J . Phys. Chem. 1991, 95. 2175-2181 predissociation rates, one cannot ignore the effects of the strength and shape of the intermode coupling function. One final question regarding observation (iv) concerns exactly when one may expect this type of "catastrophic breakdown" to occur. Within the assumptions underlying the momentum-gap law, the metastable-state wave function only has significant amplitude in the region to the right (at larger R ) of the open-channel turning point, where the associated continuum wave function is purely oscillatory in nature. However, Figure 9 shows that this breakdown occurs when the relative displacement of the effective adiabatic radial-channel potentials moves the turning point of the open-channel potential into the classically allowed region between the turning points of the metastable state's closed-channel potential. Thus, it is not the magnitude or even the sign of the associated level shift that matters, but rather the change in the average value of R for the complex. As a result, if the effective rotational constant associated with the overall rotation of the complex increases by a even a few percent on internal excitation of the monomer, the momentum-gap law should be completely disregarded. Finally, extension of the arguments developed for the two-dimensional model problem to systems with more than one internal
2175
degree of freedom was seen to introduce substantial complications. In particular (vii) When significant couplings involving more than one internal degrees of freedom contribute to the predissociation process, correlations such as that predicted by observation (vi) should disappear; their observation would be implicit evidence for a two-step impulsive mechanism for predissociation. The fact that such correlations persist for the wide range of cases surveyed by Miller,9 for which a large number of bending or internal rotation states are accessible, is thus taken as evidence that these vibrational predissociation processes are fundamentally impulsive, with the distribution of fragment internal states being decided as the separating fragments retreat along the dissociation coordinate.
Acknowledgment. We are grateful to Professor R. E. Miller for helpful discussions and for providing us with an unpublished data tabulation and to Dr. T. Slee, Mr. C. Chuaqui, and the two referees for their constructive comments. We are also pleased to acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada, through provision of both grant support to R.J.L. and an Undergraduate Summer Research Assistantship to M.E.L.
Spectroscopy of Indole van der Waals Complexes: Evidence for a Conformation-Dependent Excited State Michael J. Tubergen and Donald H. Levy* The James Franck Institute and The Department of Chemistry, The University of Chicago, Chicago, Illinois 60637 (Received: April 6, 1990: In Final Form: August 20, 1990)
Electronic excitation spectra of van der Waals complexes containing indole were obtained by using laser-induced fluorescence and resonantly enhanced, two-photon ionization. These complexes can be grouped into two classes. Acetamide, formamide, water, and methanol were found to shift the origin transition of the complex more than 400 cm-l to the red of the indole origin. Complexes of indole with methylated an+les, tetrahydrofuran, and 1,4-dioxane did not display transitions shifted as far to the red. Dispersed fluorescence spectra of the two groups of complexes were also found to differ. Complexes of indole with the amides, water, and methanol were found to fluoresce in a broad band centered 1500 cm-I to the red of excitation, while fluorescence from complexes of indole and the methylated amides, tetrahydrofuran, and 1 ,Cdioxane was sharp, with most of the intensity occurring at the excitation frequency. These differences are explained in terms of the ability of the complex partner to form a hydrogen bond with the ?r electron cloud of the indole. In such complexes, the interaction of the nearby polar solvent lowers the energy of the 'La state and allows it to mix effectively with the ILbstate. Complexes of water with I-methylindole and 3-methylindole are also shown to display the same two types of spectroscopic behavior.
Introduction The indole chromophore of the amino acid tryptophan has been the subject of extensive study because of its importance in protein spectroscopy.l In solution, the fluorescence behavior of indole is strongly affected by the environment of the chromophore. In polar solvents, the fluorescence from indole is shifted far to the red of the absorption band, but this large fluorescence shift is reduced in nonpolar solvents. One explanation of the fluorescence shift is that the excited indole molecule forms stable complexes, called exciplexes, with the solvent mole~ules.~JTwo different solvent binding sites have been identified for 3-methylindole dissolved in I - b ~ t a n o l . ~At one site nucleophilic solvents are attracted to the indole nitrogen proton, while at the second site electrophilic groups are attracted to the T electron cloud around
the indole 3 position. Molecular beam studies of indole complexed with various solvents have shown that proton donation from the indole to the solvent is common in van der Waals complexes of indole and nucleophilic solvent^.^ Similar structures have been reported for van der Waals complexes of water with 7-azaindole5 and carbazole.6 Indole has two low-lying excited electronic states, the ILb and the 'La states, which complicate the interpretation of the fluorescence spectroscopy. The 'La state is thought to be strongly stabilized by polar solvents, while the ILb state is only slightly affected. The relative location of these two excited states has recently been determined for indole dissolved in cyclohexane and I-butanol.' The effect of the polar solvent on the IL, state is so strong that the 'La state becomes the lowest energy excited
( I ) Creed, D. Photochem. Photobiol. 1984,39, 537. (2) Hershberger, M.V.; Lumry, R.W.Photochem. Photobiol. 1976,23,
(4) Hager, J.; Wallace, S . C. J. Phys. Chem. 1984, 88, 5513. (5) Fuke, K.; Yoshiuchi, H.; Kaya, K . J . Phys. Chem. 1984, 88, 5840. (6) Bombach, R.; Honegger, E.; Leutwyler, S . Chem. Phys. Left. 1985, 118, 449. (7) Rehms, A. A.; Callis, P. R. Chem. Phys. Lett. 1987, 140, 83.
