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Probing the molecular basis for solvent-induced dephasing using resonance Raman spectroscopy: betaine-30 in CH3OH, CH3OD, and CD3OD
Chemical Physics Letters 378 (2003) 582–588 www.elsevier.com/locate/cplett
Probing the molecular basis for solvent-induced dephasing using resonance Raman spectroscopy: betaine-30 in CH3OH, CH3OD, and CD3OD Xihua Zhao, Jeanne L. McHale
*
Department of Chemistry, University of Idaho, Moscow, ID 83844-2343, USA Received 2 June 2003; in final form 30 July 2003 Published online:
Abstract Absolute Raman cross-sections are reported for fourteen normal modes of betaine-30 dye in CH3 OH, CH3 OD, and CD3 OD solution, at five different excitation frequencies within the visible absorption band. Consistent with previous results from our lab, the Raman intensities of betaine-30 in CD3 OD solution are generally stronger than in CH3 OH solution. For most of the betaine-30 normal modes, however, the resonance Raman cross-sections are similar in CH3 OD and CH3 OH solution. The results suggest that methyl group motion, more so than hydroxyl group motion, is responsible for dephasing of the resonant electronic transition of betaine-30 in methanol. Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction Solution phase electron transfer and the nature of solvent–chromophore interactions are topics of much current interest in chemistry [1,2]. In our lab, we have employed resonance Raman spectroscopy to explore the influence of solvent dynamics on charge-transfer electronic transitions [3]. We have previously reported solvent isotope effects on resonance Raman intensities of betaine-30 in acetonitrile and methanol [4–6]. Resonance Raman intensities depend on the geometry change of the molecule upon electronic excitation and on the solvent-induced dephasing of the resonant elec*
Corresponding author. Fax: +1-208-885-6173. E-mail address: [email protected] (J.L. McHale).
tronic transition. Within the reasonable assumption that the ground and excited state geometries of a chromophore are independent of solvent isotopic substitution, resonance Raman cross-sections reveal differences in the solvent-induced dephasing in experiments done as a function of solvent isotopomer. Betaine-30 is a strongly solvatochromic dye with a much larger dipole moment in the ground than the excited electronic state, resulting in a dramatic blue shift of the lowest lying electronic transition with increasing solvent polarity [7]. Betaine-30 can also hydrogen bond with protic solvent molecules, leading to an additional blue shift in the absorption spectrum [8]. In the present paper, we examine the resonance Raman spectrum of betaine-30 in CH3 OH, CH3 OD, and CD3 OD so-
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0009-2614(03)01331-9
X. Zhao, J.L. McHale / Chemical Physics Letters 378 (2003) 582–588
lution in order to investigate the molecular basis for our previously reported solvent isotope effects on the Raman intensities of betaine-30 in methanol and perdeuterated methanol [4]. Raman intensities depend on solvent molecule motions which perturb the electronic transition frequency and thus cause dephasing of the resonant electronic transition. In the frequently observed inhomogeneous limit, an increase in the amplitude of such solvent motion results in a decrease in the resonance Raman intensity. In this earlier work, we found that for most of vibrational modes of betaine-30, the absolute Raman cross-sections in perdeuterated methanol are higher than in CH3 OH. This suggests that the amplitude of solvent-induced dephasing of the charge-transfer transition is larger in CH3 OH than in CD3 OD solution. The electronic absorption spectrum of the dye, on the other hand, is independent of solvent isotopic substitution. We consider this to be evidence of nonlinear solvent dynamics in the case of betaine-30 in methanol (and also in acetonitrile [5,6]). That is, the solvent dynamics depend on the electronic state of the chromophore. The electronic absorption spectrum depends on solvent motion coupled to the ground electronic state of the solute, while the resonance Raman spectrum sees the solvent response to the excited state of the dye and, due to the reversal of the betaine-30 dipole moment on excitation [9], this solvent motion initially proceeds from a highly nonequilibrium solvent configuration. Since the Raman intensities of most of the betaine-30 normal modes were found to be higher in CD3 OD than in CH3 OH, we speculated that hydrogenbond dynamics were playing a strong role in the solvent response, as the amplitude of the hydrogen bond motion (stretching or torsion) is expected to be smaller in the deuterated solvent. The purpose of the present work is to test that hypothesis by comparing the resonance Raman intensities of betaine-30 in CH3 OH and CH3 OD. Our previously reported study of the thermosolvatochromism of betaine-30 in methanol is also relevant to this work [10]. In the temperature-dependent absorption spectrum of betaine-30, a sharp isosbestic point was found in methanol solution, but not in acetonitrile or ethanol. We proposed that
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two forms of betaine-30 exist in equilibrium in methanol solution: betaine-30 hydrogen bonded to one or two methanol molecules. Most surprising however, is that temperature tuning of the solvent polarity did not obscure this isosbestic point. We concluded that the betaine-30 molecule is rather rigidly solvated by methanol molecules. In this Letter, we report for the first time resonance Raman cross-sections of normal modes of betaine-30 in CH3 OD in order to investigate the importance of hydrogen-bond dynamics in the dephasing which limits the Raman intensity. Also, resonance Raman spectra of betaine-30 in CH3 OH and CD3 OD were redetermined to compare the results more accurately. Consistent with our previous study, betaine-30 Raman intensities in CD3 OD are generally greater than those in CH3 OH. However, the absolute Raman crosssections in CH3 OH and CH3 OD are generally the same within the error of the measurement. The results suggest that, in contrast to our previous interpretation of the solvent isotope effect, the motion of the methyl group is more strongly coupled to the charge-transfer transition than the motion of the hydroxyl group.
