The 211 nm excited resonance Raman spectra of trans-stilbene and related molecules

MOLSTR

Journal

of Molecular

Journal

9115

Structure

of

MOLECULAR STRUCTURE

379 (1996) 109-120

The 211 nm excited resonance Raman spectra of trans-stilbene and related molecules’ James D. Leonard, Department

of Chemistry,The

Jr., Terry L. Gustafson*

Ohio State University,

Received

I20 West 18th Avenue, Columbus,

26 May 1995; accepted

in final form 21 August

OH 43210-l 173, USA

1995

Abstract We have obtained the 211 nm Raman spectra of trans-stilbene (tS), tvans-stilbene oxide, triphenylethylene, and 4,4’dimethoxystilbene. We compare these spectra with the 211 nm Raman spectra of biphenyl (BP) and 1,4-diphenyl-1,3butadiene (DPB). In an effort to elucidate the effect of solvent on the excited state potential energy surface, we calculate differential Raman cross-sections of the 1630 cm-’ band of DPB and the 1610 cm-’ band of BP at excitation wavelengths of 211 and 746 nm in acetonitrile, ethanol, and cyclohexane. The cross-sections for these bands with 746 nm excitation increase with increasing solvent polarity in the order gCYCl&..ane< ~,thanol < gacetonitnle. The trend is different with 211 nm interactions excitation, oacetonitrile < ~cyclohexane 5 g&hand. We attribute the different trend at 211 nm to microscopic between the solvent and the excited state of the solute molecules. We also present a comparison of the 211 nm So resonance Raman spectra and the S, resonance Raman spectra of tS and DPB. Kqwords:

Resonance

Raman

spectroscopy;

Ultraviolet

excitation;

1. Introduction

The cy, w-diphenylpolyenes are widely used as model systems for polyene photoisomerization studies [l]. Studies of these systems provide insight into photochemical isomerization reactions found in biological systems, such as the visual transduction process and photosynthesis [2]. trans-Stilbene (tS) is the simplest of these model compounds and is possibly the most widely studied. tS provides a test case for studying solvent effects on photochemical reactions that involve a large geometry involves change, as the trans ---i cis isomerization * Corresponding author. ’Dedicated to the memory

of the late Professor

0022-2860/96/$15.00 lc 1996 Elsevier SSDI 0022-2860(95)09115-7

Science

Issei Harada. B.V. All rights

trawStilbene;

Biphenyl;

Diphenylbutadiene;

Raman

cross-section

the activated twisting of the olefin bond on the lowest excited singlet state (i.e. It* + ‘p’) as first proposed by Saltiel et al. [3]. The use of tS as a model for olefin isomerization has been reviewed [4-61. Many methods have been used to study this isomerization process, including absorption, fluorescence, resonance Raman and time-resolved techniques [7-171. Of these techniques only Raman spectrometry provides direct access to the structure of the photochemically active state in solution. We are among several groups using picosecond transient Raman spectroscopy to study solutesolvent interactions using tS and its derivatives as probe molecules [ 16,18-271. In transient Raman spectra, the frequencies of the bands provide direct access to structural information on the excited state reserved

110

J.D. Leonard. Jr., T.L. Gustufson!Journal

of Molecular

molecule, and the relative intensities of these bands reflect the character of higher electronic states from which resonance enhancement occurs [28,29]. Using the transient Raman technique of optical depletion timing, Hamaguchi has observed the photochemistry of tS molecules from these higher lying electronic states [30,3 11. In order to gain a better understanding of the intensities of the Raman bands observed in transient Raman spectra, we are using 211 nm excitation to probe in the same vicinity of the electronic state from which the bands in transient Raman spectra derive intensity. We note that we are probably not probing the exact state from which the S, bands receive their intensity, owing to parity constraints. Intensities of the resonance Raman bands are a projection of the excited state geometry change on the ground state normal modes of the molecule [29]. A greater relative intensity in certain bands indicates that a larger displacement of the excited state potential surface will occur along this molecular coordinate. Ci and Myers have used resonance Raman experiments to elucidate the effect of solvent on the ground state potential

