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The 2 1A1 state of cis-hexatriene vapor: Resonance Raman spectra
Volume 202, number 5
CHEMICAL PHYSICS LETTERS
29 January 1993
The 2 ‘Al state of cis-hexatriene vapor: resonance Raman spectra Curtis Westerfield and Anne B. Myers Departmentof Chemistry, UniversityofRochester, Rochester, NY 14627-0216,USA Received 8 October 1992;in final form I 1 November 1992
Raman spectra have been obtained on resonance with the 2 ‘A, state of room temperature cis-1,3,5-hexatriene vapor at 290.8 nm. Overtones and combination bands of totally symmetric stretching and bending modes, both alone and in combination with an interval probably assignable as the overtone of the lowest frequency skeletal bending mode of b, symmetry, are strongly en-
hanced. Vibrational intervals tentatively assignedto the a2and b2methylenetorsions are also observed,consistent witha large
changein the force constant for =CH2twisting in the 2 ‘A, state.
The linear polyenes are unusual among organic molecules in having a lowest-lying singlet excited state (having Agor A, symmetry in polyenes of CZb or CZysymmetry, respectively) that is best described in terms of doubly rather than singly excited configurations [ l-31. While the state ordering 2 ‘A< 1 ‘B is now generally thought to hold for all unsubstituted and diphenyl polyenes, spectroscopic identification of the nominally one-photon forbidden A state has been extremely difficult in butadiene and hexatriene due to the virtual absence in these molecules of the vibronically induced fluorescence observed in the longer polyenes. Understanding the energetics and structure of the A states is important because of their role in polyene photochemistry [4,5] and the nonlinear optical response of polyenes and conjugated polymers [ 6-91. In addition, the lowest singlet excited A states of polyenes present a formidable test for the results of electronic structure calculations [ 3, IO,111. It has been suggested that the absence of fluorescence from dienes and trienes results from rapid twisting about a terminal double bond in the A state [ 12,131, but there is as yet little direct experimental evidence to support this. The apparent origin of the 2 IA, state in cis-hexatriene cooled in a jet expansion has recently been detected both by onephoton resonant multiphoton ionization [ 13,141 and by fluorescence excitation [ 151. In both spectra vibronic features were also resolved, but the lack of vibrational assignments for the low-frequency modes
in particular limits the structural information that can be derived from these spectra. Resonance Raman spectroscopy should be able to provide more useful structural data about the excited A state of hexatriene because the ground state vibrations are well characterized [ 16,171. Quantitative analysis of the Raman spectra on resonance with the strongly allowed B state has led to detailed conclusions about the structure and dynamics of this short-lived state in both vapor and solution phases [ 18-2 11. Even with excitation near the A-state origin, the resonance-enhanced scattering from the A state is much weaker than preresonant scattering from the B state in the fundamental region, but A-stateenhanced scattering is clearly discernible in the overtone and combination band region. Experimental conditions were identical to those in ref. [ 201 except for a few minor changes. Because of the weakness of the Raman scattering and the nearly negligible absorption at the excitation wavelengths employed, higher pressures of cis-hexatriene (x 5 Torr) and higher laser powers ( z 150 p/pulse) were used. Gas chromatographic analysis of the sample after z 5 h of irradiation showed that cis-trans isomerization of the sample was less than 5% and no other impurities were observed. Finally, because the 2 ‘A,+ 1 ‘A, origin is lower in energy than the 1 ‘B, + 1 ‘A, origin, the frequency doubled output of the dye laser could be used directly without Raman shifting.
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Fig. 1 shows spectra of cis-hexatriene in the overtone and combination band region as the laser is tuned through the reported position of the 2 ‘A, state origin band at 290.8 nm. It is clear that a number of overtones and combination bands are strongly enhanced (relative to the fundamentals such as z$, vg and v,) by resonance with the 2 ‘A, state. Fig. 2 shows a more complete spectrum obtained near the peak of the A-state excitation profile at higher signal-
291.17
nm
> : P : :
1360
1740 Raman
2100 Shift
2460
2.320
IcK?)
Fig. 1. Resonance Raman spectra of cis-hexatriene vapor at the three indicated excitation wavelengths.The spectra are scaled such that the us fundamental at 1627 cm- ’is the same height in ah three spectra. The strongest combination band at 2332 cm-’ in the 290.83 nm spectrum has an integrated area = 20 times lower than that oft+.
1470
2110 Raman
2750 Shift
3390
4030
km-1
Fig 2. Resonance Raman spectrum of cis-hexatriene vapor at 290.83 nm excitation. The strongest band in the overtone and combination band region, the vI overtone at 3254 cm-‘, has an integrated area z I I times lower than that of the + fundamental at 1627 cm-‘. The letters are for reference to table 1.
