PII: SOOO8-6223(98)00011-6

Carbon Vol. 36, No. 5-6, pp. 827-831, 1998 0 1998 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 0008-6223/98 $19.00 + 0.00

OLIGOPHENYLENES AS BUILDING BLOCKS FOR WELL-DEFINED GRAPHITE SUBUNITS

MARKUS MILLER, CHRISTIAN K~~BEL, FRANK MORGENROTH, VIVEKANANTAN S. IYER and KLAUS MULLEN*

Max-Planck-Institut ftir Polymerforschung, Ackermannweg 10, 55128 Maim, Germany

(Received 30 October 1997; accepted in revised form 9 December 1997)

Abstract-Here we present a new approach towards extremely large polycyclic aromatic hydrocarbons (PAHs), which uses smooth ambient-temperature cyclodehydrogenation conditions of oligophenylenes in contrast to conventional pyrolytic fusion methods. Vapor-deposition of these structurally well-defined, large arenes leads to supramolecular, space filling arrangements as revealed by scanning tunneling microscopy analysis. The three-dimensional ordering of PAH micro-crystals is seen by electron diffraction experiments. 0 1998 Elsevier Science Ltd. All rights reserved.

Key Words-A. Synthetic graphite, B. oxidation, C. scanning tunneling microscopy (STM), C. transmis- sion electron microscopy (TEM), D. chemical structure.

1. INTRODUCTION

The synthesis of large aromatic compounds is one of the most challenging aims since the pioneering works of Scholl [1,2], Clar [3-51 and Zander [6,7]. A systematic examination of these polycyclic aromatic hydrocarbons (PAHs) allows a correlation between the size and topology of large aromatic systems and their electronic properties. Here, we present new approaches towards very large PAHs in order to close the gap between organic molecules and graphite. In contrast to the common building up of large PAHs by thermal treatment [8-l 11, which usually forms a wide variety of products, we use a very smooth intramolecular fusing reaction at ambient temper- atures to cyclodehydrogenate soluble oligophenylene precursors yielding well-defined hydrocarbons [12,13].

2. DISCUSSION

The synthesis of the necessary oligophenylene pre- cursor molecules can be achieved generally via three different pathways:

(1)

(2)

(3)

2.1

intraiolecuiar Diels Alder reaction of suitable phenylene-vinylene derivatives [ 12,131; trimerization of suitable phenylene-ethynylene derivatives [ 14-161; and intermolecular Diels Alder reaction of suitable phenylene-ethynylene derivatives with tetraphe- nylcyclopentadienones.

Intramolecular Diels Alder reaction of suitable phenylene-vinylene derivatives

Compound 1 (Scheme 1) is subjected to a quantita- tive intramolecular [4 + 21 cycloaddition at elevated

*Corresponding author. Fax: 0049 6131 379 100; e-mail: muellen@mpip-mainz.mpg.de

temperatures to form the cyclohexene derivative 2. Aromatization of 2 with DDQ yields the correspond- ing triphenylene compound, which is finally dehydro- genated oxidatively by copper dichloride/aluminium trichloride to the well known tribenzoperylene 3.

2.2 Trimerization of suitablephenylene- ethynylene derivatives

The dicobalt octacarbonyl catalyzed trimerization of 4 (Scheme 2) results in the hexaphenylbenzene 5 with 85% yield. Depending on the substitution pattern, one can think of alkyl, alkoxy and sulfur containing side chains. In this case, we again use the Lewis acid catalyzed oxidative cyclodehydrogenation conditions to yield the hexa-peri-hexabenzocoronene (HBC) 6. These conditions were first published by Kovacic and colleagues [ 17,181 for the polymeriza- tion of benzene. Thereby, Kovacic obtained partially insoluble materials with structural defects. In con- trast, we achieve complete and nearly quantitative intramolecular cyclodehydrogenation without any intermolecular fusion reactions.

2.3 Intermolecular Diels Alder reaction of suitablephenylene-ethynylene derivatives with tetraphenylcyclopentadienones

As shown in Scheme 3, sterically accessible ethynyl functions can be reacted with suitable 1,2, 3,4-tetraphenylcyclopenta-1,3-dien-5-ones 8 in an intermolecular [4 + 21 cycloaddition under sponta- neous release of carbon monoxide. In the case of the 2,2’-bisethynylbiphenyl (7) this reaction leads to the sexiphenylene 9. Using the already described cyclodehydrogenation conditions, one reaches the C&-hydrocarbon 10 in 90% yield. As observed in the case of alkylsubstituted HBC systems 6 [ 14-161, one can also expect the formation of a discotic liquid

827

828 M. M~~LLLR el al.

1 2

Scheme I. Oligophenylene via intramolecular cyclodehydrogenation. (a) In toluene at 100°C (quantitative); (b) 2,3-dichloro-5,6-dicyano-1,4-benzoquinone in benzene at 78°C (75%); (c) copper dichloride/aluminium trichloride in carbon

disulfide at ambient temperature (99%).

R R = Ii, n-alkyl, t-butyl

5 6

(a) Dicobaltoctacarbonyl in l,4-dioxane at 100°C (85%); (b) copper Scheme 2. Oligophenylenes via cyclotrimerization. ditriflate/aluminium trichloride in carbon disulfide at ambient temperature (99%).

