4768
J . Phys. Chem. 1992, 96, 4768-4778 po(r) d r dz = 2rp arccos (r/2R) dr dz
(16)
Inserting eq 16 into eqs 8-1 1, we obtain J ( t ) = 4$2Rdr$mpr 0
(
arccos &)wcr, w(r)2,7[1
-
+
0
..
,
I
7
Here 1 w(r) = 7
RW6 (r2 zZ)3
+
For the long-time decay t / r
for S = 6
>> 1 and t >> R2/a eq
17 yields20 1 N J ( t ) 4 S Z R d r S m drp z arccos 0 0 2R 1 2z6/Rs2 2 --~r~R,d?~p (18) 3 The short-time behavior of eq 17 may be found only with limits of integraton R I 2- 2R where R I 2is the sum of the radii of two donors. We receive for small times from eq 17
(Lj
If r/2R = y, RI2/2R = a
+
u3 y4
origin V I , V21 y 3 y4
A origin 4u1 34 2Vl VI
B origin
from from from from
dual fit dual fit dual fit dual fit
13.8 f 0.2 12.0 f 1 12.2 f 0.2 11.5 f 0.3 11.5 f 0.3 11.5 f 0.5 17.7 i 0.2 15.1 f 0.3 14.4 f 0.2 14.3 f 0.2
origin origin
origin
origin origin
A conf A conf B conf 5u1 4v1 3ul 2v, C origin
lifetime, ns 17.55 f 0.1 17.6 f 0.6 17.4 f 0.6 17.7 f 0.3 17.7 f 0.4 17.5 f 0.5 17.4 f 0.6 17.7 f 0.4 17.7 f 0.4 17.7 f 0.5 21.0 f 0.2 17.7 f 0.4 17.7 f 0.4 17.7 f 0.3 17.7 f 0.4 17.7 f 0.5 21.5 f 1 >80% component 21.0 f 1 >75% component 21.5 f 1 >70% component 21.5 i 1 >70% component
+ ul
15.3 f 0.2 15.5 f 0.3 15.0 f 0.3 12.4 f 0.6 9.5 f 0.6 8.2 f 0.6 8.3 f 0.6 14.9 f 0.4 15.1 f 0.3 13.7 f 0.2 13.8 f 0.6 9.2 f 0.2 9.1 f 0.2 8.9 f 0.2 8.7 f 0.2 8.6 f 0.2
oUnless noted all lifetimes were obtained via time-correlated photon counting. Designations for vibronic peaks in some derivatives ( v , , v2, etc.) are as in ref 13. TABLE 11: Results of C N W / S Calculations for Bare Molecules transition freq, cm-I ILaJLb indole derivative 'L. 'Lh gap, cm-' e,, deg 5963 80.9 32 270 5-methoxy 38 233 5058 89.1 38495 33437 5-methyl 34 148 4440 75.7 38 588 unsubstituted 4113 76.6 33969 1-methyl 38082 3655 74.1 37652 33997 3-methyl 33 780 3623 59.8 1,2-dimethyl 37403 3155 40.6 33 682 2,3-dimethyl 36 837
The true lifetime may more closely match that of the other two progression features. 3-MI. Previous c a l c ~ l a t i o n sas~ ~well as our own (Table 11) indicate a relative 'La lowering of 700-800 cm-I compared to indole; this is only 100-300 cm-I less than for 2,3-DMI, in which the bare molecule gives evidence of 'La-'Lb interaction.' Consequently water complexes might be expected to show 'La emission, since they did with indole. The excitation spectra we obtained for water + 3-MI closely match the one reported by Tubergen and Levy.13 Scans further to the red did not reveal any other peaks. They identified the principal solvent-induced feature at AE = -233 cm-l as arising
from binding at the N site. If this is the case, the red shift should scale with solvent proton affinity in a roughly linear fashion. On the basis of red shifts induced in 3-MI following addition of NH,,l9 TMA,I9methanol,I9 and dioxane (discussed below), the red shift for water addition is roughly consistent with this correlation. The question then is whether water N site complexes with 3-MI produce 'La emission in 3-MI. They do not in indole. Repeated dual tracked LIF scans consistently indicated that all the water-induced features had IL, character emission. Figure 5 shows one such set of scans, referenced to the bare molecule origin (not shown) and including the water peaks at 34646,34667, and 34677 cm-l. Dual scans over the water induced peak at 34 763 cm-I also yielded analogous results. Lifetimes for the complex peaks at 34646 cm-' and 34667 cm-' were both about 12 ns, notably different from that of the bare molecule origin. Preliminary polarized two photon excitation spectra by the Callis group22 have also indicated that the major water band has ILa character. 1,2-DMZ. The lLa-lLb gap according to our calculations (Table 11) is comparable to the gap in 3-MI. Here, though, as in NMI, only r-site binding is a possibility. It seemed possible that the 2-position methyl could sterically interfere with m i t e solvent binding and prevent formation of complexes, but this was not the case. A dual LIF scan of 1,2-DMI with water addition is shown in Figure 6. Now a series of features is evident to the blue of
