Template-mediated synthesis of polycyclic aromatic hydrocarbons: Cyclodehydrogenation and planarization of a hexaphenylbenzene derivative at a copper surface

Article abstract of DOI:10.1002/(SICI)1521-3773(19991216)38:24<3748::AID-ANIE3748>3.0.CO;2-0

The deposition of polycyclic aromatic hydrocarbons on substrate surfaces is a key step for using such disklike molecules in nanoelectronic devices. An alternative to the (frequently problematic) preparation by evaporation or deposition from solution is to

Full text of DOI:10.1002/(SICI)1521-3773(19991216)38:24<3748::AID-ANIE3748>3.0.CO;2-0

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[
15] A. P. Spada, C. A. Fink, M. R. Myers (Rhone-Poulenc Rorer Inc.),
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Template-Mediated Synthesis of Polycyclic
Aromatic Hydrocarbons:
Cyclodehydrogenation and Planarization of a
Hexaphenylbenzene Derivative at a Copper
Surface**
Klaus Weiss, Gunda Beernink, Florian Dötz,
Alexander Birkner, Klaus Müllen, and Christof H. Wöll*
Scheme 1. Polycyclic aromatic hydrocarbons 1 ± 6.
Polycyclic aromatic hydrocarbons (PAHs) such as triphe-
nylene (1a), hexa-peri-hexabenzocoronene (HBC, 2a), and
its higher homologue 3 (Scheme 1) form two- and three-
dimensional superstructures that are attracting considerable
[1±4]
attention in the field of charge transfer processes.
The
deposition of monomolecular layers, a central step for using
such disklike molecules in nanoelectronic devices, can be
performed by evaporating PAHs in ultrahigh vacuum (UHV)
and, in case of alkyl-substituted soluble derivatives, also by
deposition from solution. However, both preparation proce-
[
*] Prof. Dr. C. H. Wöll, Dr. K. Weiss, G. Beernink, Dr. A. Birkner
Lehrstuhl für Physikalische Chemie I der Universität
Universitätsstrasse 150, D-44780 Bochum (Germany)
Fax: (‡49)234-3214182
E-mail: woell@pc.ruhr-uni-bochum.de
F. Dötz, Prof. Dr. K. Müllen
Max-Planck-Institut für Polymerforschung
Ackermannweg 10, D-55127 Mainz (Germany)
dures cannot be applied to larger PAHs which have recently
[5±8]
been synthesized.
We therefore present a method to
produce PAHs, for example, hexabenzocoronene, directly by
a thermally induced cyclodehydrogenation route starting
from a precursor molecule adsorbed on a Cu(111) surface.
This new synthetic route is based on the following concept:
[
**] This work has been funded by the Volkswagenstiftung, the TMR
project ªSISITOMASª of the European Community, and by the
Fonds der Chemischen Industrie. The authors thank J. Diedrich Brand
for fruitful discussions.
3
748
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Angew. Chem. Int. Ed. 1999, 38, No. 24

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First a precursor 4b of the target compound 5 is synthesized
and deposited in monolayers on a Cu surface. On heating this
adsorbate layer, 4b is completely dealkylated and converted
by cyclodehydrogenation into the planar product 5. The PAH
typical cyclization experiment the precursor 4a is dissolved in
dichloromethane, and a solution of FeCl in nitromethane is
3
added dropwise at room temperature. After the starting
material had disappeared (TLC) the mixture was precipitated
in methanol, and purified by column chromatography or
recrystallization. However, in the case of 4b this synthetic
protocol did not give the desired compound 2c but the
quinone 6 (Scheme 1) in 96% yield. Oxidative dealkylation
and dehydrogenation leads to an extended quinonoid system
which also incorporates the central aromatic ring. It is well
known that compounds such as 4b undergo reactions in the
presence of Lewis acids which involve both ether cleavage and
oxidation to quinonoid aromatic structures as well as for-
5
does not exist in solution, but is stabilized through the
interaction of the oxygen atoms with the Cu surface, which
also hinders its desorption.
