Stanislav Luňák

Electronic excited states of azo compounds: Strong ππ* fluorescence of bis-4,4′-diethylaminoazobenzene

Abstract The low-temperature absorption and luminescence spectra of bis-4,4′-diethylaminoazobenzene (BDAAB) in 2-methyltetrahydrofurane (MTHF) have been measured. On the basis of the spectral characteristics, this luminescence is interpreted as the fluorescence from the first excited singlet state (ππ*). A comparison of the spectral and photophysical properties of BDAAB with those of other para-substituted trans-azobenzenes enables us to draw some conclusions concerning the structure and fluorescence of these systems.

Photocatalytic degradation of 4-chlorophenoxyacetic acid in the presence of an iron complex and hydrogen peroxide

The photoinitiated transformation of 4-chlorophenoxyacetic acid (CPA) in aqueous solutions is described in detail. The photocatalytic system consists of CPA, hydrogen peroxide (no oxygen) and the complex iron(m) acetylacetonate Fe(acac)3. Special attention was paid to the choice of irradiation wavelength in order to separate the contribution of the iron-photocatalyzed reaction from that of direct photoreactions This was achieved by selecting a wavelength of 365 nm, which is absorbed exclusively by the iron complex. We specified the conditions for effective photocatalytic degradation of CPA with high quantum yields which depend on the concentration of Fe(acac)3. The reaction is primarily initiated by absorption of 365 nm radiation by Fe(acac)3 followed by the photoreduction of Fe(III)(acac)3 to Fe(II)(acac)2 and subsequently by the Fenton reaction with hydrogen peroxide producing highly reactive hydroxyl radicals OH*. The main degradation intermediates are phenol and 4-chlorophenol, which are further mineralized during continuous irradiation.

Methylene-blue-sensitized photochemical hydroxylation of 2-hydroxybenzoic acid by hydrogen peroxide: photocatalytic effect of ferric chloride

Abstract The methylene-blue-sensitized hydroxylation of 2-hydroxybenzoic acid by hydrogen peroxide initiated by visible radiation and catalysed by ferric chloride has been investigated. The major reaction product is 2,3-dihydroxy-benzoic acid. In the absence of iron(III), the reaction occurs extremely slowly. Iron(III) is observed to have an appreciable catalytic effect on the photosensitized reaction at concentrations as low as 10 −8 M. The first step of the reaction is the photochemical reduction of iron(III) to iron(II), i.e. the generation of Fentons reagent.

Photocatalytic effects of Fe(III) compounds on the hydroxylation of benzoic acid by hydrogen peroxide initiated by UV-radiation

The addition of FeCl3 and Na2[Fe(CN)5NO] increases the quantum yields of photochemical hydroxylation of benzoic acid by hydrogen peroxide. The photocatalytic effects operate especially when the radiation is absorbed by benzoic acid. When hydrogen peroxide is excited, the catalytic effects in the hydroxylation are small.AbstractДобавки FeCl3 и Na2[Fe(CN)5NO] повышают квантовые выходы фотохимического гидроксилирования бензойной, кислоты перекисью водорода. Фотокаталитические эффекты проявляются особенно тогда, когда радиация абсорбируется бензойной кислотой. Нри возбуждении перекисн водорода каталитический эффект гидроксииирования мал.

Gelation in the curing of epoxy resins with anhydrides

SummaryThe statistical theory of branching processes has been used in an analysis of gelation in the system diepoxide-cyclic anhydride-tert.-amine in step and initiated reaction mechanisms. Experimental data indicate that critical conversion depends on the concentration of tert.-amine, which suggests that a mechanism with initiation is operative.

Photocatalytic effects of Fe(III) compounds on the hydroxylation of benzoic acid by hydrogen peroxide initiated by 589 nm radiation and sensitized by methylene blue

Abstract The hydroxylation of benzoic acid by hydrogen peroxide initiated by 589 nm radiation is catalyzed by the following Fe(III) compounds: FeCl3, K3[Fe(C2O4)3], Na2[Fe(CN)5NO], and K3[Fe(CN)6]. This photochemical reaction can be effectively sensitized by methylene blue.

