- Email: [email protected]
Transient Raman spectra and structure of the “twisted” excited singlet state of tetraphenylethylene
Volume 2 17, number 4
CHEMICAL PHYSICS LETTERS
21 January 1994
Transient Raman spectra and structure of the “twisted” excited singlet state of tetraphenylethylene Tabei Tabara and Hiro-o Hamaguchi Molecular Spectroscopy Laboratory, The Kanagawa Academy of Science and Technology (K.&T), KSP East 301, 3-2-l. Sakato, Takatsu, Kawasaki 213, Japan
Received 18 September 1993; in final form 9 November 1993
Transient Raman spectra of the “twisted” Sr state of tetraphenylethylene and its “C analogue have been measured. The oletinic C,C, stretching frequency changes drastically on going from the So state to the S, state, supporting a twisted configuration of S, tetraphenylethylene. The CC stretching frequencies of the phenyl rings are discussed in relation to the zwitterionic nature of this S, state.
1. Introduction Cis-trans photoisomerization of olefins is one of the most fundamental unimolecular chemical reactions. It has been shown recently that this reaction may be categorized into two different types, “bothway” and “one-way” photoisomerization [ 11. The term “both-way” means that the isomerization takes place from the trans form to the cis, and also from the cis form to the trans. In “one-way” photoisomerization, only the cis form isomerizes to the trans while the inverse isomerization (trans-rcis) does not take place. This difference is ascribable to the different location of the excited state potential minimum, from which the excited oletins relax to the ground state. For molecules showing “both-way” photoisomerization, the excited state potential minimum is located around the perpendicular contiguration of the olefinic double bond, while it lies around the trans configuration for molecules showing “oneway” photoisomerization. In the case of “both-way” photoisomerization, photoexcited molecules initially have planar geometry and they change to the twisted form in the excited energy surface. Therefore, the planar and twisted forms in the excited state are expected to appear successively in the course of photoisomerization. The twisted excited state, which is sometimes referred to as the “phantom” state, has been attract-
ing much interest since it lies within the funnel of the potential energy surface responsible for the isomerization [ 21. Theoretical studies predict the zwitterionic nature of the twisted excited state of ethylene [ 3,4], but there is some controversy about oletins having conjugated substituents [ 5,6]. Experimental approaches to the twisted excited state are generally difficult because of its short lifetime. For example, the lifetime of the twisted S, state of stilbene, one of the most extensively studied olefins, is thought to be less than 150 fs [7]. Tetraphenylethylene gives an opportunity for studying the twisted excited state experimentally. The S, state of this molecule has been thought to have a twisted structure with a lifetime as long as a few nanoseconds in hydrocarbons [ 8-101. The zwitterionic nature of this “twisted” Si state of tetraphenylethylene has recently been suggested from picosecond absorption [ 10,111, picosecond calorimetry [ 12,131 and time-resolved microwave conductivity experiments [ 141. In spite of these advances in the elucidation of the electronic structure of this S, state, knowledge about the molecular (geometrical) structure is still lacking. To the best of the authors’ knowledge, no experimental support for the “twisted” configuration of this S1 state has been obtained from the viewpoint of the molecular structure. It is well established that time-resolved vibrational spectroscopy affords detailed information
0009-2614/94/S 07.00 0 1994 Elsevier Science B.V. All rights reserved. SSDZ OOOOS-2614(93)E1386-U
369
Volume 2 17, number 4
CHEMICAL PHYSICS LETTERS
about the molecular structure of the short-lived transient species appearing in the course of photochemical reactions [ 15 1. Thus, it is highly desirable to obtain the vibrational spectrum of this Si state. In this Letter, we report the first transient Raman spectra of this “twisted” Si state of tetraphenylethylene. In the following text, we refer to this Sr state, which has been claimed to have the twisted configuration, simply as the twisted S, state.
2. Experimental The nanosecond time-resolved Raman spectra were measured by using the apparatus described previously [ 16 1. The second Stokes line of a Dz Raman shifter (3 16 nm, 10 ns, 50 Hz) pumped by the fourth harmonic of a pulsed Q-switch Nd: YAG laser (Spectron SL801) was used to photoexcite tetraphenylethylene. The first Stokes line of a Hz Raman shifter (4 17 nm, 10 ns, 50 Hz) pumped by the third harmonic of another pulsed Q-switch Nd : YAG laser (Spectron SL804) was used to probe Raman scattering. The firing of these two lasers was synchronized by a delay generator (Stanford Research System DG535). The pump and the probe laser pulses were focused on a thin film-like jet stream of the sample solution. Typical pulse energies of the pump and probe lasers were 0.3 mJ (x4 MW/cm2) and 1.7 mJ (my22 MW/cm2) at the sample point, respectively. The scattered light at 90” to the laser beams was collected by a camera lens (Nikon), and analyzed by a triple polychromator (Jobin-Yvon T64000) equipped with an intensified photodiode array (Princeton Instruments IRY 1024G/RB). The acquired data were stored and analyzed with a personal computer (NEC PC9801 ). The nanosecond time-resolved absorption spectra were measured as described previously [ 161. A pulsed Xe lamp (Hamamatsu L2358) was used as the monitoring light source. The nanosecond pumping laser pulses (3 16 nm) were focused by a cylindrical lens onto the 1 cm quartz cell through which the sample solution was circulated. The monitoring light transmitted through the sample cell was analyzed by a monochromator (Jobin-Yvon HR320) and detected by an intensified photodiode array (Princeton Instruments IRY-700G/B/par). The de370
21 January 1994
tector was used with the 5 ns gate. Several glass Elters were used to attenuate the monitoring light and eliminate the second-order diffraction of the spectrometer. The acquired data were stored and analyzed with a personal computer (NEC PC980 1). Tetraphenylethylene ( (C,H, ),C=C( C6H5)2) was purchased from Tokyo Kasei Kogyo and recystallized from hexane several times before use. The 13C substituted tetraphenylethylene ( (C6H5)2’3C= C( C6H5)2) was prepared from “C diphenylmethane and dichlorodiphenylmethane [ 171. The “C substituted diphenylmethane was synthesized from benzene and 13C-benzyl chloride (Isotec 13C99.5%) by the Friedel-Crafts reaction [ 18 1. The synthesized sample was purified by column chromatography. Hexane and heptane (special grade or liquid chromatography grade, Wako Pure Chemical Industries) were used as obtained.