The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 4773
Induction of ILB-ILbState Coupling
I 3-MI
+
\
+
5-MI
WATER
E X C I T A T I O N FREQUENCY ( c m - l l Figure 5. Dual LIF scans for the major water induced peaks of 3-MI, as in Figure 3. Both scans are referenced to the origin of bare 3-MI (not shown).
n
WATER
EXCITATION FREQUENCY ( c m - l ) Figure 7. Conventional LIF scan of 5-MI plus water. The peak a t the blue end, truncated at 40% of its maximum in the plot, is the bare molecule origin.
n
n
1, 2 DM1
+
n
WATER
34200 34300 EXCTIATO IN FREQUENCY ( c m - l ) Figure 8. Dual LIF scans of 5-MI plus water. The reference peak at the blue end of each scan is the bare molecule origin. Each is truncated a t 35% of full scale.
5-MI
f
WATER
3 4* 060 34100 34 : 140 EXCITATION FREQUENCY (cm- '1 Figure 9. Dual LIF scans for *-site binding features of 5-MI plus water. Referencing for each scan is to the N binding site peak at 34 209 cm-I. The spectral resolution is decreased compared to Figure 7 due to inhomogeneities in sample flow averaged over each stepped acquisition interval (35 s, -0.8 cm-l/steps).
significant red shift. The reddest peak discerned in Figure 7 is at 33905 an-'. Second, the extended features give the appearance of a progression, particularly because of the extended intensity envelope. This structure could be complicated but not fully ac-
4774 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992
1 h
5-MOI + WATER
Arnold and Sulkes
II
1
3 4 150
EXCITATION FREQUENCY (cm-l) Figure 10. Dual LIF scans for the strong water induced features in 5-MOI + water. The solid line is a LIF spectrum obtained through a WG-320filter. The dotted line is for a monochromator tracked scan at A5 = -759 cm-I. The scans are ratioed to the bare molecule origin in each case. counted for by interaction of torsional and intermolecular modes. It is difficult to obtain reliable dual-tracked scans for features as weak as the red-shifted progression, but the scans in Figure 9 strongly indicate some 'La character emission. 5-MOI.The 'L,-ILb gap in 5-MOI is consistently estimated to be the largest from among the series of compounds yet considered (Table 11). The gap is roughly 800-1200 cm-' more than for indole and 1700-2000 cm-' more than for 2,3-DMI. The spectrum of 5-MOI water (Figure 10) shows a relatively large water-indud feature at 33 049 cm-' that has a fairly small redshift (A? = -87 cm-I relative to the bare molecule origin at 33 136 cm-I). Slightly to the blue, at 33054 cm-', is a weaker peak followed by progression-likefeatures at 33 068, 33 075, and 33 082 cm-I. No features were detected to the red of the large peak at 33 049 cm-'. The largest solvent-induced peak probably arises from N site binding and would strongly be expected not to show red emission. The smaller solvent peaks to the blue are not necessarily vibrations built off the complex origin at 33 049 cm-I. This interpretation seems called for by the dual tracked scans in Figure 10. All the peaks to the blue of 33 049 an-'show fairly significant relative intensity differences in the two scans; the small differences for the peak at 33 049 cm-' are apparently due to superposition with the peak at 33 054 cm-I. If the smaller peaks show a 'La-like emission component, as is indicated, the only sensible assignment is to a A-binding complex, for the following reasons: Solvent binding could be at the N site, the 5-position methoxy, or A-site. N site binding should not induce 'La emission and binding at the 5-methoxy site, on the benzene, should be ineffective in lowering 'La relative to ILb. On the other hand, ?r site binding clearly has the potential of producing this result. It is also the case that the solvent peaks to the blue of 33 049 cm-l qualitatively resemble the pattern of water-induced A site binding features seen with water addition to 1,2-DMI, and NMI, as well as indole + DCM (see below). Lifetime measurements potentially