Excited States and Acidity Scales

For the dissociation of any acid BH+, pK can be represented formally by expression (49), where aH+ represents the hydrogen-ion 

It is well known that outside the pH range almost all predeterminations for unexcited molecules have been based on the Hammett indicator method (Hammett and Deyrup, 1932). Even in concentrated acid (or alkaline) solutions, where uncertainties in the value of Qh+/b//b h+ become very serious, it is easy to measure the ratio of protonated to unprotonated indicator concentrations spectrophotometrically when the absorption peaks are sufficiently resolved. The difficulty arises in trying to extrapolate beyond the measurable [BH+]/[B] range to zero electrolyte concentration. Hammett and Deyrup assumed that the activity coefficient ratios of the type /b//bh+ were very similar for different indicators in the same acid solution. For the equilibrium (51) between two indicators A and B, a comparison of the concentration ratios [BH+]/[B] and [AH+] /[A] over an acidity range in which both could be measured would lead to a direct estimate of the difference in p.fif-values using (52) and (53). Beginning with 4-nitroaniline, which is about half- 

It is now known that consistent scales can only be hoped for if indicators of the same type are used along a series thus several different acidity function scales can be set up using different families of compounds, such as nitroanilines (H0), amides (HA), carbon acids 

As yet not enough information has been gathered to answer this question and most authors who have required an acidity scale for excited states have simply adopted the nitroaniline H0 function (Weller and Urban, 1954 Vander Donckt and Porter, 1968b Haylock et al., 1963 Hopkinson and Wyatt, 1967 Bratzel et al., 1972). Those p.K-values which have been fairly well established for excited states in the normal pH region, by direct observation of transient concentrations or by fluorescence titrations combined with lifetime measurements, have nevertheless been used to illustrate a form of test in Fig. 13. From the values of pA(St) or pA(Tj) in each 

Apart from ail the important investigations in many solvents which are helping to shape a detailed theory of acid-base interactions at the molecular level, acidity functions are constantly being revised and viewed from new points of view. For example, Marziano, Cimino, and Passerini (1974) observe that the logarithms of activity coefficient ratios of the type /b/hV/bh are proportional for 

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One of the most popular and successful scales has been developed by Dimroth and Reichardt. It is based on the pyridlnlum-N-phenoxide betaine [3], which exhibits one of the largest solvatochromic effects ever observed. The solvatochromism of this dye is negative since its ground state is considerably more polar than the excited state and is stabilized by polar solvents. Thus, in diphenylether the dye absorbs at 810 ran and appears blue-green, whereas in water the absorption is centred at 453 nm, giving an orange impression. The transition energy, expressed in kcal mol, is the so-called Ej(30) value of the solvent. Ex(30) values have been determined and tabulated for more than 270 pure solvents and many different solvent mixtures. Protonation converts the dye (Scheme 3) into a phenol as a consequence, Et(30) values cannot be measured for acidic solvents, such as carboxylic acids. 

With site-directed mutation and femtosecond-resolved fluorescence methods, we have used tryptophan as an excellent local molecular reporter for studies of a series of ultrafast protein dynamics, which include intraprotein electron transfer [64-68] and energy transfer [61, 69], as well as protein hydration dynamics [70-74]. As an optical probe, all these ultrafast measurements require no potential quenching of excited-state tryptophan by neighboring protein residues or peptide bonds on the picosecond time scale. However, it is known that tryptophan fluorescence is readily quenched by various amino acid residues [75] and peptide bonds [76-78]. Intraprotein electron transfer from excited indole moiety to nearby electrophilic residue(s) was proposed to be the quenching... 

Hc), triarylmethanols (HR), etc. Unfortunately these scales can differ by several units at the concentrated acid end. For recent reviews, the books by Rochester (1970) and Liler (1971) should be consulted. From the point of view of the acid-base properties of excited states the major question is therefore whether an Sj or Tj state of a substituted benzophenone, say, will behave like a ground-state benzophenone or like some other family of compounds with an electronic distribution, and hence polar character, rather more like that of the excited state in question. 

As more detailed studies of excited state p/ -values accumulate and our understanding of acidic and basic solutions deepens, it should become possible to discover local effects which will explain consistently both the trends in the appropriate acidity scales and the spectral effects (absorption-fluorescence spacings) related to departures from the simple Forster calculation of the pAT shift upon excitation. 

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]. 

Protonation of the olefin, or protonation and subsequent dehydration of the parent alcohol, gives cations which are then subjected to laser excitation or steady-state irradiation. Cations generated in this way were identified by their characteristic absorption spectra, which also indicated cation stability over the time scale of the individual experiments by the lack of change in their absorption spectra. Among the numerous cations generated in acidified solution for photochemical studies are the xanthyl and thioxanthyl [7-15], dibenzosuberenyl [10], triphenylmethyl [10,15], a,co-diphenylpolyenyl [16], and 1,1-diarylethyl [17] cations. Media included acetonitrile acidified with trifluoroacetic acid (TFA-ACN) or aqueous sulfuric acid [7-9,11,14,15], TEA in 2,2,2-trifluoro-ethanol (TFA-TFE) [10,12,13], n-heptane acidified with TFA [9], and BFj-etherate in methylene chloride [16]. The absorption spectral data for several cations have been previously reviewed [6]. Characterization of the cation excited states will be discussed in Section III. 

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