3. Results and discussion Fig. 1 shows nanosecond time-resolved absorption spectra of tetraphenylethylene obtained from a 4.5~ 10m4 mol drnm3 hexane solution. The excitation wavelength is 316 nm. In the spectrum at 0 ns, transient absorption showing a maximum around 420 nm is clearly observed. Negative signals appearing in the wavelength region longer than 500 nm are due to r 0.20
1
0.15 /Ons 8 5 0.10 e $2 0.05 1A u
20 ns
I
500 600 Wavelength /nm Fig. 1. Time-resolved absorption spectra of tetraphenylethylene inhexane(4.5~10-~moldm-~;pumplaser316nm).Inset:tima resolved emission spectra.
Volume 2 17, number 4
CHEMICAL. PHYSICS LETTERS
the strong fluorescence of tetraphenylethylene (see the inset in fig. 1) [ 8 19 1. The distortion of the spectrum in the region shorter than 450 nm is small since the intensity of the monitoring light is much higher than that of the fluorescence in this region. The transient absorption band around 420 nm disappears in the spectrum at a delay time of 20 ns. Greene first reported time-resolved absorption spectra of tetraphenylethylene by using picosecond excitation, and observed two transient species having different lifetimes [ 9 1. He assigned the short-lived transient species (T= 5 ps) showing absorption around 640 nm to the “unrelaxed” planar S, state, and the other longlived transient (7= 3 ns) exhibiting an absorption maximum around 417 nm to the twisted Si state of tetraphenylethylene. The transient absorption measured in the present experiment is assigned to the long-lived twisted Si state. The twisted Si state of tetraphenylethylene is accessible with the nanosecond apparatus having a time-resolution of as low as 10 ns. Fig. 2 shows nanosecond time-resolved Raman spectra obtained from a 9 x 10m4mol dme3 heptane solution. The pump and probe wavelengths are 316 and 4 17 nm, respectively. Heptane, in place of hexane, was used as the solvent in order to avoid evaporation during the measurement and resultant change of the sample concentration. In the spectra at a time delay of 0 ns, more than twenty transient Raman
I
I
15ns
I , , 1600
I
I.,
I
1200
I
I
(
I
600
I
I
(
400
I
-
-
’
Raman Shift /cm.’ Fig. 2. Time-resolved resonance Raman spectra of tetraphenylethyleneinheptane(9x10-4moldm-3;pumplaser316nm;probe laser 417 nm).
21 January 1994
bands are observed in the wavenumber region from 200 to 1700 cm-‘. They completely disappear at a time delay of 15 ns. The weak Raman bands recognized around 1600 cm- ’ in the spectrum at 15 ns are due to the So state of tetraphenylethylene. The temporal behavior of the transient Raman bands agrees with that of the transient absorption due to the twisted S1 state. Thus, the transient Raman bands observed in the spectrum at 0 ns are assignable to the twisted S, state of tetraphenylethylene. The probing laser at 417 nm is in exact resonance with S,cS, transition of the twisted Si state, and the Raman bands due to this species should be highly resonance enhanced. Ma and Zimmt suggested that the “relaxed” planar Si state is in equilibrium with the twisted S, state [ 19 1. However, the contribution from this “relaxed” planar Si state is expected to be negligible in the obtained Raman spectra because the population of this state is estimated to be as low as 1w [14]. In order to derive information about the molecular structure of the twisted S1 state from the obtained Raman spectra, we have to make vibrational assignments. Here, we do not intend to make full vibrational assignments but discuss several key vibrations which are important for the elucidation of the molecular structure. First, we focus on the oletinic C,C, stretching vibration. The Raman spectra of the normal species and the 13Canalogues of the twisted S, state of tetraphenylethylene are compared in fig. 3. In these spectra, the solvent bands are already subtracted. The Raman spectra of the ground state obtained from crystalline samples are also shown in this figure. Expanded spectra of two wavenumber regions of interest are shown in fig. 4. The most striking difference between the S,, and Si states is found in the double bond region between 1500 and 1600 cm-‘. In the So spectrum, three bands are observed in this region ( 1595, 1588 and 1564 cm-‘). The band at 1564 cm-’ of the normal species is assigned to the oleflnic C,C, stretch because this band shows a 19 cm- ’ downshift upon the 13Csubstitution. The remaining two bands are assigned to the CC stretches of the phenyl rings. However, the 8 cm-’ “C shift ofthe 1588 cm-’ band ( 1588 cm-‘+ 1580 cm-‘) indicates that there is a strong coupling between the C,C, stretch and a phenyl CC stretch in the So state. On the other hand, 371
Volume 2 17, number 4
CHEMICAL PHYSICS LETTERS
21 January 1994
,2~(s) I
DJ& ‘7” I c 1800
0
iL I
1600
s gf ’
‘3C (s,) I
1400
-
I 1200
r
I 1000
@ I 600
’
I 600
’
I 400
’
1
’
200
Raman Shift km”
Fig. 3. Raman spectra of tetraphenylethylene in the twisted S, state and those in the SOstate. From the top to the bottom, (A) normal species in the Sr state, (B) r3C analogue in the S, state, (C) normal species in the SOstate, (D) r3C analogue in the Sc state (S,: probe laser 4 17 nm; Sc: probe laser 5 14.5 nm ) .