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to-noise ratio. Tram-hexatriene also exhibits preresonance scattering throughout this region, but no new bands are enhanced when the excitation is tuned near 290.8 nm, confirming the assignment of the features in the fluorescence excitation spectrum of Petek et al. [ 151 to the cis isomer. Attempts to obtain A-stateenhanced spectra on resonance with the reported cishexatriene vibronic feature c 70 cm-’ above the origin were unsuccessful; that is, no clear enhancement above the B-state preresonant background was observed. This result is qualitatively consistent with the relative weakness of this feature in the free-jet fluorescence excitation spectrum ( < 8% relative to the integrated area of the “origin” doublet) [ 151. Table 1 summarizes our tentative assignments of the main bands between 1300 and 4200 cm-‘. The observed bands can all be assigned consistently, although not uniquely, using eight vibrational intervals: the five a, modes v5 and vg (C=C stretches), vg (CH rock), v9 (CH rock and C-C stretch), and v,, (CCC bend); an interval of 703 cm- ’assignable as two quanta of v3,, the lowest frequency b, CCC bend, or possibly as one quantum of the a, methylene torsion, v, 7;two quanta of vj5, the methylene torsion of b2 symmetry; and an interval of approximately 1400 cm-‘, which might reasonably be assigned as either one quantum of the al mode v7 (CHI scissors) or two quanta of v,,. The assignment of the 1400 cm-’ interval as v7 gives very good agreement with the observed frequencies, but we consider it unlikely that a methylene scissors mode would have this much intensity (it is not a very strong band on resonance with the 1 ‘B, state). The liquid phase frequency of v,~ appears slightly too high ( 707 cm-‘) [ 161, but the vapor phase frequency has not been reported, and activity in this mode would be consistent with the apparent contribution of the corresponding bz torsion to several weak bands in our A-state resonant spectra. The interpretation of our spectra is strongly dependent on the assignment of the 703 cm-’ interval. In our previous resonance Raman study of cishexatriene vapor on resonance with the 1 ‘B, state [ 201, we attributed a very weak feature at 7 14 cm- l to the fundamental of vr7, while noting that an alternative assignment as 2 us, was also possible. Gasplane electron diffraction studies indicate that the ground state of cis-hexatriene is slightly distorted
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29 January 1993
Table 1 Cis-hexatriene
overtone and combination
band transitions enhanced by 2 ‘A, state resonance
Expt. freq. (cm-‘) I)
Assignment ‘)
talc. freq. C) (cm-‘)
1308 (A) 1388 (B)
“0 h U6
1318 1397 1570 1627 1952 2021 2021 2100 2185 2273 2330 2415 2498 2567 2646 2724 2715 2803 2794 2876 2945 3024 3109 3201 3197 3254 3339 3418 3506 3582 3648 3648 3727
1570 1627 1950 2017
(C) (D) (E) (F)
2108 2182 2274 2330 2412 2497 2561 2652 2719
(G) (H) (I) (J) (K) (L) (M) (N) (0)
“5
b+ (2~~~ or u17)
us+PI2 hd(2Gorv17) (2~17orY7)+(2~~,or~17) W170rv7)+2v12 v6+(2hIOrh7) v3+12v3,0rh7) h+h2 24 vs+v9 Ch70rv7)+~9 udh2tW3,
or~,~)
W,7orv,)+u,
2801 (P)
v5+2v35
2v,0r4v,~ 2873 2943 3021 3109 3199
(Q) (R) (S) (T) (U)
v5+v9 v3+v0 vJ+W170r+) Choru7)+h.+u12 2%+(%
'Jrh7)
v5+v6
3254 3340 3421 3509 3578 3646
(V) (W) (X) (Y) (Z) (a)
24 v3+v8+h2 ~3+W17or~7)+~12 h+(2horV,7)+2hrr W~dVv~~orv,,) 2++42 v3+v8+(hor
3727 (b)
h7)
Y3+@'17orY7)
Intensity relative to us (at 290.83 mu) 5.4 2.0 100 1.6 2.6 1.8 0.3 0.7 5.6 0.8 2.2 0.5 1.6 3.6 0.6 4.8 1.8 1.4 1.3 2.0 9.1 1.6 1.4 0.8 4.9 5.2 4.1
+(2horv,7)
3809 (d) 3897 (e) 3953 (f)
v,+(2v,,or~,)+2v,~ r%+k+ (2Ysr or r,,) 2v9+ (2v,, or v,)
4035 (S)
2vs+ (2Qt or tk) 2ust2v,s
4115 (h) 4187 (i)
v3+us+vlz+(2v3,0rv~7) v3t2vg v,tv*tu9
3812 3900 3895 3957 4042 4042 4125 4194
1.3 2.7 7.8 2.2 3.1 1.4
‘) Letters refer to band labels in fig. 2. ‘) Tentative assignments are based on those of refs. [ 16,17 ] ; modes are numbered as in ref. [ 171. ‘) Calculatedfrequenciesarebased onv+ 1627 cm-‘, vg= 1570 cm-‘, 2u,,or v,= 1397cm-‘,vs= 1318 cm-t, us= 1249crn-‘,v,,= cm-‘, 2~ or vt7=703 cm-‘, and 2vs,= 1176 cm-‘.
from CIVsymmetry [ 22 1, and we cannot exclude the possibility that the 703 cm-’ interval present in the most strongly A-state-enhanced bands arises from u17
394
rather than 2~~~.However, the fact that the A-state resonant spectra do not seem to exhibit any other transitions involving one quantum in modes that are 411
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nontotally symmetric in the Czvpoint group leads us to prefer the assignment as 2 v3,. For nearly all of the bands listed in table 1, definitive assignments will require spectra of isotopic derivatives. For example, deuteration of the terminal hydrogens would generate very different frequency shifts in =CH2 ( =CD2) torsional and CCC bending modes [ 23 1. The lowest-frequency skeletal bending mode of b, symmetry, v3], is expected to be most active in vibronicaily coupling the 2 ‘A, state to the nearby allowed 1 ‘B, state by analogy with the longer all-trans unsubstituted and diphenyl polyenes [2,24-271. In butadiene, transitions involving two quanta of the corresponding b, bending vibration, vz4, also show enhancement in the Raman spectra excited on the low-energy side of the absorption band [28,29], but the electronic transitions are evidently too diffuse and/or too close in energy for the 2 ‘A; state to be clearly resolved. The dominant A-state-enhanced transitions in cis-hexatriene involve, besides 2v3,, mainly the same vibrations that are most active in the B-state resonant Raman [ 20,211, which have considerable C=C and/or C-C stretching character. This observation is consistent with the expected large degree of bond-order alternation leading to large changes in the C=C and C-C bond lengths in the A state [ 11,301. Our tentative assignment of a number of A-state-enhanced bands to transitions that involve the =CH2 torsional overtones 2v17and/or 2v35 seems qualitatively consistent with a twisted geometry in the 2 ‘A, state, as both ab initio calculations and the very low emission yields from this state suggest [ 131. However, quantitative simulations of the spectra, similar to those previously carried out for the B-state resonant spectra [ 18-2 11, will be needed before any definite conclusions can be drawn about the structure of the A state. Prior REMPI [ 13,I41 and fluorescence excitation [ 15] studies of jet-cooled cis-hexattiene assigned the lowest-energy feature, which is actually a doublet, as the A-state origin. This assignment was based on the fact that although the 2 ‘4-I ‘$ transition in an isolated all-trans polyene is strictly dipole-forbidden by symmetry, the 2 ‘A, t 1 ‘A, transition of cis-hexatriene is symmetry allowed in the Czy point group (through a transition dipole along the C, axis, perpendicular to the x-bond axis). This assignment is further supported by the fact that any REMPI signal 412
