Large all-hydrocarbon spoked wheels of high symmetry: Modular synthesis, photophysical properties, and surface assembly

Article abstract of DOI:10.1021/ja909229y

In a convergent modular synthesis, a very efficient pathway to shape-persistent molecular spoked wheels has been developed and applied according to the covalent-template concept. The structurally defined two-dimensional (2D) oligo(phenylene-ethynylene-butadiynylene)s (OPEBs) presented here are about 8 nm sized hydrocarbons of high symmetry. 48 alkyl chains attached to the molecular plane (hexyl and hexadecyl, respectively) guarantee a high solubility of the compounds. The structure and uniformity of these defined, stable, D_6h symmetrical compounds is verified by MALDIMS, GPC analysis, and high-temperature (HT) ¹H and ¹³C NMR. Detailed photophysical measurements of nonaggregated molecules in solution (as confirmed by dynamic light scattering (DLS)) focus on the identification of chromophores by comparison with suitable model compounds. Moreover, time-resolved measurements including fluorescence lifetime and depolarization support the chromophore assignment and reveal the occurrence of intramolecular energy transfer. Scanning tunneling microscope (STM) characterization at the solid/liquid interface demonstrates the efficient self-assembly of the OPEBs into hexagonal 2D crystalline layers with a periodicity determined by both the size of the OPEB backbone and the length of peripheral side chains. Atomic force microscope (AFM) studies show a very different assembly behavior of the two spoked wheel molecules, on both graphite and mica. While the hexylsubstituted wheel can form stacked superstructures, hexadecyl groups prevent any ordering in the film aside from the monolayer directly in contact with the surface.

Full text of DOI:10.1021/ja909229y

Published on Web 01/06/2010
Large All-Hydrocarbon Spoked Wheels of High Symmetry:
Modular Synthesis, Photophysical Properties, and Surface
Assembly
Dennis Mo¨ssinger,†,‡ Debangshu Chaudhuri,‡ Tibor Kudernac,§ Shengbin Lei,§
Steven De Feyter,*,§ John M. Lupton,*,‡ and Sigurd Ho¨ger*,†
Kekule´-Institut fu¨r Organische Chemie und Biochemie, Rheinische
Friedrich-Wilhelms-UniVersita¨t Bonn, Gerhard-Domagk-Strasse 1, 53121 Bonn, Germany,
Department of Physics and Astronomy, UniVersity of Utah, Salt Lake City, Utah 84112, and
Department of Chemistry, Laboratory of Photochemistry and Spectroscopy, and Institute for
Nanoscale Physics and Chemistry, Katholieke UniVersiteit LeuVen, Celestijnenlaan 200-F,
3001 LeuVen, Belgium
Received October 30, 2009; E-mail: hoeger@uni-bonn.de; lupton@physics.utah.edu;
steven.defeyter@chem.kuleuven.be
Abstract: In a convergent modular synthesis, a very efficient pathway to shape-persistent molecular
spoked wheels has been developed and applied according to the covalent-template concept. The
structurally defined two-dimensional (2D) oligo(phenylene-ethynylene-butadiynylene)s (OPEBs) pre-
sented here are about 8 nm sized hydrocarbons of high symmetry. 48 alkyl chains attached to the
molecular plane (hexyl and hexadecyl, respectively) guarantee a high solubility of the compounds.
The structure and uniformity of these defined, stable, D6h symmetrical compounds is verified by MALDI-
MS, GPC analysis, and high-temperature (HT) 1H and 13C NMR. Detailed photophysical measurements
of nonaggregated molecules in solution (as confirmed by dynamic light scattering (DLS)) focus on the
identification of chromophores by comparison with suitable model compounds. Moreover, time-resolved
measurements including fluorescence lifetime and depolarization support the chromophore assignment
and reveal the occurrence of intramolecular energy transfer. Scanning tunneling microscope (STM)
characterization at the solid/liquid interface demonstrates the efficient self-assembly of the OPEBs
into hexagonal 2D crystalline layers with a periodicity determined by both the size of the OPEB backbone
and the length of peripheral side chains. Atomic force microscope (AFM) studies show a very different
assembly behavior of the two spoked wheel molecules, on both graphite and mica. While the hexyl-
substituted wheel can form stacked superstructures, hexadecyl groups prevent any ordering in the
film aside from the monolayer directly in contact with the surface.
