Experimental and theoretical studies of the electronic structure of substituted and unsubstituted poly(para-phenylenevinylene) (PPV)

Synthetic Metals, 55-57 (1993) 263-268

263

EXPERIMENTAL AND THEORETICAL STUDIES OF THE ELECTRONIC STRUCTURE OF SUBSTITUTED AND UNSUBSTITUTED POLY0aARA-PHENYLENEVINYLENE) (PPV) M. FAHLMAN O. LHOST 1, F. MEYERS 1, J.L. BRI~DAS1, S.C. GRAHAM2, R.H. FRIEND2, P.L. BURN3, A.B. HOLMES3, K. KAERIYAMA4, Y. SONODA4, M. LOGDLUND, S. STAFSTROM, AND W.R. SALANECK Deparmaent of Physics, IFM, Linktping University, S-58183 Linktping, Sweden IService de Chimie des Mattriaux Nouveaux, Dtpartement des Mattriaux et Proctdts, Universit6 de Mons-Hainaut, B-7000 Mons, Belgium. 2Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, U.K. 3University Chemical Laboratory, Lensfield Road, Cambridge CB2 1EW. U.K. 4Research Institute for Polymers and Textiles, Tsukuba, Ibaraki, 305 Japan. ABSTRACT The electronic structure of poly(p-phenylenevinylene) and that of its ring-substituted derivatives, poly(2,5-diheptyl-l,4-phenylenevinylene), poly(2,5-dimethoxy-l,4-phenylenevinylene), and poly(2-methoxy-5-(2'-ethylhexoxy)-l,4-phenylenevinylene), are studied by Ultraviolet Photoelectron Spectroscopy, UPS, and X-ray Photoelectron Spectroscopy, XPS. It is observed by UPS that the x-bands closest to the valence band edge are strongly affected by the presence of the substituents. The influence of the side groups on the experimental spectra is studied theoretically using the Valence Effective Hamiltonian, VEH, model. Calculations are carried out on isolated polymer chains, including full treatment of the aliphatic side grcups. Particular attention is paid to the effect of chain torsion angles on the x-band edge. For the diheptyl derivative, the experimental results can be explained on the basis of sidegroup-induced torsions of the phenylene rings along the backbone, which influence the gband widths and contribute to differences in both optical absorption threshold and binding energy of the valence band edge. For the alkoxy derivatives, the side groups cause strong modifications in the shape of the upper two occupied g-bands, which results in significant changes in the electronic density of states. INTRODUCTION It is the purpose of this work m examine in detail, from a joint experimental and

theoretical approach, the electronic slructme of the poly(p-phenylenevinylene) compounds, in particular to understand the influence of alkyl or alkoxy ring substitutions on the top region of the valence band and on the bandgap. This is especially important in the context of determining the characteristics of an hole injection process or of the light emitted during electro- or photoluminescence phenomena. The poly(p-phenylenevinylene), or PPV, derivatives that we consider here are: (i) poly(2,5-diheptyl-l,4-phenylenevinylene), or DHPPV; (ii) poly(2,5-dimethoxy-l,4-phenylenevinylene), or DMeOPPV; and (iii) poly(20379-6779/93/$6.00

© 1993- Elsevier Seeuoia. All rights reserved

264

methoxy-5-(2'-ethylhexoxy)-l,4-phenylenevinylene),or MEHPPV. The molecular structures of the four polymers are illustrated in Figure 1. EXPERIMENTAL DETAILS Thin films of PPV, DMeOPPV, and MEHPPV, spincoated onto gold-covered Si substrates, were prepared at Cambridge. The samples were then shipped to Link6ping, sealed in glass, under N 2 atmosphere. Thin fdms of DHPPV or its precursor, synthesized at the Research Institute for Polymers and \

Textiles

[1],

LinkOping

were

by

prepared

spincoafing

in onto

aluminum-covered Si substrates. The precursor samples were thermally

n

N a) PPV

converted in situ in the ulwa-high

c) DMeOPPV

vacuum

(UHV) chamber

of the

photoelectron spectrometer. Photoelectron spectroscopies, both

\

XPS and UPS, were carried out in lJnk~ping in an insU'ument of our own design and conswaction, with a base pressure of about 10 -10 torr. An unmonochromatized M g ( K a l a 2 ) X-

n

ray source 0av = 1253.6 eV) was used for XPS, while UPS was carried out using monochromatized HeI (hv = 21.2 b) DHPPV

d) MEIIPPV

Fig. 1. Molecular mmctmeof a) PPV. b) DHPPV. c) DMeOPPV. and d) IVlEItPPV

eV)

and

HeII

(by = 40.8 eV) photons. We note that examination of the XPS core levels shows that the PPV,

DMeOPPV, and MEHPPV samples are all free from significant oxygen contamination. On the contrary, the samples of DHPPV do present some oxygen contamination, about one oxygen atom per two monomer units. THEORETICAL CALCULATIONS The electronic band mrucmre calculations are carded out on isolated polymer chain s using the nonempirical pseudopotential Valence Effective Hamiltonian model [2-3]. The bare deusity-of-valence-eleclxonic-states, DOVS, curves are directly computed by taking the inverse of the derivative of the electronic band structure with respect to momentum. The molecular geomelries used as input for the eleclronic band-structure calculations are obtained via Hartree-Fock semiempirical Austin Model 1 (AM1) [4] geometry opfimizations. The geomewy optimizatioas are carried out on oligomers, the central parts of which being used as unit cells for the polymers.

