J . Phys. Chem. 1993,97, 2609-2617
2609
Solvent-Solute Interactions Probed by Picosecond Transient Raman Spectroscopy: Band Assignments and Vibrational Dynamics of SItran~-4,4'-Diphenylstilbene Roger M. Butler,' Matthew A. Lynn, and Terry L. Gustafson' Department of Chemistry, The Ohio State University, I20 West 18th Avenue, Columbus, Ohio 43210-1 173 Received: October 7, 1992; In Final Form: December 17, I992
We present the ground- and excited-state Raman spectra of trans-4,4'-diphenylstilbene (DPS). We analyze the spectra on the basis of comparison with the Raman spectra of the ground- and excited-state and anion radical spectra of trans-stilbene and biphenyl. The excited-state Raman spectra of DPS in methylene chloride and dioxane exhibit mode-specific, solvent-dependent dynamics. Specifically, the intensities of several vibrational modes associated with the biphenyl portion of DPS are solvent dependent. We attribute the change in intensity to a variation in the Franck-Condon overlap between SIand S, caused by differences in the planarity of the biphenyl portion of DPS in the two solvents. The lower viscosity solvent, methylene chloride, results in a more planar SIstructure of DPS than does dioxane. However, the rate at which the SIgeometry achieves equilibrium is slower in methylene chloride than it is in dioxane. This result suggests that dielectric stabilization of SIDPS by the solvent, not viscosity, controls the conformational dynamics.
Introduction The investigation of solvent-solute interactions has received much attention over the past decade, both experimentally and theoretically. Much ofthis work has been reviewed.'" Ingeneral, the experimental methods involve the determination of the rates of chemical processes under differing solvent conditions. The rates of the reaction are then compared to bulk solvent properties, since the solvent is viewed from the perspective of statistical mechanics as "noninteracting". That is, the solvent molecules simply act as a heat bath and may be considered as a continuous viscous medium at liquid-phase densities. The primary spectroscopic tools to measure the rates of the reaction have been based on absorption and fluorescence spectroscopies. Recently, we have shown that picosecond transient Raman spectroscopy can provide new information concerning the microscopic interactions that occur between a reacting solute and its solvent environment.' Specifically,in the caseof trans-stilbene (ts), we compare the Raman spectra of SItS in acetonitrile and n-hexane. We observe mode-specific, solvent-dependent variations in the vibrational spectra. In acetonitrile, the vibrational bands in SItS associated with the phenyl portion of the molecule shift to higher energy and increase in bandwidth, relative to n-hexane, owing to the increased coupling of the excited state to the more polar solvent. In both solvents, the vibrational motions associated with the olefin portion of the molecule change peak position and bandwidth with delay, while the peak position and bandwidth of the phenyl modes in each solvent remain constant with delay. The change in peak position and bandwidth of the olefin modes depends on the excitation frequency used to excite the molecule. We attribute thesevariations, in part, tovibrational relaxation that occurs via resonance energy exchange involving low-frequency vibrations. Additional factors, likely attributable to conformational changes, appear to give rise to some of the spectral changes that we observe. These changes in the S Ispectra of tS (i.e., shifts in peak positions and changes in bandwidth with delay) have recently been confirmed by other worker^.^.^ Iwata and Hamaguchi have obtained the transient Raman spectra of SItS in n-heptane at one excitation wavelength using a transformlimited picosecond Raman apparatus.8 They observe a distinct difference in the dynamics of the peak position and bandwidth Author to whom correspondence should be addressed. Eli Lilly & Co., Clinton Laboratories, P.O.Box 99, Mail Drop C22K, Clinton. IN 478424099.
' Present address:
at very early times. They attribute their results both to intramolecular effects and to structural relaxation involving the motion of the surrounding solvent. There were several reasons why we chose tS as our probe molecule for studiesof solvent/solute interactions.' The dynamics of tS are well characterized in many solvents;'@'9the pump and probe wavelengths required for the transient Raman experiment are in readily accessible wavelength regions;2s27 and the assignment of the vibrational spectrum, in both the ground and excited states, is kn0wn.~*-33In addition, many substituted stilbenes are commerciallyavailable or readily synthesized. Using substituted stilbenes, we can change the physical structure without significantly altering the A* electronic properties, such as in the case of methyl deri~atives;3~ or we can change both the physical structure and the A* electronic properties, such as in the case of amino, nitro, and phenyl derivatives. For this study, we have chosen to investigate a diphenyl derivative of tS, trans-4,4'-diphenylstilbene (DPS). DPS has a longer fluorescence lifetime and higher fluorescence quantum yield than tS.35-37 The lifetime of DPS in dioxane is 1.1 ns,35 and the fluorescence quantum yield is ~ 0 . 5 The . ~ longer ~ lifetime of DPS, relative to tS, provides the opportunity to obtain higher quality transient Raman spectra at longer delays. The higher fluorescence quantum yield changes the relative contribution of conformation (Le., photoisomerization quantum yield) to the lifetime and structure of SIDPS. DPS is also structurally similar to biphenyl (BP). BP has been studied extensively as a model system for inter-ring coupling between two phenyl r i n g ~ . ~As B ~in~the case of tS, the groundstate vibrational spectra of BP and its isotopomers have been thoroughly s t ~ d i e d ; ~ ~and , ~ the ~ . ~Raman ' spectra of SI and T I BP and the anion and cation radicals of BP have been obtained.48-5' Studies in supersonic free jets and of the solid have shown that BPexists in a planar configurationin the ground state. Insolution, however, the dihedral angle is 42°.41 Supersonic jet studies by Takei et al. have shown that the SIstate is also planar, with an increased A character to the inter-ring CC u bond.40 Proutiere et al. have observed that the integrated intensity of the Y Bvibration can be used to determine the dihedral angle of BP in the ground state.52 In this work, we present the ground- and excited-state Raman spectra of DPS. We assign the ground- and excited-state spectra on the basis of comparisons to the ground- and excited-statespectra of tS and BP. We compare the SI Raman spectra of DPS in
0022-3654/93/2097-2609%04.00/0 0 1993 American Chemical Society
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2610 The Journal of Physical Chemistry, Vol. 97,No. 11, 1993
methylene chloride and dioxane. As we have previously reported for tS, we observe mode-specific, solvent-dependentdynamics in theSI vibrationalspectraofDPS. However,unlike tS, theprimary spectral changes involve changes in the relative intensities of the SIbands; there are slight shifts in the peak positions and no discernible changes in the bandwidths of the SI bands. We attribute the changes in relative intensities to the change in the Franck-Condon overlap between SI and S, as DPS goes from nonplanar to planar after photoexcitation to SI.The solvent dependence associated with the change in the relative intensities withdelay suggests thatdielectricstabilizationofS1DPScontrols the conformational dynamics.