ing photophysical features.11-15 Moreover, shape-persistent
Introduction
organic structures are expected to show reinforcement effects
in nanocomposite materials as affirmed recently by the suc-
cessful use of carbon nanotubes (CNTs) as rigid components
in polymer composites.16,17
Large two-dimensional (2D) shape-persistent, carbon-rich
molecules of defined constitution, size, and shape have gained
increasing importance in material science. 2D nanostructures
fulfilling these criteria exhibit interesting properties such as 2D
crystallinity on surfaces,1-7 liquid crystallinity,8-10 and intrigu-
(9) Ho¨ger, S.; Cheng, X. H.; Ramminger, A.-D.; Enkelmann, V.; Rapp,
A.; Mondeshki, M.; Schnell, I. Angew. Chem., Int. Ed. 2005, 44, 2801–
2805.
† Rheinische Friedrich-Wilhelms-Universita¨t Bonn.
‡ University of Utah.
§ Katholieke Universiteit Leuven.
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R. H.; MacKenzie, J. D. Science 2001, 293, 1119–1122.
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D. F.; Lindsey, J. S.; Holten, D. J. Phys. Chem. B 2006, 110, 19131–
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1410 J. AM. CHEM. SOC. 2010, 132, 1410–1423
10.1021/ja909229y  2010 American Chemical Society

Large All-Hydrocarbon Spoked Wheels of High Symmetry
A R T I C L E S
Scheme 1. Star-Shaped Dodeca-acetylene Precursor 1 and Proof-of-Principle Spoked Wheel 2a
a Estimation of the molecular diameter performed with Wavefunction Spartan ’08.
Recently, we described all-covalent molecular spoked wheels
as large defined organic compounds beyond the 5 nm range.18
In contrast to carbon-rich scaffolds such as graphene, graphyne,
graphdiyne, and the respective cutouts,19 these molecules do
not rely on a specific idealized structure. The pursuit of these
structures is on the one hand driven by scientific curiosity:
demonstration of the synthetic feasibility of such large com-
pounds along with the control over solubility. On the other hand,
we aim at creating rigid organo-based materials for potential
composite applications.
Just as shape-persistent macrocycles, spoked wheel structures
allow the introduction of functionality at the perimeter as well
as above and below the molecular plane. Although shape-
persistent macrocycles tend to pack in flat (2D) arrangements
on surfaces, single crystal structures9,20,21 and molecular dynam-
ics (MD) simulations22,23 reveal considerable deviations from
the planar conformation. By contrast, molecular spoked wheels
have been designed as reinforced (strutted) macrocycles and
combine favorable properties both from large disk-shaped
polycyclic aromatic hydrocarbons (PAHs) (with shape-persis-
tence, strict planarity, and a high degree of symmetry) and from
conventional macrocycles (with solubility, ease of characteriza-
tion, and the potential to introduce functional groups in the
molecular plane and at the periphery). Columnar stacking, as
observed for PAHs, should be successfully suppressed due to
solubilizing chains orthogonal to the molecular plane. Beside
some known covalent approaches,24,25 most spoked wheel
structures published so far were supramolecular complexes
consisting of macrocyclic rims and multidentate, star-shaped
interior scaffolds.26-30 In most cases, these scaffolds have been
utilizedtofacilitatecyclizationorstudythecomplexformation,30,31
in part along with light-harvesting behavior.13,28,32
We demonstrated that large, strutted macrocycles are acces-
sible in a covalent template approach: in a convergent synthesis,
six preformed spoke/rim modules are coupled to a hexaiodo
hub affording a star-shaped precursor molecule with 12 terminal
acetylene units (1, Scheme 1). In an intramolecular reaction,
adjacent acetylenes dimerize and form six defined butadiyne
bridges. This means that the precursor contains its own
covalently linked template, which remains bound inside the
molecular spoked wheel (Scheme 1). The proof-of-principle
(24) Marsden, J. A.; O’Connor, M. J.; Haley, M. M. Org. Lett. 2004, 6,
2385–2388.
(17) Beigbeder, A.; Linares, M.; Devalckenaere, M.; Dege´e, P.; Claes, M.;
Beljonne, D.; Lazzaroni, R.; Dubois, P. AdV. Mater. 2008, 20, 1003–
1007.
