From three-dimensional polyphenylene dendrimers to large graphite subunits

Carbon Vol.

36, No. 5-6, pp. 833-837, 1998 0 1998 Else& ScienceLtd

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

PII: SOOO&6223(98)0001&9

FROM THREE-DIMENSIONAL POLYPHENYLENE DENDRIMERS TO LARGE GRAPHITE SUBUNITS F. MORGENROTH,~ C. K~~BEL,~ M. M~~LLER,~ U.M. ‘Max-Planck-lnstitut

WIESLER,~ A.J. BERRESHEIM,~

M. WAGNER’ and K. M~~LLEN~** fi.ir Polymerforschung, Ackermannweg 10, D-5 5 128 Mainz, Germany

(Received 30 October 1997; accepted in revisedform 9 December 1997)

Abstract-Here we present a new approach to extremely large polycyclic aromatic hydrocarbons (PAHS) based on a cyclodehydrogenation method for polyphenylene dendrimers. These polyphenylene dendrimers are synthesized by repeated Diels-Alder reactions of ethynyl-substituted tetraphenylcyclopentadienones (lb) to the tetraethynylbiphenyl (2) core. Molecular mechanics and molecular dynamics calculations revealed that the dendrimers are shape-persistent due to the steric “overcrowding” of the benzene rings.

The chemical and physical properties of the dendrimers can be controlled by varying the cyclopentadienones used in the last growth step. 0 1998 Elsevier Science Ltd. All rights reserved. Key Words-A. Synthetic graphite, D. chemical structure.

A. carbon precursor,

B.

oxidation,

C. molecular simulation,

be considered the first generation G, of a polyphenylene dendrimer. After an easy deprotection of 6a with tetrabutylammonium fluoride (Bu,NF) in THF, the resulting ethynyl derivative 6b becomes available for an eightfold addition of tetraphenylcyclopentadienone la yielding the second dendrimer generation Gz (compound 7a) [ 111. Repetition of the described

1. INTRODUCTION

In recent papers we reported on a new synthetic concept for the synthesis of extremely extended polycyclic aromatic hydrocarbons (PAHs). The key step of our approach is the intramolecular oxidative cyclodehydrogenation of suitable oligophenylene precursors to PAHs under Kovacic conditions [l-lo]. A convenient access to these soluble oligophenylene precursors is provided by the intermolecular [Z + 41 cycloaddition of tetraphenylcyclopentadienone (la) to ethynyl derivatives under extrusion of carbon monoxide. PAH (5), containing 78 carbon atoms, is synthesized as shown in Scheme 1 by using the cyclopentadienone (la) as diene and 1,6bis( 1-phenylethynyl ) benzene (3) as dienophile [2e]. Here we present a new concept permitting the synthesis of even larger PAHs which constitute the most extended arenes known today.

is%

R

0

O 0

0

0

1

la +

m O=

R

3

I

4

0000

0

00000 0000

2. DISCUSSION

Our new route to PAHs employs the ethynylsubstituted tetraphenylcyclopentadienone (lb) instead of the unsubstituted la. A diene function is contained in lb like la as well as two dienophile functions. Thus lb can be considered an A,B-building block. However, since the two dienophile functions in the tetraphenylcyclopentadienone lb are protected by triisopropylsilyl (TiPS) groups it can only act as a diene. Accordingly, this tetraphenylcyclopentadienone can be reacted selectively in a fourfold [2 + 41 cycloaddition with the four dienophile functions of 2 to yield the oligophenylene 6a under extrusion of CO (Scheme 2). Oligophenylene 6a can

4

I

b)

Scheme 1. Left: building blocks for the synthesis of dendritic polyphenylenes; Right: access to large PAHs via intermolecular Diels Alder reaction, a) In diphenylether at 250°C (98%); b) copper(I1) triflate/aluminum chloride in carbondisulfide at ambient temperature (quantitative).

*Corresponding author. 833

I.‘.MORGENROTH et ul.

834

2-3-a

6a b)

6b

a)

la

b)

7b 4 I

6c

7c

8

6a. 7% 0. R= -TIPS R

R

R

66. 7b. R =aH

6 6c. 7c. R = H 6d, R = OCH,

8 Scheme 2. Dendritic polyphenylene, a) 1b in diphenylether/cc-methylnaphthalene at I80 200°C (> 80%); b) tetrabutylammonium fluoride in THF (quantitative); c) la in diphenylether/a-methylnaphthalene at reflux (> 80%); d) lc in diphenylether/ccmethylnaphthalene at 180-200°C (89%).

deprotection/cycloaddition sequence allows the synthesis of the higher dendrimer generation G3 8. Thus, the CZZObuilding block 6a with a molecular mass of 3119 g mol -’ is selectively converted into the C,,, unit 7a with a molecular mass of 7606 g mol -I. Compound 7a in turn leads in two steps to the C r2e4 unit 8 which contains 142 benzene rings. Since all generations of the dendrimer are readily soluble in common organic solvents such as CH,Cl,, CHCl,, or acetone, their characterization is achieved by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDITOF-MS) as well as ‘H and ‘%-NMR spectroscopy. Employing MALDI-TOF-MS allows potential growth imperfections during the [ 2 + 41 cycloaddition

to be detected without any ambiguity, even for the higher generations, since each unreacted ethynyl group causes a mass difference of 717 g mol (lb less CO) with respect to the completely reacted product. The perfect agreement between calculated and experimentally determined m/z ratios for G, to G3 confirms the monodispersity of the dendrimers*. Molecular mechanics and molecular dynamics calculations performed for G, and G, (without acetylene units) provide a first insight into the three*6a: calculated for CZ2,,HZSOSi8: 3119 g mot -I, found: m/z=3118; 7a: calculated for C,,aH,,eSin,, K’: 7645 g mol-‘, found in the presence of K: m/z=7645; 8: K+: 16619 g mol -‘, found in calculated for C ,204H121,,Si32r the presence of K: m/z= 16619;