I
> t v) z UI
Experimental Section We have previously described the megahertz amplified laser system that we use to obtain picosecond transient Raman ~pectra.7~2~~2~-27 Briefly, the second harmonic of a CW, modelocked Nd:YAG laser (Coherent, Inc., Model Antares 76s) pumps two independently tunable, synchronously pumped, cavity-dumped dye lasers (Coherent, Inc., Models 702-2 and 702-3, each with a Model 7220 cavity dumper) operating at a repetition rate of 1 MHz. One dye laser operates with Rhodamine 6G (Exciton Corp.) as the gain medium and DQOCI (Exciton Corp.) as the saturable absorber. The other dye laser operates with DCM (Exciton Corp.) as the gain medium and DTDCI (Exciton Corp.) as the saturable absorber. Recently, we have modified our dye lasers to incorporate self-stabilization, following the design of Cotter.53 We have found that self-stabilization reduces the timing jitter between the two dye lasers and it increases the peak power of the dye pulse, improving the frequency-doubling efficiency. The jitter between the two dye lasers is measured by cross correlation to be - 5 ps. The output of the R6G dye laser is amplified at megahertz repetition rates, using a six-pass doubleconfocal resonator geometry after the design of Fork et al.54The amplifier uses R6G as the gain medium and is pumped by -2.0 W from a cavity-dumped argon ion laser (Coherent, Inc., Model Innova 200 with Model 7208 cavity dumper). Using 3-ps pulses at a 1 MHz repetition rate, we obtain 200-400 n J / p u l ~ e . We ~~ frequency double the amplified pulses using a temperature-tuned ADA crystal (Inrad, Model 5-15) to obtain 20-40 nJ/pulse in the ultraviolet for the pump pulse. The output from the DCM dye laser (probe pulse) traverses an optical delay line (Compumotor F-L5A-P54). We adjust the polarizations of the visible andultraviolet beams with polarization rotators (Karl Lambrecht, Model MFRO2-10) to obtain magic angle (54.7O) relative polarization in order to avoid effects from rotational reorientat i ~ n The . ~ ~pump and probe beams are combined collinearly at a dichroic beam splitter. The two beams are focused onto a 1-cm quartz sample cell with a 5-cm lens. The sample cell is spun at -60 Hz in order to remove the photoexcited volume before the next pulse pair arrives. The beams impinge on the sample cell at an angle of ~ 4 and 5 the ~ backscattered Raman signal is collected. The Raman signal is detected with a single spectrograph (ISA, THR 640) equipped with a charge-coupled-deviceoptical multichannel analyzer (Photometrics, CC200 System with Thomson 576x384 chip). We align the overlap between the pump and probe by monitoring the transient absorption signal through the spinning sample cell. In order to minimize the formation of a long-lived photoproduct in DPS, we found it necessary to change sample during routine pauses in the experiment. Eachsample was exposed for a maximum of -0.5-2 min of continuous excitation. The presence of photoproduct was evidenced by an increasing fluorescence background in the Raman spectrum. Data collection times were 20 min at each delay and for each grating position. It was necessary to use three grating positions in order to collect the entire spectrum between 190 and 1700 cm-I. We collected the different time delays in random patterns to avoid biasing the
I-
E I
BP I
J
u 1.
I,A
1
WAVENUMBER SHIFT (cm.') Figure 1. Comparison of ground-state Raman spectra of trum-stilbene (tS, top), truns-4,4'-diphenylstilbene (DPS,middle), and biphenyl (BP, bottom); solid samples, 514.5-nm excitation.
peak positions for small changes in the laser wavelength or spectrograph calibration over time. CCD channel numbers were converted to wavenumber shifts by using the Raman spectra of benzene, CC14, N,N-dimethylformamide, and acetone as frequency standards. The transient Raman experiment is interfaced to an IBM AT compatible computer using software written in ASYST (Asyst Software Technologies). The spectra were analyzed using software written in ASYST57and using PeakFit (Jandel Scientific). The semiempirical molecular orbital calculations were performed using HyperChem (Autodesk) on an IBM AT compatible computer. Ground-state Raman spectra of DPS and biphenyl solids were obtained with a double monochromator (Spex, Model 1402) using 514.5-nm excitation. trans-4,4'-Diphenylstilbene was purchased from Lancaster (scintillation grade, 2039-68- 1) and was used without further purification. trans-Stilbene was purchased from Kodak (scintillation grade, 103-30-0) and was used without further purification. Biphenyl was purchased from Aldrich (99%, 92-52-4) and was used without further purification. For the transient Raman experiments, we prepared saturated solutions of DPS in methylene chloride (Mallinckrodt, spectrophotometricgrade, 7509-2) and dioxane (E. M. Science, scintillation grade, 123-91-1). The concentrations were determined by UV-vis absorption spectroscopy to be 1.5 mM in methylene chloride and 0.5 mM in dioxane. Results
We have obtained both the ground-state and excited-state Raman spectra of DPS. Because there does not exist a normalmode analysis for DPS, we analyze these data by comparing our results to the ground- and excited-state spectra of tS and BP, for which considerableanalyses exist. We compare the ground-state Raman spectra of the solid samples of tS, DPS, and BP over the range from 200 to 1700 cm-I in Figure 1. In Figure 2 we show the Raman spectrum of SIDPS in dioxane obtained at delays of -50, 0, 10, 20, 50, and 70 ps over the range from 1100 to 1700 cm-I. The pump and probe wavelengths for these data were 303
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WAVENUMBER SHIFT (cm-l) Figure 2. SItransient Raman spectra of rrans-4,4’-diphenylstilbenea t delays of -50.0, IO, 20, 50, and 70 ps over the region from 1 100 to 1700 cm-I . Pump, 303 nm; probe 650 nm; repetition rate, 1 MHz; accumulation time, 20 min.
