J. Phys. Chem. 1995,99, 11823-11829
11823
Fluorescence Spectra and Torsional Potential Energy Functions for 4-Methoxy-trans-stilbene in its SOand Sl(n@)Electronic States Whe-Yi Chiang and Jaan Laane* Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255 Received: January 26, 1995; In Final Form: May 22, 1995@
The fluorescence excitation spectra and dispersed fluorescence spectra of 4-methoxy-trans-stilbenehave been recorded and analyzed. Assignments were made for all eight of the low-frequency vibrational modes, excluding the methoxy and methyl torsions, for the SO ground state and for five of the low-frequency vibrations in the SI(n,n*) electronic excited state. Numerous transitions involving the two-phenyl torsions and the C=C torsion were observed. These were utilized to determine the two-dimensional vibrational energy surface for the phenyl torsions and to evaluate the one-dimensional ethylenic torsion for both the SOand SI states. For the phenyl torsions the barriers to simultaneous intemal rotation were calculated to be 2860 cm-' for both the SO and SI states, about 200 cm-' lower than the trans-stilbene values. The C-C torsional barrier for the SO state was estimated to be 52 f 3 kcal/mol based on its observed overtone frequency at 172 cm-I. For the excited state the C-C torsional overtone series can be observed beginning at 171 cm-I. Analysis of the potential function for this mode indicates that the barrier to trans-twist interconversion is approximately 245 cm-' higher than that for trans-stilbene.
TABLE 1: Vibrational Assignments (cm-') for the Low-Frequency Vibrations of 4-Methoxy-trans-stilbenefor Recently we presented our analysis of the fluorescence spectra the SOand SI(Z&) Electronic States of trans-stilbene.' This and related molecules have been of great trans-stilbene approximate 4-methoxy-trans-stilbene interest due to their photochemical properties (refer to ref 1 for description C, S O SI C2h so SI many of the relevant references). Zewail and c o - w o r k e r ~ ~ . ~ A, v24 273 280 C,-Phbend A' v54 234" 279 recently examined the dynamics of trans-stilbene, 4-methoxyphenyl wag (i.p.) v55 144" V25 152 (150)b trans-stilbene, and 4,4'-dimethoxy-trans-stilbeneand determined phenyl wag (i.p.) ~ 5 6 68 B, ~ 7 2 76 (70)b barriers for the trans-to-twist intemal rotation in the S I(n,n*) phenyl flap A" V ~ O 179 B, ~ 4 7 211 (200)b electronic excited state. The barriers were found to be about phenyl torsion Val 123 109 V48 118 110 C=C torsion V ~ Z 86 85.5 A, ~ 3 5 101 99 1200, 1300, and 3000 cm-I, respectively. In our previous work V36 58 47.5 phenyl flap ~ 8 3 59 46 on trans-stilbene we identified the vibronic transitions for the phenyl torsion ~ 8 4 8 28 v37 9 35 C=C torsion up to the 12th quantum state in the SI state. This a From the vapor-phase Raman spectrum. Estimated. data could be best fit with a trans twist barrier in the 2000 to 3500 cm-I, although a barrier as low as 1400 cm-' was 111. Low-Frequency Modes compatible with the data. In the ground state the trans-cis barrier is 48.3 kcavmol. We also analyzed the phenyl torsion The substitution of the methoxy group to one of the phenyl for trans-stilbene in both the SO and SIstates and determined rings reduces the symmetry of the trans-stilbene from C2h to the two-dimensional torsional potential energy surfaces for both C, (or CI depending on the orientation of the methoxy group). electronic states. The barriers to simultaneous intemal rotation In C, symmetry MS has 56A' +28A" vibrations, and the were calculated to be 3100 and 3000 cm-' for the SO and S I Sl(n,n*)electronic state is A'. The low-frequency modes of electronic states, respectively. MS, however, are expected to be very similar to those of transIn the present work we extend our investigationto 4-methoxystilbene, and the selection rules will also be affected only a little. trans-stilbene (MS) so that we can make useful comparisons Thus, the forbidden transitions for trans-stilbene should still with trans-stilbene. show little if any intensity for MS. In Figure 1 of ref 1 we presented the approximate descriptions of the eight low11. Experimental Section frequency (below 300 cm-') modes of trans-stilbene. In addition to these, MS possesses two low-frequency torsional The experimental apparatus used was