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Luminescence Studies of Quinolineand lsoquinoline
Luminescence Studies of Proton Transfer in the Excited Electronic States of Hydrogen Bonded Quinoline and lsoquinoline William R. Moomaw' and Mary F. Anton Department of Chemistry, Williams College, Wllliamsfown, Massachusetts 0 1267 (Received February 27, 1976) Publication costs assisted by The Camille and Henry Dreyfus Foundation
In the absence of a hydrogen bonder, quinoline only exhibits phosphorescence in a 3-methypentane glass at 77 K. Under these conditions isoquinoline both fluoresces and phosphoresces weakly. The addition of a weak hydrogen bonder such as ethanol induces structured ultraviolet fluorescence originating from the lowest excited singlet state. More acidic alcohols such as trifluoroethanol and hexafluoro-2-propanol induce both the structured ultraviolet fluorescence, and a now broad, violet fluorescence. This new fluorescence resembles, but differs from, that of the corresponding protonated cation. An analysis of the excitation spectrum, which differs from the excitation spectrum of the structured fluorescence, leads to the conclusion that the proton has been displaced toward the nitrogen heterocycle in the excited state.
I. Introduction Recently, we reported some detailed studies of the role of hydrogen bonding on the luminescence properties of quinoline and isoquinoline.1,2In the case of quinoline we were able to show that in a 3-methylpentane glass at 77 K, phosphorescence occurs only from non-hydrogen bonded molecules, structured fluorescence from molecules hydrogen bonded in both the ground and excited states, and that a protonated-cationlike fluorescence occurs from molecules that are probably hydrogen bonded in the ground state, but which undergo at least a partial proton transfer in the excited state. An upper limit for the fluorescence to phosphorescence ratio of 20 ppm was established for non-hydrogen bonded quinoline, by utilizing an extensive purification method. It was not possible to obtain such rigorous limits for isoquinoline, although the findings of the same three types of luminescence leads us to the conclusion that the luminescence properties of isoquinoline are similar to those of quinoline. It is the purpose of the present work to elucidate the luminescence mechanism of the protonated-cationlike fluorescence in quinoline and isoquinoline. The recently reported work of Taylor, El-Bayoumi, and Kasha3 on the excited state biprotonic tunneling in dimers of 7-azaindole has generated much interest in the hydrogen bonding and proton affinities of electronically excited states. 11. Experimental Section Quinoline was obtained from Aldrich Chemical Co. as a yellowish brown liquid. Isoquinoline obtained from J. T. Baker was yellow. Vacuum distillation produces a colorless liquid in both cases which darkens upon standing, and which contains traces of the other isomer as an impurity. Both substances were purified as the picrate: by adding the base to picric acid dissolved in a minimum of 95% ethanol. The bright yellow crystals were washed with ethanol, air dried, and recrystallized from acetonitrile. The free base was regenerated by dissolving the picrate in dimethyl sulfoxide which had been dried over 4A molecular sieves. The bright yellow solution was passed down a basic alumina column to which the picric acid adsorbed, releasing the base. The effluent was extracted with spectral grade n-pentane and distilled under vacuum. Residual amounts of solvent could be readily removed by vapor
phase chromatography. Quinoline and isoquinoline prepared in this way remained colorless even after standing in air for periods in excess of 1year. 3-Methylpentane was Phillips pure grade, which was shaken with three different aliquots of fuming sulfuric acid, washed with water, dried over anhydrous sodium sulfate, and refluxed for 12 h over metallic sodium. The 3-methylpentane was then distilled and passed through a neutral alumina column coated with silver nitrate.5 No detectable luminescence could be observed from solvent prepared in this way. Hexafluoro-2-propanol was obtained from Pierce, and found to have an impurity which luminesced around 350 nm. Distillation from 3A molecular sieves followed by storage over fresh sieves a t 0 "C proved to be an adequate remedy. Trifluoroethanol (TFE) was also obtained from Pierce, and was found to be suitable as received. It was found that traces of chromic acid cleaning solution cling to ground