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Temperature-dependent Raman spectra of polydiacetylene single crystals excited in the near infrared
Volume 198, number 3,4
CHEMICAL PHYSICS LETTERS
9 October 1992
Temperature-dependent Raman spectra of polydiacetylene single crystals excited in the near infrared ‘C. Engert, A. Materny and W. Kiefer Institutftir
Physikalische Chemie, Universitiit
Wiirzburg, Marcusstrasse
9-1 I, W-8700
Wiirzburg, Germany
Received 6 July 1992; in final form 20 July 1992
Raman spectra of the black polydiacetylene single crystals TS6 and TS/FBS were recorded by means of a conventional Raman spectrometer with excitation in the near-infrared spectral region. We observed several sidegroup vibrations below 900 cm-’ additional to the four strong vibrational modes which modulate the electronic states on the conjugated polymer backbone. The four strong bands of TS6 showed a splitting at low temperatures. The temperature dependences ofthese bands are discussed.
1. Introduction
One of the main advantages of Raman excitation in the near-infrared (NIR) spectral region is the possibility to record spectra of samples which highly absorb and/or fluoresce when excited with visible light. The advantages of this method have been widely discussed in combination with two different approaches: the conventional apparatus which uses a scanning double monochromator [l-3] and the Fourier transform technique [ 4-61. Two of us have demonstrated that by means of the dispersive method excellent polarized spectra from deeply coloured single crystals can be obtained [ 2 1. In this letter, we apply this technique to the Raman spectroscopic study of a polydiacetylene (PDA) single crystal which shows extremely high absorption in the visible spectral region. In fact the material studied 5s black. Fluorescence may arise in some cases from defect structures of the crystal lattice [ 71, Considerable attention has been paid to diacetylenes (DAs) during the last years due to their ability to undergo a topochemical solid state polymerization according to
Correspondence to: W. Kiefer, Institut ftir Physikalische Chemie, Universitit Wiirzburg, Marcusstrasse 9-11, W-8700 Wiirzburg, Germany.
[R-C=C-C=C-R’ ] ,, hv.kr
fC&C+-CR’#,
,
when treated with high energy radiation or heat. R and R’ arc the substituents of the DAs and strongly influence their properties. Great interest was focused on the K-conjugated polymerized diacetylenes (PDAs) and their properties. These are candidates for interesting nonlinear optical properties and therefore interesting materials for future applications in optoelectronics or molecular electronics [ 81. In the present work we have investigated polymer single crystals which were obtained by heating diacetylene crystals to about 60°C for several days. The DAs were of types TS/FBS (R: -CH2-O-S02-C6H4CH3 and R’: -CH,-0-SO,-C&-F) synthesized according to Strohriegl and Bertault et al. [9] and TS6 (R=R’: -CH*-O-SO&H,-CH,) synthesized according to Wegner [ 101. The PDA TS6 shows a second-order phase transition [ 11,121 below temperatures of about 180 K. This leads to a doubling of the unit cell and to a coexistence of two kinds of chains. The latter results in a partial splitting of the optical absorption lines in the order of about 400500 cm-‘. In resonance Raman investigations on this phase transition the splitting for several Raman modes of the DA polymer chains could be observed [ 131. Particularly, the modes vj and v4 show two
0009-2614/92/$ 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
395
Volume 198,number 3,4
CHEMICALPHYSICSLETTERS
separated peaks when recorded at low temperatures.
In contrast, no splitting has been found for the modes v, and v2.As Batchelder and Bloor [ 131 have shown, the splittings of the uj and v, Raman peaks depend strongly on the actual sample temperature. This is correlated to thermal expansion of the crystal lattice [141. Here we extend this work by making use of excitation out of electronic resonance. Until now, Raman investigations on PDAs were performed mostly with excitation either in strong or in pre-resonance with vibronic transitions associated with the conjugated %-electron system of the polymer backbone. The spectra observed in these cases obtained only a few (resonantly enhanced) Raman transitions which are all correlated to vibrations of the chain. One of the goals of the present study is to elucidate the other modes, In resonance Raman studies on PDAs, heating of the sample due to strong absorption plays an important role [ 151. Without applying special sample techniques it is therefore difficult or even impossible to derive reliable information on temperature-dependent properties of the crystal, e.g. phase transitions with splittings of Raman lines [ 13,161. In this regard, excitation outside the strong absorption of the material should yield better results. As will be shown below, with NIR excitation we have been able to derive more information on the vibrations of this interesting crystal.
