Synthesis and characterization of polyacene precursors

ELSEVIER

Synthetic

SYNTHESIS Department

Metals

AND CHARACTERIZATION

of Chemistry and Centerfor

84 (1997)

363-364

OF POLYACENE

C.J. Ruud, Chun Wang and G.L. Baker* Fundamental Materials Research, Michigan

PRECURSORS

State University, East Lansing, Michigan

48824, USA

Abstract Polyacenes are one-dimensional analogs of graphite and are predicted to have high conductivities without chemical doping.. The acidcatalyzed condensation of 1,2-bis(methoxymethyl)benzene gives a network polymer with idealized structure that corresponds to a linear polyacene precursor, that after elimination of H2, should yield polyacenes with extended x electron systems. In contrast, the polymerization of 1,4-bis(methoxymethyl)benzene and 1,3-bis(methoxymethyl)benzene are expected to result in networks with a proportionately higher polybenzyl content. Based on TGA and IR studies, we propose that polyacene precursors are formed in the polymerization of the ortho substituted monomer, while the meta and para-substituted monomers contain a higher fraction of the less stable polybenzyl structure. Samples heated in an inert atmosphere show conversion of the scene precursors to partially graphitized products. Keywords:

scenes, low-bandgap

conjugated polymers, graphite and related compounds.

disordered scene-like materials may enable important applications such as their use as electrode materials and antistatic agents.

1. Introduction Polyacenes are the long chain members of the family of edge-fused aromatic compounds that includes benzene, naphthalene, anthracene, tetracene, and higher homologs. The valence bond structures of true scenes are ladder-like with the rungs of the ladder connecting hvo trans-polyene chains. Theory predicts that these planar polyacenes should have small band gaps, about 0.4 eV.l Acenes can also be viewed as onedimensional analogs of graphite, a ribbon taken from a single graphene sheet. Interest in polyacenes goes beyond theoretical studies of their electronic properties. Practical applications proposed Figure 1. Ladder structure of for polyacenes and scene-like polyacenes materials include their use as optical limiters,2 as electrical conductors, and as electrodes for lithium ion batteries.3 Unfortunately, the preparation of polyacenes is difficult, owing to the low solubility of linear scenes, and their high chemical reactivity. To date, there are no examples of well-defined infinite chain polyacenes. Some related structures, however, have been prepared.4 For example, various planarized polyphenylenes have been synthesized that can be thought of as end-linked fluorene5 and phenanthrene structures. Although related structurally, their electronic properties do not match those expected for the doubly conjugated ladder structure of scenes. We have been devising routes to well-defined polyacenes and to scene-like materials. Obtaining the former should allow us to use optical methods to measure the electronic properties of scenes, and enable comparisons to the electronic structures predicted by theory. In contrast, direct routes to * Conespondiog author. 0379-67791971$17.00

0 1997

PLI SO379-6779(96)03939-2

Ekevier

Science

S.A. All

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reserved

2. Results and Discussion A simple route to scene-like materials is suggested by the acid catalyzed condensation of alkoxymethyl substituted benzenes. -As shown schematically in- Figure 2,

CH20CH3

H+

Figure 2. Idealized 2-dimensional representation of the expected products from acid-catalyzed condensation of methoxymethylbenzene, and bis(methoxymethylbenzene)s.

C.J. Ruud et al. /SyntheticMetals

364

methoxymethylbenzene should undergo electrophilic aromatic substitution to give polybenzyl, while disubstituted benzenes should yield scene precursors. Furthermore, the substitution pattern on the benzene ring should lead to different structures, with ortho substitution favoring scenes while the meta and para derivatives are expected to yield a predominately polybenzyl network. To test this idea, we prepared the ortho, meta, and para isomers of bis(methoxymethyl)benzene and polymerized them using acid catalysts. In all cases, the addition of sulfuric acid to the monomer led to the rapid formation of yellow or brown solids, which precipitated from solution. Structural characterization of these insoluble materials is difficult. Not only are they insoluble, but the structural differences between polybenzyl and the scene precursor are subtle and not readily detected by NMR, IR, and other standard spectroscopic However, thermal analysis studies provides techniques. circumstantial evidence that the ortho isomer tends to form acenelike structures, while the para and meta isomers yield predominantly polybenzyl architectures. Figure 3 shows the thermogravimetric analyses for polymers derived from the ortho and para substituted monomers. The TGA scans, taken in nitrogen containing a small percentage of oxygen, show an initial weight loss with an onset near 400 “C, that for the para derivative, is followed by the complete degradation of the sample. In contrast, the ortho sample shows remarkable stability, a property we attribute to the formation of a substantial proportion of fused rings during polymerization. For scene-like materials with ladder-like structures, the scission of a single chain does not necessarily lead to the severing of the chain, but chain scission in a single strand polymer like polybenzyl, breaks the polymer chain into smaller pieces. The ortho substituted polymer was heated in N2 to various temperatures in a TGA apparatus, and the residue was analyzed using infrared spectroscopy. The spectra, shown in Figure 4, show the progressive chemical structure evolution from that of a standard polymer with a high proportion of aliphatic C-H bonds, to an intermediate structure with predominantly aromatic C-H bonds. Further treatment of the samples at 800 “C results in the near-complete loss of C-H bonds and presumably

100 90 80 70

84 (1997)

363-364

4500

3500

2500

1500

-! 500

wavenumbers (cm”) Figure 4. Infrared absorption spectra of poly( 1,2-bis(methoxy methylbenzene)) heated under nitrogen at 10 Wmin. in a TGA to the indicated temperature. Samples for spectroscopy were prepared as KBr pellets; the broad absorption at 3500 cm-l is due to residual water in the KBr.

the formation of a graphite structure. Assuming the ortho precursor contains a large fraction of edge-fused rings, the samples taken at 500 ‘C should contain an appreciable fraction of polyacenes. One signature of polyacenes would be the appearance of materials with a relatively low bandgap, but with the retention of aromatic C-H bonds. From the IR spectra, we find that samples taken at 500 and 650 “C have a strong electronic absorption that tails into the infrared region, indicating extensive rt conjugation. In addition, we note that the IR bands corresponding to C-H stretching are retained. The bands due to aliphatic C-H bonds (2900-3000 cm-l) decrease in intensity while those due to aromatic C-H stretching (3000-3150 cm-l) increase in intensity. The shift from aliphatic to aromatic C-H bonds as expected for the formation of scenes. 3. References

200

400

temperature

600

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1000

(C)

Figure 3. Thermogravimetric analysis of poly( 1,2-bis(methoxy methylbenzene)) (ortho) and of poly( 1,4-bis(methoxymethy1 benzene)) (para) heated in a nitrogen/oxygen environment at 10 “Clmin.

1. M. Kertesz Macromolecules 1995 28, 1475-1480. 2. Y. Kojima, M. Tsuji, T. Matsuoka, and H. Takahashi Macromolecules 1994 27, 3735-3738; Y. Kojima, T. Matsuoka, N. Sato, and H. Takahashi Macromolecules 1995 28,2893-2896. 3. T. Zheng, J.S. Xue, and J.R. Dahn Chem. Mater. 1996 8, 389393. 4. U. Scherf, K. Mullen Synthesis 1992 23-38. 5. G. Grem, C. Paar, J. Stamfl, G. Leising, J. Huber, and U. Scherf Chem. Mater. 1995 7, 2-4.