J . Phys. Chem. 1994,98, 2357-2366
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Photoinduced Coupled Proton and Electron Transfers. 1. 6-Hydroxyquinoline Elisabeth Bardez,' Arnaud Chatelain, Bernadette Larrey, and Bernard Valeur' Laboratoire de Chimie GCnCrale (CNRS E R 77), Conservatoire National des Arts et MCtiers, 292 Rue Saint- Martin, 75003 Paris, France Received: October 13, 1993; In Final Form: December 14, 1993"
Excited-state processes in 6-hydroxyquinoline (6-HQ) were investigated in acidic, basic, and neutral media. When 6-HQ is in the quinolinium form (Le. with protonated ring nitrogen) in acidic aqueous solutions, the hydroxyl group behaves like a very strong acid in the excited state: deprotonation occurs even in a solution of 10 M HC104! Such a very high photoacidity is explained by the absence of proton back-recombination, as shown by time-resolved measurements, rather than by a high rate constant for deprotonation. The lack of proton recombination, surprising a t first sight in very acidic solutions, is shown to be due to intramolecular electron transfer from the hydroxylate group to the positively charged pyridinium ring as soon as the proton is ejected: this leads to an excited tautomer predominantly in a quinonoid form. Deexcitation of this tautomer occurs via reverse electron transfer, the ground-state form being predominantly zwitterionic; this route of deexcitation is mainly nonradiative as in the case of betaines. In fact, solvatochromism experiments performed on the parent compound 1-methyl-6-oxyquinoliniumconfirm the strong analogy with betaines and in particular with betaine 30 which is known to undergo nonradiative deexcitation, via ultrafast intramolecular electron transfer, toward the ground-state zwitterionic form. When 6-HQ is in the phenolate form in basic aqueous solutions, the heterocyclic nitrogen atom behaves as a very strong base in the excited state, and proton uptake is coupled to fast intramolecular electron transfer from the hydroxylate group to the adjacent ring. Finally, the photophysical behavior of 6-HQ in the neutral form, and in particular its very low quantum yield, can be interpreted in terms of double proton transfer coupled to intramolecular electron transfer. Therefore, it is concluded that all the excited-state processes in 6-HQ can be explained along the same line whatever the acidity or basicity of the solution: the cooperativity between the two functional groups -OH an 1 N in the excited state leads to apparently enhanced photoacidity and photobasicity of the molecule because of the coupling between proton and electron transfers. The high rate of the latter process in the excited state and in the transfer back to the ground state drives the proton transfer which is only limited by the ability of the water molecules to behave as proton acceptors (in concentrated acidic media) or proton donors (in concentrated basic media). No excitedstate equilibrium is ever established. This kinetic scheme relying on coupled proton and electron transfers is also shown to be valid for 5-, 7-, and 8-hydroxyquinolines. Some differences, e.g., in quantum yield of 7-hydroxyquinoline, can be explained in terms of different relative proportions of the zwitterionic and quinonoid forms of the tautomer, depending on the position of the OH group. Moreover, this scheme may be transposable to other classes of bifunctional molecules undergoing phototautomerization.
Introduction Proton transfer and electron transfer represent two of the most fundamental processes involved in chemical reactions and in living systems.' Bifunctional molecules possessing a proton donor group and a proton acceptor group undergo various interesting photoinduced processes:Z intramolecularproton transfer via H-bonded vicinal groups,3 concerted biprotonic transfer within a doubly H-bonded dimer4 or relayed by a bridge of solvent molecules between two distinct gr0ups,5-~double proton transfer at very distant groups! and, as shown in the present paper, coupled proton and electron transfers. Among bifunctional molecules, hydroxyquinolines and their derivativesdeserve special attention from both fundamental and practical points of view. In particular, 8-hydroxyquinoline (or oxine) and many of its derivatives have been extensively studied because they are very well-known chelating and fluorogenic reagents used in analytical chemistry9 although their fluorogenic effect is not yet fully understood. Another interesting feature is that some 8-hydroxyquinolinederivativesare expected to exhibit nonlinear optical properties.10 7-Hydroxyquinolinehas also been the object of interesting papers because it is one of the numerous molecules studied for a better understanding of photoinduced tautomerization: investigations focused on the mechanism by e Abstract published in Aduance ACS Abstracrs, February 1, 1994.
