E. Vander Donckt

Photochemistry of aromatic compounds. Part 1.—Acid-base properties of singlet and triplet excited states of pyrene derivatives and aza-aromatic compounds

Variations of acidity constants between the ground state G and the fluorescent state S1 of 1-, 2-, 4- and 9-azaphenanthrenes, 1-aminopyrene, 1-hydroxypyrene and pyrene-1-carboxylic acid have been measured by application of the Forster cycle. Triplet state pK values of the same derivatives were obtained by the flash photolysis technique. For 1-hydroxypyrene under flash photolysis conditions, only one transient, whose spectrum is not pH dependent could be observed. This transient spectrum has been assigned to the pyrenoxyl radical. There appears to be no relationship between the position of protonation of the various aza-phenanthrenes and the ΔpK between the ground state and the singlet or triplet excited states.

Propriétés physico-chimiques de composés à caractère aromatique—V: Etude, par spectroscopie ultraviolette, de l'interaction entre les dérivés monoazaaromatiques et les acides carboxyliques. Mise en évidence d'un potentiel à deux minimums pour le lien hydrogène

Abstract U.V. studies of hydrogen bonding between monoaza-aromatic compounds (pyridine quinoline, acridine⋯) and a series of carboxylic acids in hexane solution, confirm the existence of a tautomeric equilibrium involving the associated base and the protonated base. The fraction of both species are estimated in every case. Solvent effects on the structure of the complex of quinoline, isoquinoline and acridine with acetic acid are discussed. Our experimental results agree with a double well potential for this type of hydrogen bonds.

Electron transfer from aromatic molecules to dimethylmercury via a triplet exciplex

Whereas low concentrations of added dimethylmercury [0 < (DMM)≲ 0.1–1 mol dm–3] increase the intersystem crossing rates of electronically excited aromatic molecules, higher concentrations of the fluorescence quencher favour electron transfer from the excited aromatic to the dimethylmercury. A kinetic scheme is proposed, based on fluorescence intensity measurements, on optical densities of triplet species and radical cations as observed by flash photolysis. The data are consistent with the formation of a trimolecular complex consisting of an electronically excited aromatic species and two molecules of dimethylmercury.