Singlet state Sodium

The photohydrolysis of 2-fluoro-4-nitroanisole to 2-methoxy-5-nitrophenole is sensitized by benzophenone and completely quenched by sodium sorbate The excited state multiplicity in photoaminations has also been studied. Photolysis of mNA in liquid ammonia yields m-nitroaniline. If the amination is carried out in a large excess of benzophenone, 2-methoxy-4-nitroaniline is formed instead and thus an excited singlet state as reacting species is envisaged in the unsensitized photoamination loo.ioi). it may well be that uptake of the nucleophile present in high concentration successfully competes with intersystem crossing. 

Since neither sodium sorbate nor fluorenone were effective as quenchers, the direct photohydrolysis of 2,3-dimethoxynaphthaIene is hkely to proceed via an excited singlet state. However, sensitization by benzophenone has been achieved 102). 

Heavy atom enhancement of intersystem crossing has been used to determine the mechanism of acridine photoreduction in ethanol.115 It was found that addition of sodium iodide decreased the fluorescence intensity and the rate of disappearance of acridine to the same extent, confirming that the singlet state is responsible for photoreduction. From the increase in triplet state absorption upon addition of iodide it was found that Of for acridine was 0.76. Thus the short singlet lifetime (0.8 nsec) of acridine is due to rapid inter-system crossing to unreactive triplet states. 

The participation of the singlet states of dyes such as rose bengal (RB, tetraiodotetrachlorofluorescein, sodium salt) and MB during the sensitized oxygenation of amines was demonstrated by Davidson (40). Fluorescence quenching Stem-Volmer constants were reported for several amines, halide anions, and for 3-carotene and were consistent with charge transfer stabilization of an exciplex quenching intermediate. 

An intramolecular charge transfer excited state may be involved, and the inverse relationship between fluorescence and photoisomerisation suggests a singlet state mechanism. Irradiation of (Z)-urocanic acid, 3-(lH-imidazoI-4-yl)prop-2-enoic acid, in the presence of nitro blue tetrazolium and sodium azide promotes its photoisomerisation in a process which involves reversible addition of the azidyl radical to the double bond. ... 

Photoelectron transfer from the excited singlet state of zinc(II) tetraphenylpor-phyrin (ZnTPP) to MV " in a compartmentalised system in which the ZnTPP moieties are covalently attached to amphiphilic sodium polysulfonates carrying lauryl 2-(naphthyl)methyl, or cyclododecyl groups, is reported to be much slower... 

To keep the reaction centers open i.e. in a photoactive condition the chromatophore solutions were purged with nitrogen for an hour, after which a 10 - 50 mM sodium-dithionite solution (in 250 mM Tricine) was added to ensure that the first quinone acceptor af the RC, is maintained in the reduced state (2a)- Under these conditions illtimination of the sample leads to a primary charge separation, but this state will recombine on a nano-second time-scale. At 77 K none of the recombination will occur via the excited singlet state, but at higher temperatures slow emission decay components can be observed due to this recombination (4). However this will not interfere with the measurements of the trapping time reported here, inview of the large difference in time-scales. ( <100 ps vs. 10 -20 ns). 

Upon substitution of 30% of the sodium counterions in the SDS micelle with thallium, a similar chromatogram is observe (Fig. 12.6B). These signals also arise from fluorescence and are diirimished in intensity compared to Fig. 12.6A, because thallium is effectively depopulating the excited singlet state from which fluorescence occurs. Thus, Fig. 12.6A... 

A correlation between the rate constants k and free enthalpy change AG of electron transfer was studied by Hashimoto and Thomas [127] for quenching of excited singlet states of both pyrene and N-ethylcarbazol and of the triplet state of N-methylphenothiazine by a number of metal ions and for back electron transfer reactions in micellar sodium taurocholate and sodium dodecylsulfate solutions. Quenching rate constants were determined from Stern-Volmer plots obtained for lifetimes of excited states at high concentration of micelles, where the exponential decay... 

Recently, a clever approach was used by Foote and Wexler (1964a, b) which further substantiates the excitation transfer mechanism. The chemiluminescence produced upon reaction of hydrogen peroxide with sodium hypochlorite in aqueous solution has been, tentatively, identified as phosphorescence from a low-lying singlet state of molecular oxygen (Khan and Kasha, 1963, 1964). In preliminary studies, Foote and Wexler (1964a, b) have shown that the species produced from this thermal reaction in the presence of several substrates yields the same products as are formed in the photosensitized oxidations, e.g., Eqs. (174)-(176). 

Dithietans commonly are formed by dimerization of thiocarbonyl compounds. Intersystem crossing from the second excited singlet state ( TT- ir ") to the first triplet state is significant in the photochemical formation of the 1,3-dithietan derived from adamantanethione. Its formation on irradiation of the thione at 250 nm is quenched by l,l -azoisobutane, indicating an intermediate triplet state. Other dithietans have been obtained by irradiation of the trimer of cyclohexanethione or thioacetone, irradiation of dibenzyl thioketone, treatment of carbohydrates with phosphorus pentasulphide in boiling aqueous pyridine, treatment of perfluoropropene with sulphur and potassium fluoride at 120 in an autoclave, " and by treatment of perfluoroisobutene with sodium hydrosulphide. The adduct (212) of benzaldehyde and OO-diethyl... 

Hydrolysis is accelerated in the presence of strong acids. However, in the presence of aqueous bases such as sodium hydroxide, the rate of decomposition increases with increasing pH and teaches a maximum at the of the petoxycatboxyhc acid (ca 8.25), then decreases at higher pH (169,170). The basic decomposition products include the parent catboxyhc acid and singlet oxygen (171,172). Because the maximum rate of decomposition occurs at the p-K, the petoxycatboxyhc acid and its anion ate involved in the transition state (169). 

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