0022-365419412098-2357%04.50/0
which the excited enol form of 7-hydroxyquinoline leads to an aminoketonictautomer in alcoholicsolutionsor in apolar solvents in the presence of low fractions of alcohol. A proton transfer relayed by alcohol molecules was shown to occur in such media.w In the present paper, the discussion will be restricted to hydroxyquinolines possessing the hydroxyl group on the ring adjoining thepyridinic ring, Le. 5-, 6-, 7-, and 8-hydroxyquinolines (hereafter referred to as 5-, 6-, 7-, and 8-HQ), and particular attention is paid to 6-HQ. Basic studies of the luminescence properties of these four derivatives in various environments, and especially in aqueous solutions, were mostly published before 1980:11-19 few of them were carried out from very acidic to very basic media,12J3 but most of them dealt mainly with either concentrated acidic11J"-18or basic19 solutions. In particular, it was shown that the four derivatives exhibit similar behavior:11-18 an intense fluorescence in concentrated acids which is quenched by small additions of water. The only difference between the four compounds is the acidity range where the fluorescence intensity decreases, and the efficiency of quenching; this acidity range is the following, when expressed in terms of Hammett's acidity function: (i) HO= -10 to -6 for 5- and 8-HQ, with total quenching; (ii) Ho = -5 to -2 for 6-HQ, with the concomitant appearance of a very weak green fluorescence; (iii) HO= -4 to 0 for 7-HQ; in this case a marked green fluorescence is observed. Many explanations were put forward to account for these observations,including the existence of a low-lying either (n-a*) 0 1994 American Chemical Society
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The Journal of Physical Chemistry, Vol. 98, No. 9, 1994
or (?r-r*) excited state depending on the environment," or the influence of excited-state hydrogen-bonding interactions on the fluorescence behavior.'* The most convincing interpretations are based on the occurrence of deprotonation in the excited state because the hydroxyl group is known to be more acidic in the excited state than in the ground state. However, the fluorescence quenching in itself for 5-, 6-, and 8-HQ, Le., the fact that the excited-state reaction product is a very poorly fluorescent species, has not been yet clearly understood. Moreover, the kinetic models that were proposed to account for the excited-stateproton transfers are unsatisfactory, as this work will demonstrate. The purpose of the present investigation is then to go more deeply into the interpretation of the experimental features in aqueous solutions and to propose a coherent excited-state mechanism which is valid for media ranging from the strongly acidic to the strongly basic, a mechanism which will be shown to consist of coupled proton and electron transfers. In a first paper, special attention is paid to the 6-hydroxy derivative because, among the four aforementioned hydroxyquinolines, this compound has received the least attention until now.l3J5 The phenomena are analyzed by considering successively acidic, basic, and neutral media, which allows us to establish kinetic models that will be shown to be transposable to 5-, 7-, and 8-hydroxyquinolines.
Experimental Section Materials. 6-Hydroxyquinoline was purchased from Kodak and recrystallized from ether. 6-Methoxyquinoline from Sigma was used without further purification. 1-Methyl-6-hydroxyquinolinium iodide was prepared by the following procedure: 6-HQ in dry toluene was refluxed with methyl iodide for 48 h. The solid product was precipitated by the addition of ether and twice recrystallized from ethanol-ether. 1-Methyl-6-hydroxyquinolinium perchlorate was prepared by passing an aqueous solution of iodide salt over an anion exchanger in perchlorate form. The resulting aqueous solution was checked to see if it was free of iodide ions and used for quantum yield measurements without prior crystallization of perchlorate salt. l-Methyl-7hydroxyquinolinium iodide was purchased from Lambda Probes and used as received. f2-l.cm-l at Millipore filtered water (conductivity < 1 X 25 "C) was employed to prepare the aqueous solutions. Two different reagent grade perchloric acids (-70%) from Prolabo and from Aldrich were used because of possible impurities likely to react with hydroxyquinolines;'6 the two acids were shown to give similar results, which was considered to be satisfying. Analytical grade sodium hydroxide from Merck and anhydrous sodium perchlorate from Sigma were used as received. 1,4Dioxane from Merck was spectroscopic grade. Methods. UV-visible absorption spectra were recorded on a Kontron Uvikon-940 spectrophotometer. Corrected fluorescence spectra were obtained with a SLM 8000 C spectrofluorometer. Quantum yields @pweremeasured usingcoumarin 2 ( @ p ~ = 0.77)20 and coumarin 6 (@F = 0.78)21 as standards. A typical concentration of 8.7 X 10-5 M in 6-HQ was used for the spectral measurements. Comparison of the spectra requires that each of them is recorded at exactly the same concentration in 6-HQ. On the other hand, when looking at the evolution of the spectra as a function of the acidity or the basicity of the medium, it is not necessary to know very accurately the concentrationsin perchloric acid or sodium hydroxide. Indeed, the sample preparation procedure consisted in mixing in the cuvettes various proportions of both 1 1.7 M HC104 (or 13 M NaOH) and aqueous solutions containing exactly the same concentration (8.7 X 10-5 M) of 5-HQ. Titration performed on aliquots of the obtained mixtures showed that the acid or base concentration is known within a maximum error of 5%. Time-resolved experiments were performed with our multifrequency phase/modulation fluorometer equipped with a He-
Bardez et al.
