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Excited π states in polydiacetylenes: influence of the chain environment
ELSEVIER
Synthetic
Met&
Excited T states in polydiacetylenes:
84 (1997)
553-554
influence of the chain environment
A. Horvatht, G. Weiserf, C. Lapersonne-Meyer*, M. Schott’ and S. Spagnoli* +Dept. of Physics and Center of Material Science, University ofMarburg, D-3503 7MarbuE, Germany ‘Groupe de Physique des Solides, University of Paris i?lI, 75251 Paris Cedex 05, France Abstract Electroabsorption spectra of BCMU have been used to study the influence of disorder on the excited states in polydiacetylenes. The spectra show strong changes of transition energy and lineshape which occur by going from oriented chains diluted in monomer single crystals to polymer single crystals and spin cast films where disorder obscures the inu-insic electronic properties.
keywords:electroabsolption,UV-YIS-NLRabsorption 1. Introduction Excited states in rr conjugated polymers are widely studied because oftheir low dimensional character and as test of theoretical concepts which are based on perfect chains. Although most experiments are performed on films little is known of the influence of disorder on the spectra. We studied electroabsorption spectra of the polydiacetylene 4BCMIJ, poly [(5,7dodecadiyne-l,l2-diolbis(4-butoxy carbonyl methyl urethane)], to explore the changes which evolve with increasing disorder 2. Experimental results and discussion Fig. 1 shows absorption and electroabsorption spectra of polymer chains in a 4BCMCT monomer single crystal. The absorption is dominated by the strong 1B, exciton, vO, and its vibronic satellites, The exciton transition energy shifts from 1.81eV at low to 1.94eV at room temperature, probably related to compressive strain due to mismatch of the lattice constant and polymer repeat unit which decreases from 3% to 1% [I]. This large shift points to considerable sensitivity of the 7r states to changes in the environment. All features of the electroabsorption spectrum agree up to 2.4eV perfectly with the first derivative of the absorption spectrum, revealing a rigid red shift of the exciton and its satellites. The shift increases quadratically with the field which yields a polarizability of 6.571777,~. The Stark shift of each state is different because it involves its optical matrix elements Mij to other states:
The rigid shift of the spectrum below 2.4eV therefore implies that in this range no other electronic state of significant coupling to the ground state or the lB, state exists. The shit? results from coupling to electronic states above 2.4eV which contribute a Acr spectrum of a very di&rent lineshape. The spectrum there broadens with a F213, unique to the Franz-Keldysh effect at the gap of continuum states. The evaluation yields a dipole moment of Llnm between the exciton and the continuum which corresponds to an exciton radius [2]. The exciton thus extends over 5 repeat units and must therefore be delocalized. Below the l& exciton weaker transitions, Y, and r&, are observed which show the same redshift as the 13, exciton excluding their assignment to A, states. They most likely are lB, excitons of chains in strained regions of the monomer crystal near dislocations or stacking faults [3]. 0379-6779/97/$17.00 0 1997 Ekwier Science S.A. All rights reserved PII SO379-6779(96)04047-7
4BCMU F=21 kV/cm T=40K
Fig.1: Spectraof chainsdilutedin monomersinglecrystals The spectra broaden considerably as the polymer content increases. Fig.2 shows spectra ofa 1OOnm thin single crystal which has been polymerized by an electron beam and removed from the monomer substrate. The exciton is shifted to 1.93eV, the Same energy as in bulk crystals.The excitonic electroabsorption spectrum is described by the same redshift of the absorption spectrum as in case of single chains but the Franz-Keldysh effect is reduced to a faint signal above 2.5eV of quadratic fieldstrength dependence as observed in most bulk PDA crystals [2]. Observation of a clear Franz-Keldysh effect with its unique field broadening requires coherence of band states over tens of unit cells but most polymer crystals cannot provide such large unperturbed polymer segments. Scattering potentials create in one-dimensional bands boundaries which reduce the continuum to a set of discrete states and the FranzKeldysh effect will be replaced by the Stark effect. The smaller excitons are less sensitive but show inhomogeneous broadening of their spectra. Their large polar&ability, however~emains which indicates that the ordering of i&a-chain levels is still the same. The broad absorption peak near 2.3eV developed when the crystal was removed from the monomer Simultaneously the exciton near 2eV lost strength indicating that polymer chains were altered shifting their transitions to higher energy. Further irreversible changes occur when the sample is heated to 100 ‘C. The exciton at 2eV is replaced by a transition near 2.35eV with
A. Horvath
et al. /Synthetic Metals 84 (1997) 553-554 2
-
T=80
4BCMsJ
K
annealed
....y.+:,, n,+-dT-k lnWw AE=lO
peV
I/”
-10 1.5
2.0
2.5
