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Correlation between thermal expansion and optical band splitting in polydiacetylene-bis(toluenesulfonate) single crystals
CHEWCAL PHYSICS LETTERS
Volume 43, number 1
CORRELATION
1 October 1976
EXPANSION AND OPTICAL BAND SPLFL-TING1(N POLYDIACETYLENE-BIS(TOLUENESULFONATE) SINGLE CRYSTALS BETWEEN
THERMAL
B. REIMER, H. BASSLER Fachbereich Physikalische Chemie der Univenitiir Marbug, D 3550 MarburgjL.ahn, Germany
and T. DEBAERDEMAEKER Sektion RLintgen- und Elektronenbeugung der Universittil Ulm. D 7900 Urn, Germany Rcccived 7 J%ne 1976
The temperawe dependence of the Iattice constants of PDA-TS sin& ctystals has been me-red in the nngc 100 to 300 K. it is found that the splitting of the 2 EV absorption band varies cx~onentialiy with the distance of the polymer chains along the crystallographic c-direction. This indicates that splitting arisesfrom interaction.
Single crystals of polydiacetytenes consist of a parallel array of conjugated polymer molecules of practically infinite length [ 1] . As a consequence the lowest optical absorption band, which arises from a transition within the n-electron system of the cari‘on backbone, is polarized along the chain axis. Recem regection studies on polydiacetylenc-bis(totuenesulfo~ate) single crystals, hereafter referred to as PDA-TS, showed [2,3] that the 0-0 electronic absorption peak and its vibrational satellites split into doublets as the temperature is lowered. Between 200 and 4 K the splitting energy A increases from 15 to 32 me\‘, the existence of a finite splitting at 300 K has been inferred indirectly. Starting from the band description for delocalized electronic states (3) Reimer et al. f4] explained the splitting in terms of a weak but finite interaction between neighbouring polymer chains, i.e. a deviation of the system from the ideal one-dimensional behaviour. This interpretation seems to be contradicted by recent results of Bloor et al. [5] indicating that splitting is not inherently coupled to the presence of a fully polymerized crystal lattice but occurs also in diacetytcne monomer crystals containing only 0.1 - 1%polymer. As a consequence band
intcrmofccuiar
splitting has been interpreted as an intrinsic property of the individual chains_ Since clarification of this. problem is essential for the understanding of the optical properties of a large conjugated molecule in general and single crystals of polydiacetylenc as model substances for a one-dimensional system in particular, we undertook an attempt to distinguish between both interpretations by measuring the temperature dependence of the lattice parameters of PDA-TS. In the case that band splitting is due to molecular coupling a correlation between the temperature dependences of A and the inte~o~ecu~ar distance should exist, otherwise not. The cell parameters were obtained from the measurement of 17 strong reff exions (some were symmetry related) at different temperatures ranging between room temperature and -170°C in steps of 20” (up to -160* and a last value at --L70°C) on a Philips PWI 100 automatic fourcircle diffractometer equipped with a low temperature device. Accurate cell parameters were obtained by a least square refinement of the measured angle values. The standard deviations are O.OO7,0.004 and 0.008 for the o, b and c ceU parameters. ‘Ihe variation of the unit cell parameters in u-and 85
Volume 43. number :
c-direction with. temperature is plotted in fig-l. The variation of b is within the limits of instrumcntal accuracy. The room temperature data arc in fair agreement with literature data (61 which are a = 14.49 A, b = 4.91 A and c = 14.94 A. The following conclusions can be drawn :
(i) Thermal expansion; of PDA-TS is an anisotropic property. Above 200 K the expansion coefticicnts are
ac=(4.0i0.1)X
TDBERNUIE
c(7J strongly argues against the occurrence of a phase transition in PDA-TS. The coexistence slightly
different
equilibrium
positions
of two
for a poly-
mer molecule within the unit ccl1 can also be excluded, since the temperature dependence of the diffraction
pattern clearly indicates
the presence
of a single, well-defined crystal structure at any temperature. The fact that aQ > ac can qualitatively be understood from the molecular arrangement shown in fig. 2. Van dcr Waals interaction between neighbouring chains is larger along c- than along udirection. Consequently the slope of the potential
10s5 K-’
for the crystallographic G- and c-directions. No ncgativc values for a are obscrvcd in contrast to results obtained for the bis-(phenylurethane)polymer [7] . (ii) For T< 200 K the curve c(7) displays a pronounced decrease and levels off at T < 120 K yielding an extrapolated low-temperature expansion coefficient of 1.4 X 10-S K- t. In a-direction the fas: decrease of the lattice constant between 120 and 200 K is absent. Instead the expansion coefficient a,, decreases continuously to about 2.5 X 10m5 beldw 120 K. For T< 100 K and T> 200 K the ratio a,Jac is ! .TS, independent of tcmperature. (iii) The lack of any discontinuity in a(T) and
(K)
Fig. 1. Open circksz Temperature dcpendcnce of the lattice parameters of PDA-IS in crystagographic u- and c-direc tions (left scale). FuU circles: Splitting energy A of the optical 2 eV absorption band taken from ref. [4] (right
‘scale). Tjw dashed portion is derived from an extrapot!ion of the A (c)_curve of fii. 3.
