J. Phys. Chem. 1996, 100, 16479-16486
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Jet-Cooled Solvent Complexes with Indoles Yuhui Huang and Mark Sulkes* Department of Chemistry, Tulane UniVersity, New Orleans, Louisiana 70118 ReceiVed: April 30, 1996; In Final Form: August 12, 1996X
Cold mass-resolved excitation spectra have been obtained for substituted and unsubstituted indoles complexed with one or two polar solvent molecules. Frequency scans were carried out to more than 1000 cm-1 above the complex origins. Sharp excitation features persisted to the blue for the N-H site complexes, but no features further to the blue were attributable to the π site complexes. In 3-methylindole a number of vibrations 400-900 cm-1 above the S1 origin in the bare chromophore did not appear in corresponding locations above the S1 complex origins for n ) 1 water and methanol clusters. It is likely that these peaks underwent much greater red shifts in the complexes than the S1 origins because the former possess substantial 1La character. Any of the indoles studied that was complexed with two polar solvent molecules showed broad excitation features. The extended structure most likely arises in at least some instances because of π site interactions. In 2,3-dimethylindole, postulated to undergo N-H bond dissociation in S1, mass-resolved REMPI spectra did not disclose a lower mass product arising from N-H bond breaking. However, when the chromophore was complexed at the N-H site with a strong proton acceptor, triethylamine, evidence was found that zwitterions were formed in the S1 complex arising from N-H site proton donation to the base.
Introduction A large number of spectroscopic studies have been carried out on jet-cooled indole chromophores since the 1980s. The influence of polar interactions upon the indole chromophore has been considered extensively: (1) Side chain interactions of polar groups in analogs of tryptophan have been observed to affect properties of the indole chromophore.1-3 (2) More generally, and much more extensively, small clusters of indoles have been studied with a variety of polar solvents.4-19 Indole was found to have two separate polar binding sites.9 One, based on a correlation between the 000 frequency of the cluster species and solvent proton affinity, appears to be in the vicinity of the pyrrole N-H.8 The other has been proposed to be adjacent to the pyrrole π density in the vicinity of C-3, where the solvent acts as partial proton donor.13 (See Figure 1 for carbon numbering in indoles.) Indole chromophores have two low-lying π* excited states, 1L and 1L . In gas phase or nonpolar solvents 1L is generally b a b the lowest lying of the two, but 1La is preferentially stabilized in polar solvents and becomes the lowest excited singlet for most indoles. The character of the emission in solution depends on which of these two states is lowest lying.20-23 In a similar manner, polar solvent adducts in jet-cooled clusters should lower the energy of 1La relative to 1Lb. In recent years a number of studies have suggested that solvent adduct polar interactions may induce 1La emission in a variety of indole chromophores.10-14 Almost all the published indole cluster excitation spectroscopy is confined to the S1 origin region. One exception involves laser-induced fluorescence (LIF) excitation scans for indole complexed with one molecule of water or methanol.19 In general, ambiguities may arise in assigning peaks to particular complexes without the availability of mass-selected spectra. We have carried out, and herein report, mass-resolved excitation scans extending >1000 cm-1 to the blue of the S1 origins for indoles complexed with a variety and number of polar solvent adducts. As will be discussed, the chromophore vibronic features observed to the blue of the complex origins appear to differ X
Abstract published in AdVance ACS Abstracts, September 15, 1996.
S0022-3654(96)01244-0 CCC: $12.00
Figure 1. Carbon numbering in indoles
markedly in pattern depending on which polar binding site is involved and/or which substituted indole is involved. Experimental Section Mass-selected resonance-enhanced one-color, two-photon ionization (REMPI) excitation scans were obtained on a reflectron-based molecular beam system (Russ Jordan). Within the ion flight path, timed voltage pulses were applied to a deflector grid (Russ Jordan) in front of the detector. The effect was to accept selected mass cluster ions and to deflect ions with other masses before they reached the detector. A pulsed General Valve Series 9 solenoid valve with a 0.5 mm orifice was placed ∼2.5 cm from a 0.5 × 4 mm2 slit skimmer (Beam Dynamics) which was in turn ∼7.5 cm before the crossing of the laser beam. A 3.3 Ω coil was used on the solenoid, which enabled gas pulses of 10 µs to be attained.24 He carrier gas, typically at p ) 140 psig, was passed through a sample holder directly above the pulsed valve. For indole no sample heating was necessary. The other solids were heated ∼10 °C below their melting points, and the nozzle was heated to 75 °C. 1-Methylindole (1-MI), which is a liquid, was interspersed in a mixture of sand and diatomaceous earth. Solvent molecules were added by bubbling the carrier gas through a reservoir of the liquid. All indoles, obtained from Aldrich with stated purities > 98%, were used as received. Similarly, reagent grade solvents were used without further purification. No other mass signals due to minor impurities were found in the time-of-flight spectra. Tunable UV light was obtained using a pulsed Nd:YAG pumped dye laser (Continuum YG680 and TDL60) followed © 1996 American Chemical Society
