Syntheses of hexabenzocoronene derivatives

Article abstract of DOI:10.1055/s-2003-40526

The syntheses of four hexabenzo[bc,ef,hi,kl,no,qr]coronene (HBC) derivatives have been carried out. Double Knoevenagel condensation of the benzil 5 with the ketone 6 gave the 2,3,4,5-tetrakis(4-tert-butylphenyl)cyclopenta-2,4-dien-1-one (7). Diels-Alder a

Full text of DOI:10.1055/s-2003-40526

PAPER
1521
Syntheses of Hexabenzocoronene Derivatives
Syntheses
of
H
u
e
xabenzoco
b
ronene
D
eriv
i
atives r K. Sadhukhan,1 Christine Viala, André Gourdon*
NanoScience Group, CEMES-CNRS, BP3447, 29 Rue J. Marvig, 31055 Toulouse, France
Fax +33(562)257999; E-mail: gourdon@cemes.fr
Received 31 March 2003; revised 6 June 2003
and quartet states and their ability to take up six elec-
trons.11
Abstract: The syntheses of four hexabenzo[bc,ef,hi,kl,no,qr]coro-
nene (HBC) derivatives have been carried out. Double Knoevenagel
condensation of the benzil 5 with the ketone 6 gave the 2,3,4,5-tet-
rakis(4-tert-butylphenyl)cyclopenta-2,4-dien-1-one (7). Diels–Al-
der addition of 7 with di(4-tert-butylphenyl)acetylene, 4-[(4-tert-
butylphenyl)ethynyl]-1,1 -biphenyl (9) or 1-tert-butyl-4-(phenyl-
ethynyl)benzene (11) gave the substituted hexaphenylbenzene 8, 10
and 12 which were cyclodehydrogenated to yield 1, 2 and 3, respec-
tively. Similarly, condensation of 5 with dibenzylketone yielded the
cyclopentadienone 13 which gave, by cycloaddition with di-(4-tert-
butylphenyl)acetylene, the tetrasubstituted hexaphenylbenzene 14
which was cyclodehydrogenated to furnish 4.
These remarkable electronic properties led us to prepare
the 2D lander 2 in which R = phenyl (for electrical contact
to a metallic step edge7) and, as a reference for STM stud-
ies, the known12 hexa-tert-butyl-HBC 1. Surprisingly the
first STM images13 of co-evaporated 1 and 2 showed im-
portant image contrasts, the tunneling current through 2
being significantly smaller than through 1. This puzzling
observation led us to propose several various hypotheses
such as: a) an intramolecular interference effect induced
by the asymmetry of the molecule, or b) a surface restruc-
turing underneath the molecule with missing14 or excess
metal atom8 as observed with other landers.
Key words: arenes , ketones, cycloadditions, polycycles, dehydro-
genations, Diels–Alder reactions
In order to clarify this point we have undertaken the syn-
thesis of two other HBC derivatives 3 and 4 with 5 and 4
spacers shown on Figure 2.
The development of near-field microscopes (AFM, STM)
not only as imaging tools but also to manipulate and to
measure mechanical2,3 and spectroscopic4 properties of
single molecules has opened the way to new experiments
at the sub-molecular level.5 Along these lines, we have re-
cently investigated a series of molecules, named landers,
in which a rigid polyaromatic board is electronically de-
coupled from the metallic substrate by spacers
(Figure 1).6 So far in these molecules, the central part was
a molecular wire7,8 or a rotor9 and this work is currently
being extend to 2D landers in order to understand the tun-
nel electron propagation in the plane of a large molecule.
Like most HBC derivatives described so far in the litera-
ture,15 the target compounds 1–4 are obtained by six-fold
cyclodehydrogenation of substituted hexaphenyl benzene.
The latter precursors were prepared by Diels–Alder addi-
tion of a substituted cyclopentadienone with a diphenyl
1
2
Figure 1 General scheme of a 2D-lander on metallic surface.
Among various possible polyaromatic boards that could
be used for such a concept, hexa-peri-hexabenzocoro-
nenes (HBC) seem very attractive. These ‘superbenzenes’
are currently much studied due to interesting properties
such as high charge-carrier mobility along the -stacked
columns in the bulk mesogenic phases,10 stabilized triplet
4
3
Synthesis 2003, No. 10, Print: 21 07 2003.
Art Id.1437-210X,E;2003,0,10,1521,1525,ftx,en;T03503SS.pdf.
