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A study of the electronic excited states of fully and partially polymerized bis(p-toluene sulphonate) diacetylene crystals by resonant raman scattering
volurnc
CHEMICAL
38, nurI~hcr 1
PHYSICS
LETTERS
IS February 1976
A STUDY OF THE ELECTRONIC EXCITED STATES OF FULLY AND PbRTiALLY, POLYMERKZED bk(p-TOLUENE
SULPHONATE) DIACETYLENE CRYSTALS
BY RESONANT IUhIAN SCATTERING D.N. BATCHELDER and D. BLOOR of Phyrics. Queen Mary College. hndon
Dcpartnrent
El 4h;S. UK
Rcccivcd 13 October 1975
Certain lines in the resonant Raman spectra of fully and partMy polymerized bis(p-tolucflc sulphonatc) single crystals are observed to split into two compncnts below 160 K. The two components cspcricnw maximum intcnsily cnhanccmcnt at different frcqucncics of the incident dye laser beam. The possible origins of the splitting WCdis-
The large nearly
defect-free
single crystals
produced
by thermal polymcrimtion
of Dis(p-tolucne sulphonate) diacctylene provide a unique opportunity for the study of quasi-one-dimensional conjugated systems [ l-51. Thermal polymcrizrition initially produces the highly perfect polymer chains shown in fig. 1 which are loc;ltcd itl rando~n in the rnonorncr matrix 131. It is thus possible to study the polymer as a dilute ordered gas in monomer crystals, or as pure polymer in crystals where conversion has been carried to 100%. Optical absorption and reflectivity sttidics, X-ray diffraction measurements, and Raman spectra IEN~ shown that rhe polymer chains arc nearly identical in the two types of crystal [3, G]. Resonant Raman scattering is a useful tool for the study of both the electronic properties [7] and the lattice vibrations of semiconductors. A wide variety of polydiacetylencs have been studied near room temperature using this technique [8-l 11. These spectra
Fig. 1. Structure of the polydiacetylenc chain. In the toluene sulphonate polymer the side goups R are -CH2-O-SO*-OCHS.
are very striking since onIy whel; these relatively few charge density on the polymer backbone are created is the Raman scattering rcsoxantly enhanced. Several of the Raman lines have been identified with particular phonons by mcitns of a lattice dynamical calculation which only took into account vibrations of the carbon atoms on the backbone [S]. WC have now made studies of the rcscnant Raman spectra as a function of temperature and have observed a nzw effect which has strong implications for the electronic structure of the polymer. optic phonons which modulate the electronic
For the purposes of this investigation two single crystals of bi.s(p-toluene sulphonatc) dincetylenc mon-
omer were prepared, one being carried to 1% and the other to l(lO% conversion into polymer by thermal polymerization. The Raman spectra were studied in the back scattering mocte between 3 and 300 K by mans of a tunable dye laser and double monocbromator system. The I&man scattered radiation for all lines was strongly polarized with the electric field vector parallel to the polymer chain direction. Below 160 K at least twelve of the Raman lines between 100 and 1500 cm-l were observed to be split into two components in both the 1% and the 100% crystals. Here we only present as typical data the results for one Rnrnar~ line of the 1% converted crystais. The behaviour of other split lines in both the 1%Jand the 100% con-
37
VAunc
38. number 1
CffEMICAI. PffYSfCS IXlTERS
15 February 1976
verted crystals has been observed to be qualitatively
similar. The ordinary Kaman sca::tering from the monomer matrix in the 1% conwrted crystals is too weak to be observed as the reso!lant Raman scattering since the I% polymer is so intense. The chosen line has been tentativeIy Identified as due to a phonon which is associated with iarge amplitude stretching of the single bonds on the ~]olyn:cr ‘backbone (see rnodts 3 and 4 of fig. 3 in ref. [S]).. Fig. 2 shows tracings taken from the chart record of the two l&man components ai: 953 and 957 cm-l for the 1% polymer in monomer crystal at 4 K for three different incider.t laser frequencies. Th.e two components are observed singly or together depcnding upon the incident laser fpzquency. The upper curve in fig. 3 shows the optical absoiption spectrum appropriate to a 1% polymer in monomer crystal at 4 K and illustrates the splitting of the single elc&ronic pesk at room temperature into two peaks, A and B [6]. This absorption is due entirely to the dilute polymer as the monomer is essentially transparent in this region of the spectrum. The two lower curves in fig. 3 illustrate, on the sa171e frequency scale, the cross section
for Raman scattering by the 953 2nd 957 cm-l nons at 4 K. Where ncrcssa~
hove been corrcctcd
the raw intensity for reflection, absorption,
phodata and
index of refraction at both the incident and scattered wavelengths [ 12 J. F.