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WAVENUMBER SHIFT (cm-l)
SIRaman spectra of trans-stilbene (tS) in n-hexane (top) and trans-4,4’-diphenylstilbene(DPS) in dioxane (bottom). tram-Stilbene: pump, 303 nm; probe, 577 nm. trans-4,4’-Diphenylstilbene:pump, 303 nm; Probe 650 nm. Repetition rate, 1 MHz; delay, 20 ps. Figure 4.
to imply that a direct one-to-one correspondence exists among all the vibrational bands listed in Table I. As we have previously observed for tS,’v2’ we find that the SI Raman spectrum of DPS exhibits mode-specific, solvent-dependent vibrational dynamics. In Figure 5 we show the SI spectra of DPS in CH2C12 (solid line) and dioxane (dashed line) over the range from 1100 to 1700 cm-1 at a delay of 20 ps. The pump and probe wavelengths were 303 and 650 nm, respectively, as before. The spectra have been scaled to the band a t 1185 cm-1. We note that the spectra exhibit band shifts and substantial variations in the relative intensities of the bands between the two solvents. For a given solvent, we also observe significant changes in the SIRaman spectrum with delay. In Figure 6 we compare the spectra of SIDPS in dioxane at 10 ps (solid line) and 70 ps (dashed line). The two spectra have been scaled to the band at 1495 cm-1 in order to compare the relative intensities of the bands. All the bands at all time delays in both solvents fit well to Lorentzian line shapes; attempts to fit to Gaussian line shapes gave unacceptable results. In Figure 7 we show a representative fit to five Lorentzians in the range from 1400 to 1700 cm-1 for DPS in dioxane at a delay of 20 ps. The data are represented by dots, the sum of the five peaks by a solid line, and the individual peaks by dashed lines.
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WAVENUMBER SHIFT (cm-l) Figure 3. Comparison of the SO(bottom) and SI(top) Raman spectra of trom-4,4’-diphenylstilbene.SOspectrum obtained of the solid using 514.4-nm excitation. The SIspectrum of DPS in dioxane represents the concatenation of three spectral regions acquired for 20 min each. Pump, 303 nm; probe, 650 nm; delay, 20 ps.
and 650 nm, respectively. We note that the intensities of the vibrational bands grow according to the cross correlation of the pump and probe pulses. The lifetime of SIDPS is 790 ps in dioxane and 900 ps in CH2C12, as measured by transient absorption. For this reason, the intensities of the bands in the 70-ps spectrum are almost as intense as the bands at the peak of the transient absorption. This is a particular advantage of DPS incomparison to tS, where theshort SIlifetimereduces thesignalto-noise of the SIspectrum at longer delays. In Figure 3 we compare the ground-state spectrum of solid DPS and the excitedstate spectrum of DPS in dioxane over the range from 200 to 1700 cm-’. We compare the SIspectrum of DPS in dioxane to the SIspectrum of tS in n-hexane, both a t 20 ps, in Figure 4. Table I summarizes the observed peak positions of SOand S I DPS. For comparison we also include the peak positions observed for SO,SI, and the radical anion of tS and BP, along with an approximate assignment for the bands (vide infra). We note that we have only included thoseSobands of DPS that are pertinent to the discussion of the SI bands. We also emphasize that, while the peak positions may correlate to some extent, we do not intend
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Discussion
In order to interpret the spectra obtained from picosecond transient Raman spectroscopy in terms of interactions between a reacting solute and its solvent environment, we must first understand the vibrational spectrum of the solute molecule. To the best of our knowledge, no one has reported an analysis of the vibrational spectrum of DPS. Therefore, we first present a preliminary assignment of the ground- and excited-state Raman bands in DPS. We base our assignments primarily on a comparison to the vibrational assignments for tS and BP in both the So and SI states. We then discuss the effects of solvent on the structure of the excited state of DPS by comparing the spectra obtained in CH2C12 and dioxane. Finally, we present an analysis of the vibrational dynamics of SIDPS. A. Vibrational Assignments. In Table I we summarize the positions of the ground- and excited-state vibrational bands. We
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TABLE I: Comparison of Ground-State and Excited-State Raman Bands for tran~-4,4‘-Diphenylstilbene, trans-Stilbene, and Biphenyl and the Anion Radical Raman Bands for trans-Stilbene and Biphenyl So“
141 154 204 321 392 405 642 737 874 995 1003 1041
1028
1188
1 I88
B Pd
DPSd
tSP
226 248
197 284
229 292 3 29
412 614 730 855 982 1000 1030 1122 1 I86 1221 1253 1302
642 737 868 999 1000 1036 I I64 1206
1278 1328 1449
1276 1329 1447 1493
1486 I555 1594 1632
anion radicalh
Slh
t S‘
DPSd
1435 1496 1547 1586 1603
1510 1590, 1607‘
1595 1639
BPI
tsn
BPh
327
313
621
624
845 979
848 978
721 1007 1078 1 I50
979 1017 1171
1181
1180 I I94
1242
1251
I201 1250 I326
1333 1422 1473
1555 1528 I567
1553
1493 1568 1587
approximate assignmentsr
CO-CO-C bend CO-C-C bend ring-ring str ring-ring str ring def ring def ring def ring def (trig) ring def (trig) C-C str; C-H bend C-H bend; C-C str C-H bend; CO-CO str; CO-C str C-H bend Co-H bend; C-C str C-H bend; ring-ring str C-H bend; C-C str C-H bend; C-C str C-H bend; C-C str C-C str; C-H bend; ring-ring str C-C str; C-H bend; ring-ring str C-C str; C-H bend; ring def C r C o str; COstr
Schneinder, S.; Dorr, F.; Oxman, Solid. In solution. For tS, ref 32;for BP, ref 47. This work. References 24 and 25./Reference 50. 8 Hub,W.; J. D.; Lewis, F. D. J . A m . Chem. SOC.1984, 106, 708. Hub,W.; Kluter, U.; Schneider, S.; Dorr, F.; Oxman, J. D.; Lewis, F. D. J. Phys. Chem. 1984, 88, 2308. Reference 48. ’ Fermi doublet. [
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WAVENUMBER SHIFT (cm.’) Figure 5. SI Raman spectra of trans-4,4’-diphenylstilbenein dioxane (solid line) and CH2C12 (dashed line) scaled to the 1185-cm-I band over the region from 1100 to 1700 cm-I. Pump, 303 nm; probe, 650 nm; repetition rate 1 MHz; delay, 20 ps.