16480 J. Phys. Chem., Vol. 100, No. 41, 1996
Huang and Sulkes
TABLE 1 species
origin or reference peak, cm-1
major peaks, cm-1 relative to reference
indole indole-(H2O)1 indole-(MeOH)1 indole-(TEA)1 indole-(THF)1 3-MI 3-MI-(H2O)1 3-MI-(MeOH)1 3-MI-(TEA)1 2-MI 2-MI-(H2O)1 2-MI-(MeOH)1 2-MI-(TEA) 5-MOI 5-MOI-(H2O)1 5-MOI-(MeOH)1 5-MOI-(TEA)1
35 039 35 104 35 076 34 941 34 956 34 882 34 643 34 592 34 197 35 170 35 034 35 012 34 960 33 136 33 048 33 021 32 897
317, 366, 380, 455, 481, 541, 720, 738, 785, 910 381, 400, 438, 475, 541, 722, 746, 791 381, 465, 543, 720, 747, 787 464, 545, 654, 707, 725 381, 462, 544, 714, 730 411, 422, 429, 470, 529, 611, 619, 718, 742, 751, 823, 921, 948 21, 31, 118, 212, 246, 272, 376, 403, 451, 722, 840, 915, 958, 966 22, 44, 138, 159, 211, 235, 257, 279, 725, 745, 767, 941, 959, 980, 999, 1023 81, 101, 150, 174, 263, 343, 433, 803 432, 447, 525, 619, 766, 905 421, 447, 507, 625, 763, 910 171, 421, 505, 623, 759, 907 -120, -85, -75, 13, 300, 313, 416, 624, 675, 906 236, 362, 385, 424, 444, 506, 704, 712, 794, 890, 904, 980, 1023, 1035 19, 28, 125, 506, 525, 712, 733, 799, 902, 999, 1027, 1033 21, 150, 389, 508, 529, 713, 733, 902, 1034 -76, -49, -49, -22, 25, 388, 466, 485, 505, 633, 685, 709, 902, 1032
by a servo tracked angle tuned KDP crystal for second harmonic generation. Power levels were attenuated until optical saturation was not evident in the REMPI spectra. Time-of-flight photoionization waveforms were acquired on a Tektronix 2440 digitizer, typically averaged over 32 laser pulses at each dye laser wavelength with 0.005 nm steps after frequency doubling. The relative UV excitation intensity was measured as a function of wavelength by directing the UV beam into a laser dye cell and detecting the ensuing fluorescence with a photodiode. These files were used to correct the relative peak intensities in extended wavelength REMPI scans for all broadband features except those in Figure 9. With or without corrections, the same set of sharp and broad peaks were qualitatively evident in the REMPI scans. All REMPI spectra were taken using one-color, two-photon ionization. For cluster spectra, the excess energy of the second photon beyond the ionization threshold brings up the possibility of some cluster fragmentation. In particular, weak broad structure in a few one adduct mass spectra was actually due to loss of one solvent molecule adduct from n ) 2 clusters. Fluorescence lifetimes were measured in a supersonic free jet using time-correlated single photon counting. The quality of lifetime fitting, generally including >1500 fitting channels (either 68 or 34 channels/ns), was reckoned by examining weighted residuals and χ2 square values. A typical decay waveform fit with residuals is shown elsewhere.25 The precision of determined lifetime values is (0.1 ns. Results Solvent Complexes with Indoles. Solvent complex spectra are presented successively for each indole chromophore considered. For chromophores whose n ) 1 complexes showed discrete features, the frequencies of major peaks are tabulated in Table 1. The origin region features for indole-(H2O)1 and indole-(MeOH)1 were previously tabulated9 and are not included in Table 1. Spectral features for two separate indole-(H2O)1 complexes were found previously.9 One complex displays a strong 000 transition with a single weaker intermolecular vibration built off of it. Based on a linear correlation between complex 000 frequency and solvent adduct proton affinity,8,12 this complex arises from solvent binding near the indole N-H. The emission is 1Lb in character. Excitation of the second n ) 1 water complex produces an envelope of progression features to the red of the first complex ν00. The ensuing emission spectra display an enhanced broad red-shifted component. Similar
Figure 2. Mass-selected two-photon REMPI excitation spectra of indole and one adduct indole complexes. The complex spectra are aligned in frequency such that the origins due to the N site complexes overlay one another.
excitation and emission features had earlier been observed in certain S1 conformers of some tryptophan analogs (i.e., indoles containing polar side chains bound at C-3).1-3 For tryptophan analogs it was postulated that a polar side chain group interacted with the chromophore via polar group proton donation to the indole π cloud in the vicinity of C-3.3 Such an interaction would be geometrically feasible for some side chain conformations. By analogy with these findings, the second indole n ) 1 polar solvent complex can reasonably be supposed to bind at a similar location.13,14 Figure 2 presents a REMPI excitation scan in the indole(H2O)1 mass channel that extends >1000 cm-1 to the blue of the N-H site complex origin and >1300 cm-1 to the blue of the π site origin. This scan has been aligned such that the complex origin at 35 104 cm-1, due to ν00 of the N-H site complex, is over the origin peak of a bare indole scan, beneath it. It is qualitatively apparent that all the major vibronic features in the bare molecule indole scan are matched by features in the complex spectrum. The higher energy complex vibrations must
Jet-Cooled Solvent Complexes with Indoles
Figure 3. Mass-selected two-photon REMPI excitation spectra of indole and two adduct solvent complexes.