2. Experiment Betaine-30 was purchased from Aldrich and purified using a previously reported procedure [6]. CH3 OH was used as received, while CH3 OD and CD3 OD were distilled before use to remove fluorescent impurities. Absorption spectra were recorded in 1 cm cells using a Shimadzu UV-2501 spectrometer. Resonance Raman spectra were obtained at excitation wavelengths of 457.9, 488.0, 514.5, 576.7, and 593.1 nm using sample concentrations of about 1.1 103 M and 90° scattering geometry. The excitation source was either a Coherent argon ion laser or a Coherent ring dye laser pumped by the argon ion laser. A SPEX 14018 double monochromator with two 1800 groove/mm gratings blazed for 500 nm was used to disperse the scattered light, and a thermoelectrically cooled photomultiplier tube was employed as the detector. A Kaiser Holographic bandpass filter was put in front of the sample in order to remove argon ion
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Table 1 Total Raman cross-sections rR of solvent bands in units of lbarn, (1 lbarn ¼ 1030 cm2 ) at five excitation wavelengths used in this studya
Cyclohexane (802 cm1 ) CH3 OH (1033 cm1 ) CD3 OD (979 cm1 ) CH3 OD (1033 cm1 ) a
458 nm
488 nm
514 nm
576.7 nm
593 nm
130 31.6 40.0 35.0
97.5 22.3 27.0 24.5
76.9 19.4 23.0 21.0
46.3 10.7 15.4 11.3
40.9 9.9 12.0 10.8
Values for cyclohexane are from [11]; all other values are from this work.
plasma lines or dye laser fluorescence. A polarization scrambler was used to eliminate the polarization bias of the monochromator gratings. To avoid decomposition and overheating of the samples, solutions were circulated through a flowing cell at about 250 cm3 /min. The average power at the sample was about 80 mW. The absorption spectrum of each sample was measured before and after the Raman experiment to ensure that no concentration change or photochemical decomposition occurred. Each Raman spectrum was scanned three to five times and averaged, and then corrected for the instrument response and for differential self-absorption of the scattered light. The residual fluorescence background was subtracted after fitting it with an appropriate polynomial. Raman intensities were based on peak areas obtained by fitting each background-subtracted Raman spectrum (both solvent and solute bands) to a sum of Gaussian, Lorentzian or mixed lineshape functions using GRAMS (Thermo Galactic Version 7). Half-widths and peak positions of all solvent bands were determined in neat solvent and kept fixed in the modeling of the Raman spectra of the betaine-30 solutions. The absolute Raman cross-sections of betaine30 normal modes were obtained by referencing the corresponding peak areas to that of the 1033 cm1 band in CH3 OH and CH3 OD, or the 979 cm1 peak in CD3 OD. Depolarization ratios of betaine30 were one-third within the error of the measurement. Raman cross-sections of the solvent standard peaks in methanol and its isotopomers were determined relative to the 802 cm1 band of cyclohexane [11] in binary mixtures, and are displayed in Table 1. The absolute Raman crosssection of each internal mode was calculated using the following function:
rR;B-30 ¼
IB-30 ½ð1 þ 2qÞ=ð1 þ qÞB-30 CS rR;S Mc Sc ; IS ½ð1 þ 2qÞ=ð1 þ qÞS CB-30 ð1Þ
where I is the integrated band intensity, C is the concentration, and q is the depolarization ratio. Mc is the correction factor for the instrument response, Sc is the correction factor for the self-absorption of the sample, and the subscript S refers to the solvent band used as an internal standard. The relative uncertainties in the Raman cross-sections were estimated from replicate measurements to be about 15–25%, where strong or isolated bands have less uncertainty than bands which are strongly overlapped by nearby peaks.
