Luminescence phosphorescence quantum yield

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. 

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. 

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). 

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]. 

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 ... 

Spectroscopic Measurements. Absorption spectra were obtained using a Perkin-Elmer Model 554 Spectrophotometer and phosphorescence spectra and mean lifetimes were obtained at 77 K using a Perkin-Elmer LS-5 Luminescence Spectrometer coupled to a 3600 data station. Phosphorescence quantum yields were obtained by the relative method using benzophenone (0p = 0.74 in ethanol glass at 77 K) as a standard (11). 

Donor-acceptor separation Forster distance Rate of reaction at z Dipole orientation parameter Quantum yield of process x Quantum yield of donor emission Quantum yield of emission Quantum yield of fluorescence Quantum yield of luminescence Quantum yield of internal conversion Quantum yield of intersystem crossing Quantum yield of phosphorescence Quantum yield of triplet state formation Quantum yield of vibrational relaxation Quantum yield of singlet oxygen production Lifetime... 

Stabilisers are usually determined by a time-consuming extraction from the polymer, followed by an IR or UV spectrophotometric measurement on the extract. Most stabilisers are complex aromatic compounds which exhibit intense UV absorption and therefore should show luminescence in many cases. The fluorescence emission spectra of Irgafos 168 and its phosphate degradation product, recorded in hexane at an excitation wavelength of 270 nm, are not spectrally distinct. However, the fluorescence quantum yield of the phosphate greatly exceeds that of the phosphite and this difference may enable quantitation of the phosphate concentration [150]. The application of emission spectroscopy to additive analysis was illustrated for Nonox Cl (/V./V -di-/i-naphthyl-p-phcnylene-diamine) [149] with fluorescence ex/em peaks at 392/490 nm and phosphorescence ex/em at 382/516 nm. Parker and Barnes [151] have reported the use of fluorescence for the determination of V-phenyl-l-naphthylamine and N-phenyl-2-naphthylamine in extracted vulcanised rubber. While pine tar and other additives in the rubber seriously interfered with the absorption spectrophotometric method this was not the case with the fluoromet-ric method. 

The measurements of the luminescence of 2T and 3T were performed in dilute ethanolic solutions at 77 K. The fluorescence quantum yields were roughly estimated to be 0.07 (2T) and 0.11 (3T) from the relationship of the fluorescence intensities of dilute solutions at 77 K and room temperature. These results agree with the reported data (96JPC18683). The attempts to measure the phosphorescence were successful for 2T. A weak emission at 600 nm was found. The decay time was determined to be (800 200) (is. The attempt to measure the phosphorescence of 3T under the same experimental conditions failed. 

A complementary application to the use of Os complexes in photovoltaic cells is the use of luminescent Os complexes in electroluminescent devices. There has been a significant amount of work in this area, particularly as it applies to the development of Os complexes with high quantum yields for phosphorescence. A review of transition metal complexes used in OLED development was published in 2006 by Evans et al. [126]. Another very recent review discusses various Os(II) carbonyl complexes with diketonate, hydroxyquinolate, bipyridine, and phenanthroline ligands as emitters in OLED devices [127]. A few select examples of Os complexes in OLEDs are presented here. 

The remarkable phosphorescence properties of Ir(III)-cyclometalated species (high luminescence quantum yield and long lifetimes, tunable emission peak), prove of use in several areas in addition to OLED application. We only mention here a few representative examples regarding the Ir(CAN)2(NAN)+ chromophores in (a)-(c) below. For these complexes as incorporated in suitable materials, the emission is expected of Ir -(NAN) CT nature and the alteration of their luminescence features can be associated to other useful properties of the substrates. 

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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