Excited-State Acids

The pK values of phenols in singlet and triplet states are valuable guide to substituent effect in the excited states, specially for the aromatic hydrocarbons. In general, the conjugation between substituents and -electron clouds is very significantly enhanced by electronic excitation without change in the direction of conjugative substituent effect. The excited state acidities frequently follow the Hammett equation fairly well if exalted substituent constants a are used. 

Singlet excited state acid dissociation constants pK can be smaller or greater than the ground state constant pK by as much as 8 units. Phenols, thiols and aromatic amines are stronger acids upon excitation, whereas carboxylic acids, aldehydes and ketones with lowest >(71, ) states become much more basic. Triplet state constants pKr are closer to those for the ground state. Forster s cycle may be used to determine A pK =pK —pK) from fluorescence measurements if proton transfer occurs within the lifetime of the excited molecule. 

We have demonstrated that the novel antitumor agent 10-hydroxycamptothecin has remarkable excited-state acidity. In contrast to simple 6-hydroxyquinoline, no tautomerization is observed. The implications for the use of proton transfer dynamics in studying the microenvironment of cells remains to be demonstrated. 

According to the Forster cycle, if the longest wavelength electronic transition of the deprotonated form is of lower energy compared to that of the protonated form (red-shifted electronic absorption or emission spectrum of the deprotonated form with reference to the protonated-form spectrum), the molecule has enhanced excited-state acidity (i.e., the pK a of the molecule is lower than pKa). Equation (1) provides a quick and effective method for evaluating a molecule for its ESPT behavior. 

There are three sections in the review. This section is a brief introduction to the subject. Because aromatic hydroxy compounds are taken as prototype excited-state acids, they have been discussed in a separate subsection. Section II has subsections for each medium and each subsection is further branched according to the classes of ESPT molecules studied in the medium. Section III gives conclusions. 

It assumes that there is no entropy change between the ground- and excited-state acid-base reactions. In order for this assumption to be valid, there should be little geometric distortion of the excited molecule and the acid-base reactions must originate from the same sites in both the ground- and excited-state species. 

The observation of a fast time decay for the n = 2 mass channel excited at the cluster origin is surprising. Isotopic substitution experiments show that the decay is due to proton tunneling, yet naphthol, which is a stronger excited state acid, does not exhibit proton transfer until it is clustered with three ammonia... 

Methods of obtaining pX(S0)-values are well documented (e.g., Albert and Sargent, 1962). Since th6 molecules of interest in excited state acid-base studies absorb at different wavelengths in the B and BH+ forms, absorption spectroscopy is commonly used in the relevant ground-state pX-determination. 

If no direct measurement of the fluorescence lifetime is available the relations between the radiative lifetime and the fluorescence and absorption spectra can be used in conjunction with the quantum yield to obtain an indication of the fluorescence lifetime. Birks and Munro (1967) have reviewed the methods of calculating the radiative lifetime. In general these methods are limited to specific groups of compounds. For example, Favaro et al. (1973) applied Stickler and Berg s (1962) formula to the spectral data obtained from an excited state acid-base study of some styrylpyridines and found a lack of quantitative agreement between the measured and calculated lifetimes. 

Reinvestigation of the excited state acid-base properties of 2-naphthylamine (Schulman and Capomacchia, 1972) showed that a reported change of hybridization from sp3 to sp2 on excitation had little effect on the entropy of protonation of nitrogen in the Sx state and that therefore the Forster cycle was still applicable. A pX (S1 )-value, calculated from the fluorescence maxima of the B and BH+ form, of —8-1 is in poor agreement with the value, —2, obtained from fluorescence titration measurements. From the acidity dependence of fluorescence intensity for 1- and 2-naphthylamine Liedke and Schulman (1973a) found that the decrease in emission of the B form occurred at lower acidities than the appearance of BH+ fluorescence. [A similar titration curve for the fluorescence of the neutral molecule was obtained by Seliskar and Brand (1971), who obtained a value of 0-64 for pkr(S1) from the decrease of the... 

The excited state acid-base behaviour of molecules has direct implications in the field of analytical fluorimetry and phosphori-metry. Since the emitting species can be changed by adjusting the acidity of the solution, the sensitivity or selectivity of an analytical procedure can be increased (see e.g. McCarthy and Winefordner, 1967 Argauer and White, 1970). Thus, for example, the limit of detection by phosphorimetry of 4-nitrophenol is much lower in solutions containing the anion (Schulman and Winefordner, 1970) and the long-wavelength fluorescence of warfarin in sulphuric acid allows its selective fluorimetric determination in the presence of other 4-hydroxycoumarins (Yakatan et al., 1972). 

These examples show the widening scope of excited state acid-base applications. The necessary background of pK information is rapidly becoming consolidated but, in almost all the classes of compound studied in detail, significant inconsistencies in the interpretation call for further work. Without doubt exploitation of lifetime techniques will go a long way towards solving such problems. 

Hicks C, Ye G, Levi C, et al. Excited-state acid-base chemistry of coordination complexes Coord Chem Rev 2001 211 207-22. 

Several other photochemical reactions have been studied in micellar systems. These include excited state acid-base chemistry, photoionization and electron transfer. However, since these reactions typically do not produce permanent products, and since they have been reviewed previously, they will be discussed only briefly in this review. 

Excited state proton transfer is very likely according to the excited state acidities estimated according to eq. 6. The ground and excited state pK data of HB and 4-methoxy HB are compiled in Table 9. They follow the general order PK3 < pK.j. <... 

Hydroxyarenes become stronger acids upon electronic excitationSuch a property of an aromatic molecule is usually described as photoacidity , and the molecules undergoing such a transition upon electronic excitation are usually named photoacids . Photoacids are Brpnsted acids, and their excited state acidity may be described in terms used for ground state acids as were defined by Brpnsted some 80 years ago - . Following Brpnsted, one usually associates acidity with a proton-transfer reaction where a proton is transferred from a proton donor (an acid) to a proton acceptor (a base) (equation 1). 

---