can further corroborate the above assignments but, unfortunately,the measurable solvent peaks have nearly the same lifetime, close to the bare molecule value (Table I). The weak water complex peaks at 33 068/33 075 cm-' have a lifetime value only -0.5 ns less than the peak at 33 049 cm-I; two single-exponential components cannot meaningfully be discerned for a weak peak under these circumstances. In summary, we conclude that A site binding in 5-MOI must again be occurring and inducing 'La character emission. If this is the case, A site solvent binding must be sufficiently potent to induce 'Laemission in virtually any indole. Solvent Complexes of Dioxane with Indoles. p-Dioxane is a solvent that acts as a proton acceptor but not a proton donor. Therefore it should readily add at the N site but not the A site.
+
34250
34350
EXCITATION FREQUENCY (cm-') Figure 11. Dual LIF scans of 5-MI plus dioxane. The peak at the blue end, the bare molecule origin, truncated at 60% of its maximum in the plots, serves as the reference.
Because it has a greater proton affinity than water, thereby producing complexes with larger solvent red shifts, it has a greater potential of inducing 'La emission. Indole. Dual-tracked scans (not shown) over the largest solvent induced features (35050,35064,35081, and 35094 an-')indicate only extremely minor differences, probably due to underlying baseline components. The presence of an underlying component (single-exponential lifetime fit 12 f 3 ns) is evident from the inability to obtain good single exponential fits (Table I). We conclude, in agreement with Tubergen and Levy,13 that N site binding complexes do not show 'La emission. 5-MI.Since N site binding did not induce IL, emission in indole, it would be extremely surprising if it did in 5-MI. As was the case with water addition, there is a clear manifestation of methyl rotor transitions similar to those for the bare molecule (Figure 11). There is a trio of strong peaks at 34 149, 34 193, and 34229 an-'. The trio repeats more weakly at 34 165,34208, and 34245 cm-I. Nonetheless the dual tracked scans (Figure 11) do indicate the presence of lL, emission. However, comparison of the two scans suggests it arises from unresolved underlying structure, largely removed in the lower scan, rather than from significant 'La emission from the strong peaks. The underlying peaks could be due to n > 1 complexes. Alternatively, there may be weak van der Waals n = 1 binding of dioxane in a geometry that produces A interaction. Such complexes could produce a series of progression features beginning further to the red that do display 'La emission. 3-MI. Since we found indications of IL, emission in N-site water binding, we expect the same outcome with dioxane, a stronger proton acceptor. Dioxane addition induces a set of progression-like features beginning at 34 545 cm-' if not further to the red (Figure 12). Both their spacing and intensity envelope suggest a progression in one vibrational mode. Even when the progression features are relatively small compared to the bare molecule origin as in Figure 12 (about 15% of bare origin), a small baseline hump is evident in the top scan; this baseline component is virtually absent in the lower monochromator tracked scan. In addition, though, the size of the individual peak reductions indicates that they too must have a red emission component. This is consistent with the expectation based on water addition to 3-MI. As more dioxane is added, additional structure continues to grow in. It apparently begins to the red of the N site complex bands and extends to beyond the bare molecule origin. Dual tracked scans under these conditions show the new structure to be almost entirely eliminated in the monochromator tracked scans. Evidently a very potent A interaction is possible in these complexes. These A complexes presumably account for the baseline hump mentioned above. Weaker indications of similar A interactions were noted above for dioxane addition to indole and 5-MI.