© Georg Thieme Verlag Stuttgart · New York
Figure 2 2D landers with HBC main board, spacers: tert-butyl and
R, R = H, tert-butyl or phenyl groups.

1522
S. K. Sadhukhan et al.
PAPER
alkyne, followed by in situ decarbonylation and rearoma- 13. The latter compound was obtained by double Knoe-
tization. venagel condensation of the benzil 5 and dibenzylketone.
The synthesis of 1 and 2, outlined in Scheme 1, starts with Work in progress is directed toward UHV STM studies of
1-bromomethyl-4-tert-butylbenzene.
properties of these devices.
Reaction with tosylmethyl isocyanide (TOSMIC) and
NaH in anhyd DMSO16 and then hydrolysis of the isocy-
anide gave the acetone derivative 6 in 61% yield. This
route was preferred to the carbonylating coupling using
iron pentacarbonyl,17 which requires difficult separation
of iron hydroxide from the aqueous suspension. Then con-
densation with 5, obtained according to the Mueller–
Westerhoff procedure,18 gave the known cyclopentadi-
1H and 13C NMR spectra were recorded with a Bruker WF-250 in-
strument in CDCl3 or CD2Cl2 solutions at 20 °C and internal stan-
dard (CDCl3 at H = 7.25, C = 77.0, CD2Cl2 at H = 5.30, C = 53.8).
Mass Spectroscopy was performed with a Nermag R10-R10 (DCI).
Elemental analyses were done by the Service d’Analyse de l’ICSN
(Paris) and by the Service d’Analyse du LCC (Toulouse).
4-Ethynylbiphenyl was prepared according to ref.19 THF was dis-
tilled from Na/benzophenone. Other solvents and reagents were
enone 7. Diels–Alder addition with bis(tert-butylphe- used as obtained in the best quality available. Petroleum ether refers
to the fraction with bp 35–60 ºC.
nyl)acetylene yielded the hexaphenylbenzene 8 which
was cyclodehydrogenated12 to provide 1. The synthesis of
1,2-Bis(4-tert-butylphenyl)ethane-1,2-dione (5)
To a solution of 1-bromo-4-tert-butylbenzene (5.00 mL, 28.83
mmol) in THF (50 mL) at –78 °C was added BuLi (2.5 M; 11.6 mL)
10 was conducted in a very similar way by cycloaddition
of 7 with 9. The latter compound was obtained by Negishi
coupling of diphenylethynylzinc chloride with 1-bromo-
4-tert-butylbenzene. This reaction gave a higher yield
than the Sonogashira coupling of the alkyne to the deacti-
vated arylbromide. The cyclodehydrogenation of 10 by
FeCl3 in dichloromethane–nitromethane yielded 2 in 75%
yield.
in hexanes (29 mmol). The solution was stirred at –50 °C for 1 h and
then added to a suspension of 1,4-dimethylpiperazine-2,3-dione
(2.00 g, 14.07 mmol) in THF (50 mL). The mixture was then al-
lowed to warm up to r.t. and stirred for 1 h at r.t., and then hydro-
lyzed with aq HCl (10%; 100 mL). After 30 min of stirring, the
mixture was concentrated under vacuum and extracted with Et2O
(4 × 50 mL). The organic layer was dried (Na2SO4), and evaporated.
We achieved the synthesis of 3 and 4, shown on The residue was purified by flash chromatography (silica gel; petro-
leum ether–CH2Cl2, 3:1) to elute the yellow band of the product.
Scheme 2, in a very similar way. Cycloaddition of 7 and
1-tert-butyl-4-(phenylethynyl)benzene (11) gave the pen-
tasubstituted hexaphenylbenzene 12 in 80% yield. Cyclo-
dehydrogenation under the same conditions as for 2
yielded 3 (71%). The synthesis of 4 required the prepara-
tion of the disubstituted pentaphenylcyclopentadienone
Yield: 3.00 g (65%).
1H NMR (250 MHz, CDCl3): = 1.32 (s, 18 H), 7.50 (d, 4 H,
3J = 9.0 Hz) 7.89 (d, 4 H, 3J = 9.0 Hz).
13C NMR (62.5 MHz, CDCl3): = 30.9, 35.3, 125.9, 129.8, 130.5,
158.8, 194.5.