the same as that vibrations (of the methoxy and methyl groups). However, as described previously.' The DCM dye was utilized to record will be seen, these were not observed in any of the fluorescence the excitation spectra in the 312-325 nm region. The laser excitation or dispersed fluorescence spectra. output was focused onto the expanding supersonic jet apTable 1 presents our assignments for the observed lowproximately 8 mm downstream from the nozzle. Helium served frequency vibrations for MS in its ground and excited states as the carrier gas, typically with a backing pressure of 3 atm, and compares these to trans-stilbene. The vibrations for MS and the sanipling system was maintained at 160". A Hoya are numbered based on C, symmetry. The assignments are Optics L38 glass filter was used to eliminate scattering at and based on the fluorescence spectra and the potential energy near the excitation frequencies. calculations to be discussed later. In addition, the vapor-phase 4-Methoxy-trans-stilbene was obtained from Aldrich ChemiRaman spectra4s5of MS and the assignments for trans-stilbene' cal Co. and used without further purification. were most helpful. Below 300 cm-' the Raman spectrum of MS shows two strong, polarized bands at 234 and 144 cm-' @Abstract published in Advance ACS Abstracts, July 1, 1995. I. Introduction
~
-
~~
0022-3654/95/2099-11823$09.00/0 0 1995 American Chemical Society
Chiang and Laane
11824 J. Phys. Chem., Vol. 99, No. 31, I995 400
300
200
100
0 I
I
f
m
T
0
I
I
31200
31100 31000 30900 Wavenumber (cm")
30800
Figure 1. Fluorescence excitation spectrum of 4-methoxy-transstilbene for the lower frequency region.
and these are assigned to v54 and v55, the in-plane bending and wagging involving the phenyl rings. These values have decreased from the 273 and 152 cm-' frequencies of transstilbene, and this trend is also evident for most of the other vibrations. The other phenyl wagging vibration, v56, was observed at 68 cm-' in the single vibrational level fluorescence (SVLF) spectrum of the 38 cm-' excitation band (a hot band). , at 123 and 8 cm-I in The two phenyl torsions, vgl and v ~ 4are the SOstate and at 109 and 28 cm-' in the SIstate. Table 1 shows these frequencies to be similar to those of trans-stilbene, as expected, since the para-substitution should not have a dramatic effect on the phenyl torsions. The C - C torsion, to be discussed below, is assigned to 86 cm-' for both SO and SI states.
IV. Fluorescence Excitation and Dispersed Fluorescence Spectra Figure 1 shows the Sl(n,n*) fluorescence excitation spectra (FES)of MS for the lowest 460 cm-I. More detail is available el~ewhere.~ Table 2 lists the assignments for the first 500 cm-I; assignments of the higher frequencies are of less interest and are also less certain. The dispersed (SVLF) spectra of the ,:O 38 cm-I, and 92 cm-' bands are shown in Figure 2,and the assignments are listed in Table 3. As can be seen in Table 2, all of the assignments for the SOstate result from combinations , V M . The phenyl torsions are each of v54, V S I , VQ, v ~ 3 and assumed to be doubly degenerate as they are for trans-stilbene. Comparison with the trans-stilbene FES spectrum shows that the double-quantum jump of the C=C intemal rotation occurs at 171 cm-I band vs a value of 198 cm-' for trans-stilbene. The 171,346, 524, and 704 cm-' bands were assigned as the series of C=C torsional overtone frequencies (82:, 82:, 82:, and 82:). In the 0; SVLF spectrum, the 172, 349, and 538 cm-I bands were assigned to 82:, 82:, and 82:. According to the Franck-Condon principle, it is unusual to have a progression of appreciable length in a nontotally symmetric vibration when there is no change of point group between electronic states, but this does occur in the stilbenes as totally-symmetric doublequantum jumps. The assignment of the CSC torsional progression as double quantum jumps for both trans-stilbene and MS is supported by the fact that the phenyl torsional vibrations (v37 for trans-stilbene and vw for MS) of the same symmetry species also clearly give rise to double-quantum jumps in both FES and SVLF spectra.
0
100 200 300 400 500 600 700
Wavenumber (cm-')
Figure 2. The SVLF spectrum from the ,:O the 38 cm-I hot band, and the 92 cm-l excitation band of 4-methoxy-trans-stilbene.