The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 4775
Induction of 'L,-'Lb State Coupling I
"
~
'
"
"
"
'
I \
i
3-M1
INDOLE
+
DCM
+
-
.
:
:
:
:
,
-:
?
34520 34560 34600 EXCITATION FREQUENCY (cm-') F i p e 12. Dual LIF scans of 3-MI plus dioxane. The peak a t the blue end, the bare molecule origin, is truncated at 30% of its maximum in each plot and serves as the reference.
35250
35300
E X C I T A T I O N FREQUENCY (c m- '1 Figure 14. Dual LIF scans of indole plus DCM. The large peak at the red end serves as the reference. It is truncated to 15% of the maximum in each scan.
TABLE 111: Results of CNDO/S Calculations for Water Complexes transition INDOLE
+
indole derivative 5-methoxy
DMF
complex type
0-H a
5-methyl
0-H
unsubstituted
0-H
a a
1-methyl 3-methyl 1,2-dimethyl 2,3-dimethyl
a
0-H a a
0-H a
EXCITATION FREQUENCY (cm-') Figure 13. Conventional LIF scan of indole plus DMF. The peak at the blue end is the bare molecule origin.
other sohreata Dimethylformamide (DMF) was also reportedI3 not to induce 'La emission in indole. However, we did detect differences in dual tracked scans. An excitation spectrum is shown in Figure 13. The large feature at 34998 cm-' is followed by what looks like two progression members at 35 018 and 35 036 cm-I. It was practical to carry out a dual tracked scan over the two largest solvent peaks at 34998 and 35018 cm-I, and both showed a diminution of about one-third in the monochromator tracked scan. This reduction seems to be large enough that it cannot be attributed to weak underlying peaks. The peaks for the DMF complex clearly follow the red shift vs proton affinity correlation, which indicates that DMF is acting as a proton acceptor and binding at the N site. Therefore, in indole-DMF complexes we conclude that N site interactions can cause 'La emission. We also considered indole complexed with dichloromethane (DCM), a solvent that should be a proton donor. DCM is not expected to act as a proton acceptor and therefore should not bind at the N site. An excitation spectrum has already been reported by Hager and Wallace.23 A series of progression features with -20-cm-' spacing is evident to the blue of the bare molecule origin, beginning either slightly to the blue or red of the bare molecule origin and extending at least 160 cm-' to the blue.23 We have carried out a dual tracked LIF scan over the first four features (Figure 14) and observe a relative peak reduction of the complex features in the lower scan. The lifetimes (Table I) are close to those obtained for 'La emitting features induced in indole by formamide, acetamide, and water. The location of the peaks
frqi 'La 'Lb 37 290 32 022 32 203 31 267 37615 33 246 32 464 32015 37 692 33 974 32 466 32 328 32 047 32 222 36 657 33 838 31 831 32041 31 569 31 891 36 026 33 61 1 31 261 31 718
IL-IL~ gap, cm-I 5268 936 4369 449 3718 138 -175 2819 -210 -331 2415 -457
eabr
deg 86.1 66.0 83.9 12.8 79.5 27.8 12.0 63.4 26.0 12.2 64.9 15.9
suggests a A binding site similar to the one observed in NMI and 2,3-DMI. Computational Results. The results of the CNDO/S calculations for transitions to the 'La and 'Lb states of the bare molecules and water complexes are shown in Tables I1 and 111. As mentioned, the ordering of the energy gaps between the 'La and 'Lb states for the bare molecules studied in this work are the same as those obtained by Eftink et al." The angle between the 'La and 'Lb transition dipoles, dab, is seen to decrease upon water addition in most cases. The decrease is mostly due to a change in the direction of the 'Lb transition dipole direction, as that of 'La is largely unaffected. The effect on the transition moment directions is larger in the case of A bonding than 0-H bonding. In addition, the general features of the oscillator strengths and transition dipoles of the bare molecules are in agreement with the ones previously reported. This corroboration of the credibility of our Table I1 results (no solvent molecules) suggests that