Scheme 1 Route to 1 and 2: (a) i) BuLi, THF; 1,4-dimethylpiperazine-2,3-dione; ii) HCl–H2O, 65% (b) i) NaH, DMSO; TOSMIC; (ii) H2O,
61% (c) KOH, EtOH, 61% (d) diphenyl ether, 50% (e) FeCl3, CH2Cl2, CH3NO2, Ar bubbling, 91% (ref.12) (f) i) BuLi, ZnCl2, Et2O, ii) 1-bromo-
4-tert-butyl-benzene, Pd(PPh3)4, THF 70% (g) diphenyl ether, 80% (h) FeCl3, CH2Cl2, CH3NO2, Ar bubbling, 75%.
Synthesis 2003, No. 10, 1521–1525 © Thieme Stuttgart · New York

PAPER
Syntheses of Hexabenzocoronene Derivatives
1523
+
MS (DCI, NH3): m/z = 323 [MH+], 340 [MNH4 ].
Anal. Calcd for C23H30O (322.48): C, 85.66; H, 9.38. Found: C,
85.35; H, 9.25.
2,3,4,5-Tetrakis(4-tert-butylphenyl)cyclopenta-2,4-dien-1-one
(7)
11
To a boiling solution of 4,4 -di-tert-butylbenzil (905 mg, 2.81
mmol) and 4,4 -di-tert-butylbenzyl ketone (905 mg, 2.81 mmol) in
anhyd EtOH (5 mL) was added a solution of KOH (2 M; 1 mL) in
anhyd EtOH. The solution was refluxed 3 h and then cooled to 0 °C
overnight. The dark purple crystalline product was filtered and
washed with cold EtOH (2 × 1 mL).
a
b
7
3
Yield: 61% (1.05 g).
1H NMR (250 MHz, CDCl3): = 1.27 (s, 18 H), 1.28 (s, 18 H), 6.82
(d, 4 H, 3J = 7.5 Hz), 7.14 (d, 4 H, 3J = 7.5 Hz), 7.18–7.25 (m, 8 H).
12
13C NMR (62.5 MHz, CDCl3): = 31.2, 34.5, 124.5, 124.9, 128.0,
129.0, 129.6, 130.4, 149.9, 201.4.
+
MS (DCI, NH3): m/z = 609 [MH+] and 626 [MNH4 ].
Anal. Calcd for C45H52O (608.89): C, 88.76; H, 8.61. Found: C,
87.91; H, 8.59.
c
d
O
5
Hexa(4-tert-butylphenyl)benzene (8)
A solution of 1-tert-butyl-4-[(4-tert-butylphenyl)ethynyl]benzene
(358 mg, 1.23 mmol) and of the tetraphenyl cyclopentadienone 7
(750 mg, 1.23 mmol) in diphenyl ether (5 g) was stirred 24 h at 225–
240 °C under argon. After cooling, the red solution was diluted with
CH2Cl2 (10 mL) and poured in MeOH (100 mL). The raw product
was then filtered, washed with MeOH and vacuum dried. It can be
further purified by recrystallization in toluene.
13
e
Yield 50% (520 mg).
4
1H NMR (250 MHz, CDCl3): = 1.09 (s, 54 H), 6.68 (d, 12 H,
3J = 8.5 Hz), 6.80 (d, 12 H, 3J = 8.5 Hz)
13C NMR (62.5 MHz, CD2Cl2): = 31.2, 34.2, 123.4, 131.2, 138.3,
140.6.
14
Anal. Calcd for C66H78 (871.33): C, 90.88; H, 9.02. Found: C,
91.05; H, 9.05.
MS (EI): m/z = 870.61 (M+)
Scheme 2 Synthesis of compounds 3 and 4: (a) diphenyl ether, 80%
(b) FeCl3, CH2Cl2, CH3NO2, Ar bubbling, 71% (c) dibenzyl ketone,
KOH, EtOH, 87% (d) diphenyl ether, 50% (e) FeCl3, CH2Cl2,
CH3NO2, Ar bubbling, 71%.
4-[(4-tert-Butylphenyl)ethynyl]-1,1 -biphenyl (9)
To a suspension of 4-ethynylbiphenyl (1.00 g, 5.61 mmol) in Et2O
(5 mL) at –78 °C, was added BuLi in hexanes (2.5 M; 2.50 mL). The
solution was then warmed up to r.t. After addition of THF (2 mL),
the solution was transferred in a solution of ZnCl2 (1 M) in Et2O (10
mL) and THF (20 mL). The solution was then stirred 2 h at r.t. and
concentrated to 15 mL under vacuum. 4-tert-Butylbromobenzene
(1.2 g, 5.61 mmol) and Pd(PPh3)4 (130 mg, 0.11 mmol) in THF (10
mL) were added and the solution was then stirred 20 h at 75 °C un-
der argon. The brown solution was then poured in sat. aq NH4Cl
(100 mL) and the organic phase was washed with sat. aq NH4Cl
(2 × 25 mL). The product was then purified by chromatography
(hexane).
