David W. Ellis University of New Hampshire Durham
Excited-State Dissociation Laboratory exercise using fluorescence
In order to obtain a better understanding of photodynamic systems, the properties of electronically excited molecules are being studied increasingly. The techniques of fluorescence and phosphorescence have been applied extensively in excited-state investigations because they provide information about the excited states similar to that which absorption studies provide for the ground state. In addition, several types of fast reactions are conveniently studied by fluorescence. Investigations have confirmed that many organic compounds have acidities in their excited states that diier widely from the acidities of their ground statethe diierence can he as great as six or seven orders of magnitude. I n the following experiment the acidity of the excited 2-naphthol is measured using fluorescence techniques. It is then compared with the ground-state acidity as determined by absorption measurements or by reference to the literature. The equipment required is minimal: a pH meter, a filter fluommeter equipped with an ultraviolet source, and quartz cuvettes. The added availability of a spectrophotometer or a spectrofluorometer would permit the student to undertake a more comprehensive investigsi tion.
that the acidities of these molecules were much greater in their excited states,thau in their ground states. The acid-base properties of 2-naphthol serve as an excellent example of excited-state dissociation. When the fluorescence spectra of this molecule are obtained a t varying acidities, it is found that three phenomena occur in distinct ranges of pH: in the region from pH 1.0 to 2.8, a single fluorescence peak a t 359 mp is predominant, indicating that molecular 2-naphthol is the emitting species (see Fig. 1). Above pH 9.5, fluorescence a t 429 mp predominates. This fluorescence is characteristic of the 2-naphtholate anion fluorescence,
Discussion
The phenomenon of excited-state dissociation was first discovered and extensively studied by Fomter ( I ) . By means of fluorescence techniques with aromatic hydmxy- and ammonium-compounds, he showed
Figure I . Fluorcrsence intensity of ?-naphthol or o function of pH. M, molecular form at 359 mp; I, ionic form a t 429 mp. Meoruroment mode with a ~peOroRuommeter.
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and its appearance in this pH region would be expected since the pK. of 2-naphthol is 9.5 (see Fig. I).' In the middle pH range from 2.8 to 9.5, one would expect on the basis of absorption data that molecular 2-naphthol would be the predominant species. I n the excited state, however, some species other than 2naphthol must also be present. This is deduced from the fact that the intensities of the fluorescence a t 359 mp (molecular 2-naphthol) decreases, and the fluorescence at 429 mp (ionic) is also present. Thus in this pH range, the 2-naphthol molecule, upon excitation, may ionize to form the 2-naphtholate anion which then may emit fluorescence. The value of 2.8 has been assigned, therefore, as the excited-state dissociation constant (pK.*) of 2-naphthol (8). This phenomenon can he represented in the following manner:
An excellent review of these and other fast reactions of excited-state systems has been published (10). The Experimenl
The ground-state pK. of 2-naphthol can be obtained from the literature by the student or can be determined experimentally. I n Figure 2 are shown the absorption spectra of the molecular (M) and ionic (I) forms of 2naphthol (5). Since the2-naphtholate anion exhibits an absorption maximum in a region (app. 350 mp) where 2-naphthol has no absorption, it is a relatively simple matter to measure the pK. of 2-naphthol. Various buffer solutions are prepared differing in pH by 0.2-0.4 pH units in the pH region from 8.0 to 11.0 (the expected region of the pK. for 2-naphthol) and all containing the same concentration of 2-naphthol (lo-' M). The absorption of each solution is measured a t 350 mp and plotted versus pH. An "S-shaped" curve results; a t the inflection point of the curve, the concentration of
(1) Absowtion of ultraviolet radiation to form the excited moiecule. (2) Deactivation of the excited molecule by molecular fluorescence. (3) Radiationless deactivation of the excited molecule. (4) Excited-state dissociation of the excited molecule producing a proton and an excited anion. ( 5 ) Deaotivation of the excited anion by ionic fluorescence. (6) Radiationless deactivation of the excited anion.
Kinetic aspects of excited-state dissociation have been studied extensively by Weller (3). He has determined the intermediates and the rate constants for the different reactions and has studied the effects both of temperature and of ionic strength. He has explained the unusual behavior of 2-naphthol whereby it exhibits both molecular and ionic fluorescence over a wide pH range in terms of the lifetimes of the different excited-state species. Several workers have investigated the excited-state properties of different acid-base systems. The naphthylamines (4),the naphthalenediols (6),phenol derivatives (6), and several acridine derivatives (7) have been studied. These studies have shown that certain aromatic compounds exhibit greater acidities in the excited state than in the ground state (for example, ArOH, ArNH3+, and ArSH). Other aromatic compounds are weaker acids in the excited state [for example: ArCOOH mdAr(heterocyclic)NH+]. For the former, the pK.* is less than the pK,; for the latter, the pK,* is greater than the pK.. I n two cases, the presence of more than one functional group has produced some interesting results. Esters of salicylic acid have been shown by Weller (8)to undergo protomeric isomerization in the excited state to produce a zwitterion type structure. Certain aminonaphthol isomers (9) have also been shown to undergo a similar reaction to produce a zwitterion in the excited state.
1 There are several reasons why the molecular fluorescence intensity is less than that of the ionic species: at a given wavelength of excitation, the absorption by the molecular and the ionic species may differ; also, the quantum efficiencies of the two species are different. Various instruments1 factors also play a role.