arise from indole vibrations in the N-H site complex that have undergone red shifts similar to the complex origin. Muino and Callis made a similar correlation using LIF spectra.19 The major additional peak in the spectrum, some 25 cm-1 to the blue of the origin, arises from an intermolecular motion of the N-H site complex. While the higher energy indole vibronic features are replicated in the N-H site complex spectrum, this is not the case for the π site complex. The progression features to the red of the N-H site origin are all due to the π site complex. Beyond the initial progression envelope, no other strong peaks further to the blue are attributable to this complex. The absence of discernible higher energy peaks in the π site water complex does not seem to arise predominately from dissociation of the solvent complex. If this were the case, additional REMPI spectral structure would be expected when detection is carried out in the indole mass channel. None is observed. Methanol also can bind at two sites on indole.9 As seen in Figure 2, the extended blue scan is similar to the water case. The N-H site complex (complex origin aligned with bare indole origin) once again replicates the higher energy vibronic structure in indole, and the π site complex does not. Figure 2 also shows spectra of complexes with tetrahydrofuran (THF) and triethylamine (TEA). Since neither solvent is an H donor but both are H acceptors, only N-H site complexes would be expected. While these REMPI spectra may show some complications due to dissociation of larger complexes, both continue to display sharp spectral features to the blue that can be attributed to indole chromophore vibrations in N-H site 1L complexes. For indole-TEA some of the additional b structure, especially apparent near 34 900 and 35 600 cm-1, may be due to multiple binding orientations of the TEA at the N-H site. Finally, Figure 3 contains spectra of indole complexed with two polar solvent molecules: (H2O)2, (MeOH)2, and (THF)1(MeOH)1. To obtain these spectra, the microchannel plate detector voltage had to be increased by hundreds of volts compared to the one adduct spectra. In each instance very broad excitation features with some embedded structure were obtained.
J. Phys. Chem., Vol. 100, No. 41, 1996 16481
Figure 4. Mass selected two-photon REMPI excitation spectra of 1-MI and one adduct solvent complexes.
These spectra are qualitatively similar to those Hager et al. obtained by complexing 2,3-dimethylindole (DMI) with single polar adducts.10 N-Methylindole (1-MI). Presumably, only the π binding site is accessible for solvent binding, the N-H site being removed by methylation. Addition of either one water or one methanol molecule to 1-MI results in broad excitation peaks with some embedded sharp transitions, centered at ∼34 500 cm-1 (Figure 4). The Levy group earlier made analogous observations for 1-MI water complexes.13 There is no apparent replication of the sharp 1-MI chromophore vibronic peak pattern. Under the nozzle conditions used, no multimolecular complexation was found. 3-Methylindole (3-MI). Proceeding from water to TEA, the complexes in Figure 5 display increasing evidence of progression features. For water, a progression-like envelope appears at 212, 246, and 272 cm-1. For methanol numerous 20-24 cm-1 progression members can be seen, most prominently built off the origin, the 725 cm-1 peak, and in the range 941-1023 cm-1. The 3-MI-(TEA)1 scan shows large origin region progressionlike features, with broad mostly structureless excitation to the blue. There seem to be different progression series in the origin region, originating at 34 197, 34 278, and 34 347 cm-1. Each one may possibly correspond to a complex with a different N-H site binding geometry for TEA. Our scan continues to about 1500 cm-1 further to the blue and reveals a broad excitation maximum containing some embedded sharp structure. The broad structure, which was the dominant feature before the laser intensity correction, qualitatively resembles that for a π site complex such as 1-MI-(H2O)1. It is of interest to examine lifetimes in 3-MI-TEA as a function of excitation energy. The three sharp origin region peaks at 34 197, 34 278, and 34 371 cm-1 have singleexponential lifetimes of 9.0 ( 0.2, 8.9 ( 0.2, and 9.0 ( 0.2 ns, respectively. Excitation within the broad band at 34 920 ( 10 and 35 120 ( 10 cm-1 yielded single-exponential lifetimes of 8.4 ( 0.2 and 8.2 ( 0.2 ns, respectively. Under our nozzle conditions production of n ) 2 clusters for water and methanol was weak. No wavelength-resolved features were obtained for these clusters.
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Huang and Sulkes TABLE 2: Measured Lifetimes of S1 Levels in 2-MI freq above origin, cm-1 0 (35 170 cm 430 445
-1)
lifetime, ns
freq above origin, cm-1
lifetime, ns
15.4 13.2 11.9
524 618 764
6.8 7.1 7.4
TABLE 3: Predicted 1La-1Lb Energy Gaps in cm-1 for Isolated Molecule Indoles, by Computationa compound
Ab
Bc
Cd
2,3 DMI 3-MIe 1-MIf 2-MI indoleg 5-MOI
440 533 629 1062 1400 2163
404 729 1017 1309 1400 2564
115 615 1073 1400 2923
a Predicted gap energies have been adjusted so that each series of calculations predicts a gap of 1400 cm-1 for unsubstituted indole. b INDO/S-SDCI, ref 23. c INDO/S-SCI, ref 23. d CNDO/S, ref 14. e Experimental predictions of 100 cm-1 (ref 18), 340-1000 cm-1 (ref 22), 610 cm-1 (ref 27), and ∼500 cm-1 (ref 12). f Experimental predictions of 1150 ( 60 cm-1 (ref 22). g Experimental predictions of 1590 ( 60 cm-1 (ref 22), 1439 cm-1 (ref 30), 1400 cm-1 (ref 31), and 1400 cm-1 (ref 32).
Figure 5. Mass-selected two-photon REMPI excitation spectra of 3-MI and one adduct solvent complexes. Transition origins are aligned as in Figure 2.
