Bronislaw Marciniak

Photochemical properties of 1,3-diketonate transition metal chelates

Abstract The latest results concerning the photochemistry and sensitized photoreduction of transition metal 1,3-diketonates are examined. The physical and chemical aspects of the interaction mechanisms between excited singlet and triplet states of organic compounds and 1,3-diketonates are discussed. Some examples of the uses of transition metal 1,3-diketonates in photocatalysis are presented.

Quenching of the excited singlet state of acridine and 10-methylacridinium cation by thio-organic compounds in aqueous solution

In this work, the lowest excited singlet states of acridine (Acr), acridinium (AcrH + ) and 10-methylacridinium (AcrMe + ) are quenched by sulfur-containing amino acids and carboxylic acids in aqueous solution. Both steady-state and time-resolved fluorescence techniques were used to monitor the quenching of fluorescence. Stern–Volmer plots of the fluorescence intensity showed a static component ( KS )t o the quenching. The experimental KS values were compared to theoretical KS values for outer-sphere complexes based on Debye–Huckel theory and the Fuoss equation. The general agreement between experimental and theoretical KS values indicate that the static quenching can be attributed to non-fluorescing ion pairs associated as simple outer-sphere complexes. The computed values of the interionic distances of the ion pairs are consistent with the ion pairs of the ZAZQ =− 1 and −2 cases being solvent-separated ion pairs while those of the ZAZQ =− 3 case are contact ion pairs. The effect of the reactants’ charges on the quenching rate constants (dynamic component) was observed for the reactions of AcrMe + with the anionic forms of the quenchers (having charges ZQ =− 1, −2 and −3). The rate constants (extrapolated to ionic strength, µ = 0) for the quenching processes were determined to be 0.3–5.3 × 10 10 M −1 s −1 depending on the ionic charge (ZQ) of the quencher used. These trends in the quenching rate constants are rationalized with a quenching scheme for electron transfer. Analogous quenching rate constants for alanine and glycine were found to be at least an order of magnitude lower. Photoinduced electron transfer from the sulfur atom of the quencher molecule to the acridine excited singlet state is suggested to be the most likely mechanism of the process under discussion.

Spectroscopic and semiempirical studies of gossypol complexes with Fe2+ and Fe3+ cations

Abstract The complexes of gossypol with Fe2+ and Fe3+ cations were studied in acetonitrile by UV–Vis and FT-IR spectroscopy. Very complex equilibria and structural changes were observed for various ratios of gossypol–iron cation mixtures. With the formation of complexes, the ketol–ketol tautomer form of gossypol was favored. With increasing concentration of iron cation in the mixture, the deprotonation of hydroxyl groups was indicated. The semiempirical studies show that the most stable complexes of gossypol with iron cations are 1:4, 1:1 and 2:1. The structures of these complexes are discussed.

1H AND 13C NMR Studies of Tetrabutylammonium Salts of Gossypol in Chloroform Solution

Abstract Gossypol and its two 2:1 and 4:1 tetrabutylammonium salts were investigated by 1H and 13C NMR spectroscopy in CDCl3 solution. In this solution gossypol exists as a symmetrical aldehyde-aldehyde tautomer. In the case of the two 2:1 and 4:1 tetrabutylammonium salts of gossypol in chloroform the anhydrogossypol form was observed, instead of the ketol form which was determined in CD3 CN solution.

The tautomerization of gossypol as a function of the presence of Ni2+, Cu2+ or Zn2+ cations

Abstract 1 : 1 Complexes of gossypol with Ni(AuCl 4 ) 2 , Cu(AuCl 4 ) 2 and Zn(AuCl 4 ) 2 were studied in acetonitrile using FT-IR spectroscopy. With this complexation the tautomeric equilibrium of gossypol is completely shifted from the aldehyde-aldehyde to the lactol-lactol tautomer in the case of Ni 2+ and Cu 2+ complexes, and to the ketol-ketol tautomer with the Zn 2+ complex. The nature of the interaction of these ions with gossypol, especially that of the Zn 2+ ion, is explained in detail. Furthermore, all changes concerning hydrogen bond formation and the reasons for it are discussed.