2. Experimental The NIR-Raman system has been described in detail elsewhere [ I]. Briefly, it consists of a Jarrell-Ash model 25- 103 Czemy-Turner double monochromator fitted with a pair of 590 lines/mm conventionally ruled Bausch and Lomb gratings blazed at 1.3 urn. The output of a Nd:YAG laser (Medilas from MBB Kybernetik) was filtered by means of a narrow bandpass dielectric interference filter (type 064FS0 l25 from LOT) to remove plasma lines of the krypton flashlamps. A laser power of 200 mW and a spectral resolution of 1.2 cm-’ was used. The spectra were recorded in 90” scattering geometry with an incident angle between exciting laser and the sample surface of approximately 10’. For cooling of the sample we 396
9 October 1992
used a closed-cycle helium cryocooler from Cryogenics, The signal was modulated by chopping the laser beam mechanically and detected with an InGaAs photodetector from Electra-Optical Systems. A lockin technique in connection with a computer/slave system [ 171 has been employed. High-purity DA crystals were cleaved parallel to the (100) surface to produce thin platelets with about 50 urn thickness and about 3 mm length in chain direction. In order to polymerize the DA crystals they were heated for 36 (TS6) or 60 h (TS/FBS) at 62°C. The crystals were fixed on a copper block which was attached to the cold head of the cryocooler. The polarization of the incident laser beam was parallel to the b axes (chain direction) of the PDA crystals.
3. Results and discussion Fig. 1 shows the survey Raman spectrum of TS6 PDA crystal in the range between 5 and 2200 cm-’ taken at a temperature of 298 K. In comparison to the resonance Raman spectra of the PDA crystals [ 181 we observed some additional Raman lines below 900 cm-‘. Infrared absorption spectra of this frequency region are very complex [ 1V] and appear to be dominated by vibrations in the sidegroups, but no specific assignment was possible until now. In future works we plan to include sidegroups in a normal coordinate analysis to gain more information in regard to this aim [ 201. The employed interference filter could only remove plasma lines below 1600 cm- I, therefore the additional lines at about 1920 and 2020 cm-’ are plasma lines of the krypton flashlamps. Although the PDA crystals have 92 atoms per unit cell the conditions for nonresonant Raman spectroscopy are such that only those vibrational modes which modulate the electronic states on the conjugated polymer backbone have significant Raman activity. The Raman cross sections of those bands are much larger than those of the low-frequency modes. This is similar to the observations made for resonance Raman spectroscopy on TS6 PDA crystals [ 2 1,221 and demonstrates that the vibronic properties of the PDAs are mainly determined by the delocalized nelectron system of the polymer backbones. As already mentioned above, the present investi-
Volume198,number3,4
CHEMICAL PHYSICS LETTERS
t_
WAVENUMBER
9 October1992
[cm-‘I
Fig. 1. Surveyroom-temperature Ramanspectrumof a TS6polydiacetylene crystalwithlo= 1064nm laserexcitation.Laserpower P=200 mW, spectral slit width s- 1.2 cm-‘. The asterisk denotes residual plasma lines of the krypton flashlamps.
gation is concerned with the study of the influence of temperature on the Raman spectra of the PDA crystals. As shown in fig. 1 the nonresonant excitation yields similar Raman spectra compared to those for resonant excitation [ 2 1,221, which allows us to investigate the temperature dependence of the vibrational modes of the chains similarly to former studies [ 13,161 where resonance Raman spectroscopy has been applied. Besides the observation of a splitting of the Raman lines associated with the modes v3and v4 (see fig. 1 and ref. [ 12]), we also found that the Raman lines associated with the modes v2 and v, split. For the latter case one should mention, that there is a complication as a result of reabsorption of the scattered Raman light by vibrational bands of Hz0 from water vapor in the air. In fig. 2 we present a series of Raman spectra taken at different temperatures for the spectral range from 1450 to 1500 cm-‘. Besides the line at about 1485 cm-’ ( v2 mode, which contains a considerable amount of C=C stretching) a second line at about 1465 cm-’ can be observed in the temperature range above approximately 185 K. This line belongs to a Fermi-resonance enhanced CHI scissors vibration of the sidegroup ( vCHt) [ 231. It also shows a splitting for the temperatures below 180 K. The splitting of
Fig. 2. The v1chain vibration and the Fermi-resonance enhanced CH2 vibration of the TS6 polydiacetylene crystal as a function of temperature. Aa= 1064 nm; P=200 mW, s= 1.2cm-‘. Note that a pbase transition occurs at about 180 K.