86
1 October 1976
CIIEMICAL PHYSICS LEI-PERS
energy curve near the equilibrium position is huger along the c-direction and, concomitantly, tcmperature variation has a smaller influence on the molecular equilibrium distance. The fast variation of the lattice parameter c between 120 and 200 K must be due to slight changes in the orientation of the toluenesulfonate sidechains allowing closer molecular approach at low temperatures. At the moment this assignment is tentative and needs to be substantiated by a complete structural analysis at various temperatures which is planned for the future. The most important result of the present investigation is the correlation between the temperature variation of the splitting energy A of the optical 2 eV transition taken from previous work (41 and the lattice parameter in c-direction, the only crystallographic direction along which a significant interaction between the carbon backbone of neighbouring polymer molecules is possible. (The interchain distance along c is c/2 a 7.5 A as compared to 14.5 A along c-direction.) The inverse relationship between A(T) and c(T) clearly demonstrates that band splitting results from intermolecular coupling along cdirection which is temperature-sensitive. This confirms the model proposed in ref. [4] and argues against any interpretation of the band-splitting in terms of a single-molecule picture [5]. There is no contradiction to data obtained by resonant Raman scattering [9] indicating that the energies of the molecular vibrations coupling to each of the doublet-components are slightly different, since a sirnitar effect is also known from the vibronic absorption spectra of molecular crystals showing Davydov splitting [ lC] . Further information on theband splitting can
Volume 43, number 1
CUEMICAL
PHYSICS LEMERS
1 October 1976
%-CH2-R R-C&-F’
R : -O-i-@-%
0 Fig. 2. MoIccu~ar arrangement within the unit cell of a poiyd~cetylen~b~ (i~~uencsu~onat~) sir@ crystal (according to ref. ISi ). The central portion of the molecule is the projection of the conjugated carbonskeleton ontb tltcoc-plane. The chain axis is perpertdicular to the oc-plane. On the left side the polymer chain is shown.
be derived from a plot of Ab(7’)versus chain-distance c/2. Fig. 3 shows that an exponential dependence of the form A(n = Ah, exp (-Xc/Z) exists, Such a relationship is to be expected si;lce interaction requires overlap of tfre n-electron wavefunctions of the carbon skeleton of neighbouring molecules which for relatively large distances decay exponentially. Empirically X=5.6 X 108 cm- I is found. This value compares favourabfy with the spatial decay of the electron density of a ground state carbon atom in the configuration 3P, 2sZ2pZat large distances from the nucleus. The decay constants of the relevant Sfater functions in the linear combination for the electron wavefunc~on are 2.7 X 10” cm’ I and i .8 X 108cm-t [11].Thcyhavetobemultipliedbya
Lo-
-2
defects are required for chain formation. It is therefore likely that in the initial stage of polymerization mi~ro~bres are formed starting from defect sites and
5. 30
E g e f
20 i
t 101
735
factor of 2 to describe spatial decay of the electron density. There is no physicaf reason why the relationship between A and c/2 should be confined to the range: c/2 < 7.45 A . Extrapolation of A(c) up to mofecular distances corresponding to the room-temperature structure yields the dashed portion of the spfitting energy versus temperature. It substantiates previous data which have been derived indirectly from line broadening and which consequently were associated with a considerable error. It also confirms existence of a finite level splitting of about 16 meV at 300 K, The observation of fevef splitting also in partially polymerized diacetyfene crystals (SJ stiggests that polymer molecules incorporated in the monomer matrix are not dispersed on-a single molecular level. This is supported by the observation of a nthcr Iong induction period for thermal pofyme~~tion [ 1J indicating that nucleation centers other than point
\
*.
containing several molecules. Interchain interaction Ieading to splitting of the electronic levels can there-
=\
fort occur also in dilute samples. I
x0 cHm
745
1 750
DISTANCE (d)
Fig. 3. Splitting energy of the optical transition versus chain distance in aystilfographic c-direction.