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Figure 2.
Above, tho absorption 9peOrum of ?-naphthol. absorption spectrum of 2-naphtholate anion.
Below, tho
the molecular species equals the concentration of the ionic species. The pK, of 2-naphthol is equal numerically to the p H at that point. The measurement of the dissociation constant for the electronically excited 2-naphthol is only slightly more involved. The 2-naphthol exhibits ultraviolet fluorescence while the fluorescence of 2-naphtholate anion is in the visible region. I n a manner similar to that used to measure the ground-state pK., the visible fluorescence of 2-naphtholate anions is measured in solutions of varying pH; the resultant fluorescence intensities are plotted versus pH and a curve similar to that found in Figure 3 is obtained. The first inflection point corresponds to the excited-state dissociation constant (pKK.*= 2.8); the second occurs a t the point where 2-naphthol ionizes in the ground state (pK. =
".-,.
4 5)
More specifically, in order to measure the pK,*, it is necessary to prepare solutions of 10-8M 2-naphthol of
varying pH (from 1 to 12). I n order to avoid interferences it is suggested that buffers not be used. Rather, 1 ml of a stock solution of 2-naphthol (10-4 M) can be diluted to 100 ml with a solution which has been adjusted to the approximate p H with sodium hydroxide or sulfuric acid. Suggested solutions for dilution are one each of pH 1.0, 2.0, 2.3, 2.5, 2.8, 3.0, 3.3, 4.0, 6.0, 8.0, 8.5,9.0, 9.3, 9.5, 9.8, 10.0, 10.5, 11.0and 13.0. The pH of the resultant solutions must then be measured. The fluorescence intensities of these solutions can then be measured using quartz cells in a filter fluorometer equipped with a source capable of exciting molecular 2-naphthol (330 mw and below). The ionic fluorescence of the 2-naphtholate anion is then measured. I n Figure 3. data obtained on a standard instrument2are nlotted as described above.
spectmfluorometer. If a spectrofluorometer is available, it is a simple matter to obtain simultaneously both the molecular and the ionic fluorescence intensities as a function of pH. The excitation wavelength is usually selected in the lowest energy a* + a absorption hand (e.g., 320330 mw for 2-naphthol). Both the molecular and ionic fluorescence are scanned a t the pH's previously suggested. Typical data for 2-naphthol are presented in graphical form in Figure 1. Addendum: Two additional methods for determining excited-state dissociation constants have been described in the literature. I n one method, absorption and fluorescence maxima are used in conjunction with the Forster cycle (6). A third method utilizes only absorption data (11). Recently, Mrehry and Rogers (12) have shown that the pK.* values for aromatic hydroxy compounds obtained by the latter method differ markedly and in a consistent manner from those obtained using fluorescence techniques. A possible explanation of this discrepancy is also presented in terms of solvent reorientation effects. Acknowledgment
The author wishes to thank Professor R. W. Ramette for helpful discussions and constructive comments and to acknowledge gratefully the support of 1VSF through an undergraduate teaching equipment grant. Figure 3.
Ionic fluorescence intensity of 2-naphthol as a function of pH,
measured on o fllkr fluoromoter.
It is more difficult to measure the molecular fluorescence of 2-naphthol than the ionic fluorescence using most lilter fluorometers, because the former emission is in the ultraviolet region while the latter is in the visible. However if the molecular fluorescence is measured, a curve similar to the "M" curve in Figure 1 will be obtained. The two infiection points should correspond to those obtained when measuring the ionic fluorescence. The results obtained using either method agree well with literature values, which usually have been determined by measuring the molecular fluorescence with a 'The instrument used was a Turner Model 110 Fluorometer equipped with their ultraviolet lamp (#110-851). An ultraviolet filter (8110-$lo), transmitting from 240 mp to 420 mp, was used for selective excitation; a sharp cut filter, passing wavelengths longer than 415 mrr, was used in the detection mode. Quarte cuvettes were used.
Literature Cited (1) FORSTER, T.,Natumi88., 36, 186 (1949); 2.Eleelrochem., 54. 42. 531 1 50). ~ ~ - ~ , ~ ,19.~ W E ~ L E R ;A,, Z. phys. Chem. N . F., 17, 224 (1958). WELLER, A., Z. phys. Chem. A'. F., 15, 438 (1958). FORSTER, T.,"Photochemistry in the Liquid and Solid States," John Wileg & Sons, Inc., New York, 1960, p. 10. HERCULES, D. M., AND ROGERS, L. B., Spectrochim. A&, 15, 393 (1959). BARTOK, W., LUCCHESI, P. J., AND SNIDER, N. S., J . Am. Chem. Sac., 84, 1842 (1962). MATAGA, N., KAIFU,Y., AND K O I Z ~M., I , Bull. Chem. Soe. Japan, 29, 373 (1956). WELLER, A,, Z.Eleet~ochem.,60, 1144 (1956). ELLIS,D. W., A N D ROGERB, L. B., Spectrochim. A&, 18,265 (1962), 20, 1709 (1964). WELLER, A,, "Progress in Reaction Kinetics I," Pei-gsmon Press, New York, 1961, p. 187. JAFFE.H. H., BEVERIDGE. D. L.. AND JONES.H. L.. J . Am. (12) WEFIRY,E. L:, AND ROGERS, L. B., Spectmchim. Ada., to be published.
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