glass joints and must be rigorously excluded to avoid forming the protonated quinolinium and isoquinolinium cations. The concentration of quindine or isoquinoline and the added hydrogen bonding alcohols were kept in the concentration range of to M to avoid solubility and alcohol aggregation problems. The reported luminescence and excitation spectra are uncorrected for wavelength response of the measuring system. McPherson Model EU-700 monochrometers blazed at 500 nm for luminescence and 250 nm for excitation were used with an effective band pass of about 1.0 nm. A Hanovia 1000-W xenon lamp was used for excitation, and an EM19789 QA photomultiplier connected to an SSR 1105 photon counter was used for detection. A Cary 14 double beam spectrophotometer was used for all absorption spectra. All measurements were carried out on samples immersed in liquid nitrogen at 77 K. 111. Results
As previously reported,1,2 quinoline in 3-methylpentane (3MP) a t a temperature of 77 K only shows characteristic green phosphorescence starting a t 457.2 nm (21 870 cm-l). The addition of ethanol (pK, = 17) causes the appearance of structured naphthalene-like ultraviolet fluorescence beginning at 313.0 nm (31 950 cm-l). The addition of a more acidic The Journal of Physical Chemistry, Vol. 80, No. 20, 1976
W. R. Moomaw and M. F. Anton
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(b) lsoquinoline t H R P / x ‘O
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Figure 1. The total luminescence spectrum of a 1.6 X M solution of quinoline in 3MP containing 3.7 X M HFPP (a)and of a 1.O X M solution of isoquinoline in 3MP containing 0.55 X lou4 M HFPP (b). Both spectra were measured at a temperature of 77 K. The cationlike fluorescence near 400 nm is between the structured fluorescence near 300 nm and the phosphorescence near 450 nm.
alcohol such as hexafluoro-2-propanol (HFBP, pK, = 9.3) produces a dramatic change in the visible luminescence from green to violet. The reason for this change is readily apparent in the spectrum shown in Figure la. A strong broad band peaking a t 384.0 nm (26 040 cm-l) is now present, and the phosphorescence and regular, structured fluorescence appear weakly on either side of it. The similarity of this new emission to that of the protonated cation may be seen in Figure 2, where the quinolinium fluorescence spectrum is shown. Quinolinium cation was prepared by bubbling gaseous hydrogen chloride into a quinoline solution in 3-methylpentane, allowing the cloudly solution to clear, and then freezing it to a clear glass in liquid nitrogen. It is to be noted that the maximum of the quinolinium cation fluorescence appears at 400.0 nm (25 000 cm-l). This is lower in energy than any of the spectra appearing in the HF2P containing solutions of quinoline. The excitation spectrum of the broad cationlike fluorescence of quinoline is compared with the structured fluorescence excitation spectrum of the same solution in Figure 3. The two excitation spectra bear a superficial resemblance to each other in location and band structure (the 0,O bands of both are at 313.2 nm or 31 930 cm-l). While the structured fluorescence excitation spectrum and the absorption spectrum are essentially identical,2 the cationlike fluorescence excitation spectrum is much broader. The rather sharp drop-off in excitation intensity below 290 nm is mostly due to the lower excitation lamp intensity in this region, but the lL, band a t 275 nm is genuinely attenuated relative to the non-hydrogen bonded spectrum. Of equal importance is the comparison of the excitation spectra of the cationlike and genuine cation fluorescence. The cation excitation spectrum is broad like the fluorescence, and closely resembles the cation absorption spectra in other solvents published elsewhere.6 No quinolinium or isoquinolinium cation phosphorescence has ever been reported, and we were unable to detect any. A sizable Stoke’s shift can be seen for both the cationlike fluorescence (Figure l a ) and the cation fluorescence (Figure 2). The luminescence properties of isoquinoline are very similar to those of quinoline. A major difference is that isoquinoline in 3-methylpentane both fluoresces and phosphoresces. There is evidence1,Zto suggest that some residual hydrogen bonder The Journal of Physical Chemistry, Vol. 80, No. 20, 1976