Figure 6. Mass-selected two-photon REMPI excitation spectra of 2-MI and one adduct solvent complexes. Transition origins are again aligned.
2-Methylindole (2-MI). The REMPI spectrum of bare 2-MI (Figure 6) unexpectedly shows weak relative intensities for all vibrations above the origin. We also obtained a LIF excitation spectrum of 2-MI that showed similar peak intensities. The spectrum is similar in appearance to those of indoles such as DMI or 1,2,3,4-hydrocarbazole (HC), where it was supposed that the weakness of the bluer features arises from coupling to short-lived 1La levels.10,26 In 2-MI, however, a fairly large 1La1L gap is predicted (Table 3). Table 2 presents lifetime values b for 2-MI levels from the origin upward. In comparing the falloff in lifetime versus energy of these values to general trends for four other predominantly 1Lb emitting indoles10 (indole, 1-MI, 5-MOI, 3-MI), there is a somewhat greater drop in lifetimes
for 2-MI, ∼50% of the origin lifetime for E ≈ 700 cm-1 vs ∼30-40% in the other four cases. However, 2-MI clearly is not comparable to DMI or HC (short origin lifetime and lifetimes j1 ns at E ≈ 400 cm-1).10 In conclusion, the low relative intensities of 2-MI peaks above the S1 origin are not primarily due to a much enhanced radiationless process. This finding is consistent with the relatively large 1La-1Lb gap expected for 2-MI. A remaining possibility is that the weak higher energy peak intensities arise from weak Franck-Condon factors. If so, 2-MI is the only observed instance where a methyl substituent has this effect. The spectra for addition of single water and methanol molecules to 2-MI show no features to the red of the scans in Figure 6. At the S1 origins, 35 034 and 35 012 cm-1, there are no progression envelopes, only single intermolecular vibrations (21 and 22 cm-1, respectively) built on the origins. Although all of them are very weak, the major peaks to the blue can be correlated with bare chromophore peaks. In 2-MI-(MeOH)1 there is also a peak at 35 183 cm-1 which has no counterpart in bare 2-MI or in the 2-MI-(H2O)1 scan. The structure in the vicinity of 35 150 cm-1 in the water complex is actually due to excitation of 2-MI-(H2O)2, which has lost one H2O following ionization. The water and methanol complex spectra for 2-MI are consistent with a solvent molecule binding at only the N-H chromophore site. Evidently, the C-2 methyl, as has been found for the C-3 methyl in 3-MI,12 can interfere with formation of π site complexes. While more complicated, the 2-MI-(TEA)1 spectrum is also consistent with the pattern seen for water and methanol addition. The additional complication appears to arise from a pattern of structure that is repeated to the blue. Note the substantial repetition of the structure superimposed on ν00, between 34 800 and 35 000 cm-1 and the similar structure between 35 200 and 35 400 cm-1. In addition to solvent and intermolecular vibrations, some of these effects may be due to multiple binding geometries of TEA at the N-H site, arising from rotation of the pyramidal TEA relative to the indole plane. Included in Figure 7 are spectra of solvent complexes with two solvent adducts, at least one of them presumably at the N-H site. All three spectra, 2-MI-(H2O)2, 2-MI-(MeOH)2, and 2-MI-(TEA)1(MeOH)1, display very broad excitation features. The broad maximum becomes wider and moves further to the
Jet-Cooled Solvent Complexes with Indoles
J. Phys. Chem., Vol. 100, No. 41, 1996 16483
Figure 9. Mass-selected two-photon REMPI excitation spectra of 5-MOI and two adduct solvent complexes. Figure 7. Mass-selected two-photon REMPI excitation spectra of 2-MI and two adduct solvent complexes.
Figure 8. Mass-selected two-photon REMPI excitation spectra of 5-MOI and one adduct solvent complexes. Transition origins are aligned.