(25) Bedard, T. C.; Moore, J. S. J. Am. Chem. Soc. 1995, 117, 10662–
10671.
(26) Hoffmann, M.; Ka¨rnbratt, J.; Chang, M.-H.; Herz, L. M.; Albinsson,
B.; Anderson, H. L. Angew. Chem., Int. Ed. 2008, 47, 4993–4996.
(27) McCallien, D. W. J.; Sanders, J. K. M. J. Am. Chem. Soc. 1995, 117,
6611–6612.
(18) Mo¨ssinger, D.; Hornung, J.; Lei, S.; De Feyter, S.; Ho¨ger, S. Angew.
Chem., Int. Ed. 2007, 46, 6802–6806.
(19) Diederich, F. Nature 1994, 369, 199–207.
(20) Ho¨ger, S.; Enkelmann, V. Angew. Chem., Int. Ed. Engl. 1996, 34,
2713–2716.
(28) Jung, S.-H.; Pisula, W.; Rouhanipour, A.; Ra¨der, H. J.; Jacob, J.;
Mu¨llen, K. Angew. Chem., Int. Ed. 2006, 45, 4685–4690.
(29) Tomizaki, K.-y.; Yu, L.; Wei, L.; Bocian, D. F.; Lindsey, J. S J. Org.
Chem. 2003, 68, 8199–8207.
(21) Ho¨ger, S.; Morrison, D. L.; Enkelmann, V. J. Am. Chem. Soc. 2002,
124, 6734–6736.
(22) Ziegler, A.; Mamdouh, W.; Ver Heyen, A.; Surin, M.; Uji-i, H.; Abdel-
Mottaleb, M. M. S.; De Schryver, F. C.; De Feyter, S.; Lazzaroni, R.;
Ho¨ger, S. Chem. Mater. 2005, 17, 5670–5683.
(30) Rucareanu, S.; Schuwey, A.; Gossauer, A. J. Am. Chem. Soc. 2006,
128, 3396–3413.
(31) Hoffmann, M.; Wilson, C. J.; Odell, B.; Anderson, H. L. Angew.
Chem., Int. Ed. 2007, 46, 3122–3125.
(23) Lei, S.; Ver Heyen, A.; De Feyter, S.; Surin, M.; Lazzaroni, R.;
Rosenfeldt, S.; Ballauff, M.; Lindner, P.; Mo¨ssinger, D.; Ho¨ger, S.
Chem.-Eur. J. 2009, 15, 2518–2535.
(32) Chang, M.-H.; Hoffmann, M.; Anderson, H. L.; Herz, L. M. J. Am.
Chem. Soc. 2008, 130, 10171–10178.
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A R T I C L E S
Mo¨ssinger et al.
Scheme 2. Target Structures 3a and 3b: All-Hydrocarbon Spoked Wheels of High Symmetrya
a Estimation of the molecular diameter performed with Wavefunction Spartan ’08.
wheel 2 was moderately soluble due to 12 orthogonal side
pared to the prototype 2. To stress the influence of the alkyl
chain length on surface affinities and aggregation tendencies,
two representatives with hexyl (3a) and hexadecyl groups (3b)
have been synthesized (Scheme 2).
groups attached at the spokes. Its structure has been confirmed
1
by MALDI MS, H NMR, molecular modeling, small-angle
neutron scattering (SANS), scanning-tunneling microscopic
(STM), and atomic force microscopic (AFM) investigations. As
intended, the reinforcement of the macrocyclic rim by diagonal
spokes (or vice versa) caused a huge gain in rigidity.23 Because
of their regular 6-fold symmetric structure consisting of formal
repeating units, we denoted the wheel molecules as “2D
oligomers”.33,34 Despite their 7 nm size, individual molecules
of the prototype 2 could be accommodated inside the voids of
a honeycomb-like dehydrobenzo[12]annulene host network on
HOPG, thus allowing the observation of rotational and adsorp-
tion/desorption dynamics of “isolated” molecules.35
The successful template-directed cyclization of 1 to 2
prompted us to investigate this concept on similar systems.