is responsible for the fluorescence. Like quinoline, however, the addition of hexfluoro-2-propanol in roughly equimolar amounts produces a broad cationlike violet fluorescence. The total luminescence is shown in Figure Ib, and the excitation spectra are shown in Figure 4. As is the case for quinoline, the cationlike fluorescence excitation spectrum is seen to be broad compared to the structured fluorescence excitation spectrum. In addition, the 0,O band is shifted about 1.6 nm (160 cm-1) to lower energy from the structured fluorescence excitation at 319.4 f 0.3 nm (31 310 cm-I). The position of the isoquinoline spectrum is much more solvent dependent than is quinoline. Another interesting feature of the isquinoline cationlike fluorescence is the sensitivity of its shape to proton donor type and concentration. For an isoquinoline solution 1.0 X 10-4 M, for which the concentration of HF2P is half that amount, the cationlike fluorescence maximum appears a t 390.0 f 0.3 nm (25 640 cm-l). Increasing the concentration of HF2P eightfold causes a shift of the maximum to 396.6 nm. The 0,O band of the excitation shifts by a smaller amount from 321.0 f 0.3 nm (31 150 cm-l) to 321.8 f 0.3 nm (31 090 cm-l). In an equimolar mixture of isoquinoline and trifluoroethanol(1.0 X lod4 M) the broad fluorescence maximum is a t 396.0 nm (25 250 cm-l). The 0,O band of the excitation spectrum in this case lies at 321.0 nm.
IV. Discussion There are three important cases to be considered in determining the nature of the fluorescence in these systems. The first is that of quinoline or isoquinoline hydrogen bonded to a weak proton donor such as ethanol. The second is that of the fully protonated quinolinium or isoquinolinium cation, and the third is the apparently intermediate case of these bases hydrogen bonded to a strong proton donor such as hexafluoro-2-propanol (HF2P) or trifluoroethanol (TFE). Both quinoline and isoquinoline show well-structured absorption and fluorescence spectra when weakly hydrogen bonded in s o l ~ t i o n . l , ~Later - ~ , ~ reports of non-hydrogen bonded quinoline fluorescencegJOhave been shown to be in error.2 The hydrogen bonded induced fluorescence appears to originate from a state of TT* character: although there is strong evidence of vibronic interaction and electronic state congestionl1J2 in that region. It would appear reasonable, therefore, to assume a similar degree of hydrogen bonding in the ground and excited states. The protonation of quinoline and isoquinoline by a strong acid such as HCl produces major changes in the absorption and luminescence spectra. Both the absorption and fluorescence spectra of the protonated cations are broad structureless, roughly symmetrical bands, possessing an approximate mirror image relationship to each other (Figure 2). For quinoline the mirror image relationship is not as clear as it might be because of the strong overlapping of the IL, and l L b bands in absorption.6 The small overlap of the absorption and fluorescence of the protonated cations, and the low transition probability in the region of the 0,O transitions, suggest a large geometry difference between the ground and fluorescing states. Another possibility is that vibronic coupling not found in the unprotonated bases themselves may be playing a role. The basis for either of these explanations is not understood, nor is the lack of vibronic structure. Perhaps the broadness arises from the sensitivity of the proton normal modes to the large range of solvent environments. The situation for quinoline and isoquinoline hydrogen bonded to strong hydrogen bonders such as HF2P or T F E is
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Luminescence Studies of Quinoline and lsoquinoline
QU 1 NOLI N I UM CH LORI DE
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M quinolinium chloride solution in 3MP at 77 K.
Figure 2. The fluorescence (solid line) and fluorescence excitation (dashes) spectra of a
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Flgure 3. Quinoline fluorescence is shown by the solid line. The excitation spectrum of the structured fluorescence, which corresponds to the absorption spectrum, is shown by dots, while the excitation spectrum of the broad fluorescence is shown by dashes. Arrow denote either wavelengths monitored or excited.
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Figure 4. Isoquinoline fluorescence is shown by the solid line. The excitation spectrum of the structured fluorescence, which corresponds to the absorption spectrum, is shown by dots, while the excitation spectrum of the broad fluorescence is shown by dashes. Arrows denote either wavelengths monitored or excited.