WAVENUMBER SHIFT (cm-l) Figure 6. Comparison of St Raman spectra of trans-4,4’-diphenylstilbene in dioxane at delays of IO ps (solid line) and 70 ps (dashed line) over the region from 1400 to 1700 cm-I scaled to the peak at 1495 cm-I. Pump, 303 nm; probe, 650 nm; repetition rate, 1 MHz.
observe 23 bands in the DPS ground-state spectrum; only a portion of the observed bands are listed in Table I. We will focus our discussion on those vibrational modes that we believe to have ag symmetry, since, in general, we only observe ag modes in the excited-state spectra. We classify the vibrational bands of DPS into three general categories: “stilbene-like” modes, “biphenyllike” modes, and phenyl modes. The stilbene-like modes include those motions that couple to the olefin; the biphenyl-like modes include those bands that involve the coupling of the two adjacent phenyl rings; and the phenyl modes are those vibrations that are more localized on a single phenyl ring. We note that not all the DPS vibrational motions are able to be placed in one of these three categories, as one would expect. We also note that our classification scheme is an oversimplification, but it should provide a reasonable first pass at the assignments. More complete assignments will require a normal-mode analysis. It is also important toemphasize that the vibrational motions in the excitedstate spectra may have a substantially different potential energy
distribution compared to the ground state, even if the bands are dominated by the same vibrational motion. In the ground state, the stilbene-like modes of DPS include bands at 1632, 1449, 1188, 874, and 642 cm-I. In the excited state, the comparable bands for DPS are at 1603, 1437, 1186, 8 5 5 , and 614 cm-I. The 1632-cm-1 band in SODPS is analogous to the olefin double bond stretch at 1639 cm-I in SOtS. In the excited state, we assign the olefin double bond stretch in DPS to the band at 1603 cm-I (i.e., it acquires more single bondcharacter). In tS the olefin band shifts to an even greater extent on going from the ground to the excited state, from 1639 to 1567 cm-I. The lower double bond frequency in tS, relative to DPS, is consistent with the fact that the photoisomerization pathway dominates the decay of SI tS. The photoisomerization quantum yield in tS is -93%;10 in DPS, the photoisomerization quantum yield cannot be greater than -50%, since the fluorescence quantum yield is ~ 5 0 % The . ~ 1449-cm-’ ~ band in SODPS has a counterpart in SOtS that is at 1447 cm-I. The motion of this band in tS involves the phenyl C-H bend and C-C stretch.
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The Journal of Physical Chemistry, Vol. 97, No. 11, 1993 2613
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(cm-‘) Figure7. Representativefit to Lorentzian lineshapes for the SI transient Raman spectrum of trans-4,4’-diphenylstilbenein dioxane. Pump, 303 nm; probe, 650 nm; repetition rate, 1 MHz; delay, 20 ps. WAVENUMBER SHIFT
Analogous bands are observed in the excited state at 1437 and 1422 cm-I for DPS and tS, respectively. The band at 1188 cm-1 in the ground state for both DPS and tS and at 1 186 and 1 18 1 cm-l in the excited state for DPS and tS, respectively, contains a significant contribution from the phenyl-olefin stretch. The 874- and 642-cm-I bands in SODPS have comparable bands at 868 and 642 cm-I in SOtS. We assign these modes to ring deformation vibrations. These bands shift to lower energy in the excited state for both DPS and tS, occurring at 855 and 614 cm-1 in SIDPS and at 845 and 621 cm-I in SItS. The assignment of thestilbene-likevibrationalbands in the low-wavenumber region (100-350 cm-I) is not clear. There are four bands in SODPS at 321,204, 154, and 141 cm-I. In SOtS we observe two bands at 292 and 229 cm-I. We observe two bands at 248 and 226 cm-l in SI DPS and at 284 and 197 cm-I in SItS. We tentatively assign the excited-state bands at 248 and 226 cm-’ in SI DPS to the in-plane bending motions involving the olefin and the phenyl rings. It is unclear which of the observed ground-state bands correspond to these excited-state bands. Our analysis of the biphenyl-like vibrations is based on the work of Zerbi and Sardoni4’ for SOBP and the work of Kat0 et aL50forSIBP. Owing to the relatively small number ofvibrational modes observed in the SI spectrum of BP, we also use the anion radical spectrum of BP obtained by Takahashi and Maeda48to assist in the assignment of the SIspectrum of DPS. We have previously observed, in the case of tS, that the anion radical spectrum and the SIspectrum are very similar.24 This correspondencecan be seen in Table I by comparing the band positions of the SI and anion radical for tS and BP. We assign the groundstate bands at 1555, 1486, 1278, 1003, 737,405, and 392 cm-’ in DPS to biphenyl-like vibrations. In the excited state of DPS, bands at 1547,1496,1302,1000,730,and 412 cm-I are assigned to biphenyl-like motions. We assign the bands at 1555 cm-I in SODPS and at 1547 cm-’ in SI DPS to biphenyl-like modes based on the bands at 1555 and 1568 cm-I in the SI and anion radical spectra, respectively, of BP. We note that there is not a direct correspondence of the 1555-cm-1 band in SODPS to a band in SOBP. We assign this mode to motions involving phenyl C=C stretching and phenyl C-H bending, with a contribution from the inter-ring C C bond. The bands at 1486 cm-i in SODPS and at 1496 cm-1 in SI DPS correspond to bands at 1510 cm-I in So BP and at 1493 cm-’ in the anion radical spectrum of BP. The intensities of the bands at 1496 cm-I in SI DPS and at 1493 cm-l in the anion radical spectrum of BP are very strong. These modes also involve phenyl C=C stretching and phenyl C-H bending. Owing to the fact that this band increases in frequency upon excitation to SI, the band in DPS likely contains a contribution from the inter-ring CC bond (vide infra). The 1278-cm-I band in SO DPS can be assigned to C-H stretching and ring-ring
