RESUMEN
A novel straightforward approach has been proposed to generate in situ, under light activation and in aerated media, visible-light absorbing and well-defined titanium-based nanoparticles (NPs) in solution and in an epoxide matrix using titanium derivative complexes/iodonium salt photoinitiating systems. The nature of the solvent and oxygen plays a decisive role, and two mechanisms involved in these syntheses are operative, i.e. a photofragmentation/addition process (in toluene and isopropanol) and a photoinduced sol-gel reaction (in isopropanol).
RESUMEN
The reactivity of 11 aminoalkyl radicals toward different additives [oxygen, 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), and methyl acrylate (MA)] has been investigated through laser flash photolysis and quantum mechanical calculations. The transient absorption spectra of the radicals were recorded: good agreement was found with the spectra calculated by using quantum mechanical calculations. All the interaction rate constants were measured. A large range of values are obtained: (0.04-3) x 10(9) M(-1) s(-1) for O2, (0.002-5) x 10(8) M(-1) s(-1) for TEMPO, and (<0.004-2) x 10(7) M(-1) s(-1) for MA. Generation of the decarboxylated aminoalkyl radical derived from N-phenylglycine was unambiguously demonstrated. It was clearly found that both the addition to oxygen and the recombination with TEMPO were strongly governed by the reaction exothermicity. On the other hand, both polar and enthalpy factors have a large influence on the rate constants of the addition reaction to the acrylate unit, which were ranging over at least 4 orders of magnitude. This paper provides a set of new data to characterize the structure/reactivity relationships of aminoalkyl radicals.
RESUMEN
The formation and the reactivity of three selected sulfur-centered radicals formed from mercaptobenzoxazole, mercaptobenzimidazole, and mercaptobenzothiazole toward four double bonds (methyl acrylate, acrylonitrile, vinyl ether, and vinyl acetate) are investigated. The reversibility of the addition/fragmentation reaction in these widely used photoinitiating systems of radical polymerization was studied, for the first time, through the measurement of the corresponding rate constants by time-resolved laser spectroscopy. The combination of these results with quantum mechanical calculations clearly evidences that, contrary to previous studies on other aryl thiyl radicals, the addition rate constants (ka) are governed here by the polar effects associated with the very high electrophilic character of these radicals. However, interestingly, the back-fragmentation reaction (k-a) is mainly influenced by the enthalpy effects as supported by the relationship between the rate constants and the addition reaction enthalpy DeltaHR. The addition and fragmentation rate constants calculated from the transition state theory (TST) are in satisfactory agreement with the experimental ones. Therefore, molecular orbital (MO) calculations offered new opportunities for a better understanding of the sulfur-centered radical reactivity.
RESUMEN
The photophysical parameters controlling the cleavage process of 2-hydroxy-2,2-dimethylacetophenone (HDMA) were investigated in detail. Time-resolved picosecond absorption experiments show that the formation of the triplet state occurs within 20 ps after excitation and decays within hundreds of picoseconds depending on the solvent polarity. Molecular modeling reveals that three stable conformations exist in the ground state, the most stable one exhibiting an intramolecular hydrogen bond that modifies the electronic properties of the molecule. This explains quite well the steady-state absorption properties. The conformation of the most stable triplet state is twisted by 180 degrees with respect to the ground state. Computation of the potential energy surface along the molecular coordinate for the dissociation reaction evidences an electronic state crossing yielding a final sigmasigma* state, in perfect agreement with the state correlation diagram. Optimization of the transition state allows the calculation of the activation energy and the use of the transition-state theory leads to an estimate of 100 ps for the cleavage process in the gas phase. Single-point energy calculations using a solvent model predict an increase of the dissociation rate constant with the increase of the solvent polarity, in good agreement with the value deduced from kinetic measurements.
RESUMEN
The four component system consisting of bis[2-(o-chlorophenyl)-4,5-diphenylimidazole] (Cl-HABI), N,N'-bis(diethylamino)benzophenone (EAB), N-phenylglycine (NPG) and an iodonium salt (BIP-T) can be used for the photoinitiation of radical polymerization reactions. The excited state processes involved are reported and all the transient species, including the triplet state and ketyl radical of EAB as well as the lophyl radical, are characterized. Rate constants of interaction between the different partners in the four component system are determined. Redox potentials of the different compounds are evaluated enabling the free energies for electron transfer reactions to be calculated. An overall scheme for the evolution of the excited states and a general discussion on the role played by the different components in photoinitiating polymerization are then provided.
RESUMEN
A new process that permits the recording of holograms produced by laser sources emitting in the near infrared is presented. The recording process involves the sensitization of singlet oxygen and the oxidation of an adapted trap. Finally, a second photon initiates a photopolymerization, which induces changes in refractive index and creates the hologram.