J. Phys. Chem. 1996, 100, 11857-11862
11857
Picosecond Time-Resolved Fourier-Transform Raman Spectroscopy and Normal-Mode Analysis of the Ground State and Singlet Excited State of Anthracene† Gouri S. Jas, Chaozhi Wan,‡ Krzysztof Kuczera, and Carey K. Johnson* Department of Chemistry, The UniVersity of Kansas, Lawrence, Kansas 66045 ReceiVed: February 28, 1996; In Final Form: April 12, 1996X
Time-resolved Fourier-transform Raman spectra of the first singlet excited states of anthracene and deuterated anthracene have been measured with photoexcitation at 355 nm. Raman scattering was excited by 100-ps pulses at 1064 nm, resonant with the S3 r S1 transition. Continuous wave (CW) Fourier-transform Raman spectra were also measured for anthracene and anthracene-d10 in the ground state. Ab initio calculations were carried out at the HF/6-31G and HF/6-31G* levels for the ground state and at the CIS/6-31G and CIS/ 6-31G* levels for the excited state to generate a complete normal-mode analysis of both ground and excited states. Excellent agreement between the computational and experimental Raman frequencies is observed for anthracene and anthracene-d10 for both the ground and excited states after the computed frequencies were scaled by a single scaling factor of 0.9. In several cases, comparison with calculated frequencies allows previously ambiguous vibrational assignments to be clarified. Evidence of interaction of the excited state with the solvent is observed for alkanes, but not alcohols, in an enhanced Raman intensity of solvent C-H stretching modes.
Introduction The application of vibrational spectroscopies to the excited states of molecules has long been a goal of spectroscopists, who seek to open a window to the structures of excited states and the mechanisms of excited-state reactions. Pump-probe vibrational methods provide information about excited states that otherwise may not be readily accessible for several reasons. First, absorption spectra in solution typically lack vibrational resolution. Second, electronic transitions from the ground state probe the Franck-Condon accessible region rather than the equilibrium excited-state geometry, which in some cases differ significantly in conformation. Third, the lowest lying excited state may be only weakly allowed and masked by a nearby strongly allowed transition. These considerations have led to efforts to implement time-resolved resonance Raman spectroscopy to probe excited states.1-3 However, this approach has been limited to samples that are characterized either by low fluorescence quantum yields or by a wide spectral separation between fluorescence and the resonance Raman scattering. To broaden the applicability of this approach, we have recently developed the technique of picosecond time-resolved Fouriertransform (FT) Raman spectroscopy.4,5 This method takes advantage of the development of FT Raman spectroscopy,6 which utilizes Raman excitation in the near-infrared region and interferometric detection of the Raman scattering. For many years a major effort has been directed toward understanding the excited singlet-state structure and dynamics of polycyclic aromatic molecules. Numerous spectroscopic investigations of the excited state of anthracene have been carried out.7-13 Recently, theoretical studies have been reported with the goal of interpreting vibrational bands in the ground and excited states.14,15 However, so far the excited singlet-state Raman spectrum has not been reported for this system. We * Corresponding author: e-mail,
[email protected]. † Dedicated to Professor Robin Hochstrasser on the occasion of his 65th birthday. ‡ Current address: Department of Chemistry, California Institute of Technology, Pasadena, CA 91125. X Abstract published in AdVance ACS Abstracts, June 15, 1996.