-19_..I
(3) Hershberger, 1981, 33, 609.
M.V.; Lumry, R.; Verrall. R. Photochem. Photobiol. 0022-365419112095-2175$02.50/0
0 1991 American Chemical Society
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electronic state in polar solvents such as 1-butanol, while the 'Lb state is the lowest excited electronic state of indole dissolved in nonpolar solvents. In the isolated environment of a molecular beam, the lowest excited electronic state of indole has been identified as the 'Lb state8 Identification of features arising from direct excitation to the 'La state of jet-cooled indole has so far been ambiguous. Recent room-temperature vapor-phase studies of indole and 3-methylindole, however, place the onset of the 'La state 1400 cm-' above the origin of the 'Lb state for indole and only 100 cm-' above the 'Lb origin for 3-meth~lindole.~ Other evidence for the 'Lastate has been observed for indole derivatives in supersonic expansions. When alkyl groups are attached to the 2 and 3 positions, as in 2,3-dimethylindole (DMI) and 1,2,3,4-tetrahydrocarbazole(HC), the fluorescence lifetime decreases by almost a factor of 3 compared to Dispersed fluorescence spectra of vibrationally excited DMI and HC show broad, unstructured bands to the red of the excitation frequency. van der Waals complexes of DMI with trimethylamine, methanol, and acetonitrile have fluorescence lifetimes that are longer than the lifetime of the isolated 'DMI, and dispersed emission from these complexes is again broad and far red shifted from the excitation frequency even when the lowest energy transition is excited. These observations were interpreted" in terms of mixing between the 'La and IL, electronic states. Initially,'0 the IL, state was thought to be dissociative in the N-H stretching coordinate, so mixing of the initially excited 'Lb and the unbound 'La states would reduce the fluorescence lifetime of the uncomplexed DMI. In later work," it was postulated that the 'La state is bound in its lowest vibrational level but that the vibrational levels of the 'La state that are isoenergetic and vibrational levels of the ILbstate are above the dissociation limit. Mixing of the vibrational levels of the 'Lbstate with the higher lying vibrational levels of the I La state, therefore, would shorten the fluorescence lifetime of DMI. The mechanism responsible for the lengthening of the fluorescence lifetime in polar complexes of DMI is less certain. One suggestion" attributed the lengthened lifetime of the complexes to vibrational predissociation or internal relaxation involving the complexing partner. Either of these processes could remove sufficient vibrational energy from the chromophore to drop it below the dissociation limit of the N-H bond. In any case, the work on DMI and its complexes provided convincing evidence for mixing of the 'La and 'Lb states. Evidence for the mixing of the 'La and ' L b states in the indole chromophore was also provided by the jet spectroscopy of the amino acid tryptophan'213and tryptophan-containing peptide^.'^.^^ Of the conformations of the amino acid or peptide backbone that have been observed, one particular conformer displays a long vibrational progression in the excitation spectrum, indicating that the excited-state potential surface is shifted with respect to the ground state along one normal coordinate. The other conformers display little vibronic activity. Dispersed fluorescence spectra of the two types of conformers also differ markedly. While fluorescence from most of the conformers of tryptophan exhibit sharp dispersed fluorescence spectra, fluorescence from the progression-forming conformer is shifted far to the red (2000 cm-') of the excitation frequency and is very broad. These two types of dispersed fluorescence spectra were also observed for different conformations of the peptides.I5 Finally, the fluorescence lifetimes of the two types of conformations in tryptophan have been measured.I6 The lifetime of the progression-forming conformer (8) Bersohn, R.: Even, U.;Jortner, J . J . Chem. Phys. 1984, 80, 1050. (9) Cable, J. R. J . Chem. Phys. 1990, 92, 1627. (IO) Hager, J . W.: Demmer, D. R.;Wallace, S. C. J . Phys. Chem. 1987, 91, 1375. ( 1 1 ) Demmer, D. R.; Leach, G. W.; Outhouse, E. A.; Hager, J. W.; Wallace, S. C. J . Phys. Chem. 1990, 94, 582. (12) Rizzo, T. R.; Park, Y. D.; Peteanu, L. A.; Levy, D. H. J . Chem. Phys. 1986, 84, 2534. (13) Rizzo, T. R.:Park, Y . D.; Levy, D. H. J . Chem. Phys. 1986,85,6945. (14) Cable, J . R.:Tubergen, M. J.: Levy, D. H . J . Am. Chem. SOC.1988, 110. 7349. (15) Cable, J . R.;Tubergen, M. J.; Levy, D. H. J . Am. Chem. Soc. 1989, I l l . 9032.