166iJO ml-
16850CM-’
16950
I l
953
0 957
16000
165Oli
CM-’ LINE CM-‘LINE
17000
WAVENUMBER
17500
(CM-‘]
Fig. 3. Upper curve: op;ticat absorption spcctrurn (in arbitrary units) ofa partially polynwized crystal of his@-tolucnc sulphomtc:) diacutylene at 4 K for iight polarized parallel to Ihc polymer cflain. Lower curves: Karnan scattering cross sections (in arbitrxy units) at 4 K for the 953 ami 957 cm-l compo-
nents as a function of the incident laser frequency.
CM-’
With increasing
temperature
to 1GO K the absorpat increasing frequencies, with A shifting considerably more rapidly than B. The linewidths of both A and B increase at the same rate until just below 160 K when peak A starts to broaden at a considerably higher rate. Above 160 K it is no longer possible to distinguish two peaks. The splitting of the 953 and 957 cm-l Raman comtion peaks A and B in fig. 3 are locaicd
ponents follows a similar pattern with both decreasing in frequency as the temperature increases from 4 K.
Fig. 2. Tracings from ilie chart record ior the intensity of UE I&man components at 953 and 957 an-’ for three differunt incidsn( Iaser frequcncks FL. The salicl line below each peak is ihe true intensity zero wd indicates the ubscnce of fluorcscence.
38
The 957 CIII-~ component shifts more rapidly and above 90 K it is no longer possible to distinguish two components. Fig. 4 shows a similar behaviour in the relative splittings of the two electronic absorption peaks and the two &man components as a function
of temperature. Theoretical studies of resonant Raman scattering have predicted
that a sin&
electronic
excited
state,
Volume 3S, number 1
15 February 1976
CIEBfICAL PHYSICS I.El-FERS
gives rise to two different types of polymer chain with slightly difF:rcnt unit cell dimensions in tic chain direction [I 11. Wz can discount this explanation since X-ray stud& have shown that between 7 and 343 K there is only one value of the unit cell dimension in the chain direction [13], and that the phenomenon is observed when the poIyrner chain is either in dilute solution in a monomer crystal-or in a 1Oo%,polymer crystal. The implication is that the splitting of the electronic peaks and Raman lines is a fundamental property of the polymer chain irrespective of its environment.
Fig. 4. Tempcraturc dependence of the splitting of rhc peaks A and D in the optical nbsolpticm spectrum and &heKaman components ;It 953 and 957 an-’ relative to ti~c values at 4 K.
