Triplet formation

Photochromism Based on Triplet Formation. Upon absorption of light, many polycycHc aromatic hydrocarbons and their heterocycHc analogues undergo transitions to their triplet state which has an absorption spectmm different from that of the ground state (24). In rigid glasses and some plastics, the triplet state, which may absorb in the visible, has a lifetime of up to 20 seconds. 

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. 

Several fundamental luminescence parameters were calculated for the anion of p-aminobenzoic acid on sodium acetate (11). The triplet formation efficiency (( ), the rate constant for phosphorescence... 

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. 

Tomlinson, E. Davis, S. S., Interactions between large organic ions of opposite and unequal charge. II. Ion pair and ion triplet formation, J. Colloid Interface Sci. 74,349-357 (1980). 

If et is not known, it is possible to obtain more accurate values of d>lsc than above by using a lower flash intensity such that all the molecules are not excited during the flash (70 20-100 J). For this method the intensity of the light absorbed Ia must be accurately determined from the absorption spectrum and the incident light intensity 70 determined by actinometry. The concentration of triplet molecules [A ] can be determined from A[A ] as above. Since Ia and [A ] are smaller than in the previous case, errors due to the underlying T0 -> Tx absorption are reduced. The quantum yield of triplet formation is now... 

A peculiar effect was observed in the decomposition of 19 a with anthracene as fluorescer when oxygen was carefully removed from the solutions an increase of the chemiluminescence decay rate and of the dioxetane cleavage resulted. It was suggested that this was due to a catalytic effect of triplet anthracene (formed by energy transfer from triplet formate) on the decomposition of the dioxetane. When oxygen is present, triplet anthracene is quenched. Whether such a catalytic effect of triplet anthracene or similar compounds on dioxetane cleavage actually exists has not yet been fully established positive effects were observed by M. M. Rauhut and coworkers 24> in oxalate chemiluminescence and by S. Mazur and C. S. Foote 80> in the chemiluminescent decomposition of tetramethoxy-dioxetane, where zinc tetraphenylporphy-rin seems to exert a catalytic effect. However, the decomposition of trimethyl dioxetane exhibits no fluorescer catalysis 78h... 

AS Dhoot and NC Greenham, Triplet formation in polyfluorene derivatives, Adv. Mater., 24 1834-1837, 2002. 

In radiolysis most of the Si excited molecules (up to 90%) form in charge recombination. The singlet fraction, based on the newer experimental and theoretical studies is around 0.5-0.7. Both theoretical and experimental work indicate that /s decreases with increasing LET value of radiation, because of preferred cross-recombination and by preferred triplet formation in the high ionizing density tracks. 

Capitano (18) has shown that the absorption band for TX around 380 nm corresponds to the lowest lying — x electronic transition. Herkstroeter, Lamola and Hammond (79) calculated the triplet energy for TX to be 65.5 kcal/mol. Amirzadeh and Schnabel (20) estimated the quantum yield for triplet formation of TX to be 0.85 and determined the rate constants for a number of the photochemical reactions that take place during the photocuring reaction. We have outlined a reaction scheme for TX based on our studies (Figure 2) which includes several pathways that occur during the curing reaction. 

Triplet Lifetimes and Triplet Formation Efficiencies of Eosin Di-Aniona... 

Equation (50) can also be used for a further test of the proposed mechanism, and to derive an approximate value for 4>,a, the triplet formation efficiency at infinite dilution. If we substitute... 

This applies at low rates of light absorption, Ia, for which kh[3A ] > > k,r [ solvent viscosity (high k ) and long triplet lifetime (low kh). If two compounds have the same triplet lifetime, the one having the higher value of 0m0<2 will show the higher Ox. The maximum value will be shown by a compound for which the normal (short-lived) fluorescence efficiency (0m) is Va and the triplet formation efficiency (0,) is 2/3. 

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