The Journal of Physical Chemistry, Vol. 80, No. 20, 1976
W. R. Moomaw and M. F. Anton
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I
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Figure 5. Possible potential energy surfaces for the ground and first excited electronic states plotted as a function of proton position between the oxygen of the alcohol and the nitrogen of the heterocycle. The minima of the oxygen and nitrogen wells are indicated by Qeoand QeN. (a)Distant hydrogen bond gives rise to structured fluorescence. (b) Close hydrogen bond permits excited state proton tunneling, and broad cationlike fluorescence. Dotted curve represents possible low ground state barrier. much more complicated than either of the situations so far described. The phosphorescence can be’shown by the excitation spectrum to arise from non-hydrogen bonded molecules,2 but the presence of both broad and sharp fluorescence requires a more elaborate explanation. The excitation spectra of the structured fluorescence in the presence of HFBP or T F E are essentially the same as the absorption spectra for each molecule. The fluorescence itself is very similar to that induced by ethanol, although the position of the origin band in isoquinoline differs by as much as 1.0 nm from its value in the presence of ethanol. Since structured fluorescence is observed from the quinoline solution to which HFBP is added, hydrogen bonding must be responsible for this fluorescence just as it is for ethanol. The appearance in these solutions of broad fluorescence, which resembles that of the protonated cation, is intriguing. One possibility is that some quinoline or isoquinoline is simply protonated in the ground state either by the fluoroalcohol or perhaps even by an acidic impurity. That this is not the case can be demonstrated both by the subtle ways in which this fluorescence differs from that of the true protonated cation, and by the fluorescence excitation spectrum. As previously noted, the excitation spectrum of the broad violet fluorescence resembles that of the structured fluorescence. It differs, however, in being broader, and having significant intensity on the red side of the origin band. In isoquinoline, the maximum of the 0,O band in excitation is sensitive to the alcohol concentration, and actually lies from 1 to 3 nm below the 0,O band in absorption (or structured fluorescence excitation). This altered excitation spectrum clearly shows that the heterocycles are not protonated in the ground state, and that the broad and structured fluorescences originate from two different ensembles of molecules. The large Stokes shift or gap between the onset of the broad fluorescence and its excitation spectrum, and the lack of mirror symmetry of these spectral features imply that absorption and emission do not involve the same upper state potential surface. An attractive explanation of these observations can be had by postulating a double minimum potential for the proton in the excited state. See Figure 5. One potential minimum would be centered near the hydroxylic oxygen of the alcohol, and the second, lower The Journal of Physical Chemistry, Vol. 80, No. 20, 1976
well, would be near the heterocyclic nitrogen. For any given molecule, the distance between these two wells, and the height and thickness of the barrier between them, is fixed by the rigid glass matrix.13 For both ensembles of fluorescing molecules, according to this scheme, absorption occurs from a hydrogen bonded ground state to a hydrogen bonded excited state, Le., to the geometrical configuration in which the proton is located in the oxygen potential well. This would of course be required by the Franck-Condon principle for a conventional ground state hydrogen bond. In the case of “close” alcohol-heterocyclic complexes, the barrier is readily penetrated by the proton during the lifetime of the excited state, and a “semiprotonation” of the heterocycle takes place as the proton drops into the “nitrogen well”. The broad, violet quinolinium- or isoquinolinium-like fluorescence then takes place as these molecules return to their ground state. In the ground state, the proton then relaxes back to its original position, closer to the oxygen atom. It is not necessary that the ground state possess a double minimum, but, it may, provided that the barrier is not too high to be readily penetrated. The presence of the nearby hydroxylic oxygen and its effect on the shape of the “nitrogen well” region of the potential surface presumably accounts for the difference between the quinolinium-like and the true quinolinium fluorescence spectra. The greater breadth of the excitation spectrum of the broad quinolinium-like fluorescence relative to that of the structured naphthalene-like fluorescence may also be explained by the double minimum potential model. The vibrational levels within the oxygen potential well will be split by the presence of nearly degenerate levels in the nitrogen potential well leading to the observed broadening. This explanation also accounts for the subtle changes in the excitation spectra resulting from the use of different fluoroalcohols and concentrations. Another possible explanation of the broadening