Photochemical hydroxylation of salicylic acid by hydrogen peroxide. Photocatalytic effect of Fe(III)

The rate and quantum yields of photoinitiated hydroxylation of salicylic acid by hydrogen peroxide can be increased substantially by adding Fe(III) compounds to the reaction mixture. The UV radiation reduces the catalytically inactive Fe(III) to catalytically active Fe(II) during the reaction, thereby continuously producing the Fenton reagent.AbstractСкорость и квантовые выходы фотоинициированного гидроксилирования салициловой кислоты с помощью перекиси водорода могут быть значительно повышены добавлением соединений Fe(III) к реакционной смеси. У Ф облучение восстанавливает каталитически неактивное Fe(III) до каталитически активного Fe(II) в ходе реакции, тем самым неапрерывно вырабатывая раегент фентона.

Photophysics of PBD derivatives. II: The character of the lowest excited triplet state of 2-(biphenyl-4'-yl)-5-phenyl-1,3,4-oxadiazole

Abstract The low temperature phosphorescence spectra, polarization and the lifetime of the phosphorescence for five derivatives of 2-(biphenyl-4′-yl)-5-phenyl-1,3,4-oxadiazole (PBD) were measured. Two chemical pathways were found to force molecules of the PBD-type to phosphoresce: introducing a substituent with a localized low-energy nπ* state or substitution by a heavy atom (bromine). In both cases, the emitting, i.e. the lowest excited triplet state, is of the ππ* character. The T 1 characteristics of the non-phosphorescent PBD were estimated by analogy. The vibrational structure of the phosphorescence spectral bands yields the information on the planarity of PBD-type chromophore in the excited T 1 state.

Photophysics of PBD derivatives. I. The fluorescence of para-biphenylyl-substituted 2-(biphenyl-4′-yl)-5-phenyl-1,3,4-oxadiazoles

Abstract The room and low temperature absorption and fluorescence spectra and the fluorescence quantum yields and lifetimes of ten derivatives of 2-(biphenyl-4′-yl)-5-phenyl-1,3,4-oxadiazole with a substituent in the para-biphenylyl position are reported. The theoretical spectral characteristics of the electronic transitions of the derivatives with strong electro-releasing (-NH2) and strong electron-withdrawing (-NO2) substituents have been calculated by the PPP method. Differences between the spectral characteristics of the phenyl- and biphenyl-substituted PBDs are explained on the basis of the charge transfer character of the first electronic transition. No fluorescence has been observed for the carbonyl, thiocarbonylamide and nitro derivatives. An efficient non-radiative deactivation mechanism involving low energy nπ* states localized on the substituent is assumed for these cases. The planarity of the compounds under study both in their ground and the lowest excited singlet ππ* state is discussed on the basis of the vibrational structure of the absorption and fluorescence spectra and on the basis of quantum chemical calculations.

Fluorinated diphenylpolyenes: Ground- and excited-state geometry studied by absorption and fluorescence spectroscopy

Abstract Low-temperature absorption and fluorescence spectra of all-trans-1,4-diphenyl-1,2,3,4-tetrafluoro-1,3-butadiene (FDPB) and 1,6-diphenyl-1,2,3,4,5,6-hexafluoro-1,3,5-hexatriene (FDPH) were measured in hydrocarbon and 2-methyltetrahydrofuran solvents. The hypsochromic shift of the absorption bands, as compared with their hydrocarbon analogues, indicates a strong non-planarity in the S 0 ground state. Large Stokes shifts (about 6900 cm −1 for FDPB and about 11000 cm −1 for FDPH) depending very slightly on the solvent polarity can be explained by changes in the nuclear geometry when passing from the S 0 to the S 1 state.