stretching, as observed for the 1276-cm-I band in SOBP. The band at 1302 cm-1in SI DPS is very intense. There is also a very intense band at 1326 cm-I in the anion radical spectrum of BP that Takahashi and Maeda have correlated with the band at 1287 cm-’ in SOBP (in T H F solution).48 In the same way, we assign the band at 1302 cm-l in SIDPS to motions involving C-H bending and C-C stretching, with a substantial contribution from the inter-ring C-C bond. We assign the band at 1003 cm-I in SODPS and at 1000 cm-I in SIDPS to the concerted trigonal ring deformation of the adjacent phenyl rings. The comparable bands occur at 1000 and 1007 cm-’ in SOand SI BP, respectively. The bands at 737 cm-I in SODPS and at 730 cm-l in SI DPS can be assigned to a ring deformation vibration. There is a comparable band at 737 cm-I in SOBP. Again, no comparable band is observed in the SIspectrum of BP, but there is a band at 721 cm-I in the spectrum of the anion radical of BP. We assign the bands at 405 and 392 cm-I in SODPS and the broad band at 412 cm-1 in SI DPS to ring-ring stretching vibrations. The comparable bands occur at 329, 313, and 327 cm-1 in the SO,SIand anion radical spectra, respectively, of BP. We note that the DPS bands occur at higher energy than BP in both the ground and excited states. There are three vibrational motions that are common to DPS, tS, and BP in both the ground and excited states. We classify these bands as phenyl motions. The bands in DPS are at 1594, 1041, and 995 cm-1 in the ground state and at 1586, 1030, and 982 cm-I in the excited state. The bands at 1594 cm-1in SoDPS and at 1586 cm-I in SIDPS can be assigned to phenyl C=C stretching, C-H bending, and ring deformation motions. Most mono- and disubstituted benzene molecules, including tS and BP, have a band at -1600 cm-I in the ground-state Raman spectrum, corresponding to this type of motion.58 This band shifts to 1528 cm-I in the excited-state spectrum of tS. As in the case of the olefinic C=C stretch, the change in the band position between the ground and excited state is greater for tS than it is for DPS. Although this band was not observed in theS, spectrum of BP, the anion radical spectrum of BP has a band at 1587 cm-I. The bands at 1041 cm-I in SODPS and at 1030 cm-l in SI DPS can be assigned to phenyl C-C stretching and C-H bending motions. Comparable bands are seen as 1028 and 1036 cm-I in the ground state for tS and BP, respectively. The bands at 995 cm-I in SODPS and at 982 cm-1 in SI DPS can be assigned to the trigonal ring deformation of the phenyl ring. Comparable bands are observed at 999 cm-I in SOtS and at 979 cm-I in SI tS. The ground-state spectrum of BP has a band at 1000 cm-I (vide ante) and the anion radical spectrum of BP shows a band at 979 cm-I. On the basis of our preliminary assignments for the vibrational spectra of SOand SIDPS, we can evaluate the types of changes that occur upon photoexcitation of DPS. In general, the bands in SI DPS shift to lower frequency, relative to the ground state. This is expected, to a first approximation, because an electron has been promoted to an antibonding orbital. Therefore, the bonds will tend to weaken in the excited state. There are, however, three vibrational bands that are exceptions. The bands at 1486, 1278, and 392/405 cm-1 in So DPS shift to higher energy (1496, 1302, and 4 12 cm-I, respectively) in the excited state. We suggest that these bands involve the inter-ring CC bond. As has been observed for BP,JOexcitationof DPS to theS1 statelikelyincreases the A character of the inter-ring CC bond. Those vibrational motions that include a contribution from the inter-ring CC bond would tend to increase in frequency upon photoexcitation. We have performed an AM1 level semiempirical molecular orbital calculation with configuration interaction in order to compare with our data. In Figure 8 we show the results of the calculation for the HOMO and LUMO of DPS, where the contours represent the square of the orbital amplitude. The calculations show an increase in the orbital amplitude in the region of the inter-ring CC bonds and in the region of the olefin-phenyl
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2614 The Journal of Physical Chemistry, Vol. 97, No. 11, 1993
LUMO
positions are at higher energy and the bandwidths are greater for vibrational bands of SI DPS in dioxane, relative to CH2C12. The viscosities and dielectric constants are 0.45 and 1.44 CP(1 5 "C) and 9.08 and 2.209 for CH2C12 and dioxane, re~pectively.5~These results suggest that the more polar solvent, CH2C12,lowers the energy of the SIvibrational modes by stabilizing the intramolecular charge distribution in SI DPS. The generally greater bandwidths in dioxane would appear to arise from the increase in the density of states in dioxane, relative to CH2C12. That is, the vibrational bands tend to dephase more quickly in dioxane than they do in CH2C12owing to a greater probability of coupling between the SI vibrational states of DPS and the solvent bath modes. The greater viscosity of dioxane, relative to CH2C12, may also be a factor. Additional studies in other solvents will help to clarify this issue. We also note that the bandwidth of the phenyl C=C stretching mode in SI tS was quite broad (e.g., >33 cm-I), particularly in polar solvents.' Only the band at 1588 cm-I in SIDPS in CH2C12 appears to be significantly broadened, relative to the other vibrational bands (Table 11). We have also assigned this band to the phenyl C=C stretching mode. We attribute the increase in bandwidth for the phenyl mode to an increase in the charge localization on the rings caused by the more polar solvent. The more polar solvent will