Fluorescence Yields and Lifetimes

Equations (B5.2.8) and (B5.2.9) also show that q and q do not commute with each other  

In Sect. 5.4, we derived expressions for the total rate constant for spontaneous fluorescence k/). The reciprocal of this rate constant is called the radiative lifetime 

Because rate constants for parallel processes add, the total rate constant for decay is 

The actual lifetime of the excited state (the fluorescence lifetime,r) thus is shorter than Xr  

In general, a sample will contain molecules that interact with their surroundings in a variety ways, for example because some of the fluorescing molecules are buried in the interior of a protein while others are exposed to the solvent. The fluorescence then decays with multiphasic kinetics that can be fit by a sum of exponential terms (Eq. 1.4). Fluorescence lifetimes can be measured by time-correlated photon counting, by fluorescence upconversion, or by modulating the amplitude of the excitation beam and measuring the modulation and phase shift of the fluorescence (Chap. 1). Pump-probe measurements of stimulated emission become the method of choice for sub-picosecond lifetimes (Chap. 11). For further information on these techniques and ways of analyzing the data see [30-34]. 

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By flourescence techniques, it was observed that the fluorescence yield and lifetime of 1,8-anilinonaphthalenesulfonate decrease with an increase in the aqueous core of AOT-reversed micelles, while the position of the emission maximum shifts to longer wavelengths [64], These changes in the electronic properties were attributed to the peculiar effective polarity and viscosity of the micellar core and to their evolution with R. 

Theoretical calculations have been made for photosynthetic pigments . An extensive review of models of energy and electron transfer events of synthetic molecules for photosynthesis has been prepared by Wasielewski . Other studies have made on tetraphenylporphyrin-polypeptide pigments , photosensitization of triplet carotenoids , fluorescence yields and lifetimes for bacteriochlorophyll c , triplet yields and ESR of chlorophyll 3 8 and quenching processes of pheophytin 539 ... 

Vesicles are ordered fluids or liquid crystals, a fact which reflects well in those photoreactivities that are particular to vesicle solutions. Fatty acid derivatives 24 and 25, for example, show low quantum yields of fluorescence ((j)f) and high quantum yields of cis-trans isomerization (< )c) as well as short fluorescence life times (tf) in both methylcyclohexane and micellar SDS solutions (Table 1). In DPPC vesicles, on the other hand, cis-trans isomerizations are cumbersome and much slower, and the fluorescence yield and lifetime rise considerably (Table 1). For those stilbene derivatives which are embedded in the middle of a fatty acid backbone, isomerization is virtually eliminated in the low-temperature or gel phase of the bilayer. The vesicle thus plays the role of stabilizing trans configurations which fit into the frozen oligomethylene chain matrix. 

The experimental difficulties Involved in measurement of fluorescence yields and lifetimes for benzene, and other systems also, as a function of concentration make it certain that the various thermodynamic quantities have not yet been firmly established. 

Radiative and Nonradiative Decay Processes - Due to the potential application of these compounds as photosensitizers for photodynamic therapy" the photophysical properties of porphyrins and phthalocyanines, and their corresponding metal complexes, have been investigated extensively over the past decade. The photophysical properties of water-soluble metalloporphyrins, and especially the tetraphenylsulfonates," have been re-examined but nothing new has been found. The disulfonated metallophthalocyanines (MPcS2, where M = Al ", Ga" , or Zn") form complexes with fluoride ions for which the fluorescence yields and lifetimes are decreased with respect to the parent dyes while there are... 

NaLS) by copper(II) yields assemblies in which Cu2+ ions constitute the counter ion atmosphere of the micelle (Fig. 4.8). These may be photoreduced to the monovalent state by suitable donor molecules incorporated in the micellar interior. An illustrative example is that where D = N,N -dimethyl 5,11-dihydroindolo 3,3-6 carbazole(DI). When dissolved in NaLS micelles, DI displays an intense fluorescence and the fluorescence lifetime measured by laser techniques is 144 ns. Introduction of Cu2+ as counterion atmosphere induces a 300 fold decrease in the fluorescence yield and lifetime of DI. The detailed laser analysis of this system showed that in Cu(LS) micelles there is an extremely rapid electron transfer from the excited singlet to the Cu2+ ions. This process occurs in less than a nanosecond and hence can compete efficiently with fluorescence and intersystem crossing165. This astonishing result must be attributed to a pronounced micellar enhancement of the rate of the transfer reaction. It is, of course, a consequence of the fact that within such a functional surfactant unit regions with extremely high local concentrations of Cu2+ prevail. (Theoretical estimates predict the counterion concentration in the micellar Stem layer to be between 3 and 6 M). 

Stein and Wuerzberg1550 have reported results on the fluorescence yields and lifetimes for Tb8+, Dy3+, Sm3+, Pr3+, and Tm3+ in H20 and D20 solutions. From consideration of these and earlier results, it was shown that solvent isotope effects on the rates of radiationless decay are expected to be high when (E/h ) —... 

In aqueous neutral solutions, 47 has two emission bands corresponding to the fluorescence of the neutral = 353 nm) and anionic = 420nm) forms [168]. Fluorescence was quenched by I in water and in the presence of y-CD, the quenching was the same in the presence of a-and P-CD, the quenching was depressed [167]. Addition of a- and p-CD increased the fluorescence yield and lifetime of the neutral form of 47 and depressed the yield of the anionic form [142, 166, 167]. No variation was induced by y-CD. A depression of the proton dissociation of the included molecule explains these results. In fact, the fcj and fc, rate constants found for the protolytic equilibrium... 

The yields, lifetimes, and rate constants for relaxation from the thermal vibrational levels in 10-20 torr of benzene vapor near 300°K are given in Table III, Both fluorescence yields and lifetimes are available from several... 

Several of the fluorescence yield and lifetime studies - observe a modest pressure sensitivity. However, since the dependence is small relative to the uncertainties in the value themselves, it seems for most purposes quite appropriate to describe yields and lifetimes with single high-pressure values. 

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