Structure

379 (1996) 109-120

energy surface of tS [15]. They examined the So Raman spectra of tS in cyclohexane and methanol at excitation wavelengths of 266 and 282 nm. The relative intensity of the hydrogen out-of-plane wag at 956 cm-’ was shown to be independent of the solvent, thus indicating that the out-of-plane potential energy surfaces are similar in both cyclohexane and methanol. Studies have been performed using biphenyl (BP) that relate the relative intensities of certain Raman bands to the conformation of the molecule in solution [32]. Butler et al. used this interpretation to relate the change in relative intensity of certain “biphenyllike” Raman bands in the S, transient Raman spectrum of 4,4’-diphenylstilbene to indicate that the molecule was assuming a more planar conforma[ 181. tion with delay following photoexcitation In this paper, we present the UV resonance Raman (UVRR) spectra of tS, BP, 1,4-diphenyl1,3-butadiene (DPB), triphenylethylene, 4,4’dimethoxystilbene and trans-stilbene oxide in acetonitrile obtained with 2 11 nm excitation. The molecular structures are shown in Fig. 1. We are particularly interested in the effect of solvent on

1,4-Diphenyl-1,3-Butadiene

Biphenyl

13 c(

trans-Stilbene

trans-Stilbene

cf

Oxide

OCH,

H&O

P

4,4’-Dimethoxystilbene Fig. 1. Structures

of the molecules

c, Triphenyl examined

Ethylene

in this study.

J.D. Leonard. Jr., T.L. Gustafson/Journal

the relative intensities of the Raman bands. We calculate the relative Raman scattering crosssections of the 1610 and 1630 cm-’ bands for DPB and BP, respectively, in acetonitrile, ethanol, and cyclohexane. We compare the relative intensities of the Raman bands of BP and DPB in acetonitrile obtained with 211 nm excitation with those obtained with 746 nm excitation in order to examine the character of the S, state in the region of 211 nm. In addition, we examine the S1 resonance Raman spectra of tS and DPB and compare these spectra with the ground state resonance Raman spectra obtained with 211 nm excitation.

2. Experimental The instrumentation that we used to obtain the 211 nm Raman spectra has been previously described in detail [33]. Briefly, the output from a cw mode-locked Nd:YLF laser (Coherent, Inc., Model Antares 76s) is doubled and then tripled (Coherent, Inc., Model 7950). The 351 nm radiation is taken from the tripler and combined collinearly with residual 527 nm light. The combined beam is then focussed into a P-barium borate crystal (Cleveland Crystals, 71.5” cut, BBO) that is mounted on a rotation stage to allow for angle adjustment. We overlap the two beams temporally through the use of a fast photodiode (Ante1 Optro-

~f‘hiolecular

Structure

379 (1996) 109-120

111

nits, Inc., Model AR-S2) and a variable delay for the 351 nm radiation. The pathlength of the 211 nm beam between the BBO crystal and the sample is minimized to reduce the effects of the excitation beam’s astigmatism at the sample. For the spectra included in this paper, typical 211 nm power at the sample is 5-6 mW. We use a sample cell spinning at % 60 Hz for spectral acquisition in order to minimize photodegradation of the sample during long periods of UV exposure. The beam is focussed onto the 1 cm quartz cell with a 5 cm lens. We use a 90” backscattering geometry for collection of the Raman scatter. The Raman scatter is dispersed by a single 0.64 m spectrograph (Instruments SA, Model THR 640). We use a CCD (Photometrics, Model CC200, with a Thomson-CSF 384 x 576 chip) for spectral acquisition. We use a 3600 g mm-’ grating which yields an effective spectral resolution of 7 cm-‘. Wavenumber shift was calibrated using the Raman spectrum of cyclohexane. Data collection was controlled by software written in Asyst [34]. Data acquisition times for the spectra presented in this paper were 1600 s except for BP, for which the acquisition time was 1000 s. Raman spectra collected with 746 nm excitation were obtained with a Ti:sapphire laser (Coherent, Model 890) pumped by an argon ion laser (Coherent, Model 318). The Raman scatter was collected with a lens pair and then passed into a single 0.5 m spectrograph (Chromex, Model

1

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275

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WAVELENGTH

325

350

A

375

400

(nm)

Fig. 2. UV-visible spectra of biphenyl (dotted line), 1.4-diphenyl-l,3-butadiene (broken line) and trans-stilbene nitrile. Concentration for all compounds: 50 PM. The arrow marks the UV excitation wavelength.