3.0
2.0
Energy (eV) Fig.2: Spectra of a thinjke
standingpolylller
2.5
Energy (eV) single crystal
poorly resolvedvibronic states(fig.3). The absorptionspectrum resembles that of disorderedfiis but it is still highly anisotropic with respectto the polarizationof light. The electroabsorption spectrum,whichresolvesthevibronic statesmuchbetterandshows splittingof the exciton, hasstill a fist derivative shapewith the same’redshift.However,above2.8eV a broadpeak (shaded)is foundwhichis not reproducedby the derivative. Weattributethis signalto the residueof thebandstates. The Aa spectrumof a spin-castfin of 4BCMU (fig.4) seems a broadened versionof the spectrumin fig.3. The weakline is the spectrumderivedfromthe Starkeffectwith thepolarizabilityof the crystal,accountingonly for randomlyorienteddipoles.The poor agreement pointsto an additionaleffect which is not observedin crystals.As in otherpolymerfiis too, the Aa spectrumagrees betterwith the secondderivativeof theabsorption.Supplementing the quadraticStarkeffect by a contributionof a secondderivative lineshapereproducesthe experimentalspectrumfor a rangeof 0.5eV andfails only above2.8eV wherethe statesof the former continuumarefound. Their mixing by the field givesriseto the broadshaded peakwhichis closeto that in fig.3. Similardeviation from a fist and secondderivativesis observedin otherpolymer films[4,5]andmostlikely represents theretoothecontinuumstates. A secondderivative spectrumof quadraticfield dependence is obtainedfrom a linear Stark effect with randomlyorientated dipoles.Weproposedthat defectrelatedsmallbarriersseparated by lessthanthe meanfree path createasymmetricboundaries and leadto a redistributionof the electrondensitywhoseasymmetry is sufficient to explain the secondderivative contribution to electroabsorption spectra[6]. Lifetime broadeningof excitons yields spectraof similar shapebut this explanationmust be discardedbecausethe muchnarrowerlinesin isolatedchainsdo not not showa secondderivativelineshape. 3. Conclusions Electroabsorptionspectrareveal a strong sensitivity of rr conjugatedpolymersto disorder. Most sensitiveare continuum stateswhichbecomelocalizedby scatteringpotentials.TheFranzKeldysheffect thereforeis seenonly in exceptionalcasesof long rangeorder, Polymerfilms even when stretcheddo not provide sufficientcoherencelength. The muchsmallerexcitonsare less sensitiveand respondonly by inhomogeneous broadening.The largethermalshift of the excitonsandweak additionalexcitons resolvedfor singlechainsin a singlecrystallinematrix reveal that the gapbetweenrr andT* statesresponds to smallstructural changes in theenvironment.Thepolarizabilityof the lB, exciton,
I 3.5
3.0
Fig.3: Spectra of the thin polymer crystal after annealing
however,is not muchaffectedwhich impliesthat its extensionand its gapto higherexcitedstatesaremaintained. Disorderin fiis reducesunperturbedpolymersegments to such a smallscalethat boundaryconditionsimposedon the T states break their inversionsymmetryand lead to a contributionof a secondderivativelineshape to theelectroabsorption spectrum.This contributionseems uniqueto smallscaledisorderandprevailsin the spectraof polymerfilms whereit obscures the intrinsicproperties of the material, From the observationthat the electroabsorption spectrumover mostof the absorptionis describedby a singleset of fist andsecondderivativelineshape we conjecturethat a single exciton statewith a largegap to higherexcited statesdominates the absorptionwhich is in agreementwith calculationsmadefor infinite chains[7] but in contradictionto thosebasedon relatively smallfinite chains[8],
-20 I
I
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
Energy (eV) Fig.4 Electmabsolption
and model spectra of a spin castfilm
References [l] S.Spagnoli,ThesisParisVII, France1995 [2] G. Weiser,Phys-Rev.B 45, 14076(1992) [3] A. Horvath,G. Weiser,C. Lapersonne, M. Schottand S. Spagnoli,PhysRev.B 53, 13507(1996) [4] J.M. Leng,S. Jeglinski,X. Wei,R.E. Benner,Z.V Mrdeny, E Guo andS.Mazumdar,Phys.Rev.Lett.72, 156(1994) [5] S.J.Martin D.D.C. Bradley,J.S. OsaheniandS.A. Jenekhe, Mol.Cryst.Liq.Cryst. 256,583(1994) [6] A. Horvath,G. Weiser,G.L. Baker andS.Etemad,PhysRev. B 51,275l (1995) [7] S.Abe, M. Schreiber,WF!Su andJ. Yu,PhysRev.B 45,9432 (1992) [81 Y Kawabe,EJ arka,D. Guo, S. Mazumdar,S.N. Dixit, E Kajzar andN. Peyghambarian, PhysRev.B 44,530(1991)
Synthetic
Met&
Excited T states in polydiacetylenes:
84 (1997)
553-554
influence of the chain environment
A. Horvatht, G. Weiserf, C. Lapersonne-Meyer*, M. Schott’ and S. Spagnoli* +Dept. of Physics and Center of Material Science, University ofMarburg, D-3503 7MarbuE, Germany ‘Groupe de Physique des Solides, University of Paris i?lI, 75251 Paris Cedex 05, France Abstract Electroabsorption spectra of BCMU have been used to study the influence of disorder on the excited states in polydiacetylenes. The spectra show strong changes of transition energy and lineshape which occur by going from oriented chains diluted in monomer single crystals to polymer single crystals and spin cast films where disorder obscures the inu-insic electronic properties.
