Quantum yield determination

Intersystem crossing quantum yields determined by the methods discussed in this section and other methods<24) are presented in Table 5.6. 

Photochemical reactions of the pyrimidine polymers in solution were studied to determine the quantum yields of the intramolecular photodimerization of the pyrimidine units along the polymer chains. Photoreactions of the polymers were carried out in very dilute solutions to avoid an intermolecular(interchain) photodimerization. Quantum yields determined at 280 nm for the polymers (1-6 in Figure 1) are listed in Table I. The quantum yield of the 5-bromouracil polymer [poly(MAOU-5Br)] could not be determined because of side reactions of the base during the irradiation. 

Table 4.4 The effect of the internal heavy atom effect on the fluorescence efficiency of naphthalene and its derivatives. Fluorescence quantum yields determined in solid solution at 77K...

Fluorescence quantum yields determined in solid solu-... 

The cytotoxic and photocytotoxic effects of two water-soluble fullerene derivatives, a dendritic CL mono-adduct and the malonic acid CL tris-adduct were tested on Jurkat cells when irradiated with UVA or UVB light (Rancan et al., 2002). The cell death was mainly caused by membrane damage and it was UV dose-dependent. Tris-malonic acid fullerene was found to be more phototoxic than the dendritic derivative. This result is in contrast to the singlet oxygen quantum yields determined for the two compounds. 

Quantum yields determinations lead to analogous conclusions, although differences in the ortho-to-para ratio are found probably because of experimental reasons. However, the ortho selectivity seems to be well established. Phenyl acetate irradiated in water gives the following quantum yields of product formation 0.16 (ortho), 0.067 (para), and 0.048 (phenol) in the presence of an excess of (3-cyclodextrin, they change to 0.23 (ortho), 0.053 (para), and 0.27 (phenol) [260]. As can be seen, the ortho product is favored in the hydrophobic microenvironment of the cyclodextrin. Phenol quantum yield is enhanced with respect to the irradiation without cyclodextrin, which has been interpreted in terms of H abstraction from the inner walls of the host oligosaccharide. 

Energy transfer to photoreactive acceptors has also been widely utilized for excitation quantum yield determination (chemical titration), mainly in the decomposition of dioxetanes ° . The quantum yields are calculated from the photoproduct yield obtained at infinite energy acceptor concentrations ( = 1.0) by extrapolation of the double-reciprocal relationship between the photochemically active energy acceptor concentration and the photoproduct yield ( Lp ). H the quantum yield of the photochemical reaction (excitation quantum yield (< > ) can be calculated (equation 8) . ... 

TABLE 1. Quantum yield determination for several representative CL systems... 

The unimolecular decomposition of 1,2-dioxetanes and 1,2-dioxetanones (a-peroxylac-tones) is the simplest and most exhaustively studied example of a thermal reaction that leads to the formation, in this case in a single elementary step, of the electronically excited state of one of the product molecules. The mechanism of this transformation was studied intensively in the 1970s and early 1980s and several hundreds of 1,2-dioxetane derivatives and some 1,2-dioxetanones were synthesized and their activation parameters and CL quantum yields determined. Thermal decomposition of these cyclic peroxides leads mainly to the formation of triplet-excited carbonyl products in up to 30% yields. However, formation of singlet excited products occurs in significantly lower yields (below... 

Self-Quenching and Excimer Formation. It is often desirable to use a high concentration of sensitizer to ensure that it will absorb all of the light, particularly if the substrate has any significant absorption at the wavelength of irradiation. Recent studies have shown that quantum yields determined under these conditions can be anomalously low if the sensitizer acts as a bimolecular quencher of its own triplets. 

Some quantum yields of greater than two but less than three have been reported by Schumacher (71) for the unpublished data of Kistia-kowsky. Benson (13), however, has pointed out that the error in the quantum yield determination makes the value about 30% uncertain, and, therefore, there is no acceptable evidence that quantum yields in this wavelength region may exceed two. A chain mechanism is then not necessary to explain the results. 

Both the absolute quantum yield (determined with respect to zinc tctraphcny 1 porphvrin in ethanol) and the product of the molar absorption coefficient at the excitation wavelength with the quantum yield, e Q, which represents the overall luminescence efficiency follow the... 

In fact, alkylated succinamides were isolated in some cases, though in very poor yields, and result from radical combination, which is a chain termination step. The experimental observations, i.e. the formation of (a) 1 1 adducts, (b) telomeric products, (c) alkylated succinamides, and (d) oxamide (when an olefin is absent), are consistent with a free radical mechanism. The telomeric products obtained support the assumption that we deal here with a chain reaction, because they are characteristic products of this type of reaction. Another proof for the chain reaction mechanism is the fact that when benzophenone is used as a photoinitiator (vide infra), the amount of benzpinacol formed is smaller than the amount of the 1 1 addition product of formamide and olefin (16). Quantum yield determinations will supply extra evidence for the validity of a chain reaction mechanism for this photoaddition reaction. 

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