MS (EI): m/z = 322 [M+].
Anal. Calcd for C22H26O2 (322.44): C, 81.95; H, 8.13. Found: C,
82.08; H, 8.26.
1,3-Bis(4-tert-butylphenyl)acetone (6)
NaH (60% in oil, 3 g, 32.5 mol) was added portion wise to cooled
(5 °C) anhyd DMSO (40 mL) under argon. The mixture was then
stirred for 0.5 h followed by the dropwise addition of a solution of
TOSMIC (2.66 g, 13.6 mmol) and 4-tert-butylbromobenzene (5.00
mL, 27.21 mmol) in anhyd DMSO (40 mL). The content was stirred
for another 3 h at r.t., H2O (100 mL) was added and the mixture ex-
tracted with CH2Cl2 (3 × 50 mL), the organic layer washed with
brine, dried (MgSO4) and evaporated. The crude product was puri-
fied by flash chromatography (CH2Cl2–cyclohexane, 1:4) to get
pure product.
Yield: 70%.
1H NMR (250 MHz, CD2Cl2): = 1.33 (s, 9 H), 7.37–7.52 (m, 8 H),
7.60–7.67 (m, 7 H).
13C NMR (62.5 MHz, CDCl3): = 31.2, 34.8, 88.6, 90.2, 120.2,
122.4, 125.4, 127.0, 127.6, 128.8, 131.3, 132.0, 140.4, 140.7, 151.6.
Yield: 3.06 g (61%).
MS (DCI, NH3): m/z = 311 [MH+].
1H NMR (250 MHz, CDCl3): = 1.29 (s, 18 H), 3.67 (s, 4 H), 7.07
(d, 4H, 3J = 8.2 Hz), 7.31 (d, 4 H, 3J = 8.2 Hz).
Anal. Calcd for C24H22 (310.43): C, 92.86; H, 7.14. Found: C,
92.93; H, 7.17.
13C NMR (62.5 MHz, CDCl3): = 31.3, 34.4, 48.5, 125.5, 129.0,
130.9, 149.8, 209.0.
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1524
S. K. Sadhukhan et al.
PAPER
1-(4-Biphenyl)-2,3,4,5,6-penta(4-tert-butylphenyl)benzene (10)
A solution of the alkyne 9 (500 mg, 1.61 mmol) and of the tetraphe-
nylcyclopentadienone 10 (500 mg, 1.61 mmol) in diphenyl ether (5
g) was stirred 24 h at 225–240 °C under argon. After cooling, the
red solution was diluted with CH2Cl2 (10 mL) and poured into
MeOH (100 mL). The off-white product was then filtered, washed
with MeOH and vacuum dried. It can be further purified by slow
concentration of a solution in CH2Cl2–EtOH (1:1).
with continuous argon bubbling through the solution, concentrated
and poured into MeOH (40 mL), and filtered. The residue was puri-
fied by crystallisation from CHCl3 (20 mL) and MeOH (10 mL) to
get pure product.
Yield: 140 mg (71%).
1H NMR (250 MHz, CD2Cl2): = 1.79 (s, 18 H), 1.83 (s, 18 H),
1.85 (s, 9 H), 8.14 (t, 1 H, 3J = 7.7 Hz), 9.17 (d, 2 H, 3J = 7.7 Hz),
9.20–9.38 (m, 10 H).
13C NMR (62.5 MHz, CD2Cl2): = 32.4, 36.4, 119.8, 119.9, 122.4,
124.4, 127.2, 130.7, 130.9, 131.0, 131.2, 149.9.
Yield: 1.15 g (80%).
1H NMR (250 MHz, CDCl3): = 1.09 (s, 45 H), 6.64–6.74 (m, 10
3
H), 6.77–6.85 (m, 10 H), 6.89 (d, 2 H, J = 8.7 Hz), 7.10 (d, 2 H,
3J = 8.7 Hz), 7.24–7.42 (m, 5 H).