Tubergen and Levy is 10.4 ns, while the fluorescence lifetimes of the other tryptophan conformers are similar to each other and average 12.9 ns. These observations indicate that one special conformer experiences an excited-state interaction that is not present in the other conformers. Molecular beam studies of tryptophan derivatives have helped elucidate the interactions that cause the progressions and redshifted fl~orescence.~'*~~ The excited-state interaction in tryptophan is prevented when either the amine or the carboxylic acid is chemically blocked, and the vibrational progressions and broad fluorescence are not observed for these derivatives. One major exception is tryptophan amide. When the carboxylic acid is replaced by an amide group that has at least one free proton, evidence of the excited-state interaction is still observed. The dependence of the excited-state interaction on the presence of both the amine and the acid or amide was explained as arising from the formation of an intramolecular hydrogen bond from the acid or amide to the amine. A similar conformation of the amino acid glycine was found to have a very high dipole moment.I9 A conformation of the peptide backbone that has a large dipole moment would be expected to stabilize the 'La state with respect to the 'Lb state.18 If the two excited states have similar energies, mixing of the two electronic states would cause the broad, redshifted fluorescence. The similarities and differences between the DMI work and the tryptophan work should be noted. In both cases, mixing of the 'La and 'Lb states was used to explain the observations, and in both cases some mechanism for lowering the energy of the 'La state was needed to allow the mixing to take place. In the case of DMI, the energy lowering is brought about by methyl substitution on the indole chromophore, and in the case of tryptophan the energy lowering is brought about by the presence of a large dipole moment close to the x electron cloud in the vicinity of the indole 3 position. Also, it should be noted that the experimental observables used as diagnostics for state mixing were different in the two cases. In the studies of DMI the diagnostic was a shortening of the fluorescence lifetime, while the tryptophan studies the diagnostics were the long, low-frequency progression in the excitation spectra and the broad, red-shifted fluorescence in the emission spectra. If the model for tryptophan and tryptophan derivative spectroscopy is correct, the IL, state of indole should also be stabilized by intermolecular interactions with polar molecules. van der Waals complexes of indole and very polar solvents should therefore display the same spectroscopic behavior exhibited by the special conformer of tryptophan. For this reason, in this paper we have studied indole complexes with simple amides and with other solvents. Amides have high gas-phase dipole moments (about 3.8 D) and also are a fundamental part of the peptide backbone. In addition, we compare the results from the amide complexes to the spectra of indole complexed with water, methanol, tetrahydrofuran, and 1,4-dioxane.
Experimental Section Indole, 3-methylindole, 1 -methylindole, 1,4-dioxane, and the amides were used as purchased from Aldrich. Tetrahydrofuran was obtained from Baker. Deionized water and HPLC grade methanol were also used as solvents. The amides and indole were placed behind a 50-pm pinhole and heated to 80 OC to increase their vapor pressures. Helium (4-12 atm) was used as a carrier gas. When complexing with solvents, helium containing 0.5% solvent was passed over the heated indole sample. Mass spectra obtained by multiphoton ionization were recorded for all of the complexes examined to ensure that complexes with more than one solvent molecule were minimized. Excitation of (16) Philip, L. A.; Webb, S.P.; Martinez 111, S.J.; Flemming, G . R.; Levy, D. H . J . Am. Chem.Soc. 1988, 110, 1352. (17) Park, Y . D.: Rizzo, T. R.;Peteanu, L. A.; Levy, D. H. J . Chem. Phys. 1986,84, 6539. (18) Tubergen, M. J.; Cable, J. R.;Levy, D. H. J . Chem. Phys. 1990,92, 51.
(19) Suenram, R.D.; Lovas, F. J. J . Mol. Spectrosc. 1978, 72, 372. Suenram, R. D.; Lovas, F. J. J . Am. Chem. SOC.1980, 102, 7180.
The Journal of Physical Chemistry, Vol. 95, No. 6, 1991 2177
Indole van der Waals Complexes
1*
I
I
I
I
34650
34750
34850
34950
' ,
I
I
35050
F R E Q U E N C Y ( CM-' 1 Figure 1. Total fluorescence excitation spectra of (a) indole-formamide and (b) indoleacetamide van der Waals complexes. The features marked with asterisks arise from weak hot band transitions of the indole monomer. The other peak labels correspond to assignments made in Tables I and 11.
the skimmed molecular beam occurred in the source of a timeof-flight mass spectrometer. Resonantly enhanced two-photon ionization was also used to obtain excitation spectra of the complexes. The excitation spectra were recorded by monitoring the mass peak of the complex in the time-of-flight mass spectrometer as a Nd:YAG pumped dye laser was scanned. The ion signal was gated and sent into a Lecroy 2249 analog-to-digital converter. The output of the analog-to-digital converter was recorded by a Dell system 310 computer through a CAMAC dataway. The details of the mass spectrometer can be found in an earlier publication.20 Fluorescence excitation of a free jet occurred 5 mm downstream from the nozzle. Total fluorescence collection optics replaced the time-of-flight mass spectrometer. The fluorescence was detected by fll.0 optics imaged onto a slit in front of an RCA 8575 photomultiplier tube. Fluorescence in the opposite direction was also collected by fll.0 optics and imaged onto the entrance slit of a Spex 1 -m monochromator. The recipricol linear dispersion of the monochomator was 4 A/". Further details of the fluorescence collection optics can be found in ref 21. The fluorescence signals were processed in the same way as the ionization signals.