S0022-3654(96)00607-7 CCC: $12.00
have applied picosecond time-resolved FT Raman spectroscopy to anthracene and deuterated anthracene to measure the excited singlet-state Raman spectrum. In this paper, we present the picosecond time-resolved FT Raman measurements of anthracene (anthracene-h10) and deuterated anthracene (anthracened10) in the solvents cyclohexane, hexane, ethanol, and 2-propanol. The CW FT Raman spectra of both species were also measured for comparison. A normal-mode analysis, including the potential energy distribution16 (PED) of the observed Raman bands for both anthracene and deuterated anthracene, is given. Methodology Experimental Details. A detailed description of the timeresolved Fourier-transform (FT) Raman system was published elsewhere,4,5 and only a brief account is provided here. Pump and probe pulses at 355 (35 µJ) and 1064 nm (175 µJ), respectively, were generated by a mode-locked, Q-switched Nd: YAG laser (Coherent Antares). Picosecond time-resolved FT Raman spectra of anthracene and deuterated anthracene in their singlet excited states were measured with a step-scan interferometer (Bio-Rad FTS 60A). The step rate of the interferometer was 100 Hz. The repetition rates of the pump and probe pulses were 1 and 2 kHz, respectively. The interferometer signal was processed by a lock-in amplifier referenced at 1 kHz to generate the difference interferogram induced by the pump pulses. The total collection time for a spectrum at each time delay was 6.5 h. The spectral resolution for step-scan measurements was 8 cm-1. CW FT Raman spectra were acquired with a diodepumped Nd:YAG laser (Laser Diode LDP/1000-8) and a resolution of 1.5 cm-1 for solid anthracene and 4 cm-1 for anthracene in benzene. Zone-refined anthracene and deuterated anthracene (Aldrich) were used without further purification. The sample was flowed continuously through a 0.5-mm-diameter nozzle to ensure that each laser pulse excited a fresh region of sample. The groundstate concentration of the sample was 10-12 mM. The excitedstate concentration generated by the excitation pulse was estimated to be 3-5 mM. © 1996 American Chemical Society
11858 J. Phys. Chem., Vol. 100, No. 29, 1996
Jas et al.
Figure 1. Ground-state CW Fourier-transform Raman spectra of the pure solvents (from bottom) cyclohexane, hexane, ethanol, and 2-propanol.
Figure 2. Ground-state CW Fourier-transform Raman spectra of anthracene-h10 (bottom) and anthracene-d10 (top). Spectra were recorded at 1.5 cm-1 resolution with samples in powder form.
Computations. Ab initio calculations were performed with the Gaussian 92 program17 in the 6-31G and 6-31G* basis sets with direct SCF and direct CIS options. A full geometry optimization was performed for the ground state (1A1g), assuming D2h symmetry, followed by calculation of the harmonic force constants and normal modes in Cartesian coordinates at the HF/ 6-31G and HF/6-31G* levels. The ground-state optimized geometry was used as a starting point for the excited-state calculations. For the first excited singlet state (1B2u), a full optimization and calculation of harmonic force constants and normal modes in Cartesian coordinates were performed by using the CI singles method (CIS/6-31G and CIS/6-31G* levels).18,19 The ab initio-optimized geometries and Cartesian force constants were utilized in the normal-mode analysis of the ground and excited states of anthracene and deuterated anthracene in internal coordinates with the program MOLVIB,20 which includes the characterization of normal modes through a PED analysis.16 A scaling factor of 0.9 was used in comparing ab initio-calculated frequencies with experimental frequencies. The calculations were done on an IBM R/6000-375 workstation. The total CPU time was 4 h for ground-state calculations and 11 and 30 days for excited-state calculations in the 6-31G and 6-31G* basis sets, respectively.
TABLE 1: Ground-State Raman Frequencies of Anthracene and Anthracene-d10
Results Ground-State FT Raman Spectra. The CW Fouriertransform Raman spectra of the pure solvents cyclohexane, hexane, ethanol, and isopropyl alcohol are shown in Figure 1. These spectra were measured to identify the solvent contributions to time-resolved spectra and were acquired in the rapidscan mode with a spectral resolution of 4 cm-1. The groundstate CW FT Raman spectra of anthracene and anthracene-d10 in powder form with 1.5 cm-1 resolution are shown in Figure 2. Observed frequencies are tabulated in Table 1. Anthracene in Cyclohexane and Hexane. The picosecond time-resolved step-scan FT Raman spectra of anthracene in cyclohexane for 10 different delay times are shown in Figure 3. The spectrum at a time delay of -200 ps (i.e., the probe pulses precede the pump pulses by 200 ps) contains no excitedstate Raman signal. Strong positive and negative bands appear in the spectrum obtained at zero time delay ((100 ps). At longer time delays, these signals diminish with the 4.6-ns excited-state lifetime (as determined by a fluorescence lifetime measurement21 on an identical sample) for anthracene in cyclohexane. This decay confirms that the measured Raman signals originate in the excited singlet state of the molecule.
