, v0 is the van der Waals volume of the probe molecule, and b is a constant that is particular to the probe species. Clearly, the experimentally observed changes in both emission intensity and lifetime for/ac-ClRe(CO)3(4,7-Ph2-phen) in the TMPTA/PMMA thin film are entirely consistent with this rationale. 

In terms of photophysics, electron transfer reactions create an additional non-radiative pathway, so reducing the observed emission lifetimes and quantum yields in A-L-B dyads in comparison with a model compound. However, there are other processes, such as molecular rearrangements, proton transfer and heavy-atom effects, which may decrease the radiative ability of a compound. One of the most important experimental methods for studying photoinduced processes is emission spectroscopy. Emission is relatively easy to detect and emission intensities and lifetimes are sensitive to competing processes. Studying parameters such as emission quantum yields and lifetimes for a given supramolecular species and associated... 

The spectroscopic properties of the MCP compounds vary strongly with decreasing temperature2. One finds a very large shift of the transition energies, a reduction of the halfwidths, a change of the emission intensities and lifetimes, and for some compounds structural transformations. 

We assumed that the added excess Cd + form not only the deep trap sites but also the shallow trap sites. Accordingly, the trap sites should be originated from lattice imperfection which should be formed on the surface of CdS-DMF as a result of the adsorption of excess Cd2+. The effects on the emission intensity and lifetime proved that the maximum number of trap sites was achieved when 0.2 equivalent of excess Cd2+ was introduced into the system. 

ESPT has been identified as the main nonradiative pathway in the excited state of ethidium bromide (EB, I), a popular DNA probe [46a]. In aqueous solution, on addition of DNA, EB intercalates in the double helix of DNA [44-46]. This causes a nearly 11-fold increase in the emission intensity and lifetime of EB. The emission quantum yield and lifetime of EB are very similar in methanol and glycerol, whose viscosities differ by a factor of 2000 [46a]. Thus, the fluorescence enhancement of EB on intercalation is not due to high local viscosity. Emission intensity of EB is low in highly polar, protic solvents, such as alcohol... 

Since the dynamics of the twisted intramolecular charge transfer (TICT) process is very sensitive to the polarity of the medium, the local polarity of an organized medium may also be determined from the rate of the HCT process. For TNS, which is nearly nonfluorescent in water ((j)f = 10 and Xf = 60 ps), the emission quantum yield and lifetime increases nearly 50 times on binding to cyclodextrins and more than 500 times on binding to a neutral micelle, TX -100 [86]. Such a dramatic increase in the emission intensity and lifetime arises because of the marked reduction of the nonradiative HCT process inside the less polar microenvironment of the cyclodextrins and the micelle. Determination of the micropolarities of various organized assemblies using TICT probes has been surveyed quite extensively in several recent reviews [5b-d,f,86]. Therefore, in this chapter we will focus only on some selected works not covered in the earlier reviews. 

Ambient temperature emission intensities and lifetimes from the salts of Ru(bpy>3 + and of Ru(bpy)2(py)2 + as solids and as solutions in the polymers poly(4-vinylpyridine) (PVP) and poly(acrylic acid) (PAA) have recently been studied as a function of pressure to 7 GPa in a diamond anvil cell [33], For the Ru(bpy)3+ emission in the PAA and PVP solutions, intensities and lifetimes both decreased monotonically to about 35 and 50% of the original values, respectively, as pressure was raised from 0.1 MPa to 7 GPa. This... 

Ambient temperature emission intensities and lifetimes from Ru(bpy) + and Ru(bpy)2(py)2 salts in solutions of poly(4-vinylpyridine) (PVP) and poly (acrylic acid) (PAA) have been studied at pressures to 7 GPa in a diamond anvil cell [10],... 

Shown in Figure 7 are the Stem-Volmer plots of emission intensities and lifetimes, monitored at 630 nm, as a function of MDESA concentration. Both the static (intensity) and the dynamic (lifetime) components are nonlinear and indicate that the quenching mechanism is comphcated. The extent of the static reaction (attributed to MDESA " anions associated with Ru(bpy)3 cations)... 

FIGURE 3. Hirayama-Inokuti plot of relative phosphorescence emission Intensities ( ) and lifetimes (0), under the same conditions as In Fig. 2. Best fit Is obtained for y = 20 (see text) [from figure In J. Polym. Scl., Polym. Chem. Ed. (In press).]... 

Figure 14-14. Stern-Volmer plots of the relative emission intensity ( ) and lifetime ( ) against concentration for the electron transfer quenching of the photoexcited Ru(bpy)3 by MV in a 2 wt.% carrageenan/water solid (1x1x3cm size).

The photophysical properties of 20 are particularly interesting, in that the triplet energy of the porphyrin chromophore is comparable to the emissive state of and can be thermally repopulated at room temperature [52], This leads to a dependence of the emission intensity and lifetimes upon the degree of sample aeration. In most cases, energy transfer from the chromophore to the lanthanide occurs via the MLCT state of the chromophore, and normal behaviour is observed. However, the broad nature of the emission from luminescent MLCT states means that the tail of this emission is often superimposed upon the relatively weak luminescence from the lanthanide ion. In such situations the two signals can be resolved readily provided that their lifetimes differ significantly. This is not the case with many complexes related to [Rulbpyls] ", which have luminescence lifetimes comparable with those of neodymium and ytterbium complexes. 

In the most common scheme (Fig 12a), photoinduced electron transfer in a two-component supermolecule ("dyad") involves (i) excitation of a photosensitizer molecular component (P), (ii) electron transfer from the excited photosensitizer to an electron acceptor component (A)(a process often called "charge separation"), (iii) back electron transfer from the reduced acceptor to the oxidized photosensitizer (often designated as "charge recombination", not shown in the figure) [17,82]. The practical consequences of this sequence of processes may vary from system to system. Quenching of the excited photosensitizer is always observed (usually from emission intensity and lifetime measurements). The formation and disappearance of the charge separated state can in principle be monitored by fast spectroscopic techniques. The possibility of observation depends on both instrumental factors (sensitivity and time resolution) and on kinetic... 

Luminescent lanthanide complexes have been prepared whose emission intensity and lifetime is a sensitive and selective function of pH, p02 and pCl". By appropriate choice of excitation wavelength, the output signal is predictably determined by the local environment and the composition of the mixture of lanthanide complexes chosen. Parallel processing in solution should be possible using multivariate methods of analysis. The benefits of addressing multicomponent systems that are amenable to selective recognition and quantification are discussed. 

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