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J. Phys. Chem. 1994, 98, 11424- 11427
Ground-State Triple Proton Transfer in 7-Hydroxyquinoline. 4. Observation in Room-Temperature Methanol and Aqueous Solutions Aziz Bohra, Anita Lavin, and Susan Collins* Department of Chemistry, California State University, Northridge, Northridge, California 9131 I Received: March 9, 1994; In Final Form: July 21, 1994@
The stabilization of the ground-state keto tautomer of 7-hydroxyquinoline (7HQ) has been observed in roomtemperature methanol and aqueous solutions. Concentration studies have shown that the 7HQ cyclic dimer is formed at high 7HQ concentrations in methanol and water, and either the 7HQ-(MeOH)z or the 7HQwater complex, presumably the cyclic 7HQ-(H20)2 structure, is formed at low 7HQ concentrations in methanol or water solvents, respectively. These species give rise to the keto-tautomer excitation band centered at 398 nm (7HQ dimer in water), 420 nm (7HQ dimer in methanol), 377 nm (7HQ-water complex), and 387 nm (7HQ-methanol complex). The emission band for all species appears at 520 nm. This is the first report of the ground-state stabilized keto tautomer spectrum in methanol at room temperature. The spectrum in water has been investigated in the past, but the interpretation differs from that of this work.
Introduction
In several recent papers, we have reported on the stabilization of the keto tautomer of 7-hydroxyquinoline, 7HQ, through a ground-state triple proton transfer process involving the 7HQ(MeOH)2 cyclic complex or the ground-state double proton transfer process involving the (7HQ)2 cyclic dimer at 10 K using matrix isolation fluorescence spectroscopy (see Figure l).1-3 Before our work, the room-temperature excited-state proton transfer, ESPT, process occurring in dilute solutions of 7HQ in methanol had been studied by numerous researcher^.^-^ Excitation of the enol-normal molecule at 320 nm leads to ESPT to form the excited and ground states of the keto-tautomer form followed by regeneration of the enol-normal form. Our matrix work showed that the degree and kind of methanol aggregation gives rise to three bands in the excitation spectrum when the keto-tautomer emission is monitored at 520 nm at 10 K in argon. The band at 315 nm is due to the 7HQ-highly aggregated methanol complex of the enol-normal form. The band at 340 nm is due to the 7HQ-lesser aggregated methanol complex of the enol-normal form. The band at 420 nm is due to the cyclic 7HQ-(MeOH)z complex which undergoes groundstate proton transfer, GSPT, to form the keto tautomer. Here we report that the stabilized ground-state keto tautomer also is found in room-temperature aqueous and methanol solutions of 7HQ. Results 7HQ in Methanol Solutions at Room Temperature. Figure 2 shows the fluorescence excitation and emission spectra of a 9.0 x M solution of 7HQ in dry methanol. The excitation spectrum (Amax = 420 nm) was obtained by monitoring the emission at 520 nm. The emission spectrum (A, = 520 nm) was obtained by excitation at 420 nm. These spectra agree well with spectra of the ground-state stabilized keto tautomer in refs 1 and 2. They also agree with the transient absorption spectrum generated by initial excitation of the enol-normal band at 320 nm with subsequent formation of the transient keto tautomer by ESPT and the two-step laser-induced fluorescence spectrum obtained by Itoh et aL4 @
Abstract published in Advance ACS Abstracts, October 1, 1994.
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Figure 1. Enol-keto tautomerization for (A) the 7HQ-(MeOH)* complex and (B) the (7HQ)z dimer.
Figure 3A shows the excitation spectra of 7HQ in methanol as a function of 7HQ concentration generated by monitoring the emission at 520 nm. At the high concentrations of 9.0 x M (spectrum A) and 4.5 x lob3 M (spectrum B), the emissions are strongly quenched. The spectra exhibit two bands at 298 and 350 nm. At the intermediate concentrations of 2.3 x M (spectrum C) and 9.0 x M (spectrum D), the band at 298 nm appears to be red-shifted, presumably due to the fact that a new band at 330 nm grows in and then predominates with higher dilution. At the low concentrations M and 1.8 x M (spectra D-H), only between 9.0 x the band at 330 nm is observed. Also seen is the band at 420 nm in Figure 3A. The expanded view of this band is shown in Figure 3B,C. At the same high and intermediate concentrations (spectra A-E), only the 420 nm band is observed. At the low concentrations, spectra F-H in Figure 3C illustrate two bands, one at 387 nm and the other at 420 nm. Although the spectra are not shown, when excited at 298, 330, and 350 nm, the familiar enol-normal and ketotautomer emission bands are observed at 385 and 520 nm due to ESFT. The keto-tautomer to enol-normal emission band intensity ratio is 0.75 and does not change with 7HQ concentration. When excited at 387 or 420 nm, emission is observed at 520 nm only, due to direct excitation of the ground-state
0 1994 American Chemical Society
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Figure 2. Fluorescence excitation and emission spectra of 9.0 x M 7HQ in methanol at room temperature. The excitation curve is generated by monitoring the emission at 520 nm. The emission curve is generated by excitation at 420 nm.