say A above the ground state, will give rise to maxima in the Raman scattering cross section as a function of incident laser frequency [7] _ Two of these maxima occur when
first, the incident
laser frequency
is equal
to A, and second, the frequency of the Rarnan scattered radiation is equal to A. Fig. 3 sl2ow this prcdictcd bchaviour Tar the Raman scattering cross scction of the phonon at 957 CIII-~ if that phonon is correlated with the electronic excited state responsible for peak A alone in the optical absorption spectrum. There are maxima in the enhancement when the laser frequency is equal to 16470 cm-l, the location of peak A, and 17430 cm-1 , essentially the frequency of peak A plus the phonon frequency. In contrast fig. 3 shows that the Raman scattering cross section for the 953 cm-1 phonon is a maximum only when the incident laser frequency was 16970 cm--l , the location of peak E3in tic optical absorption spectrum (the dye laser did not have sufficient range to observe the expected second peak). The two Rvrlan components thus show selective enhancement by the two electronic excited states responsible for peaks A and B, the 953 cm-* component being coupIed to peak B and the 957 cm-1 component to peak A. It is interesting to spcculatc on the origins of the splitting of the clectranic excited states and their associated phonons. The suggestion has been made that the splitting is the result of a phase transition which
The data sugcst to us that peak B which shifts slowly and broadens uniformly with increasing temperature is due to excitation of an electron and a hole from the ground state to a band-like state 1141. For such an excited state the observation of resonantly enhanced Raman spectra’can only occur when the total wavevector is conserved, so the 953 cm-l COIIIponcnt corresponds in the normal way to a zero wavcvector phonon. By contrast peak a shifts more rapidly and has anomalous broadening when the separation between peaks A and B becomes less than kT. This behaviour suggests that peak B is due to an excited elcctronic state which is strondy coupled to lattice distortion,
for exarnplc
a polaron
or polariton
type of
excitation. If the coupling is so strong that the excitation is essentially immobile then the wavcvcctor need not be conserved for a resonant enhancement effected by that excitation. The 957 cm-l ccmponent would thus be due to the same phonon as the 953 cm-l component, but for that phonon with finite wavcvector. It is not clear what factor determines the value of the wavevector, but that value dues decrease towards zero with increasing temperature. Considerably more experimentation will be required before the interesting electronic propertics of polydiacetylene chains are thoroughly understood. In particular extension of photoconductivity studies [t 5, IS] to low temperature should be very informative.
This research was supported by a grant from the Science Research Council. We wish to thank Mr. D.J. Ando for lhc monomer preparation. and our colleagues in the Polymer Research Group tit QMC, in particular Dr. E.G. Wilson, for stimulating discussions.
39
Volume 38, number
1
CHEMICAL PHYSICS LETTERS
References [ 11 G. Wegncr, 2. Naturforscb. 24b (196s) 824. [Z] D. Moor, D.J. Ando, F.H. Preston and CX. Stcvera, Chem. Phys. Letters 24 (1974) 407. [3] D. Bloor, L. Koski, G.C. Stevens, F.H. Prestonand DJ. Ando, J. hlatcr. Sci., to be published. 13 ] D. Bloor, L. Koski and G.C. Stevens, 1. his ter. Sci., to be published. [St G.C. Stevens and D. Bloor; J. Polymer Sci. Polymer Pbys., to be publisbcd. [S] D. Bloor, J;.ft. Presto.? and D.J. Ando, Chem. Pbys. . Letters 38 (1976) 33. [7 J R.M. Martin and L.M. Falicov, in: Topics in applied physics, Vol. 8, Light scattering in solids, ed. hl. C mkma (Springer, Bcrlin, 1975).
if31D.
15 February 1976
Bloor. D.J. Ando, F.11. Preston and D.N. Batchelder, in: Structural studies of macromoIecuIes by spcctroscopic methods, cd. K.J. Jvin (Wiley, New York), to be published. A.J. hlelvepr and R.H. Baugbman, J. Poiymcr Sci. A2 ll(1973j603. 1101R.H. Baugbman, G.J. Exarbos and W.M. Risen Jr., J. Polymer Sci. A2 12 (1974) 2189. Llll R.H. Baugkman. GJ. Exarhos and WM. Risen Jr., J. Am. Chem. Sot., to be published. R.H. Callender, S.S. Sussman, M. Selders and R.K. Clrang. Phys. Rev. B7 (1973) 3788. D.N. Batchsider, to be published. E.G. Wihon, J. Phys. C 8 (1975) 727. W. Schemwm and G. Wcgncr, Makromol. Chcm. 175 (1974) 667. 11611~.Rcimer and ll. Bdssler, to be published.
