[COXTRIBETION FROX
THE
PROCESS RESEARCH DIVISION,Esso RESEARCH & ENCIXEERISC COMPAXY, LISDEY,SEIV JERSEY
Protolytic Dissociation of Electronically Excited Organic Acids* BY \V.BARTOK,P. J. LUCCIIESI A N D N. S.SNIDER RECEIVED XOVERIRER 4, 1961 Forster' and Wellcr2 havc shown t h a t excitation by thc absorption of ultraviolet light enliances the acidity of certain aromatic acids in aqueous solutions. Thus, the values of the acid dissociation constant for the excited singlets of molecules such as 1- or 2-naphthol are larger by a factor of 106-107than the corresponding ground state values. In the present study the effects of electronic excitation on the dissociation of para-substituted phenols were studied. T h e values of pK,* for the excited state were estimated from spectroscopic data. In contrast with the ground state p K values of 10.02 for phenol, 10.27 for para-cresol and 9.38 for Pam-chlorophenol which follow the Hammett Equation, excited state values of 5 . 7 , -0.ii and 3.6 were obtained for the above compounds. Apart from the large increases in acidity as a result of excitation, the dissociation constants of the excited phenols could not be correlated. From measurements in viscous glycerine solutions, a n increase in acidity of about the same magnitude as in aqueous solutions was calculated for excited 2-naphtho1, hut the corrvsponding change appeared t o be somewhat larger for glycerine solutions of phenol.
The acid dissociation constants for the electroni- and cally excited states of a number of organic acids pS, - pK,* = AE -__- AE' (4) and bases are Forster' and Weller2 2.303RT have shown that excitation by ultraviolet light greatly enhances the protolytic dissociation of where K, and K,* are the dissociation constants weak aromatic acids such as 1-naphthol and 2- for the ground and excited states. The energy naphthol in aqueous media. This is reflected both changes AE and A E ' can be estimated from the by the increased rate of the forward dissociation frequencies measured a t the maxima of the long reaction, and since equilibrium appears to be estab- wave length absorption and short wave length lished in the excited state, also by the values of the fluorescence by means of the relations dissociation constant. For excited singlet states these values are 106-107 times larger than the corresponding ground state values. In contrast with for the acid, and such large shifts in acidity observed for excited singlet states, Jackson and Porter found that the dissociation constants for the lowest triplet levels hardly differed a t all from the ground state values for the anion. Thus the value of pKa* can be calfor molecules such as 1- or 2-naphthol, acridine and culated if the value of pK has been determined independently. The results obtained by this 2-na~hthyIamine.~ As outlined by \Teller,2 the value of pK* for the spectroscopic method are in good agreement with excited state can be estimated from absorption pKa* values determined from quantum yields of and fluorescence spectra, if equilibrium is estab- fluorescence measured as a function of the pH of the system.2 The latter method is free of the aslished in the excited state. Referring to the schematic energy diagram of sumptions used in the spectroscopic calculations. I n the present work the effects of electronic Fig. 1, it follows from the thermochemistry of the system where both the acid and its conjugate base excitation on the dissociation constants of paracan be raised to electronically excited levels that, substituted phenols were studied. Absorption and fluorescence spectra of phenol, p-cresol and AE f Ed* = AE' f Ed (1) p-chlorophenol and of the corresponding anions where A E and A E ' are the energy changes for the were measured in aqueous solutions. transition from the ground state to the first singlet Measurements were also made using glycerol levels of an acid and its anion, respectively. Heats as a solvent for phenol and 2-naphthol in order to of dissociation in the ground and the excited states obtain information on the effect of increasing the are, respectively, E d and E d * . Their difference viscosity of the solvent. can be expressed as Experimental E d - Ed* = ( A G TAS) - ( A G * T A s * ) (7)
+
+
where the AG and A S terms are free energy and entropy changes (excited state values are denoted by asterisks). Assuming that the entropy changes should be about equal in the ground and the AS*,it follows that excited states, i.e., A S
+
AG - AG* = - R T (In K , - In K,*) = AE - A E ' ( 3 )
* Presented at the 140th National Meeting of the American Chemical Society, Chicago, Ill., September, 1961. (1) T. Forster, Z . Elektrochem., 54, 42 (1950). ( 2 ) A . Weller, ibid., 66, 662 (1952). ( 3 ) A . Weller, Z . P h y s . C h e m . , 3 , 238 (1955). (4) A . Weller, Z . Eleklrochem., 60, 11-14 (1950). ( 5 ) A . Weller, ibid., 61, 956 (1957). (I!) A . Weller, Disc. Faraday Soc., 27, 2 8 (1959). (7) C,. Tackson a n d C.. Porter, P r o r . Rornl .Sot. (T.ondon). 2 6 0 8 , 19 i 1 90 1) .
