Measurement of quantum yield

Fig. 3.1.5 Effects of salt concentration on the activity of Cypridina luciferase (solid lines) and quantum yield (dotted lines). In the activity measurement, Cypridina luciferin (1 pg/ml) was luminesced with a trace amount of luciferase in 2.5 mM HEPES buffer, pH 7.5, containing a salt to be tested, at 20°C. In the measurement of quantum yield, luciferin (1 pg/ml) was luminesced with luciferase (20 pg/ml) in 20 mM sodium phosphate buffer (for the NaCl data) or MES buffer (for the CaCl2 data), pH 6.7.

Measurement of Quantum Yield, Quantum Requirement, and Energetic Efficiency of the 02-Evolving System of Photosynthesis by a Simple Dye Reaction 127... 

The measurement of quantum yield is a more complicated process. Before these measurements can be made, the instrument must be calibrated. A thermopile or chemical actinometer may be used to measure the absolute intensity of incident light on the sample. Alternatively, quantum yields may be measured relative to some accepted standard. Two commonly used fluorescence standards are quinine sulfate in 0.5 M H2S04 (jQ = 0.70) and fluorescein in 0.1 M NaOH (f9 = 0.93). The quantum yield of the unknown, Q, is then calculated by Equation 5.7. 

From the studies of measurement of quantum yields it is surmised that the photo active species in the above reaction involves Ce4+ dimers and the reduced Ce3+ ion... 

The importance of acid formation requires a determination of the efficiency of this process. We have adapted an indicator dye method (see Scheme 4) to the measurements of quantum yields for acid generation. The results obtained using several sensitizers and I(i) are summarized in Fig. 1. 

Quantum yields are fundamental quantities that define the photonic economics of processes induced by light absorption. They are required to determine rate constants of photophysical and photochemical processes (Section 3.9.7). Many different techniques are used to measure quantum yields depending on the process studied. In the following, we describe some procedures commonly used in the chemical laboratory. The measurement of quantum yields is an art that has a number of pitfalls. The experimenter has few options to double-check his or her own results other than reproducibility, which will not reveal any repeated systematic errors. Therefore, it is prudent to reproduce the quantum yield of a related, well-known process in the laboratory before determining an unknown one. 

Quantum yield values measured in solution may not necessarily apply to polymer films, the usual environment for practical application of this photochemistry. McKean et al. have adapted the indicator dye method to the measurement of quantum yields for Bronsted acid photogeneration in poly-(4-tert-butoxycarbonyloxystyrene) [20], As with the solution photochemistry of diphenyliodonium salts [71], an inverse dependence of quantum yield on exposure intensity was observed absolute quantum yields from 0.26 to 0.40 were measured at 254 nm, which extrapolate to approximately 0.45 at zero intensity, comparable to the value estimated by Dektar and Hacker [82b] in solution. McKean et al. [20b] note that similar quantum yields in solution and polymer films below Tg have also been reported for photo-Fries rearrangements [84] and photodissociation of diacyl peroxides [85]. 

Nl. Nakamura, T., Standardization of an aqueous light source containing luminol Application to measurement of quantum yields of bioluminescent reactions./. Biochem. (Tokyo) 72,173-177 (1972). 

Cleavage from the singlet state is also the dominant pathway in the photo-Fries rearrangement of 1-naphthylacetate [106], as was inferred from laser flash photolysis and measurements of quantum yields. In that study, however, the authors concluded that the CIDNP signals mainly derive from the triplet precursor. This was explained by the involvement of a higher excited triplet state that is populated by a two-photon process and cleaves... 

A group of papers dealing with the intramolecular photodimerization reaction of a,a>-bis(9-anthryl)alkanes (185 n = 2—10) fails to produce agreement about the detailed mechanism of the reaction. Measurements of quantum yields for fluorescence, photoreaction, and intramolecular deactivation as a function of temperature are said to provide no support for a biradical intermediate, but rather to support a concerted mechanism. In reply, the proposers of the biradical mechanism reinterpret these data and find them consistent with their mechanism. A third research group reports results in fluid solution at room tempera-ture " their concern is more with the question of excimer involvement in the mechanism, and they report that in many of the systems unidentified photoproducts are formed via excimers that do not lead to the normal 9,9 -linked photodimer. The internal photodimer from (185 n = 3) has been studied in a matrix at 10 and photodissociation is shown to lead to two different modifications of (185 n = 3) with different reactivities. A geometrical constraint on the intramolecular photoreactions of 9,9 -linked bisanthracenes is demonstrated by the failure of the di-substituted ethylenes (186) to give internal dimers. 

The oxygen and 9-fluorenone-l-carboxylic acid quenching experiments on the photohydration of p-NDHD show that the triplet excited states are involved to give the primary photoproducts. The photohydration of p-NDHD exhibits a maximum efficiency at Hq = -1.0 for both primary photoproducts, as shown by the measurements of quantum yields as a function of acidity. The suggested mechanism involves a nucleophilic attack of water, synchronous with protona-... 

Maricq et al. [180] have studied the self-reaction of FCO radicals. Although the self-reaction will be of minor importance for upper atmospheric chemistry, this reaction is critical in laboratory studies involving FCO radicals. For example, in the measurement of quantum yields from CFjO, CFCIO, and HFCO photodissociation, the self reaction could lead to a gross underestimation of the quantum yields. The rate coefficient of 1.9 0.2 X 10 cm s suggests that the self-reaction is quite rapid. In the self-reaction, the FCO radicals recombine to form oxylfluoride, which is produced with excess energy above the barrier for molecular dissociation to carbonyl fluoride and carbon monoxide [192]. 

Measurement of Quantum Yields in Polychromatic Light Dinitroaniline Herbicides... 

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