Mónica Barra

PHOTOINDUCED TRANSIENT PHENOMENA IN CYCLODEXTRIN SOLID COMPLEXES: PHOTOCHEMISTRY OF AROMATIC KETONES

Abstract—

Study of xanthone-cyclodextrin inclusion complexes in the solid state using time-resolved diffuse reflectance-laser flash photolysis

The photochemistry of xanthone complexed with cyclodextrins (α‐, β‐ and γ‐CD) in the solid state has been examined by diffuse reflectance laser flash photolysis. The first half‐life for xanthone triplet decreases as the size of the CD cavity increases; long‐lived ketyl radicals are observed in the cases of β‐ and γ‐CD. These changes in photobehavior are attributed to the restrictions on the mobility of included xanthone imposed by the CD structure.

Excited triplet states as probes in organized systems. An overview of recent results

Abstract Triplet molecules have been employed as probes in organized systems in solution and in the solid state. This article present an account of our recent work in this area. In particular, these probes are very useful when studying processes involving relatively slow mobility, where the short lifetime of singlet states would make fluorescence a poor monitoring technique. Among the molecules examined, xanthone and 4,4′-dimethylbenzophenone have proven useful in cyclodextrin complexes, leading to information on polarity, mobility and excited state reactivity. Other triplet probes based on the β-phenylpropiophenone moiety yield information on intracavity mobility, inclusion geometries and generally on the importance of steric factors in the formation and stability of cyclodextrin complexes. Finally, delayed fluorescence form pyrene derivatives has been employed as a probe for mobility and exchange in reversed micelles.

Kinetic Studies on the Thermal cis-to-trans Isomerization of 1-Phenyltriazenes Derived from Cyclic Amines

Rate constants for thermal cis-to-trans isomerization of N-(phenylazo)-substituted nitrogen ring heterocyles were determined as a function of phenyl ring substitution, cyclic amine ring size, and organic solvents. Observed first-order rate constants are found to increase with increasing electron-withdrawing character of the para substituent, with larger amine rings, and with increasing solvent polarity. Overall, trends observed are consistent with geometrical isomerization taking place through rotation around the nitrogen-nitrogen double bond via a polarized transition state.

Diffuse Reflectance Laser-flash Photolytic Study of Aromatic Ketones within Calixarene Solid Matrices

Abstract The photochemistry of benzophenone and xanthone within calix[6]arene solid matrices is dominated by their triplet excited states. Triplet decays within hexa-t‐butyl-37,38,39,40,41,42-hexaethoxycalix[6]arene host matrices are remarkably simple; while benzophenone triplet decays can be adequately described by a two-exponential function, decays for xanthone (a less flexible ketone than benzophenone) are very well reproduced by a single discrete lifetime model.

Application of diffuse reflectance laser-flash photolysis to the study of aromatic probes in β-cyclodextrin solid complexes

Time-resolved diffuse reflectance laser-flash photolysis techniques were employed to study the photochemistry of benzil, naphthalene, and pyrene within β-cyclodextrin solid complexes. The photochemistry of benzil and naphthalene is dominated by their triplet state. In the case of pyrene, the formation of radical cations and radical anions is observed, in addition to the signals due to the triplet state. In all three cases triplet excited states exhibit non-exponential (multi-component) kinetics. Analysis of triplet decays by means of lifetime distributions shows that quenching by oxygen is significantly restricted within β-cyclodextrin solid complexes. This result is attributed to the protection offered by the cyclodextrin cavity, which limits the space available for diffusion and contact of reactants.

Deuterium Isotope Effect on the Complexation of β-Cyclodextrin and Triplet Xanthone in Aqueous Solution

Abstract The replacement of H2O by D2O as solvent leads to a significant decrease in the equilibrium constant for complexation of excited triplet xanthone with β-cyclodextrin, mainly as the result of the decrease in the corresponding association rate constant.

Photochemical studies of atomic species (F, Br, O) in solution

Abstract The reactions of fluorine, bromine and oxygen atoms in solution have been examined using nanosecond laser flash photolysis techniques. In each system it was necessary to identify a suitable precursor (XeF2 for fluorine, vicinal dibromides for bromine, and pyridine-N-oxide for oxygen) and a convenient probe species having a readily observable absorption band in the UV—vis region. The kinetics of the debromination of β-bromoalkyl radicals have been examined for silicon-substituted radicals.

Kinetic studies on the proton-catalysed hydrolytic decomposition of quinone-imine dyes

Abstract Kinetic studies on the hydrolytic decomposition of 4-[[4-(dimethylamino)phenyl]imino]-2,5-cyclohexadien-1-one [phenol blue (PB)], 4-[(4-hydroxyphenyl)imino]-2,5-cyclohexadien-1-one [indophenol (IP)], and 4-[[4-(dimethylamino)phenyl]imino]-naphthalen-1-one [indophenol blue, (IPB)] in HCl solutions are consistent with nucleophilic addition of water to the protonated imine functionality. The reactivity increases in the order PB∼IP

Erythromycin as a supramolecular receptor

Abstract When Erythromycin A (E) is added to a solution of Rhodamine B base (Z) dissolved in chloroform, dioxane or DMSO the color disappears immediately and this effect is attributed to the formation of a Host-Guest complex between Z and E followed by a fast lactonization of Z.