V. Calculations A. Kinetic Energy Functions. The kinetic energy operator for the phenyl rotors in two dimensions was defined previously.' It is assumed that the form of the kinetic energy operator and that of the potential energy function for trans-stilbene are still applicable to MS even though the two phenyl groups are no longer identical in MS. Only the single term of the kinetic energy function at the equilibrium structure was utilized in the following potential energy calculations, since the exact orientation of the methoxy group during the intemal rotation of the phenyls is not known. The intemal rotor constants for the C=C torsion of MS were determined to be 0.128and 0.132cm-' for the SOand SIstates, respectively. These values are smaller than the 0.2034and 0.203 cm-' values for trans-stilbene because of the added mass of the methoxy group. B. Potential Energy Function for the Phenyl Torsions. The two-dimensional potential energy functions for the phenyl torsions of MS were assumed to have the same form used for trans-stilbene
(6:)
where 41 and & are the torsional angles for the phenyl groups and VZ,VI,, and VI{ are the potential energy constants. The previously described computer program was used to calculate the energy levels.' The potential energy parameters, for the two-dimensional surface for the phenyl rotors in the SO state, were determined to be VZ = 1430 cm-I, VIZ= 195 cm-I, and VI*' = 515 cm-I. For the SIstate, they are V2 = 1430 cm-I, VIZ= 17.5 cm-l, and VIZ' = 157.5 cm-'. Table 4 presents the observed data used to determine these functions along with the frequencies calculated from the above potential surfaces. As can be seen, the frequency fit is quite good and within experimental error. Unlike the trans-stilbene spectra, the combination bands of the two phenyl torsions were not observed for the SO state. The combination bands can be helpful for determining the interaction between the two phenyl torsions. Furthermore, the observed frequencies in the SIstate are nearly harmonic. These two factors result in more uncertainty in the determination of the potential parameters, but the potential
4-Methoxy-trans-stilbene in SO and SI States
J. Phys. Chem., Vol. 99, No. 31, 1995 11825
TABLE 2: Assignments for the SI(Z@) Fluorescence Excitation Frequencies (cm-') Relative to the Band Origin at 30 768 cm-l of 4-Methoxy-trans-stilbene obsd calcd" re1 intens assignt obsd calcd" re1 intens assignt -18 -18 3 292 293 12 84: 0 0 100 296 11 296 0: 20 20 7 299 299 8 84: 38 38 16 302 301 18 83A84: 22 56 56 306 307 15 84: 67 66 46 313 313 32 84: 74 74 24 318 317 19 83i84: 92 58 320 321 28 (92) 83: 104 104 6 327 327 10 84; 109 8 330 329 19 (109) 81: 112 112 6 336 54 335 84: 122 7 122 338 338 57 83h84: 128 129 7 340 34 1 37 81i84: 130 130 346 15 346 88 83A84: 134 134 352 13 353 54 84p 11 148 147 357 32 356 81i84: 150 360 11 359 83A84: 72 158 365 366 158 13 83:84:,83;84: 373 17 373 163 162 49 82:84: 165 374 165 27 8 1:84: 377 171 71 378 23 (171) 82: 7 16 384 179 385 180 83A84F 388 30 389 22 184 184 83; 55 399 192 8 398 193 81:84:,81i83;84: 40 1 40 35 1 196 8 196 83i84: 402 201 14 201 81:83: 408 17 408 40 210 209 82:83:84: 12 409 216 216 83i84: 41 1 217 41 1 37 82i83: 413 22 1 414 36 222 13 8 1:84; 419 222 418 67 83:84: 427 42 27 229 426 229 81:83:84: 432 232 432 47 17 232 83:84; 437 237 437 38 32 236 82:84: 440 440 29 37 240 239 83:84: 18 444 247 443 54 246 82i84: 14 445 255 445 32 254 82:84: 257 450 53 17 257 450 81:83:84: 144 458 47 458 262 263 81:82;84: 30 12 463 268 464 267 83i84: 27 1 17 270 464 54:84: 127 470 470 24 279 54: (279) 42 476 23 285 286 475 81:83:84: Based on observed values indicated by parentheses. energy surface nonetheless is accurate at lower energies where the spectroscopic data are available. Figure 3 shows the potential energy surface for the SO state, and Figure 4 shows slices of the same surface taken along the diagonals (representing the SIand S2 coordinates defined in eqs 1 and 2 of ref 1) while Figures 5 and 6 present the surface for the SIstate and slices of the surface, respectively. The barrier to simultaneous phenyl torsion is 2860 cm-' in either the SOor S I state, while the barrier to internal rotation of a single phenyl group is 1040 cm-I in the SO state and 1395 cm-' in the SI state. The YE4 out-ofphase phenyl torsion has a flatter potential function than the
YE^ in-phase phenyl torsion. This was also found to be the case for trans-stilbene. C. Potential Energy Functions for the C=C Torsion. SO State. The previously described potential energy function'
v(e) = V2v1(i- COS e)
+ l12v2(1- 213)+ + l12v8(i -
V2v4(i - COS 4e)
COS
COS
8e) (2)
where 0 is the torsion angle for the C=C bond, was also used to characterize this vibration for MS. This allows us to study the effect of the substituent group on the translcis isomerization.