Raman Shift /cm”
Fig. 4. Raman spectra of tetraphenylethylene in the twisted S, state and those in the Sc state. From the top to the bottom, normal species in the S, state, r3C analogue in the S1 state, normal species in the So state, and ‘“C analogue in the SOstate ( S1: probe laser417nm;S,:probelaser514.5nm).
372
in the spectrum of the S, state, only two Raman bands are observed at 1587 and 1543 cm-’ in this frequency region. These two Raman bands are assigned to the phenyl CC stretch since they show no shift with the 13C substitution. A small change in the shape of the valley between these two phenyl ring bands is due to the unexcited ground state molecules. No Raman band assignable to the C,C, stretch is observed in this frequency region. In addition, a complete lack of the 13C shift implies that there is no coupling between the C,C, stretch and the phenyl CC stretches in the S, state. The absence of coupling between the C,C, stretch and the phenyl CC stretch can be rationalized if the C,C, stretching frequency drastically changes on going from the ground state to the S, state. The twisted geometry of the S, state is expected to induce a marked decrease of the bond order of the C,C, bonding and to result in the significant decrease of the C,C, stretching frequency. The obtained vibrational data are consistent with this expectation, and support the twisted configuration of S, tetraphenylethylene. We sought the C&C, stretch band in the single bond region of the Si spectra, but no Raman band ascribable to this vibration was found. The only Raman band exhibiting a significant 13C shift is the weak
Volume 2 17, number 4
CHEMICAL PHYSICS LETTERS
band located at 1255 cm-’ (fig. 4). This band shows a broadening in addition to the frequency downshift upon the 13C substitution. A similar broadening is found for the 1264 cm- ’ band in the S,, Raman spectrum. The asymmetrical 13Csubstitution is expected to give rise to small splitting in the band due to the C,-phenyl stretch because the substituted species has two inequivalent C,-phenyl bondings ( “C,-phenyl and ‘3C,-phenyl). If the splitting is small, it should be observed as a band broadening in the Raman spectra. Thus, it is concluded that the Raman bands showing band broadening upon the “C substitution (1255 cm-’ in S1 and 1264 cm-’ in So) are due to the vibrational modes having contributions from the C,-phenyl stretch. However, the effect of the “C substitution on the 1255 cm-’ band of the S, state is somewhat different from that on the 1264 cm- ’ band in the So state. It seems that the 13Cshift of the 1255 cm-l band is significantly larger than that of the 1264 cm-’ band of the S,-,state although the signal-to-noise ratio of the S1 spectrum of the i3C analogue is low. It is probable that the S, state has much lower C,C, stretching frequency and it causes mode mixing between the C,C, stretch and the C,-phenyl stretch. A larger 13Cshift of the 1255 cm-’ band of the S, spectrum may be due to this mode mixing and might be an indication of the C,C, stretching frequency in the single bond region. Next, we discuss the vibrational frequencies of the phenyl CC stretch bands in the S1 state. As described before, two Raman bands assignable to the phenyl CC stretches are observed at 1587 and 1543 cm-’ in the S, spectra. The frequency difference of these two bands (44 cm-‘) is large, compared with the frequency difference of the two phenyl CC stretch bands in the So state (7 cm- ’ ). It suggests that a significant structural change is also induced in the phenyl ring parts with the excitation from the So state to the S, state. Recently, the zwitterionic nature of the twisted S, state was experimentally clarified [ 10-141. It means that tetraphenylethylene has both anionic and cationic diphenyl methyl moiety in the twisted S1 state. These two electronically different moieties having the same atomic frame are expected to give different vibrational frequency, if the exchange of the cationic and the anionic parts (A+-A-*A--A+ ) is slower than the Raman optical process. It is known that the anion and the cation radicals of aromatic
21 January 1994