(solid line) in aceto-

J.D. Leonard, Jr.. T.L. GustafsoniJournal

112

SOOIS). For the near IR work we used a CCD (Princeton Instruments, Model LN/CCD-1152, with a 298 x 1152 chip) for spectral acquisition. Data acquisition times for the NIR spectra were 480 s. Spectra obtained at 746 nm were corrected for spectrograph response using the spectrum of a tungsten bulb. trans-Stilbene [C.A. Registry No. 103-30-O] (scintillation grade), 4,4’-dimethoxystilbene [ 1563814-91 (97%), trans-stilbene oxide [ 1439-07-21 (99%), biphenyl [92-52-41 (99%) and the solvents cyclohexane [l lO-82-71 (spectrophotometric grade) and ethanol [64-17-51 (spectrophotometric grade) were purchased from Aldrich and used without further purification. Triphenylethylene [58-72-O] and diphenylbutadiene [886-65-71 (scintillation grade) were obtained from Eastman Kodak. Tetra[ 109-99-91 (analytical reagent grade) hydrofuran was purchased from Mallinckrodt. Acetonitrile [75-05-81 (spectrophotometric grade) was obtained from Burdick and Jackson. Differential Raman cross-sections of DPB and BP (1 mM concentrations) in acetonitrile,

211 nm

l1700

1600

1500

1400

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1300

1200

1100

1000

SHIFT (cm-‘)

Fig. 3. Comparison of Raman spectra of trans-stilbene in acetonitrile under resonance (top) and off-resonance conditions (bottom).

of Molecular

Structure 379 (1996)

109-120

cyclohexane, and ethanol were obtained by comparison with appropriate bands of the solvent, whose cross-sections had been referenced to the 992 cm-’ band of benzene in a separate experiment. The Raman cross-section of the 992 cm-l band in benzene was calculated through the use of equations presented by Asher and Johnson [35]. Raman spectra were fitted with Peakfit (Jandel Scientific) to determine peak intensities.

3. Results and discussion 3.1. E.xcitation

and solvent efects

In order to determine the resonance enhancement associated with the use of 211 nm radiation as an excitation source, we will compare the relative intensities of certain Raman bands in the Raman spectra of tS, BP and DPB obtained with 211 nm excitation with those obtained under offresonance conditions with 746 nm excitation. Fig. 2 presents the UV-visible absorption spectra of these molecules in acetonitrile at a concentration of 50 PM. Fig. 3 presents plots for tS in acetonitrile at excitation wavelengths of 211 and 746 nm. The band positions agree well with those reported in the literature with the exception of bands present at 1520, 1410, and 1335 cm-‘, which will be discussed later in this paper [36,37]. In particular, we note relative intensity increases in the 1594 and 1572 cm-’ bands in the 211 nm spectrum. These bands have been assigned to the 8a and 86 phenyl ring stretching modes, respectively, in the tS molecule [38]. Fig. 4 presents plots for DPB obtained at the same excitation wavelengths. We observe that bands present at 1182, 1220, 1601, and 1613 cm-’ in DPB are all preferentially enhanced with the 211 nm excitation. The bands at 1182 and 1220 cm-’ have been assigned to phenyl C-C-H bending motions, with the 1220 cm-’ band also containing contributions from the olefinic portions of the molecule [39]. The 1601 and 1613 cm-’ bands are assigned to phenyl C=C stretching vibrations mixed with small contributions from phenyl CCH bending motions [39]. The picture that emerges when we consider data from both tS and

J.D. Leonurd, Jr.,T.L. Gusra~~onlJournalofMolecular Struciure379 (1996) 109-120

1700

1600

1500

1400

WAVENUMBER Fig. 4. Comparison of Raman spectra diene in acetonitrile under resonance conditions (bottom).