29 January 1993
from the trans isomer is far weaker than that from the cis [ 13,141. The alternative interpretation, supported by our tentative assignment of the 703 cm-’ interval as 2v3,, is that the 290.8 nm band is a false origin built on one quantum of v31as a promoting mode. This would be consistent with the observation by Petek et al. [ 151 that the rotational structure of the apparent origin more closely resembles a parallel than a perpendicular transition, indicating that the intensity is largely borrowed from the 1 ‘B, state. The assignment as a false origin is also supported to some extent by the fact that the vibronic progression in our emission spectrum is quite different from that observed in excitation [ 131; no analog of the strong 703 cm-’ interval in emission seems to be present in the REMPI excitation spectrum, although large vibrational frequency changes could mask the correspondence between ground and excited state vibrations. The 2 ‘Al/l ‘B, vibronic coupling to lowest order requires the vibronically active mode to change by one quantum in the electronic transition. Thus, if the intensity is vibronically induced the emission spectrum on resonance with the 2 ‘A, state should exhibit transitions involving both two quanta of vl,, when one quantum is gained in both the upward and downward transitions, and zero quanta of Vet,when the quantum gained in the upward transition is lost in the downward one (fig. 3 ). Transitions of the latter type, however, are also enhanced through the allowed 1 ‘B, state via the usual Franck-Condon mechanism, and while overtones and combination hands are relatively weak when the excitation is only preresonant with the 1 IB, state, they still derive some intensity from this source. This would explain our observation that transitions involving the 703 cm-’ interval generally have more sharply peaked excitation profiles than do overtones and combination bands of a, modes alone (fig. 4). The excitation profiles for the strong transitions involving the 703 cm-’ interval are quite narrow, having a full width at half maximum of only about 10 cm-’ (uncorrected for laser bandwidth). These profiles are broader than the jet-cooled spectra, which exhibit two bands in the “origin” region each having widths of no more than 2-3 cm-‘, but are still perhaps surprisingly sharp for a polyene at room temperature. The relative Raman excitation profiles peak at 343842 4 cm-‘, in reasonable agreement with the
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CHEMICAL PHYSICS LETTERS
Vibronic Ext.
29 January 1993
Franck-Cmdon
Emission
Excitation
Emission
Fig. 3. Possible mechanisms giving rise to the 703 cm-’ interval observed in the A-state resonant emission spectra: vibronic coupling (if 703 cm-’ =2us,) or Franck-Condon (if 703 cm-‘= u,,). In the vibronic case, the lowest-frequency transition in absorption or excitation is a false origin transition to one quantum of us,, while emission occurs to modes involving either zero or two quanta of u,,. In the Franck-Condon case, both the true origin and transitions involving one or more quanta of vi, arc allowed in both excitation and emission.
$
0.06-
2
$ 0.04._ t;; ;ij a 0.02-
0.001 . . . . 34340
I
.
34360
Excitation
.
.
’
I
.
.
34360
.
I
.
-’
34400
Frequency
(l/cm)
Fig. 4. Relative Raman excitation profiles (integrated intensity relative to that of the q fundamental) of 2 vs (squares, dashed line),us+(2~,,or~,~) (circles,solidline),andthebandat2017 cm-i comprised of vs+ (215, or vi?) and/or vs+ his (triangles, dotted line). The intensities at 290.83 nm are slightly different from those listed in table I because this tigure is based on spectra obtained at a lower signal-to-noise ratio.
in fluorescence excitation (34385 and 34390 cm-‘) and REMPI (34388 and 34393 cm-’ ). Note that the relative Raman profiles may be slightly skewed in frequency because the band to which the intensity is ratioed, v5, gains intensity with increasing excitation frequency due to the preresonance enhancement from the 1 ‘B, state, and also should itself have some res-
frequencies
of the “origin”
doublet
observed
onance enhancement from the 2 ‘Al state. We cannot resolve the doublet structure observed in the jet spectra. However, the assumption that this doublet structure does underlie our profiles, together with the expectation that the rotational envelope at room temperature should be at least several wavenumbers wide, leads us the conclude that the linewidth of each rovibronic feature does not exceed a few wavenumhers. Finally, we address the question of whether this emission should be considered as resonance Raman or fluorescence. By the usual experimental definition, we are clearly looking at Raman spectra because the Raman shift, and not the absolute emission frequency, remains fixed as the excitation frequency is tuned. From a theoretical viewpoint, a Raman process is one in which the system’s density matrix evolves from start to finish without any loss of phase coherence. Pure dephasing, caused either by interactions with a material bath [ 31,321 or by fluctuations in the phase of the radiation used to drive the process [33,34], will convert some of this coherent superposition of states into an incoherent population of levels and result in fluorescence emission. Our molecules are in the gas phase and should be essentially collision free during the 2 ‘Al state lifetime, but the highly excited vibrational levels of the ground state that are isoenergetic with the 2 ‘A, state resonance might act as a source of dephasing, and our laser spectral bandwidth is probably comparable 413
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CHEMICAL PHYSICS LETTERS
to or greater than the widths of the individual rovibronic transitions. Thus our spectra may involve a superposition of theoretically “Raman” and “fluorescence” type of emission. More quantitative information about the 2 ‘A, state will require a more definitive assignment of the observed transitions followed by computational modeling of the resonance Raman intensities. The calculations will need to account properly for the 2 ‘A,/ 1 ‘B, vibronic coupling, which for frequencies well below the 1 ‘B, state origin can be handled by the usual perturbative Herzberg-Teller approach. To model the intensities of bands not involving 2 v3,, we will also need to treat the interference between the 2 ‘Al-state resonant and 1 ‘B,-state preresonant scattering, as observed in previous excitation profile studies on longer polyenes [35,36] and modeled quantitatively in our work on the alkyl iodides [ 37,381. The parameters needed to simulate the spectra should allow us to evaluate the strength of the vibronic coupling and characterize the excited state potential surface of the 2 ‘A1 state, at least in the region near the ground state geometry.