Photo-oxidation of Methionine-containing Peptides by the 4-Carboxybenzophenone Triplet State in Aqueous Solution. Competition Between Intramolecular Two-centered Three-electron Bonded (S∴S)+ and (S∴N)+ Formation¶

Abstract Quantum yields for the formation of transients were measured following the quenching of triplet 4-carboxybenzophenone (3CB*) by methionine-containing peptides in aqueous solutions. Ketyl radicals (CBH·), ketyl radical anions (CB·−) and various sulfur radical cations were identified following the triplet-quenching events. The presence of these intermediates indicated that the triplet-quenching mechanism can be characterized as mainly electron-transfer in nature. The quenching rate constants were of the order of 2 × 109 M−1 s−1. There were small, but significant, differences in the triplet-quenching rate constants, and these trends indicate the existence of multiple sulfur targets in the quenchers. The absorption of the transient products was followed in detail by using spectral-resolution analysis. From the absorption data, quantum yields were estimated for the formation of the various transients. There were differences found in the yields of the transient products between the experiments, where the quenchers were the “mixed” stereoisomers of methionylmethionine (l,d and d,l) and experiments where the quenchers were l,l and d,d stereoisomers. Triplet-quenching data from several other methionine-containing small oligopeptides were analyzed in an analogous manner. Systematic variations were observed, and these patterns were discussed in terms of competitive donation of protons to the CB·− within the charge-transfer complex. The competition was between protons on carbons adjacent to the sulfur-radical center and protons on the protonated amino groups of the radical cation. In addition, there was a competition between the two intramolecular two-centered, three-electron bonded species (S∴S)+ and (S∴N)+ that play roles in the secondary kinetics.

A reevaluation of the photolytic properties of 2-hydroxybenzophenone-based UV sunscreens: are chemical sunscreens inoffensive?

The excited states of a set of popular sunscreen agents (2-hydroxybenzophenone, oxybenzone, and sulisobenzone) are studied by using femto- and nanosecond time-resolved spectroscopy. Upon excitation, the compounds undergo an ultrafast excited-state intramolecular proton transfer (ESIPT) reaction as the major energy-wasting process and the rate constant of this reaction is k=2×10(12) s(-1) . The ESIPT yields a keto conformer that undergoes a fast, picosecond internal conversion decay. However, a photodegradative pathway is a monophotonic HO bond breakage that subsequently leads to trace yields of phenoxyl radicals. Because potentially harmful phenoxyl radicals are formed upon irradiation of sunscreen agents, care should be taken about their reactivity towards biologically relevant compounds.

Photochemical studies of a photodissociative initiator based on a benzophenone derivative possessing a thioether moiety

Abstract S -(4-Benzoyl)phenylthiobenzoate (BpSBz) and 4-benzoyl-phenyl benzoate (BpOBz) in acetonitrile were photolyzed under steady-state and laser-flash conditions. Analysis of transients absorptions and final products from the photolysis of BpSBz showed that 4-benzoylphenylthiyl and 4-benzoyl radicals were formed with an initial quantum yield of 0.45. Benzaldehyde was the main final product. In contrast, photolysis of BpOBz did not yield benzaldehyde, and nanosecond laser-flash photolysis of BpOBz showed only a long-lived triplet state. These results suggest that CO bond cleavage does not occur as a primary photochemical reaction of BpOBz. The implications for the use of BpSBz as a photoinitiator of polymerization are discussed.

Photoreduction of benzophenone by 2,4,6-trimethyl-1,3,5-trithiane in solution. Laser flash photolysis study

Abstract The role of 2,4,6-trimethyl-1,3,5-trithiane (TMT) in accelerating the initiation of polymerization induced by benzophenone (BP) was pursued by studying the primary photochemical reactions occurring between BP and TMT. Kinetic aspects of the quenching of the BP triplet by TMT were followed by laser flash photolysis. Triplet quenching rate constants and quantum yields for electron-transfer products were measured. In addition the mechanism of the photoreduction of BP by organic sulfides in acetonitrile and mixed acetonitrile/water solutions is discussed. Photoinitiated polymerizations with a model monomer were carried out with some of the sulfide co-initiators investigated, and the polymerization behavior was correlated with the kinetic results.

CIDNP Spectroscopic Observation of (S:.+N) Radical Cations with a Two‐Center Three‐Electron Bond During the Photooxidation of Methionine

Cyclic radical cations Met-(S…︁█+ N) of methionine could play an important role in processes such as long-range electron transfer across cell membranes and oxidative damage to cells. CIDNP spectroscopy (CIDNP = chemically induced dynamic nuclear polarization) furnishes direct structural proof and allows experimental investigation of the spin-density distribution in the two-center three-electron bonds.