is the shortened lifetime of the upper state of the “oxygen well” resulting from proton tunneling. Additional measurements clearly need to be done to either confirm or deny these speculations. The simultaneous appearance of the structured naphthalene-like fluorescence is also readily explained by this model. Alcohol-heterocycle complexes separated by large distances or possessing unfavorable geometries will have an upper state potential barrier between the oxygen and nitrogen potential wells which is too thick or high for the proton to penetrate during the lifetime of the excited states. For these systems the net result will be similar to that which occurs when the heterocycle is more weakly hydrogen bonded by ethanol. In this latter case, the barrier is presumably high because of the depth of the “oxygen-well” as evidenced by the low ethanol acidity and weaker hydrogen bonding capability in the ground state. Previous work by three other research groups bears on the observations reported here. Ermolaev and Kotlyar14 report that the “violet-blue luminescence of the quinolinium ion” appears in petroleum ether solutions to which 0.5-2% of a variety of alcohols have been added. When these alcohols are added in lower concentration, 0.25-0.5%, or when isopropyl alcohol is used, only structured fluorescence is observed. They report similar effects when water or acetic acid are used. Unfortunately, they report no fluorescence or excitation spectra, and, we are unable to reproduce their results using ethanol in 3-methylpentane. We find that 1%ethanol added to a M quinoline solution in 3-methylpentane produces a cracked glass, and only structured fluorescence. A similar solution
Luminescence Studies of Quinoline and Isoquinoline which had been saturated with water showed only phosphorescence. Perhaps the structure of the glass matrix permits closer approaches of the alcohol in petroleum ether than in 3MP. Whatever the source of the cationlike fluorescence in their experiments, Ermolaev and Kotlyar were the first to explain it as an excited state protonation. For some unknown reason, their observations appear ’tohave been largely ignored, and few researchers today seem aware of their work. O’Donnell and co-workers15 have studied the luminescence of a series of quinoline derivatives. Among them is 2-(2-hydroxyethyl)quinoline, which is a quinoline molecule to which an ethanol moiety has been attached in the /3 position, next to the nitrogen. This molecule shows broad cationlike fluorescence and a structured excitation spectrum in 3MP solvent “identical with that observed for quinoline”.15 Since these authors were using fairly low resolution, and did not publish their excitation spectra, it is difficult to know whether their excitation spectra differed in the manner which we observed. They assign the luminescence to emission from the substituted quinolinium cation formed by intramolecular proton transfer from the hydroxy group during the lifetime of the excited state. This result is particularly interesting since we fail to find any evidence that ethanol can effect an excited state protonation of quinoline in 3MP or in EPA (ether, pentane, ethyl alcohol). This suggests that the nitrogen-proton distance in the ground state hydrogen bond of Z-(Z-hydroxyethyl)quinoline is much shorter than is the vast majority of ethanolquinoline hydrogen bonds in the solutions we studied. This observation is consistent with our suggestion that excited state protonation occurs from closely hydrogen bonded HFZPquinoline complexes. The very much lower acidity of the ethyl alcohol moiety on the substituted quinoline would require that the proton be much closer to the nitrogen than is the case for HFBP. On the other hand, it may be that the six-membered ring resulting from the intramolecular hydrogen bond of the substituted quinoline simply places the ethanolic proton near the plane of the quinoline ring where the nitrogen lone-pair density is the greatest. Incidentally, O’Donnell and his co-workers erroneously assign the observed broad luminescence from both the quinolinium cation and from the substituted quinoline as phosphorescence. It is well known6 that the quinolinium luminescence is fluorescence. The long lifetime of 50 ms measured by these workers is probably a measuring artifact, as we find the broad luminescence lifetimes to be considerably less than 1ms. The general appearance of the broad fluorescence and its energy is also very reminescent of excimer fluorescence. In fact, excimer fluorescence has recently been reported for quinoline at 160 K and for isoquinoline at 241 K in ethanol by Blaunstein and Gant.lG The excimer fluorescence maxima which they observe is near 400 nm in both cases, while we observe the maximum to be at 348 nm for quinoline and 390 nm for isoquinoline. These authors made their observations in fluid media at concentrations between 10-1 and M. We observe IN evidence of broad emission in a rigid EPA glass at 77 K, which has a high ethanol concentration, even at 10-3 M. Since our solutions were 10 to 100 times less concentrated in rigid 3MP at 77 K, it is difficult to see how the addition of HFBP under these conditions could produce excimers.