3. Results and discussion Fig. 1 shows the room temperature absorption spectrum of betaine-30 in CH3 OH. Identical absorption spectra are obtained in CH3 OD and CD3 OD. Raman spectra of betaine-30 in the three solvents at an excitation wavelength 457.9 nm are shown in Fig. 2. The peak frequencies of the betaine-30 bands were found to be independent of solvent isotopic substitution. The absolute Raman cross-sections for fourteen vibrational modes of betaine-30 in three solvent isotopomers are shown in Figs. 3 and 4, as a function of excitation frequency. Cross-sections for two moderately strong bands at 1000 and 1455 cm1 are not reported here because they are strongly overlapped by solvent lines in CH3 OH and CH3 OD. Raman cross-sections reported here for betaine-30 in CH3 OH and CD3 OD are somewhat higher than reported in our previous study [4]. For example, the Raman crosssection of the most intense peak at 1320 cm1 in
X. Zhao, J.L. McHale / Chemical Physics Letters 378 (2003) 582–588
2 ) of betaine-30 Fig. 1. Absorption spectrum (cross-section in A in methanol at room temperature. The structure of betaine-30, where R is the phenyl group, is shown in the inset.
Fig. 2. Resonance Raman spectrum of betaine-30 excited at 457.9 nm in three different isotopomers of methanol. Solvent lines are labeled ÔSÕ and peak frequencies of betaine-30 modes are labeled in the top spectrum.
CH3 OH is almost half an order of magnitude greater than the previously reported value. These differences probably result from differences in the data treatment. In the present study, all solvent and solute bands were subjected to modeling to obtain more accurate peak areas. Improved purification procedures also resulted in less fluorescence background in the present study and we feel the present Raman cross-sections are more accurate as a result.
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However, the general trends in both studies are quite similar: for most betaine-30 modes the Raman intensities in CD3 OD are higher than in CH3 OH, and this solvent isotope effect becomes smaller when the excitation wavelength moves to the red side of the absorption band. As the excitation wavelength moves away from resonance with the visible absorption band, the effects of solventinduced dephasing of the electronic transition are diminished and the solvent isotope effect is reduced. There are some discrepancies between the present results and those of [4]. In this work, the Raman cross-sections for the mode at 289 cm1 are observed to be generally stronger in CD3 OD than in the other two solvents, while previously very little difference between the profile of this mode in CH3 OH and CD3 OD was observed. Similarly, the profiles for the 605 and 645 cm1 modes in CD3 OD and CH3 OH in the previous study showed little solvent isotope effect, but in this work the errors bars for the cross-sections in perdeuterated methanol show no overlap with those in the other two solvents except at red excitation wavelengths. For the most part however, the trends reported previously are upheld, and in all cases where the difference in Raman intensities is outside the range of the error bars, the Raman cross-sections are largest in CD3 OD. In contrast to our expectations at the outset of this study, Raman cross-sections of betaine-30 in CH3 OD and CH3 OH are found to be quite similar. Thus the motion of the hydroxyl groups (either free or hydrogen bonded) does not appear to contribute strongly to the solvent-induced dephasing. The Raman cross-sections do however depend on isotopic substitution of the methyl group, indicating that motion of this group is strongly coupled to the electronic transition. Our previous interpretation relied on the expectation of stronger hydrogen bonds in CD3 OD compared to CH3 OH [12]: it was thought that the lower amplitude of zero-point vibrational motion in perdeuterated methanol was responsible for the increased Raman cross-sections. Also, since it is well known that hydrogen bonding at the phenoxide functional group of betaine-30 leads to a large blue shift of the absorption spectrum, it seemed reasonable to suppose that the dynamics of the hydrogen bonded methanols are
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Fig. 3. Raman cross-sections of eight betaine-30 modes in CD3 OD (black triangles), CH3 OH (blue squares), and CH3 OD (red circles), at five excitation frequencies. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
strongly coupled to the charge-transfer transition. However, this picture clearly conflicts with the present experimental results. Our recent observation of the temperaturedependent absorption spectrum of betaine-30 in methanol [10] may shed some light. The sharp isosbestic point attributed to two different hydrogen-bonded forms of the dye was not obscured by a temperature variation from about 5 to 50 °C. This suggests that the relative amplitudes, but not the lineshapes, of the component spectra of the two forms of betaine-30 changes with temperature. This is in contrast to the thermosolvatochromic behavior of betaine-30 in ethanol and in other aprotic solvents [10,13], where an increase in
temperature results in a general red-shift in the absorption spectrum as a result of decreased bulk solvent polarity. The bulk dielectric constant of methanol clearly decreases as temperature increases, so we conclude that the absorption spectrum of betaine-30 depends more strongly on the local solvation structure than the bulk solvent, and that this local structure is ordered by hydrogen bonding interactions. If the dipole moment of methanol is resolved into components along the OH (OD) and OCH3 (OCD3 ) moities, the two components are found to be similar in magnitude; for example, about 2 Debye using the atomic charges and positions given in [14]. For a configuration in which the OH or
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587
Fig. 4. Raman cross-sections of six betaine-30 modes in CD3 OD (triangles), CH3 OH (blue squares), and CH3 OD (red circles), at five excitation frequencies. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
OD group is pinned down by hydrogen bonding to the phenoxide group, a significant change in dipole moment direction can still be achieved by the rotation of the methyl group about the hydrogenbonded hydroxyl group as shown in Scheme 1. The zero-point energy and thus the amplitude of this motion would be lower for a CD3 group than for a CH3 group, perhaps explaining the generally larger Raman intensities of betaine-30 in CD3 OD solution.