We used near edge X-ray absorption fine structure (NEX-
AFS) spectroscopy for the characterization of molecules
adsorbed on metal surfaces and for the analysis of products
formed by chemical modifications. This method allows a
determination of the molecular orientation and provides
detailed information on the electronic structure of the
molecules. Scanning tunneling microscopy (STM) provides
additional information on the lateral order of the molecular
adsorbate.
mation of new carbon ± carbon bonds between adjacent
phenyl rings.[
11, 12]
Which of the two reactions occurs first
depends on the system.[
12]
The synthetic route to hexaether 4b is shown in Scheme 2.
Starting from commercially available 4-iodophenol (7), ether-
ification with 1-bromodecane affords 4-(dodecyloxy)iodoben-
zene (8), which is then coupled to (trimethylsilyl)acetylene
The thermogravimetric analysis of 4b shows that this
substance is stable up to temperatures of 490 K; in the
temperature range between 490 K and 570 K about 1% loss in
weight can be observed. Monolayers of 4b with a film
thickness of about 3 Š were deposited at a temperature of
Tˆ 540 K from a stainless steel Knudsen cell onto a clean,
structurally well-defined Cu(111) surface. The deposition rate
�
1
of 3 Šmin was controlled by a quartz crystal microbalance.
The samples were heated for 5 ± 10 min to different temper-
atures and subsequently investigated by NEXAFS spectro-
scopy. For each temperature the layer thickness was deter-
mined by relating the height of the edge jump to the signal
intensity at 280 eV.[ The results reveal that the amount of
molecules present on the surface is essentially unchanged.
Figure 1 shows a series of NEXAFS spectra recorded at the
C(1s) absorption edge at normal and grazing photon inci-
13]
Scheme 2. Synthesis of the precursor molecule 4b. R ˆ OC12
25
H .
(
TMSA) by using the method developed by Hagihara,
[
9]
Sonogashira et al. Deprotection of the trimethylsilyl group
with KF in DMF afforded the acetylene 9, which, after
renewed Hagihara ± Sonogashira coupling with 8 was con-
verted into 4,4'-di(dodecyloxy)diphenylacetylene (10). The
cyclotrimerization of 10 with [Co (CO) ] in dioxane finally
Figure 1. C(1s) NEXAFS spectra of 4b monolayers adsorbed on Cu(111)
and heated for 5 ± 10 min to different temperatures. The spectra were
recorded for normal and grazing photon incidence, where the angle
between the electric field vector of the linearly polarized synchrotron light
and the surface normal is 908 and 308, respectively. The spectra are
normalized to the absorption step at 325 eV and focus on the NEXAFS
spectral features in close proximity to the absorption edge which is located
at around 291 eV. The bottom spectrum was recorded for 4b deposited
from solution.
2
8
[
10]
gives the precursor 4b. The key step on the path to the PAH
c is the oxidative cyclodehydrogenation of the oligophenyl-
ene precursors. Optimization of the appropriate cyclization
2
conditions finally proved FeCl to be superior to all other
Lewis acids on the way to hexaalkyl-substituted HBCs. In a
3
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dence. The spectra recorded for 4b heated to 500 K reveal a
well-defined resonance at 285.1 eV, which is assigned to an
excitation of C(1s) core level electrons into the lowest
unoccupied p* orbital of the phenyl subunits. Following
previous studies on linear saturated hydrocarbons[
14]
an
additional weak resonance R at 287.4 is attributed to
excitations of C(1s) electrons into Rydberg(R) orbitals of
the alkyl chains. The comparison with spectra recorded for 4b
deposited from solution shows that most of the alkyl chains
were removed during evaporation. For the sample heated to
temperatures above 500 K the spectra of 4b recorded at
normal and grazing incidence are virtually identical. This
absence of dichroism is consistent with a wide distribution of
phenyl orientation angles as expected from the nonplanar
structure of the molecules. In contrast, spectra recorded for
the surface heated to higher temperatures reveal an increasing
amount of dichroism, which provides direct evidence for a
[
15]
thermally induced planarization of the molecule 4b. Since
such a planarization is ± because of steric constraints ± not
possible for the unmodified molecule, this observation
provides strong evidence for the onset of dehydrogenation.
This effective planarization is completed after heating the
sample to 710 K, at which temperature essentially all mole-
cules are oriented coplanar with the surface.
