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Lattice simulations of thermochromic distortions in poly(alkylthiophene)s
ELSEVIER
SyntheticMetals 101(1999) 318-319
Lattice simulations of thermochromic distortions in poly(alkylthiophene)s Hongwei Xiea, J. Co&h”, S. G. All”, D.A. Morton-Blakea> , and K. Aasmundtveitb aChemishy Department, bInstituff forfidtlr,
Trinip
College Dublin, Ireland.
Norges Teknisk Natzrrvitenskapeiige
Universitet, Trondheim, Norway.
Abstract
The thermochromism of poly(3-alkylthiophene)s is explainedasa bucklingof the mainchainby torsionsaroundthe inter-ring bonds resultingin a dilation of the rc bandenergygap. Atomi>tic simulationcalculationsshowthat the necessityto preservetranslational periodicity imposes restrictionson the permittedtorsions.In our modelthe effect of heatingcauses a relative shift of adjacentpolymLr chains,changingthe ring stackingfrom ‘staggered’to ‘eclipsed’,in which the chainsundergoa spontaneous torsionof 20”. Kqvwords:Thermochromic polymers,Atomistic simulations, Latticestructures,Main chaindistortions,
1. Introduction
The substitutionof flexible side chainsinto the electroactive materialpolythiopheneconfersseveralinterestingpropertieson the polymer. One of tfiese is thermochromism, as a result of which the polymer undergoesa colour change on heating. Probably, reversible torsionsoccur in the conjugatedpolymer backbone,but the detailedmechanism is riot known [l]. Arbitrary torsionsin a chain initially in the most stablea$’ cotiguration (Fig. 1) wouldresultin a lossnot only of the main-
a
Fig. 1. The PSBTlattice showingthe staggered relationshipof
the two chainsin the unit cell, which are distinguished by shading.The b axis is normalto the figure.
chain’speriodicity, but alsoof the directionin which the polymer is aligned.Alternatively, imposinga set of constant torsionson the polymerbackbonewouldcreatea helical chain [Z]. In either case,the presenceof long alkyl side chainswould makethese processes highly activated.Jnfact the therrnochromictransitionis reportedto preserve the orientationof the mainchain131. = Poly(3,3’-dioctyl-2,2’-bithiophene),PDOBT, differs from poly(3-allqlthiophene)(P3AT) in that the connectionsof the alhylthiopheneunits are ‘headto head’ rather than the ‘headto tail’ arrangementof Fig. 1. A~diffraction investigationof the latter showsthat the PDOBT backboneis permanentlytwisted [4], and is not tlxxrnochromic. The repeatsegmentof the chain consistsof four alkylthiophenerings with a periodic torsional pattern [cp, O”, -9, O”,cp,O”, . . .], which conferstranslational periodicityon the polymerchainfor anyvalue of cp. 2. Lattice structure While unsubstituted polythiophenechainsform a lattice in which
they occupytwo non-parallelplanes[5], the a&y1 side chainsin P3ATswould generatetoomuchemptyspacein sucha structure. The substitutionof the hydrogenat the 3 positionby a groupeven assmallasmethyl causes the chainsto stack in (almost)parallel planes[6]. An importantstructuralfeature of the P3ATs is the number of chains in the unit cell. Several difiaction investigationsconcludethat there are two chainsin a stacked arrangement,but that their relative positionsshow a shift %C (half the chainrepeatunit of Fig. 1) [3]. This impliesthat the thiophenerings in adjacentchainsalong the stackingdirection (the b axis) showa staggeredrelationshipasshownin the Figure. Otherdiffractionworkersdescribechainsthat aresimplystacked alongb with no displacement alongc, resultingin a structurein
0379~6779/991$ - seefrontmatter0 1999ElsevierScience S.A.-AlIrightsreserved. PII: 50379-6779(98)01 176-X
S.G. Ali
et al. / Synthetic
Metals 101
(1999)
318-319
319
A: the 4-&g segment [cp,O", -cp> 0'1, B: the 6-ring segment [+(p,-‘p,O", +Q-‘p,0'1 C: the lo-ring segment[+cp,O”,O”,-cp,O”,+cp,O”,O”,-9, 0”]
Torsional angle $ (degrees) Fig. 2. Lattice energiesof ‘staggered’(--D-)
and ‘eclipsed’
(-- a-) chainstructuresfor torsionsA, B and C, whichthe stackedthiophenesareeclipsed [6]. The relationshipbetweenadjacentchains along the stacking directionb plays an importantrole in the thermocluomism of the P3ATs.Recallingthat the inter-thiophenebondsin polythiophene are non-coaxial,the long side chains would require a large amountof spacein a torsion aroundthesebonds.Moreover, a relative displacementof the adjacentchainsby %c as found in a staggered arrangement, would inhibit torsionsof the chains. But if‘ the cell containeda sin& chain, concertedtorsionswould be moreeasilyaccommodated by the resultingeclipsed dispositionof the chains.This is becausethe repetition of a torsion in one segmentof the polymer by those in adjacent chains would dovetailthe segmental torsions,leadingto better latticepacking.