From three-dimensional polyphenylene dendrimers to large graphite subunits

dimensional structure of the dendrimers* [ 121. The calculations reveal that selected inner distances of the molecule vary only in a range of 5-10% during the molecular dynamics simulations, thus indicating that the overall shape of the molecule does not change significantly throughout the simulation time. Accordingly, the higher dendrimer generations Gz and G3 represent shape-persistent molecules. In contrast to other approaches, which employ rigid building blocks in order to create shape-persistent architectures [ 13- 161, our dendrimers exhibit an invariant overall shape due to the dense packing of benzene rings [ 121. According to the molecular mechanics calculations,_the diameter of the dendrimer generation G2 is 36 A. As an indication of the remarkable size of our dendrimers, the diameter of G3 is found to be approximately 55 A. In Fig. 1 the three-dimensional structure of G2 (without acetylene units) is shown as revealed by molecular mechanics calculations. As a hitherto unknown feature for dendrimers it is possible to planarize the three-dimensional architecture. Under cyclodehydrogenation conditions (AlCl,/CuCl,/C,H,Cl,) the polyphenylene 6e, which represents the first dendrimer generation, forms 28 new bonds yielding the planar polycyclic aromatic hydrocarbon 9 (Scheme 3). Owing to its extremely low solubility in all common solvents PAH 9 is characterized by laser desorption mass spectrometry *The molecular mechanics calculations and molecular dynamics simulations were carried out using the Cerius’ program package with the MM2 (85) force field. For more details see [6].

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based on its M + peak. As expected, one observes a peak at m/z= 1621 (calculated for C1JZH34 1620 g mol-‘). This peak corresponds to the mass of the starting material (1676 g mol - ‘) minus 56 mass units (2 x 28 hydrogen atoms) [ 111. First experiments give evidence that it is also possible to planarize the polyphenylene precursor 7c in order to obtain large polybenzoid discs such as 10 (Schemes 2 and 3). However, the reaction conditions have to be optimized in order to avoid partially cyclized and/or chlorinated byproducts. For the planarization of any dendrimer generation higher than G2 the following needs to be considered: as indicated in Scheme 2, only the dendrimer generations Gi and Gz can be projected into two dimensions without spatial overlap of the benzene rings, since eight benzene rings overlap in the two-dimensional projection of GJ. According to our experience with such “overcrowded” systems the benzene rings migrate under the cyclodehydrogenation conditions leading to PAHs isomers [2d]. Due to the extremely low solubility of extended polybenzoid discs, a sufficient processability of the PAHs is a precondition for their full characterization as well as the investigation of their properties. Enhanced processability has been achieved by substitution with solubilizing alkyl chains [2c], [2e]. However, in order to tune the electronic properties of PAHs and their polyphenylene precursors a substitution with donor or acceptor groups, such as -OCHJ or NOz, would be required. A straightforward approach to introduce the aforementioned groups is to use substituted tetraphenylcyclopentadienones, such as Ic (Scheme 1). This concept avoids synthesiz-

836

6a

Scheme 3. Cyclodehydrogenation

9

reactions, a) copper dichloride/aluminum

ing the substituted dendrimers using for example organometallic reagents which may suffer from a low yield and side-products. In contrast, one can benefit from the almost qantitative Diels-Alder reaction of substituted tetraphenylcyclopentadienones with ethynyl compounds. Employing this concept, we obtained the octamethoxy-substituted polyphenylene 6d via the Diels-Alder reaction of tetraethynylbiphenyl 2 with lc. Oligophenylene (6d) is soluble in common organic solvents and is characterized by field desorption (FD) mass spectrometry as well as ‘H and % NMR spectroscopy. The FD mass spectrum of 6d consists of two peaks at m/z= 1917(M ‘) and m/z= 958(M2+), which is in good accordance with the calculated mass of 1916 g mol -I. The ‘H NMR of the branched oligophenylene 6d reflects the correct relative intensifies of aromatic to aliphatic protons (H aromatic:Haaphatic= 3.4). 3. CONCLUSION Our experiments confirm that it is possible to flatten a three dimensional polyphenylene dendrimer into large two-dimensional polybenzoid discs via cyclodehydrogenation reactions. This means that a chemical switching from three to two dimensions can be achieved. Focussing on graphite itself as a reference system for the analysis of large PAHS, the preparation of polybenzoid discs that can be considered as the missing link between structurally undefined macroscopic graphite and large, well defined, PAHs is challenging. This aim is in reach using our new synthetic approach.

trichloride in tetrachloroethane

at IOO’C, 9 25 h.

Acknowledgements ~-This work was supported Iinancially by the Volkswagenstiftung and the Bundesministerium fiir Forschung und Bildung. C.K. and F.M. thank the Fond der Chemischen Industrie for scholarships. We wish to thank Dr J. Rader and K. Martin (Dip]. Chem.) for recording the LD-TOF mass spectra.

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