508
G. J. YAKATAN AND S. G. SCHULMAN
Electronic Spectral Study of the Ionizations of the Naphthalene Monosulfonic Acids by G. J. Yakatan and S. G. Schulman* College of Pharmacy, University of Florida, Gainaville, Florida 38801 (Received July 16, 1971) Publication costs borne completely by The Journal of Physical Chemistry
Failure of the protonation of the 1-and 2-naphthalenesulfonatesto shift their absorption and fluorescence spectra is attributed to negligible conjugation between the site of protonation and the aromatic system. Protonation of the neutral naphthalenesulfonic acids, however, apparently occurs at a site strongly coupled to the aromatic system as both fluorescence and absorption spectra are affected. The fluorescences of all species studied arise from the I L b state of naphthalene except for the I-naphthalenesulfonicacidium ion fluorescence, which may originate from the 1La state.
Introduction There is, at present, a paucity of quantitative information about the chemical properties and electronic structures of sulfonic acids. Early electrometric determinations of the dissociation constants of the naphthalenesulfonic acids yielded pK, values of 0.74 and 0.60 for the 1 and 2 isomers, respective1y.l Indicator methods have been employed to determine a pK, of 0 for methanesulfonic acid.2 These vaiues appear reasonable by comparison with the pK, values of -3 and $2.0 for sulfuric acid3 and suggest that OH is a weaker electron donor, while - 0- is a stronger electron donor than an alkyl or aryl group attached to the -S03H moiety. I n a recent study of the solvent dependence of anthracene monosulfonate fluorescence4it was found that changes in acidity had no effect on the absorption spectra and only a very small effect on the fluorescence spectra of these compounds. The deactivating and meta directing properties of the sulfonic acid group, with respect to aromatic electrophilic substitution, have long been attributed to the ability of the hexavalent sulfur atom to expand its valence shell by d orbital participation, thereby withdrawing electrons from the aromatic ring. It would then be apparent that the sulfonic acid group would be essentially a part of the aromatic system, so that a change in the electronic distribution a t the sulfonic acid group, such as that produced by protonation of the sulfonate anion, would perturb the entire aromatic system. Moreover, due to the dipole nature of electronic transitions, the electronic distributions of the aryl sulfonic acids are expected to be different in the lower electronically excited singlet states than in the ground state. Consequently, the protonation energies in ground and excited states should be different, as a result of which the neutral sulfonic acid and the sulfonate anion should absorb or fluoresce a t different frequencies.6 Obviously The Journal of Physical Chemistry, Vol. 76,No. 4, 197.9
this is not the case for the anthracene monosulfonates. Studies in this laboratory indicate that it is also not the case for the naphthalene monosulfonates. Since this phenomenon obviously is related to the nature of the electronic structure of arylsulfonic acids, it was decided to undertake the present study of the acidity dependence of the electronic transitions in the naphthalenesulfonic acids.
Experimental Section 1-Waphthalenesulfonic acid and 2-naphthalenesulfonic acid were purchased from Eastman Organic Chemicals, Inc., Rochester, N. Y., and were recrystallized from chloroform. Mallinckrodt reagent grade sulfuric acid and distilled deionized water were employed as the solvents in these studies. Absorption spectra were taken on a Becliman DB-GT spectrophotometer. Fluorescence measurements were performed on a Perkin-Elmer MPF-ZA fluorescence spectrophotometer whose monochromators were calibrated against the xenon line emission spectrum and whose output was corrected for instrumental response by means of a rhodamine-B quantum counter.
Results and Discussion I n the region pH 14 to HO-2 there is no change in either the absorption or fluorescence spectra of the naphthalenesulfonic acids. At acidities greater than HO - 2 the fluorescence of 1-naphthalenesulfonic acid diminishes in intensity and shifts to the rcd (Figure 1). No vibrational structure is apparent for the 1-isomer in any solution. A corresponding change in the fluores(1) H.E.Fierz and P. Weissenbach, HeEv. Chim. Acta, 3 , 305 (1920). (2) K.N. Bascombe and R. P. Bell, J. Chem. Soc., 1096 (1959). (3) A.Albert and E. P. Serjeant, “Ionization Constants of Acids and Bases,” Wiley, New York, N. Y., 1962,p 151. (4) K.K.Rohatgi and B. P. Singh, J. Phys. Chem., 75,595 (1971). (5) T.Forster, 2. Elektrochem., 54,42 (1950).
