The acid—base equilibria of dimethylphenols in the ground and the first excited singlet states

Spectrochimica Acta, Vol. 43A, No. 9, pp. 1161-1163, 1987.

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The acid-base equilibria of dimethyiphenols in the ground and the first excited singlet states E. TOMAS VERT, P. MEDINA CASAMAYOR*, A. OLBA TORRENT and A. CODOlqER CASTELLOTE Departamento de Quimiea Fisiea, Facultad de Ciencias Quimicas, Universidad de Valencia, 46100 Burjassot (Valencia), Spain (Received 6 November 1986; in final form 5 February 1987; accepted 13 February 1987) Abstract--Measurements of the acidity constants in the ground (pK, (So)) and the lowest excited singlet (pKo (S~)) states for 2,3-, 2A-, 2,5-, 2,6-, 3,4- and 3,5-dimethylphenois in aqueous solution have been carried out spectrophotometrieaily at 25°C. The pKo values in SOhave been derived from the absorption spectra and the pKo values in S~ were estimated by means of the Forster cycle. It is found that the hydroxy group is more acidic in the first excited singlet than in the ground state.

INTRODUCTION This paper deals with the acid-base equilibrium A O H , = ~ A O - + H +, corresponding to the deprotonation of the phenolic group in the ground and the first excited singlet states, a comparison of interest because it is well known that the acid-base behaviour of most organic molecules is strongly affected by electronic excitation [1, 2]. G r o u n d state ionization constants pKo(So) were calculated using the standard formula [3]: pKa (So) = pH - log

AAOa -- A -

-

A - AAO-

(1)

EXPERIMENTAL

where A, AAOH and AAO- are the absorbances of a solution containing both A O H and A O - , and a solution of pure A O H and a solution of pure A O - , respectively, all solutions having the same overall concentration. The pK~ (S~) values for the first singlet excited state were estimated using the following relation derived from the Forster formula [4]: pKa ( S t ) = pK~ (S0) +

0.625 T

Av (cm - l )

sition towards large v in absorption and towards shorter v in fluorescence. These pKo ($1) values are not in full agreement with the pKo ($1) values calculated by HAZRA and LAHIRI [51 obtained by using the absorption frequencies only in the Forster equation, and therefore these values will be less accurate than the values calculated in this paper. The pKa ($1) obtained for the phenol by these authors is also different from the reported by BARTOK et al. [6], who use the same procedure to average the absorption and emission maximum frequencies as us and similar to those obtained results reported here.

(2)

where Av is the difference between the frequency of the O-0 electronic transition in the base A O - and the corresponding frequency in the acid molecule A O H and T is the absolute temperature. We have observed the fluorescence emission of the base A O - species in strongly alkaline solutions as a band of low intensity. The fluorescence of the acid A O H species was obtained in an aqueous solution at pH values below 7. Fluorescence of A O - has not previously been reported in the literature. pKa (St) values were calculated from Eqn. (2), the 0-0 frequency being estimated by averaging the frequencies of absorption and fluorescence maxima of both A O - and A O H species, since the F r a n k Condon transition is shifted relative to 0-0 tran*Author to whom correspondence should be addressed.

Materials All dimethylphenols were supplied by Fluka Chemicals and Biochemicals, as purum grade reagents, except the 2,4- and 2,6-dimethylphenol isomers which were supplied as practical grade materials. 2,4-Dimethyiphenol was purified by two fractional distillations to a constant boiling point of 210.0°C. The other isomers were purified by a standard fractional sublimation method until no further change in their melting points was observed. The buffer solutions were 0.1 M glycine-NaOH mixtures for the pH range 8 < pH < 12, 0.1 M NaOH for pH > 12 and 0.1 M HCI for pH < 7. We have chosen these particular buffer solutions because they are appropriate in such a range of pH and do not interact with the compounds under study. The solute concentration for all the solutions was 10 -4 M. All the experiments were carried out using thermostatted solutions at a temperature of 25 _ 0.1°C. Apparatus Ultraviolet absorption spectra were recording using a Cary 219 spectrophotometer. Corrected fluorescence emission spectra were recorded using a Perkin-Elmer MPF-44A spectrofluorometer equipped with a corrector accessory (DCSU-2). The pH values were measured using a calibrated Radiometer 26 pH meter to within 0.01 of a pH unit. RESULTS AND DISCUSSION Electronic absorption spectra The absorption spectra of the compounds studied are strongly dependent on the pH of the solution, as shown as an example for 2,4-dimethylphenol in Fig. 1. 1161

1162

F. TOMASVERT et al. Table 1. Wavelengths of the absorption spectra and fluorescen¢~ maxima and the frequencies of the Forster equation

0.4

Compound

Species

(~'ab)max

(nm)

()'era)max

(nm)

A7

(cm - 1)

i

AOH AO AOH AOAOH AO AOH AO AOH AO AOH AO-

2,3-DP 2,4-DP 2,5-DP 2,6-DP

0.2

3,4-DP 3,5-DP

0.0

I 240

I 280

I 320 k (nm)

Fig. 1. Absorption spectrum of 2,4-dimethylphenoi (ionic strength, 0.1): curve a, pH 6.13; curve b, pH 8.92; curve c, pH 9.33; curve d, pH 9.72; carve e, pH 10.08; curve f, pH 10.34; curve g, pH 10.55; curve h, pH 10.75; curve i, pH 12.81.