keywords:electroabsolption,UV-YIS-NLRabsorption 1. Introduction Excited states in rr conjugated polymers are widely studied because oftheir low dimensional character and as test of theoretical concepts which are based on perfect chains. Although most experiments are performed on films little is known of the influence of disorder on the spectra. We studied electroabsorption spectra of the polydiacetylene 4BCMIJ, poly [(5,7dodecadiyne-l,l2-diolbis(4-butoxy carbonyl methyl urethane)], to explore the changes which evolve with increasing disorder 2. Experimental results and discussion Fig. 1 shows absorption and electroabsorption spectra of polymer chains in a 4BCMCT monomer single crystal. The absorption is dominated by the strong 1B, exciton, vO, and its vibronic satellites, The exciton transition energy shifts from 1.81eV at low to 1.94eV at room temperature, probably related to compressive strain due to mismatch of the lattice constant and polymer repeat unit which decreases from 3% to 1% [I]. This large shift points to considerable sensitivity of the 7r states to changes in the environment. All features of the electroabsorption spectrum agree up to 2.4eV perfectly with the first derivative of the absorption spectrum, revealing a rigid red shift of the exciton and its satellites. The shift increases quadratically with the field which yields a polarizability of 6.571777,~. The Stark shift of each state is different because it involves its optical matrix elements Mij to other states:
The rigid shift of the spectrum below 2.4eV therefore implies that in this range no other electronic state of significant coupling to the ground state or the lB, state exists. The shit? results from coupling to electronic states above 2.4eV which contribute a Acr spectrum of a very di&rent lineshape. The spectrum there broadens with a F213, unique to the Franz-Keldysh effect at the gap of continuum states. The evaluation yields a dipole moment of Llnm between the exciton and the continuum which corresponds to an exciton radius [2]. The exciton thus extends over 5 repeat units and must therefore be delocalized. Below the l& exciton weaker transitions, Y, and r&, are observed which show the same redshift as the 13, exciton excluding their assignment to A, states. They most likely are lB, excitons of chains in strained regions of the monomer crystal near dislocations or stacking faults [3]. 0379-6779/97/$17.00 0 1997 Ekwier Science S.A. All rights reserved PII SO379-6779(96)04047-7
4BCMU F=21 kV/cm T=40K
Fig.1: Spectraof chainsdilutedin monomersinglecrystals The spectra broaden considerably as the polymer content increases. Fig.2 shows spectra ofa 1OOnm thin single crystal which has been polymerized by an electron beam and removed from the monomer substrate. The exciton is shifted to 1.93eV, the Same energy as in bulk crystals.The excitonic electroabsorption spectrum is described by the same redshift of the absorption spectrum as in case of single chains but the Franz-Keldysh effect is reduced to a faint signal above 2.5eV of quadratic fieldstrength dependence as observed in most bulk PDA crystals [2]. Observation of a clear Franz-Keldysh effect with its unique field broadening requires coherence of band states over tens of unit cells but most polymer crystals cannot provide such large unperturbed polymer segments. Scattering potentials create in one-dimensional bands boundaries which reduce the continuum to a set of discrete states and the FranzKeldysh effect will be replaced by the Stark effect. The smaller excitons are less sensitive but show inhomogeneous broadening of their spectra. Their large polar&ability, however~emains which indicates that the ordering of i&a-chain levels is still the same. The broad absorption peak near 2.3eV developed when the crystal was removed from the monomer Simultaneously the exciton near 2eV lost strength indicating that polymer chains were altered shifting their transitions to higher energy. Further irreversible changes occur when the sample is heated to 100 ‘C. The exciton at 2eV is replaced by a transition near 2.35eV with
A. Horvath
et al. /Synthetic Metals 84 (1997) 553-554 2
-
T=80
4BCMsJ
K
annealed
....y.+:,, n,+-dT-k lnWw AE=lO
peV
I/”
-10 1.5
2.0
2.5
3.0
2.0
Energy (eV) Fig.2: Spectra of a thinjke
standingpolylller
