Sensitized phosphorescence yield

At least four applications of this technique can be cited. Quantum yields for triplet formation in benzene108 and fluorobenzene109 have been estimated by comparing the phosphorescence yields of biacetyl produced by sensitization to that produced by direct irradiation. Intersystem crossing yields of a number of organic molecules in solution have been obtained by measuring the quantum yield with which they photosensitize the cis-trans isomerization of piperylene (1,3-pentadiene) and other olefins.110 As will be discussed later, the triplet states of... 

In benzene solution, quantum yields for formation of olefin triplets from 2537 A irradiation are only 0.20.235 There is some doubt whether triplet benzene is the actual energy donor in solution, since the quantum yield for sensitized phosphorescence of biacetyl depends on a greater than first order of benzene concentration.238 Dubois has suggested that an excited dimer is responsible for energy transfer.236... 

If it can be shown that only one energy transfer step occurs in a system, and if the properties of the donor are established, information of great mechanistic value can be obtained. For example, if step 47 in Figure 6 could be demonstrated to be the only energy transfer or quenching step (i.e., if steps 57 and 46 are absent), the limiting quantum yield for sensitized phosphorescence is [see eq. (20)]... 

Considerable data are available for triplet yields of benzene in dilute solutions of different solvents (see Table 13). In the main, two techniques have been used sensitized phosphorescence of biacetyl, sensitized cis-trans isomerization of butene-2, octene-2, and stllbene. All yield comparable results. In saturated hydrocarbon solvents at room temperature, the triplet yield for CgHg is found to be about 0.24 0.01. There is a solvent dependence of this quantity, the yield dropping to 0.15 in ethanol, 0.13 in methanol, and 0.09 in acetonitrile (91). In determining the effect of environment on the rate constant controlling intersystem crossing, values for emission lifetimes in the various systems are needed. These are, as mentioned previously, often unreliable. Cundall and Pereira (91) have reported... 

Figure 12. Relative quantum yield of sensitized phosphorescence of biacetyl (top), and nonradiative decay rate of naphthalene vapor (bottom), as a function of the excess vibrational energy in Sl naphthalene. (From ref. [40] with permission.)...

From the sensitized phosphorescence of blacetyl, the quantum yield of Inter-system crossing ( ( isc ) was found to be 0.610.1. Prom the slope of the Stern-Volraer plot it can be concluded that the lifetime of the triplet state Is about 30 ns (figure 1). This result is in good agreement with the absence of inhibition by oxygen of dissolved air. The heterolytical scission of the C-Cl bond may be concerted (or not) with the heterolysis of the H-OH bond (18,19). 

The quantum yield of this reaction has been shown to be independent of oxygen pressure (Brunet and Noyes, 1958) and, therefore, occurs prior to intersystem crossing to a triplet state. In agreement with this conclusion is the fact that sensitized phosphorescence from biacetyl is accompanied by a decrease in the quantum yield of Reaction (46). 

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... 

Solid-surface room-temperature phosphorescence (RTF) is a relatively new technique which has been used for organic trace analysis in several fields. However, the fundamental interactions needed for RTF are only partly understood. To clarify some of the interactions required for strong RTF, organic compounds adsorbed on several surfaces are being studied. Fluorescence quantum yield values, phosphorescence quantum yield values, and phosphorescence lifetime values were obtained for model compounds adsorbed on sodiiun acetate-sodium chloride mixtures and on a-cyclodextrin-sodium chloride mixtures. With the data obtained, the triplet formation efficiency and some of the rate constants related to the luminescence processes were calculated. This information clarified several of the interactions responsible for RTF from organic compounds adsorbed on sodium acetate-sodium chloride and a-cyclodextrin-sodium chloride mixtures. Work with silica gel chromatoplates has involved studying the effects of moisture, gases, and various solvents on the fluorescence and phosphorescence intensities. The net result of the study has been to improve the experimental conditions for enhanced sensitivity and selectivity in solid-surface luminescence analysis. 

