Banca de DEFESA: ROMULO CORREIA FERREIRA
Uma banca de DEFESA de DOUTORADO foi cadastrada pelo programa.DISCENTE : ROMULO CORREIA FERREIRA
DATA : 22/07/2019
HORA: 09:00
LOCAL: Pavilhão de Química sala 48
TÍTULO:
STUDY OF THE REACTIVITY OF THE TRIPLET EXCITED STATE OF DIMETHOXY XANYHONES EMPLOYING THE LASER FLASH PHOTOLYSIS TECHNIQUE
PALAVRAS-CHAVES:
dimethoxyxanthones, laser flash photolysis, triplet excited state, energy transfer, electron transfer, hydrogen transfer, radical cation, singlet oxygen.
PÁGINAS: 205
GRANDE ÁREA: Ciências Exatas e da Terra
ÁREA: Química
SUBÁREA: Química Orgânica
ESPECIALIDADE: Fotoquímica Orgânica
RESUMO:
Studies by UV/Vis absorption spectroscopy, fluorescence emission and 77K phosphorescence emission indicated that the dimethoxylated xanthones 1,2-dimethoxy-9H-xanthen-9-one (1,2MX), 2,3-dimethoxy- 9H-xanthen-9-one (2,3MX) and 3,4-dimethoxy-9H-xanthen-9-one (3,4MX) present triplet excited state with a pp* configuration. Studies by laser flash photolysis (lexc=355 nm) were performed to investigate the reactivity of the triplet excited state of the dimethoxylated xanthones 1,2MX, 2,3MX and 3,4MX toward different types of quenchers. Quenching of 1,2MX, 2,3MX and 3,4MX by 1-methylnaphthalene and 1,3-cyclohexadiene, in acetonitrile, results in the formation of the corresponding triplet excited states through a triplet-triplet energy transfer process controlled by diffusion. The hydrogen transfer reaction of these dimethoxylated xanthones employing alcohols (methanol, ethanol, and 2-propanol) and 1,4-cyclohexadiene occurs through the pure mechanism, with rate constants consistent with the involvement of a triplet excited state with pp* character, confirming the experimental results of the phosphorescence studies. In all cases, the formation of the corresponding ketyl radicals was observed. Using the electron donors DABCO and triethylamine as quenchers of the triplet excited state of 1,2MX, 2,3MX and 3,4MX the formation of the corresponding radical anion by electron transfer was observed. On the other hand, quenching with phenol and its derivatives led to the formation of the ketyl/phenoxyl radical pair, while using indole or the methyl ester of N-acetyl tryptophan (NATME) the ketyl/indolyl radical pair was formed. In all cases, the reaction takes place through a mechanism involving an electron transfer coupled to an ultra-fast proton transfer. Comparing the quenching rate constants for the dimethoxylated xanthones1,2MX, 2,3MX and 3,4MX, it can be concluded that there is no relevant difference between their reactivities. Photolysis of 2,3MX in H2O/acetonitrile solution (1:1 v/v) (lexc=266 nm) resulted in its photoionization with the formation of the corresponding radical cation (lmax~390 nm). The characterization of this radical cation through the use of the nucleophilic quenchers chloride, bromide and acetate leads to quenching rate constants with diffusion-controlled values. Finally, 1,2MX, 2,3MX and 3,4MX were used as photosensitizers for singlet oxygen formation, although presenting very low efficiency (F D1O2: 1,2MX = 0.19; 2,3MX = 0.12 and 3,4MX = 0.06).
MEMBROS DA BANCA:
Presidente - 3278519 - DARI CESARIN SOBRINHO
Interno - 386645 - AURELIO BAIRD BUARQUE FERREIRA
Externo ao Programa - 386996 - FRANCISCO DE ASSIS DA SILVA
Externo à Instituição - FABIO DA SILVA MIRANDA
Externo à Instituição - NANCI CAMARA DE LUCAS GARDEN