2.5
Energy (eV) single crystal
poorly resolvedvibronic states(fig.3). The absorptionspectrum resembles that of disorderedfiis but it is still highly anisotropic with respectto the polarizationof light. The electroabsorption spectrum,whichresolvesthevibronic statesmuchbetterandshows splittingof the exciton, hasstill a fist derivative shapewith the same’redshift.However,above2.8eV a broadpeak (shaded)is foundwhichis not reproducedby the derivative. Weattributethis signalto the residueof thebandstates. The Aa spectrumof a spin-castfin of 4BCMU (fig.4) seems a broadened versionof the spectrumin fig.3. The weakline is the spectrumderivedfromthe Starkeffectwith thepolarizabilityof the crystal,accountingonly for randomlyorienteddipoles.The poor agreement pointsto an additionaleffect which is not observedin crystals.As in otherpolymerfiis too, the Aa spectrumagrees betterwith the secondderivativeof theabsorption.Supplementing the quadraticStarkeffect by a contributionof a secondderivative lineshapereproducesthe experimentalspectrumfor a rangeof 0.5eV andfails only above2.8eV wherethe statesof the former continuumarefound. Their mixing by the field givesriseto the broadshaded peakwhichis closeto that in fig.3. Similardeviation from a fist and secondderivativesis observedin otherpolymer films[4,5]andmostlikely represents theretoothecontinuumstates. A secondderivative spectrumof quadraticfield dependence is obtainedfrom a linear Stark effect with randomlyorientated dipoles.Weproposedthat defectrelatedsmallbarriersseparated by lessthanthe meanfree path createasymmetricboundaries and leadto a redistributionof the electrondensitywhoseasymmetry is sufficient to explain the secondderivative contribution to electroabsorption spectra[6]. Lifetime broadeningof excitons yields spectraof similar shapebut this explanationmust be discardedbecausethe muchnarrowerlinesin isolatedchainsdo not not showa secondderivativelineshape. 3. Conclusions Electroabsorptionspectrareveal a strong sensitivity of rr conjugatedpolymersto disorder. Most sensitiveare continuum stateswhichbecomelocalizedby scatteringpotentials.TheFranzKeldysheffect thereforeis seenonly in exceptionalcasesof long rangeorder, Polymerfilms even when stretcheddo not provide sufficientcoherencelength. The muchsmallerexcitonsare less sensitiveand respondonly by inhomogeneous broadening.The largethermalshift of the excitonsandweak additionalexcitons resolvedfor singlechainsin a singlecrystallinematrix reveal that the gapbetweenrr andT* statesresponds to smallstructural changes in theenvironment.Thepolarizabilityof the lB, exciton,
I 3.5
3.0
Fig.3: Spectra of the thin polymer crystal after annealing
however,is not muchaffectedwhich impliesthat its extensionand its gapto higherexcitedstatesaremaintained. Disorderin fiis reducesunperturbedpolymersegments to such a smallscalethat boundaryconditionsimposedon the T states break their inversionsymmetryand lead to a contributionof a secondderivativelineshape to theelectroabsorption spectrum.This contributionseems uniqueto smallscaledisorderandprevailsin the spectraof polymerfilms whereit obscures the intrinsicproperties of the material, From the observationthat the electroabsorption spectrumover mostof the absorptionis describedby a singleset of fist andsecondderivativelineshape we conjecturethat a single exciton statewith a largegap to higherexcited statesdominates the absorptionwhich is in agreementwith calculationsmadefor infinite chains[7] but in contradictionto thosebasedon relatively smallfinite chains[8],
-20 I
I
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
Energy (eV) Fig.4 Electmabsolption
and model spectra of a spin castfilm
References [l] S.Spagnoli,ThesisParisVII, France1995 [2] G. Weiser,Phys-Rev.B 45, 14076(1992) [3] A. Horvath,G. Weiser,C. Lapersonne, M. Schottand S. Spagnoli,PhysRev.B 53, 13507(1996) [4] J.M. Leng,S. Jeglinski,X. Wei,R.E. Benner,Z.V Mrdeny, E Guo andS.Mazumdar,Phys.Rev.Lett.72, 156(1994) [5] S.J.Martin D.D.C. Bradley,J.S. OsaheniandS.A. Jenekhe, Mol.Cryst.Liq.Cryst. 256,583(1994) [6] A. Horvath,G. Weiser,G.L. Baker andS.Etemad,PhysRev. B 51,275l (1995) [7] S.Abe, M. Schreiber,WF!Su andJ. Yu,PhysRev.B 45,9432 (1992) [81 Y Kawabe,EJ arka,D. Guo, S. Mazumdar,S.N. Dixit, E Kajzar andN. Peyghambarian, PhysRev.B 44,530(1991)