265

RESULTS AND DISCUSSION The DOVS closest to the band edge obtained from the UPS HeI spectra are compared with the two highest occupied bands obtained from the VEH band structure calculations. In all cases, the highest occupied band is the fourth n-band, denoted 4~, delocalized along the polymer backbone. The top of this band has almost equal contributions coming from the phenylene moieties and the vinylene linkages. The band immediately below 4x, is the fiat 3re band localized within the phenyl rings. The case of PPV is illustrated in Figure 2. There occurs an avoided crossing between the two bands because of the lack of symmetry within the unit cell. In the experimental spectrum, the difference between the locations of the two O peaks with lowest binding energies thus reflects the difference between the top part of band 4~ and the fiat 37t band, i.e., about 1.8 eV. Note that the fiat band appears only 0.2 eV above what would constitute the bottom part of the wide band in the absence of an avoided crossing. Along a chain of DHPPV, steric hindrance occurs between the hydrogen atoms on the vinyl bridges and the heptyl -

-4

-3

-2

-I

0

BINDING ENERGY (eV)

groups on the phenylene rings. These interactions force the rings to twist out of the plane of the backbone, the optimal torsion angle being 34" at the AM1 level. The AM1 potential barrier towards a coplanar

Fig. 2. Energybandsvs UPS spectrum for PPV conformation is about 2.3 kcal/mol per ring and is relatively fiat around the minimum. Solidstate packing effects can thus be expected to drive the conformation towards a more coplanar character, at least in the b e t ~ ordered materials, In this case, x-electron delocalization along the backbone will be effective and lead to a wide 4g band; large ring torsions would; on the contrary, diminish significantly the 4x bandwidth and increase both the ionization potential and the bandgap. There were appareafly significant variations in the average ring torsion angles among the DHPPV samples that we examined. In general, the DHPPV samples made by converting the precursor polymer in lJnkqSping had larger average torsions (i.e., for instance larger bandgaps) than the samples based on the DHPPV eonvexted in Tsulmba; there were also variations within the two sample groups. This sensitivity to sample preparation is an indication that the degree of order, and thus the influence of solid state packing, is different in different samples. (We note that an alternate explanation to this behavior would come from different degrees of conversions to the conjugated polymer, this, however, appears to be less plausible).

266

ilIil .............

v

[-., 2; o

o

<~ z [.., -5

-4

-3

-2

-1

0

BINDING ENERGY (eV) Fig. 3a. Energybands vs UPS spectrum for DHPPV (0 = 0")

E

-

-4

-3

-2

BINDING E N E R G Y

-I

0

(eV)

Fig. 3b. Energybandsvs UPS spectrum for DHPPV (0 = 34")

In Figure 3, the VEH-calculated 4~ and 37t bands arc shown for DHPPV, first in a coplanar conformation, then in a conformation with ring torsion angles of 34". In the former case, Figure 3a, there is very good agreement with one of the UPS spectra taken on a D H P P V sample converted in Tsukuba. This confirms that in this particular film, the polymer backbones are approximately coplanar. The major influence of the heptyl groups on the electronic structure of the upper part of the valence band, is on the position of the flat 3x band: with respect to PPV, the flat band is shifted upward by about 0.3 cV; analyzing thc wave functions, this is due to antibonding interactions between the carbon starting the aliphafic chain and the ring ortho carbon to which it is attached. The total width of the two bands (2.0 eV) as well as the io;tiT~6on potential remain at exactly the same values as in PPV. The shift of the flat band nicely correlates the shift of the main peak in the UPS spectrum, to 3.7 eV up fi'om -4.0 eV in PPV. In Figure 3b, the UPS spectrum of another DHPPV sample, converted from the precursor in Link~ping, is shown and compared to the theoretical bands of twisted DHPPV. The total dispersion of the two bands is vastly reduced, decreasing to 0.8 eV. The ionization potential increases by 0.8 eV relative to PPV or COpIAnsr DHPPV. This results in the observation of only one, but broader, UPS peak close to the Fermi energy. The other samples of DHPPV studied were found to vary between these two extremes. In DMeOPPV and MEI-IPPV, the hydrogen-type bonds between the alkoxy oxygens and the vinyl hydrogens tend to lock the polymer backbones into coplanar conformations [4]. From Figure 4, we obserge that the presence of the oxygen atoms strongly destabilizes the highest two occupied bands (which are identical in the two compounds), the flat band being pushed up by 1.2-1.4 eV relative to PPV. This destabiliTsdon comes from the antibonding character of the interaction between the ring and mcthoxy wavefunctions for the two bands

267 t6]. The effect is, however, more pronounced for the flat band (note that in DHPPV, only the flat band was affected in the coplanar system). The avoided crossing between the two bands is also increased by the presence of the alkoxy groups, separating further the two bands [6]. Consistent with the consideration of coplanar conformations is the fact that the full width of the two bands is 2.0 eV.