compounds have quite different frequencies for the phenyl CC stretch. For example, the frequency of the phenyl CC stretch of the biphenyl cation radical is 1616 cm-‘, and that of the anion radical is 1587 cm- ’ [ 20,2 11. A similar frequency difference is also found between the cation and the anion radicals of trans stilbene ( 1605 cm-’ in the cation radical and 1577 cm-’ in the anion radical) [ 15,22-251. The frequency differences between the cation and the anion radicals of these two compounds (29 and 28 cm- ’ ) are similar to that of the two phenyl vibration of S1 tetraphenylethylene (44 cm-‘) although the absolute frequencies are different by a few tens of wavenumbers. It is likely that the large difference in the frequency of the two phenyl CC stretch bands reflect the existence of the cationic and the anionic moieties in S, tetraphenylethylene, and it may be a manifestation of the zwitterionic nature of this twisted excited state. Finally, we briefly mention the character of the S,+-S, transition revealed in the Raman spectrum. It is well known that resonance Raman intensities provide information about the electronic transition with which the Raman probing wavelength is in resonance. For the Franck-Condon (Albrecht’s A term) type resonance, the vibrational modes along which the potential minimum is displaced with the electronic transition give rise to strong Raman bands in the resonance Raman spectra [26]. In the Raman spectra of twisted S, tetraphenylethylene, only the intensities due to the phenyl ring vibrations are highly enhanced, and that related to the C,C, stretch is low. This means that the structural change is induced primarily on the phenyl rings along with the S,tS, transition, and indicates that this transition is localized on the phenyl ring moiety. Schilling and Hilinski pointed out the similarity of the absorption spectra among S1 tetraphenylethylene, the diphenylmethyl cation, and its anion, and argued that this similarity is a manifestation of the zwitterionic nature of S1 tetraphenylethylene [lo]. The selective resonance enhancement of the phenyl Raman bands is consistent with localized electronic transitions characteristic to the zwitterionic structure of the S1 state of tetraphenylethylene.
373
Volume 2 17, number 4
CHEMICAL PHYSICS LETTERS
References
[ 1 ] T. Arai and K. Tokumaru, Chem. Rev. 93 (1993) 23. [2] J. Saltiel and J.L. Charlton, in: Rearrangement in ground and excited states, ed. P. de Mayo (Academic Press, New York, 1980) p. 25. [3] R.J. Buenker and SD. Peyerimhoff, Chem. Phys. 9 (1976) 75. [4] B.R. Brooks and H.F. Schaefer III, J. Am. Chem. Sot. 101 ( 1979) 307. [ 5 ] I. Nebot-Gil and J. Malrieu, Chem. Phys. Letters 84 ( 198 1) 571. [ 61 I. Nebot-Gil and J. Mahieu, J. Am. Chem. Sot. 104 (1982) 3320. [ 7 ] S. Abrash, S. Repinec and R.M. Hochstrasser, J. Chem. Phys. 93 (1990) 1041. [ 81 P.F. Barbara, S.D. Rand and P.M. Rentzepis, J. Am. Chem. Sot. 103 (1981) 2156. [ 91 B.I. Greene, Chem. Phys. Letters 79 ( 198 1) 5 1. [ lo] C.L. Schilling and E.F. Hilinski, J. Am. Chem. Sot. 110 (1988) 2296. [ll]Y.-P. Sun and M.A. Fox, J. Am. Chem. Sot. 115 (1993) 747. [ 121 M.B. Zimmt, Chem. Phys. Letters 160 (1989) 564. [ 131 J. Morais, J. Ma and M.B. Zimmt, J. Phys. Chem. 95 ( 1991) 3885.
374
21 January 1994
[ 141 W. Schuddeboom, S.A. Jonker, J.M. Warman, M.P. de Haas, M.J.W. Vermeulen, W.F. Jager, B. de Lange, B.L. Feringa and R.W. Fessenden, J. Am. Chem. Sot. 115 (1993) 3286. [ 151 H. Hamaguchi, in: Vibrational spectra and structure, ed. J.R. Durig (Elsevier, Amsterdam, 1987) p. 227. [ 161 T. Tahara and H. Hamaguchi, J. Phys. Chem. 96 ( 1992) 8252. [ 17 ] D.A. Shirley, Preparation of organic intermediates (Wiley, New York, 195 1) p. 272. [ 181 L.F. Fieser, Experiments in organic chemistry (Heath, Lexington, 1957) ch. 28. [19]J.MaandM.B.Zimmt,J.Am.Chem.Scc.114(1992)9723. [ 201 C. Kato, H. Hamaguchi and M. Tasumi, Chem. Phys. Letters 120 (1985) 183. [ 2 1 ] C. Takahashi and S. Maeda, Chem. Phys. Letters 24 ( 1974) 584. [22]L.R. Dosser, J.B. Pallix, G.H. Atkinson, H.C. Wang, G. Levin and M. Sxwarc, Chem. Phys. Letters 62 ( 1979) 555. [23] W. Hub, S. Schneider, F. Doerr, J.T. Simpson, J.D. Oxman andF.D. Lewis, J.Am. Chem.Soc. 104 (1982) 2044. [ 24 ] W. Hub, S. Schneider, F. Doerr, J.D. Gxman and F.D. Lewis, J. Am. Chem. Sot. 106 (1984) 708. [25] W. Hub, U. Klueter, S. Schneider and F. Doerr, J. Phys. Chem. 88 (1984) 2308. [ 261 A.C. Albrecht, J. Chem. Phys. 34 ( 196 1) 1476.