the trends in our Table I11 results (solvent interactions present) are worth considering. In both tables, however, the predicted red shifts and excited-state energy gaps are greatly exaggerated. The red shifts of transitions to 'Lb are predicted to be on the order of 1700 cm-'rather than the hundreds of wavenumber shifts which are observed. The 'L,-'Lb gaps likewise are predicted to be in the thousands rather than the hundreds which are observed. These errors are not surprising, given the level of the computational theory and the fact that the calculated values correspond to the absorption maxima and not the electronic origins. The trends of the numbers in Tables I1 and I11 are nonetheless clearly consistent with the assertion that the red shift of the 'Lb transition upon addition of a solvent molecule is accompanied by a larger red shift of the 'La transition. It is also evident from the calculations that addition of a water molecule to the A cloud of the pyrrole double bond results in a very much larger red shift
-
4776 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992
than does 0-H bonding. While the absolute values of the predicted 'La-'Lb gaps may be uniformly high, it is satisfying that the calculations nonetheless predict an inversion of the 'La-ILb state ordering upon a bonding of a water molecule to the molecules that have the smaller 'La-'Lb energy gaps. The results of our calculations agree with the experimental that 0-H bonding observations made by us and by Callis et of water to 3-MI causes 'La emission. The calculated 'La-ILb energy gap for an 0-H bonded 3-MI water complex is -300 an-' lower than the gap calculated for bare 2,3-DMI, which itself shows evidence of 'La e f f e c t ~ .The ~ gap calculated for the indole 0-H bonded water complex, on the other hand, is -550 cm-' higher than that of bare 2,3-DMI, seemingly in agreement with the observation that this water complex does not exhibit 'La emission. When the computational results are viewed in a general, relative sense, there is a strong correlation with the experimental observations. We believe that this computational agreement occurs through a cancellation of errors and is not merely fortuitous. Wetime Changes Due to 'LaEffects in Tryptophan Derivatives. Tryptophan in solution displays multiexponential fluorescence d e ~ a y . ~An~ explanation ,~~ put forward to explain this behavior is the conformer which hypothesizes that a distribution of conformers exists with different conformers displaying different lifetimes. One of the major factors presumed to cause the conformation dependent lifetime differences is variable fluorescence quenching arising from charge transfer to electron-accepting groups on the side chain such as the carbonyl.25 The conformer theory is potentially subject to direct study using supersonic gas expansions, since they produce "frozen" distributionsof conformers existing in Boltzmann equilibrium prior to expansion; each generally shows a separate 0 transition. As a result, lifetimes of separate conformers can be obtained for a variety of chosen derivatives; charge-transfer effects as a function of conformation should be observable and variable. If, however, 'L,-'Lb state mixing may potentially be induced for certain conformations, the conformers studied must first be carefully assessed for 'La effects. Our current results indicate that a consideration of conformation lifetimes should always be accompanied by dual tracked LIF scans, to identify possible 'La emission. Bare tryptophan illustrates these points. Seven conformers can be discerned via vibrationally resolved excitation spectroscopy, and all but one have lifetimes close to 14 ns.15,26The remaining conformer has a lifetime of around 11 ns.15-26At first sight, these results have an encouragingly straightforward correspondence to the conformer theory. One could suppose that the conformer with the shorter lifetime has a geometry which enhances charge transfer from the