11826 J. Phys. Chem., Vol. 99, No. 31, I995
Chiang and Laane
TABLE 3: Assignments for the SODispersed Fluorescence Frequencies (cm-I) of 4-Methoxy-trans-stilbene excitation lines excitation lines SI
+
Ocm-'
38cm-'
92cm-l
assignt
0
0
SI
+
Ocm-I 248
18
259
42
266 278
56 67
assignt
264
30 42
92cm-I
253
8
18
38cm-I
69
280 289
68 73
293 296
85
100
306
113
317
123 127 168 172
330 349 35 1 361 370
174 179
375 186
410 416
187 422
194 198 205
203 208 212 218
428 493 517 538 590
224 232 244
664 709
TABLE 4: Observed and Calculated Frequencies (cm-') for the Phenyl Torsions of 4-Methoxy-trans-stilbene spacing So SI (O,o)-(Y84.2181) obsd calcd" obsd calcdb 8 8 28 28 18 18 56 56 30 30 84 84 42 42 112 112 56 56 142 141 71 73 85 86 102 123 109 109 123 136 129 164 138 161 a V in eq 1 with V2 = 1430 cm-I, Vl2 = 195 cm-I, and VI2' = 515 cm-I; FII= 1.06 cm-] and F12 = 0.87 cm-I. * V in eq 1 with V2 = 1430 cm-', V12 = 17.5 cm-l, and VI; = 157.5 cm-l; FI1= 0.99 cm-' and F12 = 0.80 cm-I.
Figure 3. Two-dimensional potential energy surface for the phenyl torsions of the SO ground state of 4-methoxy-tmns-stilbene.
A series of bands observed at 172, 349, 538, and possibly at 740 cm-' were assigned to the C=C torsional bands in the SO state, as shown in Table 5. Even though there are not as many observed frequencies for MS as there were for trans-stilbene, we were able to make a reasonable comparison of the MS function with that of trans-stilbene. We first used the potential energy parameters (VI = 1605 cm-I, V2 = 16 089 cm-I, and V4 = -900 cm-I) previously calculated for trans-stilbene along with the internal rotor constant calculated for MS. This resulted
in a calculated frequency of 81.2 cm-I, somewhat lower than the observed value of 86 cm-I. This indicates that MS has a higher banier for the translcis isomerization than trans-stilbene in the SOstate. By use of only the V2 value in eq 2 to fit the one observed frequency, a value of V2 = 14500 cm-' was calculated. The same procedure for trans-stilbene gave a value of V2 = 13 303 cm-I, about 9% lower. As discussed previously,' the actual barrier for trans-stilbene is 48.3 kcal/mol (16 900 cm-I). If the MS barrier to isomerization is assumed
J. Phys. Chem., Vol. 99, No. 31, 1995 11827
4-Methoxy-trans-stilbene in SOand SI States
TABLE 5: Observed and Calculated Frequencies (cm-') for the C-C Torsion of 4-Methoxy-Srans-stilbene
3000
so spacing 0-vsz 7- 2000
0- 1
E
0-2 0-4 0-6
s
>r
P
15
obsd
0-8
lo00
172 349 538 740?
