Phosphorescence quantum yield values

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. 

Solid-surface fluorescence and phosphorescence quantum yield values were obtained from +23° to -180°C for the anion of p-aminobenzoic acid adsorbed on sodium acetate (11). Fhosphorescence lifetime values were also obtained for the adsorbed anion from +23° to -196°C. Table 1 gives the fluorescence and phosphorescence quantum yield values acquired. The fluorescence quantum yield values remained practically constant as a function of temperature. However, the phosphorescence quantum yield values changed substantially with temperature. The phosphorescence lifetime experiments indicated two decaying components. Each component showed a gradual increase in phosphorescence lifetime with cooler temperatures, but then the increase appeared to level off at the coldest temperatures. 

Table I. Fluorescence and Phosphorescence Quantum Yield Values for the Anion of p-Aminobenzoic Acid Adsorbed on Sodium...

The value of the phosphorescence quantum yield can be determined by measuring the total luminescence spectrum under steady irradiation. If the fluorescence quantum yield is known then the phosphorescence quantum yield may be found by comparing the relative areas under the two corrected spectra. 

Tryptophan at 77 K in rigid solution has a phosphorescence quantum yield of 0.17(20) and a lifetime of 6 s. These values at 77 K are relatively invariant from protein to protein and do not vary significantly between buried and exposed tryptophans.(21,22) If one assumes that the intersystem crossing yield is a constant, a calculation of the quantum yield of indole phosphorescence can be roughly estimated from the lifetimes. The phosphorescence yield is related to lifetime by... 

Although these values are extremely important quantities, there is no established, general method for measuring them on an absolute basis. Experimental difficulty arises because the species of interest is often a transient, and hence the precise values of its desired physical properties, such as (1) the T-T absorption coefficient and (2) the phosphorescence quantum yield for the specified excitation energy and the specified eigenstate, are not available for the desired spectroscopic analysis. However, some limited methods may be used with adequate caution. These methods involve either luminescence measurement or product analysis ... 

The phosphorescence spectra of the two aryl polysilylenes studied are shown in Figure 9 and their fluorescence at room temperature in Figure 10. Although phosphorescence quantum yields for these two polymers were not measured, estimates based on comparison with the alkyl intensities indicate that these polymers emit with substantially greater yield. Todesco and Kamat (21 ) have measured the phosphorescence yield of a copolymer of a-naphthyl methyl and dimethyl silylene units to be 0.39. Our naphthyl polymer gives clearly the most intense phosphorescence and probably has a quantum yield near the 0.39 value for the copolymer. We estimate the phenyl polymer yield to be = 1/10 of the naphthyl polymer. 

The simultaneous measurements of phosphorescence and ESR spectra in combination with the results of triplet lifetime measurements allow us to separate out the nonradiative decay rate constant Wnr from the observed lifetime x. Eor this, we need the phosphorescence quantum yield (])pj, for each DAP. For phenanthrene dissolved in ethanol at 77 K, ( )pjj = 0.12, [208], which was determined on the basis of an ISC quantum yield of 0.86 [209]. As a consequence of the similarity of experimental conditions (solvent, temperature), we may choose these values as a standard for the DAPs from among a number of available date [208, 210-213]. The phosphorescence quantum yield (])pjj is given by the general relation [214]... 

Sodium Acetate-Sodium Chloride Mixtures. Ramasamy and Hurtubise (12) obtained RTF and RTF quantum yields, triplet formation efficiency, and phosphorescence lifetime values for the anion of p-aminobenzoic acid adsorbed on sodium acetate and on several sodium acetate-sodium chloride mixtures. Rate constants were calculated for phosphorescence and for radiationless transition from the triplet state. The results showed that several factors were important for maximum RTF from the anion of p-aminobenzoic acid. One of the most important of these was how efficiently the matrix was packed with sodium acetate molecules. A similar conclusion was found for RTF however, the RTF quantum yield increased more dramatically than the RTF quantum yield. 

Details of nitrobenzene photochemistry reported by Testa are consistent with the proposal that the lowest triplet excited state is the reactive species. Photoreduction, as measured by disappearance quantum yields of nitrobenzene in 2-propanol is not very efficient = (1.14 0.08) 10 2 iD. On the other hand, the triplet yield of nitro benzene in benzene, as determined by the triplet-counting method of Lamola and Hammond 28) is 0.67 0.10 2). This raises the question of the cause of inefficiency in photoreduction. Whereas Lewis and Kasha 29) report the observation of nitrobenzene phosphorescence, no long-lived emission from carefully purified nitrobenzene could be detected by other authors i4,3o). Unfortunately, the hterature value of Et for nitrobenzene (60 kcal mole i) is thus based on an impurity emission and at best a value between 60 and 66 kcal mole can be envisaged from energy-transfer experiments... 

The phosphorescence decay kinetics of the triplet excited states of CuP molecules (Fig. 14) is adequately described by Eq. (16). Using this equation one can obtain the values of the parameter p = (Tra /2) In2 veT from the initial non-exponential part of the phosphorescence decay curves and the values of t = l/ k, i.e. the characteristic time of phosphorescence decay, from the final exponential part. Then the data on the dependence of the quantum yield of CuP phosphorescence on the concentration of C(N02)4 have been used to estimate the effective radii of electron tunneling from triplet excited copper porphyrins to C(N02)4 within the time x R, = (ac/2) In vet (Table 3). In doing so, the quenching of CuP luminescence by electron abstraction was assumed to be the only process leading to a decrease in the quantum yield of CuP phosphorescence in the presence of C(N02)4. From Table 3 an electron is seen to tunnel, within the lifetime of triplet excited states x at 10-4s, from CuP particles to C(N02)4 molecules over the distance R, 11 A. Further, the parameter vc and ae for different porphyrins were estimated from the values of (3, Rt, and x. These values are also cited in Table 3. 

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