CHEMICAL
38, nurI~hcr 1
PHYSICS
LETTERS
IS February 1976
A STUDY OF THE ELECTRONIC EXCITED STATES OF FULLY AND PbRTiALLY, POLYMERKZED bk(p-TOLUENE
SULPHONATE) DIACETYLENE CRYSTALS
BY RESONANT IUhIAN SCATTERING D.N. BATCHELDER and D. BLOOR of Phyrics. Queen Mary College. hndon
Dcpartnrent
El 4h;S. UK
Rcccivcd 13 October 1975
Certain lines in the resonant Raman spectra of fully and partMy polymerized bis(p-tolucflc sulphonatc) single crystals are observed to split into two compncnts below 160 K. The two components cspcricnw maximum intcnsily cnhanccmcnt at different frcqucncics of the incident dye laser beam. The possible origins of the splitting WCdis-
The large nearly
defect-free
single crystals
produced
by thermal polymcrimtion
of Dis(p-tolucne sulphonate) diacctylene provide a unique opportunity for the study of quasi-one-dimensional conjugated systems [ l-51. Thermal polymcrizrition initially produces the highly perfect polymer chains shown in fig. 1 which are loc;ltcd itl rando~n in the rnonorncr matrix 131. It is thus possible to study the polymer as a dilute ordered gas in monomer crystals, or as pure polymer in crystals where conversion has been carried to 100%. Optical absorption and reflectivity sttidics, X-ray diffraction measurements, and Raman spectra IEN~ shown that rhe polymer chains arc nearly identical in the two types of crystal [3, G]. Resonant Raman scattering is a useful tool for the study of both the electronic properties [7] and the lattice vibrations of semiconductors. A wide variety of polydiacetylencs have been studied near room temperature using this technique [8-l 11. These spectra
Fig. 1. Structure of the polydiacetylenc chain. In the toluene sulphonate polymer the side goups R are -CH2-O-SO*-OCHS.
are very striking since onIy whel; these relatively few charge density on the polymer backbone are created is the Raman scattering rcsoxantly enhanced. Several of the Raman lines have been identified with particular phonons by mcitns of a lattice dynamical calculation which only took into account vibrations of the carbon atoms on the backbone [S]. WC have now made studies of the rcscnant Raman spectra as a function of temperature and have observed a nzw effect which has strong implications for the electronic structure of the polymer. optic phonons which modulate the electronic
For the purposes of this investigation two single crystals of bi.s(p-toluene sulphonatc) dincetylenc mon-
omer were prepared, one being carried to 1% and the other to l(lO% conversion into polymer by thermal polymerization. The Raman spectra were studied in the back scattering mocte between 3 and 300 K by mans of a tunable dye laser and double monocbromator system. The I&man scattered radiation for all lines was strongly polarized with the electric field vector parallel to the polymer chain direction. Below 160 K at least twelve of the Raman lines between 100 and 1500 cm-l were observed to be split into two components in both the 1% and the 100% crystals. Here we only present as typical data the results for one Rnrnar~ line of the 1% converted crystais. The behaviour of other split lines in both the 1%Jand the 100% con-
37
VAunc
38. number 1
CffEMICAI. PffYSfCS IXlTERS
15 February 1976
verted crystals has been observed to be qualitatively
similar. The ordinary Kaman sca::tering from the monomer matrix in the 1% conwrted crystals is too weak to be observed as the reso!lant Raman scattering since the I% polymer is so intense. The chosen line has been tentativeIy Identified as due to a phonon which is associated with iarge amplitude stretching of the single bonds on the ~]olyn:cr ‘backbone (see rnodts 3 and 4 of fig. 3 in ref. [S]).. Fig. 2 shows tracings taken from the chart record of the two l&man components ai: 953 and 957 cm-l for the 1% polymer in monomer crystal at 4 K for three different incider.t laser frequencies. Th.e two components are observed singly or together depcnding upon the incident laser fpzquency. The upper curve in fig. 3 shows the optical absoiption spectrum appropriate to a 1% polymer in monomer crystal at 4 K and illustrates the splitting of the single elc&ronic pesk at room temperature into two peaks, A and B [6]. This absorption is due entirely to the dilute polymer as the monomer is essentially transparent in this region of the spectrum. The two lower curves in fig. 3 illustrate, on the sa171e frequency scale, the cross section
for Raman scattering by the 953 2nd 957 cm-l nons at 4 K. Where ncrcssa~
hove been corrcctcd
the raw intensity for reflection, absorption,
phodata and
index of refraction at both the incident and scattered wavelengths [ 12 J. F.
