Photoacids formation

Products from the photolysis of the cyclopropyl ketone (44) are dependent on the pH of the solvent.(42) In aqueous dioxane only the 2,3-diphenyl-phenol (45) is formed along with the photoacid (46). Bond cleavage at c takes place via both the singlet and triplet states, whereas bond cleavage at a with phenol formation takes place in the triplet state ... 

A second and more recent example, the photochemical rearrangement of 4,4-diphenylcyclohexadienone (VIII), was provided by the present author and co-workers (4, 5,14). This compound (VIII) when photolyzed in aqueous dioxane with light of wavelength above 310 mp. was found (4, 5) to afford the bicyclic ketone IX, 2,3-diphenylphenol (X) and an acid whose structure was shown (14) to correspond to XI. Additionally, 3,4-diphenylphenol (XII) was shown (14) to be a minor by-product. Strikingly and reminiscent of the dependence of product distribution on solvent in santonin photolysis, it was found (14) that approximately equal quantities of 3,4-diphenylphenol and 2,3-diphenylphenol (X) were formed when the photolysis was run in 50% aqueous acetic acid. [Control experiments (14) demonstrated that neither 4,4-diphenylcyclohexadienone nor bicyclic ketone IX were reactive in the dark under the aqueous dioxane or aqueous acetic acid reaction conditions, in the presence or absence of acid XI.] Furthermore, the bicyclic ketone IX has been demonstrated to afford 2,3-diphenylphenol (X) and the photoacid XI on photolysis in aqueous dioxane, and consequently this ketone may be formulated as a reaction intermediate in the formation of X and XI from 4,4-diphenylcyclohexadienone (VIII) (4, 5, 14). 

The UV photochemistry of phenol and related systems (such as indole, pyrrole, imidazole) is dominated by a hydrogen detachment reaction which is driven by repulsive 1ira states [33,35 10], For the isolated chromophores, the 1 mr -driven photodissociation has been explored in unprecedented detail by high-resolution photofragment translational spectroscopy [40], The OH (or NH) bond is broken homolytically, resulting in the formation of two radical species, the hydrogen atom and the phenoxy (or indolyl, etc.) radical. Ion pair formation (abstraction of protons) is energetically not feasible for isolated photoacids. 

The discovery of photoacidity was made by Forster more then 50 years ago . Forster correctly explained the unusual large Stokes shift found in the fluorescence of several classes of aromatic dyes, including 1- and 2-naphthol derivatives as an indication of excited state proton-transfer reaction which results in the formation of the molecular anion still in the excited state. Thus, it become clear that excited-state proton transfer may compete with other radiative and non-radiative decay routes of the photoacid. The main modern-day importance of photoacids lies in their ability to initiate and then to follow acid-base reactions so they may be regarded as optical probes for the study of general proton-transfer reactions. 

This stage may involve some further electronic rearrangement in the photoacid toward the formation of the photobase. 

One approach to provide resists with high sensitivity and contrast involves the principle of chemical amplification (CA) (6-8), The amplification effect is achieved by employing a photo generated acid as a catalyst to carry out a cascade of chemical reactions in the resist film. Catalytic chain lengths of >1000 have been reported in the literature (9,10), However, the high catalytic chain lengths also enhance the resist s sensitivity to airborne basic contaminants and basic moieties on the substrate (11,12), In a positive tone resist such deactivation of the photoacid by airborne or substrate bound basic contaminants results in T-tops or foot formation respectively. 

Figure 5 shows the effect of incorporation of hydrophilic units such as amide or ether units into the photoacid generating polymers on the polysiloxane formation rate at the irradiated film surface. NIS unit fractions of 1, 2b, 3 and 6 were 0.17, 0.24,... 

Plate 2. Image formation (upon photoacid generation) within photosensitive polymer films for assembly of multi-layered structures. Two-photon fluorescence LSCM imaging usingfs pulsed near-IR pump allows for 3-D volumetric imaging of the layered structure. 

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