1300

113

211 nm

211 nm

746 nm

746 nm

1200

1100

SHIFT (cm-‘) of 1,4-diphenyl-1,3-buta(top) and off-resonance

DPB is that the resonance Raman intensities indicate that the greatest change in the redistribution of electron density in the state near 211 nm occurs in the phenyl rings. BP has been found to become more planar in the S, excited state [40,41]. We have obtained 211 and 746 nm spectra of BP in acetonitrile in order to compare these results with those obtained for tS and DPB. Fig. 5 presents Raman spectra of biphenyl at these wavelengths. We observe from these plots that the 1585 and 1610 cm-’ bands that contain contributions from phenyl C=C stretching motions and small contributions from phenyl C-C-H bending are enhanced in the 211 nm spectra relative to the 746 nm spectra [42]. These bands compose a Fermi doublet [42]. We also note that the bands at 1188 and 1510 cm-’ are resonantly enhanced with 211 nm excitation. The 1188 cm-’ band has been attributed to phenyl C-CH bending motions [42]. The 1510 cm-’ band has been assigned to phenyl C=C stretching motions [42]. The resonance enhancement patterns that we observe for all three of these

,

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1400

1300

1200

1100

1000

WAVENUMBER

9'00

SHIFT (cm-l)

Fig. 5. Comparison of Raman spectra of biphenyl in acetonitrile under resonance (top) and off-resonance conditions (bottom).

molecules support the reasoning that the state accessed by 211 nm radiation has a large change in geometry on the phenyl rings. We will compare these data with those obtained from studies on the Si electronic states of tS and DPB later in this paper. In order to study the effect of solvent on the ground state potential energy surface of BP and DPB, we have calculated the differential Raman cross-sections of the 1630 cm-’ band of DPB and the 1610 cm-’ band of BP at excitation wavelengths of 211 and 746 nm. We have examined spectra in three different solvents: acetonitrile, ethanol, and cyclohexane. We chose the 1630 cm-’ band of DPB (which has been assigned to olefinic stretching motions with small contributions from olefin carbon-hydrogen bending modes [39]) because this band exhibits broadening in the Si excited state that has been attributed to the presence of a distribution of s-tram conformers in solution [23]. We chose to examine the 1610 cm-’ band of BP because this band has been used in other studies to determine the planarity of the molecule [32]. Therefore, both bands should relate

114 Table 1 Differential 746 nm

J.D. Leonard. Jr., T.L. Gu.~t~~sotl,‘Jounlul qf‘Moleculur Structure 379 11996) 109-120

cross-section

Differential Biphenyl

211 nm 746 nm

data for biphenyl

Raman

and 1.4-diphenyl-1,3-butadiene

in different solvents and at excitation

of 211 and

cross-section”

(1610cm-‘)

Diphenylbutadiene

Acetonitrile

Ethanol

Cyclohexane

7.2 (10.6) x lo-l6 1.3 (kO.7) x 10-j’

9.2 (f0.7) x 10-‘h 7.6 (51) x lo-”

8.3 (*0.5) 2.3 (50.9)

’ Data are presented

wavelengths

in units of cm’jmolecules

x 10mz6 x 10m3’

Acetonitrile

Ethanol

Cyclohexane

1.5 (10.5) x IO-*’ 1.1 (kO.1) x 1O-3’

4.0 (10.7) x lo-” 5.7 (1tO.4) x lOm32

3.7 (*0.6) 2.6 (10.2)

sr. Error values listed in parentheses

to the planarity of the excited state molecule. The results of our cross-section calculations are presented in Table 1. We observe that, for the values obtained from the 211 nm spectra, the crosssections for the 1610 cm-’ band of BP and the 1630 cm-’ band of DPB increase in the order acetonitrile, cyclohexane, ethanol. Cross-sections calculated from 746 nm excitation spectra show that the bands in both BP and DPB yield the highest values in acetonitrile. The dielectric constants for acetonitrile, ethanol, and cyclohexane are 37, 24, and 2.3, respectively [43]. We observe that the trend in the cross-section values for the 746 nm spectra follows that of polarity for the solvents used in this study. We note that the trends for the crosssections at the two excitation wavelengths are different. We attribute this to the fact that the different excited states accessed by these wavelengths are influenced by the solvents in different ways. Rice and Baronavski have recently observed an anomalously long lifetime for the St state of cis-

Vans-Stilbene

(1630cm-‘)

are 12~ from the fit.