T h e absorption and fluorescence spectra of phenol, p n r a cresol and para-chlorophenol and of the phenolate, paracresolate and para-chlorophenolate anions were measured in neutral water and in 0.1 ;1/ aqueous sodium hydroxide solutions, respectively. Fluorescence spectra of the acid molecules were also obtained using 0.1 A4 aqueous hydrochloric acid solutions. In addition to the phenols, the absorption and fluorescence spectra of 2-naphthol and of phenol were also determined in neutral glycerol (Eastman Kodak spectro-grade) and in glycerol solutions of 0.1 Jf sodium methoxide. Solutions of all compounds (obtained from the Matheson Company, Matheson, Coleman and Bell Division) were prepared in 0.001 molar concentrations for spectroscopic measurements. I n the preparative work a dry box under a nitrogen atmosphere was used in order t o insure freedom from contamination by oxygen or by moisture in the case of nonaqueous systems. The solutinns were thoroughly purged with nitrogen before use.
1.2
mi*
ROH*
-2
, /
IiOH
,
Fig. 1.-Schematic thermochemical diagram for the dissociation reactions: ROH HsO KOHIOf in the ground state; ROH* H?O F?KO-* HyO in the excited state [after ref. 2 ) .
+
+
+
+
Both absorption and fluorescence spectra were obtained using a Cary Model 11 Spectrophotometer. The instrument was converted for fluorescence studies by means of a special attachment, h mercury arc (Hanovia type "LO" burner) was used as the light source for excitation. The 2537.1 mercury line was isolated using a 1.0 cm. path of 500 g./l. NiSO, solution in combination with a Corning 9863 red-purple Cores filter. Since the optical arrangement of the Car? attachment w a s such t h a t the fluorescence was measured from the irradiated face of the solutions, the effects of self-quenching were kept to a minimum. The fluorescence spectra were obtained with corrections for background as required. Independent checks using the fluorescence modification of a n hminco-Keirs spectrophosphorimeter produced results in agreement with these measuretnents.
Results and Discussion The data obtained on ultraviolet absorption by the compounds studied are summarized in Table I . A s shown in the table, the results are in generally good agreement with the ultraviolet spectra of phenols published in the literature.a-'o TABLE I t:I.TRAVIOI-ET
I
28,000
Phenol Phenolate p-Cresol p-Cresolate p- Chlorophenol p-Chlorophenolate 2-Saphthol 2-Saphtholate Phenol Phenolate 2-Xaphthol 2-Xaphtholate lief. 10. c R e f . 2.
298"
3325, 328" 24.5',34ii"
270 288 27; 295 280 298
32,000 cm
34,000
36,000
-l
Fig 2 -Fluorescence bands correspiinding to the phenol molecule ( K O H * ) and t o the phenolate anion (KO-*) T h e intensity values are normalized and do not represent the true relative intensities of the excited species involved Curve - - is for KC)-* and - for ROH*
flected in all further calculations based on these spectroscopic measurements. The type of information obtained is illustrated in Fig. 2 by the fluorescence spectra of the phenol molecule and of the phenolate anion. TABLE I1 FLUORESCENCE MEASUREMENTS
Compound
.lqueous solutions
Long wave length absorption maximum mr Present Literature work
270".~ 288", 287" 277b 2 9 5" 279 . 5"
30,000 vfi,
XBSORPTION hfEASUREMESTS
Compound
Aqueous solution
I
0:
Glycerol solutions
fl
Ref.2.