A more detailed investigation of the NEXAFS spectra
reveals that the occurrence of the dichroism is accompanied
by two thermally induced effects. The significant broadening
of the phenylene p*-resonance is attributed to the interaction
of the phenyl groups with the Cu(111) surface and indicates a
direct interaction of all ring systems with the surface. This is a
general feature in NEXAFS spectra of p-electron systems in
direct contact with metal surfaces and has previously been
Figure 2. STM images (300 Š  300 Š) of 4b submonolayers adsorbed on
Cu(111) and heated for 10 ± 15 min to different temperatures. Right: Line
profiles which were taken along the white lines indicated in the images. All
images were recorded in the constant current mode. No filtering
procedures were used. The images (a) ± (c) were recorded with a tip bias
voltage of � 0.3 V and a tunneling current of 0.03 nA. Image (d) was
recorded with a tip bias voltage of � 1.0 Vand a tunneling current of 0.5 nA.
[
16, 17]
seen for benzene adsorbed on Cu substrates.
The
620 K) to 1.5 Š (after heating to 670 K) and then to 1.2 Š
(after heating to 720 K).
The results of our NEXAFS and STM studies provide very
strong evidence for the reaction scenario depicted in Figure 3.
disappearance of the resonance at 287.4 eV at temperatures
above 570 K supports the complete dealkylation of the
hexaether on the Cu surface. This finding is supported by
the results of a detailed X-ray photoelectron spectroscopy
(XPS) study, where in addition ± due to the formation of a
O�Cu bond ± a significant O(1s) binding energy shift has been
[
18]
observed.
To obtain independent information on the products of this
temperature-induced reaction we have carried out STM
measurements on submonolayers of 4b using a UHV-based
instrument. No stable imaging was observed directly after
deposition of the molecule. This behavior is expected since
the as-deposited molecules are bound to the surface only
through weak van der Waals forces and the molecules should
thus be very mobile. Reproducible STM results were only
obtained after heating the samples above 500 K; typical STM
topographs are presented in Figure 2. After the samples had
been heated to 570 K, the STM data reveal the formation of
well-ordered islands of adsorbed molecules. The diameter of
the spherical objects within these islands (bright areas)
amounts to 17 Æ 1 Š, which within the error bars agrees with
the diameter of a single hexaphenylbenzene unit. At this stage
the height of the islands amounts to 2.1 Š (see Figure 2).
When the samples were heated further, the STM images
reveal a substantial reduction in contrast and a significant
decrease of the island heights from 1.8 Š (after heating to
Figure 3. Schematic representation of the stepwise planarization of the
hexaphenylbenzene unit of 4b by thermally induced cyclodehydrogena-
tion. The molecule is anchored through the oxygen atoms to the Cu(111)
surface. The formation of hexabenzocoronene unit 5 (shown at the bottom,
right) is completed at 710 K.
3
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Heating 4b to a temperature of 470 K leads to sublimation
accompanied by substantial dealkylation, and a physisorbed
film is deposited on the substrate at room temperature.
Heating the substrate to temperatures above 570 K leads to
complete dealkylation followed by the formation of an O�Cu
4-Dodecyloxy(trimethylsilylethynyl)benzene:
zene (8) (70.02 g, 180.31 mmol), triphenylphosphane (2.36 g, 9.03 mmol),
copper(i) iodide (1.71 g, 8.98 mmol), and trans-[Pd(PPh
4-(Dodecyloxy)iodoben-
3 2 2
) Cl ] (1.90 g,
2
.71 mmol) were dissolved in piperidine (700 mL) under an argon
atmosphere. TMSA (19.51 g, 197.92 mmol) was added dropwise, and the
mixture stirred for 6 h and then cooled to room temperature. The mixture
was then washed with aqueous ammonium chloride, extracted with
dichloromethane, and the combined extracts were washed again with
aqueous ammonium chloride and water, and dried over MgSO . After
4
removal of the solvent in vacuo, the crude product was purified by column
chromatography (petroleum ether/ethyl acetate) to give a colorless oil
bond. A very similar reaction mechanism has been found in a
recent study on the temperature-induced reaction of hexabu-
[
19]
tyloxytriphenylene with a Cu(111) surface. In this case the
cleavage of the alkyl�O bond and the formation of a O�Cu
1
3
bond was also found to occur at 570 K. At this temperature
the height of the islands is 2.1 Š, which is consistent with the
presence of the unmodified hexaphenylbenzene unit. The
NEXAFS data demonstrate that the structure of the phenyl
rings is unmodified at this temperature.
