Quantum yield -

The initial quantum yields for the oligomerization and the polymerization of DSP, P2VB, and p-PDA Me are summarized in Table 2. 

Since the scattered light is included in the absorbed light, the quantum yields may be expected to be higher than those listed in Table 2. The quantum yields of oligomerization and polymerization of DSP and p-PDA Me are between 0.7 and 1.6. 

Holm and Zienty have measured the quantum yield for the overall polymerization process of a, a -bis(4-acetoxy-3-methoxybenzylidene)-p-benzenediacetonitrile (AMBBA) crystals in slurries where scattered light was calibrated and the conversion determined by UV and IR spectroscopy or by the weight of isolated solid residues7-1. The quantum yield of the polymerization of AMBBA is 0.7 which was determined on the basis of the disappearance of two double bonds (1.4 if assigned on the basis of the number of double bonds disappeared). 

These quantum yields indicate that these photoreactions belong to single-photon reactions in which the theoretical maximum value is equal to 2, and that some of these reactions proceed very efficiently. Such a high quantum yield may reflect a high probability of effective collisions in the four-center photopolymerization in the crystalline state. 

Higuchi et al. have shown that the reactivity of DSP and P2 VB crystals is explained by the stabilization energy of the transient complex which, in turn, depends on both the electronic structure of the monomer molecule and the intermolecular arrangement in the monomer crystal35.  

Johnson et al. (1962) measured the quantum yield of Cypridina luciferin in the luciferase-catalyzed reaction for the first time, using a photomultiplier calibrated with two kinds of standard lamps. The measurement gave a value of 0.28 0.04 at 4°C in 50 mM sodium phosphate buffer, pH 6.5, containing 0.3 M NaCl. The quantum yield 

Luciferin (1 pg) plus C. hilgendorfii luciferase (80 pg) in 20 mM sodium phosphate buffer containing 0.3 M NaCl, pH 6.5, at 10°C 0.30 

Luciferin (unspecified amount) in diglyme containing 5 pi of 1 M sodium acetate buffer, pH 5.6, at room temperature 0.03  

Efforts by Harvey (1931) to find luciferin and luciferase in euphausiids were unsuccessful, but J. D. Doyle (1966, personal communication) succeeded in obtaining the extracts of luciferin and luciferase from the photophores of M. norvegica and Thysanoessa raschii. 

Isolation of F and P. The first attempt to isolate and purify the substances responsible for the light emission of M. norvegica was made by Shimomura and Johnson (1967). They isolated two substances, a protein (P) and a fluorescent compound (F), which produce a blue light 

A key concept in photochemistry is that of the quantum yield for process n, I , defined as in Eq. 16.6. For example, the fluorescence quantum yield, I f, is equal to the number of photons emitted divided by the number of photons absorbed. If all possible processes are considered, then the sum of all quantum yields should be 1.0 in typical systems. A formal exception to this rule is the case of a photochemically initiated polymerization, photopolymerization. In this case, if we define the quantum yield as the number of couplings between monomers divided by the number of photons absorbed, we expect to see d 1. 

Number of molecules that undergo process n Total number of photons of light absorbed 

Much of photochemistry is an exercise in relative rates, and that is certainly true when considering quantum yields. As we show in Eq. 16.7, Eq. 16.6 could be rewritten as a ratio of rate constants, where k is the rate constant for the process under consideration, and k/ are the rate constants for all possible processes from the excited state. In either case, the key is that we must consider all possible processes in the denominator in order to get the proper quantum yield for the process in the numerator. As the next Going Deeper highlight describes, internal conversion is a particularly important process competing with fluorescence. 

The quantum yield is an indicator of how efficient a particular process is. However, some care must be taken in comparing quantum yields for different systems, because the quantum yield is always measured relative to other processes in the molecule. For example, bf (f for fluorescence) is roughly 0.2 for both benzene and pyrene-3-carboxaldehyde. This might lead to the conclusion that fluorescence is equally efficient for the two compounds. However, the fluorescence rates, kf, are 2 X 10 s and 1 X 10 for benzene and pyrene-3-carboxaldehyde, respectively. Fluorescence is in a sense more efficient for the pyrene derivative. The reason this is not reflected in the quantum yields is that we must also consider competing processes. The ISC rate for pyrene-3-carboxaldehyde is also much faster than that for benzene due to the ability to access a ( ,tt ) state that is not available for benzene. This competing ISC process limits the amount of fluorescence, and by coincidence the two compounds end up with the same fluorescence quantum yield. Thus, while the quantum yield tells you about the efficiency of a process for a given molecule, it alone cannot tell you why the process is or is not efficient. 

For qualitative photochemical investigations concerned only with the nature of the products the measurement of the quantum 

The accuracy of the quantum yield measurements is limited by several factors. The error in absolute values may be up to five per cent but the relative accuracy and the reproducibility are probably good to one per cent. Usually the chemical analysis involves the greatest error. 

Further consideration of photochemistry as a means of understanding chemical kinetics can be obtained best from the study of a few specific photochemical reactions. 