red (from ∼35 150 to 34 900 cm-1) with increasing collective proton affinity of the two solvent adduct molecules. 5-Methoxyindole (5-MOI). Major chromophore vibrations well to the blue of the origin are evident for one molecule addition of water, methanol, and TEA (Figure 8). For addition of TEA, as noted earlier with 2-MI, a packet of well-defined structure repeats for major chromophore vibrations. Solvents that can donate a proton, like water and methanol, should have the potential of forming π site complexes. To find evidence of them, careful scans were carried out as much as 400 cm-1 to the red of the origin features shown in Figure 8, but no additional peaks were found. Red-shifted π site
complexes, if present at all, are not conspicuous. One other possibility is that weak structure slightly to the blue of the origin in the water complex may arise from π site binding.14 Spectra of complexes involving two polar solvent adducts, 5-MOI-(H2O)2 and 5-MOI-(MeOH)2 (Figure 9), as for other indoles all have broad excitation features. 2,3-Dimethylindole (DMI). This indole has been extensively studied by the Wallace group.10,11 For 2,3-dialkylindoles such as DMI, 1La is expected to be close to 1Lb. The very short lifetimes observed in bare DMI (6.0 ns origin and lifetimes e1.0 ns for E > 300 cm-1) were attributed to coupling of 1Lb levels to 1La; a highly efficient nonradiative process was associated with 1La.26 Wallace’s group initially proposed indolylic N-H bond dissociation in 1La as the radiationless process.10,11 Evidence pointing to this conclusion is the significant increase in bare molecule lifetimes in DMI-d1 and also in HC-d1, where the N-H has been replaced by N-D.11 We tried to verify an N-H bond dissociation channel in bare DMI using mass-resolved spectra. Excitation spectra were carried out in which time-of-flight mass waveforms were taken to cover the parent m/e ) 147 channel as well as lower mass channels. As the wavelength of excitation proceeded to the blue of the 0-0 transition, one might anticipate the appearance of an excitation peak in the m/e ) 146 channel. In fact, a scan from ∼ 400 to 1100 cm-1 above the origin did not reveal any correlation between the 146 and 147 amu channels. The N-H dissociative channel in 1La, if it exists, is too weak to be detected. We note that our results, if the detection limits are meaningful, do not necessarily mean that N-H bond breaking is not occurring, only that it is apparently not occurring within the 1La manifold. Our findings are consistent with an amended Wallace group hypothesis concerning the 1La radiationless channel.12 They propose that N-H dissociation occurs through coupling to levels of another state, a triplet or high-energy dissociative levels of the ground state. Another possibility not ruled out by our results is excited state tautomerization.27 The Wallace group experiments suggest that the pKa* of the DMI N-H group might be lower than is the case for other indoles, and we wished to address this possibility experimentally. As was seen previously for some solvent complexes with phenol28,29 and also 4- and 5-hydroxyindole,27 complexation with
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sufficiently basic proton acceptor molecules can produce excited state zwitterions. For DMI, the following steps would take place:
(DMI-NH)-B + hν f (DMI-NH)*-B
(1A)
(DMI-NH)*-B T (DMI-N)-*-H+B
(1B)
B indicates a basic solvent molecule. The indolylic-NH is explicitly indicated for DMI. A second photon could then produce the following photoionization outcomes:
(DMI-NH)*-B + hν f (DMI-NH)+-B
(2A)
(DMI-N)-*-H+B + hν f DMI-N + H+B
(2B)
Formation of an excited state zwitterion in (1B) would then mean that a 1 + 1 REMPI excitation spectrum monitored in the H+B mass channel (2B) should match the one in the (DMINH)+-B mass channel (2A). We complexed DMI with TEA, a potent proton acceptor, and measured wavelength scanned onecolor 1 + 1 REMPI excitation spectra. Ion signal monitoring took in both the 102 amu channel, corresponding to H-TEA+, and 246 amu, corresponding to DMI-(TEA)+. There is a correlation between the two spectra although the former is much weaker than the latter. These results indicate that some of the REMPI ionized DMI-(TEA)1 complexes dissociate to form H-TEA+. Analogous complexation experiments carried out on TEA complexes with indole and 4-methoxyindole did not yield a signal in the H-TEA+ mass channel. Evidently, then, the excited state N-H acidity in DMI is enhanced compared to other indoles. Discussion Polar solvent adducts can induce 1La lowering in indoles. This has been evidenced fairly convincingly in DMI, possessing a small 1La-1Lb gap in the jet-cooled monomer.10,11 (Table 3 contains some computed and experimental 1La-1Lb gaps for all the indoles considered in this paper.) For DMI, clustering with one molecule of methanol or trimethylamine (TMA) at the N-H site evidently brings about 1La-1Lb inversion. Subsequently, Levy’s group found a number of instances where polar group interactions with an indole chromophore, evidently near C-3, induced broad, red-shifted emission.1-3 Initially, these effects were seen in selected jet-cooled conformers of tryptophan and some tryptophan analogs. Subsequently, similar observations were made in a number of instances where indoles were complexed with H bond donor solvent molecules.13,14 The emission characteristics were taken to indicate 1L . To explain these observations, it was proposed that π site a interactions bring 1La slightly below 1Lb, resulting in a double well surface. Initial optical excitation was presumed to be to the 1Lb character well followed by curve crossing to the 1La well, from which the emission was broad and red-shifted. More recently, however, it has been suggested by the Callis group that the broad and red-shifted emission from the π site indole-H2O complex need not be assigned to 1La.18 Instead, they proposed that an increased number of emission transitions from 1Lb combined with monochromator resolution can produce similar spectral features. Further, on the basis of two-photon polarization results, they now definitely assign the progression features to 1Lb in the indole-H2O complex.33 At this point it is apparent that π site binding with H donor solvents generally can produce extended Franck-Condon envelopes in the excitation spectra. This, incidentally, is not the case with benzene-
