Chain quantum yield

The intrinsic UV fluorescence of proteins is dominated by the tryptophan indole rings. The absorption maximum is 280-290 nm with the fluorescence maximum ranging from 315-355 nm, depending on the local environment of the indole side-chains. Quantum yields range from 0.04 to 0.50 0.10 is a common value. As the local environment polarity or dielectric constant increases, the fluorescence maximum shifts up to 355 nm, such as for an indole ring in water or buffer. Trp moieties in highly hydro-phobic environments fluoresce at 315-320 nm. Thus the fluorescence emission maximum (and the quantum yield) provide indirect information as to the local environment of the Trp fluors. 

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. 

Autooxidation. Liquid-phase oxidation of hydrocarbons, alcohols, and aldehydes by oxygen produces chemiluminescence in quantum yields of 10 to 10 ° ein/mol (128—130). Although the efficiency is low, the chemiluminescent reaction is important because it provides an easy tool for study of the kinetics and properties of autooxidation reactions including industrially important processes (128,131). The light is derived from combination of peroxyl radicals (132), which are primarily responsible for the propagation and termination of the autooxidation chain reaction. The chemiluminescent termination step for secondary peroxy radicals is as follows ... 

In aqueous solution, riboflavin has absorption at ca 220—225, 226, 371, 444 and 475 nm. Neutral aqueous solutions of riboflavin have a greenish yellow color and an intense yellowish green fluorescence with a maximum at ca 530 nm and a quantum yield of = 0.25 at pH 2.6 (10). Fluorescence disappears upon the addition of acid or alkah. The fluorescence is used in quantitative deterrninations. The optical activity of riboflavin in neutral and acid solutions is [a]=+56.5-59.5° (0.5%, dil HCl). In an alkaline solution, it depends upon the concentration, eg, [a] J =—112-122° (50 mg in 2 mL 0.1 Ai alcohohc NaOH diluted to 10 mL with water). Borate-containing solutions are strongly dextrorotatory, because borate complexes with the ribityl side chain of riboflavin = +340° (pH 12). 

For copolymers of structure I, for both types of side-chains, there is a striking similarity with the optical properties of the corresponding models the absorption and photoluminescence maxima of the polymers arc only 0.08-0.09 eV red-shifted relative to those of the models, as shown in Figure 16-9 (left) for the octyloxy-substituted compounds. The small shift can be readily explained by the fact that in the copolymers the chromophorcs are actually substituted by silylene units, which have a weakly electron-donating character. The shifts between absorption and luminescence maxima are exactly the same for polymers and models and the width of the emission bands is almost identical. The quantum yields are only slightly reduced in the polymers. These results confirm that the active chro-mophores are the PPV-type blocks and that the silylene unit is an efficient re-conjugation interrupter. 

Poly[2,5-dialkoxy-l,4-phenylene) vinylenejs with long solubilizing alkoxy chains dissolve in conventional organic solvents such as chloroform, toluene, or tetrahydrofuran [21, 28, 32-36]. Their emission and absorption spectra are red-shifted relative to PPV itself, and the polymers fluorescence and electroluminescence quantum yields are greater than parent PPV. This benefit may be a consequence of the long alkyl chains isolating the polymer chains from each other. 

The optical properties can be tuned by variations of the chromophores (e.g. type of side-chains or length of chromophorc). The alkyl- and alkoxy-substituted polymers emit in the bluc-gnecn range of the visible spectrum with high photolu-inincsccncc quantum yields (0.4-0.8 in solution), while yellow or red emission is obtained by a further modification of the chemical structure of the chromophores. For example, cyano substitution on the vinylene moiety yields an orange emitter. 

On photolysis in benzene solution, the bisamides 22 (n > 14) form internal [2 + 2] photodimers 23, the maximum quantum yield being noted with n = 26. The shorter alkyl chain analogs 22 (n = 2-12), under the same conditions, form intcrmolccular dimers.236... 

The reported quantum yields of the long-chain aldehydes in the luminescence reaction catalyzed by P. fischeri luciferase are 0.1 for dodecanal with the standard I (Lee, 1972) 0.13 for decanal with the standard I (McCapra and Hysert, 1973) and 0.15-0.16 for decanal, dodecanal and tetradecanal with the standard III (Shimomura et al., 1972). Thus, the quantum yield of long-chain aldehydes in the bacterial bioluminescence reaction appears to be in the range of 0.10-0.16. 

