Theoretical investigation of phenylene-based materialsin their pristine and doped state

Egbert Zojer a,b,*, J�erome Cornil b, G�unther Leising a, Jean-Luc Br�edas b

a Institut f�ur Festk�orperphysik, Technische Universit�at Graz, Petersgasse 16, 8010 Graz, Austriab Service de Chimie des Mat�eriaux Nouveaux, Centre de Recherche en Electronique et Photonique Mol�eculaires,

Universit�e de Mons Hainaut, B-7000 Mons, Belgium

Abstract

Phenylene-based organic materials play an important role in organic device technology, especially in light emitting

diodes and displays. We have investigated their geometries and optical transitions in both pristine and doped states,

paying special attention to chain-length e�ects as well as to the implications of inter-ring twists considering also bridged

ladder type molecules. Our calculations give an extent of four benzene rings for the geometry modi®cations associated

with the formation of polarons and six to eight rings for bipolarons. We calculate two sub-gap absorption features for

polarons in short-chain molecules and a single peak for bipolarons. In longer chains and for interacting bipolarons, this

situation changes considerably within the theoretical framework we use. Ó 1999 Elsevier Science B.V. All rights re-

served.

PACS: 36.20.Kd; 36.20.Hb; 31.25.Qm; 78.66.Qn

Keywords: Quantum-chemical simulations; Oligophenylenes; Ladder-type molecules; Polarons; Bipolarons

1. Introduction

Conjugated organic materials based on phen-ylene repeat units play an important technologicalrole due to their wide range of possible applica-tions in various devices, especially in light emittingdiodes (LEDs) [1,2], light emitting electrochemicalcells (LECs) [3] and lasers [4]. A big advantage ofthese materials is their emission in the blue spectralrange, which can e.g. be utilized to achieve red±green±blue (RGB) full color displays via colorconversion techniques [5]. Ladder type oligo- andpolyphenylenes, in which the rotational degree offreedom of the phenylene rings linked in para-

position is eliminated by an additional inter-ringbridge [6], are especially interesting because oftheir high quantum e�ciencies [7], a high degree ofintra-chain order [8], a steep absorption onset andlow sub-gap absorption [9]. The main goal of thepresent work is to investigate the in¯uence of chainlength and inter-ring bridging on the geometry andoptical properties of phenylene-based systems andto highlight the e�ect of doping in these materials.

2. Theoretical methodology

We have investigated para-phenylene oligomerswith 2±12 repeat units (hereafter denoted nP andnlP, with n corresponding to the number of ringsand l denoting the bridged ladder-type character ±

Optical Materials 12 (1999) 307±310

* Corresponding author: Tel.: +43-316-873-8475; fax: +43-

316-873-8478; e-mail: egbert@�phal01.tv-graz.ac.at

0925-3467/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 5 - 3 4 6 7 ( 9 9 ) 0 0 0 3 9 - 7

see Fig. 1) in twisted, planar and bridged confor-mations.

The geometry of the neutral and charged olig-omers is optimized with the semiempirical Har-tree±Fock Austin Model 1 (AM1) method. Thisapproach provides good estimates of geometriesand heats of formation for organic molecules intheir ground state [10]. The neutral and doublycharged molecules are treated within the Restrict-ed Hartree-Fock formalism (RHF). For singlycharged species a Restricted Open-shell Hartree±Fock (ROHF) approach is chosen. All geometryoptimizations are performed for isolated moleculesneglecting the e�ect of counter-ions. This bestcorresponds to the situation encountered in solu-tions and photoinduced absorption experimentsand to a certain extent in ®lms of materials withlong aliphatic side-chains. The e�ect of counter-ions is studied in detail e.g. in Refs. [11,12].

The optical absorption spectra are calculated byapplying a Gaussian broadening to the transitionenergies and oscillator strengths calculated on thebasis of the semiempirical Hartree±Fock Interme-diate Neglect of Di�erential Overlap (INDO)method [13] coupled to a Single Con®gurationInteraction (SCI) technique. The latter allows us toaccount for electron-correlation e�ects (for a moredetailed description of the calculations, compareRefs. [14,15]).

3. Geometric structure

Geometry optimizations of planar and twistedoligophenylenes yield a strongly aromatic structureof the molecules with AM1 bond lengths between1.391 and 1.404 �A for the intra-ring bonds and

1.464 �A (1.461 �A in the twisted conformation) forthe inter-ring bonds. An inter-ring twist angle of40° corresponds to the global minimum in energyfor isolated molecules in good agreement withexperimental data [16]. In ®lms a more planarconformation has to be expected due to inter-mo-lecular interactions [17]. A second bond betweenadjacent rings via an additional carbon atom doesnot only result in a planarization of the moleculesbut also leads to a distortion of the para-phenylenebackbone as shown in Fig. 1 for 3lP [18]. The AM1geometry of the central rings in longer ladder typeoligomers is equivalent to that of the central ring in3lP. The para inter-ring bond length is nearly un-changed compared to unbridged molecules and thebonds in the bridge display a marked single-bondcharacter (length of 1.505 �A). Interestingly, thebenzene rings themselves are not signi®cantly dis-torted due to the formation of the ladder-typestructure as is indicated by the intra-ring bondangles (although the C±C bond in the benzene ringsis calculated to elongate to up to 1.429 �A in theladder type molecule).