the v2line itself can be observed in this example only by the occurence of a weak shoulder. The splitting of the u2 and V, lines (the latter is not shown here) is smaller than the one observed for the V~and v4modes [ 131, which contain a relatively large amount of sidegroup motions [ 161. This is in accordance with the results from crystallographic studies of the second-order phase transition of the TS6 PDA crystal [ 121 which revealed that mainly the sidegroups of 397
polymer chains are influenced. The fact that in resonance Raman spectroscopic investigations no splitting of the v,, v2 and vCH2modes could be observed may be a consequence of sample heating caused by the laser irradiation despite the much lower power level needed to record spectra near resonance. Apparently, because there is no absorption at 1064 nm this is not the case for the nonresonant excitation. In former investigations it could be shown that sample heating due to excitation in the visible spectral region ieads to a temperature increase up to about 30 K [15]. In order to test this assumption we/determined the shift of the Raman lines belonging to the high-temperature phase as a function of temperature. In fig. 3 the peak positions of tke lines belonging to the u,, vZ, vCH2and v4 modes are given for the TS6 PDA crystal. Fig. 4 shows the same for a TS/FBS PDA crystal. In fig. 3 we also present data for the line belonging to the vCH2mode in the low-temperature
the
-
7
Phase transition
4
Phase transition
954
I-.
952 0
L
00
o 50
I 0
”
0
Y
Y
100 150 TEMPERATURE
0
0
0
200 (K) mm->
0
*
250
0
0
300
Fig. 3. Wavenumbers of the v,, v2and vqchain vibrations of TS6 polydiacetylene crystal as a function of temperature; A,,= 1064 nm.
398
9 October 1992
CHEMICAL PHYSICS LETTERS
Volume 198, number3,4
J
2087 2085
VI
n
2083zoai-
f
f
tz=
=t
957-
I i
955-
vq
q51 0L-.0
50
100 TEMPERATURE
(") mm->
Fig. 4. Wavenumbers of the IQ,uLand vqchain vibrations of TS/ FBS polydiacetylene crystal as a function of temperature; &,= 1064 nm. phase (marked by plus signs). The data were taken by increasing and decreasing the temperature. No hysteresis was observed which is in accordance with the observations by Cottle et al. [ 161. The temperature dependence of the line positions are similar for TS6 and TS/FBS PDA crystals. There are only slight deviations. Comparing our results for the TS6 PDA crystal with those given by Cottle et al. we notice differences for the quantitative temperature dependence particularly for the v2and v4mode but also for the other observed Raman lines. We explain these differences by heating effects of the PDA crystals when resonant excitation is applied which shifts the line positions and additionally makes them dependent on the laser intensity. In our experiment we can exclude such effects because the wavelength of the exciting laser light is in the NIR spectral region for the PDA crystals, where no absorption is present. Summarizing we can say that Raman spectroscopy of PDAs with excitation in the NIR can provide better information when definite temperature conditions are required. Especially temperature dependences of Raman modes can be determined with much higher confidence. Also the possibility to gain
Volume 198, number 3,4
CHEMICAL PHYSICS LETTERS
Raman spectra from samples which are very sensitive against heating and which would be destroyed by laser excitation in the visible spectral region makes this form of Raman spectroscopy very interesting.
Acknowledgement We thank Professor Markus Schwoerer, Universit& Bayreuth, for making the DA crystals available to us. Thanks are also due to Mrs. Irene Mtiller for growing the excellent crystals and to Mr. Thilo Michelis for the development of software for data acquisition. One of us (AM) thanks the Stiftung Volkswagenwerk and the Fonds der Chemischen Industrie for a postgraduate scholarship. We gratefully acknowledge financial support from the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 347, Project C2) and from the Fonds der Chemischen Industrie.