4. Results The energyprofiles plotted per monomerunit in Fig. 2 show that imposinga torsioncpin the staggered-chain lattice increases the lattice energywith cp, Someof the profiles showa dip when cpis about50” - 60”. In the staggeredstructurethe expansionof the latticeat this torsionreducesthe lattice densityby morethan 50%. Such an abnormallylow lattice density makesthis an unlikely mechanism,In the eclipsedstructure,onthe otherhand, the lattice energyat cp= 0” is higherthan in the staggered.But with increasingcp,the energyof the eclipseddecreases belowthat of the staggered,producinga local minimumat cp= 20”. The crystaldensityat this energywell is morethan 75% of that at cp= O”, indicating that the concertedmotion of the side chains generatessmallerlattice spacesthan before.The profilessuggest that on supplyingthermalenergythe chainsundergochangesso that every secondchain is displacedby %c along c: attaining eclipsedconformations. They canthen undergotorsion. Orthogonalcells containingsingle chains(implying eclipsed chains)have in fact beenproposedfrom the interpretationof Xray diffractograms[6]. This is often accruedto a polymorphic stabilisationdueto samplepreparation,but it may alsoimply the presenceof the eclipsedformsat lowertemperatures. The commonfeatureof a dip in the three lattice energycurves suggestthat the mechanism predictedis insensitiveto the precise pattern of torsions- torsional patterns which preservechain periodicity apparentlyleadto a twisting cp= 20”. This value is smallerthan the 40” estimatedfrom spectralshifts [9], but since the latter relieson calculationson isolated chainsit is difficult to obtainreliablemeasurements. It is interestingthat planar di- and fyi- thiophe units have beendetectedabovethe thermochromictransition [IO], as these are generatedrespectivelyby the [. , 0”. , .] and [. O”,0”:. .] components of the A, B andC sequences describedin thiswork. References [I] J.Corish, D.A.Morton-Blake, D.E.Feeley, F.Beniere, M.Marchetti, J.Phys.Chem. B 101(1997) 10075. [2] J.Corish, D.A.Morton-Blake, K.Veluri, F.Beniere, Molecular Simulation14 (1995) 381; K.Veluri, J.Corish, D.A.Morton-Blake, F.BCniQe, J.Mol. Struct. (Theochem) 365 (1996) 13.
KTashiro, K.Ono, YMinagawa, M.Kobayashi,T.Kawai, KJYoshino, Polym.Sci. B: Polym.Phys.29 (1991)1223. [4] H.J.Fell, E.J.Samuelsen, J.M&rdalen, E.Bakken, P.H.J.Carlsen,Synth.Metals 69 (1995)301. [5] S.Briickner,W.Porzio, Makromol. Chem.189(1988)961. [6] W. Luzny andA. Pron,Synth.Metals 79 (1996) 37. [7] J.D.Gale,J. Chem.Sot., FaradayTrans.93 (1997) 629. [8] J.Corish,J.; D.A.Morton-Blake, K.Veluri, F.BCniere, 3. Computationalmethod J. Mol. Struct. (Theochem)283 (1993)121. The perfect-lattice part of the atomistic lattice simulation [9] C.Roux,M.Leclerc, Macromolecules 25 (1992)2141. program GULP [q and the atomistic potentials described [lo] W.R.Salaneck,O.Inganas,B.ThCmans, elsewhere[S] were used to relax the staggeredand eclipsed J.O.Nielsson,B.Sjogren,J.-E.&terholm, lattice structuresof poly(3-butylthiophene)(P3BT). The chains J.-L.BrCdas,S.Svensson, J. Chem.Phys.89 (1988)4613. weresubjectedto the periodictorsionschemes A, B andC: [3]
SyntheticMetals 101(1999) 318-319
Lattice simulations of thermochromic distortions in poly(alkylthiophene)s Hongwei Xiea, J. Co&h”, S. G. All”, D.A. Morton-Blakea> , and K. Aasmundtveitb aChemishy Department, bInstituff forfidtlr,
Trinip
College Dublin, Ireland.