CHEMICAL PHYSICS LETTERS
21 January 1994
Transient Raman spectra and structure of the “twisted” excited singlet state of tetraphenylethylene Tabei Tabara and Hiro-o Hamaguchi Molecular Spectroscopy Laboratory, The Kanagawa Academy of Science and Technology (K.&T), KSP East 301, 3-2-l. Sakato, Takatsu, Kawasaki 213, Japan
Received 18 September 1993; in final form 9 November 1993
Transient Raman spectra of the “twisted” Sr state of tetraphenylethylene and its “C analogue have been measured. The oletinic C,C, stretching frequency changes drastically on going from the So state to the S, state, supporting a twisted configuration of S, tetraphenylethylene. The CC stretching frequencies of the phenyl rings are discussed in relation to the zwitterionic nature of this S, state.
1. Introduction Cis-trans photoisomerization of olefins is one of the most fundamental unimolecular chemical reactions. It has been shown recently that this reaction may be categorized into two different types, “bothway” and “one-way” photoisomerization [ 11. The term “both-way” means that the isomerization takes place from the trans form to the cis, and also from the cis form to the trans. In “one-way” photoisomerization, only the cis form isomerizes to the trans while the inverse isomerization (trans-rcis) does not take place. This difference is ascribable to the different location of the excited state potential minimum, from which the excited oletins relax to the ground state. For molecules showing “both-way” photoisomerization, the excited state potential minimum is located around the perpendicular contiguration of the olefinic double bond, while it lies around the trans configuration for molecules showing “oneway” photoisomerization. In the case of “both-way” photoisomerization, photoexcited molecules initially have planar geometry and they change to the twisted form in the excited energy surface. Therefore, the planar and twisted forms in the excited state are expected to appear successively in the course of photoisomerization. The twisted excited state, which is sometimes referred to as the “phantom” state, has been attract-
ing much interest since it lies within the funnel of the potential energy surface responsible for the isomerization [ 21. Theoretical studies predict the zwitterionic nature of the twisted excited state of ethylene [ 3,4], but there is some controversy about oletins having conjugated substituents [ 5,6]. Experimental approaches to the twisted excited state are generally difficult because of its short lifetime. For example, the lifetime of the twisted S, state of stilbene, one of the most extensively studied olefins, is thought to be less than 150 fs [7]. Tetraphenylethylene gives an opportunity for studying the twisted excited state experimentally. The S, state of this molecule has been thought to have a twisted structure with a lifetime as long as a few nanoseconds in hydrocarbons [ 8-101. The zwitterionic nature of this “twisted” Si state of tetraphenylethylene has recently been suggested from picosecond absorption [ 10,111, picosecond calorimetry [ 12,131 and time-resolved microwave conductivity experiments [ 141. In spite of these advances in the elucidation of the electronic structure of this S, state, knowledge about the molecular (geometrical) structure is still lacking. To the best of the authors’ knowledge, no experimental support for the “twisted” configuration of this S1 state has been obtained from the viewpoint of the molecular structure. It is well established that time-resolved vibrational spectroscopy affords detailed information
0009-2614/94/S 07.00 0 1994 Elsevier Science B.V. All rights reserved. SSDZ OOOOS-2614(93)E1386-U
369
Volume 2 17, number 4
CHEMICAL PHYSICS LETTERS
about the molecular structure of the short-lived transient species appearing in the course of photochemical reactions [ 15 1. Thus, it is highly desirable to obtain the vibrational spectrum of this Si state. In this Letter, we report the first transient Raman spectra of this “twisted” Si state of tetraphenylethylene. In the following text, we refer to this Sr state, which has been claimed to have the twisted configuration, simply as the twisted S, state.
2. Experimental The nanosecond time-resolved Raman spectra were measured by using the apparatus described previously [ 16 1. The second Stokes line of a Dz Raman shifter (3 16 nm, 10 ns, 50 Hz) pumped by the fourth harmonic of a pulsed Q-switch Nd: YAG laser (Spectron SL801) was used to photoexcite tetraphenylethylene. The first Stokes line of a Hz Raman shifter (4 17 nm, 10 ns, 50 Hz) pumped by the third harmonic of another pulsed Q-switch Nd : YAG laser (Spectron SL804) was used to probe Raman scattering. The firing of these two lasers was synchronized by a delay generator (Stanford Research System DG535). The pump and the probe laser pulses were focused on a thin film-like jet stream of the sample solution. Typical pulse energies of the pump and probe lasers were 0.3 mJ (x4 MW/cm2) and 1.7 mJ (my22 MW/cm2) at the sample point, respectively. The scattered light at 90” to the laser beams was collected by a camera lens (Nikon), and analyzed by a triple polychromator (Jobin-Yvon T64000) equipped with an intensified photodiode array (Princeton Instruments IRY 1024G/RB). The acquired data were stored and analyzed with a personal computer (NEC PC9801 ). The nanosecond time-resolved absorption spectra were measured as described previously [ 161. A pulsed Xe lamp (Hamamatsu L2358) was used as the monitoring light source. The nanosecond pumping laser pulses (3 16 nm) were focused by a cylindrical lens onto the 1 cm quartz cell through which the sample solution was circulated. The monitoring light transmitted through the sample cell was analyzed by a monochromator (Jobin-Yvon HR320) and detected by an intensified photodiode array (Princeton Instruments IRY-700G/B/par). The de370
21 January 1994
tector was used with the 5 ns gate. Several glass Elters were used to attenuate the monitoring light and eliminate the second-order diffraction of the spectrometer. The acquired data were stored and analyzed with a personal computer (NEC PC980 1). Tetraphenylethylene ( (C,H, ),C=C( C6H5)2) was purchased from Tokyo Kasei Kogyo and recystallized from hexane several times before use. The 13C substituted tetraphenylethylene ( (C6H5)2’3C= C( C6H5)2) was prepared from “C diphenylmethane and dichlorodiphenylmethane [ 171. The “C substituted diphenylmethane was synthesized from benzene and 13C-benzyl chloride (Isotec 13C99.5%) by the Friedel-Crafts reaction [ 18 1. The synthesized sample was purified by column chromatography. Hexane and heptane (special grade or liquid chromatography grade, Wako Pure Chemical Industries) were used as obtained.