However, to perform detailed spectroscopic studies (UV-vis),
we searched for a geometry with a higher degree of symmetry,
yet accessible in a more facile synthesis. Here, we present an
efficient synthesis toward pure hydrocarbon spoked wheels,
which is based on a simple modular approach (Scheme 2). With
a limited number of building blocks and reduced synthetic effort,
a compound with a diameter of nearly 8 nm is obtained. 48
alkyl chains attached orthogonally to the molecular plane
considerably improve the solubility of the new systems com-
The wheel’s highly symmetrical structure, originating from
identical linear segments employed both as spokes and as rim
segments, and a regular substitution pattern, provides simple
UV-vis absorption and emission spectra. Thus, this system
permits the construction of simple model compounds to allocate
different chromophores within the wheel and the noncyclic
precursors. These models can be applied to study basic (static
and time-resolved) photophysical properties of spoked wheel
compounds in general. In addition, the organization of 3 on solid
surfaces as a function of the alkyl group length is investigated
by means of STM and AFM.
Synthesis
As compared to the previously described synthesis of the
prototype 2, the new synthetic approach toward 3 has several
advantages. First, the number of 6-fold transformations has been
reduced and the synthetic toolbox of the critical several-fold
C-C coupling steps was limited to high-yield Sonogashira-
Hagihara and Glaser reactions.36 The hub, previously synthe-
sized in five steps including two 6-fold transformations, has been
replaced by 4, which is available in one step and good yields
through iodination of hexaphenylbenzene with [bis(trifluoro-
acetoxy)iodo]benzene (PIFA) (Scheme 3).37 Furthermore, pre-
(33) Jenkins, A. D.; Kratochv´ıl, P.; Stepto, R. F. T.; Suter, U. W. Pure
Appl. Chem. 1996, 68, 2287–2311.
(34) Sakamoto, J.; van Heijst, J.; Lukin, O.; Schlu¨ter, A. D. Angew. Chem.,
Int. Ed. 2009, 48, 1030–1069.
(36) The several-fold Suzuki coupling steps could not be optimized further
than 90% conversion per bond (52% in total).
(37) Kobayashi, K.; Kobayashi, N.; Ikuta, M.; Therrien, B.; Sakamoto, S.;
Yamaguchi, K. J. Org. Chem. 2005, 70, 749–752.
(35) Tahara, K.; Lei, S.; Mo¨ssinger, D.; Kozuma, H.; Inukai, K.; van der
Auweraer, M.; De Schryver, F. C.; Ho¨ger, S.; Tobe, Y.; De Feyter, S.
Chem. Commun. 2008, 3897–3899.
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Large All-Hydrocarbon Spoked Wheels of High Symmetry
A R T I C L E S
Scheme 3. Module Construction, Convergent Assembly, and Template-Directed Cyclizationa
a (a) I2, PIFA, CH2Cl2, room temperature, 36 h, recryst. n-hexane/CHCl3 (1:1, v/v), 74%; (b) [PdCl2(PPh3)2], CuI, PPh3, piperidine, THF, room temperature,
16 h, 7a, 89%, 7b, 98%; (c) Na2[PdCl4], CuI, P(t-Bu)3, HN(i-Pr)2, 80 °C, 16 h, 9a, 90%, 9b, 88%; (d) K2CO3, THF, MeOH, room temperature, 2 h, 10a,
99%, 10b, 96%; (e) [PdCl2(PPh3)2], CuI, PPh3, piperidine, 80 °C, 16 h, 11a, 73%, 11b, 78%; (f) excess TBAFd[NBu4]F, 1.0 M in THF, 5% H2O,
0-20 °C, 3 h, 12a, 86%, 12b, 92%; (g) 3a, CuCl, CuCl2, pyridine, DBU (2%, v/v), 50 °C, 4 d, 75%; 3b, [PdCl2(PPh3)2], CuI, I2, THF, HN(i-Pr)2, 50 °C,
3 d, recycling GPC separation, 64%.
