Photoinitiators-bond dissociation

A lot of information are currently available, for example, transient absorption spectra, lifetimes of excited states x, interaction rate constants k, intersystem crossing quantum yield <3>jSC, triplet-state energy levels Zsx, dissociation quantum yields of cleavable photoinitiators, bond dissociation energies (BDEs) of amines or thiols used... 

TABLE 10.1 Examples of Available Data Concerning Usual Photoinitiators Triplet State Lifetimes r , Bond Dissociation Energies BDE, Quantum Yields of Dissociation diss> Triplet State Energy Levels, Rate Constants of Interaction kq with Oxygen, Monomer and Hydrogen Donor, Quantum Yields of Intersystem Crossing isc. 

Recently Fouassier and coworkers, presented a general photo-thermal methods for studying both kinetic and thermodinamic properties of the photopolymerization processes. Photoacoustic and thermal lensing spectroscopies allow the determination, of triplet quantum yields and energy levels of photoinitiators. Beyond the possibility of determining easily and accurately bond dissociation energies of coinitiators, the methods provide important information on their reactivity. The application of photoacoustics was extended by Fouassier and coworkers, to the study of the initiation step. A specific data treatment was developed to determine the rate constants and the enthalpy of the reaction of addition of a radical to a monomer unit. 

Although these compounds absorb more strongly in the near-UV than most of the other aromatic photoinitiators, their use as photoinitiators is limited, as they are thermally not very stable. The relatively weak N-O bond dissociates both photochemically and thermally at moderate temperatures. 

Initiation of the reaction of l-chloro-2-naphthoxide anion with NajSO, has been proposed to occur by ET between the excited triplet state of the substrate and its ground state. This reaction can be dye-photoinitiated-" or initiated by visible light with a Ru complex as sensitizer and a Co complex as the intermediate electron carrier.- - For the 1-bromo derivative, photohomolytic CBr bond dissociation is proposed. "... 

Irradiation of a CO-adduct of cytochrome oxidase induces the transfer of a CO molecule, originally bound to an iron(II) center, to copper(I) within 1 ps [108]. Photoinitiated dissociation of CO actually occurs in less than 100 fs, probably on the time scale comparable with a vibrational period of the Fe-CO stretch ( 64 fs). Since the involved reaction centers, Fe(II) and Cu(I), are very close, the mechanism in which the Cu-CO bond begins to form as the Fe-CO bond breaks, seems to be a plausible description of the CO transfer. A similar mechanism is believed to hold for the 02 transfer in biological processes in which cytochrome oxidase participates. 

More recently, the use of picosecond and femtosecond lasers in reaction dynamics opened up the field of femtochemistry, which was pioneered by Zewail [51-54]. The idea of these reactions is to photoinitiate the reactive process in a van der Waals complex. Sometimes, the process that is initiated is a simple dissociation or the isomerization of a free molecule. In each case, the reaction is initiated by a first ultrashort laser pulse (the pump pulse). It is analyzed after a certain delay by a second pulse (the probe pulse). This gives access to the reaction dynamics on the pertinent time-scale where chemical bonds are broken and others are formed. Depending on the system, this typically lasts between a few tenths of femtoseconds to hundredths of picoseconds. Recently the techniques of stereodynamies have been combined by Zewail and co-workers with femtosecond analysis [55, 56] to label specific reaction channels in electron-transfer reactions. 

Free-radical polymerizations can be initiated thermally by thermal initiators, by redox initiators, by photo initiators, or electrolytically. The polymerization process starts with the generation of radicals, high-energy species, which are capable of interacting with the double bond of vinyl, acrylic or olehn monomers. The source of these species is a molecule called the initiator. Thermal initiators dissociate homolytically into two radicals at elevated temperature, usually 60-80°C, whereas redox initiators form radicals by a redox mechanism, normally at lower temperatures than thermal initiators. Photoinitiators form radicals by action of UV light. 

In each system, the primary photo-event is dissociation of the cationic photoinitiator to produce an acid. This reaction proceeds with a quantum efficiency that is characteristic of the particular initiator. The photogenerated acid then interacts with a carefully chosen polymer matrix to initiate a chain reaction, or acts as a catalyst, such that a single molecule of photogenerated acid serves to initiate a cascade of bond making or breaking reactions. The effective quantum efficiency of the overall process is the product of the photolysis reaction efficiency times the length of the chain reaction (or the catalytic chain length). This multiplicative response constitutes... 

After geometry optimization, the transition state is found higher (by 3.5 kcal/mol) than the relaxed triplet state. As a consequence, the cleavage process is thermally activated. The dissociation energy of the C-C bond is computed to be 65.5 kcal/mol. Excitation of HAP quantitatively leads to the triplet state from which the dissociation can occur. The very low energy barrier explains the high value of the reported dissociation quantum yield (0.8). All these properties explain the high efficiency of HAP as a photoinitiator. 

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