Solvent dynamics in methanol and other alcohols are known to be more complex than those of aprotic solvents [14–19]. In addition to overall reorientation there are contributions from hydroxyl group torsions and breaking and forming of hydrogen bonds. Fonseca, Ladanyi and coworkers [14–16] have used molecular dynamics to investigate solvation dynamics in CH3 OH and CH3 OD. In [14], it was concluded that the initial phase of the solvent response results from reoriR1
R1
-
O
R2
-
R2
O
H
H
R1 O
CH3
R1 O H3C
Scheme 1.
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entational motion of free hydroxyl groups. The model solute used in that calculation, however, had zero dipole moment in the ground state, and it was concluded that the solute exerted a structurebreaking effect on methanol. In contrast, it appears that betaine-30, with its large ground state dipole moment (15 Debye), exerts a structuremaking effect on the solvent which could limit the rotation of the hydroxyl groups. The time scale for hydrogen bond rearrangements in ground state betaine-30 in alcohol solution has been reported to be about 100 ps [20,21]. The breaking and forming of hydrogen bonds, which certainly would perturb the transition frequency, is probably too slow to contribute to dephasing on the resonance Raman time scale. It is still surprising, however, that lowfrequency bending and stretching motions of hydrogen bonds do not result in a solvent isotope effect in our studies. It is also important to remark that the solvent isotope effects observed here are not identical for all normal modes of the chromophore. For example, there is no clear solvent isotope effect on the intensities of modes at 1209 and 1320 cm1 . If dephasing dynamics were independent of vibrational mode, then the solvent isotope effect would be similar for all modes of the chromophore. As has been previously noted [3], this observation suggests that there are mode-dependent relaxation processes that contribute to the Raman intensities. However, in no case do the differences in the Raman intensities in CH3 OD and CH3 OH exceed the estimated uncertainty in the measurement. Thus we conclude that the dynamics of hydroxyl group motion makes little contribution to the dephasing seen by the resonance Raman experiment, a conclusion consistent with our picture of a fairly rigid solvation structure ordered by solute–solvent hydrogen bonds. This structure still permits motion of the methyl group as suggested in Scheme 1, and thus methyl group deuteration has an effect on the dephasing dynamics.
4. Conclusions The molecular basis for solvent-induced dephasing of the electronic transition of betaine-30 in
methanol has been investigated using resonance Raman spectroscopy and solvent isotopic substitution. Resonance Raman intensities of most betaine-30 normal modes are observed to be more sensitive to methyl group deuteration than Odeuteration. The results support the idea that the dephasing dynamics in this system are decided by short range interactions between betaine-30 and methanol molecules in a solvation shell which is highly ordered by solute–solvent hydrogen bonds.
Acknowledgements The support of the National Science Foundation is gratefully acknowledged. Dr. Jim Burt is thanked for very helpful discussions on the project.