SI
calcd" 86 172 349 538 740
obsd
calcdb
171 346 524 704
85 171 346 524 104
a SO: b2 = 14 634 cm-I, b4 = -63 479 cm-I, and bg = 3 267 880 cm-l in eq 3, F(cm-I) = 0.128cm-'. S I : bz = 13 245 cm-', bq = 47 742 cm-I, and bg = -287 327 cm-l in eq 3;F(cm-') = 0.132 cm-I.
0 -90
0
90
180
Degree
Figure 4. Potential energy of 4-methoxy-rruns-stilbeneas a function of the (41 &)/2 and (4, - &)/2 symmetry coordinates for the SO
+
state. 3aoo
-
1500
'E,
d
r
e $
0
equation and obtain the calculated frequencies. Three potential energy parameters (h, b4, and b6) were used, and these are given in Table 5 along with the observed and calculated frequencies. For our limited number of observed frequencies, the extrapolation of the potential energy function to higher values is not very reliable. From the observed data alone and these calculations, it could at best be concluded that the C-C torsional banier of MS has a value of 52 f 20 kcdmol. However, the data clearly indicate that this barrier is somewhat (about 9%) higher than the known value of 48.3 kcdmol of trans-stilbene. Thus, the uncertainty limits may be considered to be much smaller (perhaps f 3 kcaumol). SI Stare. For the C% torsion in the SIstate, the observed frequencies are at 171, 346, 524, and 704 cm-I. Here again, the potential energy function of trans-stilbene was adapted to MS. First, the potential parameters for trans-stilbene (VI = lo00 cm-', V2 = -5411.5 cm-I, V4 = 5411.5 cm-I, and Vg = -262 cm-l in eq 1 for the SI state) together with the C=C internal rotor constant (0.132 cm-') calculated for MS were used to calculate the 0 1 frequency. This was found to be 80 cm-', while the observed value is 85.5 cm-' (half of 171 cm-I). Then a least squares fit was carried out for the coefficients b2, b4, and b6 to obtain the calculated frequencies. The results of the calculation are summarized in Table 5 . The correspondingpotential energy curve has a banier of 2269 cm-', which is 245 cm-' higher than the 2024 cm-' value for transstilbene (curve a in Figure 13 of ref 1). The dynamics study by Baiiares, Heikal and Zewai13 reported the barrier of MS to be 140 cm-' higher than that of trans-stilbene, in reasonable agreement with the results here since the uncertainties in the determination of the barriers here are at least several hundreds of cm-I. As with trans-stilbene, where we found that barriers in the 1400 to 3500 cm-I range could give rise to the observed spectra, the absolute value of the barrier for MS is difficult to determine precisely. However, our results do indicate quite clearly that the MS barrier is about 10% higher than that for trans-stilbene. It should be emphasized that the vibrational models assumed for trans-stilbene and MS ignore vibrational coupling between the low-frequency modes and with higher frequency modes. Hence, the potential energy surfaces are "effective" surfaces rather than pure ones in terms of the assumed coordinates.
-
Figure 5. Two-dimensional potential energy surface for the phenyl torsions of the S I ground state of 4-methoxy-trans-stilbene. I
I
3000
0 -160
-90
0
90
180
Degree Figure 6. Potential energy of 4-methoxy-truns-stilbeneas a function of (41 4412 and (91 - 42)/2symmetry coordinates for the SI state.
+
to be 9% higher than that for trans-stilbene, as indicated by this calculation, it would have a value of 52.6 kcaYmol. A calculation with V2 = 18 396 cm-I (52.6 kcdmol) and V4 = -950 cm-I reproduces the 86 cm-' frequency for the 0 1 transition. We also utilized the Taylor's series expansion'
-.
V(6)= b202-k b484 -k b6@
+ ...