3. Results and discussion Fig. 1 shows nanosecond time-resolved absorption spectra of tetraphenylethylene obtained from a 4.5~ 10m4 mol drnm3 hexane solution. The excitation wavelength is 316 nm. In the spectrum at 0 ns, transient absorption showing a maximum around 420 nm is clearly observed. Negative signals appearing in the wavelength region longer than 500 nm are due to r 0.20
1
0.15 /Ons 8 5 0.10 e $2 0.05 1A u
20 ns
I
500 600 Wavelength /nm Fig. 1. Time-resolved absorption spectra of tetraphenylethylene inhexane(4.5~10-~moldm-~;pumplaser316nm).Inset:tima resolved emission spectra.
Volume 2 17, number 4
CHEMICAL. PHYSICS LETTERS
the strong fluorescence of tetraphenylethylene (see the inset in fig. 1) [ 8 19 1. The distortion of the spectrum in the region shorter than 450 nm is small since the intensity of the monitoring light is much higher than that of the fluorescence in this region. The transient absorption band around 420 nm disappears in the spectrum at a delay time of 20 ns. Greene first reported time-resolved absorption spectra of tetraphenylethylene by using picosecond excitation, and observed two transient species having different lifetimes [ 9 1. He assigned the short-lived transient species (T= 5 ps) showing absorption around 640 nm to the “unrelaxed” planar S, state, and the other longlived transient (7= 3 ns) exhibiting an absorption maximum around 417 nm to the twisted Si state of tetraphenylethylene. The transient absorption measured in the present experiment is assigned to the long-lived twisted Si state. The twisted Si state of tetraphenylethylene is accessible with the nanosecond apparatus having a time-resolution of as low as 10 ns. Fig. 2 shows nanosecond time-resolved Raman spectra obtained from a 9 x 10m4mol dme3 heptane solution. The pump and probe wavelengths are 316 and 4 17 nm, respectively. Heptane, in place of hexane, was used as the solvent in order to avoid evaporation during the measurement and resultant change of the sample concentration. In the spectra at a time delay of 0 ns, more than twenty transient Raman
I
I
15ns
I , , 1600
I
I.,
I
1200
I
I
(
I
600
I
I
(
400
I
-
-
’
Raman Shift /cm.’ Fig. 2. Time-resolved resonance Raman spectra of tetraphenylethyleneinheptane(9x10-4moldm-3;pumplaser316nm;probe laser 417 nm).
21 January 1994
bands are observed in the wavenumber region from 200 to 1700 cm-‘. They completely disappear at a time delay of 15 ns. The weak Raman bands recognized around 1600 cm- ’ in the spectrum at 15 ns are due to the So state of tetraphenylethylene. The temporal behavior of the transient Raman bands agrees with that of the transient absorption due to the twisted S1 state. Thus, the transient Raman bands observed in the spectrum at 0 ns are assignable to the twisted S, state of tetraphenylethylene. The probing laser at 417 nm is in exact resonance with S,cS, transition of the twisted Si state, and the Raman bands due to this species should be highly resonance enhanced. Ma and Zimmt suggested that the “relaxed” planar Si state is in equilibrium with the twisted S, state [ 19 1. However, the contribution from this “relaxed” planar Si state is expected to be negligible in the obtained Raman spectra because the population of this state is estimated to be as low as 1w [14]. In order to derive information about the molecular structure of the twisted S1 state from the obtained Raman spectra, we have to make vibrational assignments. Here, we do not intend to make full vibrational assignments but discuss several key vibrations which are important for the elucidation of the molecular structure. First, we focus on the oletinic C,C, stretching vibration. The Raman spectra of the normal species and the 13Canalogues of the twisted S, state of tetraphenylethylene are compared in fig. 3. In these spectra, the solvent bands are already subtracted. The Raman spectra of the ground state obtained from crystalline samples are also shown in this figure. Expanded spectra of two wavenumber regions of interest are shown in fig. 4. The most striking difference between the S,, and Si states is found in the double bond region between 1500 and 1600 cm-‘. In the So spectrum, three bands are observed in this region ( 1595, 1588 and 1564 cm-‘). The band at 1564 cm-’ of the normal species is assigned to the oleflnic C,C, stretch because this band shows a 19 cm- ’ downshift upon the 13Csubstitution. The remaining two bands are assigned to the CC stretches of the phenyl rings. However, the 8 cm-’ “C shift ofthe 1588 cm-’ band ( 1588 cm-‘+ 1580 cm-‘) indicates that there is a strong coupling between the C,C, stretch and a phenyl CC stretch in the So state. On the other hand, 371
Volume 2 17, number 4
CHEMICAL PHYSICS LETTERS
21 January 1994
,2~(s) I
DJ& ‘7” I c 1800
0
iL I
1600
s gf ’
‘3C (s,) I
1400
-
I 1200
r
I 1000
@ I 600
’
I 600
’
I 400
’
1
’
200
Raman Shift km”
Fig. 3. Raman spectra of tetraphenylethylene in the twisted S, state and those in the SOstate. From the top to the bottom, (A) normal species in the Sr state, (B) r3C analogue in the S, state, (C) normal species in the SOstate, (D) r3C analogue in the Sc state (S,: probe laser 4 17 nm; Sc: probe laser 5 14.5 nm ) .