Interactions in Solid-Surface Luminescence Temperature Variation. Solid-surface luminescence analysis, especially solid-surface RTF, is being used more extensively in organic trace analysis than in the past because of its simplicity, selectivity, and sensitivity (,1,2). However, the interactions needed for strong luminescence signals are not well understood. In order to understand some of the interactions in solid-surface luminescence we recently developed a method for the determination of room-temperature fluorescence and phosphorescence quantum yields for compounds adsorbed on solid surfaces (27). In addition, we have been investigating the RTF and RTF properties of the anion of p-aminobenzoic acid adsorbed on sodium acetate as a model system. Sodium acetate and the anion of p-aminobenzoic acid have essentially no luminescence impurities. Also, the overall system is somewhat easier to study than compounds adsorbed on other surfaces, such as filter paper, because sodium acetate is more simple chemically. 

Phosphorescence is readily detectable from most types of proteins at room temperature. Tryptophan phosphorescence lifetimes and yields are very sensitive to environment, and therefore phosphorescence is sensitive to conformational changes in proteins. Fundamental questions concerning exactly what parameters affect lifetime and spectra of tryptophan in proteins remain still to be answered. 

Explanation of Table ET = triplet energy in kcal/mole from O—O phosphorescence band. Je = intersystem crossing yield from sensitized olefin isomerization method. tp - phosphorescence lifetime in a rigid glass at —196° (sec). Repetition of the data compiled by Arnold63 has in general been avoided. 

The observed quenching effects suggest that the excitation energy of the reactive triplet may be close to the 71 kcal Zimmerman estimated from the phosphorescence spectrum of 35. On the other hand, lumi-santonin, also an enone, must have a relatively low-energy reactive triplet, since its rearrangement is sensitized by Michler s ketone.401 From a Stern-Volmer plot of the effect of naphthalene on the quantum yield for photorearrangement of 35, kjkr was measured to equal 7000.416 Thus if naphthalene quenches triplet 35 at the diffusion-con-... 

Fluorescein and related quinonelike dyes such as eosin have found extensive use as low-energy sensitizers, especially in photooxidation studies. The efficiency of intersystem crossing in fluorescein varies markedly with pH. For the neutral molecule, the quantum yield of fluorescence is only 0.31, while it is 0.91 for the dianion.449 In strong acid solutions, however, protonated fluorescein is strongly phosphorescent. In this state (dissolved in boric acid) fluorescein was the first compound for which T-T absorption in an excited triplet state was observed.7... 

Platinum porphyrin complexes can be prepared by reaction with PtCl2(PhCN)2. Purification of the final complex is by medium pressure liquid chromatography on alumina. The strongly phosphorescent platinum(II) porphyrin complexes are efficient sensitizers for stilbene isomerization. The quantum yields for the cis to trans process are greater than unity because of a quantum chain process in which the metalloporphyrin serves both as an energy donor and an acceptor.1110 Picosecond laser spectroscopy has been used to obtain time-resolved excited-state spectra of platinum octaethylporphyrin complexes, and to probe the excited-state energy levels.1111 Tetrabenzoporphyrin complexes have been prepared for platinum in both the divalent and tetravalent oxidation states. The divalent complex shows strong phosphorescence at 745 nm.1112... 

Biacetyl sensitized photoisomerization of l,2-di-9-anthrylethane 7a does not lead to the 4n + 4n cyclomer 8a but yields exclusively the An + 2n cycloadduct 26 with a quantum yield of 0.1 [72]. Since the phosphorescence of biacetyl is quenched by dianthrylethane 7a at nearly diffusion controlled rate, the photochemical Diels-Alder reaction is explicable by triplet energy transfer from biacetyl to 7a. The photochemical isomerization of 10-benzoyl-l,2-di-9-anthrylethane 27 also proceeds exclusively by An + 2n cycloaddition and gives cycloadduct 28 with a quantum yield of 0.005 [73], The low fluorescence quantum yield of 27 (excited triplet state. Biacetyl sensitization of 27 leads to 28... 

Photon counting detection reaches the ultimate limits of sensitivity in light detection at the present time. It is useful for the detection of very weak luminescence of quantum yields below 10-4 some phosphorescence emissions in liquids at ordinary temperatures can be measured in this way (Figure 7.28). 

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