6 < 1 radian
(3)
together with the observed frequencies to solve the Schrodinger
VI. Conclusion The fluorescenceexcitation and dispersed fluorescence spectra of 4-methoxy-trans-stilbene have provided insight into the torsional potential energy functions for this molecule and allowed them to be compared to those of trans-stilbene. For MS the barrier to simultaneous phenyl torsion was calculated to be 2860 cm-' for both SOand Sl(n,n*)states as compared to values of 3100 and 3000 cm-', respectively, for tram-stilbene. The single phenyl torsional barriers for MS were determined
Chiang and Laane
11828 J. Phys. Chem., Vol. 99, No. 31, 1995
to be 1040 and 1395 cm-] for the SOand SIstates. For transstilbene the corresponding values are 825 and 1670 cm-'. For both molecules the internal rotation of a single phenyl group is considerably easier in the ground state than in the excited state. The observed transitions for the C=C torsion indicate small increases in the barriers for MS in both the SO and SIstates as compared to trans-stilbene. The SO barrier increases from approximately 49 to 52 kcallmol, while in the SI state an increase of about 250 cm-l can be estimated for the trans-twist barrier. These results are consistent with the small increase observed in the barrier from the dynamics study of Zewail and c o - ~ o r k e r swho , ~ present explanations for their findings in terms of relative conformational stabilization of the trans and twisted structures by the methoxy substitution. It should be clear that our analyses of the 4-methoxy-transstilbene spectra are based to a large extent on the assignments and conclusions derived from our previous work on transstilbene.' In that work we presented detailed arguments for the revised assignments of the eight low-frequency vibrational modes and compared these to previous studies. These arguments will not be repeated here, but since our conclusions differ considerably from those of previous workers, we will add some perspective to our previous discussion, particularly for the analyses of the three torsional modes. We are totally confident of the assignments for the out-ofphase phenyl torsions for the SO states. These are observed at 9 and 8 cm-' for trans-stilbene and MS, respectively. This assignment is in accord with that of previous worke;s.6-8 The potential energy surfaces nicely fit the observed data and are well determined at lower energies. The energy barriers, however, which are obtained by extrapolation of the surfaces to higher energies, may have uncertainties of several hundred cm-I. For the SIstate we assign the out-of-phase phenyl torsion to 35 and 28 cm-' for trans-stilbene and MS, respectively, while the phenyl flap is ascribed to higher values at 47.5 and 46 cm-I, respectively. These assignments provide the best fit with our model of the potential energy surface. If the reversed assignment were used, torsional barriers which we feel to be unrealistically high would be calculated. Our assignments for the in-phase phenyl torsions at 118 and 110 cm-I for the SO and SI states, respectively, for trans-stilbene agree with those of Ito and co-w~rkers.~ The MS assignments at 123 and 109 cm-' are similar. The vapor-phase Raman spectra4 confirm the ground state assignments. Spanglefi~~ argues that this mode (B, for trans-stilbene) should be forbidden in the fluorescence spectra. However, as we have demonstrated, each of these levels is 4-fold degenerate (with A,, A,,, B,, and B,, symmetry species) so that conventional symmetry analysis need not be applicable. Perhaps the most difficult, and most important, feature of our analyses of the trans-stilbene and MS spectra is associated with the assignment of the ethylenic torsion which governs the trans cis isomerization. Previous workers (Tables 1 and 2, ref 1) have estimated this to be in the 229 to 306 cm-] range for the SO and SI states of trans-stilbene. However, our calculation of the kinetic energy expression' demonstrates that this value for either state should be about 115 30 cm-I. In order to assist us with the assignment, we recorded the hightemperature vapor-phase Raman ~ p e c t r aof ~ ?both ~ trans-stilbene and MS. trans-Stilbene has two (and only two) strong polarized Raman bands below 300 cm-' at 273 and 152 cm-I. Since there are only two expected A, modes below 300 cm-', the two observed bands are logically assigned to these. We have been concerned about the possibility that these bands could arise
-