stabilized keto tautomer. Although it would be desirable for a complete picture of the 7HQ system to investigate it in a hydrocarbon or ether solvent with varying amounts of added alcohol, it was not possible due to the low solubility of 7HQ in hydrocarbon and ether solvents. 7HQ in Aqueous Solutions at Room Temperature. Figure 4 shows the fluorescence excitation and emission spectra of a 1.0 x lop4M solution of 7HQ in H20. The excitation spectrum (A = 398 nm) was obtained by monitoring the emission at 500 nm. The emission spectrum (2- = 510 nm) was obtained by excitation at 400 nm. These spectra are identical to those of Figure 2 except for small wavelength difference. Figure 5A shows the excitation spectra of 7HQ in H20 as a function of 7HQ concentration generated by monitoring the emission at 500 nm. A dramatic effect is that the ground-state stabilized keto-tautomer band at 398 nm appears with greater intensity than the enol-normal band at 330 nm. Note in Figure 3A the opposite trend for methanol solutions. Also, the enolnormal band position does not change with concentration as in the case of methanol, whereas the keto-tautomer band position does. At the high concentrations, 1.0 x 10-4-8.0 x M (spectra A-C), the keto band appears at 398 nm. At the low concentrations of 8.0 x and 4.0 x low6M (spectra D and E in Figure 5B),an additional band is seen at 377 nm. Figure 6 shows the emission spectra generated by excitation at 330 nm as a function of 7HQ concentration. The ESPT fluorescence band at 510 nm is 7 times more intense than the enol-normal band at 417 nm for the 1.0 x M solution in Figure 6A. The enol-normal to keto-tautomer relative band intensity ratio decreases with concentration and drops to 0.9 for the 4.0 x M solution in Figure 6B. Finally we note that the solubility of 7HQ in HzO is less than in methanol. The highest possible 7HQ concentration used in H20 was 1.0 x lop4M and in methanol was 9.0 x M.
Discussion The wavelength region of 370-480 nm in the excitation spectra of Figures 2-5 clearly show that the ground-state keto tautomer of 7HQ is stabilized to a small extent in methanol and to a large extent in water at room temperature. We assume that the 7HQ-water complex is cyclic and it involves two water molecules as in the case of methanol, but we have not investigated this in detail. The existence of the water complex
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Figure 3. (A, top) Fluorescence excitation spectra of 7HQ in methanol solutions generated by monitoring the emission at 520 nm. The following concentrations generated the spectra: A (9.0 x lo-’ M), B (4.5 x 10-3 MI, c (2.3 x 10-3 MI, D (9.0 x 10-4 MI, E (1.75 x 10-4 M), F (4.5 x 10-4 M), G (9.0 x 10-5 M), H (1.8 x 10-5 M). (B, middle) Expanded view of Figure 3A. (C, bottom) Expanded view of Figure 3B. had been missed by us in our previous studies and by other
researchers studying the ESPT process. The fact that the keto form of the complex is stabilized at room temperature opens
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Figure 4. Fluorescence excitation and emission spectra of 1.0 x M 7HQ in water at room temperature. The excitation curve is generated by monitoring the emission at 500 nm. The emission curve is generated by excitation at 400 nm.
up the possibility that our tunneling hypothesis, suggested in ref 3, can be tested by variable temperature studies. Also note that in our previous matrix studies involving methanol, we have emphasized the point that the cyclic complex was matrix isolated. Now it appears not to be a requirement for the GSPT process. Figure 3 shows that in methanol there are high- and lowconcentration regimes. The enol-normal excitation band at 298 nm is associated with high 7HQ concentrations. We attribute it to be due to 7HQ molecules hydrogen bonded to each other, which upon excitation rearrange to form the 7HQ dimer. Proton transfer occurs from the excited state of the dimer. This twostep process is similar to that proposed by Konijnenberg et al. for room-temperature solutions of the 7HQ-methanol system and also in refs 1-3. The keto-tautomer band at 420 nm is associated with high 7HQ concentration. We attribute it to the 7HQ cyclic dimer complex which undergoes GSFT from the enol-normal form. The band at 350 nm is present at high concentration also. It disappears more rapidly than the band at 298 nm upon dilution of 7HQ, thus it is probably another species involving aggregated 7HQ. The band at 330 nm in Figure 3A is associated with dilute 7HQ solutions. We attribute it to 7HQ bonded to methanol in a noncyclic fashion. These complexes after excitation and subsequent rearrangement to form the 7HQ-(MeOH)z cyclic complex undergo ESPT. This is the complex that has been investigated by many researchers over the year^.^-^ As mentioned above, Konijnenberg et al. determined it to follow a two-step process, involving solvent rearrangement to the “proper” two-methanol complex with excited 7HQ followed by proton t r a n ~ f e r .The ~ band at 387 nm in Figure 3C is associated with dilute solutions only and therefore is attributed to the 7HQ-(MeOH)z bridged complex in the keto form. It forms in the ground state from the enol molecule and subsequently undergoes GSPT to form the stabilized keto tautomer. The analysis of the excitation spectra of 7HQ in water in Figure 5A,B is similar to that of Figure 3A,B except that the enol-normal excitation band position at 330 nm does not change with 7HQ concentration. Thus the loose configuration of 7HQ molecules that rearrange upon excitation to the cyclic dimer absorbs at the same position as the loose configuration of 7HQwater molecules that rearrange upon excitation to the 7HQwater cyclic complex.