Results Since indole is a rigid ring system, its excitation spectrum contains features from only one conformation. The fluorescence excitation spectrum of supersonically cooled indole shows very few vibrational features less than 700 cm-I above the origin transition.* Dispersed fluorescence produced by exciting the origin transition of indole is sharp and has most of its intensity at the excitation frequency.22 Several indole complexes discussed in this paper have multiple conformations, and the excitation spectra of most conformations of these complexes show low-frequency structure. Since low-frequency vibrations do not appear in the indole monomer spectrum, the low-frequency bands of the complex must be vibrations involving intermolecular coordinates. Two types of spectra were observed when fluorescence from the origin bands of the complexes was dispersed. Some conformers produced sharp and resonant fluorescence, similar to the fluorescence from the indole origin, while other conformers produced a strong, broad fluorescence band centered far to the red (1 500 cm-') of the excitation frequency. Amide Complexes. Fluorescence excitation spectra of the van der Waals complexes of indole with formamide and acetamide (20) Carrasquillo, M. E.; Zwier, T. S.;Levy, D. H.J . Chem. Phys. 1985, 83. 4990. (21) Sharfin, W.; Johnson, K . E.; Wharton, L.; Levy, D. H.J . Chem. Phys. 1979, 71, 1292. (22) Bickcl, G . A.; Demmer. D. R.; Outhouse, E. A.; Wallace, S.C. J . Chem. Phys. 1989. 91, 6013.
TABLE I: Assignment of Features in tbe Excitation Spectrum of the Indole-Formamide van der Wash Complex freq shift from 34 774 cm-l, cm-' intensity assignment -83 12 indole hot band 0 100 origin 37 34 VI 51 58 5s 64 Y3 65 11 indole hot band 13 16 2v1 88 23 V I + Y2 91 26 VI + Y3 101 34 2VZ 106 36 Y2 + Y3 110 42 29 1 I6 19 ? 124 86 Y4 137 16 Y, 2v2 141 18 V I + Y* + Y, I46 22 Y , + 2v, IS1 18 3 ~ 2 IS5 3s ? 160 45 V I + v4 165 32 34 174 so v2 + Y4 178 57 Y3 + Y4 243 58 2v4 257 114 indole hot band
+
"Normalized such that the 34774-cm-' peak has intensity 100. TABLE 11: Assignment of Features Found in the Excitation Spectrum of the Indole-Acetamide van der Waals Complex freq shift from intensity' assignmentb 34760 cm-', cm-I -68 36 indole hot band 0 100 conformer A origin 43 44 VI 51 89 Y2 56 66 VI 70 52 "4 79 47 indole hot band 95 78 VI + u2 101 I00 2Y2 107 63 v2 + V 3 112 54 2u3 120 56 y2 + Y4 12s 60 Y3 + Y4 138 58 2Y4 144 59 Y I + 2Y2 152 86 3 ~ 2 161 120 conformer B origin V I + u2 + u4 164 70 181 7s ? 200 68 4v2 226 110 indole hot band 'Normalized such that 34760-cm-I peak has intensity 100. bVibrational modes are all from the A conformation of the complex.
are presented in Figure 1. Both spectra contain substantial vibronic activity. The spectrum of indole and formamide shown in the upper portion of the figure has the origin transition at 34774 cm-I, 463 cm-' to the red of the indole origin. Progressions in different vibrational modes of 37, 51, 55, and 124 cm-I are observed. Second harmonic bands of all four modes can be found in this excitation spectrum, and combination bands of these modes account for most of the remaining features. Specific assignments of the features in this excitation spectrum can be found in Table 1.
The excitation spectrum of the indoleacetamide complex (lower trace) is more complex. The origin of this spectrum is at 34 760 cm-I, 4 7 7 cm-l to the red o f the indole origin. Progressions in four van der Waals vibrational modes with frequencies of 43, 51, 56, and 70 cm-l are present. Overtones up to the fourth harmonic are observed for the 51-cm-' vibration, and many of the remaining
2178 The Journal of Physical Chemistry, Vol. 95, No. 6, 1991
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FREQUENCY ( C M-' ) Figure 2. Dispersed fluorescence spectra of the lowest energy transitions of the (a) indole-formamide (34 774 cm-') and (b) indole-acetamide (34 760 cm-I) van der Waals complexes. The resolution of the monochromator was 50 cm-' for the upper trace and 75 cm-I for the lower
trace. peaks can be assigned as other overtones and combinations. The large peak at 34 921 cm-l is assigned as the origin of a second conformation of the indole-acetamide complex, this origin being 316 cm-I to the red of the indole monomer origin. At frequencies above the second conformer origin, assignment of vibrations becomes difficult since vibrations built on both conformer origins are present. A detailed assignment of the features in the excitation spectrum of the indole-acetamide complex can be found in Table 11. Figure 2a shows the dispersed fluorescence spectrum obtained when the 34 774-cm-' peak of the indole-formamide complex is excited. The resolution of this spectrum is 50 cm-I. Fifty percent of the intensity of the sharp spike located at the excitation frequency is due to scattered laser light, but the broad tail on this feature is due to fluorescence to many low-frequency ground-state vibrational levels similar to the vibrational levels observed in the excitation spectrum. Most of the intensity in this spectrum is in a broad band shifted about 1700 cm-' from the excitation frequency. Although the dispersed fluorescence spectrum of the indole monomer origin has much sharp structure in this frequency range, the dispersed fluorescence of the indoleacetamide complex has only two broad peaks 800 and 1330 cm-I to the red of the excitation frequency which rise above the broad fluorescence band. The frequencies of these bands are similar to ground electronicstate vibrational frequencies of the indole monomer.23 The dispersed fluorescence spectrum obtained by exciting the lowest frequency band of the indole-acetamide complex is shown in Figure 2b. The resolution of this spectrum is 75 cm-l. Again, about 50% of the fluorescence intensity at the excitation frequency (34760 cm-I) is due to scattered laser light, and a broad fluorescence feature centered about 1700 cm-I to the red of excitation is again present. The other features in this spectrum are also found in the dispersed fluorescence spectrum of the formamide complex and are interpreted in the same way. On the other hand, fluorescence from the 34 921 - c d origin of the indole acetamide complex was found to be sharp and strongest near the excitation frequency, and no broad, red-shifted feature was observed. The difference in fluorescence behavior of these two complex transitions justifies their assignment to different conformers. The fluorescence excitation spectrum of the complex of methylformamide with indole has its origin transition at 34929 cm?, 304 cm-' to the red of the indole origin, and is followed by many weak vibrations at higher frequencies. These vibrations are not (23) LautiE, A.; Lauti6, M. F.; Gruger, A.; Fakhri, S.A. Spectrochim. Acta 1980,36A, 8 5 . Takeuchi, H.; Harada, 1. Spectrochim. Acta 1986,42A, 1069.