*
Phenol Phenolate p-Cresol p-Cresolate p- Chlorophenol p-Chlorophenolate 2-hTaphthol 2-Saphtholate Phenol Phenolate 2-Saphthol 2-Xaphtholate Ref.8.
Short-wave length fluorescence maximum Xmax, mp Present 1,iterature work
......
...... ......
...... ...... ...... 350", 35gh 425", 42gh ......
...... ...... ......
302 320 310 417 313 351
356 421 298 341 357 423
As expected, the fluorescence bands of the anions were located a t lower frequencies than those corresponding to the undissociated molecules. Since ...... the shift of the absorption bands of the phenol ...... molecules and of the anions are of about the same ...... magnitude for all these systems, the fluorescence "Ref. 8 " Ref. 9. shift is a measure of the change in acidity between solution). ground and excited states. As discussed later, Table I1 presents a summary of the results ob- the unusually large change from 310 mp for paratained from the fluorescence spectra. For 2- cresol to 417 m,u for the anion leads to a very naphthol and 2-naphtholate anions the agreement large increase of acidity in the excited state. is satisfactory with previously published values.2,a In addition to the fluorescence spectra of phenols In general, the fluorescence spectra were not suf- listed in Table 11, the determination of the value ficiently sharply defined to pinpoint the frequency of pK* by this method was also attempted for paraa t which the short wave length band had maximum nitrophenol. However, no fluorescence could be intensity. This uncertainty was, of course, re- detected with solutions of this compound a t any ( 8 ) D. M . Hercules a n d L. B. Rogers, Spectrochim. A d a , 393 (1959). pH value, probably because of the quenching effect (9) L. Doub and J , M . Vandenbelt, J. A m . Chem. Soc., 69, 2714 of the nitro-group. (1947) The values of 9Ka* were calculated by combining (10) E. F. G. Herrington and W. Kynaston, Trans. F a r a d a y Soc , Equations 4, 5 and 6, using the known PKa values 6.3, 138 (1955). Glycerol solution
......
.. 272 290 330 349 ( i n ethanol
w.BARTOK,P. J. LUCCHESI AND N. s. SNIDER
1844
for ground state dissociation reactions. These results are presented in Table 111. The values indicate that the acidity of the compounds studied is greatly increased by excitation. However, while the Hammett equation holds very well for ground state acid dissociation constants,11 it does not appear to be obeyed by the fluctuating pKa* values obtained for the excited species. In the case of para-cresol the very large change from pKa = 10.27 to pK* = -0.6 is unexpected in the light of the relatively small inductive effects of parasubstituents compared with the influence of electronic excitation. This change in the pK values is several orders of magnitude larger than the estimated accuracy of the spectroscopic calculations Furthermore, the same trend was qualitatively confirmed for para-cresol and para-chlorophenol by estimating energy changes for the electronic transitions from the short wave length limits of fluorescence. TABLE 111 AQUEOUSDISSOCIATION OF PHENOLS Compound
Ground state
pKaa
Phenol 10.02 p-Cresol 10.27 p-Chlorophenol 9.38 a Ref. 11. Ref. 10.
Hammett c b
0 -0.170 0.227
Excited singlet state
pKa*
5.7 f 0.7
- 0 . 6 f. 0.5 3.6 f 0.8
As shown in Tables I and 11, the absorption and fluorescence spectra of 2-naphthol and of phenol were also obtained in neutral and basic glycerol solutions. In all cases, fluorescence was observed with higher intensities than measured from aqueous solutions of the same species. The location of the frequencies of maximum absorption and fluorescence intensities permitted the calculation of (pK - pK*) for these acids. Thus (PK - pK*) = 6.3 for 2-naphthol, a value which is in good agreement with the (pK - pK*) in aqueous salu(11) H. H . Jaffe, Chem. Revs., 63, 199 (1953).
Vol. 84
tion. However, for phenol a value of (pK - pK*) = 6.8 was calculited, L e . , about the ‘‘usual” increase due to excitation instead of (PI< - PI