This work was supported in part by NIH grant GM 39724. ABM is the recipient of a Packard Fellowship in Science and Engineering, a Sloan Research Fellowship, an NSF Presidential Young Investigator Award, and a Dreyfus Teacher-Scholar Award. Helpful discussions with Professor Bryan Kohler regarding the symmetry-allowedness of the A-state transition are acknowledged. References [ 1 ] B.S.Hudsonand B.E. Kohler, J. Chem. Phys. 59 (1973)
4984. [2] B.S. Hudson, B.E. Kohler and K Schulten, in: Excited states, Vol. 6, ed. E.C. Lint (Academic Press, New York, 1982) p. [3] k. Orlandi, E Zerbetto and M-2. Zgierski, Chem. Rev. 91 (1991) 867. [41B.E. Kohler, P. Mitm and P. West, .I.Chem. Phys. 85 ( 1986) 4436. 151W.G. Dauben, B. Disanayaka, D.J.H. Funhoff, B.E. Kobler, DE. Schilke and B. Zhou, J. Am. Chem. Sot. 113 (199 1) 8367. [6] J.R. Heflin, K.Y. Wong, 0. Zamani-Khamiri and A.F. Garito, Phys. Rev. B 38 (1988) 1573. [7] Z.G. Soos and S. Ramasesha, J. Chem. Phys. 90 (1989) 1067.
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[ 8 ] J.L. Bredas and J.M. Toussaint, I. Chem. Phys. 92 ( 1990) 2624. [ 9 ] T. BaUy,K. Roth, W. Tang, R.R. Schrock, K. Knoll and L.Y. Park, J. Am. Chem. Sot. 114 (1992) 2440. 110 BE. Kohler, J. Chem. Phys. 93 (1990) S838. (11 M. Aoyagi, I. Ohmine and B.E. Kohler, J. Phys. Chem. 94 ( 1990) 3922. 112F. Zerbetto and M.Z. Zgierski, J. Chem. Phys. 93 (1990) 1235. 1131W.J. Buma, B.E. Kohler and K. Song, J. Chem. Phys. 94 ( 1991) 6367. [I4,I W.J. Buma, B.E. Kohler and K. Song, J. Chem. Phys. 92 (1990) 4622. [15 H. Petek, A.J. Bell, R.L. Christensen and K. Yoshihara, J. Chem. Phys. 96 (1992) 2412. [Is F.W. Langkilde, R. Wilbrandt, O.F. Nielsen, D.H. Christensen and F.M. Nicolaisen, Spectrochim. Acta 43A (1987) 1209. [ 171R. McDiarmid and A. Sabljic, J. Phys. Chem. 91 (1987) 276. [ 181A.B. Myers and K.S. Pmnata, I. Phys. Chem. 93 (1989) 5079. [ 191X. Ci, M.A. Per&a and A.B. Myers, 1. Chem. Phys. 92 (1990) 4708. [20] X. Ci and A.B. Myers, J. Chem. Phys. 96 (1992) 6433. [ 2 I] B. Amstrup, F. W. Langkilde, K. Bajdor and R. Wilbrandt, J. Phys. Chem. 96 (1992) 4794. [22] M. Traetteberg, Acta Chem. Stand. 22 ( 1968) 2294. [23 ] F.W. Langkilde, R. Wilbrandt and AM. Brouwer, J. Phys. Chem. 94 (1990) 4809. [24] H. Petek, A.J. Bell, K. Yoshihara and R.L. Christensen, J. Chem. Phys. 95 ( 1991) 4739. [25]M.F. Granville, G.R. Holtom and B.E. Kohler, J. Chem. Phys. 72 (1980) 4671. [ 261 B.E. Kohler and T.A. Spiglanin, J. Chem. Phys. SO (1984) 5465. [27] L.A. Heimbrook, B.E. Kohler and T.A. Spiglanin, Proc. Natl. Acad. Sci. US 80 (1983) 4580. [ 281 R.R. Chadwick, D.P. Gerrity and B.S. Hudson, Chem. Phys. Lettersll5(1985)24. [29] R.R. Chadwick, M.Z. Zgienki and B.S. Hudson, J. Chem. Phys. 95 (1991) 7204. [ 301F.Zerbetto, M.Z. Zgierski, F. Negri and G. Orlandi, J. Chem. Phys. 89 (1988) 3681. [ 3 I] J. Sue, Y.J. Yan and S. Mukamel, J. Chem. Phys. 85 ( 1986) 462. [ 321 B. Li and A.B. Myers, J. Phys. Chem. 94 (1990) 4051. [33] Y.P. Zhang and L.D. Ziegler, J. Chem. Phys. 93 (1990) 8605. [ 341 B.LiandA.B.Myers,J. Chem. Phys. 94 ( 1991) 2458. 1351R.J. Thrash, H.L.B. Fang and G.E. Leroi, J. Chem. Phys. 67 (1977) 5930. 1361I.W. Sztainbuch and GE. Leroi, J. Chem. Phys. 93 ( 1990) 4642. [37] D.L. Phillips and A.B. Myers, J. Chem. Phys. 95 (1991) 226. [381 F. Markel and A.B. Myers, J. Chem. Phys., in press.