anthracene-h0 ∆ν (cm-1) 243 285 353(?) 395 478 520 578 623 753 771 775
intensity 5.8 3.1 1.6 31.5 2.4 3.1 0.4 0.5 26.5 1.2 1.1
anthracene-d10 ∆ν (cm-1)
intensity
assignmenta
227 259 364 381 413 501 487 or 510(sh) 600 or 612 708
6.1 2.9 8.5 36.3 4.0 7.1 0.8, 1.8 0.7, 1.2 26.9
600 or 612 659 or 694 814 775 836b 885 845b
0.7, 1.2 0.4, 0.7 5.5 3.1 19.3 1.6 16.5
4b1g 6b2g 11b3g 12ag 3b1g 10b3g 5b2g 11ag 10ag 4b2g 2b1g 3b2g 9b3g 1b1g(?) 9ag 8b3g 8ag
916 0.4 956 1.2 1007 13.8 1103 1.1 1163 6.7 1170 2.3 1186 11.4 940 1260 6.4 1155 1274 0.9 1121(?) 1376 3.7 1230 1402 100.0 1386 1412 1.3 1481 17.4 1400 or 1417 1496 1.9 1504 1.8 1558 14.2 1530 1575 1.1 1539 or 1554 1628, 1633 0.8, 0.7 1612 3006 1.7 2254 3028 9.1 2256 3050 14.6 2265 3071 3.3 2274 3076 3.4 2285
2.2 4.6 0.5 1.1 100.0
7b3g 7ag 6b3g 5b3g 6ag
28.4, 10.3
5ag
21.7 8.8, 1.1 1.3 8.7 9.2 7.8 10.6 18.8
4ag 4b3g 3b3g 2b3g 3ag 2ag 1b3g 1ag
a Assignments are based largely on the calculated frequencies in refs 22 and 28-31. b These frequencies are taken from the FT Raman spectrum of anthracene in benzene, where these two bands are resolved.
The negative bands at 800, 1027, 1265, and 1443 cm-1 match the frequencies of the strong solvent bands in this region of the spectrum (see Figure 1). The intensity decrease in these bands is a result of excited-state absorption of the 1064-nm pulses following the excitation of anthracene at 355 nm. The intensities of these bands recover with the same lifetime as the decay of the excited-state Raman bands. An apparently anomalous feature in the time-resolved Raman spectra of anthracene in cyclohexane appears in the cyclohexane bands at 2851, 2897, and 2934 cm-1. Unlike the solvent bands at lower frequency
FT Raman Spectroscopy of Anthracene
Figure 3. Time-resolved Fourier-transform Raman spectra of anthracene in cyclohexane at time delays (from bottom) of -200, -100, -50, 0, 1000, 2000, 3000, 3500, 4000, and 5500 ps. The concentration of anthracene in cyclohexane was about 10 mM. The Fourier-transform interferometer was stepped at 100 steps/s. A total of 170 scans was summed at each time delay.
Figure 4. Time-resolved Fourier-transform Raman spectra of anthracene in hexane at time delays (from bottom) of -100, 0, 3500, and 5500 ps. The concentration of anthracene in hexane was about 10 mM. The interferometer was stepped at 100 steps/s. A total of 170 scans was summed at each time delay.