O

4~

<,5_.

Z

.

-

.

-4

.

.

-3

.

-2

, -1

0

BINDING ENERGY (eV) Fig. 4a. Energybandsvs UPS spectrum for DMeOPPV

Z

....

-5

i..

-4

.,

-3

-2

-I

o

BINDING ENERGY (eV) Fig.4b. Energybandsvs Ut~ spoctrum for IVIEHPPV

The calculated upper two bands of DMeOPPV and MEHPPV are compared with the corresponding UPS spectra in Figure 4a and 4b. Because of the strong avoided crossing, there occurs a larger separation between the two bands although their total width remains at 2 eV. The flat parts of 4x and 3g create a broader peak in the DOVS than for DHPPV and PPV, in excellent agreement with the experimental spoetra. This peak broadening results in the fact that the peak closest to the Fermi energy (that was observed in PPV and DI-IPPV) becomes

unresolved in the experimental spectrum of DMeOPPV and only sfighfly perceptible in MEHPPV. The observation that the calculated ~-bands of DMeOPPV and MEHPPV are essentially identical, is an expected feature. Indeed, the difference in lengths of the alkyl groups between these two compounds impacts on the ~-bands at higher binding energies and can only affect the characteristics of the upper ~-bands if there were changes in the ring torsion angles along the backbone. However, this does not happen for DMeOPPV and MEHPPV, since the oxygen atoms lead to bonding effects which tend to lock these polymers into planar geometries. Based on these results, the length of the alkyl chains in the alkoxy groups is seen to have little effect on the electronic structnre near the Fermi energy, although it can influence the solubility and thermal stability properties of the polymers. The overall agreement with the UPS spectra indicates that there are no major solid-state effects which negate the results of the calculations carried out on isolated chains.

268

SYNOPSIS The electronic structures of PPV, DHPPV, DMeOPPV, and MEHPPV have been described through the results of experimental studies by means of ultra-violet and X-ray photoelectron speclroscopies and theoretical investigations via AM1 and VEH quantumchemical calculations. The densities of states obtained from the calculations are in excellent agreement with the UPS spectra. The evolutions in energy shifts, bandwidths, and band separations calculated for the different substituents allow for a detailed interpretation of the experimental data. Two aspects can be distinguished with regard to the influence of the ring substituents on the characteristics of the upper two valence bands. On one hand, there is a direct influence in the case of coplanar conformations, whereby the energy position of either the flat band (in DHPPV) or the two bands (in the alkoxy derivatives) is affected by the substitution; the total width of the two bands, however, remains the same as in PPV. On the other hand, there is an indirect effect which occurs when the presence of the substituents leads to ring torsions along the backbone, as is the case in some of the DHPPV samples; the dispersion of the wide band is then reduced, the more so, the larger the torsion angle. ACKNOWLEDGEMENTS The IAnkiSping-Mons and Mons-Cambridge collaborations are supported by the Commission of the European Community, respectively within the SCIENCE progrmm~ (project 0661 POLYSURF) and the BRI'IT~JEURAM programme (project 0148 NAPOLEO). The LinkiSping-RIPT collaboration is supported by the Swedish Board for Technical Development (NuTek). Research on conjugated polymers in Link0ping is supported in general by grants from the Swedish Natural Sciences Research Council (NFR), the Swedish National Technical Research Board (TFR), and the Neste Corp., Finland. The work in Mons is partly supported by the Belgian Government "Ptle d'Attraction en Chinfie Supramoltculaire et Catalyse", the SPPS "Progrmmne dTanpulsion eat Technologie de l'Information (contract IT/SC/22)", and FNRS. REFERENCES [1] Y. Sonoda and K. Kaefiyama, Bull. Chem. Soc. Jon.. in press [2] J.L. Brhlas, R.IL Chance, IL Silbey, G. Nicolas and Ph. Durand, J, Chem. Phys.. 75 (1981) 255. [3] J.M. Andrt, J. Delhalle and J.L. Brtdas, "Quantum Chemistry Aided Design of Organic Polymers" (World Scientific, Singapore, 1991). [4] M.J.S. Dewar, E.G. Zoebish, 1LF. Healy, and JJ.P. Stewart, J. Am. Chem. Sot.. 107 (1985) 3902. [5] O. Lhost and J.L. Brtdas, J. Chem. Phys., 96 (1992) 5279. [6] F. Meyers, A.J. Hecger, and J.L. Brtdss, J. Chem. Phys.. in press.