fabricated bisacetylene rods of defined length were used to
construct the spoke and rim segments.38,39 These bisacetylenes
can be statistically monoprotected40 either with the stable (3-
cyanopropyl)diisopropylsilyl (CPDIPS)41 protective group or
with the more labile (3-cyanopropyl)dimethylsilyl (CPDMS)42
protective group to give versatile linear building blocks such
as 6 and 8. In general, polar acetylene protective groups
dramatically simplify chromatographic separations of otherwise
nonpolar (hydrocarbon) coupling products.43
As pointed out in Schemes 2 and 3, both spoked wheels 3a
and 3b rely on the same building blocks 4 and 5, but differ in
building blocks 6 and 8, which either bear hexyl (a) or hexadecyl
(b) groups. The reaction sequence starts with the assembly of
the spoke/rim modules. Because the pyrylium salt route44,45 still
marks one of most efficient pathways to functionalized m-
terphenyl cornerpieces,46 we retained the synthesis of the
bromodiiodo compound 5 (two steps, 27% yield).23 Using
standard Sonogashira conditions ([PdCl2(PPh3)2], CuI, piperi-
dine, THF) at room temperature, the iodo functionalities of 5
were selectively coupled with the monoprotected bisacetylenes
6 (Scheme 3).47 7 was obtained in very good yields (89-98%)
and could be easily separated from the byproducts due to its
polar protective groups. In the next step, the complete spoke
should be introduced by Sonogashira coupling of the arylbro-
mide 7 with the monoprotected bisacetylene dimer 8.38 In
preceding optimization efforts, the remaining bromo group of
a similar test compound scarcely reacted with 8 under previously
used Sonogashira conditions (50 °C). The conversion rate
improved when the bromide coupling conditions were applied
following Herrmann et al. ([Pd2(dba)3], P(t-Bu)3, NEt3, 80 °C,
(38) Mo¨ssinger, D.; Jester, S.-S.; Sigmund, E.; Mu¨ller, U.; Ho¨ger, S.
Macromolecules 2009, 42, 7974–7978.
(44) Balaban, A. T.; Balaban, T. S. In Science of Synthesis; Houben-Weyl
Methods of Molecular Transformations; Thomas, E. J., Ed.; Thieme:
Stuttgart, 2003; Vol. 14, pp 11-200.
(39) The time-consuming three-step extension of the spokes in the previous
synthesis toward prototype 2 strongly demanded optimization.
(40) Lopez, S.; Fernandez-Trillo, F.; Midon, P.; Castedo, L.; Saa, C. J.
Org. Chem. 2005, 70, 6346–6352.
(45) Zimmermann, T.; Fischer, G. W. J. Prakt. Chem. 1987, 329, 975–
984.
(41) Gaefke, G.; Ho¨ger, S. Synthesis 2008, 2008, 2155–2157.
(42) Ho¨ger, S.; Bonrad, K. J. Org. Chem. 2000, 65, 2243–2245.
(43) In the previous synthesis, the tedious product separation from side
products and starting material with similar polarity further decreased
the yields of most steps.
(46) Ho¨ger, S.; Rosselli, S.; Ramminger, A.-D.; Enkelmann, V. Org. Lett.
2002, 4, 4269–4272.
(47) The 2-fold Suzuki reaction in the previous synthesis demanded more
laborious conditions as well as a more difficult chromatographic
purification and afforded lower yields.
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J. AM. CHEM. SOC. VOL. 132, NO. 4, 2010 1413

A R T I C L E S
Mo¨ssinger et al.
33% yield).48 The best results for the test compound were
obtained by applying an in situ variant of Plenio’s ready-made
catalyst mixture (69% yield),49 that is, with free phosphine
-
instead of (t-Bu)3PH+BF4 and without (i-Pr)2NH2+Br- as a
catalyst matrix salt. These optimized conditions could be
transferred one-to-one to the current synthesis. Na2[PdCl4], CuI,
and (t-Bu)3P smoothly catalyzed the coupling of 7 and 8 in (i-
Pr)2NH at 80 °C and gave yields around 90%. Treatment of 9
with potassium carbonate selectively cleaved the CPDMS
protective group and produced the final spoke/rim module 10
in close to quantitative yields. In the critical 6-fold Sonogashira
coupling of the hub module 4 and the spoke/rim module 10
toward 11 (Scheme 3), we slightly changed the well-established
conditions from the previous synthesis.23 Although still 10 mol
% of palladium(II) catalyst was employed per reactive site (0.6
equiv in total), we further increased the overall concentration
of the reaction partners to approximately 0.15 M (in the case
of 10), omitted THF as solvent, raised the reaction temperature
to 80 °C, and stopped the reaction after one night. 11 could be
completely separated from small amounts of the 4- and 5-fold
coupling products by column chromatography and was obtained
in yields from 73% to 78%.50 If the reaction was conducted
with significantly lower catalyst load (0.1 equiv in total), 11a
could still be isolated in 57% yield, but the separation from
incomplete coupling side products became more tedious.51
Initially, the desilylation of 11 toward 12 also caused some
unexpected trouble. To completely remove all CPDMS groups,
we added a 1.0 M solution of TBAF in THF at room temperature
and stirred overnight. However, this harsh treatment produced
12 along with oligomeric impurities, which could be removed
by preparative gel permeation chromatography (GPC) only.