water π complexes.34 Based on the Callis group results, however, the extent to which π site interactions in indoles may lower 1La relative to 1Lb is not clear. I. One Solvent Adduct Results. DMI. Complexation at the N-H site with one proton accepting solvent molecule lowers 1L to an extent that optical transitions proceed directly to 1L , a a resulting in broad excitation features. The 1La radiationless process in bare DMI was postulated to involve N-H bond scission.10,11 We did not find a mass corresponding to this direct dissociation product. Somewhat analogously, however, using TEA solvent molecules, we found that N-H site excited state proton transfer in the complex is also a possibility. No evidence for an excited state proton transfer process was found in TEA complexes with indoles possessing larger 1La-1Lb separations (indole, 4-MOI). Following N-H solvent complexation of DMI and HC, lifetimes were found to increase.11 As a result, the inference was made that IVR brought about energy removal from the N-H coordinate to vibronic degrees of freedom in the adjacent solvent molecule, thereby inhibiting the radiationless process.11 More generally, however, we point out that other coordinates evidently must be considered. This is evidenced by lifetime measurements on W(1a), a 2,3-dialkylindole which contains a second N-H site in its 2,3-dialkyl ring. Separate water or alcohol molecule addition to either N-H site in W(1a) (both are distinguishable in terms of different spectral red shifts) resulted in lengthened lifetimes of 8-10 ns.15 Indole. N-H site complexation results in excitation and emission spectra attributable to 1Lb. This holds true even for a strong proton accepting solvent such as TEA and speaks to the relatively large 1La-1Lb gap of 1400 cm-1. In each excitation spectrum, characteristic 1Lb chromophore vibrations continue well to the blue of the complex origins. The new observation that emerges from the water and methanol complex scans of Figure 2 is that no additional peaks attributable to the π site complexes appear further to the blue of the initial intermolecular mode progression envelopes. As noted in Results, excitation of the indole complexes further to the blue produced no new higher energy peaks in the indole mass channel. This fact rules against dissociation of the π site indole complexes at higher energies and suggests the possibility of some other radiationless process higher in the S1 manifold. 5-MOI. This indole, with its large 1La-1Lb gap, has N-H complexes that follow the characteristic 1Lb pattern of chromophore vibronic activity persisting to the blue. The absence of excitation peaks clearly attributable to a second, π, binding site may simply arise from a very small S0 population fraction, due to more favorable energetics of N-H site binding. 1-MI. Complexation with one water or methanol molecule results in broad excitation features qualitatively similar to those seen for DMI. At least part of the broadness of these features can be attributed simply to closely spaced progression features arising from π site binding. However, there is at least the possibility that S1 is now 1La. Lami and Glasser found that 1-MI emits from 1La in water.22 Their overall solvent polarity results for five indoles, including 1-MI, could be explained in terms of a model assuming formation of ground state solventsolute complexes. More definitive testing for the possibility of π site induction of 1La (two-photon polarized excitation spectra) should be done for polar n ) 1 complexes. 2-MI. With a relatively large 1La-1Lb gap, N-H binding site complexes have peaks to the blue of the S1 complex origins that continue to have 1Lb character. No progression envelope features appear in the spectra, indicating the likelihood that formation of π complexes is inhibited by the presence of a
Jet-Cooled Solvent Complexes with Indoles methyl group at C-2. In some circumstances, however, π site complexes can be formed, as was evidenced in earlier experiments with 1,2-dimethylindole (1,2-DMI). In the latter instance, with N-H site binding now removed, typical π site progression features were seen following the addition of water.14 3-MI. 3-MI is a particularly interesting case because of the intermediate 1La-1Lb gap. A detailed study of 1La-1Lb coupling in 3-MI and its polar clusters has been published fairly recently by the Wallace group.12 In this work, by plotting n ) 1 solvent complex origin shifts versus solvent proton affinity for a number of cases, they showed that the complexes observed could be ascribed to N-H binding sites. These cases included water, methanol, and TMA. Although π site binding is not expected, we observed increasing evidence of progression features in the order water f methanol f TEA. As will be further discussed, the above trend appears to be related to increasing solvent proton affinity. When 3-MI is complexed with TEA, the spectrum qualitatively resembles previously observed cases of π site binding, even though the TEA must bind near the N-H based on steric factors, basicity, and correlations between red shifts and solvent proton affinity for N-H site binding. For instances such as 3-MI(TEA), Demmer et al. show convincingly that excitation is to 1La.12 Their studies relate the lowering of 1La to the basicity of the solvent adduct at the N-H, and they also address the reason why progression structure is seen concomitant with 1La lowering. For a number of solvent adducts considered, they found that until the solvent adduct proton affinity exceeded 200 kcal/mol (values for water and methanol are 166.5 and 181.9 kcal/mol, respectively), they continued to find complex lifetimes that exceed 10 ns. At higher solvent proton affinities shorter lifetimes, associated with 1La, predominated. For complexation with trimethylamine (TMA, proton affinity 225.1 kcal/mol, more than sufficient to make 1L the S state and very comparable to TEA), the published 1 a origin region excitation spectra closely resemble our spectra of the TEA complex. Because the 1La state is expected to have a larger dipole moment than 1Lb or the ground state, increasing 1L character should correlate with an increase in the strength a of excited state hydrogen bonding. For a solvent such as TMA or TEA, the differences in equilibrium binding coordinates for a more strongly bound complex in 1La S1 versus a less strongly bound one in S0 results in extended Franck-Condon progressions. Our 3-MI-(TEA)1 scan carries on well into the blue. After the initial Franck-Condon envelope, there is a reduction in intensities and a spectral broadening in the REMPI spectrum. These indications might be associated with an enhanced radiationless process at higher energies. However, as seen in Results, lifetimes in the vicinity of 35 000 cm-1 are still more than 8 ns. In analogy with the lengthened lifetimes seen by Demmer et al. for the TMA complex,12 although the 3-MI(TEA)1 levels accessed are probably 1La, the radiationless process is apparently inhibited by IVR from the N-H dissociative mode to solvent modes. With one solvent, CH3CN, Demmer et al. did see evidence of π site addition for 3-MI. They reported a very broad excitation spectrum extending ∼1000 cm-1 to the red of the bare molecule origin.12 The emission was reported to be even broader, with a maximum 5000 cm-1 to the red of the excitation frequency. The foregoing spectral indications are characteristic of π complexes and indicate that π site complexation in 3-MI may not always be precluded. Previously, the spectral indications for the CH3CN complex would definitely have been associated with induction of 1La emission. Given the intermedi-