tend to stabilize the charge density on the phenyl rings, increasing the coupling to the solvent. The stronger coupling to the solvent will result in faster dephasing of the vibrational band (Le., greater bandwidth) . The analysis of the changes in the relative peak intensities with different solvents is more complicated. The intensities of the vibrational bands that are observed in the SI state are derived from resonance enhancement with the S, excited state. Typically, the intensities represent the change in the electronic structure that occurs when the molecule undergoes the transition from SI to S,; those vibrational modes that involve coordinates where the electronic displacement is greatest will have larger intensities in the resonance Raman spectrum. Presumably, the two electronic states, SIand S,, are the same in each solvent. Therefore, the intensity change between the two solvents is likely due to a change in the Franck-Condon overlap between SIand S,. A change in the Franck-Condon overlap could occur if the conformation of the molecule is different in the two solvents. In order to compare the relative intensities of the bands in the SIspectra of DPS, we scaled the intensities to the band at -1185 cm-I. We have assigned the 1185-cm-1 band to a stilbene-like vibration (vide ante). In our previous study, the comparable band in tS did not exhibit any change in relative intensity with delay or solvent.' The band in SI DPS at 1185 cm-I in each solvent does not change its peak position or bandwidth significantly with delay (vide infra). We note that the choice of which band to scale the intensities to is somewhat arbitrary; however, the choice of a stilbene-like vibration provides a self-consistent picture of the changes that we observe. The two bands that exhibit the largest change in intensity between the two solvents, relative to the band at -1 185 cm-I, areat -1301 and -1545cm-1 (TableII). Both of these bands have been assigned to biphenyl-like vibrations (Table I). We suggest that the change in the relative intensities between the solvents arises from a difference in the conformation of DPS in CH2C12 and dioxane. Specifically, since the greatest changes in the relative intensities involve biphenyl-like vibrations, we propose that the intensity changes arise from differences in the degree of planarity in the biphenyl portion of DPS between the two solvents in the S Istate. The planarity of DPS in the SI state affects the Franck-Condon overlap with the S, state from which the resonance enhancement is occurring. Changes in the Franck-Condon overlap will give rise to variations in the relative intensities of the bands observed in the SI resonance Raman spectra. There is precedent in the work of Proutiere et al. for correlating the intensity of So Raman bands, specifically the Yg
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I
HOMO Figure 8. H O M O and L U M O molecular orbitals for rruns-4,4'diphenylstilbene. AM1 level semiempirical calculation with configuration interaction: contours represent the square of the orbital amplitude.
TABLE 11: Comparison of the Peak Positions, Bandwidths, and Relative Amplitudes of Selected Raman Bands in SI truL44,4'-Diphenylstilbene in CHzC12 and Dioxane at a Delay of 20 ps CH?CI?
dioxane
peak position,' bandwidth* re1 (cm I) amplitude, (cm I )
peak position" bandwidthh re1 (cm I ) (cm I) amplitude,
1 I83 f 1 I215 f 1 1251 f 1
1301 f 1 1438 f 1 I493 f 1 1544 f 1 1588 f 5 1603 f 1
14.8 f 1.0 1.00 f 0.06 1186 f 1 20.5 f 3.1 1221 f 1 1253 f 1 26.5 f 3.7 14.1 f 0.5 2.02 f 0.04 1302 f 1 14.5 f 4.0 1435 f 2 14.8 f 0.6 1.62 f 0.05 1496 f I 17.5 f 0.8 1.40 f 0.05 1547 f 1 36.9 f 7.6 0.50 f 0.36 I586 f I 16.7 f 3.9 0.87 f 0.25 1603 f 1
18.4 f 0.8 1.00 f 0.03 23.1 f 4.2 22.3 f 3.3 17.1 f 0.4 1.67 f 0.03 23.0 f 6.4 18.2 f 0.4 1.6 I f 0.03 18.8 f 0.7 1.19f 0.03 22.4 f 3.2 0.53 f 0.12 23.8 f 1.8 0.88 k 0.08
Theuncertaintyin thepeakpositionisfl o r f 2 u i n thefit,whichever is greater. The uncertainty in the bandwidth is f 2 u in the fit. The amplitudes are scaled to the band at I185 cm-I in each solvent. The uncertainty is f 2 u in the fit. (I
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CC bonds. These results support the hypothesis that there is increased r character in the inter-ring CC bond in SI DPS. These calculations also would suggest that DPS is more planar in the excited state than it is in the ground state, just as occurs in BP.40 It is important to emphasize that the assignment of the vibrational spectra of DPS in the ground and excited states, as discussed above, is only preliminary. A more complete analysis will require the generation of an appropriate force field and a normal-coordinate analysis for DPS in the ground and excited states. However, we believe that the major spectral features observed in the ground- and excited-state Raman spectra of DPS can be reasonably assigned by comparison to the spectra obtained for the ground and excited states of tS and BP, as presented above. B. Solvent Dependence. TheSl Raman spectrumof DPSshows significant differences between dioxane and methylene chloride (Figure5). InTableIIwecompare thepeakpositions, bandwidths and relative amplitudes of selected bands in SIDPS obtained in CH2Cl2 and dioxane at a delay of 20 ps. There are several trends that are apparent in the data in Table 11. In general, the peak
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SolventSolute Interactions
The Journal of Physical Chemistry, Vol. 97, No. 11, 1993 2615
1549
5
v
1548
z
0
‘v) 11
1547
2 Y
a
W
1546
n
U
A
1545
I 0.00
0
10
20
30
40
50
60
70
DELAY (ps) Figure9. Changeinpeakpositionasafunctionofdelayfor the 1546-cm-’ band of rrons-4,4’-diphenylstilbenein dioxane; the uncertainty corresponds to f 2 0 in the fit. Pump, 303 nm; probe, 650 nm; repetition rate, 1 MHz.