Anal. Calcd for C62H58 (742.02): C, 92.72; H, 7.28. Found: C 92.58;
H, 7.02.
MS (DCI, NH3): m/z = 803 [MH+].
13C NMR (62.5 MHz, CD2Cl2): = 31.7, 34.7, 123.9, 124.0, 125.6,
127.3, 127.6, 129.3, 131.7, 132.7, 138.7, 141.3, 148.5.
MS (DCI, NH3): m/z = 890 [M+], 908 [MNH4 ]
+
3,4-Bis(4-tert-butylphenyl)-2,5-diphenylcyclopenta-2,4-dien-1-
one (13)
Anal. Calcd for C68H74 (891.32): C, 91.63; H 8.37. Found: C, 91.25;
H, 8.15.
To a refluxing solution of dibenzylketone (652 mg, 3.10 mmol) and
5 (1.00 g, 3.10 mmol) in anhyd EtOH (5 mL) under argon was added
KOH in anhyd EtOH (2 M; 2 mL). After 1 h of reflux, the suspen-
sion was cooled and filtered. The precipitate was then washed with
cold EtOH giving 13.
2,5,8,11,14-Penta-tert-butyl-17-phenylhexabenzo-
[bc,ef,hi,kl,no,qr]coronene (2)
To a solution of 10 (100 mg, 0.11 mmol) in anhyd CH2Cl2 (50 mL)
was added a solution of anhyd FeCl3 (0.50 g, 3.08 mmol) in anhyd
nitromethane (10 mL). The red solution was then stirred 10 h at r.t.
with argon bubbling to remove HCl. The solution was concentrated
(15 mL) and then poured into MeOH (50 mL) and the resulting mix-
ture was filtered. The product was purified by crystallisation from
CHCl3 (10 mL) and MeOH (10 mL) to get pure product.
Yield: 1.54 g (87%).
1H NMR (250 MHz, CD2Cl2): = 1.27 (s, 18 H), 6.85 (d, 4 H,
3J = 8.5 Hz), 7.19 (d, 4 H, 3J = 8.5 Hz), 7.24–7.26 (m, 10 H).
13C NMR (62.5 MHz, CD2Cl2): = 31.7, 35.3, 125.4, 128.0, 128.6,
129.8, 130.9, 132.1, 152.6, 155.7, 201.2.
+
MS (DCI, NH3): m/z = 497 [MH+], 514 [MNH4 ].
Yield: 74 mg (75%).
1H NMR (250 MHz, CDCl3): = 1.81 (s, 18 H), 1.83 (s, 27 H), 7.60
(t, 1 H, 3J = 7.5 Hz), 7.75 (t, 2 H, 3J = 7.5 Hz), 8.17 (d, 2 H, 3J = 7.5
Hz), 9.3–9.37 (m, 12 H).
Anal. Calcd for C37H36O (496.68): C, 89.47; H, 7.31. Found: C,
90.01; H, 7.02.
13C NMR (62.5 MHz, CD2Cl2): = 32.0, 35.6, 118.7, 119.0, 120.2,
120.4, 123.6, 127.5, 128.2, 129.1, 129.9, 130.1, 130.2, 130.3, 130.8,
138.8, 148.6.
1,4-Diphenyl-2,3,5,6-tetra(4-tert-butylphenyl)benzene (14)
A solution of 13 (500 mg, 1.00 mmol) and 8 (293 mg, 1.00 mmol)
in diphenyl ether (5 g) was stirred for 24 h at 225–240 °C under ar-
gon. After cooling, the red solution was diluted with CH2Cl2 (20
mL) and poured into stirred MeOH (150 mL). The off-white prod-
uct was then filtered, washed with MeOH and vacuum dried.
MS (DCI, NH3): m/z = 879 [MH+].
Anal. Calcd for C68H62 (879.22): C, 92.72; H, 7.11. Found: C,
92.76; H, 6.93.
Yield: 50% (380 mg).
1H NMR (250 MHz, CD2Cl2): = 1.10 (s, 36 H), 6.69 (d, 8 H,
1-Phenyl-2,3,4,5,6-penta(4-tert-butylphenyl)benzene (12)
A solution of 1-tert-butyl-4-(phenylethynyl)benzene (192 mg, 0.82
mmol) and of the tetraphenyl cyclopentadienone 7 (500 mg, 0.82
mmol) in diphenyl ether (5 g) was stirred 24 h at 225–240 °C under
argon. After cooling, the red solution was diluted with CH2Cl2 (10
mL) and poured in stirred MeOH (100 mL). The off-white product
was then filtered, washed with MeOH and vacuum dried. It can be
further purified by slow concentration in a solution of CH2Cl2–
EtOH (1:1).