Tubergen and Levy
34700
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FREQUENCY(CM" ) Figure 3. Total fluorescence excitation spectra of indole complexed with (a) methanol and (b) water.
regularly spaced and do not form a vibrational progression in any particular vibrational mode. Since there are no long progressions in any of the vibrational modes, the potential energy surfaces of the ground and excited states must be only slightly shifted from each other. The origin transition in the fluorescence excitation spectrum of the complex of indole and methylacetamide is located at 34899 cm-'. Other transitions involving low-frequency van der Waals modes occur within 35 cm-I, and overlapping these features is an unresolved feature centered at ca. 34925 cm-I. In the dispersed fluorescence spectrum of the origin transition of indole-methylformamide, the broad, red-shifted fluorescence band observed in the dispersed fluorescence spectra of formamide-indole and acetamide-indole is greatly reduced. Most of the intensity in this spectrum is at the excitation frequency with weaker bands at -625, -770, and -1300 cm-l. The dispersed fluorescence spectrum of the origin transition of indole-methylacetamide is also composed primarily of sharp, resonant fluorescence. The vibrational frequencies of the two complexes are the same within the resolution of the spectra. The fluorescence excitation spectrum of the van der Waals complex of indole and dimethylformamide has only two peaks at 34995 and 35015 cm-I. The intensities of these two features are similar, and they are therefore assigned as two separate conformers of the complex. The fluorescence excitation spectrum of the indole and dimethylacetamide complex has only one strong transition at 34962 cm-I and three weak vibronic bands within 30 cm-' of the strong transition. The dispersed fluorescence spectra of the origin transitions of indole complexed with dimethylformamide and dimethylacetamide are similar in that they have strong resonance fluorescence and sharp vibrational features. Indole ring vibrations are observed 750 and 1300 cm-I from the excitation frequency for both complexes. The broad, red-shifted fluorescence band is not observed for either complex. Fluorescence excitation and dispersed fluorescence spectra of the complexes of indole with methylformamide, methylacetamide, dimethylformamide, and dimethylacetamide are available as supplementary material (see the paragraph at the end of the paper). Solvent Complexes. Figure 3a shows the fluorescence excitation spectrum of indole complexed with one methanol molecule. As observed by Hager and Wallace," there is a strong transition at 35 076 cm-I followed by a 23-cm-I vibration. Also, there are many small features at lower frequencies. The lowest energy transition is at 34763 cm-I and has several weak vibrational transitions associated with it. Figure 3b presents the fluorescence excitation spectrum of complexes of indole and water. This spectrum is also very similar to excitation spectra of the indole-water complex presented e l s e ~ h e r e . * ~The . ~ ~lowest energy transition is at 34 782
The Journal of Physical Chemistry, Vol. 95, No. 6, 1991 2179
Indole van der Waals Complexes
35000
34000
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FREQUENCYK M - ' )
FREQUENCY(CM-')
Figure 4. Dispersed fluorescence spectra of (a) the lowest energy transition (34782 cm-I) and (b) the strongest transition (35 104 cm-I) of the indolewater complex. The resolution of the monochromator was 50 cm-' for both spectra.