(58.76 g, 92%). H NMR (300 MHz, CDCl
3
): d ˆ 7.43 (d, J(H,H) ˆ 8.5 Hz,
3
3
2
6
0
H; 3-H, 5-H), 6.82 (d, J(H,H) ˆ 8.5 Hz, 2H; 2-H, 6-H), 3.94 (t, J(H,H) ˆ
.4 Hz, 2H; OCH
2
), 1.80 (m, 2H, OCH
2
CH
2
), 1.33 ± 1.21 (m, 18H; Halkyl),
); 13C NMR
.96 (t, 3H, 3J(H,H) ˆ 6.3 Hz; CH
), 0.23 (s, 9H; SiCH
3
3
(
50 MHz, CDCl
3
): d ˆ 159.90 (C-1), 133.84, 115.69 (C-2), 114.84, 105.97
(Ar�CꢀC), 92.60 (Ar�CꢀC), 68.47 (OCH
), 32.48, 30.20, 30.15, 29.93,
), 0.61 (SiCH ); MS (FD, 8 kV):
38OSi ˆ 358.6).
2
2
9.75, 26.56, 23.23, (all Calkyl), 14.62 (CH
3
3
Although the lateral mobility of the molecules is expected
to decrease significantly as a result of the formation of
covalent O�Cu bonds, the high degree of lateral order seen in
‡
m/z (%): 358.2 (100) [M ] (calcd for C23
H
9: 4-Dodecyloxy(trimethylsilylethynyl)benzene (37.83 g, 105.48 mmol) was
dissolved in DMF (450 mL), and a solution of potassium fluoride (18.61 g,
the STM topographs demonstrates the presence of a mobility
which is sufficiently high to allow the formation of well-
ordered islands. The NEXAFS data (shape and polarization
dependence of the p*-resonance) as well as the STM
measurements (height of the islands) reveal that the dehy-
drogenation/planarization process is completed at 710 ±
3
21 mmol) in water (65 mL) was added. The solution was stirred for 3 h,
diluted with water, and extracted with toluene. The combined extracts were
washed with water, dried over MgSO , and the solvent evaporated in
4
vacuo. Column chromatography (petroleum ether/ethyl acetate) afforded 9
1
as a colorless oil (30.06 g, 98%). H NMR (200 MHz, CDCl
3
): d ˆ 7.46 (d,
3
3
J(H,H) ˆ 9.0 Hz, 2H; 3-H, 5-H), 6.85 (d, J(H,H) ˆ 9.0 Hz, 2H; 2-H, 6-H),
4
.95 (t, 3J(H,H) ˆ 7.4 Hz, 2H; OCH
), 3.01 (s, 1H; ꢀCH), 1.81 (m, 2H;
2
720 K. The substrate is covered with hexabenzocoronene
), 1.34 ± 1.21 (m, 18H; Halkyl), 0.98 (t, 3H, 3J(H,H) ˆ 7.1 Hz;
2 2
OCH CH
units, which are bound (as shown in 5) through oxygen atoms
to the surface. The importance of graphite segments and
molecularly defined graphitic structures suggests the transfer
of this novel strategy, namely the surface-induced PAH
CH ); 13C NMR (50 MHz, CDCl ): d ˆ 160.09 (C-1), 134.01, 114.95 (C-4),
3
3
114.66, 84.30 (Ar
�
CꢀC), 76.21 (Ar�CꢀC), 68.51 (OCH ), 32.55, 30.28,
2
3
2
0.02, 29.81, 26.63, 23.29 (all Calkyl), 14.65 (CH
3
); MS (FD, 8 kV): m/z (%):
‡
86.1 (100) [M ] (calcd for C20
H
30O ˆ 286.5).
[
6, 8]
10: 4-(Dodecyloxy)iodobenzene (8) (33.55 g, 86.40 mmol), copper(i) iodide
1.64 g, 8.63 mmol), and [Pd(PPh ] (1.36 g, 1.72 mmol) were dissolved in
synthesis, to considerably larger oligophenylenes.