The values of A^o a and A^o- were taken from the spectra solutions at pH 6.13 and pH 12.81, respectively. The absorption maxima for solutions of each species on the compounds studied are summarized in Table 1. Fluorescence spectra

The fluorescence spectra recorded for the six isomers are invariable at pH values lower than 7, and they correspond to acid A O H species. The spectra for alkaline solutions (pH > 12) correspond to basic A O species, which, showing a very low intensity, necessitated a high sensitivity setting to enable them to be recorded. As an example, the fluorescence spectra of both 3,5-dimethylphenoi species are shown in Fig. 2, the other isomers behaved similarly.

277 288 276 294 278 290 275 286 280 292 277 288

299 344 309 353 305 347 297 345 306 350 298 345

J30O

L

I

380

X (nm)

- 2977 - 3187

The ground and excited state pK= values for the acid-base equilibria present in aqueous solution of dimethylphenols, together with the values found by us for phenol (for comparative purposes) are summarized in Table 2. The pK= (S0) values obtained in this work accord with values found in the literature [7, 8].

/ 340

- 2908 - 3270

pK=(S0) and pKe(St ) values

A

;amav

- 3277

In all cases the 2ex chosen was 270 nm, which was found to be the best for the acid A O H species. Also, )-¢x = 270 nm has been used in recording the fluorescence spectra of the basic A O - forms, in order to avoid interference from the Raman band of water, although the optimal wavelength would have been 290 nm. To confirm the emission maxima for the A O - forms of the compounds studied, we have carried out additional experiments using an e t h a n o i - N a O H solvent (Fig. 2). We have found that the intensity of fluorescence is larger than in aqueous solution although the emission maxima of the basic forms are similar in both solvents• The fluorescence maxima for the two species of all dimethylphenols in aqueous solution are summarized in Table 1.

I/Io

I/Io

- 3077

I 28O

I 320

i 360 k ( n m )

Fig. 2. Fluorescence spectrum of 3,5-dimethyiphenol (2ex = 270 nm, ionic strength 0.1): (a) acidic form (AOH); (b) basic form (AO-): curve A in H20-NaOH solution and curve B in ethanoI-NaOH solution.

Acid-base equilibria of dimethyl phenols Table 2. Ground and excited singlet state pK, values Compound 2,3-DP 2,4-DP 2,5-DP 2,6-DP 3,4-DP 3,5-DP Phenol

pK, (So)

pK° (Si)

10.42 10.54 10.43 10.45 10.24 10.07 9.90

3.97 3.67 4.33 3.58 4.05 3.69 3.30

1163

The combination of both effects can explain the greater destabilization of the anionic form of the 2,4-dimethylphenol and its lessening, through the series of dimethylphenols, to the 3,5-dimethylphenol anion which is the most stabilized. The pKa ($1) values show the 2,4- and 2,6-dimethylphenols molecules to be the most acidic in the first excited state, whereas the opposite was found in the study of the acid-base behaviour of the dimethylphenols in their ground state. REFERENCES

The acidity of these molecules is much greater in the first excited singlet ($1) than in the ground state (So). This behaviour agrees well with the pattern observed for the other phenolic systems [9, 10]. The pKa(So) values obtained seem to reflect an orientation effect of two methyl groups upon the phenolic OH. Thus, the least acidic molecule is the 2,4dimethylphenol which has CH 3 substituents at ortho and parr positions and the most acidic molecule is the 3,5-dimethylphenol which has these substituents at the meta position. The correlation between the substituent position and pKa (So) values can be explained by the ortho, para orientation character of the OH group and the electron-donatingcharacter of the methyl substituents.

[1] A. WELLES,[email protected] Kinet. 1, 199 (1961). [2] G. JACKSONand G. PORTER,Proc. R. Soc. A 260, 13 (1960). [3] H. H. J Ar:FEand M. OacalN, Theory and Applications of Ultraviolet Spectroscopy. Wiley, New York (1962). [4] T. FORSTER,Z. Electrochem. 54, 531 (1950). [5] D.K. HAZRAand S. C. LAalRi,Indian J. Chem. 10, 753 (1972). [6] W. BARTOK,R. B. HARTMANand P. J. LUCCaESI, Photochem. Photobiol. 4, 499 (1965), [7] E. F. G. HERINTONand W. KYNASTON,Trans. Faraday Soc. 53, 138 (19571 [8] D. T. Y. CHENand K. J. LAIDLER,Trans. Faraday Soc. 58, 480 0962). [9] A. WELLES, Z. Electrochem. 56, 662 (1952). [10"1 E. L. WEARYand L. B. ROGERS,J. Am. chem. Soc. 87, 4234 (1965).