stilbene (cS) in cyclohexane that they attribute to the specific way in which the solvent molecules interact with the CS molecule [44]. These specific solvent-solute interactions may be reflected in the trends that we observe in the cross-section values for BP and DPB. The off-resonance excitation at 746 nm reflects resonance enhancement from a “weighted average” of excited states that contribute to the Raman intensities, with a greater contribution from the lower lying excited states. The 211 nm excitation accesses a specific higher-lying electronic state directly. Our results would indicate that the change in the electron density between the ground state and the state probed by 211 nm excitation is affected less directly by solvent polarity in comparison with the change between the ground state and lower lying electronic states. Other solvent properties, such as solvent structure, appear to influence the magnitude of the displacement between the ground state and the state in the region of 211 nm.

cis-Stilbene

4a, 4b-Dihydrophenanthrene

Phenanthrene Fig. 6. Photochemical

reaction

x 10-l’ x 1O-32

scheme for rrans-stilbene.

J.D. Lemard.

Jr., T.L. Gusr~f~~or~iJ~~urnalqf’Molecular

Structure

379 (1996)

115

109-120

Ethanol

Acetonitrile

_J

1800

1400

1600

WAVENUMBER

1200 SHIFT (cm-‘)

Fig. 7.211 nm resonance Raman spectra of trans-stilbene in ethanol (top) and acetonitrile 1578 cm-’ band. Accumulation time for both spectra was 1600 s.

3.2. Photochemistry

of tS

As we noted in the previous section, the bands present at 1520, 1410, and 1335 cm-’ in the 211 nm spectrum shown in Fig. 3 are not assignable to tS [38]. These bands correspond to those reported by Schrader for the phenanthrene (PA) photoproduct [36]. It is known that tS undergoes facile photochemica1 conversion to cis-stilbene, which can then react to form 4a,4b_dihydrophenanthrene (DHP) [45]. DHP, in the presence of air and irradiation

1000

(bottom).

Spectra have been normalized

to the

at wavelengths below 310 nm, is irreversibly oxidized to PA [46]. This reaction sequence is shown in Fig. 6. Since we did not deaerate the sample solutions, it is reasonable to assume that some PA would be formed during the time scale of the experiment. A slight yellow color observed in the solutions after irradiation confirmed the presence of PA. We do not observe any bands that we can attribute to DHP [47] in these data because the concentration is low and there is evidently little, if any, resonance enhancement of DHP at 211 nm.

, 200

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Fig. 8. W-visible spectra of4,4’-dimethoxystilbene (solid line), trams-stilbene oxide (broken acetonitrile. Concentration for all compounds: 50 PM. The arrow marks the UV excitation

375

400

line), and triphenylethylene wavelength.

(dotted

line) in

116

J.D. Leonard, Jr.. T.L. Gustc~f~~o,~~Jou~nui qf MolerulorStructure 379 (19961 109-120 1

1900

1700

1300

1500

WAVENUMBER Fig. 9. 211 nm resonance

Raman

spectrum

of 4,4’-dimethoxystilbene

We note that the bands due to PA are probably observed at moderate intensity owing to resonance enhancement. We also find that the apparent yield of PA is solvent dependent. In Fig. 7, we compare the Raman spectra of tS in acetonitrile and ethanol. These spectra have been normalized to the 1597 cm-’ band of tS. It is clear that the relative intensity of the bands attributed to PA is greater in the spectrum of the ethanol solution as compared with that of the acetonitrile solution. (Note, for example, the intensity of the PA band at 1520 cm-’ .) This observation is supported by Myers and co-workers, who report that the quantum

Raman

spectrum

of trcms-stilbene

900

(cm-‘)

in acetonitrile (20 mM). Accumulation

yield for DHP production solvents [48,49].

time 1600 s

is lower in more polar

3.3. Raman spectra of tS analogues We have obtained UVRR spectra of 4,4’dimethoxystilbene (DMS), trans-stilbene oxide (TSO) and triphenylethylene (TPE) in acetonitrile in order to examine the effect on the Raman spectra of varying the substituents on the tS molecule. The UV-visible absorption spectra for these compounds in acetonitrile over the region 190-400 nm are presented in Fig. 8. To the best of our

WAVENUMBER Fig. 10. 211 nm resonance

SHIFT

1100

SHIFT

(cm-‘)

oxide in acetonitrile

(20 mM). Accumulation

time 1600 s

J.D. Leonard.Jr.. T.L. GusrafsonlJournalq~Molecular Smxture

1900

1700

1500

1300

379 11996) 109-120

1100

117

900

WAVENUMBER SHIFT (cm-‘) Fig.