J. Phys. Chem., Vol. 100, No. 41, 1996 16485 ate 1La-1Lb gap, more definitive testing for 1La emission would be interesting in this case. The 1La-1Lb gap of DMI was sufficiently small that one polar adduct evidently brought 1La well below 1Lb. 3-MI, with its intermediate gap, is a case where 1La and 1Lb might be nearby in some instances of polar solvent complexation. Indeed, of the indoles considered, only for 3-MI when complexed with water and methanol was it the case that some but not all chromophore vibrations maintained nearly constant frequencies relative to the origin. Muino and Callis earlier recorded extended LIF scans of indole complexes in an effort to implicate 1L behavior in optically excited levels of indoles.19 The a rationale for the experiments is that the larger dipole change associated with 1La transitions should mean that polar solvent complexation red shifts 1La levels significantly more than 1Lb levels. In indole N-H site complexes with water and methanol, chromophore vibrations showed red shifts that nearly matched those of the 1Lb complex origins. This continues to be true for several vibrations in 3-MI complexessthose bare molecule levels that are presumably 1Lbsbut is not true for a number of others. The largest features common to the water and methanol complexes that appear to shift in tandem with the origins are for water at 722 and 935/958 cm-1 relative to the complex origin (34 643 cm-1) and for methanol at 725 and 959 cm-1 relative to the complex origin (34 592 cm-1). In contrast, bare 3-MI has some possibly corresponding structure at 718, 742, and 751 cm-1 as well as at 921 cm-1 relative to the origin (34 882 cm-1). At least some of the bare 3-MI structure in the 700 cm-1 region must presumably have substantial 1Lb character as must either the 921 or 948 cm-1 peak. Sammeth et al. have previously obtained polarized two photon fluorescence spectra of jet-cooled 3-MI.17 Based on peak intensity ratios for linear versus circular polarized excitation, peaks could be assigned as having 1La or 1Lb character. The three 700 cm-1 region peaks are assigned 718 cm-1 T 1Lb 742 cm-1 T 1La, and 751 cm-1 T 1La (all frequencies adjusted to our values). The locations above the origin for the persisting features in the complexes, at 722 and 725 cm-1, are close in frequency to the bare molecule 1Lb assigned peak at 718 cm-1. Sammeth et al. found the intense bare molecule peak at 921 cm-1 to be 1La. The next relatively intense peak in the bare 3-MI spectrum is at 948 cm-1, actually a closer match in frequency to the complex peaks at 935/958 cm-1 for water and 959 cm-1 for methanol. Sammeth et al. did not assign the 948 cm-1 peak. On the basis of our comparative results, we expect it to be 1Lb. Major bare 3-MI peaks that do not consistently repeat in the complex spectra are at 411, 422, 429, 470, 611, 619, and 823 cm-1 above the origin. All were earlier assigned to be 1La except for the peak at 429 cm-1.17 While there are peaks in the water complex spectrum at 403 and 451 cm-1 that could conceivably be associated with the 429 cm-1 3-MI peak, there appears to be no corresponding peak in the methanol complex. More recently, Demmer et al., based on fluorescence decay measurements, assigned bare 3-MI peaks at 614 and 718 cm-1 as having 1La character.12 Their quoted frequency of 614 cm-1 falls between peaks at 611 and 619 cm-1, both of which we and Sammeth et al. find to have 1La character. However, the 718 cm-1 peak was found to be 1Lb by Sammeth et al., and we found a corresponding Lb peak in the complex spectra. If the peak at 34 278 cm-1 in the 3-MI (TEA)1 spectrum were considered to be the origin of the predominant complex geometry present and if the peak at 35 000 cm-1 belonged to this complex, its displacement from the origin would be 722
16486 J. Phys. Chem., Vol. 100, No. 41, 1996 cm-1. This peak might then be the analog of the 722 and 725 cm-1 peaks seen in the water and methanol complexes with 3-MI. That is, it might still possess 1Lb character. If so, the associated vibration consistently shows poor coupling to 1La. II. Two Adduct Complexes. In each observed instance the addition of two polar molecules to an indole chromophore resulted in broad excitation spectra similar to those observed for n ) 1 polar complexes with DMI. Reasonable arguments could be made in the latter case that optical excitation was occurring directly to 1La.10,11 A similar argument could credibly be made for all the two solvent complexes observed except those with 5-MOI. This indole, which has a large 1La-1Lb spacing, has been found to emit from 1Lb in water and methanol solutions.22 An alternative explanation for some of the broad features, not involving 1La inversion, can be made that applies to 5-MOI as well as to the other cases. If one of the two solvent adducts added to the π site, an extensive Franck-Condon progression would be expected. The other site would superimpose additional features, contributing to a broad excitation spectrum. Summary and Conclusions Solvent shift data in water and methanol complexes of 3-MI indicate the presence of a number of 1La levels at E > 300 cm-1 above the S1 origin. The set of levels indicated to be 1La compares quite closely with assignments made earlier by the Callis group.17 Polar solvent molecules binding to indoles can arise from solvent H bond donation to the chromophore π density between C-2 and C-3. Due to binding geometry differences between the ground and excited state complexes, the excitation and emission features are spectrally broadened. Further study of the indole π site water complex by Callis et al. has indicated that the emission, despite its broadness and enhanced red component, is not due to 1La. If emission spectra alone are not a reliable indicator of 1La, the general effectiveness of π complexes in lowering 1La is unclear. Further characterization of possible 1La emission should be carried out in π site complexes of 1-MI. An even more interesting case might be π site complexes of 1,2-DMI, since this indole apparently has a 1L -1L gap similar to the one in 3-MI. a b Acknowledgment. This work was supported by PHS Grant GM32777. We gratefully acknowledge support from the Center for Photoinduced Processes, funded by the National Science Foundation and the Louisiana Board of Regents. References and Notes (1) Rizzo, T. R.; Park, Y. D.; Levy, D. H. J. Chem. Phys. 1986, 85, 6945.