CHEMICAL PHYSICS LETTERS
29 January 1993
The 2 ‘Al state of cis-hexatriene vapor: resonance Raman spectra Curtis Westerfield and Anne B. Myers Departmentof Chemistry, UniversityofRochester, Rochester, NY 14627-0216,USA Received 8 October 1992;in final form I 1 November 1992
Raman spectra have been obtained on resonance with the 2 ‘A, state of room temperature cis-1,3,5-hexatriene vapor at 290.8 nm. Overtones and combination bands of totally symmetric stretching and bending modes, both alone and in combination with an interval probably assignable as the overtone of the lowest frequency skeletal bending mode of b, symmetry, are strongly en-
hanced. Vibrational intervals tentatively assignedto the a2and b2methylenetorsions are also observed,consistent witha large
changein the force constant for =CH2twisting in the 2 ‘A, state.
The linear polyenes are unusual among organic molecules in having a lowest-lying singlet excited state (having Agor A, symmetry in polyenes of CZb or CZysymmetry, respectively) that is best described in terms of doubly rather than singly excited configurations [ l-31. While the state ordering 2 ‘A< 1 ‘B is now generally thought to hold for all unsubstituted and diphenyl polyenes, spectroscopic identification of the nominally one-photon forbidden A state has been extremely difficult in butadiene and hexatriene due to the virtual absence in these molecules of the vibronically induced fluorescence observed in the longer polyenes. Understanding the energetics and structure of the A states is important because of their role in polyene photochemistry [4,5] and the nonlinear optical response of polyenes and conjugated polymers [ 6-91. In addition, the lowest singlet excited A states of polyenes present a formidable test for the results of electronic structure calculations [ 3, IO,111. It has been suggested that the absence of fluorescence from dienes and trienes results from rapid twisting about a terminal double bond in the A state [ 12,131, but there is as yet little direct experimental evidence to support this. The apparent origin of the 2 IA, state in cis-hexatriene cooled in a jet expansion has recently been detected both by onephoton resonant multiphoton ionization [ 13,141 and by fluorescence excitation [ 151. In both spectra vibronic features were also resolved, but the lack of vibrational assignments for the low-frequency modes
in particular limits the structural information that can be derived from these spectra. Resonance Raman spectroscopy should be able to provide more useful structural data about the excited A state of hexatriene because the ground state vibrations are well characterized [ 16,171. Quantitative analysis of the Raman spectra on resonance with the strongly allowed B state has led to detailed conclusions about the structure and dynamics of this short-lived state in both vapor and solution phases [ 18-2 11. Even with excitation near the A-state origin, the resonance-enhanced scattering from the A state is much weaker than preresonant scattering from the B state in the fundamental region, but A-stateenhanced scattering is clearly discernible in the overtone and combination band region. Experimental conditions were identical to those in ref. [ 201 except for a few minor changes. Because of the weakness of the Raman scattering and the nearly negligible absorption at the excitation wavelengths employed, higher pressures of cis-hexatriene (x 5 Torr) and higher laser powers ( z 150 p/pulse) were used. Gas chromatographic analysis of the sample after z 5 h of irradiation showed that cis-trans isomerization of the sample was less than 5% and no other impurities were observed. Finally, because the 2 ‘A,+ 1 ‘A, origin is lower in energy than the 1 ‘B, + 1 ‘A, origin, the frequency doubled output of the dye laser could be used directly without Raman shifting.
0009-2614/93/$06.00 Q 1993 ElsevierSciencePublishers B.V. All rights reserved.
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CHEMICAL PHYSICS LETTERS
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Fig. 1 shows spectra of cis-hexatriene in the overtone and combination band region as the laser is tuned through the reported position of the 2 ‘A, state origin band at 290.8 nm. It is clear that a number of overtones and combination bands are strongly enhanced (relative to the fundamentals such as z$, vg and v,) by resonance with the 2 ‘A, state. Fig. 2 shows a more complete spectrum obtained near the peak of the A-state excitation profile at higher signal-
291.17
nm
> : P : :
1360
1740 Raman
2100 Shift
2460
2.320
IcK?)
Fig. 1. Resonance Raman spectra of cis-hexatriene vapor at the three indicated excitation wavelengths.The spectra are scaled such that the us fundamental at 1627 cm- ’is the same height in ah three spectra. The strongest combination band at 2332 cm-’ in the 290.83 nm spectrum has an integrated area = 20 times lower than that oft+.
1470
2110 Raman
2750 Shift
3390
4030
km-1
Fig 2. Resonance Raman spectrum of cis-hexatriene vapor at 290.83 nm excitation. The strongest band in the overtone and combination band region, the vI overtone at 3254 cm-‘, has an integrated area z I I times lower than that of the + fundamental at 1627 cm-‘. The letters are for reference to table 1.
