Quantum yield measurement

The measurement of the d p of a sample is generally carried out by comparing the sample fluorescence to that of a standard molecule whose I f has been determined using an absolute method. The basis of this secondaiy method of comparison of of the sample to that of a standard is the following 

The determination of d p of a molecule by a primary method is a very difficult task and has been the subject of several reviews (4, 5). The quantum yields of several molecules under rigorous experimental conditions have been determined (6, 7). A selection that is generally useful is listed in Table 3. When such a standard molecule is chosen in order to measure of the molecule of interest, 

When the 4 p of the sample is determined, fl( ) of the sample and the standard should cover a similar range. Obviously, both the sample and standard 

When one is measuring 4 p of a fluorophore in an aqueous solution and the standard is in a non-aqueous solvent then a refiractive index correction is recommended. This correction is applied according to  

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The experiment is performed with a spectrofluorometer similar to the ones used for linear fluorescence and quantum yield measurements (Sect. 2.1). The excitation, instead of a regular lamp, is done using femtosecond pulses, and the detector (usually a photomultiplier tube or an avalanche photodiode) must either have a very low dark current (usually true for UV-VIS detectors but not for the NIR), or to be gated at the laser repetition rate. Figure 11 shows a simplified schematic for the 2PF technique. 

Karstens, T. and Kobs, K. (1980). Rhodamine-B and rhodamine-101 as reference substances for fluorescence quantum yield measurements. J. Phys. Chem. 84, 1871-1872. 

Furthermore, kinetic analysis of the decay rate of anthracene cation radical, together with quantum yield measurements, establishes that the ion-radical pair in equation (76) is the critical reactive intermediate in osmylation reaction. Subsequent rapid ion-pair collapse then leads to the osmium adduct with a rate constant k 109 s 1 in competition with back electron-transfer, i.e.,... 

Several luminescent ( 650 nm) dioxorhenium(V) systems(25) have been investigated as potential 0-atom transfer agents. The emission quantum yields measured with 436 nm excitation are about 0.03 for trans-ReO (pyridine)u and its isotopically-substituted derivatives in pyridine solution. The excited state lifetimes of these ions vary from 4 to 17 ys. 

The assumption of a simple primary dissociation (2) has been cast in doubt by the results of an application of the Cundall technique to the photolysis9, which suggest that excited-molecule reactions are very important at 2537 A. There is clearly a need here for some careful quantum yield measurements to establish the nature of the primary step. 

Table 4.2 The effect of molecular rigidity on fluorescence quantum yield measured in solution at room temperature...

This photoreaction has been investigated by laser flash photolysis58 and quantum yield measurements that identify the triplet state (r = 6 nanoseconds) as the reactive species, and show intermediate 82 is sensitive to hydroxylic molecules, but the logical precursor biradical intermediate 81 could not be detected owing to a short lifetime (equation 48). 

From photoreduction (> 280 nm) in diethylamine, low yields of 1-naphthyl-amine and the corresponding azo- and azoxy compounds have been obtained Photolysis (366 nm) in acidified 50% aqueous 2-propanol at varied HCl-concentrations results in remarkable enhancement of photoreduction compared to neutral 2-propanol. The highest disappearance quantum yield measured was 1.28 X 10 2 for 6 M HCl 4-chloro-l-naphthylamine is formed as main product 74.75). 

TABLE 4.29 Some Product Quantum Yield Measurements in the Photolysis of C10N02 ... 

Excitation-wavelength-dependent emission polarization studies indicate the presence of an overlapping xy polarized transition in the bluer part of the 290-315-nm range, as indicated in Fig. 5. The combination of static absorption, time-resolved emission, and emission quantum yield measurements suggests that the emitting state has the same polarization (z axis, linear), but is not the same state as that giving rise to the 362-nm absorption peak. These assignments for the 3.5-nm particles are summarized in Fig. 5. 

Photolysis of carbethoxymethylenetriphenylphosphine in cyclohexene yields benzene, ethyl acetate, ethyl cyclohexylacetate, ethyl cyclohex-2-ene-l-acetate, phenylcyclohexane l,T-bicyclohex-2-ene (Quantum yield measured by use of a low pressure mercury lamp as a light source no yield in material) and diphenyl phosphinic acid. In this case, no triphenylphosphine is produced. On the other hand, pyrolysis of this carbethoxymethy-lene compound shows that only P=C bond fission occurs91. Using acetylmethylene-triphenylphosphine, the observed products are analogous20. However, the irradiation of... 

The quantum yield measured at 313 nm for CO elimination from [Ru-ClH(CO)(PPh3)3] is 0.06 0.02. Because of the air sensitivity of [RuClH(PPh3)3], the yield was determined by irradiating a CH2CI2 solution of [RuClH(CO)-(PPh3)3] in a degassed and sealed uv cell. Since the reaction vessel was sealed, reverse reaction with CO was not prevented, and the measured quantum yield should be considered a lower limit. 

No experiments have been conducted to determine the elimination mechanism (although it is likely concerted), and poor spectral properties have precluded quantum yield measurements. The reaction does appear efficient,... 

Study of the common situation in which there is no useful emission to use as a handle in kinetic analysis requires resourceful experimental programs. Measurement of the quantum yield of an A -> B reaction is of limited value, since quantum yields measure only the ratio of the nonradiative decay rates to A and B. Since both rates are expected to vary widely as a function of structure, the quantum yield alone tells next to nothing about the individual decay rates. The most popular approach to dissection of the kinetic problem involves the use of quenchers. Some third species, C, is introduced into the system in an attempt to intercept A. The most common interception process is energy transfer. 

Most opinions and theories on this question have been formulated from data consisting only of quantum yield measurements, and perhaps suffer from too little knowledge of actual rate constants for excited-state reactions. Quantum yields do not give accurate measures of the relative rates at which two excited species undergo a particular reaction in at least two cases (7) if the chief competing reactions of the two excited species are different or occur at different rates and (2) if the reactions of interest occur so much faster than physical decay that all quantum yields are unity. 

The photochemical processes of triatomic molecules have been extensively studied in recent years, particularly those of water, carbon dioxide, nitrous oxide, nitrogen dioxide, ozone, and sulfur dioxide, as they are important minor constituents of the earth s atmosphere. (Probably more than 200 papers on ozone photolysis alone have been published in the last decade.) Carbon dioxide is the major component of the Mars and Venus atmospheres. The primary photofragments produced and their subsequent reactions are well understood for the above-mentioned six triatomic molecules as the photodissociation involves only two bonds to be ruptured and two fragments formed in various electronic states. The photochemical processes of these six molecules are discussed in detail in the following sections. They illustrate how the knowledge of primary products and their subsequent reactions have aided in interpreting the results obtained by the traditional end product analysis and quantum yield measurements. 

Absolute luminescence quantum yield measurements are not made in photophysical practice and are left to specialized laboratories such as the National Physical Laboratory (UK) or the National Bureau of Standards (USA). These provide the quantum yields of a variety of primary standards that are used in practice to determine an unknown quantum yield e. First the luminescence spectrum of the primary standard is measured, and then that of the unknown sample is compared with it as the ratio of the integrated spectra. 

This method of luminescence quantum yield measurement against a standard emitter is simple and easy to implement with computer-linked fluorimeters, in particular for the integration of the spectra. Its accuracy should however not be over-rated. It is, in the best cases, of the order of 5% (and often far worse). It remains at this time the most widely used technique in photophysics. 

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