Scheme 3. Oligophenylenes via intermolecular cyclodehydrogenation. (a) In diphenylether at 230°C (quantitative); (b) (0 copper dichloride/aluminium trichloride in 1,1,2,2-tetrachloroethane at 145°C; (2) r-butyl lithium in hexane at 50°C (90%).

R = H, n-alkyl, t-butyl

crystalline mesophase by introducing eight n-alkyl chains into the periphery of large PAHs like 10 or 13.

Due to the size of the tetraphenylcyclopentadie- none unit 8, only a few intermolecular cycloaddition reactions with appropriate phenylene-ethynylene derivatives already yield extraordinary large oligo- phenylene precursors. The resulting oligophenylene precursors can be subjected to quantitative cyclode- hydrogenation as shown in Schemes 4 and 5 [ 19-221.

1,3,5_Triethynylbenzene (11) containing 12 carbon atoms gives rise to the PAH 13 with 96 carbon atoms in two reaction steps (Scheme4). An analogous

growth is reached when the precursor 14 is trans- formed into “supertriphenylene” 16, which with 132 carbon atoms is one of the largest PAHs ever synthe- sized [ 19,201.

One of the most fascinating aspects is the two- and three-dimensional supramolecular ordering of those large aromatic compounds. The assembly of PAHs is studied using the rhombic C,,H,,-hydrocarbon 20 as an example, which is synthesized via a double intramolecular Diels Alder reaction of 17 followed by aromatization and oxidative cyclodehydrogena- tion [23,24]. A few ordered domains of 20 are already

II

A Go 11 3 a)

Oligophenylenes as building blocks 829

built during the cyclodehydrogenation and micro- crystallites can be grown by in situ deposition of the reaction product. Figure 1 shows the transmission electron microscopy (TEM) image of such a crystal. The three-dimensional ordering of the crystal is seen in the corresponding electron diffraction pattern, which reveals that the unit cell contains at least two molecules of 20 (Scheme 6).

Scheme 4. Synthesis of the C,,H,,-hydrocarbon 13. (a) 3 Eq. 1,2,3,4-tetraphenylcyclopenta-I ,3-dien-5-one (8) in diphenylether at 250°C (quantative); (b) copper ditriflate/ aluminium trichloride in carbon disulfide at ambient

A well ordered monomolecular layer of 20 was prepared by fractionated sublimation of &Hz2 at temperatures between 550 and 650°C in ultra-high vacuum (UHV) onto a (0001) face of an MO!& single crystal. Figure 2 shows the scanning tunneling micro- scopy (STM) image of this vapor-deposited mono- layer and one can clearly recognize the shapes of single molecules of 20. The topology of 20 allows a space filling arrangement of the single molecules on the substrate. In further experiments we will sublime arene 20 onto a palladium surface in order to receive a similar space filling grouping, which should be dehydrogenated thermally by catalytic aid of the palladium crystal.

3. CONCLUSION temperature (90%).

The present approach from suitable oligopheny- lenes to large, planar, polycyclic aromatic compounds

a)

Scheme 5. Synthesis of the C,,,H,,-hydrocarbon 16. (a) 3 Eq. 1,2,3,4-tetraphenylcyclopenta-1,3-dien-5-one (8) in diphenylether at 250°C (95%); (b) copper ditriflate/aluminium trichloride in carbon disulfide at ambient temperature (90%).

Fig. I. Electron microscopy of the C,,H,,-hydrocarbon 20. TEM brightfield image of a lathpshaped crystal of 20 surrounded by partially ordered domains of the C,,H,,-hydrocarbon, The corresponding electron diffrac- tion image shows the diffraction pattern of the crystallite. In addition one notices Debye-rings due to partially ordered material surrounding the crystal (Philips CM 12. 120 kV

accelerating voltage).

via oxidative intramolecular cyclodehydrogenation

promises to become a very general method to synthe- size aromatic molecules with a wide variety of topolo- gies. This smooth method combines the advantages

of a straightforward synthesis with a high yield fol all reactions involved. The synthesis allows to vary

F’ig. 2. Scanning tunneling microscopy of the C,,H,,-hydrocarbon 20. STM image of a nominally 3 A thick vapor-deposited layer of 20 on a (0001) face of a MO& single crystal, taken in constant height mode (bias: t I .8 v: tunneling current: 90 pA; image area

lOOAxlOOA).

the topology of the resulting polycyclic hydrocarbons in order to achieve different two- and three-dimen- sional supramolecular arrangements.

.~lc.l;no~~~/ed~rmrnt.s This work was supported financially by the Volkswagenstiftung and the Bundesministerium fur Forschung und Bildung. C. K. and F. M. thank the Fonds dcr Chemischem lndustric for a scholarship. The authors wish to thank Dr J. RPder and Dip]. Chem. K. Martin for recording the LD-TOF mass spectra. Professor Dr N. Karl and Dipl. Phys. C. Gunther for the vapor deposition of the monomolecular layers and Dr R. Strohmaier and Dr J. Petersen for recording the STM images.

20 19

Scheme 6. Synthesis of the C&H,,-rhombus 20. (a) In I. 1,2,2Xetrachloroethane at l35’C (98%); (b) 2,3-dichloro- 5,6-dicyano-1,4-benzoquinone in l,l,2,2-tetrachloroethane at l35’C (96%); (c) copper dichloride/aluminium trichloride in

carbon disulfide at ambient temperature (99%).

Oligophenylenes as building blocks 831

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