509
IONIZATIONS OF THE NAPHTHALENE MONOSULFONIC ACIDS 0 90
80
W z
250
2%
300
325
350 WAVELENGTH, nm.
W
30
Figure 2. A. Absorption spectra of 2-naphthalenesulfonic acid: 1, at pH 7 and HO-2; 2, at Ho - 10. B. Absorption spectra of 1-naphthalenesulfonic acid: 1, at pH 7 and Ho -2; 2, at Ho - 10. The naphthalenesulfonic acid concentration in each case was 1.0 X 10-4 M .
V W
20 LL
300 340 380 420
3(
WAVELENGTH, nm
Figure 1. A. Fluorescence spectra of 2-naphthalenesulfonic acid: 1, at pH 7 and Ho -2; 2, at Ha- 10. B. Fluorescence spectra of 1-naphthalenesulfonic acid: 1, at pH 7 and HO -2; 2, at HO- 10. The naphthalenesulfonic acid concentration in each case was 1.0 x 10-6 M .
Table I : Absorption ( i s )and Fluorescence ( i t ) Frequencies of Naphthalene and the Naphthalenesulfonic Acids in the Anion (pH 4), Neutral (Ha -2), and Protonated (Ho -9) Forms (Frequencies Are Given in cm-1 X PsflLb)"
log e
Fa('h)'
IOCl e
if"
Naphthalene (in 10% aqueous methanol)
cence of 2-naphthalenesulfonic acid occurs beginning a t Ho < -3 (Figure 1). The fluorimetric titration of the 1-isomer reaches a minimum and constant fluorescence intensity at Ho w - 6 with a total red shift of 3300 cm-I while that for the 2-isomer reaches a minimum a t HO 8, and exhibits a red shift of 1500 cm-' and a loss of vibrational structure present in the fluorescence at low acidities. The absorption spectra of the 1-isomer do not begin to change appreciably until HO - 6 is attained whereupon the 'Laband begins to red shift and overlaps the 'Lb band envelope (Figure 2). The absorption spectra of the 2-isomer do not change until an acidity of HO -7 is reached (Figure 2). Appreciable shifting of neither the *Laof 'Lb band is observed for the 2-isomer but an increase in absorbance with increasing acidity occurs. For neither isomer is the absorptiometric titration complete by the time the most acidic sulfuric acid solution (Ho - 10) is employed. The spectral features of the naphthalenesulfonic acids in some representative solutions along with those of naphthalene are presented in Table I. The failure of the absorption spectra to change with acidity in the same region where the fluorescence spectra change indicates that the quantum yields of fluorescence of the naphthalenesulfonic acids are acidity dependent and thus the fluorimetric titrations correspond to ioninations of the sulfonic acids in an electronically excited state. The pK,* values, estimated from the midpoints of the fluorimetric titration curves, are found to be - 3.7 for the 1-isomer and - 5.1 for the 2-isomer. That the fluorescence spectra of both isomers shift to longer wavelengths with increasing acidity suggests that these molecules become more basic in the fluoresN
-
3.20
2.40
3.62
3.62
2.99
3.61 3.61 3-56'
3.02 3.02 2.69
3.62 3.62 3.1gb
2.95 2.95 2.80
1-Naphthalenesulfonic Acid Anion Neutral Cation
3.50 3.50 3.31'
2.65 2.65 3.34'
3.15 3.15 3.15'
2-Naphthalenesulfonic Acid Anion Neutral Cation
3.10 3.10 3.06b
3.62 3.62 3.62b
2.83 2.83 3.22'
"i,(lLb) was taken at the 0-0 band of the 1Lb absorption. Due to the lack of distinct structure in the 'L, absorption and in some of the fluorescence spectra i,(lLa) and i f were taken at the band maxima. This corresponds to the second vibronic feature in the structured fluorescence spectra. These are approximate values, ao the cation absorptions are not isolated at this acidity.
'
cent state, It is possible, employing the shifts in the fluorescence spectra and the excited state dissociation constants in conjunction with the Forster cycle15to estimate the ground state pK, values for the corresponding equilibria. The ground state pKa value of the 1-isomer is found to be -10.6, while that for the 2isomer is estimated to be -8.3. These results are in accord with the partial absorptiometric titration data obtained and are far more acidic than the pK, values obtained for the equilibria between the neutral sulfonic acids and the sulfonate anions. It is suggested that the pK, values of - 10.6 and -8.3 correspond to prototropic equilibria, in the ground state, between the neutral sulfonic acids and the corresponding sulfonic acidium ions: RSOaH H+ RSOsH2f.