Doping (single and double oxidation) signi®-cantly modi®es the geometries of the organicbackbone and leads to an increased quinoidcharacter, especially in the central rings of themolecules, as shown for 6lP in Fig. 2. The mag-

Fig. 1. AM1 optimized bond length (in �A) and bond angles in

bridged terphenyl.

Fig. 2. Change in the bond lengths of 6lP upon polaron and

bipolaron formation (the bonds in the graph correspond to the

dark lines in the schematic representation of the molecule).

308 E. Zojer et al. / Optical Materials 12 (1999) 307±310

nitude of the bond length modi®cations is similarin the bridged and unbridged molecules [14] andthe additional inter-ring bonds in the ladder typespecies are virtually not a�ected by the dopingprocess. From the extent of the changes in bondlengths calculated in singly charged molecules upto 6P and 6lP and in the doubly charged case up to10P and 12lP, respectively, an extent of four ben-zene rings can be estimated for polarons and six toeight rings for bipolarons.

4. Optical absorption

The positions of the lowest optically allowedtransitions in twisted, planar and bridged oligo-phenylene molecules in the neutral state are shownin Fig. 3. For the twisted oligophenylenes we ob-tain an excellent agreement between the INDO/SCI calculations and experimental data for un-substituted and substituted oligophenylenes insolution [19]. There is also a very good corre-spondence between the simulations for bridgedoligophenylenes and the data we have obtained forthe corresponding substituted oligomers in hexanesolution. In all investigated cases the energy of thetransition is proportional to the inverse number ofbenzene rings in the molecules, as also observedand predicted for other conjugated systems (see

references in Ref. [14]). In spite of the associateddistortions the inter-ring bridge further reduces theenergy of the lowest transition compared to anunbridged but also planarized conformation. Ananalysis of the CI expansion coe�cients of the ®rstexcited state shows that its description in all mol-ecules is dominated by the transition from theHighest Occupied Molecular Orbital (HOMO) tothe Lowest Unoccupied Molecular Orbital(LUMO) and therefore is of strong single particlecharacter [14].

The geometry relaxations associated with theoxidation in a doping process result in a destabi-lization of the HOMO (becoming the lower pola-ron or bipolaron level) and a stabilization of theLUMO (becoming the upper polaron or bipolaronlevel). This gives rise to new sub-gap absorptionfeatures, as shown in Fig. 4: (i) The formation ofpolarons leads to the appearance of two newpeaks. The ®rst one originates from a transitionfrom the highest doubly occupied level to thesingly occupied lower polaron level. For the sec-ond transition correlation e�ects do play an im-portant role. The CI expansion is dominated by atransition between the two polaron levels but thereare also signi®cant contributions of excitationsfrom lower-lying occupied states to the upperpolaron level [14]. (ii) In short-chained systems thecreation of bipolarons induces a single sub-gappeak governed by a transition from the new HO-MO to the lower bipolaron level. However, the

Fig. 3. Evolution of the lowest INDO/SCI transition energy of

neutral oligophenylenes in their fully optimized, planar and

bridged conformations as a function of inverse chain length.

The simulations are compared to experimental data for nP

molecules from the literature [19] and to the energies at the

center of the vibronic progressions of the lowest lying transi-

tions of 5lP and 7lP in hexane solution.

Fig. 4. INDO/SCI simulated absorption spectra of 6lP in the

neutral, singly (polaron) and doubly (bipolaron) positively

charged states.

E. Zojer et al. / Optical Materials 12 (1999) 307±310 309

situation changes in longer, polymer-like mole-cules, for which a second peak involving transi-tions from energetically lower-lying occupiedorbitals to the lower bipolaron level gains signi®-cant oscillator strength. A second sub-gap ab-sorption peak is also observed, when consideringinteracting bipolarons generated by removing fourcharges from a single oligophenylene molecule[14].

5. Conclusion

Geometry optimizations performed at the AM1level yield an only slight quinoid character in thebenzene rings of planar and twisted oligophenyl-enes. In bridged systems a distortion of the mo-lecular backbone is observed. Upon doping thequinoid character of the central benzene ringsstrongly increases and the unbridged systemsadopt a more planar conformation. INDO/SCIsimulations of the optical absorption of neutralchains are in good agreement with experimentaldata and yield the lowest transition energies forbridged oligophenylenes. Doping leads to two newsub-gap absorption features in short chains sup-porting polarons and to a single sub-gap peak inshort doubly oxidized molecules.