References [ I] C. Engert, T. Michelis and W. Kiefer, Appl. Spectry. 45 (1991) 1333. [2] C. Engertand W. Kiefer, J. Raman Spectry. 22 (1991) 715. [ 3 ] D.R. Porterfield and A. Campion, J. Am. Chem. Sot. 110 (1988) 408. [4] T. Hirschfeldand D.B. Chase, Appl. Spectry. 40 ( 1986) 133. [5] D.B. Chase, J. Am. Chem. Sot. 108 (1986) 7485. [6] C.G. Zimba, V.M. Hallmark, J.D. Swalen and J.F. Rabolt, Appl. Spectry. 41 (1987) 721. [7] D. Bloor, D.N. Batchelder and F.H. Preston, Phys. Stat. Sol. (a) 40 (1977)279;
9 October 1992
M. Dudley, J.N. Sherwood, D.J. Ando and D. Bloor, in: Polydiacctylenes, Vol. E2 eds. D. Bloor and R.R. Chance, NATO ASI Series (Nijhoff, Dordrecht, 1985) p. 87. [S] J.L. Brtdas and R.R. Chance, eds., Conjugated polymeric materials: opportunities in electronics, optoelectronics, and molecular electronics, NATO ASI Series E I82 (Kluwer, Dordrecht, 1990). [ 91 P. Strohriegl, Makromol. Chem. Rapid Commun. 8 (1987) 437; M. Bertault, L. Toupet, J. Canccill and A. Collet, Makromol. Chem. Rapid Commun. 8 ( 1987) 443. [lo] G. Wegner, in: Molecular metals, ed. W.E. Hatfield (Plenum Press, New York, 1979). [ 111D. Bloor, F.H. Preston and D.J. Ando, Chem. Phys. Letters 38 (1976) 33. [ 121R.L. Williams, D. Bloor, D.N. Batchelder, M.B. Hursthouse and W.B. Daniels, Faraday Discussions Chem. Sot. 69 (1980) 49. [ 131D.N. Batchelder and D. Bloor, Chem. Phys. Letters 38 (1976) 37. [ 141B. Reimer, H. Bsssler and T. Debaerdemaeker, Chem. Phys. Letters 43 ( 1976) 85. [ 151M. Lankers, D. GBttges, A. Materny, K. Schaschek and W. Kiefer, Appl. Spectry. 46 (1992),in press. [ 161A.C. Cottle, W.F. Lewis and D.N. Batchelder, J. Phys. C 11 (1978) 605. [ 171T. Michelis and W. Kiefer, to be published. [ 181D. Bloor, F.H. Preston, D.J. Ando and D.N. Batchelder, in: Structural studies of macromolecules by spectroscopic methods, ed. K.J. Ivin (Wiley, New York, 1976) p. 91. [ 191J. Kiji, J. Kaiser, G. Wegner and R.C. Schultz, Polymer I4 (1973) 433. [20] C. Engert, A. Materny and W. Kiefer, in progress. [21] W.F. Lewis and D.N. Batchelder, Chem. Phys. Letters 60 (1979) 232. [ 221 D.N. Batchelder and D. Bloor, in: Advances in infrared and Raman spectroscopy, Vol. 11, eds. R.J.H. Clark and R.E. Hester (Wiley, New York, 1984) ch. 4. [23] D.N. Batchelder and D.J. Bloor, J. Polym. Sci. Polym. Phys. Ed. 17 (1979) 569.
399
CHEMICAL PHYSICS LETTERS
9 October 1992
Temperature-dependent Raman spectra of polydiacetylene single crystals excited in the near infrared ‘C. Engert, A. Materny and W. Kiefer Institutftir
Physikalische Chemie, Universitiit
Wiirzburg, Marcusstrasse
9-1 I, W-8700
Wiirzburg, Germany
Received 6 July 1992; in final form 20 July 1992
Raman spectra of the black polydiacetylene single crystals TS6 and TS/FBS were recorded by means of a conventional Raman spectrometer with excitation in the near-infrared spectral region. We observed several sidegroup vibrations below 900 cm-’ additional to the four strong vibrational modes which modulate the electronic states on the conjugated polymer backbone. The four strong bands of TS6 showed a splitting at low temperatures. The temperature dependences ofthese bands are discussed.