indole to the carbonyl. However, the conformer with an 11-ns lifetime is the reddest in the excitation spectrum and shows extensive progression structure; the emission shows a broad red-shifted component. Thus it seems that the lifetime difference may be due mainly to emission from a second electronic state. Tubergen et al.Ik identified, by using dual LIF scans,a number of tryptophan derivatives showing 'La character emission. We have measured lifetimes of conformer origins in these cases whenever practicable and present the results in Table I. [In NAT, included in the table, the large peak at 34 928 cm-' was earlier studied by observing its dispersed emission and was found not to show 'La state character.'& We were able to resolve smaller peaks and to measure one lifetime in addition to the lifetime of the large peak. These values are similar and do not suggest emission from two different electronic states.] For 'La emission in tryptophan derivatives, as in solvent complexes, there appears to be a general trend for the IL,-character emission to have shorter lifetimes than the ILb emissions. Induction of 'L,Emissionin Tryptopben Derivatives. If the polar side chain in tryptophan derivatives can in certain conformations bring 'La below 'Lb, it followed that solvent complexation with an indole might potentially produce the same effect and this turned out to be the case, as demonstrated here and by Tubergen and Levy.13 Similarly, polar solvent binding at the tryptophan derivative side chain, not the indole chromophore itself, might also conceivably produce this effect. Solvent complexes binding at the
Arnold and Sulkes side chain are distinguishable from binding at the chromophore because of much smaller spectral shifts for the former.14 Use of dual LIF scans can again indicate the character of the solvent complex emission. We in fact already carried out studies of this sort for addition of water, methanol, and chloroform to trypt~phan.'~ With water addition, some of the peaks gave clear indications of enhanced red emission while the other solvents showed no such effect. We now have also tried dual-tracked scans for water and acetone added to tryptamine; the solvent-induced peaks following polar side-chain addition were previously ~haracterized.'~In neither case did the solvent peaks show enhanced red emission. These generally negative results are not really surprising. Because the polar side chain is generally further removed from the chromophore in tryptophan derivatives, the effect of solvent interactions is diminished. Evidently, the conformation-imposed placement of the solvent adduct allows it to be effective in lowering 'La only in certain side chain conformations. Similarly, the differences between 'La emission in indole-solvent complexes and in tryptophan derivatives probably arise from geometric d i f f e r e w in polar group placement imposed by side-chain conformation. The interactions in the latter case are a-cloud type, but the "solvent" placement is not now determined solely by "solvent"chromophore interactions. The results of previous workI2 have established the observed trends we now set forth for 'La emission in derivatives of tryptophan and indole. Of the derivatives in which there is only one polar group, either amine or carboxylic, there is no instance in which 'La emission is evident. No 'La emission is evident in the spectra of the tryptophan methyl and ethyl esters. Strong progressions and red-shifted emission are present in the cases of tryptophan, tryptophan amide and tryptophan methyl amide. Tryptophan dimethyl amide, however, shows no signs of 'La emission. NATA exhibits 'La emission, and NAT may exhibit some 'La emission. Indole complexed with acetamide and formamide exhibits complex peaks which indicate both N and a site binding. The a-binding complexes exhibit strong 'La emission in both cases. The methyl and dimethyl derivatives of formamide and acetamide produce only N site binding complexes.