+
+
The Journal of Physical Chemistryr Vol. 76, No. 4, 1973
510
G. J. YAKATAN AND 8. C. SCHULMAN
That the protonations of the naphthalenesulfonates are reflected ncither in the absorption nor in the fluorescence spectra while the protonations of the neutral sulfonic acids are apparent in both types of spectra, leads to the conclusion that the site of protonation (oxygen atom) in the sulfonate anion is essentially uncoupled, electronically, to the aromatic system while the oxygen atom protonatcd in the neutral sulfonic acid is strongly coupled to the aromatic system, Thus the three oxygen atoms in each sulfonate anion are not equivalent to one another. That the excited state pK,* values of the protonated sulfonic acids are less acidic than the ground state pK, values indicated that in the fluorescent state, charge transfer from the naphthyl moieties to the sulfonic acid group occurs to a greater extent than in the ground state. The absorption spectra of the naphthalenesulfonic acids are similar in appearance to those of naphthalene, the ratios of the molar absorptivities of the 'Lb bands to those of the IL, bands being about the same as that for naphthalene (-0.1). This fact suggests that the transitions in the anions and neutral acids are localized on the naphthalene rings. The weakness of the 'Lb transition is due to its symmetry forbiddeness. However, substituents on or in the naphthyl ring, as in quinoline, isoquinoline and the naphthylamines, which couple with the aromatic system, tend to reduce the symmetry of the naphthalene ring and thereby reduce the forbiddenness of the 'Lb transitiona6 Consequently, in quinoline, isoquinoline, and the naphthyamines dLb/€'L& 1. Since enhancement of the 'Lb transition is small for the naphthalenesulfonates, it can be concluded that sulfonate or sulfonic acid substitution does not substantially affect the symmetry of the Telectron system of naphthalene, However, in the sulfonic acidium ions, charge transfer to the exocyclic groups, in the excited state, reduces the symmetry of the T-systems, removing the forbiddenness of the 'Lb transitions. Relative to the 'Lb and 'La absorption bands of naphthalene, substitution of a sulfonate anion or sulfonic acid group in the 2-position of naphthalene produces a greater shift of the 'Lb band than does substitution in the 1-position of naphthalene. The opposite is true for the 'L, band. This is in accord with the longitudinal (long axis) polarization of the 'Lb -+ 'A transition and with the transverse (short axis) polarization of the 'L, + 'A transition in naphthalene. The fluorescences of the 2-substituted naphthalene sulfonate and sulfonic acid are shifted relative to the fluorescence of naphthalene to R greater extent than are the fluorescences of the
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The Journal of Physical Chemistry, Vol. 76, No. 4, 107.9
corresponding 1-substituted compounds. This suggests that thc fluorescences of these compounds arise from the 'IJb state since the 'Lb state is lowest in the absorption spectrum. The electronic transition is localized on the naphthalene so that no dramatic excited state thermal relaxation processes which may alter the ordering of electronic states should occur. The fluorescences show approximate mirror image relationships with the 'Lb absorption bands, and substituent orientation and transition polarization correlate approximately the same for fluorescence as for absorptive transitions between the same states. However, it is obvious that protonation of the 1-sulfonic acid produces a much greater fluorescence shift than protonation of the 2isomer. The opposite is to be expected if the fluorescent states in both protonated isomers are 'Lb. As a result of the greater shifting of the fluorescence of the 1-naphthalene sulfonic acid upon protonation, which parallels (approximately) the shifting of the 'L, absorption band of the 1-isomer upon protonation, and the greater sensitivity of the 'L, band of naphthalene to 1-substitution, it is proposed that while fluorescence of the protonated 2-naphthalenesulfonic acid occurs from the 'Lb state, in the protonated l-naphthalenesulfonic acid fluorescence occurs from the 'L, state. This is a result of the red shift of the 'L, state relative to the 'Lb state, in the 1-isomer, upon protonation, which probably leaves the long wavelength end of the 'La band a t lower energy than the 0-0 band of the 'Lb band. Thus the 'La state would be the lowest excited singlet state in the Protonated 1-isomer. If the neutral 1-naphthalenesulfonic acid does, in fact, fluoresce from the 'Lb state while the protonated 1-isomer fluoresces from the 'L, state, it is possible that the application of the Forster cycle to equilibria involving these species is not, strictly speaking, valid since interconversion between the two excited conjugate species entails a change in electronic configuration along with the chemical reaction,T a process which violates the assumption of equal protonation entropies in ground and electronically excited states, upon which the validity of the Forster cycle rests. However, that there is good agreement between the Forster cycle calculation for the l-isomer and the fluorimetric and absorptiometric titrations suggests that the entropy error due to fluorescence from noncorresponding states may be too small to affect the general conclusions, a t least in this case, obtained from Forster cycle calculations. (6) H. H. Jaffb and,,M. Orohin, "Theory and Applications of Ultraviolet Spectroscopy, Wiley, New York, N. Y., 1962. (7) H. H.Jaff6 and H , L. Jones, J . Org. Chem., 30,964 (1965).