166iJO ml-
16850CM-’
16950
I l
953
0 957
16000
165Oli
CM-’ LINE CM-‘LINE
17000
WAVENUMBER
17500
(CM-‘]
Fig. 3. Upper curve: op;ticat absorption spcctrurn (in arbitrary units) ofa partially polynwized crystal of his@-tolucnc sulphomtc:) diacutylene at 4 K for iight polarized parallel to Ihc polymer cflain. Lower curves: Karnan scattering cross sections (in arbitrxy units) at 4 K for the 953 ami 957 cm-l compo-
nents as a function of the incident laser frequency.
CM-’
With increasing
temperature
to 1GO K the absorpat increasing frequencies, with A shifting considerably more rapidly than B. The linewidths of both A and B increase at the same rate until just below 160 K when peak A starts to broaden at a considerably higher rate. Above 160 K it is no longer possible to distinguish two peaks. The splitting of the 953 and 957 cm-l Raman comtion peaks A and B in fig. 3 are locaicd
ponents follows a similar pattern with both decreasing in frequency as the temperature increases from 4 K.
Fig. 2. Tracings from ilie chart record ior the intensity of UE I&man components at 953 and 957 an-’ for three differunt incidsn( Iaser frequcncks FL. The salicl line below each peak is ihe true intensity zero wd indicates the ubscnce of fluorcscence.
38
The 957 CIII-~ component shifts more rapidly and above 90 K it is no longer possible to distinguish two components. Fig. 4 shows a similar behaviour in the relative splittings of the two electronic absorption peaks and the two &man components as a function
of temperature. Theoretical studies of resonant Raman scattering have predicted
that a sin&
electronic
excited
state,
Volume 3S, number 1
15 February 1976
CIEBfICAL PHYSICS I.El-FERS
gives rise to two different types of polymer chain with slightly difF:rcnt unit cell dimensions in tic chain direction [I 11. Wz can discount this explanation since X-ray stud& have shown that between 7 and 343 K there is only one value of the unit cell dimension in the chain direction [13], and that the phenomenon is observed when the poIyrner chain is either in dilute solution in a monomer crystal-or in a 1Oo%,polymer crystal. The implication is that the splitting of the electronic peaks and Raman lines is a fundamental property of the polymer chain irrespective of its environment.
Fig. 4. Tempcraturc dependence of the splitting of rhc peaks A and D in the optical nbsolpticm spectrum and &heKaman components ;It 953 and 957 an-’ relative to ti~c values at 4 K.