We gratefully acknowledge cooperation with the Fachbereich Ceowissenschaften of tie PhilippsU&versittit, ,Marburg, when performing the X-ray
87
Volume 43. number
1
CHEMICAL
measurements. This work was supported by the Deutsche Forschungsgemeinschaft.
Zcferenccs [l] C. Wcgncr, Makromoi. Chcm. 145 (1971) 85. [ 21 3. Bloor. D.J. Ando. F.H. Preston and CC. Stevens, Chcm. Phys. Letters 24 (1974) 407. [ 3 1 E.G. Wilson. J. Phys. C 8 ( 1975) 727. 14 J B. Rcimer. H. Bkler. J. Iiessc and G. Weircr, Phys. Stat. Sol. 73 b (1976) 709.
88
PHYSICS LElTERS
1 October
1976
[ 51 D. Bloor. F.H. Preston and D.J. Ando. Chem. Phys. Lctters 38 (1976) 33. [6J D. Kobclt and F. Paulus. ActaCryst. B 30 (1974) 231. 171 P-H. Baughman, J. Chcm. Phys. 58 (1973) 2876. [8] W. Schcrmann. G. Wegner. J.O. Williams and J.M. Thomas, J. Polymer Sci. Polymer Phys. Ed. 13 (1975) 753. [9] D-N. Batchelder and D. B!oor, Chcm. Phys. Letters 38 ( 1976) 37. [IO] A Brec and LE. Lyons, J. Chem. Sot. (1960) 5206. (I I] E. Ctemcnti and C.C.J. Roothaan, Phys. Rev. 127 (1962) 1618.
Volume 43, number 1
CORRELATION
1 October 1976
EXPANSION AND OPTICAL BAND SPLFL-TING1(N POLYDIACETYLENE-BIS(TOLUENESULFONATE) SINGLE CRYSTALS BETWEEN
THERMAL
B. REIMER, H. BASSLER Fachbereich Physikalische Chemie der Univenitiir Marbug, D 3550 MarburgjL.ahn, Germany
and T. DEBAERDEMAEKER Sektion RLintgen- und Elektronenbeugung der Universittil Ulm. D 7900 Urn, Germany Rcccived 7 J%ne 1976
The temperawe dependence of the Iattice constants of PDA-TS sin& ctystals has been me-red in the nngc 100 to 300 K. it is found that the splitting of the 2 EV absorption band varies cx~onentialiy with the distance of the polymer chains along the crystallographic c-direction. This indicates that splitting arisesfrom interaction.
