Triplet state acidity

The triplet state acidities, pkl, are found to be not much different from ground state pK s. But feterocyclics like o-phenanthroline and quinoline are more basic in triplet state than in ground state, although less basic than in the first excited singlet state. It follows that the singlet transition in these molecules has considerably more charge transfer character than the transition in the triplet state. 

Triplet state acidities, pKj, have been obtained by Porter and his school from phosphorescence studies, using flash photolysis techniques. The singlet, triplet and ground state acidity constants of some organic molecules are given in Table 4.2. 

In both groups the triplet state acidity is much more similar to ground state acidity than to excited singlet state acidity. 

Adiabatic protolytic equilibria in the triplet state are generally fully established due to the intrinsically longer lifetimes of triplets. Soon after Forster s work, Jackson and Porter determined the acidity of 2-naphthol (10) in the triplet state by flash photolytic titration.382 The triplet triplet absorption of 10 changes from 2max = 432 to 460 nm as the pH is moved above the triplet p/Ta (10) of 8.1. Triplet state acidity can also be predicted using the Forster cycle. The triplet excitation energies ET of the acid and its conjugate base are determined from the 0 0 bands of their phosphorescence spectra. 

Richtol, H.H., Fitch, B.R., Triplet state acidity constants for hydroxy and amino substituted anthraquinones and related compounds. Anal. Chem. 46 1860-1863. 

Catalytic oxidation of isobutyraldehyde with air at 30—50°C gives isobutyric acid [79-31-2] ia 95% yield (5). Certain enzymes, such as horseradish peroxidase, cataly2e the reaction of isobutyraldehyde with molecular oxygen to form triplet-state acetone and formic acid with simultaneous chemiluminescence (6). 

Irradiation of 5-acetyldibenz[i>,/]azepine (19) in the presence of. V-methylmaleimide results in a mixture of the homo dimer 20 and the mixed dimer 21. No [2 + 2] cycloadducts are obtained, however, with other alkenes such as styrene, acetylenedicarboxylic acid, dimethyl fumarate or acrylonitrile.32 Later studies have demonstrated that the dimerization occurs via the excited triplet state.196... 

Fluorescence and phosphorescence spectra of poly(propynoic acid)(FPA), polyphenylene (PP), and DPAcN show that the difference of energies between the lower excited singlet and triplet states, as observed in the case of PP (583 nm) and DPAcN (528 nm), is considerably greater than that of poly(propynoic acid) (270—300 nm) which besides transitions may undergo rr - transitions. PCSs showing only... 

In weaker acid systems, other reactions involving the triplet state supervene to the exclusion of dimerization. Photolysis of 85 in 3-3% sulfuric acid, 96-5% acetic acid, and 0-2% water gave as products tri-phenylmethane (93), 9-phenylfluorene (94), 6is-9-phenylfluorenyl peroxide (95) and benzophenone (96). When benzene was present, tetra-phenylmethane (97) was also formed in addition to the other products. When the triphenylmethyl cation is irradiated in 3-3% H2SO4, 80 1% HOAc, 16-4% toluene, and 0-2% H2O, the products observed were... 

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. 

That the high degree of torsional and other types of strain inherent in the triplet states or trans conformers of cyclohexene and cycloheptene may be responsible for their photochemical behavior is suggested by the reactions of compound (50), a moderately twisted olefin according to molecular models. Compound (50) quantitatively yields bicyclo[3.3.1]non-l-yl acetate (51) within 15 sec after being dissolved in glacial acetic acid(83> ... 

In acidic solution the rearrangement of a-santonin to lumisantonin and then ultimately to photosantonic acid is not as efficient as rearrangement to the hydroxy ketone, isophotosantonic lactone (5), shown on page 308. Fisch and Richards(6) found that the photorearrangements of a-santonin could be sensitized with benzophenone or Michler s ketone. Moreover, if the irradiation of a-santonin (3660 A) is carried out in piperylene as solvent, the photoreaction is completely quenched. This suggests that the rearrangements proceed via triplet states. 

Enthalpies of formation for the singlet and triplet states of methylene were obtained from the photodissociation of ketene.131 The data for CH2 (3Bi) were recently confirmed by methods which do not rely on ketene.132,133 In a widely applicable procedure, threshold collision energies for the loss of halide ion from RR C-X- were combined with gas phase acidities of RR CH-Cl to give AHf (RR C ) (Eq. 11).134 Similarly, gas phase acidities of the radicals RR CH were combined with ionization energies of the radical anions RR C -, or electron affinities of the carbenes RR C (Eq. 12).135136... 

The quenching of benzophenone phosphorescence has been used by Mar and Winnik (1981) as a photochemical probe of hydrocarbon chains in solution. The bimolecular reaction for quenching the triplet state of 4-methoxy-carbonylbenzophenone [24] by 1-pentene occurs at rates which are below the diffusion limit by two to three orders of magnitude. Consequently, the intramolecular quenching reactions of to-alkenyl esters of benzophenone-4-carbo-xylic acid [25] occurs under conformational control. In [25] the point of... 

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