STEPHEN SCHULMAN AND QUIPU’TUS FERNANDO
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Prototropic Equilibria in the First Excited Singlet States of Halogenated 8-Quinolinols
by Stephen Schulman and Quintus Fernando Dcpnrtment of Chemistry, University of Arizona, Tucson, Arizona
(Received February 7 , 1967)
The prototropic equilibrium constants of a series of mono- and dihalogenated 8-quinolinols in their first excited singlet states have been measured by a fluorescence titration technique. The values of the equilibrium constants indicate that in the 5-halogen-substituted 8-quinolinols, the acidity order in the excited state is the reverse of that observed in the corresponding halophenols.
The conversion of a molecule from its ground state to an excited state alters the electronic distribution in the molecule to such an extent that the excited state of the molecule may be considered to be a species that is chemically different from the ground state. It is not surprising, then, that the acidities of the molecules in their excited states are quite different from the acidities in their ground states. Two methods are available for the determination of prototropic equilibrium constants in electronically excited states. The first of these methods is due to Forster’ and is based on the energy equivalence of two paths for obtaining the excited state of a base from the ground state of its conjugate acid. The acid dissociation constant in the excited state can be obtained from the Forster cycle only if several assumptions are made. The bases of these assumptions have been discussed extensively by Jaff6.2 If the alternative method for the determination of dissociation constants in the excited state is used, these assumptions need not be made. The method involves the measurement of the variation in concentration of an excited-state species as a function of the solution acidity. For equilibria occurring in excited singlet states, fluorescence, when it occurs, is a valuable tool for the measurement of the concentration of the luminescent species and its variation with acidity. If one or both members of a conjugate acidbase pair is fluorescent, the variation in fluorescence intensity of either species can be determined as a function of pH or of the Hammett acidity scale Ho. If prototropic equilibrium is established w-ithin the lifetime of the first excited singlet state (the fluorescent The Journal of Physical Chemistry
state), a plot of the fluorescence intensity vs. the solution acidity will give a sigmoid curve whose point of inflection is a measure of the dissociation constant in the excited state. This method, which is in effect a fluorescence titration, gives meaningful results only if there are no other processes, such as ground-state prototropic equilibria, occurring in such a way as to alter the concentration of the absorbing species. In addition to this restriction it is a necessary condition that all components of the fluorescent solution be free of luminescent impurities. In this work the fluorescence titration technique has been employed in order to determine the influence of substituents on the excited-state dissociation constants of a series of substituted 8-quinolinols.
Experimental Section Fluorescence spectra were obtained with an AmincoBowman spectrophotofluorometer with a 150-w mercury-xenon lamp as the light source. The emission monochromator of the instrument was calibrated with a Spectroline “pen-ray” low-pressure mercury discharge lamp. This was accomplished by allowing the discharge from the lamp to pass through the narrowest slit setting of the monochromator while rotating the monochromator drum from 800 to 200 mp. The most intense maxima recorded at 730, 365, and 313 mp were used in the calibration. The excitation monochromator was calibrated by placing a small mirror in the cell com(1) T. Forster, 2. Elektrochem., 54, 42 (1950). (2) H. H. Jaff6 and H. L. Jones, J . Org. Chem., 30, 964 (1965).
PROTOTROPIC EQUILIBRIA IN HALOGENATED 8-QUIKOLINOLS
partment at 45" to both the mercury-xenon lamp slit and the emission monochromator slit. The emission monochromator was set at 365 mp and the maximum response obtained by rotating the excitation monochromator was recorded. The emission monochromator was then set at 313 mp and the process repeated. In this may the positions of the 365- and the 313-mp band maxima on the excitation monochromator dial were established. Pure samples of 5-fluoro-, 5-chloro-, 5-bromo-, and 5iodo-S-yuinolinols were obtained from Dr. Herman Gershori of the Boyce Thompson Institute for Plant Research Inc., Yonkers, N. Y. The dihalogenated 8quinolinols were obtained from I< and K Laboratories and purified before use. Many of these halogenated 8-quinolinols have been used for other studies in our laboratory and their purity has been ~ e r i f i e d . ~The sulfuric acid used in this work was obtained from Nallinckrodt Chemical Co.
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Figure 1. Fluorescence spectra of 5-fluoro-8-quinolinol in HzSOl solutions of varying acidity. (Hovalues in order of increasing fluorescence intensity: -5.87; -7.28; -7.72; -8.02; -8.33; -9.37; -10.02; -10.20; -10.40.)
Results All of the halogenated 8-quinolinols were found to fluoresce at room temperature only in concentrated acid solutions. Figure 1 shows the fluorescence spectra obtained for one of the 5-halogen-substituted S-quinolinols. The variation of the fluorescence intensity with the acidity of the solution is shown in Figure 2. The Ho values that have been used in this work correspond to the Ho scale that has been defined for uncharged bases. In Table I are summarized the pK* values that correspond to the Ho values a t the inflection points in the sigmoid curves (Figure 2).
