Initiators Based on Benzaldoximes: Bimolecular and Covalently Bound Systems.

Typical bimolecular photoinitiators (PIs) for radical polymerization of acrylates show only poor photoreactivity because of the undesired effect of back electron transfer. To overcome this limitation, PIs consisting of a benzaldoxime ester and various sensitizers based on aromatic ketones were introduced. The core of the photoinduced reactivity was established by laser flash photolysis, photo-CIDNP, and EPR experiments at short time scales. According to these results, the primarily formed iminyl radicals are not particularly active. The polymerization is predominantly initiated by C-centered radicals. Photo-DSC experiments show reactivities comparable to that of classical monomolecular type I PIs like Darocur 1173.

Photolysis of 4-chlororesorcinol in water: competitive formation of a singlet ketene and a triplet carbene.

Research on photoinduced reactions of halogenated phenols is of interest for environmental photochemistry and for synthetic organic chemistry. Previous studies have uncovered interesting mechanistic features, including ring contraction from benzene to cyclopentadiene from ortho derivatives and two-electron processes forming carbocations and carbenes from para derivatives. In the present work, we studied the aqueous photochemistry of 4-chlororesorcinol (1), which combines the conformational properties of both types of derivatives, using nanosecond transient absorption spectroscopy and photoproduct analysis. The absorption spectra obtained upon pulsed laser excitation of 1 showed the occurrence of both o-Cl and p-Cl elimination, the first observed transients being the ketene 3-hydroxy-6-fulvenone (2, λ(max) = 255 nm) and the carbene 2-hydroxy-4-oxo-2,5-cyclohexadienylidene (3, λ(max) = 405 and 395 nm). The reactivities of 2 and 3, the spectra of the secondary transients and the analysis of the final products showed that the two HCl elimination pathways take place concurrently. Most probably, the bifurcation step is the competition between intersystem crossing on the molecular level and o-Cl elimination on the singlet surface; p-Cl elimination proceeds on the triplet surface. Remarkably, the quantum yield of p-Cl elimination from 1 is lower by one order of magnitude compared to that found in para-halogenated phenols, while that of o-Cl elimination from 1 is comparable to ortho-halogenated phenol. To explain this result, we propose that o-Cl elimination is the major deactivation step, forming an intermediate singlet cation which is able to recombine to ground state 1, thereby limiting the observed photochemical quantum yields.

Photoreactivity of psoralen derivatives in micelles: the roles of hydrophobicity and heavy atom substitution.

Intersystem crossing quantum yields of two psoralen derivatives, bromopsoralen (BrMOP) and hexylpsoralen (8-HOP) were measured in ethanol and water by means of laser flash photolysis. Compared to 8-methoxypsoralen (8-MOP), the triplet quantum yields of BrMOP and 8-HOP in ethanol are increased by a factor of 5 and 30, respectively, while BrMOP in aqueous solution shows a twofold enhancement with respect to 8-MOP. Radical cations and hydrated electrons were generated by photoionization in micellar solution upon excitation at 266nm. A nonlinear relationship between transient yield and photon fluence was obtained for each compound, indicating that a two-photon mechanism is predominant in the photoionization of the sensitizers. The photoionization efficiencies are significantly higher in anionic sodium dodecyl sulfate (SDS) than in cationic cetyltrimethylammonium bromide (CTAB) micelles, reflecting the influence of micelle charge on the efficiency of the separation of the photoproduced charge carriers. The photoionization efficiencies of 8-HOP and 8-MOP are similar. It is concluded that the intersystem crossing properties of BrMOP show a substantial heavy atom effect, whereas the effects of hydrophobic substitution on photoionization efficiency are minor.

Revealing phenylium, phenonium, vinylenephenonium, and benzenium ions in solution.