1. Introduction
One of the main advantages of Raman excitation in the near-infrared (NIR) spectral region is the possibility to record spectra of samples which highly absorb and/or fluoresce when excited with visible light. The advantages of this method have been widely discussed in combination with two different approaches: the conventional apparatus which uses a scanning double monochromator [l-3] and the Fourier transform technique [ 4-61. Two of us have demonstrated that by means of the dispersive method excellent polarized spectra from deeply coloured single crystals can be obtained [ 2 1. In this letter, we apply this technique to the Raman spectroscopic study of a polydiacetylene (PDA) single crystal which shows extremely high absorption in the visible spectral region. In fact the material studied 5s black. Fluorescence may arise in some cases from defect structures of the crystal lattice [ 71, Considerable attention has been paid to diacetylenes (DAs) during the last years due to their ability to undergo a topochemical solid state polymerization according to
Correspondence to: W. Kiefer, Institut ftir Physikalische Chemie, Universitit Wiirzburg, Marcusstrasse 9-11, W-8700 Wiirzburg, Germany.
[R-C=C-C=C-R’ ] ,, hv.kr
fC&C+-CR’#,
,
when treated with high energy radiation or heat. R and R’ arc the substituents of the DAs and strongly influence their properties. Great interest was focused on the K-conjugated polymerized diacetylenes (PDAs) and their properties. These are candidates for interesting nonlinear optical properties and therefore interesting materials for future applications in optoelectronics or molecular electronics [ 81. In the present work we have investigated polymer single crystals which were obtained by heating diacetylene crystals to about 60°C for several days. The DAs were of types TS/FBS (R: -CH2-O-S02-C6H4CH3 and R’: -CH,-0-SO,-C&-F) synthesized according to Strohriegl and Bertault et al. [9] and TS6 (R=R’: -CH*-O-SO&H,-CH,) synthesized according to Wegner [ 101. The PDA TS6 shows a second-order phase transition [ 11,121 below temperatures of about 180 K. This leads to a doubling of the unit cell and to a coexistence of two kinds of chains. The latter results in a partial splitting of the optical absorption lines in the order of about 400500 cm-‘. In resonance Raman investigations on this phase transition the splitting for several Raman modes of the DA polymer chains could be observed [ 131. Particularly, the modes vj and v4 show two
0009-2614/92/$ 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
395
Volume 198,number 3,4
CHEMICALPHYSICSLETTERS
separated peaks when recorded at low temperatures.
In contrast, no splitting has been found for the modes v, and v2.As Batchelder and Bloor [ 131 have shown, the splittings of the uj and v, Raman peaks depend strongly on the actual sample temperature. This is correlated to thermal expansion of the crystal lattice [141. Here we extend this work by making use of excitation out of electronic resonance. Until now, Raman investigations on PDAs were performed mostly with excitation either in strong or in pre-resonance with vibronic transitions associated with the conjugated %-electron system of the polymer backbone. The spectra observed in these cases obtained only a few (resonantly enhanced) Raman transitions which are all correlated to vibrations of the chain. One of the goals of the present study is to elucidate the other modes, In resonance Raman studies on PDAs, heating of the sample due to strong absorption plays an important role [ 151. Without applying special sample techniques it is therefore difficult or even impossible to derive reliable information on temperature-dependent properties of the crystal, e.g. phase transitions with splittings of Raman lines [ 13,161. In this regard, excitation outside the strong absorption of the material should yield better results. As will be shown below, with NIR excitation we have been able to derive more information on the vibrations of this interesting crystal.