Raman Shift /cm”
Fig. 4. Raman spectra of tetraphenylethylene in the twisted S, state and those in the Sc state. From the top to the bottom, normal species in the S, state, r3C analogue in the S1 state, normal species in the So state, and ‘“C analogue in the SOstate ( S1: probe laser417nm;S,:probelaser514.5nm).
372
in the spectrum of the S, state, only two Raman bands are observed at 1587 and 1543 cm-’ in this frequency region. These two Raman bands are assigned to the phenyl CC stretch since they show no shift with the 13C substitution. A small change in the shape of the valley between these two phenyl ring bands is due to the unexcited ground state molecules. No Raman band assignable to the C,C, stretch is observed in this frequency region. In addition, a complete lack of the 13C shift implies that there is no coupling between the C,C, stretch and the phenyl CC stretches in the S, state. The absence of coupling between the C,C, stretch and the phenyl CC stretch can be rationalized if the C,C, stretching frequency drastically changes on going from the ground state to the S, state. The twisted geometry of the S, state is expected to induce a marked decrease of the bond order of the C,C, bonding and to result in the significant decrease of the C,C, stretching frequency. The obtained vibrational data are consistent with this expectation, and support the twisted configuration of S, tetraphenylethylene. We sought the C&C, stretch band in the single bond region of the Si spectra, but no Raman band ascribable to this vibration was found. The only Raman band exhibiting a significant 13C shift is the weak
Volume 2 17, number 4
CHEMICAL PHYSICS LETTERS
band located at 1255 cm-’ (fig. 4). This band shows a broadening in addition to the frequency downshift upon the 13C substitution. A similar broadening is found for the 1264 cm- ’ band in the S,, Raman spectrum. The asymmetrical 13Csubstitution is expected to give rise to small splitting in the band due to the C,-phenyl stretch because the substituted species has two inequivalent C,-phenyl bondings ( “C,-phenyl and ‘3C,-phenyl). If the splitting is small, it should be observed as a band broadening in the Raman spectra. Thus, it is concluded that the Raman bands showing band broadening upon the “C substitution (1255 cm-’ in S1 and 1264 cm-’ in So) are due to the vibrational modes having contributions from the C,-phenyl stretch. However, the effect of the “C substitution on the 1255 cm-’ band of the S, state is somewhat different from that on the 1264 cm- ’ band in the So state. It seems that the 13Cshift of the 1255 cm-l band is significantly larger than that of the 1264 cm-’ band of the S,-,state although the signal-to-noise ratio of the S1 spectrum of the i3C analogue is low. It is probable that the S, state has much lower C,C, stretching frequency and it causes mode mixing between the C,C, stretch and the C,-phenyl stretch. A larger 13Cshift of the 1255 cm-’ band of the S, spectrum may be due to this mode mixing and might be an indication of the C,C, stretching frequency in the single bond region. Next, we discuss the vibrational frequencies of the phenyl CC stretch bands in the S1 state. As described before, two Raman bands assignable to the phenyl CC stretches are observed at 1587 and 1543 cm-’ in the S, spectra. The frequency difference of these two bands (44 cm-‘) is large, compared with the frequency difference of the two phenyl CC stretch bands in the So state (7 cm- ’ ). It suggests that a significant structural change is also induced in the phenyl ring parts with the excitation from the So state to the S, state. Recently, the zwitterionic nature of the twisted S, state was experimentally clarified [ 10-141. It means that tetraphenylethylene has both anionic and cationic diphenyl methyl moiety in the twisted S1 state. These two electronically different moieties having the same atomic frame are expected to give different vibrational frequency, if the exchange of the cationic and the anionic parts (A+-A-*A--A+ ) is slower than the Raman optical process. It is known that the anion and the cation radicals of aromatic
21 January 1994