Single crystals of polydiacetytenes consist of a parallel array of conjugated polymer molecules of practically infinite length [ 1] . As a consequence the lowest optical absorption band, which arises from a transition within the n-electron system of the cari‘on backbone, is polarized along the chain axis. Recem regection studies on polydiacetylenc-bis(totuenesulfo~ate) single crystals, hereafter referred to as PDA-TS, showed [2,3] that the 0-0 electronic absorption peak and its vibrational satellites split into doublets as the temperature is lowered. Between 200 and 4 K the splitting energy A increases from 15 to 32 me\‘, the existence of a finite splitting at 300 K has been inferred indirectly. Starting from the band description for delocalized electronic states (3) Reimer et al. f4] explained the splitting in terms of a weak but finite interaction between neighbouring polymer chains, i.e. a deviation of the system from the ideal one-dimensional behaviour. This interpretation seems to be contradicted by recent results of Bloor et al. [5] indicating that splitting is not inherently coupled to the presence of a fully polymerized crystal lattice but occurs also in diacetytcne monomer crystals containing only 0.1 - 1%polymer. As a consequence band
intcrmofccuiar
splitting has been interpreted as an intrinsic property of the individual chains_ Since clarification of this. problem is essential for the understanding of the optical properties of a large conjugated molecule in general and single crystals of polydiacetylenc as model substances for a one-dimensional system in particular, we undertook an attempt to distinguish between both interpretations by measuring the temperature dependence of the lattice parameters of PDA-TS. In the case that band splitting is due to molecular coupling a correlation between the temperature dependences of A and the inte~o~ecu~ar distance should exist, otherwise not. The cell parameters were obtained from the measurement of 17 strong reff exions (some were symmetry related) at different temperatures ranging between room temperature and -170°C in steps of 20” (up to -160* and a last value at --L70°C) on a Philips PWI 100 automatic fourcircle diffractometer equipped with a low temperature device. Accurate cell parameters were obtained by a least square refinement of the measured angle values. The standard deviations are O.OO7,0.004 and 0.008 for the o, b and c ceU parameters. ‘Ihe variation of the unit cell parameters in u-and 85
Volume 43. number :
c-direction with. temperature is plotted in fig-l. The variation of b is within the limits of instrumcntal accuracy. The room temperature data arc in fair agreement with literature data (61 which are a = 14.49 A, b = 4.91 A and c = 14.94 A. The following conclusions can be drawn :
(i) Thermal expansion; of PDA-TS is an anisotropic property. Above 200 K the expansion coefticicnts are
ac=(4.0i0.1)X
TDBERNUIE
c(7J strongly argues against the occurrence of a phase transition in PDA-TS. The coexistence slightly
different
equilibrium
positions
of two
for a poly-
mer molecule within the unit ccl1 can also be excluded, since the temperature dependence of the diffraction
pattern clearly indicates
the presence
of a single, well-defined crystal structure at any temperature. The fact that aQ > ac can qualitatively be understood from the molecular arrangement shown in fig. 2. Van dcr Waals interaction between neighbouring chains is larger along c- than along udirection. Consequently the slope of the potential
10s5 K-’
for the crystallographic G- and c-directions. No ncgativc values for a are obscrvcd in contrast to results obtained for the bis-(phenylurethane)polymer [7] . (ii) For T< 200 K the curve c(7) displays a pronounced decrease and levels off at T < 120 K yielding an extrapolated low-temperature expansion coefficient of 1.4 X 10-S K- t. In a-direction the fas: decrease of the lattice constant between 120 and 200 K is absent. Instead the expansion coefficient a,, decreases continuously to about 2.5 X 10m5 beldw 120 K. For T< 100 K and T> 200 K the ratio a,Jac is ! .TS, independent of tcmperature. (iii) The lack of any discontinuity in a(T) and
(K)
Fig. 1. Open circksz Temperature dcpendcnce of the lattice parameters of PDA-IS in crystagographic u- and c-direc tions (left scale). FuU circles: Splitting energy A of the optical 2 eV absorption band taken from ref. [4] (right
‘scale). Tjw dashed portion is derived from an extrapot!ion of the A (c)_curve of fii. 3.
86
1 October 1976
CIIEMICAL PHYSICS LEI-PERS
energy curve near the equilibrium position is huger along the c-direction and, concomitantly, tcmperature variation has a smaller influence on the molecular equilibrium distance. The fast variation of the lattice parameter c between 120 and 200 K must be due to slight changes in the orientation of the toluenesulfonate sidechains allowing closer molecular approach at low temperatures. At the moment this assignment is tentative and needs to be substantiated by a complete structural analysis at various temperatures which is planned for the future. The most important result of the present investigation is the correlation between the temperature variation of the splitting energy A of the optical 2 eV transition taken from previous work (41 and the lattice parameter in c-direction, the only crystallographic direction along which a significant interaction between the carbon backbone of neighbouring polymer molecules is possible. (The interchain distance along c is c/2 a 7.5 A as compared to 14.5 A along c-direction.) The inverse relationship between A(T) and c(T) clearly demonstrates that band splitting results from intermolecular coupling along cdirection which is temperature-sensitive. This confirms the model proposed in ref. [4] and argues against any interpretation of the band-splitting in terms of a single-molecule picture [5]. There is no contradiction to data obtained by resonant Raman scattering [9] indicating that the energies of the molecular vibrations coupling to each of the doublet-components are slightly different, since a sirnitar effect is also known from the vibronic absorption spectra of molecular crystals showing Davydov splitting [ lC] . Further information on theband splitting can
Volume 43, number 1
CUEMICAL
PHYSICS LEMERS
1 October 1976
%-CH2-R R-C&-F’
R : -O-i-@-%
0 Fig. 2. MoIccu~ar arrangement within the unit cell of a poiyd~cetylen~b~ (i~~uencsu~onat~) sir@ crystal (according to ref. ISi ). The central portion of the molecule is the projection of the conjugated carbonskeleton ontb tltcoc-plane. The chain axis is perpertdicular to the oc-plane. On the left side the polymer chain is shown.