2. Experimental The NIR-Raman system has been described in detail elsewhere [ I]. Briefly, it consists of a Jarrell-Ash model 25- 103 Czemy-Turner double monochromator fitted with a pair of 590 lines/mm conventionally ruled Bausch and Lomb gratings blazed at 1.3 urn. The output of a Nd:YAG laser (Medilas from MBB Kybernetik) was filtered by means of a narrow bandpass dielectric interference filter (type 064FS0 l25 from LOT) to remove plasma lines of the krypton flashlamps. A laser power of 200 mW and a spectral resolution of 1.2 cm-’ was used. The spectra were recorded in 90” scattering geometry with an incident angle between exciting laser and the sample surface of approximately 10’. For cooling of the sample we 396
9 October 1992
used a closed-cycle helium cryocooler from Cryogenics, The signal was modulated by chopping the laser beam mechanically and detected with an InGaAs photodetector from Electra-Optical Systems. A lockin technique in connection with a computer/slave system [ 171 has been employed. High-purity DA crystals were cleaved parallel to the (100) surface to produce thin platelets with about 50 urn thickness and about 3 mm length in chain direction. In order to polymerize the DA crystals they were heated for 36 (TS6) or 60 h (TS/FBS) at 62°C. The crystals were fixed on a copper block which was attached to the cold head of the cryocooler. The polarization of the incident laser beam was parallel to the b axes (chain direction) of the PDA crystals.
3. Results and discussion Fig. 1 shows the survey Raman spectrum of TS6 PDA crystal in the range between 5 and 2200 cm-’ taken at a temperature of 298 K. In comparison to the resonance Raman spectra of the PDA crystals [ 181 we observed some additional Raman lines below 900 cm-‘. Infrared absorption spectra of this frequency region are very complex [ 1V] and appear to be dominated by vibrations in the sidegroups, but no specific assignment was possible until now. In future works we plan to include sidegroups in a normal coordinate analysis to gain more information in regard to this aim [ 201. The employed interference filter could only remove plasma lines below 1600 cm- I, therefore the additional lines at about 1920 and 2020 cm-’ are plasma lines of the krypton flashlamps. Although the PDA crystals have 92 atoms per unit cell the conditions for nonresonant Raman spectroscopy are such that only those vibrational modes which modulate the electronic states on the conjugated polymer backbone have significant Raman activity. The Raman cross sections of those bands are much larger than those of the low-frequency modes. This is similar to the observations made for resonance Raman spectroscopy on TS6 PDA crystals [ 2 1,221 and demonstrates that the vibronic properties of the PDAs are mainly determined by the delocalized nelectron system of the polymer backbones. As already mentioned above, the present investi-
Volume198,number3,4
CHEMICAL PHYSICS LETTERS
t_
WAVENUMBER
9 October1992
[cm-‘I
Fig. 1. Surveyroom-temperature Ramanspectrumof a TS6polydiacetylene crystalwithlo= 1064nm laserexcitation.Laserpower P=200 mW, spectral slit width s- 1.2 cm-‘. The asterisk denotes residual plasma lines of the krypton flashlamps.
gation is concerned with the study of the influence of temperature on the Raman spectra of the PDA crystals. As shown in fig. 1 the nonresonant excitation yields similar Raman spectra compared to those for resonant excitation [ 2 1,221, which allows us to investigate the temperature dependence of the vibrational modes of the chains similarly to former studies [ 13,161 where resonance Raman spectroscopy has been applied. Besides the observation of a splitting of the Raman lines associated with the modes v3and v4 (see fig. 1 and ref. [ 12]), we also found that the Raman lines associated with the modes v2 and v, split. For the latter case one should mention, that there is a complication as a result of reabsorption of the scattered Raman light by vibrational bands of Hz0 from water vapor in the air. In fig. 2 we present a series of Raman spectra taken at different temperatures for the spectral range from 1450 to 1500 cm-‘. Besides the line at about 1485 cm-’ ( v2 mode, which contains a considerable amount of C=C stretching) a second line at about 1465 cm-’ can be observed in the temperature range above approximately 185 K. This line belongs to a Fermi-resonance enhanced CHI scissors vibration of the sidegroup ( vCHt) [ 231. It also shows a splitting for the temperatures below 180 K. The splitting of
Fig. 2. The v1chain vibration and the Fermi-resonance enhanced CH2 vibration of the TS6 polydiacetylene crystal as a function of temperature. Aa= 1064 nm; P=200 mW, s= 1.2cm-‘. Note that a pbase transition occurs at about 180 K.