from thermal decomposition, but we have found no evidence for this. Previous worker^^^^^^ have assigned v24 (C,-Ph bend) to 292 cm-' from dispersed fluorescence studies, but that band appears to be 273 19 cm-I, where the 19 cm-' is the double energy jump for the out-of-phase phenyl torsion. The other A, mode is ~ 2 (in-phase 5 phenyl wag) which has generally been assigned to the dispersed fluorescence band at 202 cm-'. However, the 152 cm-I Raman band is the most intense band in the spectrum below 1000 cm-' and certainly appears to be a fundamental. It could be assigned as 2v72 (out-of-phase phenyl wag; 2 x 76 cm-I), but it appears to be much too intense to be an overtone. In MS, the intense band in the Raman spectrum is at 144 cm-I, and this frequency does not match 2v56 (outof-phase phenyl wag; 2 x 68 cm-') very well. The Raman spectrum of trans-stilbene also shows weaker depolarized bands at 21 1 and 118 cm-' which were assigned to the B, modes, v47 (phenyl flap) and v48 (phenyl in-phase torsion). A very weak, probably polarized, Raman band appears at 202 cm-I, and this apparently corresponds to the observed dispersed fluorescence band. Thus, the Raman spectra strongly indicate that the A, modes are at 273 and 152 cm-I, and the 202 cm-' level must be something else, likely an overtone (based on the weakness and polarization of the band). The only vibration below 300 cm-' which was not clearly assigned from the Raman and dispersed fluorescence data then was the v35 C=C torsion expected near 115 cm-I. Since the 202 cm-' band appears to be the overtone of a 101 cm-' band, this then logically must be 2~35. Since 2v35 has A, symmetry, this can be expected to give rise to reasonable fluorescence intensity in transition to the B, electronic state. In fact, at 19 cm-' (and with higher double energy level jumps) gives rise to a strong series of bands. Since v35 and ~ 3 are 7 both of A, symmetry, the behavior of v35 can be expected to be similar. For the SI state, the 198 cm-' band then also corresponds to 2%. This all appears clear-cut, but there are two concerns. First, the 202 and 198 cm-' bands each have relatively harmonic overtones for the SO and SI states. Second, the similarity of the frequencies suggests they are both due to the same mode which is not significantly altered in the excited state. However, the v35 torsional potential is quite different for the SO and SI states. Furthermore, Spangler also argues that the Sielbrand theory'O would not allow substantial intensity for the 2v35 since the SO and SIfrequencies are similar, since the vibrations are relatively harmonic, and since there is "no displacement in any out-of-planemode". This appears to be a valid argument except that we have clearly observed a similar case where this does not apply. For cyclopentanone' the SO ring-twisting and ringbending modes occur at 236 and 95 cm-', respectively, whereas they are at 238 and 91 cm-' in the SIstate. These frequencies are clearly similar for both states, and both modes are quite harmonic in both states. Nonetheless, significant intensity was observed for both modes in the excitation spectra. Thus, violations of the Sielbrand theory may not be uncommon. These may be due to mixing of the different vibrational modes and to the anharmonicity and large amplitudes of seemingly harmonic vibrations. In a separate study, Frederick and c o - ~ o r k e r s ' ~ ~ ' ~ have found that vibrations which appear to be nearly harmonic in both the ground and excited electronic states may actually have very different (and actually anharmonic) potential functions for the two states. In summary, we feel the arguments presented above logically lead to our conclusions. We hope that future experiments will help to resolve some of the apparent conflicts in the interpretation.
+
4-Methoxy-trans-stilbene in SOand S I States Acknowledgment. The authors acknowledge financial support from the National Science Foundation, the Robert A. Welch Foundation, and the Texas ARP program. We also acknowledge discussions with Professor Ahmed Zewail and thank him for bringing this problem to our attention. References and Notes (1) Chiang, W.-Y.; Laane, J. J . Chem. Phys. 1994, 100, 8755, and references therein. (2) Syage, J. A.; Felker, P. M.; Zewail, A. H. J . Chem. Phys. 1984, 81, 4685; J. Chem. Phys. 1984, 81, 4706. (3) Baliares, L.; Heikal, A. A.; Zewail, A. H. J . Phys. Chem. 1992, 96, 4127.
J. Phys. Chem., Vol. 99, No. 31, 1995 11829 (4) Haller, K.; Chiang, W. Y.;Laane, J. To be published. (5) Chiang, W.-Y. Ph.D. Thesis, Texas A&M University, College Station, TX, 1994. (6) Spangler, L. H.; Zee, R.; Zwier, T. S. J . Phys. Chem. 1987, 91, 2782. (7) Suzuki, T.; Mikami, N.; Ito, M. J . Phys. Chem. 1989, 93, 5124. (8) Urano, T.; Maegawa, M.; Yamanouchi, K.; Tsuchiya, S. J . Phys. Chem. 1989, 89, 3459. (9) Spangler, L. H. Private communication. (10) Sielbrand, W. J . Chem. Phys. 1967, 46, 440. (11) Zhang, J.; Chiang,W.-Y.; Laane, J. J . Chem. Phys. 1995,98,61296137. (12) Frederick, J. H.; Fujiwara, Y.; Penn, J. H.; Yoshihara, K.; Petek, Hrvoje. J . Phys. Chem. 1991, 95, 2845. (13) Frederick, J. H. Private communication. JP950275E