Table I : Excited State Acid Dissociation Constants of Halogenated 8-Quinolinols
4 0
Figure 2. Fluorescence titration of 5-bromo-&quinolinol.
55,7-
5-
5,7-
5-
5-
Iodo
Chloro7-iodo
Dibromo
Bromo
Dichloro
Chloro
Fluoro
-6.7
-7
-7.6
-7.8
-8.2
-9
-11
5-
Several experimental difficulties were encountered in this work. 5,7-Diiodo-8-quinolinol decomposed so rapidly in concentrated HzS04that it was not possible to obtain a fluorescence titration curve for this compound. 5-Chloro-7-iodo-8-quinolinol also decomposed in concentrated HzS04, but its rate of decomposition was slower than that of the 5,7-diiodo compound. Therefore the excited-state dissociation constant reported for this compound is only an approximate value. All the monohalogenated 8-quinolinols reacted with the most concentrated HzS04 solution ( H , =
- 10.41) that was used in this work. This did not present a serious problem with the 5-bromo and 5-iOdO compounds. With the 5-chloro and 5-fluor0 compounds, however, it was found that the fluorescence intensity in an HzS04 solution of Ho = - 10.41 dropped rapidly with time. Moreover, by measuring the fluorescence intensities of these solutions immediately after preparation it was found that the inflection points in the fluorescence titration curves for these compounds could not be obtained even in HzS04 solutions of Ho = -10.41. From the few experimental data points that were obtained, it was apparent that the (3) G. O'Dom and Q.Fernando, Anal. Chem., 38, 844 (1966).
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5-fluoro compound had a larger dissociation constant in the excited state than the 5-chloro compound. Approximate pK* values for the 5-fluoro- and 5-chloro8-quinolinols were obtained from the intercept of the linear plot obtained when ho was plotted against ho/lf where H o = -log hoand I f is the fluorescence intensity. From this work it has been found that the order of increasing excited-state dissociation constants in the substituted S-quinolinols is as given in Table I.
Discussion Ballard and Edwards have shown that in 8-quinolinols and in 8-quinolinol-5-sulfonic acid, the equilibrium involved in the fluorescence titration is that between the excited states of the zwitterion and the cation of the 8-quin0linol.~
Hence, in the excited singlet state, the hydroxy proton of the quinolinium ion is more acidic than the proton on the ring nitrogen. This result is in agreement with the Huckel molecular orbital calculations of Burton and Davis15 which indicate a migration of electronic charge from the homocyclic to the heterocyclic ring of the 8-quinolinol upon excitation from the ground singlet to the first singlet excited state. This calculation also predicts that the heterocyclic ring nitrogen atom will become more basic in the first excited singlet state. Ballard and Edwards have shown this to be true also; the calculated value of the pK* for the equilibrium between the quinolinium ion and the neutral species is 15.3. From the foregoing it seems
reasonable to assume that the pK* values determined by the fluorescence titration technique in this work
The Journal of Physical Chemietry
STEPHEN SCHULMAN AXD QUINTUS FERNANDO
correspond to the equilibria between the cation and the zwitterion species of all the 8-quinolinols studied. The pK* values of two of the monohalogenated 8quinolinols are not known accurately. Therefore it would be meaningless to correlate the pK* values obtained in this work with any type of substituent parameters. A trend, however, can be observed; the pK* values of the &halogenated 8-quinolinols decrease with increasing electronegativity of the halogen atom. Thus, in the first excited singlet state the 5-iodo-8-quinolinol is the weakest acid and the 5fluoro-8-quinolinol the strongest acid. In the series of substituted phenols investigated by Wehry and Rogers16it was found that of the three halogen-substituted compounds, p-fluorophenol was the weakest acid and p-bromophenol the strongest acid in the first excited singlet state. Clearly, the presence of the heterocyclic ring in 8-quinolinol is responsible for the reversal of the acidity order in the excited states of the S-quinolinols as compared with the phenols. The interpretation of the pK* values of the dihalogen-substituted 8-quinolinols is complicated by steric effects that the substituent in the 7 position may have on the phenolic group, The acidities of these compounds do, however, follow an order which seems to be dependent upon the electronegativity of the substituent in the 5 position and either the electronegativity or the steric effect of the substituent in the 7 position. The 5,7-dichloro compound is a weaker acid than the 5-chloro compound and the 5,7-dibromo compound a slightly weaker acid than the 5-bromo compound.
Acknowledgment. The authors are grateful to the National Institutes of Health for financial assistance and to Dr. L. S. Forster of this department for helpful discussions.
(4) R. E. Ballard and J. W. Edwards, J . Chem. SOC.,4868 (1964). (5) R. E.Burton and W. J. Davis, ibid., 1766 (1964). (6) E. L. Wehry and L. R . Rogers, J . Am. Chem. SOC.,87, 4234 (1965).