Norges Teknisk Natzrrvitenskapeiige
Universitet, Trondheim, Norway.
Abstract
The thermochromism of poly(3-alkylthiophene)s is explainedasa bucklingof the mainchainby torsionsaroundthe inter-ring bonds resultingin a dilation of the rc bandenergygap. Atomi>tic simulationcalculationsshowthat the necessityto preservetranslational periodicity imposes restrictionson the permittedtorsions.In our modelthe effect of heatingcauses a relative shift of adjacentpolymLr chains,changingthe ring stackingfrom ‘staggered’to ‘eclipsed’,in which the chainsundergoa spontaneous torsionof 20”. Kqvwords:Thermochromic polymers,Atomistic simulations, Latticestructures,Main chaindistortions,
1. Introduction
The substitutionof flexible side chainsinto the electroactive materialpolythiopheneconfersseveralinterestingpropertieson the polymer. One of tfiese is thermochromism, as a result of which the polymer undergoesa colour change on heating. Probably, reversible torsionsoccur in the conjugatedpolymer backbone,but the detailedmechanism is riot known [l]. Arbitrary torsionsin a chain initially in the most stablea$’ cotiguration (Fig. 1) wouldresultin a lossnot only of the main-
a
Fig. 1. The PSBTlattice showingthe staggered relationshipof
the two chainsin the unit cell, which are distinguished by shading.The b axis is normalto the figure.
chain’speriodicity, but alsoof the directionin which the polymer is aligned.Alternatively, imposinga set of constant torsionson the polymerbackbonewouldcreatea helical chain [Z]. In either case,the presenceof long alkyl side chainswould makethese processes highly activated.Jnfact the therrnochromictransitionis reportedto preserve the orientationof the mainchain131. = Poly(3,3’-dioctyl-2,2’-bithiophene),PDOBT, differs from poly(3-allqlthiophene)(P3AT) in that the connectionsof the alhylthiopheneunits are ‘headto head’ rather than the ‘headto tail’ arrangementof Fig. 1. A~diffraction investigationof the latter showsthat the PDOBT backboneis permanentlytwisted [4], and is not tlxxrnochromic. The repeatsegmentof the chain consistsof four alkylthiophenerings with a periodic torsional pattern [cp, O”, -9, O”,cp,O”, . . .], which conferstranslational periodicityon the polymerchainfor anyvalue of cp. 2. Lattice structure While unsubstituted polythiophenechainsform a lattice in which
they occupytwo non-parallelplanes[5], the a&y1 side chainsin P3ATswould generatetoomuchemptyspacein sucha structure. The substitutionof the hydrogenat the 3 positionby a groupeven assmallasmethyl causes the chainsto stack in (almost)parallel planes[6]. An importantstructuralfeature of the P3ATs is the number of chains in the unit cell. Several difiaction investigationsconcludethat there are two chainsin a stacked arrangement,but that their relative positionsshow a shift %C (half the chainrepeatunit of Fig. 1) [3]. This impliesthat the thiophenerings in adjacentchainsalong the stackingdirection (the b axis) showa staggeredrelationshipasshownin the Figure. Otherdiffractionworkersdescribechainsthat aresimplystacked alongb with no displacement alongc, resultingin a structurein
0379~6779/991$ - seefrontmatter0 1999ElsevierScience S.A.-AlIrightsreserved. PII: 50379-6779(98)01 176-X
S.G. Ali
et al. / Synthetic
Metals 101
(1999)
318-319
319
A: the 4-&g segment [cp,O", -cp> 0'1, B: the 6-ring segment [+(p,-‘p,O", +Q-‘p,0'1 C: the lo-ring segment[+cp,O”,O”,-cp,O”,+cp,O”,O”,-9, 0”]
Torsional angle $ (degrees) Fig. 2. Lattice energiesof ‘staggered’(--D-)
and ‘eclipsed’
(-- a-) chainstructuresfor torsionsA, B and C, whichthe stackedthiophenesareeclipsed [6]. The relationshipbetweenadjacentchains along the stacking directionb plays an importantrole in the thermocluomism of the P3ATs.Recallingthat the inter-thiophenebondsin polythiophene are non-coaxial,the long side chains would require a large amountof spacein a torsion aroundthesebonds.Moreover, a relative displacementof the adjacentchainsby %c as found in a staggered arrangement, would inhibit torsionsof the chains. But if‘ the cell containeda sin& chain, concertedtorsionswould be moreeasilyaccommodated by the resultingeclipsed dispositionof the chains.This is becausethe repetition of a torsion in one segmentof the polymer by those in adjacent chains would dovetailthe segmental torsions,leadingto better latticepacking.