11.211 nm resonance

Raman

spectrum

of triphenylethylene

knowledge, there are no vibrational analyses for these compounds. Therefore we will make preliminary band assignments based on the Raman spectrum of tS. We will concentrate our attention on the 1450-1650 cm-’ region of the Raman spectra. We note that, just as we observed for tS, DPB, and BP, the resonance Raman spectra of DMS, TSO, and TPE obtained with 211 nm excitation appear to be dominated by phenyl vibrations. The UVRR spectrum of DMS is shown in Fig. 9. We observe that the 8b mode, at 1572 cm-’ in tS, shifts 5 cm-’ to 1577 cm-’ in DMS and there is a shift of 13 cm-’ to 1607 cm-’ for the 8a mode (1594 cm-’ for tS). These results are anticipated because of the electron donating character of the methoxy groups. We present the 211 nm Raman spectrum of TSO in Fig. 10. Comparing this spectrum with that of tS in Fig. 3, we find that the 8b mode shifts to 1584 cm-’ for the TSO spectrum. This change of 12 cm -’ suggests that the potential energy distribution for this mode in TSO contains a larger contribution from the phenyl ring stretch than in tS. This observation is consistent with the loss of conjugation through the olefin region in TSO. The UVRR spectrum of TPE is presented in Fig. 11. For this compound, we observe a shift of 10 cm-’ to 1604 cm-’ for the 8a mode and no shift for the 8b mode as compared with band positions in the tS spectrum. We observe that the olefinic C=C stretch, present at 1639 cm-’ in tS, shifts to 1623 cm -’ in TPE. A similar shift was observed by Meic

in acetonitrile

(20 mM). Accumulation

time 1600 s

and Gusten for this band in the spectrum of the deuterated cq a’-tS molecule [38]. We attribute this shift to the increase in mass attached to one of the olefinic carbons.

I

1700

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1600

1500

1400

WAVENUMBER

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1200

1100

1000

SHIFT (cm-l)

Fig. 12. Comparison of So and S, Raman spectra of transstilbene in acetonitrile. The S,, spectrum was obtained with 211 nm excitation. The S, spectrum represents a time delay of 10 ps at a pump wavelength of 294 nm and a probe wavelength of 588 nm.

118

3.4. Comparison

J.D. Leonard, Jr., T.L. GustaJion~JournaI

of MolecularStructure

379 (1996) 109-120

of S, and S, Raman spectra

Fig. 12 presents a comparison of the So and S, Raman spectra of tS in acetonitrile. The Si spectra reported in this paper were obtained with the picosecond transient Raman instrumentation described waveelsewhere [ 16,18,50]. The photoexcitation length for the Si spectrum was 294 nm, with a probe wavelength of 588 nm at a delay of 10 ps following photoexcitation. In general, the bands in the S, state shift to lower frequency relative to the ground state. This is expected because, to a first approximation, excitation into the S, electronic state can be viewed as promotion of an electron into an anti-bonding orbital, causing a general weakening of the bonding framework. For example, in tS the olefinic C=C stretch at 1639 cm -’ in the ground state shifts to 1567 cm.-’ in the Si electronic state. In addition, the 8a phenyl mode of tS in the ground state shifts 53 cm-’ to 1544 cm-’ in the Si state. The assignment of the So and Si bands in tS has been analyzed in detail elsewhere [37]. The emphasis in this work is on the changes in the relative intensities of the So and the Si Raman bands when both are excited in the region of the same excited electronic state, S,, at 211 nm. We note that the relative intensity pattern in the S, spectrum is different from that observed in the ground state spectrum. In general, we observe that the resonance Raman spectrum of St, tS obtained with 211 nm excitation is dominated by strong intensities in vibrational bands associated with phenyl motion. And the resonance Raman spectrum of S, tS is dominated by vibrational bands associated with olefin character. For example, the olefin C=C stretch at 1567 cm-’ in the Si spectrum of tS demonstrates greater relative intensity than the phenyl C=C stretch at 1544 cm-i. We suggest two possible reasons for the change in the relative intensity patterns between the So and the S, state. As mentioned previously, one reason could be that simple parity considerations would suggest that the allowed transition between St, and “S,,” does not involve the same S, state as the transition between Si and “S,*“. Another reason is that the S, spectrum represents the molecule after the initial geometric distortion involved in the transition between St, and S,. The

