A series of Ag-TiO 2 photocatalysts were obtained in microemulsion system (water/AOT/cyclohexane), using several Ag precursor amount ranging from 1.5 to 8.5 mol.%. The photocatalysts' characteristics by X-ray diffraction, STEM microscopy, UV-Vis spectroscopy, X-ray photoelectron spectroscopy, BET methods showed that a sample with the highest photo-and bioactivity had anatase structure, about 90 m 2 /g specific surface area, absorbed light for λ>400 nm and contained 1.64 at.% of silver (0.30 at.% of Ag 0 and 1.34 at.% of Ag 2 O) and about 13 at.% of carbon in the surface layer.The photocatalytic activity of the catalysts was estimated by measuring the decomposition rate of phenol in 0.21 mM aqueous solution under visible and ultraviolet light irradiation.The bioactivity of silver-doped titanium dioxide nanocomposites was estimated using
Photophysical studies of a series of over 20 compounds based on pyrrole, indole, and carbazole chromophores were carried out in protic, nonpolar, and polar solvents. Absorption spectra revealed the formation of groundstate hydrogen bonding with protic solvent partners. The equilibrium constants were determined by spectrophotometric titration. Strong fluorescence quenching was observed when azaaromatic proton acceptors, pyridine and quinoline, were used as protic solvents. No quenching occurs for nonaromatic protic partners such as dimethyl sulfoxide, morpholine, and piperidine, even though the ground-state equilibrium constants are not smaller. The rates of quenching in pyridine solutions at 293 K span a range from 1.2 × 10 9 to 5.9 × 10 10 s -1 and are sensitive to minor structural variations. The mechanism of quenching involves an electron transfer from a photoexcited chromophore to a hydrogen-bonded partner, followed by a rapid internal conversion (back electron transfer) to the ground state. The quenching rates are larger for systems with stronger hydrogen bonds. This model was confirmed by theoretical time-dependent DFT and semiempirical studies, in which the pattern of excited states of an isolated chromophore was compared with that of a hydrogen-bonded pyridine complex. For the latter, low-lying charge transfer (CT) states were predicted, with an electron transferred to pyridine.
A series of heteroazaaromatic molecules possessing both a hydrogen bond donor and acceptor groups linked by a single bond was studied using electronic absorption and fluorescence techniques, combined with quantum--chemical calculations. Fluorescence lifetimes and quantum yields strongly depend on the solvent. Double exponential decays are observed in alcohols. This was interpreted in terms of alcohol-induced syn-anti rotamerization in the ground electronic state. The driving force for the conformational change is provided by two factors: (a) solvent polarity increase; (b) the formation of two separate hydrogen bonds in the anti species. In the syn structure, formation of cyclic hydrogen bonds leads to rapid depopulation of the lowest excited singlet state via two competing mechanisms: proton transfer and enhanced internal conversion. The experimental results are rationalized by calculations which include specific and nonspecific solvent effects.
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