Discussion The first convincing evidence for 'La state effects in jet-cooled indole chromophoresg apparently involved a few fairly unusual instances. Subsequently, Levy's groupI2showed that in tryptophan derivatives instances of 'La emission were not overly unusual. They then showed the same for solvent complexes with ind01e.I~Our work goes beyond this in demonstrating that jet-cooled solvent complexes with most indoles more commonly show signs of 'La emission than not. 'La emission from jet-cooled indole chromophores has turned out to occur in a wide set of instances. Now it seems to be useful, insofar as it is possible, to rationalize the rather diverse range of 'Lastate attributions within a small set of generalizing rules. We have found that all cases of 'La emission reported prior to this paper fit in very well within these rules. A few of the new cases we report, all involving particular T interactions, do not conform to all the rules. I. 'Locharacter emission has the characteristic signature of being broad and red shifted, and 'Lolifetimes are generally not drastically different from the 'Lb lifetimes. The red shift is affected by Franck-Condon factors, arising from the displacement of 'La in some molecular coordinate, as well as the actual inversion of 'La and 'Lb. The spectral broadness is a result of the displacement of 'La. An overview is afforded by the potential energy diagrams presented by Demmer et al.IOand by Tubergen and (except in the latter the 'Lb state should be shifted to larger displacement following solvent addition). When broad red emission occurs, solvent or side-chain interactions with the chromophore have acted to bring 'La beneath ILb The 'La state is further displaced than 'Lb in some molecular coordinate, and curve crossing occurs in this coordinate. Unless 'La has been significantly lowered beneath 'Lb, vertical excitation will be to the 'Lb portion of a combined adiabatic curve, giving rise to
The Journal of Physical Chemistry. Vol. 96, NO. 12, 1992 4111
Induction of 'La-'Lb State Coupling excitation features that are structured. Subsequent emission takes place from the 'La portion of the surface. II. For a given indole, the extent of red shijt of a feature in the excitation spectrum is usually somewhat reflective of the extent to which the 'Lastate is lowered relative to 'Lb.This is why solvent complexes or conformers showing 'La character emission usually are the reddest peaks in the excitation spectra. This effect comes about because both 'Laand 'Lb have dipole moments, though the latter is smaller. As a result, dipolar solvent or side chain interactions will have an energy lowering effect on 'Lb which is smaller than the effect on 'La but which generally tracks with it. Some variation can occur when a solvent adduct is closer to the dipole of one state than the other. The amount of 'La state lowering needed for 'La character emission to be possible depends on the initial 'Lb-lLa energy gap, which varies for different indoles. The precision with which this rule applies will depend on the angle between the 'La and 'Lb dipoles (closest tracking effects for parallel dipoles) and on the assumption that dipole moments increase in the order ground < 'La < 'Lb. The rule can also potentially be blurred by breakdown of the dipoledipole approximation at short distances. In some instances of potent solvent-indole a interaction reported herein (NMI water, 1,2-DMI water, 5-MOI water, indole DCM) this rule may not be well obeyed. III. Strong progression features in the excitation scans will usually display 'La character emission. The presence of progressions can be related to a crossing of the 'La and 'Lb potential energy surfaces. For crossing to occur 'La must be brought at least somewhat below 'Lb. Second, and evidently equally important, 'Lb must be sufficiently displaced toward the minimum of IL, in the displaced coordinate; otherwise the curve crossing is minimal. As a result of this coordinate displacement in 'Lb, red-emitting features would tend to show progressions in the excitation spectra. The usefulness of the above picture is in providing some generalization as well as the ability to make simple predictions. For example, it is evident that a solvent that produces the requisite IL, lowering with, say, 3-MI, may not do so for 5-MI. Conversely, a solvent that cannot induce 'La character emission in 5-MI may do so for 3-MI. Generally, rule I1 is accompanied by rule 111, but rule I11 is not necessary if the lowering of 'La is sufficient. This seems to be the case when 2,3-DMI is complexed with polar solvents such as ammonia and TMA? In those cases the excitation features are broad and weak, which seems to suggest direct excitation to 'La. It seems possible at this point to put the earliest 'La evidence in a clearer perspective. In 1987 Hager et al? presented evidence suggesting the presence of 'La state effects in jet-cooled indoles. Spectral shifts and lifetimes for solvent complexes with indole, 2-MI, 3-MI, 5-MI, 7-MI, and 5-MOI were presented for the solvents methanol, ammonia, and TMA. In two cases the lifetimes were drastically shorter, and in each instance the spectral shift was also much greater than otherwise observed. These two complexes were for 3-methylindole with ammonia (A? = -440 cm-I, lifetime