When a more dilute solution of TBAF (0.25 M in THF) was
added at 0 °C and the reaction was stopped after 3 h, pure 12
could be obtained, even though in suboptimal yields (86-92%).
According to the covalent-template principle, 12 bears 12
adjacent terminal acetylenes and supports its 6-fold intramo-
lecular oxidative coupling. Intuitively, we applied the previ-
ously reported copper(I)-catalyzed cyclization conditions on
12a.23 A solution of the crude product was filtered over a
silica gel column to remove the copper catalyst. GPC analysis
revealed a mixture of 85% of the desired spoke wheel 3a
(11 900 g mol-1) and 15% of the dimer and higher oligomers
(Figure 1a). However, purification by preparative recycling
GPC separation and precipitation with methanol afforded only
small amounts of the pure spoked wheel (yields around 20%).
Apart from substance losses during the purification procedure,
this poor recovery rate must be due to partial polymerization
of 12a during the cyclization reaction and retention of the
polymer and higher oligomers by the silica gel column.
However, under pseudo high-dilution conditions, intermo-
lecular side reactions can only occur if the intramolecular
reaction proceeds slowly. We assumed that pyridine is not
basic enough to deprotonate the terminal acetylenes of
electron-rich 12a. Consequently, the overall yield and the
cycle/oligomer ratio could be improved in consecutive
reactions by the addition of 1,8-diazabicyclo[5.4.0]undec-7-
ene (DBU) as strong auxiliary base (Figure 1b) and even
Figure 1. GPC analyses (in THF vs PS) of the crude cyclization products
of 3a (a-c) and 3b (d,e). Conditions were (a) CuCl, CuCl2, pyridine, room
temperature, 4 d; (b) CuCl, CuCl2, DBU (2%, v/v), pyridine, room
temperature, 4 d; (c) CuCl, CuCl2, DBU (2%, v/v), pyridine, 50 °C, 4 d;
(d) CuCl, CuCl2, DBU (2%, v/v), pyridine, 50 °C, 4 d; (e) [PdCl2(PPh3)2],
CuI, I2, THF, HN(i-Pr)2, 50 °C, 3 d; (f) pure 3b after recycling GPC
purification of the above crude products. See the Supporting Information
for GPC analyses of pure 3a, 12a, and 12b.
more when the latter reaction was performed at 50 °C (Figure
1c). Under optimized reaction conditions, subsequent column
chromatography, and precipitation with methanol, 3a was
obtained in 75% yield without detectable oligomer contami-
nation and did not require further purification steps such as
preparative GPC.
Oddly, the optimized conditions for the cyclization of 12a
could not be transferred to the hexadecyl precursor 12b, but
produced an oligomeric mixture that contained less than 55%
of 3b (16 200 g mol-1, Figure 1d). As a consequence, we
completely discarded our established set of reagents and
switched to palladium-catalyzed cyclizations in diisopropyl-
amine/THF with iodine as oxidant.52 This approach brought
considerable improvements toward a wheel/oligomer ratio of
about 17:3 (Figure 1e) when a catalyst system consisting of
[PdCl2(PPh3)2] and CuI was applied.53 Still, after column
chromatography, the product was not obtained as pure as 3a
and thus required recycling GPC separation. Subsequent pre-
cipitation afforded pure 3b in 64% yield (Figure 1f; for details,
see the Supporting Information). Despite the recycling GPC
separation during the last step, a 50 mg amount of substance
was easily obtained in a simple run of the complete synthesis
over seven steps (see TOC graphic). Still, all C-C coupling
reactions took place in miniature flasks (1-5 mL) aside from
the cyclization step, which was performed under pseudo high-
dilution conditions (20 mL reaction volume).