Acknowledgements

The work in Graz is ®nancially supported by aDissertationsstipendium of the Austrian Academyof Science, by the Spezialforschungsbereich El-ektroaktive Sto�e and by a F�orderungsstipendiumof the Technical University Graz. The work inMons is partly supported by the Belgian FederalGovernment ``Pole d'Attraction Interuniver-sitaire en Chimie Supramol�eculaire et Catalyse(PAI 4/11)'', the Belgian National Fund for Sci-enti®c Research (FNRS-FRFC), and an IBMAcademic Joint Study. J.C. is an FNRS researchfellow.

References

[1] Y. Ohmori, M. Uchida, K. Muro, K. Yoshino, Jpn. J.

Appl. Phys., Part 2, 30 (1991) L1941; G. Grem, G.

Leditzky, B. Ullrich, G. Leising, Adv. Mater. 4 (1992) 36;

W. Graupner, G. Grem, F. Meghdadi, C. Paar, G. Leising,

U. Scherf, K. M�ullen, W. Fischer, F. Stelzer, Mol. Cryst.

Liq. Cryst. 256 (1994) 549; M. Era, T. Tsutsui, S. Saito,

Appl. Phys. Lett. 67 (1996) 2436; Y. Yang, Q. Pei, A.J.

Heeger, J. Appl. Phys. 79 (1996) 934.

[2] S. Tasch, A. Niko, G. Leising, U. Scherf, Appl. Phys. Lett.

68 (1996) 1090.

[3] F.P. Wenzl, S. Tasch, J. Gao, L. Holzer, U. Schert, A.J.

Heeger, G. Leising, Mat. Res. Soc. Symp. Proc. 488 (1998)

57.

[4] C. Zenz, W. Graupner, S. Tasch, G. Leising, U. Scherf,

Appl. Phys. Lett. 71 (1997) 2566.

[5] S. Tasch, C. Brandst�atter, F. Meghdadi, G. Leising, G.

Froyer, L. Athouel, Adv. Mater. 9 (1997) 33.

[6] U. Scherf, K. M�ullen, Makromol. Chem. Rapid. Commun.

12 (1991) 489.

[7] J. Stamp¯, S. Tasch, G. Leising, U. Scherf, Synth. Met. 71

(1995) 2125.

[8] W. Graupner, S. Eder, M. Mauri, G. Leising, U. Scherf,

Synth. Met. 69 (1995) 419.

[9] M. Moser, S. Tasch, G. Leising, Synth. Met. 84 (1997) 651.

[10] M.J.S. Dewar, E.G. Zoebisch, E.F. Healy, J.J.P. Stewart,

J. Am. Chem. Soc. 107 (1985) 3902.

[11] S. Irle, H. Lischka, J. Chem. Phys. 107 (1997) 3021.

[12] J.L. Br�edas, B. Th�emans, J.G. Fripiat, J.M. Andr�e, R.R.

Chance, Phys. Rev. B 29 (1984) 6761.

[13] J.A. Pople, D.L. Beveridge, P.A. Dobosh, J. Chem. Phys.

47 (1967) 2026.

[14] E. Zojer, J. Cornil, J.L. Br�edas, G. Leising, Phys. Rev. B

59, in press.

[15] J. Cornil, D. Beljonne, J.L. Br�edas, J. Chem. Phys. 103

(1995) 834; J. Cornil, D. Beljonne, J.L. Br�edas, J. Chem.

Phys. 103 (1995) 842.

[16] J. Almlof, Chem. Phys. 6 (1974) 135.

[17] K.N. Baker, A.V. Fratini, T. Resch, H.C. Knachel, W.W.

Adams, E.P. Socci, B.L. Farmer, Polymer Papers 34 (1993)

1571; C. Ambrosch-Draxl, J.A. Majewski, P. Vogl, G.

Leising, Phys. Rev. B 51 (1995) 9668; S. Guha, W.

Graupner, R. Resel, M. Chandrasekhar, R. Glaser, G.

Leising, J. Chem. Phys., submitted.

[18] For AM1 simulations on bridged oligophenylenes, com-

pare also: N. Johansson et al., to be published.

[19] R.K. Khanna, Y.M. Jiang, B. Srinivas, Ch.B. Smithhart,

D.L. Wertz, Chem. Mater. 5 (1993) 1792 and references

therein; J. Grimme, M. Kreyenschmidt, F. Uckert, K.

M�ullen, U. Scherf, Adv. Mater. 7 (1995) 292; H. Gregorius,

W. Heitz, K. M�ullen, Adv. Mater. 4 (1993) 279.

310 E. Zojer et al. / Optical Materials 12 (1999) 307±310