band at 1607/1590 cm-I, to the planarity of BP52they observed that more planar biphenyl derivatives had a greater intensity in the Yg band. For these reasons, we attribute the changes in the relative intensities with solvent to changes in planarity of the biphenyl portion of DPS, with DPS in CH2C12 being more planar than DPS indioxane (Le., thegreater relative intensity corresponds to the more planar conformation). We have insufficient data a t this point to quantify the planarity of DPS in these experiments. However, a qualitative picture, involving the relative planarity of SIDPS in the two solvents, is consistent with our interpretation of the vibrational spectrum and the vibrational dynamics. C. Vibrational Dynamics. As we have observed for SItS, the S I Raman spectrum of DPS exhibits mode-specific, solventdependent vibrational dynamics. In tS the primary changes that we observed were changes in the peak positions and bandwidths with delaye7 We observed no changes in the relative intensities of thevibrational bands in S ItS with delay. However, the primary change with delay that we observe in SIDPS is in the relative intensities of the bands. We do observe small changes in the peak positions for some bands with delay. In Figure 9 we show the peak position of the 1546-cm-l band of SIDPS in dioxane as a function of delay; the uncertainty indicated on the graph is f 2 u in the fit to the data. We note that the overall change in the peak position between 0 and 70 ps is -2 cm-I. The data in Figure 9 are representative of the magnitude of the change in the peak positions that we observe in SI DPS in both CHzCl2 and dioxane: of the bands that change peak position with delay, none varies by more than -2 cm-I over the range from 0 to 70 ps. At comparable excess vibrational energies (-3500 cm-I) in tS, we observed changes of -8 cm-I in the peak positions of certain vibrational modes over the same delay range. Within experimental uncertainty, we observe no change in the bandwidth with delay for any vibrational mode in SIDPS. Again, thisisincontrast towhatweobserveinS, tS,where thebandwidths of several vibrational bands narrow with increasing delay.7 We attribute the changes in peak position and bandwidth with delay in SItS, in part, to intramolecular vibrational relaxation and to conformational changes in the SI state. In this work, we are exciting DPS in the SIstate with >3500 cm-1 of excess vibrational energy, comparable to the excess vibrational energy in SI tS that gave the greatest changes in the Raman spectra. One possible reason for the difference between tS and DPS may be the fact that DPS has more vibrational degrees of freedom and the effective temperature of the low-frequency vibrations that couple to the solvent may be reduced since the same energy is distributed among more vibrational modes. The difference may also arise from the fact that the quantum yield for photoisomerization is different between tS and DPS. Therefore, the changes in the SI Raman spectrum of tS with delay may include a substantial contribution
‘
0
10
20
30
40
50
60
70
DELAY (ps) Figure 10. Relative peak intensity, scaled to the 1 186-cm-1 band, a s a function of delay for rrons-4,4’-diphenylstilbenein dioxane; the uncertainty corresponds to i 2 a in the fit: 1185-cm-I band ( 0 ) ;1496-cm-1 band (A); 1547-cm-l band (+); 1586-cm-I band (m); pump, 303 nm; probe, 650 nm; repetition rate, 1 MHz. 2.50
i W
a
0.50
0.00
I
1 ‘
I
1 0
10
20
30
40
50
60
70
DELAY (ps) Figure 11. Relative peak intensity, scaled to the 1183-cm-I band, as a function of delay for trans-4,4’-diphenylstilbenein CH2C12;the uncertainty corresponds to f 2 a in the fit: 1183-cm-I band ( 0 ) ;1301-cm-I band (W); 1493-cm-I band (A);1544-cm-I band (+); pump, 303 nm; probe, 650 nm; repetition rate, 1 MHz.