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Figure 5. (A, top) Fluorescence excitation spectra of 7HQ in water at room temperature generated by monitoring the emission at 500 nm. The following concentrations generated the spectra: A (1.0 x MI, B (4.0x 10-5 M), c (2.0 x 10-5 M), D (8.0 x 10-6 MI, E (4.0 x M). (B, bottom) Expanded view of Figure 5A.
The fact that the stabilized tautomer band intensity at 398 nm in the excitation spectrum is greater than the enol-normal band intensity for all 7HQ concentrations in aqueous solutions is unusual. From the literature we know that the 7-azaindole monoaquo cyclic complex is very hard to form in the ground or excited states in the presence of competitive hydrogen bonding between water molecule^.^ Consequently ESPT does not occur. Hydrogen bonding occurs, but in a noncyclic fashion resulting in normal-molecule exciplex emission. Here it appears to be easier to form the 7HQ-water cyclic complex which undergoes GSFT, than to form noncyclic water hydrogenbonded to 7HQ which would stay in the enol form until excitation. Perhaps there is not much energy difference between the keto forms involving cyclic and noncyclic water hydrogen bonds, and both are in fact more stable than the enol molecule. Then the keto molecule would be formed in the presumed cyclic geometry due to a kinetic effect, rather than a thermodynamic effect. Also the higher probability of finding the keto tautomer in water may possibly be because two water molecules would make a more compact structure with 7HQ than would two methanol molecules, thereby contributing to the ease of formation of the complex. Figures 6 shows that at high concentrations of 7HQ in water
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formed from the ESPT process to give a higher yield of tautomer molecule emission than the normal molecule emission. The emission from the 7HQ-water complex formed from the ESPT process gives a higher yield of normal molecule emission than the tautomer emission. It is not clear at this point why ESPT in the cyclic dimer would give a higher keto tautomer molecule emission yield in aqueous solutions than in methanol solutions. The fact that the keto tautomer absorbs at 420 nm accounts for the fact that highly concentrated solutions of 7HQ in methanol and water are green. Mason had previously attributed this effect as being due to a ground-state zwitterionic structure.* While the structure of the tautomer has not yet been proven, we point out here that the excitation of the stabilized tautomer at 420 nm gives the samepuorescence spectrum at 520 nm as that obtained by ESPT when the enol-normal molecule is excited at 320 nm. The ESPT literature exclusively refers to the emission as being from the keto tautomer. We plan to investigate the structural aspects of the keto tautomer using matrix-isolation FTIR techniques.
Experimental Section Fluorescence excitation and emission spectra were obtained using a SPEX 1681 Fluorolog spectrometer. One-millimeterpath-length cells were used. 7HQ (Karl Industries, 99% pure) was recrystallized from methanol. Reagent-grade methanol (Mallinckrodt) was purified and dried by distillation over Mg(s) in a N2 atmosphere. It was stored over 3A vacuum dried molecular sieves. During some runs, the methanol was found to still have a fluorescent impurity. It was then run down a 8 in. basic alumina column (Aldrich) and then redistilled under Nz without Mg. DI water was boiled to remove COS. For some runs N2 was bubbled through the solutions to remove 0 2 . No differences in the spectra were seen.
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Figure 6. (A, top) Fluorescence emission spectra of 7HQ at room temperature generated by excitation at 330 nm. The following concentrations generated the spectra: A (1.0 x M), B (4.0 x 10-5 M), c (2.0 x 10-5 MI, D (8.0 x 10-6 M), E (4.0x 10-6). (B, bottom) Expanded view of Figure 6A.
the keto-tautomer emission band is more intense than the enolnormal molecule emission band when excited at 330 nm. At low 7HQ concentrations the opposite trend is found. We interpret this as emission from the 7HQ cyclic dimer in water
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. References and Notes (1) Lavin, A,; Collins, S. Chem. Phys. Lett. 1993, 204, 94. (2) Lavin, A,; Collins, S. Chem. Phys. Left. 1993, 207, 513. (3) Lavin, A,; Collins, S. J . Phys. Chem., in press. (4) Itch, M.; Adachi, T.; Tokumura, K. J . Am. Chem. SOC. 1984, 106, 850. (5) Konijnenberg, J.; Eckelmans, G.; Huzier, A,; Varma, C. J . Chem. SOC., Faraday Trans. 1989, 85, 39. (6) Thistlethwaite, P. Chem. Phys. Len. 1983, 96, 509. (7) Chou, P.; Martinez, M.; Cooper, W.; Collins, S.; McMorrow, D.; Kasha, M. J . Phys. Chem. 1992, 96, 5203. (8) Mason, S. J . Chem. SOC. 1957, 5010.