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V. Conclusions Both structured naphthalene-like fluorescence and broad structureless fluorescence resembling the protonated cations of quinoline and isoquinoline have been observed in 3MP glasses when hexafluoro-2-propanol or trifluoroethanol are used as hydrogen bonders. These observations can be explained by assuming a double minimum potential for the proton in the excited state. The proton in close alcohol-heterocycle complexes may undergo tunneling of the barrier between the oxygen and nitrogen potential wells. The broad fluorescence arises from these “semiprotonated” excited state species. The barrier between the wells for distant alcoholheterocycle cdmplexes is too large to be penetrated during the life of the excited state, and only structured naphthalene-like fluorescence is observed from such complexes. The glass matrix structure is important in determining the separation of the two potential wells and the shape of the barrier. Acknowledgments. This research was made possible by funds from the Camille and Henry Dreyfus Foundation through their awarding of a grant to William R. Moomaw as a Dreyfus Teacher-Scholar. Our thinking about the role of the solvent matrix in shaping the upper state potential was greatly stimulated by conversations with Dr. H. B. Dellinger and Professor M. Kasha. We also gratefully acknowledge several helpful discussions with Professor E. C. Lim. References and Notes (1)Preliminary results were presented at the 30th Symposium on Molecular Structure and Spectroscopy as paper RH8, Columbus, Ohio, June 19. 1975. (2) M. F. Anton and W. R. Moomaw, J. Chem. Phys., in press. (3)C. A. Taylor, M. A. El-Bayoumi, and M. Kasha, Proc. Natl. Acad. Sci. U.S.A., 63,253 (1969). (4)G. J. Brealey and M. Kasha, J. Am. Chem. SOC.,77,4462(1955). (5) E. C. Murray and R. N. Keller, J. Org. Chem., 34, 2234 (1969). (6)H. Zimmermann and N. Joop, Z.Elektrochem., 65,61 (1961). (7)N. Mataga, Y. Kaifu, and M. Koizumi, Bull. Chem. SOC.Jpn., 29, 373 (1956). (8)M. A. El-Sayed and M. Kasha, Specfrochim. Acta, 15,758 (1959). (9)E. C.Lim and J. M. H. Yu, J. Chem. Phys., 47, 3270 (1967). (10)Y. H. Li and E. C. Lim, Chem. Phys. Lett., 9,279 (1971). (11)G.Fisher, J. Mol. Spectrosc., 49,201 (1974). (12)G.Fisher and R. Naaman, private communication. (13)H. B. Dellinger, Ph.D. Thesis, Florida State University, 1976. (14)V. L. Ermolaev and I. P. Kotlyar, Opt. Spectrosc., 9, 183 (1960). (15)C. M. O’Donnell, G. A. Knessel, T. S. Spencer, and F. R. Stermitz, J. Phys. Chem., 74,3555 (1970). (16)R. P. Blaunsteinand K. S. Gant, Photochem. Photobiol, 18, 347 (1973).
Discussion M. A. EL-SAYED.Have you used deuterated alcohol? W. R. MOOMAW.No, we have not, but we have plans to do so. The deuterated alcohols coupled with temperature dependent studies should tell us a great deal about the tuneling process. J. W. LONGWORTH. Have you used a more acidic fluoroalcohol, in particular, I had in mind CF&(OH)&F3, hexafluoroacetone sesquihydrate? W. R. MOOMAW.No, I have only used the less acidic trifluoroethanol which has a pK, of about 1 2 compared to a pK, of 17 for ethanol and 9 for hexafluoro-2-propanol.
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