The 4-methoxyphenylium ion has been generated in the triplet state ((3)An(+)) by photolysis of 4-chloroanisole in polar media and detected by flash photolysis (lambda(max)=400 nm). This is the first detection of a phenylium ion in solution by flash photolysis and the assignment is supported by time-dependent density functional theory (TD-DFT) calculations. In neat solvents, the cation was reduced to anisole, a process initiated by electron transfer from the starting compound ((3)An(+)+AnCl-->An(*)+AnCl(*+), with the radical cation detected at 470 nm, then An(*)-->AnH). Addition of pi nucleophiles to the (3)An(+) cation offers a novel access to a number of other cationic intermediates under mild, nonacidic conditions. Two intermediates are successively formed with alkenes, a diradical cation and the phenonium ion, which are detected at 440 and 320 nm, respectively, by flash photolysis and are in accordance with calculations. Allylanisoles or beta-alkoxyalkylanisoles are the end products, with a small amount of alpha-alkoxyalkylanisoles that arises from a Wagner-Meerwein rearrangement to form benzyl cations. Further intermediates that have been predicted and detected are the phenylvinylium ion, possibly in equilibrium with the vinylenephenonium ion, with 1-hexyne (lambda(max)=340 nm) and the benzenium ion with benzene (lambda(max)=380 nm). The final products were anisylhexyne and methoxybiphenyl (an analogous product and intermediate were detected with thiophene).

Efficient singlet-state deactivation of cyano-substituted indolines in protic solvents via CN--HO hydrogen bonds.

The photophysical properties of indoline (I) and three of its derivatives, namely, N-methylindoline (MI), 5-cyanoindoline (CI), and 5-cyano-N-methylindoline (CMI), are studied in H-donating solvents of varying polarity. Based on measurements of fluorescence yield and lifetime, and of triplet yield and hydrated-electron formation, two distinct mechanisms of solvent-induced fluorescence quenching are evidenced. The first mechanism involves the cyano substituent and leads to an increase in the rate constant of internal conversion of one order of magnitude in ethanolic solution and of more than two orders of magnitude in water, as compared to solutions in n-hexane or acetonitrile. A similar trend had previously been observed in the case of 4-N,N-dimethylaminobenzonitrile (DMABN). The second mechanism reduces the fluorescence lifetimes of the non-cyanated derivatives in aqueous solution by one order of magnitude and is related to the formation of hydrated electrons. Neither of these mechanisms is influenced by methylation at the ring nitrogen. Quantum chemical calculations are performed on the ground and excited states of the hydrogen-bonded complexes between protic solvents and MI as well as CMI. Stable hydrogen-bonded configurations involving the CN substituent and a solvent OH group are found; these configurations are stable both in the ground and the first excited singlet states, whereas the corresponding complex at the ring amino nitrogen is stable in the ground state only. The CN--HO configuration is therefore a prime candidate for a mechanistic explanation of the observed quenching by the first mechanism. These findings may have useful applications for the design of fluorescence probes for water in biological systems.

Ultraviolet photoionization of the photosensitizers khellin and visnagin in aqueous solution and in micelles: one-photon ionization is a minor process.

One-photon ionization, leading to formation of hydrated electrons and radical cations, has been proposed as a possible mechanism of action of some sensitizers in photobiology. In this contribution, we have investigated this proposal for the compounds khellin and visnagin, used in photomedical applications. Nanosecond transient absorption spectroscopy covering a wide range of laser pulse energies was employed to measure the formation of radical cations and hydrated electrons in aqueous solution and in cationic (CTAB) as well as anionic (SDS) micellar solutions. A model allowing for simultaneous one- and two-photon processes and fully accounting for the nonlinearity of the pulse energy dependence was used to simulate the data. The results did not support the hypothesis of a significant role of one-photon ionization, the upper limits of the quantum yields of radical cation formation being phi < 0.01 for visnagin and phi < 0.004 for khellin.

G-protein-coupled glucocorticoid receptors on the pituitary cell membrane.