(800-1500 cm-1), these bands show an increase in intensity following excitation. This increase appears to be superimposed on the decreased solvent signal exhibited by lower frequency solvent bands. At time delays greater than 3.5 ns, the positive signal disappears. Anthracene in hexane yields time-resolved FT Raman spectra (Figure 4) similar to those in cyclohexane, with nearly identical S1 vibrational frequencies. As in cyclohexane, solvent bands appear as negative peaks with the exception of the C-H stretching modes (primarily at 2852 cm-1), which gain intensity following photoexcitation. As in cyclohexane, the solvent stretching bands become negative at longer time delays. Deuterated Anthracene in Cyclohexane. Picosecond timeresolved FT Raman spectra of deuterated anthracene in cyclohexane at six different time delays are shown in Figure 5. The characteristics of the solvent bands are similar to those for anthracene in cyclohexane in Figure 2. Anthracene in Ethanol and 2-Propanol. Time-resolved FT Raman spectra were also measured for anthracene in ethanol (Figure 6) and 2-propanol (Figure 7). The concentration of the sample in each solvent, the collection time, and the spectral resolution at each time delay were the same as those in cyclohexane. An intensity increase in the solvent C-H stretching region similar to that in cyclohexane and hexane was not observed in ethanol or 2-propanol, where the solvent modes in
J. Phys. Chem., Vol. 100, No. 29, 1996 11859
Figure 5. Time-resolved Fourier-transform Raman spectra of deuterated anthracene in cyclohexane at time delays (from bottom) of -200, -100, 0, 2000, 4000, and 5500 ps. The concentration of deuterated anthracene in cyclohexane was about 10 mM. The interferometer was stepped at 100 steps/s. A total of 170 scans was summed at each time delay.
Figure 6. Time-resolved Fourier-transform Raman spectra of anthracene in ethanol at time delays (from bottom) of -100, 0, and 4000 ps. The concentration of anthracene in ethanol was about 5 mM. The interferometer was stepped at 100 steps/s. A total of 170 scans was summed at each time delay.
Figure 7. Time-resolved Fourier-transform Raman spectra of anthracene in 2-propanol at time delays (from bottom) of -100, 0, and 4000 ps. The concentration of anthracene in isopropyl alcohol was about 5 mM. The interferometer was stepped at 100 steps/s. A total of 170 scans was summed at each time delay.
this region (2880, 2927, and 2966 cm-1) appear as negative bands. Normal-Mode Analysis for the Ground- and Excited-State Raman Bands. A normal-mode analysis with PED and the
11860 J. Phys. Chem., Vol. 100, No. 29, 1996 TABLE 2: CW FT Raman and ab Initio Frequencies (cm-1) of the S0 ag Vibrational Modes of Anthracene, C14H10 assignment
experimenta
HF/ 6-31G
HF/ 6-31G*
12 11 10 9 8 7 6
395 623 753 1007 1163 1260 1402
387 637 749 990 1187 1254 1418
380 626 741 978 1162 1235 1410
5 4
1481 1558
1489 1588
1480 1590
3 2 1
3028 3050 3076
3014 3021 3049
3017 3026 3048
a
PED(HF/6-31G*) sC-C(53) + dCCC(47) dCCC(95) sC-C(70) + dCCC(28) sC-C(82) + dCCH(17) dCCH(87) + sC-C(13) sC-C(77) + dCCH(22) sC-C(84) + dCCH(8) + dCCC(8) sC-C(44) + dCCH(54) sC-C(71) + dCCH(10) + dCCC(18) sC-H(100) sC-H(100) sC-H(99)
Jas et al. TABLE 5: Time-Resolved FT Raman and ab Initio Frequencies (cm-1) of the S1 ag Vibrational Modes of Perdeuterioanthracene, C14D10 assignment
experiment
CIS/ 6-31G
CIS/ 6-31G*
PED(CIS/6-31G*)
12 11 10 9 8 7 6 5 4
372a 567a 690a 826a 874, 882b 1131a 1375a 1433a 1462(?)a
370 590 684 842 856 1120 1336 1411 1519
364 569 675 825 844 1115 1304 1407 1519
sC-C(51) + dCCC(48) dCCC(95) sC-C(65) + dCCC(24) sC-C(14) + dCCH(86) dCCH(53) + sC-C(44) sC-C(77) + dCCH(20) sC-C(91) sC-C(78) + dCCH(18) sC-C(72) + dCCC(20)
a
This work. b Reference 27.
This work.
TABLE 3: Time-Resolved FT Raman and ab Initio Frequencies (cm-1) of the S1 ag Vibrational Modes of Anthracene, C14H10 assignment
experiment
CIS/ 6-31G
CIS/ 6-31G*
12 11 10 9 8 7 6
386a 587a 733a 1019b 1168b 1242a 1389a
384 612 725 1019 1175 1235 1344
377 590 715 1013 1145 1226 1315
5 4
1497a 1543a
1497 1558
1491 1550
a
PED(CIS/6-31G*) sC-C(51) + dCCC(49) dCCC(96) sC-C(74) + dCCC(24) sC-C(82) + dCCH(17) dCCH(73) + sC-C(27) sC-C(66) + dCCH(33) sC-C(83) + dCCH(11) + dCCC(6) sC-C(51) + dCCH(42) sC-C(62) + dCCH(21) + dCCC(17)
This work. b Reference 10.