Figure 5. Excitation spectra of van der Waals clusters of water with (a) 3-methylindole and (b) I-methylindole. The excitation spectrum of the 3-methylindole complex was recorded by monitoring total fluorescence, while the spectrum of the I-methylindole complex was obtained by using resonantly enhanced two-photon ionization. The large feature that goes off the top of the figure in spectrum (a) is the origin of the 3-methylindole monomer.
cm-' and is part of a 35-cm-l vibrational progression containing seven members. There is also a weak 49-cm-' vibrational progression of one member which appears in combination with the members of the 35-cm-' progression. The strongest transition is at 35 104 cm-' and has a weak 24-cm-' vibration. Dispersed fluorescence spectra of two indole-water transitions are shown in Figure 4. Figure 4a shows the dispersed fluorescence of the lowest energy transition of the complex. Scattered light accounts for 90% of the intensity of the peak at the excitation frequency, although its tail again represents fluorescence to lowfrequency vibrational levels in the ground state. This spectrum has weak individual vibratonal bands (only the 750-cm-' band is observed) and a broad fluorescence feature that peaks about 1700 cm-' from the excitation frequency. The dispersed fluorescence spectrum of the strongest water complex transition is shown in Figure 4b. Because the broad fluorescence feature is not observed in this spectrum, individual vibrational bands appear strong, and the indole ring vibrations at 750 and 1300 cm-' are easily identified. Only about 1% of the feature at the excitation frequency is due to scattered light. The two different dispersed spectra for the indole-water complex suggest that there are two different conformations of the complex which have different kinds of excited states. Fluorescence from indole-methanol complex transitions was also dispersed. Dispersed fluorescence of the large feature at 35 076 cm-' was found to be similar to the dispersed fluorescence of the strong band in the excitation spectrum of indole-water. Its spectrum has well-resolved vibrations and no broad, red-shifted background. The lowest energy methanol complex transition, however, was found to fluoresce in a broad band to the red of the excitation frequency with weak individual vibrational bands. Fluorescence at the excitation frequency of the lowest energy transition was also weak. The methanol complex, therefore, has the same types of conformers as the indole-water complex. The fluorescence excitation spectrum of the complex of indole with tetrahydrofuran has a strong origin transition at 34956 cm-l and several weak van der Waals vibrations at slightly higher frequencies: there are five transitions within 85 cm-' of the origin. No overtone or combination bands of these transitions appear in t h e excitation spectrum. The fluorescence excitation spectrum of indole complexed with I ,4-dioxane has a strong origin transition a t 35049 cm-' and five weak vibrations with frequencies of 13, ~
~~
(24) Hager, J.; Ivanco, M.; Smith, M. A.; Wallace, S. C. Chem. Phys. Lett. 1985, 113, 503. Hager, J.; Ivanco, M.; Smith, M. A.; Wallace, S. C. Chem. Phys. 1986, 105, 391. (25) Montoro, T.;Jouvet, C.; Lopez-Campillo, A.; Soep, E.J . Phys. Chem. 1983, 87, 3582.
17, 31,43, and 78 cm-' built on this origin. The position of the origin transition of the complex and the vibrational frequencies built on it are in agreement with a spectrum published by Nibu et Dispersed fluorescence spectra of the origin transitions of the tetrahydrofuran and 1,4-dioxane indole complexes are similar in that the strongest fluorescence occurs at the excitation frequency. Also, indole vibrational bands are observed in both spectra at 625, 750, and 1300 cm-' from the excitation frequency. No broad background fluorescence is present in either spectrum. Nibu et a1.26also dispersed the indole-dioxane complex origin and observed a broad band centered 19 cm-' from the excitation frequency, which they attribute to relaxation to vibrational levels of a conformational isomer of the complex. We do not observe this broad band since the resolution of these spectra is 50 cm-'. Fluorescence excitation and dispersed fluorescence spectra of the complexes of indole with tetrahydrofuran and 1,4-dioxane are available as supplementary material. Other Complexes. Complexes of water and methylated indoles were prepared by using the same conditions that maximized the intensity of the spectrum of the indole-water complex. Replacing the nitrogen-bound proton with a methyl group to form 1methylindole blocks the site usually assumed to be the point of attachment for hydrogen-bond-accepting solvents. In fact, the previous conclusion24that both indole-water complexes involved hydrogen bonding at the nitrogen-bound hydrogen was based on the inability to observe a 1 -methylindole-water complex. Since methyl groups are bulkier than hydrogen atoms, methyl substitution at other positions of indole may sterically prevent formation of certain types of complex geometries. Figure Sa shows a fluorescence excitation spectrum of the complex of 3-methylindole and water. The origin transition of the complex is at 34643 cm-' and is followed by three low-frequency vibrations. These vibrations are within 120 cm-' of the origin and are due to single-member progressions involving van der Waals vibrational modes. The feature at 34 879 cm-l which goes off the top of the figure is the 3-methylindole monomer origin. It is preceded at lower frequencies by two small hot-band transitions. Our excitation spectrum of the 3-methylindole-water complex is similar to one published earlier.* We have now observed an excitation spectrum of a complex of I-methylindole and water, which is shown in Figure 5b. The (26) Nibu,
3898.