compounds have quite different frequencies for the phenyl CC stretch. For example, the frequency of the phenyl CC stretch of the biphenyl cation radical is 1616 cm-‘, and that of the anion radical is 1587 cm- ’ [ 20,2 11. A similar frequency difference is also found between the cation and the anion radicals of trans stilbene ( 1605 cm-’ in the cation radical and 1577 cm-’ in the anion radical) [ 15,22-251. The frequency differences between the cation and the anion radicals of these two compounds (29 and 28 cm- ’ ) are similar to that of the two phenyl vibration of S1 tetraphenylethylene (44 cm-‘) although the absolute frequencies are different by a few tens of wavenumbers. It is likely that the large difference in the frequency of the two phenyl CC stretch bands reflect the existence of the cationic and the anionic moieties in S, tetraphenylethylene, and it may be a manifestation of the zwitterionic nature of this twisted excited state. Finally, we briefly mention the character of the S,+-S, transition revealed in the Raman spectrum. It is well known that resonance Raman intensities provide information about the electronic transition with which the Raman probing wavelength is in resonance. For the Franck-Condon (Albrecht’s A term) type resonance, the vibrational modes along which the potential minimum is displaced with the electronic transition give rise to strong Raman bands in the resonance Raman spectra [26]. In the Raman spectra of twisted S, tetraphenylethylene, only the intensities due to the phenyl ring vibrations are highly enhanced, and that related to the C,C, stretch is low. This means that the structural change is induced primarily on the phenyl rings along with the S,tS, transition, and indicates that this transition is localized on the phenyl ring moiety. Schilling and Hilinski pointed out the similarity of the absorption spectra among S1 tetraphenylethylene, the diphenylmethyl cation, and its anion, and argued that this similarity is a manifestation of the zwitterionic nature of S1 tetraphenylethylene [lo]. The selective resonance enhancement of the phenyl Raman bands is consistent with localized electronic transitions characteristic to the zwitterionic structure of the S1 state of tetraphenylethylene.
373
Volume 2 17, number 4
CHEMICAL PHYSICS LETTERS
References
[ 1 ] T. Arai and K. Tokumaru, Chem. Rev. 93 (1993) 23. [2] J. Saltiel and J.L. Charlton, in: Rearrangement in ground and excited states, ed. P. de Mayo (Academic Press, New York, 1980) p. 25. [3] R.J. Buenker and SD. Peyerimhoff, Chem. Phys. 9 (1976) 75. [4] B.R. Brooks and H.F. Schaefer III, J. Am. Chem. Sot. 101 ( 1979) 307. [ 5 ] I. Nebot-Gil and J. Malrieu, Chem. Phys. Letters 84 ( 198 1) 571. [ 61 I. Nebot-Gil and J. Mahieu, J. Am. Chem. Sot. 104 (1982) 3320. [ 7 ] S. Abrash, S. Repinec and R.M. Hochstrasser, J. Chem. Phys. 93 (1990) 1041. [ 81 P.F. Barbara, S.D. Rand and P.M. Rentzepis, J. Am. Chem. Sot. 103 (1981) 2156. [ 91 B.I. Greene, Chem. Phys. Letters 79 ( 198 1) 5 1. [ lo] C.L. Schilling and E.F. Hilinski, J. Am. Chem. Sot. 110 (1988) 2296. [ll]Y.-P. Sun and M.A. Fox, J. Am. Chem. Sot. 115 (1993) 747. [ 121 M.B. Zimmt, Chem. Phys. Letters 160 (1989) 564. [ 131 J. Morais, J. Ma and M.B. Zimmt, J. Phys. Chem. 95 ( 1991) 3885.
374
21 January 1994
[ 141 W. Schuddeboom, S.A. Jonker, J.M. Warman, M.P. de Haas, M.J.W. Vermeulen, W.F. Jager, B. de Lange, B.L. Feringa and R.W. Fessenden, J. Am. Chem. Sot. 115 (1993) 3286. [ 151 H. Hamaguchi, in: Vibrational spectra and structure, ed. J.R. Durig (Elsevier, Amsterdam, 1987) p. 227. [ 161 T. Tahara and H. Hamaguchi, J. Phys. Chem. 96 ( 1992) 8252. [ 17 ] D.A. Shirley, Preparation of organic intermediates (Wiley, New York, 195 1) p. 272. [ 181 L.F. Fieser, Experiments in organic chemistry (Heath, Lexington, 1957) ch. 28. [19]J.MaandM.B.Zimmt,J.Am.Chem.Scc.114(1992)9723. [ 201 C. Kato, H. Hamaguchi and M. Tasumi, Chem. Phys. Letters 120 (1985) 183. [ 2 1 ] C. Takahashi and S. Maeda, Chem. Phys. Letters 24 ( 1974) 584. [22]L.R. Dosser, J.B. Pallix, G.H. Atkinson, H.C. Wang, G. Levin and M. Sxwarc, Chem. Phys. Letters 62 ( 1979) 555. [23] W. Hub, S. Schneider, F. Doerr, J.T. Simpson, J.D. Oxman andF.D. Lewis, J.Am. Chem.Soc. 104 (1982) 2044. [ 24 ] W. Hub, S. Schneider, F. Doerr, J.D. Gxman and F.D. Lewis, J. Am. Chem. Sot. 106 (1984) 708. [25] W. Hub, U. Klueter, S. Schneider and F. Doerr, J. Phys. Chem. 88 (1984) 2308. [ 261 A.C. Albrecht, J. Chem. Phys. 34 ( 196 1) 1476.