from changes in the conformation of tS associated with the isomerization The conformational contribution of the isomerization process would be less in DPS owing to the reduced photoisomerization quantum yield. The changes in the relative intensities of selected bands of SI DPS with delay in dioxane and CH2C12 are shown in Figures 10 and 11, respectively. The uncertainties shown in the figures represent f 2 0 in the fit; if no vertical markings are visible, the uncertainty is less than the size of the data marker. As we discussed in the previous section, we scaled the intensities to the band at 1185 cm-l in each solvent. For dioxane (Figure lo), we show the changes in the relative intensities of the bands at 1496and 1547 cm-1(biphenyl-like modes) and 1586cm-1(phenyl mode), in addition to the 1186-cm-1 band (stilbene-like mode). The intensity of the biphenyl-like band at 1302 cm-l in dioxane also chapges intensity with delay; the data are not shown in Figure 10 because they are nearly coincident with the data for the 1496-cm-1band. For CH2C12(Figure 1l), we show the changes in the relative intensities only for the biphenyl-like bands at 1301, 1493, and 1544 cm-1, in addition to the stilbene-like band at 1183 cm-I. Within experimental uncertainty, only the vibrational motions associated with the biphenyl-like modes change in relative intensity with delay in both solvents. Although the same vibrational modes change relative intensity with delay in both dioxane and CH2C12, the dynamics of the change between the solvents are significantly different. For
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2616 The Journal of Physical Chemistry, Vol. 97, No. I I, I993 dioxane the changein the relative intensity is essentiallycomplete after -20 ps. However, in CH2C12 the change in the relative intensity is still not complete after 70 ps. We note that in our discussion of the solvent dependence of the intensities only the bands at 1301 and 1545 cm-I showed substantial differences between dioxane and CH2C12 (Table 11). We would have predicted that the band at 1495 cm-1 would have also shown an increasein relativeintensity sinceit is a biphenyl-likevibration. As can be seen in Figures 10 and 11, the band at -1495 cm-1 is, in fact, more intense in CHzCl2 than it is in dioxane, but it takes it longer to evidence this. Fortuitously, at a delay of 20 ps, the band is approximately the same intensity in both solvents (TableII). However, at 70 ps the relative intensity is 1.98 f 0.05 in CHZC12 and 1.69 f 0.03 in dioxane. The relative intensity of the band at 1495 cm-I at 70 psis consistent with our discussion in the previous section of the solvent dependence of the relative intensities of the bands at -1301 and -1545 cm-I. On the basis of our interpretation of the solvent dependence (it-., the bands become more intense as DPS becomes more planar), the dynamics suggest that it takes longer for SIDPS to reach its equilibrium structure in CH2C12 than it does in dioxane, and the final structure in CH2Clz is more planar than it is in dioxane. Thedynamical result is somewhat counterintuitivesince dioxane is more viscous than CH2Cl2 (1.44 vs 0.45 CPat 15 "C). Assuming that thechange in intensity is monitoring thedynamics of the biphenyl rings becoming more planar, it would be natural to anticipate that thesolvent viscosity would control the dynamics. Our results suggest that dielectric stabilization of SIDPS by the solvent, not viscosity, controls the conformational dynamics. The concept of dielectric friction has received much attention.14 However, it is normally applied to electron-transfer reactions and to charged species undergoing reactions in solution. In this case, we have a neutral molecule undergoing a very simple chemical reaction: two phenyl rings rotating relative to each other. Yet, stabilization of the electron density by the solvent appears to control the rate for the SI conformation to reach equilibrium. Solvent viscosity may control the equilibrium geometry (Le., degree of planarity), but dielectric friction appears tocontrol the rate at whichconformationalequilibriumisachieved. We note that we do not consider conformational changes in the olefin portion of the molecule to be contributing to the observed changes in the relative intensities since the relative intensities of the vibrational bands we assign to the olefin do not appear to be changing. An alternative explanation for the changes in the relative intensities of the biphenyl-like vibrations with delay could involve specific structural interactions of the solvent with the solute. Recently, Rice and Baronazski observed an anomalously long lifetime for SIcis-stilbene in cyclohexane.60They attributed this to the structure of the solvent and the specific way the solvent interacts microscopically with the solute. The exact type of microscopic interaction is not clear. Dioxane and cyclohexane are structurally similar and may give rise to comparable specific interactions. More work remains to be done to clarify this issue.
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Conclusions We have presented an analysis of the ground- and excitedstate Raman spectra of DPS based on a comparison with the known spectra of tS and BP. We have observed mode-specific, solvent-dependent changes in the SIspectrum of DPS. Unlike SItS, where we observed changes in the peak positions and bandwidths,' the greatest change in the SIspectrum of DPS involves changes in the relative intensities of the bands, specifically in those bands that contain a significant contribution from the inter-ring CC bond. We attribute the changes in the relative intensities to variations in the degree of planarity of SIDPS in CH2C12and dioxane. We suggest that dielectric stabilization of SIDPS by the solvent, not viscosity, controls the conformational
Butler et al. dynamics. The lower viscosity of CH2C12, relative to dioxane, results in a more planar SI geometry, but the rate of conformational equilibrium appears to be controlled by the dielectric properties of the solvent. We have recently obtained the timeresolved Raman excitation profiles of DPS6I The changes in the relative intensities with probe wavelength are consistent with our interpretation involving the Franck-Condon overlap between S I and S,. More work remains to be done. A normal-mode analysis of So and SI DPS would be very useful. However, in order for a normal-mode analysis to be successful,ground- and excited-state spectra of several isotopomers of DPS need to be obtained. The vibrational analysis would benefit from supersonicjet spectra of DPS. Jet spectra would also provide insight into the planarity of DPS in the gas phase in the ground and excited states. Picosecond transient Raman studies of DPS in different solvents would assist in sorting out the relative contributions of viscosity, dielectric constant, and solvent structure to the observed dynamics. In addition, our understanding of the solvation dynamics in DPS would benefit from comparable studies of simpler molecules, such as biphenyl and terphenyl. Some of these studies are in progress. It is important to reemphasize the wealth of information about solvation dynamics that is available from picosecond transient Raman spectroscopy. Whereas transient absorption spectroscopy and time-resolved fluorescence spectroscopy primarily give information about solvation dynamics via rates of reactions, timeresolved vibrational spectroscopy provides microscopic details above solvent-solute interactions through mode-specific changes in multiple parameters: peak positions, bandwidths, and intensities. Although the interpretation of the transient vibrational spectra is presently not as unequivocal, the method will ultimately provide a wider range of correlations among the various solvent properties as a larger data base becomes available. Acknowledgment. We thank Paul F. Barbara for helpful discussions. We acknowledge Jane Rice for providing us with the excited-state absorption spectrum of DPS. We thank Mick Jacupca and Prabir Dutta for their assistance in obtaining the ground-state Raman spectra. We thank Bruce E. Bursten for providing us access to his copy of HyperChem. We acknowledge Coherent, Inc., for the loan of some of the equipment used in these experiments. We acknowledge the National Science Foundation for support of portions of the instrumentation used in this work under grant CHE-9108384. We also acknowledge The OhioState University for partial support of this work through the Seed Grant Program. References and Notes ( I ) Fleming, G. R. Chemical Applications of Ultrafast Spectroscopy; Oxford University Press: New York, 1986. (2) Maroncelli, M.; Maclnnis, J.; Fleming, G. R. Science 1989, 243, 1674.
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