4. Results The energyprofiles plotted per monomerunit in Fig. 2 show that imposinga torsioncpin the staggered-chain lattice increases the lattice energywith cp, Someof the profiles showa dip when cpis about50” - 60”. In the staggeredstructurethe expansionof the latticeat this torsionreducesthe lattice densityby morethan 50%. Such an abnormallylow lattice density makesthis an unlikely mechanism,In the eclipsedstructure,onthe otherhand, the lattice energyat cp= 0” is higherthan in the staggered.But with increasingcp,the energyof the eclipseddecreases belowthat of the staggered,producinga local minimumat cp= 20”. The crystaldensityat this energywell is morethan 75% of that at cp= O”, indicating that the concertedmotion of the side chains generatessmallerlattice spacesthan before.The profilessuggest that on supplyingthermalenergythe chainsundergochangesso that every secondchain is displacedby %c along c: attaining eclipsedconformations. They canthen undergotorsion. Orthogonalcells containingsingle chains(implying eclipsed chains)have in fact beenproposedfrom the interpretationof Xray diffractograms[6]. This is often accruedto a polymorphic stabilisationdueto samplepreparation,but it may alsoimply the presenceof the eclipsedformsat lowertemperatures. The commonfeatureof a dip in the three lattice energycurves suggestthat the mechanism predictedis insensitiveto the precise pattern of torsions- torsional patterns which preservechain periodicity apparentlyleadto a twisting cp= 20”. This value is smallerthan the 40” estimatedfrom spectralshifts [9], but since the latter relieson calculationson isolated chainsit is difficult to obtainreliablemeasurements. It is interestingthat planar di- and fyi- thiophe units have beendetectedabovethe thermochromictransition [IO], as these are generatedrespectivelyby the [. , 0”. , .] and [. O”,0”:. .] components of the A, B andC sequences describedin thiswork. References [I] J.Corish, D.A.Morton-Blake, D.E.Feeley, F.Beniere, M.Marchetti, J.Phys.Chem. B 101(1997) 10075. [2] J.Corish, D.A.Morton-Blake, K.Veluri, F.Beniere, Molecular Simulation14 (1995) 381; K.Veluri, J.Corish, D.A.Morton-Blake, F.BCniQe, J.Mol. Struct. (Theochem) 365 (1996) 13.
KTashiro, K.Ono, YMinagawa, M.Kobayashi,T.Kawai, KJYoshino, Polym.Sci. B: Polym.Phys.29 (1991)1223. [4] H.J.Fell, E.J.Samuelsen, J.M&rdalen, E.Bakken, P.H.J.Carlsen,Synth.Metals 69 (1995)301. [5] S.Briickner,W.Porzio, Makromol. Chem.189(1988)961. [6] W. Luzny andA. Pron,Synth.Metals 79 (1996) 37. [7] J.D.Gale,J. Chem.Sot., FaradayTrans.93 (1997) 629. [8] J.Corish,J.; D.A.Morton-Blake, K.Veluri, F.BCniere, 3. Computationalmethod J. Mol. Struct. (Theochem)283 (1993)121. The perfect-lattice part of the atomistic lattice simulation [9] C.Roux,M.Leclerc, Macromolecules 25 (1992)2141. program GULP [q and the atomistic potentials described [lo] W.R.Salaneck,O.Inganas,B.ThCmans, elsewhere[S] were used to relax the staggeredand eclipsed J.O.Nielsson,B.Sjogren,J.-E.&terholm, lattice structuresof poly(3-butylthiophene)(P3BT). The chains J.-L.BrCdas,S.Svensson, J. Chem.Phys.89 (1988)4613. weresubjectedto the periodictorsionschemes A, B andC: [3]