9 i=

4

9

SO

D!

/ 1900

1700

1500

WAVENUMBER

1300

1100

SHIFT (cm-l)

Fig. 13. Comparison of So and S, Raman spectra of diphenylbutadiene in tetrahydrofuran. The S,, spectrum was obtained with 211 nm excitation. The S, spectrum represents a time delay of 20 ps at a pump wavelength of 303 nm and a probe wavelength of 650 nm.

relative intensities of the resonance Raman bands in the S, spectrum represent the geometrical distortions that occur between St and S,, as has been discussed previously [28]. In Fig. 13, we compare the UVRR spectrum of So DPB with the 20 ps transient Si spectrum of DPB (both in THF solution). DPB has been utilized as a probe molecule to determine the effect of solvent on the S, structure where there are two closely lying electronic states in the vicinity of S, [23,24,51]. The assignment of the vibrational bands of S, DPB are consistent with the presence of bands with both l’B, and 2’A, character [23]. The broad features present in the Si spectrum at ~1620 cm-’ and 1250 cm-’ have been attributed to vibrations in the olefinic region in the excited state; the breadth of the bands arises from the presence of a distribution of s-trans conformers in the excited state [23]. We observe that the 1601 cm-’ band in the So spectrum, corresponding to phenyl C=C stretching motions, shifts to 1573 cm-’ in the Si spectrum. Another band at 1182 cm-’ in the So

J.D. Leonard, Jr., T.L. GustafsonlJournal

spectrum, which has been assigned to phenyl CC-H bending motions, shifts to 1177 cm-’ in the S, spectrum. There is an interesting contrast between tS and DPB in the enhancement of the Raman bands in the So and S, states. The ground state resonance Raman spectra of tS and DPB obtained with 211 nm excitation are dominated by vibrational motions of the phenyl rings. However, in the Si spectra the olefin bands are most strongly enhanced in tS, but both the olefin and phenyl modes are enhanced for DPB. The difference is consistent with the interpretation that Si DPB involves the mixing of the 1 ‘B, and 2’A, states. The reduced symmetry of Si DPB provides additional opportunity for obtaining enhancement of the S, Raman bands.

4. Conclusions We have obtained UV resonance Raman spectra of trans-stilbene, 1,4-diphenyl- 1,3-butadiene, and biphenyl at an excitation wavelength of 211 nm and compared these spectra with those obtained at 746 nm under off-resonance conditions in order to examine the vibrational coordinates that exhibit resonance enhancement under 211 nm excitation. We have calculated the relative Raman cross-sections for the 1630 cm-’ band in DPB and the 1610 cm-’ band in BP. We have discovered that the cross-sections obtained from the 211 nm spectra do not demonstrate the same trend as those obtained from the 746 nm data. We attribute these different trends to differences in the polarities of the solvent and also to microscopic interactions between the solvent and solute molecules. We have presented preliminary vibrational analyses for three analogues of tS: 4,4’-dimethoxystilbene, trans-stilbene oxide, and triphenylethylene. We have also presented a comparison between the So and Si Raman spectra of tS and DPB.

Acknowledgements We gratefully acknowledge support by NSF (CHE-9108384) for instrumentation used in this research. We also acknowledge The Ohio State

qf‘Molecular Structure 379 (1996)

109-120

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University Center for Materials Research, the Dow Chemical Company, and NIH for partial support of this work. We thank C. Russell Hille and Craig Hemann for the use of their equipment to obtain the 746 nm Raman spectra. We dedicate this paper to the memory of Professor Issei Harada, who contributed greatly to the field of Raman spectroscopy.

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