TABLE 4: CW FT Raman and ab Initio Frequencies (cm-1) of the S0 ag Vibrational Modes of Perdeuterioanthracene, C14D10 assignment
experimenta
HF/ 6-31G
HF/ 6-31G*
12 11 10
378 612 or 600 708
373 612 706
366 591 696
835 851 1146 1373 1417 1560 2227 2233 2264
822 836 1134 1363 1410 1562 2229 2236 2263
9 8 7 6 5 4 3 2 1 a
837 845 1155 1386 1417 1539 2254 2274 2285
PED(HF/6-31G*) sC-C(53) + dCCC(45) dCCC(92) sC-C(59) + dCCC(29) + dCCH(12) sC-C(49) + dCCH(48) dCCH(84) + sC-C(14) sC-C(71) + dCCH(24) sC-C(78) + dCCH(16) sC-C(89) sC-C(76) + dCCC(17) sC-H(100) sC-H(100) sC-H(99)
This work.
observed and computed ground- and excited singlet-state frequencies for the totally symmetric modes of anthracene and deuterated anthracene are presented in Tables 2-5. The nuclear motion comprising each normal mode for both ground and excited singlet states is displayed schematically in Figure 8. A complete normal-mode analysis for all vibrations of anthraceneh10 and anthracene-d10 in the ground and excited (S1) states has been reported elsewhere.22 Discussion The first excited state, designated 1La (Platt notation), has symmetry.23 The 1Lb (1B2u) state is located within about
1B 1u
Figure 8. Schematic representation of the ground- and excited singletstate normal modes for anthracene, with harmonic force constants calculated in Gaussian 92.
2500 cm-1 of S1, and a 1B3g state (S3) is predicted to lie at about 8200 cm-1 above S1.24,25 Consequently, the 1064-nm probe pulse (9395 cm-1) is resonant with the S3 r S1 transition. Absorption of the probe pulses accounts for the decreased scattering observed from solvent bands after photoexcitation, since the probe intensity is reduced by excited-state absorption.
FT Raman Spectroscopy of Anthracene
J. Phys. Chem., Vol. 100, No. 29, 1996 11861
TABLE 6: Correspondence between Symmetries in the Mulliken and Pariser Conventions molecular plane
short axis (1La)
long axis (1Lb)
Mulliken
yz
z(1B1u)
y(1B2u)
Pariser
xy
y(1B2u)
x(1B3u)
vibrational modes planar: ag, b3g, b1u, b2u nonplanar: au, b3u, b1g, b2g planar: ag, b1g, b2u, b3u nonplanar: au, b1u, b2g, b3g
Accordingly, the decrease in Raman intensity of the solvent bands at 800, 1027, 1265, and 1443 cm-1 recovers with the anthracene excited-state lifetime. While excited-state Raman scattering is resonantly enhanced, ground-state Raman scattering is nonresonant. As a result, bleaching of ground-state anthracene modes was not observable. Excited-State Raman Spectrum of Anthracene. The modes observed in the excited-state FT Raman spectra can be assigned to totally symmetric vibrations. Vibrational frequencies in the first excited state of anthracene have been determined previously by fluorescence excitation spectroscopy of anthracene in mixed crystals,7,8 matrices,9,13,26 and a supersonic free jet expansion.10 The frequencies observed by time-resolved FT Raman spectroscopy (see Tables 3 and 5) are in good agreement with the results from mixed-crystal, matrix, and jet spectroscopy in most cases. A strong mode observed at 1242 cm-1 in the excited-state Raman spectrum was not observed in the jet fluorescence excitation spectrum10 or the mixed-crystal absorption spectrum.8 On the basis of the calculated frequency and PED (Table 3), this band can be assigned to mode 7ag and associated with the ground-state band at 1260 cm-1. Our ab initio calculations (1235 and 1226 cm-1) and a semiempirical calculation (1226 cm-1)15 support this assignment of the excitedstate experimental frequency. However, this assignment disagrees with the proposed assignment of Gruner et al. of mode 7ag to a band at 1291 cm-1 in the jet excitation spectrum15 and also disagrees with the reassignment by Zilberg et al.14 of mode 7ag to a band observed at 1168 cm-1 in the jet excitation spectrum. We assign the band at 1168 cm-1 in the jet excitation spectrum to mode 8ag. The assignment of the excited-state band at 1497 cm-1, which is confirmed by comparison