Y.;Abe, H.; Mikami, N.; Ito, M. J. Phys. Chem. 1983, 87,
2180 The Journal of Physical Chemistry, Vol. 95, No. 6,1991
Tubergen and Levy TABLE 111: Summarv and ComDarison of Indole Cluster Swctrn conformer u0 proton excitedorigins, vdndalC, affinity, state indole complex cm-' cm-' kcal/mol mixing.' acetamide 34760 -477 206.3' yes 34921 -316 no methy lacetamide 34899 -338 212.7' no dimethylacetamide 34962 -275 216.8' no formamide 34774 -463 198.8" yes methylformamide 34929 -308 204.8' no dimethylformamide 34 995 -242 21 1.4' no 35015 -222 no water 34782 -455 173.0b yes 35104 -133 no methanol 34763 -474 184.9b Yes 35076 -161 no tetrahydrofuran 34956 -281 199.6b no 35049 -188 194.Ib no dioxane ~~
1
1
I
I
I
35000
34000
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32000
31000
I
FREQUENCY (CM") Figure 6. Dispersed fluorescencespectrum of the origin transition (34 643 cm-') of the 3-methylindole-water van der Waals complex. The resolution of the monochromator was 50 cm-'.
spectrum of this complex was very weak compared to the spectrum of the I-methylindole monomer. Since both species have strong transitions a t these frequencies, the fluorescence excitation spectrum of the complex cannot be observed due to overlap with the spectrum of the monomer. Resonantly enhanced multiphoton ionization, however, avoids the problem of interference by the monomer. The detector of the time-of-flight mass spectrometer was gated to accept only the mass peak of the complex as the excitation/ionization laser was scanned, thus generating the excitation spectrum of 1-methylindole-water shown in Figure 5b. The spectrum of the I-methylindole complex has two distinguishing characteristics. First, there is a long progression of weak features that starts at 34 180 cm-I. There are at least 10 members of this progression, and they have a constant 26-cm-I spacing. Such a long vibrational progression indicates a substantial shift in the electronic potential energy surfaces upon excitation. If the shift of the excited-state potential surface is very large, then transitions to the vibrationless level of the excited state will be weak, and we cannot identify the origin transition for this reason. There is also a broad feature in the excitation spectrum centered at 34 500 cm-' and spanning several hundred cm-'. The vibrational progression forms the low-frequency side of the broad feature, while no discernible peaks are higher in frequency. It seems probable that the broad band arises from transitions to a dense region of vibrational states high on the excited-state potential surface. Figure 6 presents the dispersed fluorescence spectrum obtained by exciting the origin transition of the 3-methylindole-water complex. The strongest fluorescence transition in this spectrum occurs at the excitation frequency, only 10%of this feature being due to scattered laser light. Most of the peaks in this dispersed fluorescence spectrum are also observed in the dispersed fluorescence spectrum of the origin transition of the 3-methylindole monomer, indicating that these transitions are to ground-state vibrational levels primarily involving the 3-methylindole. The vibration 140 cm-' from the excitation, however, does not appear in the dispersed fluorescence of the 3-methylindole origin and is probably a transition to a van der Waals vibrational level in the ground electronic state.
Discussion We now will examine the results on the spectroscopy of indole complexes presented in the previous section to see if they are consistent with the model1*used to explain the unusual spectroscopy of tryptophan and its derivatives. That model assumed the existence of a special conformer in which the 'La state was lowered in energy by interaction with a polar substituent to the point where it could mix effectively with the 'Lbstate. This mixing produced long, low-frequency progressions in the excitation spectra and broad, red-shifted fluorescence in the emission spectra. Table 111 presents a summary and comparison of the excitation spectra of the various indole complexes studied in this paper, and (27) Meot-Ner, M. J . Am. Chem. Sm. 1984, 106, 278. (28) Aue, D. H.; Bowers, M. T. In Gas Phase Ion Chemistry; Bowers, M. T. Ed.:Academic Press: New York, 1979; Vol. 2, pp 1-51.
~~~~~~~
' From ref 27. From ref 28. .'"yes" in this column indicates that the conformer has low-frequency progressions in the excitation spectrum and broad, red-shifted emission. TABLE IV: Comparison of Tryptophan Peptide and Derivative Data
sample tryptophan (Trp)' TrpGlyb
Trp amide' Trp methylamide.' Trp dimethylamide( Gly-Trpb
conformer origins, cm-' 34 873 34 909 34519 34 948 34 724 34 944 34 660 34 932 34 807 34716 34 907
uo
- u$dolc , excited-state cm-' -364 -328 -718 -289 -513 -293 -577 -305 -430 -521 -330
mixingd Yes no
Yes no
Yes no Yes
no no Yes no
"From ref 12. bFrom ref 14. (From ref 18. d"yes" in this column indicates that the conformer has low-frequency progressions in the excitation spectrum and broad, red-shifted emission. for comparison Table IV gives similar summary of tryptophan and its derivatives. In both cases (with the exception of tryptophan itself), conformers in which excited-state mixing has been identified are shifted more than 500 cm-' to the red of the indole origin while conformers with no excited-state mixing are shifted less than 400 cm-I. Hager and Wallace4 found that in complexes of indole, there was a linear relationship between the spectral red shift of the complex and the proton affinity of the solvent. All of the solvents studied by Hager and Wallace were hydrogen-bond acceptors and were presumed to be bound to the proton on the indole nitrogen. The complexes reported in Table I11 roughly follow the Hager and Wallace correlation if only the conformers not showing excited-state mixing are considered. Those conformers identified in Table 111 as having mixed excited states do not follow the correlation between red shift and proton affinity, suggesting that in these complexes the solvent is not the hydrogen-bond acceptor. Our model for the spectroscopy of indole complexes is summarized in Figure 7. Complexes in which the solvent accepts a hydrogen bond from the indole have the solvent molecule attached to the side of the indole molecule via the hydrogen on the indole nitrogen, while complexes that can donate a hydrogen bond have the solvent molecules interacting with the ?r cloud above the ring near the basic indole 3 position. These two geometries are those suggested to explain the spectroscopy of indolesolvent complexes in s o l ~ t i o n . ~ The effect of complexing a polar solvent molecule is to lower the energy of the 'La state thus enhancing mixing with the 'Lb state. The amount of energy lowering depends on the dipole moment of the solvent, the distance from the solvent dipole to the indole dipole, and the orientation of the two dipoles. Since the interaction energy scales inversely as the cube of the distance