(
3 4
)
piperidine (350 mL) under an argon atmosphere. 4-(Dodecyloxy)ethynyl-
benzene 9 was also dissolved in piperidine (100 mL), added, and the
mixture was stirred for 3 h. The resulting mixture was diluted with water
Experimental Section
(
2 4
300 mL), extracted with dichloromethane, and dried over Na SO . The
The NEXAFS measurements were recorded at the synchrotron radiation
organic solvent was removed in vacuo and the residue was purified by
chromatography with petroleum ether to yield a colorless solid (46.12 g,
9
8
6
[20]
facility BESSY I (Berlin) using beamline HE-TGM2
and a multi-
�
10
chamber UHV system operated in the low 10 mbar pressure range. The
spectra were recorded in partial electron yield (PEY) detection mode by
using a channeltron with a retarding voltage of � 150 V. After calibrating
the photon energy scale with respect to the NEXAFS data for highly
oriented pyrolytic graphite and correcting the spectra for the monochro-
mator transmission function by division through the spectrum of the clean
7%,). M.p. 978C; 1H NMR (300 MHz, CDCl
3
3
): d ˆ 7.42 (d, J(H,H) ˆ
3
.4 Hz, 4H; 2-H, 2'-H, 6-H, 6'-H), 6.85 (d, J(H,H) ˆ 8.4 Hz, 4H; 3-H, 3'-H,
3
-H, 6'-H), 4.98 (t, J(H,H) ˆ 6.2 Hz, 4H; OCH
2
13
), 1.80 (m, 4H; OCH
2
CH
2
),
):
1
3
.60 ± 1.20 (m, 36H; Halkyl), 0.92 (m, 6H; CH ); C NMR (75 MHz, CDCl
3
d ˆ 159.09 (C-4, C-4'), 133.03, 116.08 (C-1, C-1'), 115.02, 84.00 (CꢀC), 68.58
(
(
OCH
2
), 32.41, 30.13, 29.88, 29.73, 26.52, 23.17 (all Calkyl), 14.57 (CH
3
); MS
[
21]
Cu(111) surface, the spectra were normalized to the edge jump at 325 eV.
The STM data were recorded at room temperature with a UHV Omicron
‡
FD, 8 kV): m/z (%): 547.1 (100) [M ] (calcd for C38
H
58
O
2
ˆ 546.9).
�
11
Micro STM (base pressure lower than 1 Â 10 mbar) in the constant
current mode with tip bias voltages of � 0.3 ± 1.0 V and tunneling currents
4b: 4,4'-(Dodecyloxy)diphenylacetylene (10) (30.50 g, 54.95 mmol) and
[Co
(600 mL) under an argon atmosphere and refluxed for 4 h. Solvent was
removed in vacuo and the brown residue purified by column chromatog-
�
3
2
(CO)
8
] (0.51 mg, 1.49 Â 10 mmol) were dissolved in 1,4-dioxane
�
1
of 0.03 ± 0.5 nA. Imaging was performed with a scanning speed of 900 Šs
.
‡
In both UHV systems the Cu(111) surface was cleaned by extensive Ar
sputtering (ion energy 1 keV) and annealing cycles prior to the experi-
ments. Measurements using thermogravimetry were carried out using a
Mettler TG 50 thermobalance.
raphy (petroleum ether/dichloromethane) to give a yellowish solid (26.72 g,
89%). M.p. 528C; 1H NMR (250 MHz, CD
Cl
): d ˆ 6.68 (d, J(H,H) ˆ
3
2
2
3
8.5 Hz, 12H; H meta to O), 6.40 (d, J(H,H) ˆ 8.5 Hz, 12H; H ortho to O),
3
4
1
.74 (t, J(H,H) ˆ 6.6 Hz, 12H; OCH
2
), 1.64 (m, 12H; OCH
2
CH
2
), 1.45 ±
):
8
: 4-Iodophenol (7) (100.01 g, 454.59 mmol), 1-bromododecane (226.82 g,
1
3
.15 (m, 108H; Halkyl), 0.87 (m, 18H; CH
3
); C NMR (75 MHz, CDCl
3
910.08 mmol), and potassium carbonate (125.72 g, 911.01 mmol) were
d ˆ 156.80 (Carene�O), 140.75, 134.05, 132.95, 113.38 (all Carene), 68.24
dissolved in dimethylformamide (DMF) (500 mL) and refluxed for 3 h. The
solution was then diluted with water, hydrochloric acid (2n), and extracted
with dichloromethane. The combined extracts were washed with aequeous
(
(
OCH
2
), 32.45, 30.09, 29.96, 29.82, 26.53, 23.16 (all Calkyl), 14.57 (CH
3
); MS
‡
FD, 8 kV): m/z (%): 1639.5(100) [M ] (calcd for C114
H
174
O
6
ˆ 1639.3).