References [1] P.F. Barbara, T.J. Meyer, M.A. Ratner, J. Phys. Chem. 100 (1996) 13148. [2] M. Cho, G.R. Fleming, Adv. Chem. Phys. 107 (1999) 311. [3] J.L. McHale, Acc. Chem. Res. 34 (2001) 265. [4] Y. Zong, J.L. McHale, J. Chem. Phys. 107 (1997) 2920. [5] Y. Zong, J.L. McHale, J. Chem. Phys. 106 (1997) 4963. [6] X. Zhao, J.A. Burt, F.J. Knorr, J.L. McHale, J. Phys. Chem. A 105 (2001) 11110. [7] C. Reichardt, Chem. Rev. 92 (1994) 2319. [8] C.A. Coleman, C.H. Murray, J. Org. Chem. 57 (1992) 33578. [9] M.C. Beard, G.M. Turner, C.A. Schmuttenmaer, J. Phys. Chem. A 106 (2002) 878. [10] X. Zhao, F.J. Knorr, J.L. McHale, Chem. Phys. Lett. 356 (2002) 214. [11] M.O. Trulson, R.A. Mathies, J. Chem. Phys. 84 (1986) 2068. [12] G. Nemethy, H.A. Scheraga, J. Chem. Phys. 41 (1964) 680. [13] C. Laurence, P. Nicolet, C. Reichardt, Bull. Soc. Chim. Fr. (1987) 125. [14] M.S. Skaf, T. Fonseca, B.L. Ladanyi, J. Chem. Phys. 98 (1993) 8929. [15] T. Fonseca, B.L. Ladanyi, J. Phys. Chem. 95 (1991) 2116. [16] T. Fonseca, B.L. Ladanyi, J. Mol. Liq. 60 (1994) 1. [17] J. Barthell, K. Bachhuber, R. Buchner, H. Hetzenauer, Chem. Phys. Lett. 165 (1990) 369. [18] J.T. Kindt, C.A. Schmuttenmaer, J. Phys. Chem. 100 (1996) 10373. [19] D. Bingemann, N.P. Ernsting, J. Chem. Phys. 102 (1995) 2691. [20] P.J. Reid, S. Alex, W. Jarzeba, R.E. Schlief, A.E. Johnson, P.F. Barbara, Chem. Phys. Lett. 93 (1994) 229. [21] P.J. Reid, P.F. Barbara, J. Phys. Chem. 99 (1995) 3554.
Probing the molecular basis for solvent-induced dephasing using resonance Raman spectroscopy: betaine-30 in CH3OH, CH3OD, and CD3OD Xihua Zhao, Jeanne L. McHale
*
Department of Chemistry, University of Idaho, Moscow, ID 83844-2343, USA Received 2 June 2003; in final form 30 July 2003 Published online:
Abstract Absolute Raman cross-sections are reported for fourteen normal modes of betaine-30 dye in CH3 OH, CH3 OD, and CD3 OD solution, at five different excitation frequencies within the visible absorption band. Consistent with previous results from our lab, the Raman intensities of betaine-30 in CD3 OD solution are generally stronger than in CH3 OH solution. For most of the betaine-30 normal modes, however, the resonance Raman cross-sections are similar in CH3 OD and CH3 OH solution. The results suggest that methyl group motion, more so than hydroxyl group motion, is responsible for dephasing of the resonant electronic transition of betaine-30 in methanol. Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction Solution phase electron transfer and the nature of solvent–chromophore interactions are topics of much current interest in chemistry [1,2]. In our lab, we have employed resonance Raman spectroscopy to explore the influence of solvent dynamics on charge-transfer electronic transitions [3]. We have previously reported solvent isotope effects on resonance Raman intensities of betaine-30 in acetonitrile and methanol [4–6]. Resonance Raman intensities depend on the geometry change of the molecule upon electronic excitation and on the solvent-induced dephasing of the resonant elec*
Corresponding author. Fax: +1-208-885-6173. E-mail address: [email protected] (J.L. McHale).
tronic transition. Within the reasonable assumption that the ground and excited state geometries of a chromophore are independent of solvent isotopic substitution, resonance Raman cross-sections reveal differences in the solvent-induced dephasing in experiments done as a function of solvent isotopomer. Betaine-30 is a strongly solvatochromic dye with a much larger dipole moment in the ground than the excited electronic state, resulting in a dramatic blue shift of the lowest lying electronic transition with increasing solvent polarity [7]. Betaine-30 can also hydrogen bond with protic solvent molecules, leading to an additional blue shift in the absorption spectrum [8]. In the present paper, we examine the resonance Raman spectrum of betaine-30 in CH3 OH, CH3 OD, and CD3 OD so-
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0009-2614(03)01331-9
X. Zhao, J.L. McHale / Chemical Physics Letters 378 (2003) 582–588
lution in order to investigate the molecular basis for our previously reported solvent isotope effects on the Raman intensities of betaine-30 in methanol and perdeuterated methanol [4]. Raman intensities depend on solvent molecule motions which perturb the electronic transition frequency and thus cause dephasing of the resonant electronic transition. In the frequently observed inhomogeneous limit, an increase in the amplitude of such solvent motion results in a decrease in the resonance Raman intensity. In this earlier work, we found that for most of vibrational modes of betaine-30, the absolute Raman cross-sections in perdeuterated methanol are higher than in CH3 OH. This suggests that the amplitude of solvent-induced dephasing of the charge-transfer transition is larger in CH3 OH than in CD3 OD solution. The electronic absorption spectrum of the dye, on the other hand, is independent of solvent isotopic substitution. We consider this to be evidence of nonlinear solvent dynamics in the case of betaine-30 in methanol (and also in acetonitrile [5,6]). That is, the solvent dynamics depend on the electronic state of the chromophore. The electronic absorption spectrum depends on solvent motion coupled to the ground electronic state of the solute, while the resonance Raman spectrum sees the solvent response to the excited state of the dye and, due to the reversal of the betaine-30 dipole moment on excitation [9], this solvent motion initially proceeds from a highly nonequilibrium solvent configuration. Since the Raman intensities of most of the betaine-30 normal modes were found to be higher in CD3 OD than in CH3 OH, we speculated that hydrogenbond dynamics were playing a strong role in the solvent response, as the amplitude of the hydrogen bond motion (stretching or torsion) is expected to be smaller in the deuterated solvent. The purpose of the present work is to test that hypothesis by comparing the resonance Raman intensities of betaine-30 in CH3 OH and CH3 OD. Our previously reported study of the thermosolvatochromism of betaine-30 in methanol is also relevant to this work [10]. In the temperature-dependent absorption spectrum of betaine-30, a sharp isosbestic point was found in methanol solution, but not in acetonitrile or ethanol. We proposed that