the v2line itself can be observed in this example only by the occurence of a weak shoulder. The splitting of the u2 and V, lines (the latter is not shown here) is smaller than the one observed for the V~and v4modes [ 131, which contain a relatively large amount of sidegroup motions [ 161. This is in accordance with the results from crystallographic studies of the second-order phase transition of the TS6 PDA crystal [ 121 which revealed that mainly the sidegroups of 397
polymer chains are influenced. The fact that in resonance Raman spectroscopic investigations no splitting of the v,, v2 and vCH2modes could be observed may be a consequence of sample heating caused by the laser irradiation despite the much lower power level needed to record spectra near resonance. Apparently, because there is no absorption at 1064 nm this is not the case for the nonresonant excitation. In former investigations it could be shown that sample heating due to excitation in the visible spectral region ieads to a temperature increase up to about 30 K [15]. In order to test this assumption we/determined the shift of the Raman lines belonging to the high-temperature phase as a function of temperature. In fig. 3 the peak positions of tke lines belonging to the u,, vZ, vCH2and v4 modes are given for the TS6 PDA crystal. Fig. 4 shows the same for a TS/FBS PDA crystal. In fig. 3 we also present data for the line belonging to the vCH2mode in the low-temperature
the
-
7
Phase transition
4
Phase transition
954
I-.
952 0
L
00
o 50
I 0
”
0
Y
Y
100 150 TEMPERATURE
0
0
0
200 (K) mm->
0
*
250
0
0
300
Fig. 3. Wavenumbers of the v,, v2and vqchain vibrations of TS6 polydiacetylene crystal as a function of temperature; A,,= 1064 nm.
398
9 October 1992
CHEMICAL PHYSICS LETTERS
Volume 198, number3,4
J
2087 2085
VI
n
2083zoai-
f
f
tz=
=t
957-
I i
955-
vq
q51 0L-.0
50
100 TEMPERATURE
(") mm->
Fig. 4. Wavenumbers of the IQ,uLand vqchain vibrations of TS/ FBS polydiacetylene crystal as a function of temperature; &,= 1064 nm. phase (marked by plus signs). The data were taken by increasing and decreasing the temperature. No hysteresis was observed which is in accordance with the observations by Cottle et al. [ 161. The temperature dependence of the line positions are similar for TS6 and TS/FBS PDA crystals. There are only slight deviations. Comparing our results for the TS6 PDA crystal with those given by Cottle et al. we notice differences for the quantitative temperature dependence particularly for the v2and v4mode but also for the other observed Raman lines. We explain these differences by heating effects of the PDA crystals when resonant excitation is applied which shifts the line positions and additionally makes them dependent on the laser intensity. In our experiment we can exclude such effects because the wavelength of the exciting laser light is in the NIR spectral region for the PDA crystals, where no absorption is present. Summarizing we can say that Raman spectroscopy of PDAs with excitation in the NIR can provide better information when definite temperature conditions are required. Especially temperature dependences of Raman modes can be determined with much higher confidence. Also the possibility to gain
Volume 198, number 3,4
CHEMICAL PHYSICS LETTERS
Raman spectra from samples which are very sensitive against heating and which would be destroyed by laser excitation in the visible spectral region makes this form of Raman spectroscopy very interesting.
Acknowledgement We thank Professor Markus Schwoerer, Universit& Bayreuth, for making the DA crystals available to us. Thanks are also due to Mrs. Irene Mtiller for growing the excellent crystals and to Mr. Thilo Michelis for the development of software for data acquisition. One of us (AM) thanks the Stiftung Volkswagenwerk and the Fonds der Chemischen Industrie for a postgraduate scholarship. We gratefully acknowledge financial support from the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 347, Project C2) and from the Fonds der Chemischen Industrie.
References [ I] C. Engert, T. Michelis and W. Kiefer, Appl. Spectry. 45 (1991) 1333. [2] C. Engertand W. Kiefer, J. Raman Spectry. 22 (1991) 715. [ 3 ] D.R. Porterfield and A. Campion, J. Am. Chem. Sot. 110 (1988) 408. [4] T. Hirschfeldand D.B. Chase, Appl. Spectry. 40 ( 1986) 133. [5] D.B. Chase, J. Am. Chem. Sot. 108 (1986) 7485. [6] C.G. Zimba, V.M. Hallmark, J.D. Swalen and J.F. Rabolt, Appl. Spectry. 41 (1987) 721. [7] D. Bloor, D.N. Batchelder and F.H. Preston, Phys. Stat. Sol. (a) 40 (1977)279;
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