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to-noise ratio. Tram-hexatriene also exhibits preresonance scattering throughout this region, but no new bands are enhanced when the excitation is tuned near 290.8 nm, confirming the assignment of the features in the fluorescence excitation spectrum of Petek et al. [ 151 to the cis isomer. Attempts to obtain A-stateenhanced spectra on resonance with the reported cishexatriene vibronic feature c 70 cm-’ above the origin were unsuccessful; that is, no clear enhancement above the B-state preresonant background was observed. This result is qualitatively consistent with the relative weakness of this feature in the free-jet fluorescence excitation spectrum ( < 8% relative to the integrated area of the “origin” doublet) [ 151. Table 1 summarizes our tentative assignments of the main bands between 1300 and 4200 cm-‘. The observed bands can all be assigned consistently, although not uniquely, using eight vibrational intervals: the five a, modes v5 and vg (C=C stretches), vg (CH rock), v9 (CH rock and C-C stretch), and v,, (CCC bend); an interval of 703 cm- ’assignable as two quanta of v3,, the lowest frequency b, CCC bend, or possibly as one quantum of the a, methylene torsion, v, 7;two quanta of vj5, the methylene torsion of b2 symmetry; and an interval of approximately 1400 cm-‘, which might reasonably be assigned as either one quantum of the al mode v7 (CHI scissors) or two quanta of v,,. The assignment of the 1400 cm-’ interval as v7 gives very good agreement with the observed frequencies, but we consider it unlikely that a methylene scissors mode would have this much intensity (it is not a very strong band on resonance with the 1 ‘B, state). The liquid phase frequency of v,~ appears slightly too high ( 707 cm-‘) [ 161, but the vapor phase frequency has not been reported, and activity in this mode would be consistent with the apparent contribution of the corresponding bz torsion to several weak bands in our A-state resonant spectra. The interpretation of our spectra is strongly dependent on the assignment of the 703 cm-’ interval. In our previous resonance Raman study of cishexatriene vapor on resonance with the 1 ‘B, state [ 201, we attributed a very weak feature at 7 14 cm- l to the fundamental of vr7, while noting that an alternative assignment as 2 us, was also possible. Gasplane electron diffraction studies indicate that the ground state of cis-hexatriene is slightly distorted
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Table 1 Cis-hexatriene
overtone and combination
band transitions enhanced by 2 ‘A, state resonance
Expt. freq. (cm-‘) I)
Assignment ‘)
talc. freq. C) (cm-‘)
1308 (A) 1388 (B)
“0 h U6
1318 1397 1570 1627 1952 2021 2021 2100 2185 2273 2330 2415 2498 2567 2646 2724 2715 2803 2794 2876 2945 3024 3109 3201 3197 3254 3339 3418 3506 3582 3648 3648 3727
1570 1627 1950 2017
(C) (D) (E) (F)
2108 2182 2274 2330 2412 2497 2561 2652 2719
(G) (H) (I) (J) (K) (L) (M) (N) (0)
“5
b+ (2~~~ or u17)
us+PI2 hd(2Gorv17) (2~17orY7)+(2~~,or~17) W170rv7)+2v12 v6+(2hIOrh7) v3+12v3,0rh7) h+h2 24 vs+v9 Ch70rv7)+~9 udh2tW3,
or~,~)
W,7orv,)+u,
2801 (P)
v5+2v35
2v,0r4v,~ 2873 2943 3021 3109 3199
(Q) (R) (S) (T) (U)
v5+v9 v3+v0 vJ+W170r+) Choru7)+h.+u12 2%+(%
'Jrh7)
v5+v6
3254 3340 3421 3509 3578 3646
(V) (W) (X) (Y) (Z) (a)
24 v3+v8+h2 ~3+W17or~7)+~12 h+(2horV,7)+2hrr W~dVv~~orv,,) 2++42 v3+v8+(hor
3727 (b)
h7)
Y3+@'17orY7)
Intensity relative to us (at 290.83 mu) 5.4 2.0 100 1.6 2.6 1.8 0.3 0.7 5.6 0.8 2.2 0.5 1.6 3.6 0.6 4.8 1.8 1.4 1.3 2.0 9.1 1.6 1.4 0.8 4.9 5.2 4.1
+(2horv,7)
3809 (d) 3897 (e) 3953 (f)
v,+(2v,,or~,)+2v,~ r%+k+ (2Ysr or r,,) 2v9+ (2v,, or v,)
4035 (S)
2vs+ (2Qt or tk) 2ust2v,s
4115 (h) 4187 (i)
v3+us+vlz+(2v3,0rv~7) v3t2vg v,tv*tu9
3812 3900 3895 3957 4042 4042 4125 4194
1.3 2.7 7.8 2.2 3.1 1.4
‘) Letters refer to band labels in fig. 2. ‘) Tentative assignments are based on those of refs. [ 16,17 ] ; modes are numbered as in ref. [ 171. ‘) Calculatedfrequenciesarebased onv+ 1627 cm-‘, vg= 1570 cm-‘, 2u,,or v,= 1397cm-‘,vs= 1318 cm-t, us= 1249crn-‘,v,,= cm-‘, 2~ or vt7=703 cm-‘, and 2vs,= 1176 cm-‘.
from CIVsymmetry [ 22 1, and we cannot exclude the possibility that the 703 cm-’ interval present in the most strongly A-state-enhanced bands arises from u17
394
rather than 2~~~.However, the fact that the A-state resonant spectra do not seem to exhibit any other transitions involving one quantum in modes that are 411
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nontotally symmetric in the Czvpoint group leads us to prefer the assignment as 2 v3,. For nearly all of the bands listed in table 1, definitive assignments will require spectra of isotopic derivatives. For example, deuteration of the terminal hydrogens would generate very different frequency shifts in =CH2 ( =CD2) torsional and CCC bending modes [ 23 1. The lowest-frequency skeletal bending mode of b, symmetry, v3], is expected to be most active in vibronicaily coupling the 2 ‘A, state to the nearby allowed 1 ‘B, state by analogy with the longer all-trans unsubstituted and diphenyl polyenes [2,24-271. In butadiene, transitions involving two quanta of the corresponding b, bending vibration, vz4, also show enhancement in the Raman spectra excited on the low-energy side of the absorption band [28,29], but the electronic transitions are evidently too diffuse and/or too close in energy for the 2 ‘A; state to be clearly resolved. The dominant A-state-enhanced transitions in cis-hexatriene involve, besides 2v3,, mainly the same vibrations that are most active in the B-state resonant Raman [ 20,211, which have considerable C=C and/or C-C stretching character. This observation is consistent with the expected large degree of bond-order alternation leading to large changes in the C=C and C-C bond lengths in the A state [ 11,301. Our tentative assignment of a number of A-state-enhanced bands to transitions that involve the =CH2 torsional overtones 2v17and/or 2v35 seems qualitatively consistent with a twisted geometry in the 2 ‘A, state, as both ab initio calculations and the very low emission yields from this state suggest [ 131. However, quantitative simulations of the spectra, similar to those previously carried out for the B-state resonant spectra [ 18-2 11, will be needed before any definite conclusions can be drawn about the structure of the A state. Prior REMPI [ 13,I41 and fluorescence excitation [ 15] studies of jet-cooled cis-hexattiene assigned the lowest-energy feature, which is actually a doublet, as the A-state origin. This assignment was based on the fact that although the 2 ‘4-I ‘$ transition in an isolated all-trans polyene is strictly dipole-forbidden by symmetry, the 2 ‘A, t 1 ‘A, transition of cis-hexatriene is symmetry allowed in the Czy point group (through a transition dipole along the C, axis, perpendicular to the x-bond axis). This assignment is further supported by the fact that any REMPI signal 412