sodium bicarbonate, sodium chloride, and dried (MgSO
the solvent in vacuo and column chromatography (petroleum ether/
dichloromethane) of the crude product yielded a colorless solid (149.56 g,
4
). Evaporation of
6: Hexaphenylbenzene (4b) (1.01 g, 0.62 mmol) was dissolved in dichloro-
methane (150 mL) in a Schlenk flask under an argon atmosphere.
Throughout the whole reaction a constant stream of argon was bubbled
through the mixture to remove HCl formed in situ. A solution of FeCl
3
(1.80 g, 11.10 mmol) in nitromethane was added dropwise and the mixture
stirred for 30 min at room temperatue. Methanol (100 mL) was added and a
5%,). M.p. 378C; 1H NMR (200 MHz, CDCl
): d ˆ 7.56 (d, J(H,H) ˆ
3
8
3
3
8
.0 Hz, 2H; 3-H, 5-H), 6.68 (d, J(H,H) ˆ 8.0 Hz, 2H; H-2, 6-H), 3.39 (t,
3
J(H,H) ˆ 7.0 Hz, 2H; OCH
2
), 1.90 ± 1.70 (m, 2H; OCH
2
CH
2
), 1.60 ± 1.10
3
13
(
m, 18H; Halkyl), 0.91 (t, J(H,H) ˆ 7.9 Hz, 3H; CH
3
); C NMR (50 MHz,
beige solid precipitated which was filtered and dried in vacuo (768 mg,
1
CDCl
3
): d ˆ 159.56 (C-1), 138.64, 117.47 (Carene), 82.88 (C-4), 68.65 (OCH
2
),
96%). M.p. 1788C; H NMR (500 MHz, [D
6
]DMSO, 393 K): d ˆ 7.32 (d,
3
3
32.43, 30.15, 30.09, 29.87, 29.68, 26.51, 23.20 (all Calkyl), 14.61 (CH
3
); MS
J(H,H) ˆ 8.4 Hz, 4H; Harene), 7.04 (d, J(H,H) ˆ 8.4 Hz, 4H; Harene), 6.75
(dd, J(H,H) ˆ 4.4 Hz, J(H,H,) ˆ 1.0 Hz, 2H; ortho-Hquinone), 6.70 (d,
‡
3
4
(
FD, 8 kV): m/z (%): 388.3 (100) [M ] (calcd for C18
H
29OI ˆ 388.3).