Huang and Sulkes (2) Cable, J. R.; Tubergen, M. J.; Levy, D. H. J. Am. Chem. Soc. 1989, 111, 9032. (3) Tubergen, M. J.; Cable, J. R.; Levy, D. H. J. Chem. Phys. 1990, 92, 51. (4) For references on solvent complexation of indoles germane to this paper, see refs 5-19. See also additional references cited therein. (5) Montoro, T.; Jouvet, C.; Lopez-Compillo, A.; Soup, B. J. Phys. Chem. 1983, 87, 3582. (6) Nibu, Y.; Abe, H.; Mikami, N.; Ito, M. J. Phys. Chem. 1983, 87, 3898. (7) Bersohn, R.; Even, U.; Jortner, J. J. Chem. Phys. 1984, 80, 1050. (8) Hager, J. W.; Wallace, S. C. J. Phys. Chem. 1984, 88, 5513. (9) Hager, J.; Ivanco, M.; Smith, M. A.; Wallace, S. C. Chem. Phys. 1986, 105, 397. (10) Hager, J. W.; Demmer, D. R.; Wallace, S. C. J. Phys. Chem. 1987, 91, 1375. (11) Demmer, D. R.; Leach, G. W.; Outhouse, A. E.; Hager, J. W.; Wallace, S. C. J. Phys. Chem. 1990, 94, 582. (12) Demmer, D. R.; Leach, G. W.; Wallace, S. C. J. Phys. Chem. 1994, 98, 12834. (13) Tubergen, M. J.; Levy, D. H. J. Phys. Chem. 1991, 95, 2175. (14) Arnold, S.; Sulkes, M. J. Phys. Chem. 1992, 96, 4768. (15) Teh, C. K.; Gharavi, A.; Sulkes, M. Chem. Phys. Lett. 1990, 165, 460. (16) (a) Hays, J. R.; Henke, W. E.; Selzle, H. L.; Schlag, E. W. Chem. Phys. Lett. 1983, 97, 347. (b) Sammeth, D. M.; Yan, S.; Spangler, L. H.; Callis, P. R. J. Phys. Chem. 1990, 94, 7340. (17) Sammeth, D. M.; Siewert, S. S.; Spangler, L. H.; Callis, P. R. Chem. Phys. Lett. 1992, 193, 532. (18) Muino, P. L.; Callis, P. R. Chem. Phys. Lett. 1994, 222, 156. (19) Muino, P. L.; Callis, P. R. SPIE Proc. 1994, 2137, 278. (20) See refs 21-23 below and references therein for considerable additional work which has been done. (21) Meech, S. R.; Phillips, D.; Lee, A. G. Chem. Phys. 1984, 80, 317. (22) Lami, H.; Glasser, N. J. Chem. Phys. 1986, 84, 597. (23) Eftink, M. R.; Selvidge, L. A.; Callis, P. R.; Rehms, A. A. J. Phys. Chem. 1990, 94, 3469. (24) Huang, Y.; Sulkes, M. ReV. Sci. Instrum. 1994, 65, 3868. (25) Teh, C. K.; Sipior, J.; Sulkes, M. J. Phys. Chem. 1989, 93, 5393. (26) Additional lifetime measurements on S1 levels of 2,3-dialkylindoles are reported in: Teh, C. K.; Gharavi, A.; Sulkes, M. SPIE Proc. 1990, 1204, 820. In some instances reported here different S1 origin peaks display drastically different lifetimes (e.g., 10.9 ns vs 4.9 ns for two peaks 8 cm-1 apart in 3-carboxy-1,2,3,4-tetrahydro-2-carboline, designated W(1). The 2,3dialkyl substituents for these compounds, like HC, are in the form of a closed ring. Unlike HC, however, the closed rings contain polar groups. Origin peaks with drastically different lifetimes may belong to S1 conformations with different geometries, resulting in different admixtures of 1La. (27) Huang, Y., Sulkes, M. Chem. Phys. Lett. 1996, 254, 242. (28) Syage, J. A.; Steadman, J. J. J. Chem. Phys. 1991, 95, 2497. (29) Steadman, J.; Syage, J. A. J. Am. Chem. Soc. 1991, 113, 6788. (30) Strickland, E. H.; Horwitz, J.; Billips, C. Biochemistry 1990, 9, 4914. (31) Cable, J. R. J. Chem. Phys. 1990, 92, 1627. (32) Fender, B. J.; Sammeth, D. M.; Callis, P. R. Chem. Phys. Lett. 1995, 239, 31. (33) Callis, P. R. Private communication. (34) Gotch, A. J.; Zwier, T. S. J. Chem. Phys. 1992, 96, 3388.
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