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two forms of betaine-30 exist in equilibrium in methanol solution: betaine-30 hydrogen bonded to one or two methanol molecules. Most surprising however, is that temperature tuning of the solvent polarity did not obscure this isosbestic point. We concluded that the betaine-30 molecule is rather rigidly solvated by methanol molecules. In this Letter, we report for the first time resonance Raman cross-sections of normal modes of betaine-30 in CH3 OD in order to investigate the importance of hydrogen-bond dynamics in the dephasing which limits the Raman intensity. Also, resonance Raman spectra of betaine-30 in CH3 OH and CD3 OD were redetermined to compare the results more accurately. Consistent with our previous study, betaine-30 Raman intensities in CD3 OD are generally greater than those in CH3 OH. However, the absolute Raman crosssections in CH3 OH and CH3 OD are generally the same within the error of the measurement. The results suggest that, in contrast to our previous interpretation of the solvent isotope effect, the motion of the methyl group is more strongly coupled to the charge-transfer transition than the motion of the hydroxyl group.
2. Experiment Betaine-30 was purchased from Aldrich and purified using a previously reported procedure [6]. CH3 OH was used as received, while CH3 OD and CD3 OD were distilled before use to remove fluorescent impurities. Absorption spectra were recorded in 1 cm cells using a Shimadzu UV-2501 spectrometer. Resonance Raman spectra were obtained at excitation wavelengths of 457.9, 488.0, 514.5, 576.7, and 593.1 nm using sample concentrations of about 1.1 103 M and 90° scattering geometry. The excitation source was either a Coherent argon ion laser or a Coherent ring dye laser pumped by the argon ion laser. A SPEX 14018 double monochromator with two 1800 groove/mm gratings blazed for 500 nm was used to disperse the scattered light, and a thermoelectrically cooled photomultiplier tube was employed as the detector. A Kaiser Holographic bandpass filter was put in front of the sample in order to remove argon ion
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Table 1 Total Raman cross-sections rR of solvent bands in units of lbarn, (1 lbarn ¼ 1030 cm2 ) at five excitation wavelengths used in this studya
Cyclohexane (802 cm1 ) CH3 OH (1033 cm1 ) CD3 OD (979 cm1 ) CH3 OD (1033 cm1 ) a
458 nm
488 nm
514 nm
576.7 nm
593 nm
130 31.6 40.0 35.0
97.5 22.3 27.0 24.5
76.9 19.4 23.0 21.0
46.3 10.7 15.4 11.3
40.9 9.9 12.0 10.8
Values for cyclohexane are from [11]; all other values are from this work.
plasma lines or dye laser fluorescence. A polarization scrambler was used to eliminate the polarization bias of the monochromator gratings. To avoid decomposition and overheating of the samples, solutions were circulated through a flowing cell at about 250 cm3 /min. The average power at the sample was about 80 mW. The absorption spectrum of each sample was measured before and after the Raman experiment to ensure that no concentration change or photochemical decomposition occurred. Each Raman spectrum was scanned three to five times and averaged, and then corrected for the instrument response and for differential self-absorption of the scattered light. The residual fluorescence background was subtracted after fitting it with an appropriate polynomial. Raman intensities were based on peak areas obtained by fitting each background-subtracted Raman spectrum (both solvent and solute bands) to a sum of Gaussian, Lorentzian or mixed lineshape functions using GRAMS (Thermo Galactic Version 7). Half-widths and peak positions of all solvent bands were determined in neat solvent and kept fixed in the modeling of the Raman spectra of the betaine-30 solutions. The absolute Raman cross-sections of betaine30 normal modes were obtained by referencing the corresponding peak areas to that of the 1033 cm1 band in CH3 OH and CH3 OD, or the 979 cm1 peak in CD3 OD. Depolarization ratios of betaine30 were one-third within the error of the measurement. Raman cross-sections of the solvent standard peaks in methanol and its isotopomers were determined relative to the 802 cm1 band of cyclohexane [11] in binary mixtures, and are displayed in Table 1. The absolute Raman crosssection of each internal mode was calculated using the following function:
rR;B-30 ¼
IB-30 ½ð1 þ 2qÞ=ð1 þ qÞB-30 CS rR;S Mc Sc ; IS ½ð1 þ 2qÞ=ð1 þ qÞS CB-30 ð1Þ
where I is the integrated band intensity, C is the concentration, and q is the depolarization ratio. Mc is the correction factor for the instrument response, Sc is the correction factor for the self-absorption of the sample, and the subscript S refers to the solvent band used as an internal standard. The relative uncertainties in the Raman cross-sections were estimated from replicate measurements to be about 15–25%, where strong or isolated bands have less uncertainty than bands which are strongly overlapped by nearby peaks.
