Phosphorescence quantum yields

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. 

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...

Temperature ( C) Fluorescence quantum yield, Phosphorescence quantum yield,... 

Figure 4. Graphs of fluorescence quantum yield and phosphorescence quantum yield ( ) versus log of the ratio of millimoles of dissolved sodium acetate to millimoles of p-aminobenzoic acid anion. (Reproduced from reference 12. Copyright 1988 American Chemical Society.)...

When D and A are similar molecules emission-reabsorption cannot be very important due to the usually small overlap of the emission and absorption spectra. Also, this mechanism should not be important for triplet-triplet energy transfer because of (a) low phosphorescence quantum yields in fluid solutions and (b) the low oscillator strengths for singlet-triplet absorption. 

Thus we would expect the phosphorescence efficiency to be greater for the first case than the second. In agreement with this conclusion, Similar effects are observed for heterocycles for example, the phosphorescence quantum yield for pyrazine (lowest n, it triplet) is 0.30(119) while that for quinoline in a hydroxylic solvent (lowest 77,77 triplet) is 0.19/305... 

Outline the essential features needed to determine a fluorescence quantum yield and a phosphorescence quantum yield. 

The phosphorescence quantum yield (0P) (the fraction of photons emitted from Ti when Si is excited) is given by ... 

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. 

On the other hand, if the calorimetric cell is transparent, then Et, E[, E, and E[ have to be determined for example by using data for the transmittance of the media and fluorescence and phosphorescence quantum yields (see following discussion). 

As mentioned above, phosphorescence is observed only under certain conditions because the triplet states are very efficiently deactivated by collisions with solvent molecules (or oxygen and impurities) because their lifetime is long. These effects can be reduced and may even disappear when the molecules are in a frozen solvent, or in a rigid matrix (e.g. polymer) at room temperature. The increase in phosphorescence quantum yield by cooling can reach a factor of 103, whereas this factor is generally no larger than 10 or so for fluorescence quantum yield. 

Another possibility is deactivation through increased intersystem crossing,(1) which would occur via enhanced spin—orbit coupling.(28) If intersystem crossing is enhanced, then the phosphorescence quantum yield of a... 

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... 

Bands in the emission spectrum—numbers to right show relative intensities. h The phosphorescence quantum yield is designated as the phosphorescence lifetime is designated... 

Research Focus Preparation of high phosphorescence quantum yield iridium-containing organic light-emitting elements. 

Phosphorescence quantum yields can also be measured in the same way, but require choppers for eliminating fluorescence (Figure 10.4). 

The only experimental data on the excited heterocyclic molecules relate to emission (or the lack of emission). Most of these investigations are qualitative. The fluorescent and phosphorescent quantum yields have never been accurately measured. Moreover, the lowest triplet states have been only detected by phosphorescence (except for... 

Pyrimidine (1,3-diazine) and pyrazine (1,4-diazine) exhibit weak fluorescences73,74 in solutions or as vapors at room temperature, and strong phosphorescences 76-79 in dilute solid solutions at low temperatures (77 or 90°K). The phosphorescent quantum yields have never been accurately measured in these solid solutions. In the vapor phase or in ordinary solutions, at room temperature, these two compounds do not phosphoresce. Radiationless deactivation processes must be considered again and a deactivation through an isomer cannot be excluded. 

In molecules which contain only C-D bonds, the situation is different because the heavier D atom leads to much smaller spacings of vibrational levels, something like 2000 cm-1. The crossing from Ti (v = 0) to S0(v = n) must reach a vibrational level of much higher quantum number so that the overlap of the nuclear wavefunctions is much smaller. The phosphorescence quantum yields and lifetimes are therefore greater in the deuterated compounds. To take one example, the observed phosphorescence lifetime of naphthalene-A8 is 2.3 s, but that of naphthalene-rf8 is 18.4 s, both measured in a rigid glass at 77 K. 

With 26 the Si —> S0, internal conversion deactivation channel becomes operative (inoperative with 25) which leads to high triplet yield from 0.97 in 26 to 0.66 in 25 and also by a decrease in the phosphorescence quantum yield, ( ) >h. 

The phosphorescence emission of 30 is red shifted by 82 nm compared to the emission of 24, but the emission is only 60 nm red shifted for 32, while 30 and 33 resulted in a 100 nm bathochromic shift. The phosphorescence lifetimes were about twofold longer in the syn isomers compared to the anti-isomers. Only a negligible phosphorescence quantum yield of below 5% was observed for anti-30 and -33 in a glass matrix at 77 K. Higher phosphorescence quantum yields are observed only for the syn (32 = 0.10, 34 = 0.57). For 24, 25, and 34 >40% of the excited molecules decay via phosphorescence. 

The optical and PL spectroscopies have been undertaken to understand the structure-property correlations of this important family of triplet-emitting polymers. The red shift in the absorption features upon coordination of the metal groups is consistent with there being an increase in conjugation length over the molecule through the metal center. The trade-olf relationship between the phosphorescence parameters (such as emission wavelength, quantum yield, rates of radiative and nonradiative decay) and the optical gap will be formulated. For systems with third-row transition metal chromophores in which the ISC efficiency is close to 100%,76-78 the phosphorescence radiative (kr)y, and nonradiative (/cm)p decay rates are related to the measured lifetime of triplet emission (tp) and the phosphorescence quantum yield ([Pg.300]

It is worth noting that the argon-degassed dichloromethane solutions of 44 and 45 show bright luminescence in a lighted room, and display unusual phosphorescence quantum yields of 80 10% in solution at room temperature. The emission spectral profile is independent of excitation wavelength, and the emission of 44 and 45 decayed as a single exponential with lifetimes of 2 and 4 is in dichloromethane solution, respectively [108]. 

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