say A above the ground state, will give rise to maxima in the Raman scattering cross section as a function of incident laser frequency [7] _ Two of these maxima occur when
first, the incident
laser frequency
is equal
to A, and second, the frequency of the Rarnan scattered radiation is equal to A. Fig. 3 sl2ow this prcdictcd bchaviour Tar the Raman scattering cross scction of the phonon at 957 CIII-~ if that phonon is correlated with the electronic excited state responsible for peak A alone in the optical absorption spectrum. There are maxima in the enhancement when the laser frequency is equal to 16470 cm-l, the location of peak A, and 17430 cm-1 , essentially the frequency of peak A plus the phonon frequency. In contrast fig. 3 shows that the Raman scattering cross section for the 953 cm-1 phonon is a maximum only when the incident laser frequency was 16970 cm--l , the location of peak E3in tic optical absorption spectrum (the dye laser did not have sufficient range to observe the expected second peak). The two Rvrlan components thus show selective enhancement by the two electronic excited states responsible for peaks A and B, the 953 cm-* component being coupIed to peak B and the 957 cm-1 component to peak A. It is interesting to spcculatc on the origins of the splitting of the clectranic excited states and their associated phonons. The suggestion has been made that the splitting is the result of a phase transition which
The data sugcst to us that peak B which shifts slowly and broadens uniformly with increasing temperature is due to excitation of an electron and a hole from the ground state to a band-like state 1141. For such an excited state the observation of resonantly enhanced Raman spectra’can only occur when the total wavevector is conserved, so the 953 cm-l COIIIponcnt corresponds in the normal way to a zero wavcvector phonon. By contrast peak a shifts more rapidly and has anomalous broadening when the separation between peaks A and B becomes less than kT. This behaviour suggests that peak B is due to an excited elcctronic state which is strondy coupled to lattice distortion,
for exarnplc
a polaron
or polariton
type of
excitation. If the coupling is so strong that the excitation is essentially immobile then the wavcvcctor need not be conserved for a resonant enhancement effected by that excitation. The 957 cm-l ccmponent would thus be due to the same phonon as the 953 cm-l component, but for that phonon with finite wavcvector. It is not clear what factor determines the value of the wavevector, but that value dues decrease towards zero with increasing temperature. Considerably more experimentation will be required before the interesting electronic propertics of polydiacetylene chains are thoroughly understood. In particular extension of photoconductivity studies [t 5, IS] to low temperature should be very informative.
This research was supported by a grant from the Science Research Council. We wish to thank Mr. D.J. Ando for lhc monomer preparation. and our colleagues in the Polymer Research Group tit QMC, in particular Dr. E.G. Wilson, for stimulating discussions.
39
Volume 38, number
1
CHEMICAL PHYSICS LETTERS
References [ 11 G. Wegncr, 2. Naturforscb. 24b (196s) 824. [Z] D. Moor, D.J. Ando, F.H. Preston and CX. Stcvera, Chem. Phys. Letters 24 (1974) 407. [3] D. Bloor, L. Koski, G.C. Stevens, F.H. Prestonand DJ. Ando, J. hlatcr. Sci., to be published. 13 ] D. Bloor, L. Koski and G.C. Stevens, 1. his ter. Sci., to be published. [St G.C. Stevens and D. Bloor; J. Polymer Sci. Polymer Pbys., to be publisbcd. [S] D. Bloor, J;.ft. Presto.? and D.J. Ando, Chem. Pbys. . Letters 38 (1976) 33. [7 J R.M. Martin and L.M. Falicov, in: Topics in applied physics, Vol. 8, Light scattering in solids, ed. hl. C mkma (Springer, Bcrlin, 1975).
if31D.
15 February 1976
Bloor. D.J. Ando, F.11. Preston and D.N. Batchelder, in: Structural studies of macromoIecuIes by spcctroscopic methods, cd. K.J. Jvin (Wiley, New York), to be published. A.J. hlelvepr and R.H. Baugbman, J. Poiymcr Sci. A2 ll(1973j603. 1101R.H. Baugbman, G.J. Exarbos and W.M. Risen Jr., J. Polymer Sci. A2 12 (1974) 2189. Llll R.H. Baugkman. GJ. Exarhos and WM. Risen Jr., J. Am. Chem. Sot., to be published. R.H. Callender, S.S. Sussman, M. Selders and R.K. Clrang. Phys. Rev. B7 (1973) 3788. D.N. Batchsider, to be published. E.G. Wihon, J. Phys. C 8 (1975) 727. W. Schemwm and G. Wcgncr, Makromol. Chcm. 175 (1974) 667. 11611~.Rcimer and ll. Bdssler, to be published.