3. Results and discussion Fig. 1 shows the room temperature absorption spectrum of betaine-30 in CH3 OH. Identical absorption spectra are obtained in CH3 OD and CD3 OD. Raman spectra of betaine-30 in the three solvents at an excitation wavelength 457.9 nm are shown in Fig. 2. The peak frequencies of the betaine-30 bands were found to be independent of solvent isotopic substitution. The absolute Raman cross-sections for fourteen vibrational modes of betaine-30 in three solvent isotopomers are shown in Figs. 3 and 4, as a function of excitation frequency. Cross-sections for two moderately strong bands at 1000 and 1455 cm1 are not reported here because they are strongly overlapped by solvent lines in CH3 OH and CH3 OD. Raman cross-sections reported here for betaine-30 in CH3 OH and CD3 OD are somewhat higher than reported in our previous study [4]. For example, the Raman crosssection of the most intense peak at 1320 cm1 in
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2 ) of betaine-30 Fig. 1. Absorption spectrum (cross-section in A in methanol at room temperature. The structure of betaine-30, where R is the phenyl group, is shown in the inset.
Fig. 2. Resonance Raman spectrum of betaine-30 excited at 457.9 nm in three different isotopomers of methanol. Solvent lines are labeled ÔSÕ and peak frequencies of betaine-30 modes are labeled in the top spectrum.
CH3 OH is almost half an order of magnitude greater than the previously reported value. These differences probably result from differences in the data treatment. In the present study, all solvent and solute bands were subjected to modeling to obtain more accurate peak areas. Improved purification procedures also resulted in less fluorescence background in the present study and we feel the present Raman cross-sections are more accurate as a result.
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However, the general trends in both studies are quite similar: for most betaine-30 modes the Raman intensities in CD3 OD are higher than in CH3 OH, and this solvent isotope effect becomes smaller when the excitation wavelength moves to the red side of the absorption band. As the excitation wavelength moves away from resonance with the visible absorption band, the effects of solventinduced dephasing of the electronic transition are diminished and the solvent isotope effect is reduced. There are some discrepancies between the present results and those of [4]. In this work, the Raman cross-sections for the mode at 289 cm1 are observed to be generally stronger in CD3 OD than in the other two solvents, while previously very little difference between the profile of this mode in CH3 OH and CD3 OD was observed. Similarly, the profiles for the 605 and 645 cm1 modes in CD3 OD and CH3 OH in the previous study showed little solvent isotope effect, but in this work the errors bars for the cross-sections in perdeuterated methanol show no overlap with those in the other two solvents except at red excitation wavelengths. For the most part however, the trends reported previously are upheld, and in all cases where the difference in Raman intensities is outside the range of the error bars, the Raman cross-sections are largest in CD3 OD. In contrast to our expectations at the outset of this study, Raman cross-sections of betaine-30 in CH3 OD and CH3 OH are found to be quite similar. Thus the motion of the hydroxyl groups (either free or hydrogen bonded) does not appear to contribute strongly to the solvent-induced dephasing. The Raman cross-sections do however depend on isotopic substitution of the methyl group, indicating that motion of this group is strongly coupled to the electronic transition. Our previous interpretation relied on the expectation of stronger hydrogen bonds in CD3 OD compared to CH3 OH [12]: it was thought that the lower amplitude of zero-point vibrational motion in perdeuterated methanol was responsible for the increased Raman cross-sections. Also, since it is well known that hydrogen bonding at the phenoxide functional group of betaine-30 leads to a large blue shift of the absorption spectrum, it seemed reasonable to suppose that the dynamics of the hydrogen bonded methanols are
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Fig. 3. Raman cross-sections of eight betaine-30 modes in CD3 OD (black triangles), CH3 OH (blue squares), and CH3 OD (red circles), at five excitation frequencies. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
strongly coupled to the charge-transfer transition. However, this picture clearly conflicts with the present experimental results. Our recent observation of the temperaturedependent absorption spectrum of betaine-30 in methanol [10] may shed some light. The sharp isosbestic point attributed to two different hydrogen-bonded forms of the dye was not obscured by a temperature variation from about 5 to 50 °C. This suggests that the relative amplitudes, but not the lineshapes, of the component spectra of the two forms of betaine-30 changes with temperature. This is in contrast to the thermosolvatochromic behavior of betaine-30 in ethanol and in other aprotic solvents [10,13], where an increase in