of the PEDs and normal-mode displacements (see Figure 8) calculated for the ground and excited states, agrees with the reassignment of this band by Zilberg et al.14 to mode 5ag rather than with its assignment by Lambert et al.10 and Wolf and Hohlneicher13 to mode 4ag. We assign mode 4ag to a relatively weak band at 1543 cm-1. This assignment is supported by the frequencies calculated by us and Gruner et al.15 In the excited-state FT Raman spectra of anthracene-d10 (Table 5 and Figure 5), the band at 1131 cm-1 is assigned to mode 7ag. One member of a pair of bands at 874 and 882 cm-1 in the jet excitation spectrum of anthracene-d10 can be assigned to mode 8ag. We tentatively assign a weak band at 1462 cm-1 in the FT Raman spectrum of anthracene-d10 as mode 4ag, although this assignment may be questionable given the large difference (57 cm-1) between this value and the calculated frequency. Ground-State Raman Spectrum of Anthracene. Assignments of the frequencies observed in the CW FT Raman spectrum of the ground states of anthracene-h10 and anthracened10 are given in Table 1. A comparison with the ab initiocalculated frequencies is given in Table 2 and 4 for the totally symmetric models. Assignments were based on ab initio calculations22 and previous fluorescence measurements8-10,13 and normal-mode calculations.28-31 The anthracene-h10 (anthracened10) bands at 395 (381), 623 (612?), 753 (708), 1007 (836),
1163 (845), 1260 (1155), 1402 (1386), 1481 (1417), and 1558 cm-1 (1530 cm-1) can be assigned as ag vibrations. Another set of prominent bands can be assigned as planar b3g modes. These are observed for anthracene-h10 (anthracene-d10) at 353(?) (364), 520 (501), 916 (814), 1103 (885), 1186 (940), 1274 (1121(?)), 1376 (1230), 1575 (1539), and 1628 cm-1 (1612 cm-1). The ag and b3g assignments in Table 1 are, for the most part, in close agreement with the normal-mode calculations of Ohno31 and Jas and Kuczera.22 Many of these bands have been observed in the CW Raman spectra of anthracene.32,33 Some, however, were not assigned, notably the 9ag and 8ag modes at 835 and 845 cm-1 in anthracene-d10, respectively, which were not resolved in previously reported spectra. The location of mode 11b3g in anthracene-h10 is uncertain. This mode may correspond to a weak band at 353 cm-1 or may be coincident with 12ag mode at 395 cm-1. These alternatives have been discussed by Wolf and Hohlneicher.13 Several out-of-plane b1g and b2g fundamentals are also observed. Assignments for these bands in Table 1 have been based on normal-mode calculations.22 Normal-Mode Analysis and Comparison with Calculations. Computational vibrational frequencies for the ground states (S0) of anthracene and deuterated anthracene are found to be in excellent agreement with the experimental Raman frequencies. Our computational frequencies for the groundstate molecule are also in close agreement with Ohno’s valence force field calculations.31 The normal-mode analysis for the excited singlet-state Raman bands of anthracene and deuterated anthracene has been performed. Singlet excited-state (S1) computational vibrational frequencies for anthracene and deuterated anthracene are also in good agreement with the experimental Raman frequencies. A full account of the calculated excited- and ground-state structures and normal modes will appear elsewhere.22 The frequency shifts between the ground- and the singlet excited-state ag Raman bands of anthracene and deuterated anthracene range from 5 to 36 cm-1 and from 6 to 77 cm-1, respectively. This suggests that the structural change between the ground and the singlet excited states of the molecule is minor. Most of the totally symmetric modes shift to lower frequencies upon excitation from S0 to S1. The exceptions for anthracene-h10 are modes 9ag, 8ag, and 5ag. The ab initiocalculated frequencies correctly predict the direction of the frequency shift in all cases except mode 