Product quantum yields are much easier to measure. The number of quanta absorbed can be determined by an instrument called an actinometer, which is actually a standard photochemical system whose quantum yield is known. An example of the information that can be learned from quantum yields is the following. If the quantum yield of a product is finite and invariant with changes in experimental conditions, it is likely that the product is formed in a primary rate-determining process. Another example In some reactions, the product quantum yields are found to be well over 1 (perhaps as high as 1000). Such a finding indicates a chain reaction (see p. 895 for a discussion of chain reactions). 

They are initiated or accelerated by typical free-radical sources, such as the peroxides referred to, or by light. In the latter case the concept of quantum yield applies (p. 1316). Quantum yields can be quite high, for example, 1000, if each quantum generates a long chain, or low, in the case of nonchain processes. 

Chain reactions ivere discovered around 1913, ivhen Bodenstein and Dux found that the reaction betv een H2 and CI2 could be initiated by irradiating the reaction mixture ivith photons. They ivere surprised to find that the number of HCl molecules per absorbed photon, called the quantum yield, is around 10 Nernst explained this phenomenon in 1918 the photon facilitated the dissociation of CI2 into Cl radicals (the initiation step), which then started the following chain process ... 

The oxidation of oxalic acid by mercuric chloride to give CO2 and mercurous chloride is a classic example of an induced reaction. This reaction is extremely slow unless small quantities of chromic acid and manganous ions are added, whereon facile reduction takes place Addition of permanganate or persulphate and some reducing agents is also effective and the oxidation proceeds readily under photo- or X-irradiation (Eder s reaction). The large quantum yield points to a chain mechanism , which could also function with an inducing oxidant, viz. 

The processes by which unsaturated monomers are converted to polymers of high molecular weight exhibit the characteristics of typical chain reactions. They are readily susceptible to catalysis, photoactivation, and inhibition. The quantum yield in a photoactivated polymerization in the liquid phase may be of the order of 10 or more, expressed as the number of monomer molecules polymerized per quantum absorbed. The efficiency of certain inhibitors is of a similar magnitude, thousands of monomer molecules being prevented from polymerizing by a single molecule of the inhibitor. ... 

Side-chain photochlorination of toluene isocyanates yields important industrial intermediates for polyurethane synthesis, one of the most important classes of polymers [6]. The motivation for micro-channel processing stems mainly from enhancing the performance of the photo process. Illuminated thin liquid layers should have much higher photon efficiency (quantum yield) than given for conventional processing. In turn, this may lead to the use of low-intensity light sources and considerably decrease the energy consumption for a photolytic process [6] (see also [21]). 

The limitation of this method is the relatively low quantum yield of radical formation by chain scission for most polymers. It will take high doses at short wavelengths (< 300 nm) to produce enough initiating radicals for a complete surface coverage of grafted... 

These quantum yield values appear to be much higher than unity and therefore demonstrate that carbonization occurs by a chain reaction process. The mechanism of the laser-induced dehydrochlorination of photodegraded C-PVC can be schematically represented by the follo-... 

The dimerization of iV-vinylcarbazole and its derivatives proceeds with quantum yields well in excess of unity, indicating a chain mechanism such as shown below(123) ... 

The extent of the photodegradation reaction is measured by the photodegradation quantum yield, pd, which is defined as the fraction of molecules degraded in relation to how many photons were absorbed, and quantifies the light sensitivity of the molecule (Turro 1978). Usually, pd < 1 but values higher than unity can indicate more complex processes, such as radical chain reactions. 

In addition, the results indicated that the efficiency of cis —> trans increased as the initial cis double bond configuration is shifted from the center of the polyenic chain, consistent with the 7j, triplet excited state potential curve that has a very shallow minimum at the 15-cis position compared to the deep minima at the all-trans position. The results strongly suggest that isomerization takes place via the 7j state of the carotenoid even in the case of direct photoexcitation, with their photosensitized process because of the very low intersystem crossing quantum yield, isc ([Pg.246]

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