Rapid, nongenomic actions of glucocorticoids (GCs) have been well documented, but information about putative membrane receptors that mediate them is scarce. We used fluorescence correlation spectroscopy to search for membrane GC-binding on the mouse pituitary cell line AtT-20. A slowly diffusing fraction (tau3; diffusion constant 3x10(-10) cm2 s-1) of fluorescein-labeled dexamethasone on the cell membrane corresponds to fluorescein-dexamethasone binding. Preincubation experiments were performed to test binding specificity: a 500-fold excess of unlabeled dexamethasone abolished subsequent fluorescein-dexamethasone membrane binding from 58+/-2 (control) to 8+/-1 (% of tau3, mean+/-s.e.m.), the natural ligand corticosterone prevented it partially (29+/-2), while the sex steroids estradiol (56+/-4) and progesterone (50+/-4) and the GC-receptor antagonist RU486 (56+/-2) had no effect. Preincubation with pertussis toxin resulted in disappearance of the slowest diffusion component (11+/-4) suggesting association of the receptor with a G-protein. Varying the concentration of fluorescein-dexamethasone showed that membrane binding is highly cooperative with an apparent Kd of 180 nM and Bmax of 230 nM. Taken together, these results demonstrate high-affinity GC-binding on the cell membrane of AtT-20 cells with characteristics distinct from intracellular binding.

Antioxidant properties of natural and synthetic chromanol derivatives: study by fast kinetics and electron spin resonance spectroscopy.

[structure: see text] Chromanol-type compounds act as antioxidants in biological systems by reduction of oxygen-centered radicals. Their efficiency is determined by the reaction rate constants for the primary antioxidative reaction as well as for disproportionation and recycling reactions of the antioxidant-derived radicals. We studied the reaction kinetics of three novel chromanols: cis- and trans-oxachromanol and the dimeric twin-chromanol, as well as ubichromanol and ubichromenol, in comparison to alpha-tocopherol and pentamethylchromanol. The antioxidant-derived radicals were identified by optical and electron spin resonance spectroscopy (ESR). The kinetics of the primary antioxidative reaction and the disproportionation of the chromanoxyl radicals were assessed by stopped-flow photometry in different organic solvents to simulate the different polarities associated with biomembranes. Furthermore, the reduction of the chromanoxyl radicals by ubiquinol and ascorbate was measured after laser-induced one-electron chromanol oxidation in ethanol and in a micellar system, respectively. The rate constants showed that twin-chromanol had better radical scavenging properties than alpha-tocopherol and a significantly slower disproportionation rate of its corresponding chromanoxyl radical. In addition, the radical derived from twin-chromanol is reduced by ubiquinol and ascorbate at a faster rate than the tocopheroxyl radical. Finally, twin-chromanol can deliver twice as many reducing equivalents, which makes this compound a promising new candidate as artificial antioxidant in biological systems.

Degradation of supramolecular complexes photoinduced by iron(III): the influence of the host on the reactivity of the guest.

Fe(III)-photoinduced degradation of two different supramolecular host-guest complexes, naphthalene with alpha-cyclodextrin and 4-chloroaniline with p-sulfonic-calix[4]arene; was studied in aqueous solution. The stoichiometries, equilibrium constants and conformations of these complexes were characterized. In both cases, the degradation of the guest could not be accounted for by simple competition kinetics of OH radical attack on the guest and host molecules, respectively. This observation was explained by the occurrence of subsequent radical reactions of transients resulting from an OH attack onto the host molecule, which are assumed to be able to induce additional degradation reactions of the guest. An inhibiting influence of the calix[4]arene host on the abundance of oligomeric degradation intermediates of 4-chloroaniline was observed.

Microenvironmental effects in the excited state properties of p-dimethylaminobenzonitrile complexed to alpha- and beta-cyclodextrin.

The steady state and time resolved fluorescence and the triplet-triplet absorption of p-dimethylaminobenzonitrile (DMABN) in presence of alpha- and beta-cyclodextrin (CD) were investigated at various host and guest concentrations and temperatures. The formation of 1:1 and 1:2 DMABN:alpha-CD and 1:1 and 2:2 complexes DMABN:beta-CD complexes was ascertained by applying global analysis methods. The "pure" fluorescence spectra as well as the emission quantum yields and lifetimes and the triplet properties of the various associates were determined. The role of environmental features in the radiative and non-radiative deactivation of the LE and ICT excited states of the complexed DMABN was elucidated.