be derived from a plot of Ab(7’)versus chain-distance c/2. Fig. 3 shows that an exponential dependence of the form A(n = Ah, exp (-Xc/Z) exists, Such a relationship is to be expected si;lce interaction requires overlap of tfre n-electron wavefunctions of the carbon skeleton of neighbouring molecules which for relatively large distances decay exponentially. Empirically X=5.6 X 108 cm- I is found. This value compares favourabfy with the spatial decay of the electron density of a ground state carbon atom in the configuration 3P, 2sZ2pZat large distances from the nucleus. The decay constants of the relevant Sfater functions in the linear combination for the electron wavefunc~on are 2.7 X 10” cm’ I and i .8 X 108cm-t [11].Thcyhavetobemultipliedbya
Lo-
-2
defects are required for chain formation. It is therefore likely that in the initial stage of polymerization mi~ro~bres are formed starting from defect sites and
5. 30
E g e f
20 i
t 101
735
factor of 2 to describe spatial decay of the electron density. There is no physicaf reason why the relationship between A and c/2 should be confined to the range: c/2 < 7.45 A . Extrapolation of A(c) up to mofecular distances corresponding to the room-temperature structure yields the dashed portion of the spfitting energy versus temperature. It substantiates previous data which have been derived indirectly from line broadening and which consequently were associated with a considerable error. It also confirms existence of a finite level splitting of about 16 meV at 300 K, The observation of fevef splitting also in partially polymerized diacetyfene crystals (SJ stiggests that polymer molecules incorporated in the monomer matrix are not dispersed on-a single molecular level. This is supported by the observation of a nthcr Iong induction period for thermal pofyme~~tion [ 1J indicating that nucleation centers other than point
\
*.
containing several molecules. Interchain interaction Ieading to splitting of the electronic levels can there-
=\
fort occur also in dilute samples. I
x0 cHm
745
1 750
DISTANCE (d)
Fig. 3. Splitting energy of the optical transition versus chain distance in aystilfographic c-direction.
We gratefully acknowledge cooperation with the Fachbereich Ceowissenschaften of tie PhilippsU&versittit, ,Marburg, when performing the X-ray
87
Volume 43. number
1
CHEMICAL
measurements. This work was supported by the Deutsche Forschungsgemeinschaft.
Zcferenccs [l] C. Wcgncr, Makromoi. Chcm. 145 (1971) 85. [ 21 3. Bloor. D.J. Ando. F.H. Preston and CC. Stevens, Chcm. Phys. Letters 24 (1974) 407. [ 3 1 E.G. Wilson. J. Phys. C 8 ( 1975) 727. 14 J B. Rcimer. H. Bkler. J. Iiessc and G. Weircr, Phys. Stat. Sol. 73 b (1976) 709.
88
PHYSICS LElTERS
1 October
1976
[ 51 D. Bloor. F.H. Preston and D.J. Ando. Chem. Phys. Lctters 38 (1976) 33. [6J D. Kobclt and F. Paulus. ActaCryst. B 30 (1974) 231. 171 P-H. Baughman, J. Chcm. Phys. 58 (1973) 2876. [8] W. Schcrmann. G. Wegner. J.O. Williams and J.M. Thomas, J. Polymer Sci. Polymer Phys. Ed. 13 (1975) 753. [9] D-N. Batchelder and D. B!oor, Chcm. Phys. Letters 38 ( 1976) 37. [IO] A Brec and LE. Lyons, J. Chem. Sot. (1960) 5206. (I I] E. Ctemcnti and C.C.J. Roothaan, Phys. Rev. 127 (1962) 1618.