250
270
290
310 330 350 wavelength (nm)
370
390
410
Figure I. Absorption spectra of 6-HQ in acidic ([HCIOI] = 0.1 MI), neutral (pH = 7), and basic ([NaOH] = 1 M) aqueous solutions.
Cd (kc = 325 nm) laser and a Pockels cell operatingat frequencies ranging from 0.1 to 200 M H Z . ~The concentration of the solutions was of the order of 2 X le5 M. Identical concentrations of 6-HQ are not necessary for these measurements. The only constraint is that the absorbance at the excitation wavelength is kept low enough ([HQ] < 2 X 10-5 M) so as to avoid parasitic effects. On the other hand, in the perchloric media, quantitative use of the data requires accurate values of HC104 concentrations in order to determine the corresponding water activity. Therefore, large quantities of the relevant perchloric solutions were prepared in volumetric flasks, and a proper quantity of 6-HQ from a concentrated aqueous stock solution was added directly in the cuvettes. For these measurements, the error in the concentration of HC104 is estimated to be less than 1%. The determination of the two ground-state pKa values of 6-HQ, with a gap of only 4 units, was performed spectrophotometrically using the treatment recommended by ONeal and S ~ h u l m a n ? ~ and the values were found to be 5.1 f 0.1 (>NH+/ZN) and 9.2 f 0.1 (-OH/+) in quite close agreement with the values of 5.17 and 8.88 previously determined.24 The pKa value of the hydroxyl group of 1-methyl-6-hydroxyquinoliniumion was found tobe 7.0f0.1,correspondingcloselytopreviouslyobtained results (7.15;257.1-7.226); it is noteworthy that this value is more than 2 units lower than in the parent molecule 6-HQ, which shows that the electron-withdrawing effect of the quaternated nitrogen atom increases the acidity of the hydroxyl group in the ground state. Solvatochromism measurements were performed on the UVvisible absorption spectra of 1-methyl-6-oxyquinolinium (or 1-methyl-7-oxyquinolinium)solutions obtained by addition of sodium hydroxide up to pH 10 to solutions of l-methyl-6-(or 7-)hydroxyquinolinium in 1,4-dioxane/water mixtures.
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Results and Discussion The ground-state prototropic equilibria established with the bifunctional molecule 6-HQ in aqueous solutions are governed by the PKa values of 5.1 and 9.2 (see Experimental Section). Consequently, 6-HQ exists almost exclusively in the cationic (quinolinium) form at pH values lower than 3.1 and in the anionic (hydroxylate) form at pH values larger than 1 1.2. At a pH value of ca. 7, the predominant form is the neutral form. These three forms will be hereafter denoted 6-HQ(C), 6-HQ(A), and 6-HQ(N)*
Figure 1 shows the UV absorption spectra of the three forms in dilute solutions. The long-wavelength absorption maxima of 6-HQ(C), 6-HQ(N), and 6-HQ(A) are located at 313 (and 344 nm), 326, and 358 nm, respectively, in accordance with previous re~u1ts.l~ In this investigation, each of the three forms of 6-HQ
The Journal of Physical Chemistry, Vol. 98, No. 9, 1994 2359
Coupled Proton and Electron Transfers
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