Corrections for reflections at the interfaces were largely eliminated by taking the zero readings of the thermopile-galvanometer with an empty cell included in the path of the light. This cell s dimensions were the same as those of the reaction chamber. Some of the light which passed through the reaction cell was reflected back from the rear window and from the thermostat and thermo- 

Quantum Yields with N206, N02, and N204 Wave Length 3,660 A, Temp. 0° 

Geometrically, with the destruction of the x bonding orbital, the molecule tends toward a pyramidal rather than a planar configuration. Spectroscopic determinations show also that the C = O bond length changes in the excited state toward that of a C-O single bond. 

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After the primary step in a photochemical reaction, the secondary processes may be quite complicated, e.g. when atoms and free radicals are fcrnied. Consequently the quantum yield, i.e. the number of molecules which are caused to react for a single quantum of light absorbed, is only exceptionally equal to exactly unity. E.g. the quantum yield of the decomposition of methyl iodide by u.v. light is only about 10" because some of the free radicals formed re-combine. The quantum yield of the reaction of H2 -f- CI2 is 10 to 10 (and the mixture may explode) because this is a chain reaction. 

The attachment of pyrene or another fluorescent marker to a phospholipid or its addition to an insoluble monolayer facilitates their study via fluorescence spectroscopy [163]. Pyrene is often chosen due to its high quantum yield and spectroscopic sensitivity to the polarity of the local environment. In addition, one of several amphiphilic quenching molecules allows measurement of the pyrene lateral diffusion in the mono-layer via the change in the fluorescence decay due to the bimolecular quenching reaction [164,165]. 

In order to probe the importance of van der Waals interactions between reactants and solvent, experiments in the gas-liqnid transition range appear to be mandatory. Time-resolved studies of the density dependence of the cage and clnster dynamics in halogen photodissociation are needed to extend earlier quantum yield studies which clearly demonstrated the importance of van der Waals clnstering at moderate gas densities [37, 111]... 

Figure A3.6.12. Photolytic cage effect of iodme in snpercritical ethane. Points represent measured photodissociation quantum yields [37] and the solid curve is the result of a numerical simnlation [111].

The simple difhision model of the cage effect again can be improved by taking effects of the local solvent structure, i.e. hydrodynamic repulsion, into account in the same way as discussed above for bimolecular reactions. The consequence is that the potential of mean force tends to favour escape at larger distances > 1,5R) more than it enliances caging at small distances, leading to larger overall photodissociation quantum yields [H6, 117]. 

Schwarzer D, Schroeder J and Schroder Ch 2000 Quantum yields for the photodissociation of iodine in compressed liquids and supercritical fluids Z. Phys. Chem. 214... 

Dardi P S and Dahler J S 1990 Microscopic models for iodine photodissociation quantum yields in dense fluids J. Chem. Phys. 93 242-56... 

Similar to the fullerene ground state the singlet and triplet excited state properties of the carbon network are best discussed with respect to the tliree-dimensional symmetry. SurjDrisingly, the singlet excited state gives rise to a low emission fluorescence quantum yield of 1.0 x 10 [143]. Despite the highly constrained carbon network,... 

Sun Y-P, Wang P and Hamilton N B 1993 Fluorescence spectra and quantum yields of Buckminsterfullerene (Cgg) in room-temperature solutions. No excitation wavelength dependence J. Am. Chem. Soc. 115 6378-81... 

The vast majority of single-molecule optical experiments employ one-photon excited spontaneous fluorescence as the spectroscopic observable because of its relative simplicity and inlierently high sensitivity. Many molecules fluoresce with quantum yields near unity, and spontaneous fluorescence lifetimes for chromophores with large oscillator strengths are a few nanoseconds, implying that with a sufficiently intense excitation source a single... 

Table 7.16 Fluorescence Spectroscopy of Some Organic Compounds Table 7.17 Fluorescence Quantum Yield Values...

The intensity of fluorescence. If, is proportional to the amount of the radiation from the excitation source that is absorbed and the quantum yield for fluorescence... 

Molecular fluorescence and, to a lesser extent, phosphorescence have been used for the direct or indirect quantitative analysis of analytes in a variety of matrices. A direct quantitative analysis is feasible when the analyte s quantum yield for fluorescence or phosphorescence is favorable. When the analyte is not fluorescent or phosphorescent or when the quantum yield for fluorescence or phosphorescence is unfavorable, an indirect analysis may be feasible. One approach to an indirect analysis is to react the analyte with a reagent, forming a product with fluorescent properties. Another approach is to measure a decrease in fluorescence when the analyte is added to a solution containing a fluorescent molecule. A decrease in fluorescence is observed when the reaction between the analyte and the fluorescent species enhances radiationless deactivation, or produces a nonfluorescent product. The application of fluorescence and phosphorescence to inorganic and organic analytes is considered in this section. 

Accuracy The accuracy of a fluorescence method is generally 1-5% when spectral and chemical interferences are insignificant. Accuracy is limited by the same types of problems affecting other spectroscopic methods. In addition, accuracy is affected by interferences influencing the fluorescent quantum yield. The accuracy of phosphorescence is somewhat greater than that for fluorescence. 

Sensitivity From equations 10.32 and 10.33 we can see that the sensitivity of a fluorescent or phosphorescent method is influenced by a number of parameters. The importance of quantum yield and the effect of temperature and solution composition on f and p already have been considered. Besides quantum yield, the sensitivity of an analysis can be improved by using an excitation source that has a greater... 

Table 7.11 Fluorescence quantum yield

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