29 January 1993
from the trans isomer is far weaker than that from the cis [ 13,141. The alternative interpretation, supported by our tentative assignment of the 703 cm-’ interval as 2v3,, is that the 290.8 nm band is a false origin built on one quantum of v31as a promoting mode. This would be consistent with the observation by Petek et al. [ 151 that the rotational structure of the apparent origin more closely resembles a parallel than a perpendicular transition, indicating that the intensity is largely borrowed from the 1 ‘B, state. The assignment as a false origin is also supported to some extent by the fact that the vibronic progression in our emission spectrum is quite different from that observed in excitation [ 131; no analog of the strong 703 cm-’ interval in emission seems to be present in the REMPI excitation spectrum, although large vibrational frequency changes could mask the correspondence between ground and excited state vibrations. The 2 ‘Al/l ‘B, vibronic coupling to lowest order requires the vibronically active mode to change by one quantum in the electronic transition. Thus, if the intensity is vibronically induced the emission spectrum on resonance with the 2 ‘A, state should exhibit transitions involving both two quanta of vl,, when one quantum is gained in both the upward and downward transitions, and zero quanta of Vet,when the quantum gained in the upward transition is lost in the downward one (fig. 3 ). Transitions of the latter type, however, are also enhanced through the allowed 1 ‘B, state via the usual Franck-Condon mechanism, and while overtones and combination hands are relatively weak when the excitation is only preresonant with the 1 IB, state, they still derive some intensity from this source. This would explain our observation that transitions involving the 703 cm-’ interval generally have more sharply peaked excitation profiles than do overtones and combination bands of a, modes alone (fig. 4). The excitation profiles for the strong transitions involving the 703 cm-’ interval are quite narrow, having a full width at half maximum of only about 10 cm-’ (uncorrected for laser bandwidth). These profiles are broader than the jet-cooled spectra, which exhibit two bands in the “origin” region each having widths of no more than 2-3 cm-‘, but are still perhaps surprisingly sharp for a polyene at room temperature. The relative Raman excitation profiles peak at 343842 4 cm-‘, in reasonable agreement with the
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Vibronic Ext.
29 January 1993
Franck-Cmdon
Emission
Excitation
Emission
Fig. 3. Possible mechanisms giving rise to the 703 cm-’ interval observed in the A-state resonant emission spectra: vibronic coupling (if 703 cm-’ =2us,) or Franck-Condon (if 703 cm-‘= u,,). In the vibronic case, the lowest-frequency transition in absorption or excitation is a false origin transition to one quantum of us,, while emission occurs to modes involving either zero or two quanta of u,,. In the Franck-Condon case, both the true origin and transitions involving one or more quanta of vi, arc allowed in both excitation and emission.
$
0.06-
2
$ 0.04._ t;; ;ij a 0.02-
0.001 . . . . 34340
I
.
34360
Excitation
.
.
’
I
.
.
34360
.
I
.
-’
34400
Frequency
(l/cm)
Fig. 4. Relative Raman excitation profiles (integrated intensity relative to that of the q fundamental) of 2 vs (squares, dashed line),us+(2~,,or~,~) (circles,solidline),andthebandat2017 cm-i comprised of vs+ (215, or vi?) and/or vs+ his (triangles, dotted line). The intensities at 290.83 nm are slightly different from those listed in table I because this tigure is based on spectra obtained at a lower signal-to-noise ratio.
in fluorescence excitation (34385 and 34390 cm-‘) and REMPI (34388 and 34393 cm-’ ). Note that the relative Raman profiles may be slightly skewed in frequency because the band to which the intensity is ratioed, v5, gains intensity with increasing excitation frequency due to the preresonance enhancement from the 1 ‘B, state, and also should itself have some res-
frequencies
of the “origin”
doublet
observed
onance enhancement from the 2 ‘Al state. We cannot resolve the doublet structure observed in the jet spectra. However, the assumption that this doublet structure does underlie our profiles, together with the expectation that the rotational envelope at room temperature should be at least several wavenumbers wide, leads us the conclude that the linewidth of each rovibronic feature does not exceed a few wavenumhers. Finally, we address the question of whether this emission should be considered as resonance Raman or fluorescence. By the usual experimental definition, we are clearly looking at Raman spectra because the Raman shift, and not the absolute emission frequency, remains fixed as the excitation frequency is tuned. From a theoretical viewpoint, a Raman process is one in which the system’s density matrix evolves from start to finish without any loss of phase coherence. Pure dephasing, caused either by interactions with a material bath [ 31,321 or by fluctuations in the phase of the radiation used to drive the process [33,34], will convert some of this coherent superposition of states into an incoherent population of levels and result in fluorescence emission. Our molecules are in the gas phase and should be essentially collision free during the 2 ‘Al state lifetime, but the highly excited vibrational levels of the ground state that are isoenergetic with the 2 ‘A, state resonance might act as a source of dephasing, and our laser spectral bandwidth is probably comparable 413
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to or greater than the widths of the individual rovibronic transitions. Thus our spectra may involve a superposition of theoretically “Raman” and “fluorescence” type of emission. More quantitative information about the 2 ‘A, state will require a more definitive assignment of the observed transitions followed by computational modeling of the resonance Raman intensities. The calculations will need to account properly for the 2 ‘A,/ 1 ‘B, vibronic coupling, which for frequencies well below the 1 ‘B, state origin can be handled by the usual perturbative Herzberg-Teller approach. To model the intensities of bands not involving 2 v3,, we will also need to treat the interference between the 2 ‘Al-state resonant and 1 ‘B,-state preresonant scattering, as observed in previous excitation profile studies on longer polyenes [35,36] and modeled quantitatively in our work on the alkyl iodides [ 37,381. The parameters needed to simulate the spectra should allow us to evaluate the strength of the vibronic coupling and characterize the excited state potential surface of the 2 ‘A1 state, at least in the region near the ground state geometry.
