Quantum yield triplet formation

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. 

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... 

Electron transfer can be accomplished by quenching of a micelle trapped chromophore by ions capable of ion pairing with the micelle surface. For example, excited N-methylphenothiazine in sodium dodecylsulfate (SDS) micelles can exchange electrons with Cu(II). The photogenerated Cu(I) is rapidly displaced by Cu(II) from the aqueous phase so that intramicellar recombination is averted, Fig. 5 (266). Similarly, the quantum yield for formation of the pyrene radical cation via electron transfer to Cu(II) increases with micellar complexation from 0.25 at 0.05 M SDS to 0.60 at 0.8 M SDS (267). The electron transfer quenching of triplet thionine by aniline is also accelerated in reverse micelles by this mechanism (268). 

The quantum yields of formation of tert-butyl N-(ethoxycyclopropyl)carbamate (13) from irradiation of 12 and of tert-butyl N-(ethoxyethylidene)carbamate (27) and glutarimide (29) from irradiation of 14 are moderately high and are shown in Table 2. The effect of the triplet quencher c/s-piperylene on the quantum yields has been examined. Over the concentration range 0.01—0.1 OM c/s-piperylene, no ap-... 

We have studied several aminoketones and the results are not totally understandable 168>. For a,y, and b dialkylaminosubstituted ketones, the quantum yield for formation of long-lived triplet increases as the amino group gets farther away from the carbonyl and is higher in protic solvents than in aprotic solvents. 

Numerous papers dealing with the photoreduction of viologens have appeared. Rate constants for electron transfer between aromatic hydrocarbon triplets and bipyridinium and pyridinium salts have been measured. The results suggest that the process is both facile and efficient. All the rate constants necessary for the determination of the quantum yield of formation of the viologen cation-radical in the methyl viologen-Eosin-EDTA system are also now available. " The high activity of dihydroxytin(rv) uroporphyrin in the photoreduction of methyl viologen has been ascribed to the failure of a photochemi-cally inactive tt—tt complex to be formed between substrate and sensitizer. 

The selective one-electron reduction of C70 to C70" is also attained through photoinduced electron transfer from BNAH or (BNA)2 to the triplet excited state of C70 [227]. The limiting quantum yields for formation of C70" in the photoreduction... 

The quantum yield provides information about the effectiveness of a certain photo-induced process. The quantum yield of loss of starting material or product formation will provide valuable information about structure-activity relationships. Fluorescence and phosphorescence quantum yields will indicate the fraction of molecules likely to be found in the excited singlet and triplet state. The quantum yield is a useful parameter to predict the importance of a certain reaction for example, an isolated degradation product can have a long phosphorescence lifetime and should therefore be considered as a possible sensitizer. If the quantum yield of formation of this product is very low, however, it is less likely to be formed in a biologically active concentration and may therefore play a minor role in phototoxicity reactions. 

Two papers have been presented on the photochemistry of 5-methylphena-zinium salts in aqueous solution. Fluorescence, optical flash photolysis, and electron paramagnetic resonance (e.p.r.) techniques have been used to elucidate various aspects of product formation and quantum yield. Two products have been identified, namely the 5-methyl-10-hydrophenazinium cation radical (MPH ) and the pyocyanine (l-hydroxy-5-methyl-phenozinium) cation (PyH ) in a stoicheiometric ratio of 2 1. The quantum yield of formation of (MPH ) was found to be 0.29 0.03 at pH 7.0 and 1.1 0.1 at pH 3.0. The triplet state of MP (Ti) has also been detected by triplet-triplet absorption and is found to have a lifetime of 0.5 ns. Flash photolysis and e.p.r. have also been used to study a geminate triplet radical pair obtained from hydrogen abstraction by excited triplet acetone from propan-2-ol. The authors demonstrate that the geminate pairs contribute most of the polarization in photochemically-induced dynamic electron polarization (CIDEP) as compared with free random-phase pairs. 

Quantitative photophysical, photochemical and photopolymerisation data are presented on five novel water soluble benzophenone photoinitiators. Phosphorescence quantum yields, triplet lifetimes and transient formation on conventional flash photolysis correlate with the ability of the initiators to photoinduce the polymerisation of 2-hydroxyethylmethacrylate and a commercial monoacrylate resin in aqueous media. The results indicate that the lowest excited triplet state of the initiator is abstracting an electron from the tertiary amine cosynergist probably via a triplet exciplex followed by hydrogen atom abstraction. This is confirmed by a detailed analysis on the effect of oxygen, pH and the ionisation potential of the amine on transient formation and photopolymerisation. Using photocalorimetry a linear correlation is found between the photopolymerisation quantum yields of the initiators and their photoreduction quantum yields in aqueous media. 

A novel face-to-face dimeric porphyrin system was reported recently by Lehn et al. [5,6] in which the two porphyrin rings are held apart by [18]-N204-aza-oxamacrocycles (1). Spectroscopic studies showed that the two porphyrin rings interact together via exciton coupling [7]. This effect serves to shorten the excited singlet state lifetime (Xg = 1.22 ns) and to reduce the fluorescence quantum yield with respect to the corresponding monomeric porphyrin. For 1 in acetonitrile, the quantum yield for formation of the porphyrin excited triplet state was only 0.18, compared to a value of 0.67... 

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. 

Table 5.6 Properties of three typical photoredox-active molecules. bpy denotes 2,2 bipyridine, TMPP is tetra N-methylpyridine porphyrin Amax is the wavelength of the absorption maximum, e is the extinction coefficient at Amax, cpT is the quantum yield of the formation of the excited triplet state, r0 is its lifetime, and E0 are standard redox potentials...

The photoreduction of cyclobutanone, cyclopentanone, and cyclohexanone by tri-n-butyl tin hydride was reported by Turro and McDaniel.<83c> Quantum yields for the formation of the corresponding alcohols were 0.01, 0.31, and 0.82, respectively. Although the results for cyclopentanone and cyclohexanone quenching were not clear-cut (deviations from linearity of the Stem-Volmer plots were noted at quencher concentrations >0.6 M), all three ketone photoreductions were quenched by 1,3-pentadiene, again indicating that triplets are involved in the photoreduction. 

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