Angew. Chem. Int. Ed. 1999, 38, No. 24
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
1433-7851/99/3824-3751 $ 17.50+.50/0
3751

COMMUNICATIONS
3
4
J(H,H) ˆ 9.8 Hz, 4H; Harene), 6.60 (d, J(H,H) ˆ 1.0 Hz, 2H; ortho-Hquinone
Fluorescence Detection from Single
Dendrimers with Multiple Chromophores**
3
3
)
4
, 6.37 (d, J(H,H) ˆ 4.4 Hz, 2H; meta-Hquinone), 6.14 (d, J(H,H) ˆ 9.8 Hz,
H; Harene), 1.91(m, 4H; OCH
CH CH ), 1.58 ± 1.32 (m, 68H; Halkyl), 1.04 ± 0.94 (m, 12H; CH
C NMR (125 MHz, [D
2
), 1.75 (m, 4H; OCH
2
CH
2
), 1.62 (m, 4H;
OCH
2
2
2
3
);
1
3
Thomas Gensch, Johan Hofkens, Andreas Heirmann,
Kenji Tsuda, Wendy Verheijen, Tom Vosch,
Thomas Christ, Thomas Basch e , Klaus Müllen,* and
Frans C. De Schryver*
]THF): d ˆ 185.98 (Cquinone), 161.75 (Carene�O),
8
1
5
(
(
61.36 (Carene�O), 147.32, 145.05, 140.90, 137.00, 135.94 130.60 (all Carene),
8.57 (OCH
2
), 34.08, 31.82, 31.45, 28.25, 24.75 (all Calkyl), 15.63 (CH
3
); MS
‡
FD, 8 kV): m/z (%): 1300.4 (100) [M ] (calcd for C90
H
122
O
6
ˆ 1300.0); IR
�
1
KBr pellet): nÄ [cm ] ˆ3062(w), 3041(w), 2918(s), 2847(s), 1663 [(CO)st-
retch], 1603(m), 1495(m) [n(C
8
ˆ
C)
arene
], 1244(s) [(Ar�O�R)], 1170(m),
� 1 � 1
Following the pioneering studies of Moerner and Kador as
well as Orrit and Bernard[ a wealth of experiments have been
performed using high-resolution single-molecule spectrosco-
py (SMS) at low temperature. Single molecules that are
immobilized on and in thin polymer films and gels, as well as
25(m); UV/Vis (CHCl
3
): l[nm] (lge [L mol cm ]): 314 (sh, 4.52), 330
1]
(
4.69), 347 (4.74).
Received: June 1, 1999 [Z13492IE]
German version: Angew. Chem. 1999, 111, 3974 ± 3978
[2]
on glass, have been investigated at room temperature with
Keywords: cyclodehydrogenations ´ graphitic segments ´
scanning tunneling microscopy ´ template synthesis ´ X-ray
absorption spectroscopy
various types of optical microscopies.[
3, 4]
The spectroscopic
study of single molecules in solution has attracted substantial
attention and is currently being developed as a new tool in
analytical chemistry.[
5]
In the majority of the SMS studies the single molecules are
investigated through their fluorescence properties. One of the
most prominent observations in SMS studies is the occurrence
of sudden changes in fluorescence intensity, often called the
on ± off behavior of a single molecule. This is in contrast to
bulk measurements where the fluorescence intensity decreas-
es exponentially with irradiation time as a result of photo-
bleaching. Herein we investigate the difference between SMS
of a single chromophore and of multichromophoric systems
with the emphasis on the passage from the typical on ± off
behavior of a single molecule to the behavior of the ensemble.
To this end a system has to be synthesized in which chromo-
phores can be placed at a specific distance without being coupled
through bonds. One way to obtain multiple chromophores in a
well arranged environment is based on dendrimer synthesis.
We have recently introduced a new type of dendrimers and
hyperbranched 3-dimensional polyphenylenes consisting of
penta- or hexaphenylbenzene building blocks with strongly
twisted benzene units.[ The polyphenylene dendrimers are
synthesized by repetitive Diels ± Alder reactions using ethynyl-
substituted aromatic cores such as 3,3',5,5'-tetraethynylbi-
phenyl (1) and the cyclopentadienone derivative 2 as the
[
[
[
[
1] P. Herwig, C. W. Kayser, K. Müllen, H. W. Spiess, Adv. Mater. 1996, 8,
510.
2] A. M. van de Craats, J. M. Warman, K. Müllen, Y. Geerts, J. D. Brand,
Adv. Mater. 1998, 10, 36.
3] A. Stabel, P. Herwig, K. Müllen, J. P. Rabe, Angew. Chem. 1995, 107,
1768; Angew. Chem. Int. Ed. Engl. 1995, 34, 1609.
4] D. Adam, P. Schumacher, J. Simmerer, L. Häussling, K. Siemensmey-
er, K. H. Etzbach, H. Ringsdorf, D. Haarer, Nature 1994, 371, 141; J.
Simmerer, B. Glüsen, W. Paulus, A. Kettner, P. Schumacher, D. Adam,
K. H. Etzbach, K. Siemensmeyer, J. H. Wendorff, H. Ringsdorf, D.
Haarer, Adv. Mater. 1996, 8, 815.
[
5] M. Müller, J. Petersen, R. Strohmaier, C. Günther, N. Karl, K. Müllen,
Angew. Chem. 1996, 108, 947; Angew. Chem. Int. Ed. Engl. 1996, 35,
886.
