RESUMO
A mechanistic study of the photochromic properties and photodegradation processes of an asymmetrical diarylcyclopentenone bearing thiophene and benzothiophene units using stationary photolysis, nanosecond laser flash photolysis and time-resolved luminescence was performed. It was found that the light-induced reversible isomerization of (3-(2,5-dimethyltiophen-3-il)-2-(2-methyl-1-benzylthiophen-3-il)cyclopent-2-en-1-one, compound 1) from open to closed form is a common photochromic transformation inherent to diarylethenes, while the photodegradation process proceeds in two ways. The first is a formal 1,2-dyotropic rearrangement, proceeding without the participation of oxygen. The second is the oxygen-dependent mechanism involving the excitation of the open form 1A into the triplet state, quenching of the latter by dissolved oxygen, and oxidation of the initial compound by singlet oxygen.
RESUMO
The fluorescent properties of a recently synthesized photochromic naphthopyran containing a 1-aza-15-crown-5 moiety (1b) and its crownless analogue (1a) were studied. 1b emits fluorescence with a maximum at 528 nm, quantum yield 0.1 and characteristic lifetime 2.4 ns (in acetonitrile at room temperature). Its luminescence could be switched off photochemically in two ways using two parallel photochemical reactions characteristic for this type of naphthopyran. The first way is the irreversible transcis photoisomerization of a closed form ("stilbene-like reaction"). The second way is the thermally reversible reaction of closed form transition to the open form ("chromene-like reaction"). The fluorescence of 1b is quenched by alkali earth metal cations by the mechanism of static quenching. Stability constants for 1 : 1 complexes of 1b with Mg(2+) and Ba(2+) determined from Stern-Volmer plots are in agreement with that obtained by UV spectroscopy.