Photoacids reactivity

Structure (1-100 (itn). An acidreactive modification of the polymer blend film converted part of the organic template to a sUica network through exposure to precursor vapors. The impact of the polymer blend composition on the structure of the porous silica films was examined with the stmcture of the porous silica films determined using X-ray diffraction (XRD), SEM, transmission electron microscopy (TEM), spectroscopic ellipsometry (SE), and ellipsometric porosimetry (EP). 

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). 

Since the transfer of protons is truly contact, this reaction is best suited for comparison with the contact and spinless theory given above. However, the authors who first monitored it in the time domain tried to fit the fluorescence signal as a biexponential one [59]. The similar reaction but of a more reactive photoacid (2-naphthol-6-sulfonate) with an acetate anion has been studied, and its kinetics, which is neither exponential nor biexponential, was fitted to the tme theory of contact quenching [60]. It is especially important that the fluorescence... 

V. FREE-ENERGY CORRELATIONS BETWEEN PHOTOACIDITY AND REACTIVITY... 

Pines, Fleming and coworkers have utilized a free-energy correlation between the excited-state equilibrium constant of the photoacid and the proton dissociation rate . Such correlations are extensions of similar correlations existing between the equilibrium constant and reactivity of ground-state acids (the Br0nsted relation ). 

More direct evidence for the inherent microscopic reversibility of an excited-state proton transfer reaction was found in ps-time-resolved measurements of a strongly reactive photoacid, namely HPTS (Fig. 12.2). With its conjugated-base, fourfold charged, the observation of the back (geminate) recombination of the pro-... 

Eq. (12.18) for the correlation between the proton transfer rate, kp and (the dependence on enters through the free energy of activation term AG see below) usually result in very good agreement between the observed reactivity of the photoacid and its Forster cycle value. 

It follows that one may discuss the effect of various types of substituents on photoacidity using arguments and terminology that have been traditionally used for ground state acids. In particular, Hammett [127,128] and Taft [129,131] have contributed much to the discussion of the substituent effect on equilibrium and reactivity of aromatic acids in the ground electronic state. Their arguments seem to be valid also for the excited state of aromatic acids but with different scaling factors (i.e., different values in the Hammett Equation) [24]. 

The properties of the photoacid can be readily modified by varying the coim-terion in these salts, generally accomplished through a metathesis reaction. This exchange usually has no significant impact on the photochemical reactivity of the... 

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