temperature results in a general red-shift in the absorption spectrum as a result of decreased bulk solvent polarity. The bulk dielectric constant of methanol clearly decreases as temperature increases, so we conclude that the absorption spectrum of betaine-30 depends more strongly on the local solvation structure than the bulk solvent, and that this local structure is ordered by hydrogen bonding interactions. If the dipole moment of methanol is resolved into components along the OH (OD) and OCH3 (OCD3 ) moities, the two components are found to be similar in magnitude; for example, about 2 Debye using the atomic charges and positions given in [14]. For a configuration in which the OH or
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Fig. 4. Raman cross-sections of six betaine-30 modes in CD3 OD (triangles), CH3 OH (blue squares), and CH3 OD (red circles), at five excitation frequencies. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
OD group is pinned down by hydrogen bonding to the phenoxide group, a significant change in dipole moment direction can still be achieved by the rotation of the methyl group about the hydrogenbonded hydroxyl group as shown in Scheme 1. The zero-point energy and thus the amplitude of this motion would be lower for a CD3 group than for a CH3 group, perhaps explaining the generally larger Raman intensities of betaine-30 in CD3 OD solution.
Solvent dynamics in methanol and other alcohols are known to be more complex than those of aprotic solvents [14–19]. In addition to overall reorientation there are contributions from hydroxyl group torsions and breaking and forming of hydrogen bonds. Fonseca, Ladanyi and coworkers [14–16] have used molecular dynamics to investigate solvation dynamics in CH3 OH and CH3 OD. In [14], it was concluded that the initial phase of the solvent response results from reoriR1
R1
-
O
R2
-
R2
O
H
H
R1 O
CH3
R1 O H3C
Scheme 1.
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entational motion of free hydroxyl groups. The model solute used in that calculation, however, had zero dipole moment in the ground state, and it was concluded that the solute exerted a structurebreaking effect on methanol. In contrast, it appears that betaine-30, with its large ground state dipole moment (15 Debye), exerts a structuremaking effect on the solvent which could limit the rotation of the hydroxyl groups. The time scale for hydrogen bond rearrangements in ground state betaine-30 in alcohol solution has been reported to be about 100 ps [20,21]. The breaking and forming of hydrogen bonds, which certainly would perturb the transition frequency, is probably too slow to contribute to dephasing on the resonance Raman time scale. It is still surprising, however, that lowfrequency bending and stretching motions of hydrogen bonds do not result in a solvent isotope effect in our studies. It is also important to remark that the solvent isotope effects observed here are not identical for all normal modes of the chromophore. For example, there is no clear solvent isotope effect on the intensities of modes at 1209 and 1320 cm1 . If dephasing dynamics were independent of vibrational mode, then the solvent isotope effect would be similar for all modes of the chromophore. As has been previously noted [3], this observation suggests that there are mode-dependent relaxation processes that contribute to the Raman intensities. However, in no case do the differences in the Raman intensities in CH3 OD and CH3 OH exceed the estimated uncertainty in the measurement. Thus we conclude that the dynamics of hydroxyl group motion makes little contribution to the dephasing seen by the resonance Raman experiment, a conclusion consistent with our picture of a fairly rigid solvation structure ordered by solute–solvent hydrogen bonds. This structure still permits motion of the methyl group as suggested in Scheme 1, and thus methyl group deuteration has an effect on the dephasing dynamics.
4. Conclusions The molecular basis for solvent-induced dephasing of the electronic transition of betaine-30 in
methanol has been investigated using resonance Raman spectroscopy and solvent isotopic substitution. Resonance Raman intensities of most betaine-30 normal modes are observed to be more sensitive to methyl group deuteration than Odeuteration. The results support the idea that the dephasing dynamics in this system are decided by short range interactions between betaine-30 and methanol molecules in a solvation shell which is highly ordered by solute–solvent hydrogen bonds.
Acknowledgements The support of the National Science Foundation is gratefully acknowledged. Dr. Jim Burt is thanked for very helpful discussions on the project.
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