8ag. For anthracened10, the direction of the shift is predicted correctly for all modes except 9ag and 5ag. Modes 9ag and 8ag are predicted to shift up upon excitation to S1. Experimentally, one of these (mode 9ag if the assignment in Table 5 is correct) is observed to shift down, while the other shifts up. It can be observed from Figure 8 that, for most of the ag fundamentals, the ground-state normal modes are similar to their corresponding excited-state normal modes. The modes showing the greatest differences between the excited and ground states are modes 4ag, 5ag, and 6ag. The ground-state frequency predicted at 1410 cm-1 for mode 6ag is shifted to 1315 cm-1 in the excited state. This substantial frequency shift (74 cm-1) predicted for the excited state is reflected by the somewhat different directions of the displacements. However, the experimentally observed frequency shift is much smaller (13 cm-1), suggesting that the excited-state force field for this mode may require further refinement. Solvent Interaction in Cyclohexane and Hexane. A surprising feature of the time-resolved FT Raman spectra in Figures 3-5 is the enhanced scattering from solvent C-H stretching modes observed after excitation of anthracene in
11862 J. Phys. Chem., Vol. 100, No. 29, 1996 cyclohexane and hexane, but not in ethanol and 2-propanol. We have observed a similar phenomenon for 9,10-diphenylanthracene,5 where increased intensity is observed for the C-H stretching modes of cyclohexane (but not ethanol) following the photoexcitation of diphenylanthracene. We do not think that thermal effects can account for this phenomenon because (1) the enhanced scattering is observed only in the highfrequency C-H stretching modes and not in lower frequency vibrations and (2) observation of the enhanced scattering depends on the nature of the solvent. The increased intensity of these modes in cyclohexane and hexane may occur due to an interaction of the solvent with the excited state of anthracene that induces vibronic coupling (intensity borrowing). This interaction may not occur in alcohols due to the existence of stronger hydrogen-bonded solvent structures, which are not readily disrupted. Conclusions Time-resolved FT Raman spectroscopy has been successfully used to measure the totally symmetric excited singlet-state vibrational frequencies of anthracene. Raman excitation in the near-IR region allows the excited-state Raman scattering to be spectrally separated from fluorescence in the near-UV and blue regions, so that the excited-state Raman spectrum of anthracene can be detected in spite of the high fluorescence quantum efficiency. The normal-mode analyses for the excited singletstate Raman bands of anthracene and deuterated anthracene are also presented. Ground- and singlet excited-state ab initio computational vibrational frequencies are found to be in good agreement with the experimental Raman frequencies. These calculations have led us to assignments of several modes in the S1 state. We conclude that ab initio calculations of vibrational frequencies for anthracene can be used with a high degree of confidence for the interpretation of vibrational spectra. An intriguing enhancement in Raman scattering was observed in the C-H stretching modes of cyclohexane and hexane upon the excitation of anthracene. These bands may reflect the interaction of the excited state with the solvent. Acknowledgment. We thank Prof. Thomas Squier and Mr. Greg Hunter for the fluorescence lifetime measurements. This work was supported by NSF under EPSCoR Grant No. 9255223 and by University of Kansas General Research Allocation No. 3614. This work also received matching support from the State of Kansas EPSCoR program. The FT Raman instrumentation was funded by Grant CHE-9023773 from the NSF Chemical Instrumentation Program. References and Notes (1) Takahashi, H., Ed. Time-ResolVed Vibrational Spectroscopy VI; Springer: Berlin, 1991.
