Estudo da termólise de dioxetanos catalisada por trasferência intramolecular de elétron / Studies on the intramolecular electron transfer catalyzed thermolysis of 1,2-dioxetanes

AUTOR(ES)
DATA DE PUBLICAÇÃO

1997

RESUMO

The generation of visible light by a living organism or by a chemical reaction is a sufficiently rare and unusual event to attract the interest of biologists, biochemists and chemists. The chemiluminescent reaction most extensively investigated over the past 25 years, is the thermolysis of 1,2-dioxetanes. These high energy molecules generate mainly triplet excited carbonyl compounds on thermal decomposition, except when the dioxetane ring bears electron-donating substituents. In the latter case, such labile dioxetanes follow an intramolecular electron transfer mechanism, probably via the CIEEL (Chemically Initiated Electron Exchange Luminescence) pathway. This work reports our results on the synthesis and the chemiluminescence properties of the dioxetanes I to IV. These compounds serve as models to distinguish between resonance electron transfer (dioxetane II) and through space electron transfer (dioxetane IV) in their fluoride-catalyzed decomposition. Note: Dioxetanes I-IV illustrations available at the PDFs Summary The classical Kopecky method for the preparation of dioxetanes from the corresponding alkenes via β-bromohydroperoxides was the route used for the synthesis of I to IV. The dioxetanes were purified by low-temperature silica gel cromatography and characterized on the basis of their spectral data (1H-NMR and 13C-NMR). Thermal decomposition of I-IV gave the expected cleavage products, the corresponding carbonyl compounds. For I and II, direct emission was observed with a maximum at 412 nm, while for III and IV it was necessary to use sensitizers for the detection and quantification of the eletronically excited carbonyl compounds. Upon unimolecular decomposition, the dioxetanes I-IV show activation parameters and excited state yields expected for trisubstituted 1,2-dioxetanes; relatively high stability (ΔG≠approximately equal to 25 kcal.mol-1) and preferential formation of triplet excited states are observed. The excitation yields were deterrnined by the well-stablished methods, in which 9,10-diphenylantracene (DPA) is employed for singlet and 9,10-dibromoantracene (DBA) for triplet state counting. In the presence of tetrabutylammonium fluoride, which causes the deprotection of the phenol group to the corresponding phenolate, the decomposition rates of II and IV are increased drastically. The deprotected derivatives decompose about 104 times faster than the protected ones, and a strong direct emission with a maximum at 560 nm is observed. The singlet quantum yields for the catalyzed decomposition of II and IV were 100 and 1,0 % respectively. The activation parameters for the fluoride catalyzed decomposition are considerably lower than that for the unimolecular one. These facts indicate the occurrence of an intramolecular "Chemically Initiated Electron Exchange Luminescence" (CIEEL) mechanism, leading to the preferential formation of singlet excited states. The proposed mechanism, based on the CIEEL scheme, involves removal of the silyl protecting group, promoted by fluoride, to generate the phenoxy-anion, which subsequently acts as an electron donor to the dioxetane moiety, leading to dioxetane cleavage and excited states formation. The first electron transfer from the phenolate to the peroxide ring occurs with the same rate in both cases. Therefore, the observed difference between the singlet quantum yields indicate that the back electron transfer occurs with lower efficiency in the case of IV. The observation of fluoride catalysis in the decomposition of dioxetane IV constitutes the first example of an intramolecular CIEEL mechanism in 1,2-dioxetanes initiated by electron transfer from a donor which is not directly bond to the peroxide ring ("through space electron transfer"). Moreover, as the chemiluminescence parameters, specifically the quantum yields of II and IV, can be explained based on the proposed pathways for the CIEEL mechanism, this work supplies additional evidence for the validity of this widely cited but still polemic mechanism.

ASSUNTO(S)

chemiluminescence cieel luminescence luminescência quimiluminescência cieel dioxetanes dioxetanos

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