[
6] V. S. Iyer, M. Wehmeier, J. D. Brand, M. A. Keegstra, K. Müllen,
Angew. Chem. 1997, 109, 1675; Angew. Chem. Int. Ed. Engl. 1997, 36,
1604.
[
[
7] M. Müller, C. Kübel, K. Müllen, Chem. Eur. J. 1998, 4, 2099.
8] V. S. Iyer, K. Yoshimura, V. Enkelmann, R. Epsch, J. P. Rabe, K.
Müllen, Angew. Chem. 1998, 110, 2843; Angew. Chem. Int. Ed. 1998,
3
7, 2696.
9] S. Takahashi, Y. Koroyama, K. Sonogashira, N. Hagihara, Synthesis,
980, 627.
[
6, 7]
1
[
[
[
[
10] K. P. C. Vollhardt, Acc. Chem. Res. 1970, 10, 1.
11] P. Henderson, H. Ringsdorf, P. Schuhmacher, Liq. Cryst. 1995, 18, 191.
12] O. C. Musgrave, Chem. Rev. 1969, 69, 499.
13] The pre-edge signal at a photon energy of 280 eV is dominated by Cu
photoelectrons. At this energy the carbon contributions (C(2s), C(2p)
photoelectrons) amount to less than 5% of the edge jump observed at
branching (A B-type) reagent. The stepwise approach to
2
3
25 eV. Since the edge jump is proportional to the absolute amount of
[
*] Prof. Dr. K. Müllen, A. Heirmann
Max-Planck-Institut für Polymerforschung
Ackermannweg 10, D-55128 Mainz (Germany)
Fax: (‡49)6131-379350
carbon adsorbed on the surface, the ratio beween the edge jump and
the pre-edge signal can be used to estimate the overall coverage.
14] P. S. Bagus, K. Weiss, A. Schertel, C. Wöll, W. Braun, C. Hellwig, C.
Jung, Chem. Phys. Lett. 1996, 248, 129.
15] The transition dipole moment T of the p*-resonance is perpendicular
to the phenyl ring plane. Since the intensity of the p*-resonance is
[
[
E-mail: muellen@mpip-mainz.mpg.de
Prof. Dr. F. C. De Schryver, T. Gensch, J. Hofkens, K. Tsuda,
W. Verheijen, T. Vosch
Department of Chemistry
2
given by Ip* /j< f j TE j i >j with the electric field vector of the linear
polarized synchrotron radiation E, the p*-resonance vanishes under
normal incidence (908), when all phenyl rings are oriented parallel to
the surface.
Katholieke Universiteit Leuven
Celestijnenlaan 200F, B-3001 Heverlee (Belgium)
Fax: (‡32)16-327989
[
[
[
16] K. Weiss, S. Gebert, M. Wühn, H. Wadepohl, C. Wöll, J. Vac. Sci.
E-mail: frans.deschryver@chem.kuleuven.ac.be
Technol. A 1998, 16, 1017.
T. Christ, T. BascheÂ
Fakultät für Chemie der Universität
Welderweg 11, D-55099 (Germany)
17] M. Weinelt, N. Wassdahl, T. Wiell, O. Karis, J. Hasselström, P.
Bennich, A. Nilsson, Phys. Rev. B 1998, 58, 7351.
18] G. Beernink, K. Weiss, F. Dötz, A. Birkner, K. Müllen, C. Wöll,
unpublished results.
19] H. Wegner, K. Weiss, Ch. Wöll, Surf. Rev. Lett. 1999, 6, 183.
20] S. Bernstorff, W. Braun, M. Mast, W. Peatman, T. Schroeter, Rev. Sci.
Instrum. 1989, 60, 2097.
[**] The authors gratefully acknowledge the FWO, the Flemish Ministry of
Education for the support (GOA/1/96), the EC (TMR Sisitomas and
TMR Marie Curie), the VW Stiftung and the support of DWTC
(Belgium; IUAP-IV-11). J.H. thanks the FWO for a post-doctoral
fellowship.
[
[
[
21] J. Stöhr, NEXAFS Spectroscopy, Springer, Berlin, 1992.
3
752
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
1433-7851/99/3824-3752 $ 17.50+.50/0
Angew. Chem. Int. Ed. 1999, 38, No. 24