3J = 8.5 Hz), 6.82–6.86 (m, 18 H).
13C NMR (62.5 MHz, CD2Cl2): = 31.1, 34.0, 123.1, 124.7, 126.3,
131.0, 131.6, 137.7, 139.9, 140.4, 140.9, 147.4.
+
MS (DCI, NH3): m/z = 776 [MNH4 ].
Anal. Calcd for C58H62 (759.11): C, 91.77; H, 8.23. Found: C,
91.21; H, 8.16.
2,5,11,14-Tetra-tert-butylhexabenzo[bc,ef,hi,kl,no,qr]coronene
(4)
Yield: 80% (1.15 g).
1H NMR (250 MHz, CD2Cl2): = 1.10 (s, 45 H), 6.68–6.75 (m, 10
H), 6.83–6.87 (m, 15 H).
13C NMR (62.5 MHz, CDCl3): = 31.1, 334.0, 122.9, 123.1, 124.7,
126.3, 131.0, 131.69, 137.8, 139.76, 140.1, 140.4, 140.5, 140.9,
147.2, 147.4.
FeCl3 (200 mg, 1.20 mmol) in nitromethane (5 mL) was added to a
stirred solution of compound 14 (50 mg, 0.06 mmol) in anhyd
CH2Cl2 (50 mL), the reaction mixture was stirred for 6 h with con-
tinuous argon bubbling through the reaction mixture. Then the con-
tent was concentrated and poured into MeOH (20 mL), the
precipitated product was filtered. The residue was purified by boil-
ing in THF, cooling and the suspended material was centrifuged to
get the pure product.
+
MS (DCI, NH3): m/z = 832 [MNH4 ].
Anal. Calcd for C62H70 (815.22): C, 91.35; H, 8.65. Found: C,
90.81; H, 8.62.
Yield: 35 mg (71%).
1H NMR (250 MHz, CDCl3): = 1.23 (s, 36 H), 8.22 (t, 2 H,
2,5,8,11,14-Penta-tert-butylhexabenzo[bc,ef,hi,kl,no,qr]coro-
nene (3)
FeCl3 (800 mg, 4.90 mmol) in nitromethane (10 mL) was added to
a stirred solution of the compound (200 mg, 0.24 mmol) in anhyd
CH2Cl2 (100 mL).The reaction mixture was stirred at r.t. for 24 h
3J = 8.0 Hz), 9.20–9.27 (m, 12 H)
13C NMR not recorded due to the very low solubility of the product.
MS (DCI, NH3): m/z = 747 [MH+].
Synthesis 2003, No. 10, 1521–1525 © Thieme Stuttgart · New York

PAPER
Syntheses of Hexabenzocoronene Derivatives
1525
Anal. Calcd for C58H50 (747.02): C, 93.25; H, 6.75. Found: C,
93.10; H, 6.50.
(10) See for instance: (a) Biasutti, M. A.; Rommens, J.; Vaes, A.;
De Feyter, S.; De Schryver, F. D.; Herwig, P.; Müllen, K.
Bull. Soc. Chim. Belg. 1997, 106, 659. (b) Brand, J. D.;
Kubel, C.; Ito, S.; Müllen, K. Chem. Mat. 2000, 12, 1638.
(c) Brown, S. P.; Schnell, I.; Brand, J. D.; Müllen, K.; Spiess,
H. W. J. Am. Chem. Soc. 1999, 121, 6712. (d) Brown, S. P.;
Schnell, I.; Brand, J. D.; Müllen, K.; Spiess, H. W. PCCP
Phys. Chem. Chem. Phys. 2000, 2, 1735. (e) Fechtenkotter,
A.; Saalwachter, K.; Harbison, M. A.; Müllen, K.; Spiess, H.
W. Angew. Chem. Int. Ed. 1999, 38, 3039. (f) Fischbach, I.;
Pakula, T.; Minkin, P.; Fechtenkotter, A.; Müllen, K.;
Spiess, H. W.; Saalwachter, K. J. Phys. Chem. B 2002, 106,
6408.
Acknowledgment
This work was supported by the CNRS, the European Community
FET-NID project BUN (Bottom-Up Nanomachines) IST-1999-
11565.
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