The Journal of Physical Chemistry, Vol. 95, No. 6, 1991 2181
Indole van der Waals Complexes
Solvent accepting hydrogen bond
x
Solvent donating hyrogen bond
'La
'A
Q
vibration is the stretching vibration between the two partners in the complex. The extent of the potential energy shift can be estimated by comparing the Franck-Condon factors for a vibronic transition with the observed intensities in the progression. The intensity pattern for a given mode is determined by the displacement of the excited-state potential energy surface and by the force constants of the two surfaces. We used the recursion relations for Franck-Condon factors of Henderson et al.B to model the intensity pattern of the progressions in the excitation spectra of indole complexed with acetamide, formamide, and water using the reduced mass of the complex as the mass of the oscillator. Fitting the intensity atterns of the progressions gave displacements of 0.28 f 0.04 for indole-acetamide, 0.20 f 0.04 A for indoleformamide, and 0.61 f 0.07 A for indole-water. The error was estimated by varying both the displacement and the ratio of ground- and excited-state force con$ants until the intensity patterns of the spectra were no longer reproduced.
8:
Q
Figure 7. Potential energy diagrams describing the effects of the two types of van der Waals complexes on the excited states of indole. The right diagram shows the case where the solvent can donate a hydrogen bond to the indole 7 cloud and lower the energy of the 'Lastate of the indole. The left diagram shows the case where the solvent accepts a hydrogen bond from the indole nitrogen and is unable to perturb the excited states of indole. In both diagrams the coordinate Q refers to some intramolecular coordinate in indole, probably the N-H stretch".
between the two dipoles, distance is likely to be the most important factor. The distance between the solvent and the C,-C, bond in indole is much greater for bonding at the nitrogen proton than it is for bonding at the A cloud. Of course, in many cases there will be binding at both sites, producing both ordinary and special complexes. The relative amounts of the two complexes are more likely to be determined by the kinetics of formation in the supersonic expansion than by the relative strength of the bonds in the two complexes. In the case of I-methylindole, the indole nitrogen is blocked, the usually more abundant ordinary complex cannot form, and only the special complex is observed. In the case of 3-methylindole or in the case of the methyl amides complexing with indole, the bulk of the additional methyl groups inhibits formation of the special complex and only the ordinary complex is observed. The potential diagrams in Figure 7 are slices through the multidimensional surface showing the variation of energy with respect to one indole coordinate identified as the N-H stretch by Demmer et a1.I' The displacement along this coordinate of the ILa state from the ground state is responsible for the broad, red-shifted emission as long as the 'Lastate is mixed with the only slightly displaced ' L b state (which is responsible for the Franck-Condon allowed absorption). I f we were to show a different slice of the potential surface plotting the energy as function of the distance between the solvent and the indole, the ground- and excited-state curves would also be displaced. This displacement along the intermolecular coordinate is responsible for the long, low-frequency progressions observed in the excitation spectra; that is, this low-frequency
Conclusions Excitation spectra of indole van der Waals complexes with amides and other solvents show that two types of conformations of the complex are formed in the supersonic expansion. One type has a spectral shift between the complex and uncomplexed indole that is proportional to the proton affinity of the complex partner. This type of complex contains a hydrogen bond from the indole nitrogen proton to an electron-rich site on the solvent molecule. Fluorescence from this type of complex is sharp and dominated by a peak at the excitation frequency. In the other type of complex geometry the solvent acts as a proton donor to the indole A cloud near the indole 3 position. In this type of complex, the spectral shift from uncomplexed indole is larger than in the first type of complex, and the excitation spectrum has one or more long, low-frequency progressions. The fluorescence from this type of complex occurs in a broad band far to the red of the excitation frequency. The broad fluorescence band for the second type of complex is due to a mixing of the ' L b state with the IL, state. The energy of the 'La state is lowered by the interaction with a polar solvent so that it mixes effectively with the 'Lbstate. When the solvent accepts a hydrogen bond from the indole, it is held away from the ring and prevented from lowering the energy of the IL, state. The spectroscopy of the indole complexes with polar solvents is similar to the sEtroscopy of tryptophan peptides and derivatives, which show a similar excited-state mixing.
Acknowledgment. This work has been supported by the National Science Foundation under Grant No. CHE-88 18321. Supplementary Material Available: Fluorescence spectra of indole van der Waals complexes (7 pages). Ordering information is given on any current masthead page. (29) Henderson, J. R.; Muramoto, M.; Willett, R. A. J. Chem. Phys. 1964, 41, 580.
