C. A. Parker

Determination of triplet formation efficiencies by the measurement of sensitized delayed fluorescence

Measurement of the relative intensities and lifetimes of the P-type delayed fluorescence of the same acceptor compound sensitized by two donor compounds allows the ratio of the triplet formation efficiencies of the two donor compounds to be calculated. The method has been applied to a selection of aromatic hydrocarbons using perylene as acceptor and anthracene as the standard donor. With five of the six compounds studied, the sum of the triplet formation efficiency and the fluorescence efficiency exceeds 0.9. The results are compared with previous data and the application of triplet formation efficiencies to the calculation of other photochemical parameters is briefly discussed.

DELAYED FLUORESCENCE AND SOME PROPERTIES OF THE CHLOROPHYLL TRIPLETS

–P‐Type delayed fluorescence and sensitised P‐Type delayed fluorescence have been observed from solutions of chlorophylls a and b in ethanol. E‐Type delayed fluorescence has been observed from solutions in propylene glycol. The measurements in propylene glycol have yielded approximate values for the triplet energies of chlorophylls a and b. Measurements of the emissions from the ethanolic solutions have been used to calculate approximate values of the triplet formation efficiencies in this solvent. The sums of the fluorescence and triplet formation efficiencies in ethanol fall far short of unity and the triplet formation efficiencies therefore need to be confirmed by an independent method before they can be accepted with confidence.

P-type delayed fluorescence from ionic species and aromatic hydrocarbons☆

Abstract P -type delayed fluorescence has been observed from ethanolic solutions of the hydrochlorides of proflavine and acridine orange at −70°C. The results are interpreted by a mechanism involving energy transfer between cationic and/or neutral triplet molecules. P -type delayed fluorescence has also been observed from ethanolic solutions of acenaphthene, fluoranthene, and 1:2-benzanthracene at 22°C. Efficiencies and lifetimes of delayed fluorescence are compared with those previously reported for anthracene, pyrene, naphthalene, and 3:4-benzpyrene.

Triplet behaviour in fluorocarbon solvents

The phosphorescence efficiencies and lifetimes of 9 aromatic carbonyl compounds in fluorocarbon solvents at 20°C are reported, and triplet radiative lifetimes derived. With one exception the measured lifetimes fall in the range 0.3–1.0 ms, i.e., close to the range of lifetimes observed for the “unreactive” triplets in conventional solvents. Measurement of phosphorescence in fluorocarbon solvents provides a convenient method for investigating the triplets which react with conventional solvents by hydrogen abstraction. Constants for quenching of benzophenone triplet by ethanol, cyclohexane and benzene, and for quenching of acetophenone triplet by ground-state acetophenone have been measured. Irradiation of perfluorocarbon solutions of benzophenone containing small amounts of cyclohexane, or concentrated perfluorocarbon solutions of acetophenone alone, causes precipitation of photoproducts. The solvents thus show promise for investigating the photochemical reactions of such aromatic ketones.

Prompt and delayed fluorescence of some DNA adsorbates.

Abstract— The absorption and prompt fluorescence spectra of twelve cationic compounds in the free and bound states were surveyed, and four compounds selected for detailed study, viz: proflavine, benzoflavine, acridine orange and thioflavine T. The absorption spectra, prompt fluorescence spectra and delayed fluorescence and phosphorescence spectra of these four compounds in fluid solutions in the free and bound states were measured. The shifts in the promp fluorescence bands of the diaminoacridines on adsorption were found to differ significantly from those of the other compounds, and the behaviour of acridine orange was found to be different from that of the other two diaminoacridines. All three diaminoacridines gave E‐type delayed fluorescence in the free and bound states. The ordered structure of the strongly binding adsorption sites in native DNA is associated with a higher intensity of delayed fluorescence than is observed from the diaminoacridines in the free state or adsorbed on denatured DNA. Quenching experiments with proflavine indicated that, in the weakly binding sites, its triplet is easily quenched by unadsorbed quenchers, but in the strongly binding sites its triplet is quenched only with great difficulty.

Artifacts and trivial effects in the measurement of delayed fluorescence

A spurious delayed fluorescence from perylene solutions is reported, and the experimental pitfall is discussed in relation to previous observations of delayed fluorescence.