This work was supported in part by NIH grant GM 39724. ABM is the recipient of a Packard Fellowship in Science and Engineering, a Sloan Research Fellowship, an NSF Presidential Young Investigator Award, and a Dreyfus Teacher-Scholar Award. Helpful discussions with Professor Bryan Kohler regarding the symmetry-allowedness of the A-state transition are acknowledged. References [ 1 ] B.S.Hudsonand B.E. Kohler, J. Chem. Phys. 59 (1973)
4984. [2] B.S. Hudson, B.E. Kohler and K Schulten, in: Excited states, Vol. 6, ed. E.C. Lint (Academic Press, New York, 1982) p. [3] k. Orlandi, E Zerbetto and M-2. Zgierski, Chem. Rev. 91 (1991) 867. [41B.E. Kohler, P. Mitm and P. West, .I.Chem. Phys. 85 ( 1986) 4436. 151W.G. Dauben, B. Disanayaka, D.J.H. Funhoff, B.E. Kobler, DE. Schilke and B. Zhou, J. Am. Chem. Sot. 113 (199 1) 8367. [6] J.R. Heflin, K.Y. Wong, 0. Zamani-Khamiri and A.F. Garito, Phys. Rev. B 38 (1988) 1573. [7] Z.G. Soos and S. Ramasesha, J. Chem. Phys. 90 (1989) 1067.
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[ 8 ] J.L. Bredas and J.M. Toussaint, I. Chem. Phys. 92 ( 1990) 2624. [ 9 ] T. BaUy,K. Roth, W. Tang, R.R. Schrock, K. Knoll and L.Y. Park, J. Am. Chem. Sot. 114 (1992) 2440. 110 BE. Kohler, J. Chem. Phys. 93 (1990) S838. (11 M. Aoyagi, I. Ohmine and B.E. Kohler, J. Phys. Chem. 94 ( 1990) 3922. 112F. Zerbetto and M.Z. Zgierski, J. Chem. Phys. 93 (1990) 1235. 1131W.J. Buma, B.E. Kohler and K. Song, J. Chem. Phys. 94 ( 1991) 6367. [I4,I W.J. Buma, B.E. Kohler and K. Song, J. Chem. Phys. 92 (1990) 4622. [15 H. Petek, A.J. Bell, R.L. Christensen and K. Yoshihara, J. Chem. Phys. 96 (1992) 2412. [Is F.W. Langkilde, R. Wilbrandt, O.F. Nielsen, D.H. Christensen and F.M. Nicolaisen, Spectrochim. Acta 43A (1987) 1209. [ 171R. McDiarmid and A. Sabljic, J. Phys. Chem. 91 (1987) 276. [ 181A.B. Myers and K.S. Pmnata, I. Phys. Chem. 93 (1989) 5079. [ 191X. Ci, M.A. Per&a and A.B. Myers, 1. Chem. Phys. 92 (1990) 4708. [20] X. Ci and A.B. Myers, J. Chem. Phys. 96 (1992) 6433. [ 2 I] B. Amstrup, F. W. Langkilde, K. Bajdor and R. Wilbrandt, J. Phys. Chem. 96 (1992) 4794. [22] M. Traetteberg, Acta Chem. Stand. 22 ( 1968) 2294. [23 ] F.W. Langkilde, R. Wilbrandt and AM. Brouwer, J. Phys. Chem. 94 (1990) 4809. [24] H. Petek, A.J. Bell, K. Yoshihara and R.L. Christensen, J. Chem. Phys. 95 ( 1991) 4739. [25]M.F. Granville, G.R. Holtom and B.E. Kohler, J. Chem. Phys. 72 (1980) 4671. [ 261 B.E. Kohler and T.A. Spiglanin, J. Chem. Phys. SO (1984) 5465. [27] L.A. Heimbrook, B.E. Kohler and T.A. Spiglanin, Proc. Natl. Acad. Sci. US 80 (1983) 4580. [ 281 R.R. Chadwick, D.P. Gerrity and B.S. Hudson, Chem. Phys. Lettersll5(1985)24. [29] R.R. Chadwick, M.Z. Zgienki and B.S. Hudson, J. Chem. Phys. 95 (1991) 7204. [ 301F.Zerbetto, M.Z. Zgierski, F. Negri and G. Orlandi, J. Chem. Phys. 89 (1988) 3681. [ 3 I] J. Sue, Y.J. Yan and S. Mukamel, J. Chem. Phys. 85 ( 1986) 462. [ 321 B. Li and A.B. Myers, J. Phys. Chem. 94 (1990) 4051. [33] Y.P. Zhang and L.D. Ziegler, J. Chem. Phys. 93 (1990) 8605. [ 341 B.LiandA.B.Myers,J. Chem. Phys. 94 ( 1991) 2458. 1351R.J. Thrash, H.L.B. Fang and G.E. Leroi, J. Chem. Phys. 67 (1977) 5930. 1361I.W. Sztainbuch and GE. Leroi, J. Chem. Phys. 93 ( 1990) 4642. [37] D.L. Phillips and A.B. Myers, J. Chem. Phys. 95 (1991) 226. [381 F. Markel and A.B. Myers, J. Chem. Phys., in press.