Jas et al. (2) Morris, D. L., Jr.; Gustafson, T. L. J. Phys. Chem. 1994, 98, 6725 and references cited therein. (3) Johnson, C. K.; Dalickas, G. A.; Payne, S. A.; Hochstrasser, R. M. Pure Appl. Chem. 1985, 57, 195. (4) Jas, G. S.; Wan, C. Z.; Johnson, C. K. Spectrochim. Acta 1994, 50, 1825. (5) Jas, G. S.; Wan, C. Z.; Johnson, C. K. Appl. Spectrosc. 1995, 48, 645. (6) Chase, D. B., Rabolt, J. F., Eds. Fourier Transform Raman Spectroscopy; Academic: San Diego, 1994. (7) (a) Bree, A.; Katagiri, S. J. Mol. Spectrosc. 1965, 17, 24. (b) Bree, A.; Katagiri, S.; Suart, S. R. J. Chem. Phys. 1966, 44, 1788. (8) Small, G. J. J. Chem. Phys. 1970, 52, 656. (9) Carter, T. P.; Gillespie, G. D. J. Phys. Chem. 1982, 86, 2691. (10) Lambert, W. R.; Felker, P. M.; Syage, J. A.; Zewail, A. H. J. Chem. Phys. 1984, 81, 2195. (11) Tripathi, G. N. R.; Fisher, M. R. Chem. Phys. Lett. 1984, 104, 297. (12) Bree, A.; Leyderman, A.; Taliani, C. J. Mol. Struct. 1986, 142, 151. (13) Wolf, J.; Hohlneicher, G. Chem. Phys. 1994, 181, 185. (14) (a) Zilberg, S.; Samuni, U.; Fraenkel, R.; Haas, Y. Chem. Phys. 1994, 186, 303. (b) Zilberg, S.; Haas, Y.; Shaik, S. J. Phys. Chem. 1995, 99, 16558. (15) Gruner, D.; Nguyen, A.; Brumer, P. J. Chem. Phys. 1994, 101, 10366. (16) (a) Keresztury, G.; Jalosovszky, G. J. Mol. Struct. 1971, 10, 304. (b) Aliz, A. J. P. Spectrosc. Lett. 1981, 14, 441. (17) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzales, C.; Martin, R. L.; Fox, D. S.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian 92, ReVision C; Gaussian Inc.: Pittsburgh, PA, 1992. (18) Foresman, J. B.; Head-Gordon, M.; Pople, J. A.; Frisch, M. J. Phys. Chem. 1992, 96, 135. (19) Foresman, J. B.; Schlegel, H. B. Recent Experimental and Computational AdVances; Fausto, R., Ed.; NATO-ASI Series C; Kluwer Academic: Dordrecht, The Netherlands, 1993; p 11. (20) Kuczera, K.; Kuczera-Wiorkiewicz, J. MOLVIB, a Program for Analysis of Molecular Vibrational Spectra, 1995. (21) Hunter, G.; Squier, T. C.; Jas, G. S. Unpublished results. (22) Jas, G. S.; Kuczera, K. Manuscript submitted. (23) We follow the axis convention of Mulliken, R. S. J. Chem. Phys. 1955, 23, 1997 where the molecule is in the yz plane, with the long axis along y and the short axis along z. The correspondence between symmetries in the Mulliken and Pariser conventions is shown in Table 6. (24) Dick, B.; Hohlneicher, G. Chem. Phys. Lett. 1981, 83, 615. (25) Salvi, P. R.; Marconi, G. J. Chem. Phys. 1986, 84, 2542. (26) Fraenkel, R.; Samuni, U.; Haas, Y.; Dick, B. Chem. Phys. Lett. 1993, 203, 523. (27) Peng, L. W.; Keelan, B. W.; Semmes, D. H.; Zewail, A. H. J. Phys. Chem. 1988, 92, 5540. (28) Krainov, E. P. Opt. Spectrosc. 1963, 16, 532. (29) Evans, D. J.; Scully, D. B. Spectrochim. Acta 1964, 20, 891. (30) Cyvin, B. N.; Cyvin, S. J. J. Phys. Chem. 1969, 73, 1430. (31) Ohno, K. J. Mol. Spectrosc. 1979, 77, 329. (32) Ra¨sa¨nen, J.; Stenman, F.; Penttinen, E. Spectrochim. Acta 1973, 29, 395. (33) Bree, A.; Kydd, R. A. Chem. Phys. Lett. 1969, 3, 357.
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