As for spoked wheel 2, the target compounds appeared as
slightly yellow solids, which can be easily handled in solution
and stored under air exposure. The large number of alkyl chains
afforded a better solubility than for 2. 3a’s hexyl chains lead to
(48) Bo¨hm, V. P. W.; Herrmann, W. A. Eur. J. Org. Chem. 2000, 3679–
3681.
(52) Marsden, J. A.; Miller, J. J.; Haley, M. M. Angew. Chem., Int. Ed.
2004, 43, 1694–1697.
(49) Ko¨llhofer, A.; Plenio, H. AdV. Synth. Catal. 2005, 347, 1295–1300.
(50) Note that 78% in a 6-fold reaction corresponds to 96% in each of the
six coupling reactions per molecule.
(53) [PdCl2(dppe)] was only tried once. The HT 1H NMR spectrum of the
GPC-separated main fraction showed a minor singlet peak at δ ) 3.34
ppm, possibly a hint at an incomplete terminal acetylene conversion
during the cyclization reaction.
(51) Those conditions were not applied for the synthesis of 11b.
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Large All-Hydrocarbon Spoked Wheels of High Symmetry
A R T I C L E S
Table 1. GPC Peaks (Mp,i) and Molar Masses (Mabs) of 10, 12, and
3
Mabs/g mol-1
MGp PC (vs PS)/g mol-1
M
/Mabs
GPC
vs PS
compound
10a
12a
3a
10b
12b
3b
1971.13
10 173.19
10 161.09
3093.26
16 905.95
16 893.85
2800
12 900
11 900
4500
16 200
15 400
1.42
1.27
1.17
1.45
0.96
0.91
increased solubility in chloroform (>50 mg mL-1), but are
obviously not that effective in THF, toluene, and dichlo-
romethane. By contrast, the hexadecyl-substituted wheel (3b)
showed enhanced solubility in all solvents discussed above and
could even be dissolved in cyclohexane as monomeric species
(see DLS investigations). 3a does not melt below 250 °C and
decomposes above that temperature. Because the noncyclic
precursor 12b was obtained as a resin even after precipitation
with methanol, we anticipated finding a low melting point for
3b. Instead, the material does not undergo any change apart
from a solid/liquid-crystalline transition at 225 °C. Unfortu-
nately, decomposition already occurred to some extent in the
liquid crystalline state, so that neither could a transition into an
isotropic melt be observed, nor could the LC phase be analyzed
in detail (see the Supporting Information for differential scanning
calorimetry (DSC) thermograms and polarization microscopy).
Figure 2. 1H NMR spectra (500 MHz, C2D2Cl4): the aromatic region of
the open precursor (12a, 25 °C, top) exhibits some signals, which are masked
in the better resolved high-temperature spectrum of the spoked wheel (3a,
100 °C, bottom) and vice versa. The absence of terminal acetylene signals
(around δ ) 3.35 ppm) in the spectrum of 3a is an indication of complete
cyclization. For full spectra of 3a, 3b, 12a, and 12b, see the Supporting
Information.
of 12b and 3b with PS calibration reflects the correct molecular
weight within an error of 10%.
High-Temperature NMR Investigations
Typically, room-temperature (RT) NMR spectra of 3 show
broadened signals, as the phenylene units directly attached to
the hub and the corner pieces are expected to have considerable
rotational barriers.23 As shown in Figure 2, the aromatic region
of the wheel spectrum (3a, bottom) obtained from high-
temperature (HT) 1H NMR measurements at 100 °C in deuter-
ated 1,1,2,2-tetrachloroethane is comparable to the RT spectra
of the precursor (12a) (top, for spectra of 3b and 12b, see the
Supporting Information).55Apart from significant temperature
shifts, which reveal a merged signal group in the spectrum of
3a (δ ) 6.95 ppm), the aromatic regions of the precursor and
the wheel are similar. The small number of signal groups despite
180 aromatic hydrogen atoms per molecule accounts for the
high symmetry of both 3 and 12. The small cutout of the alkyl
region (Figure 2, right) shows terminal acetylene signals at δ
) 3.35 ppm in the spectrum of 12a (top) and the absent signal
as one proof of cyclization in the spectrum of 3a (bottom).
Substance quantities were sufficient for HT 13C NMR measure-
ments of 3a and 3b in deuterated tetrachloroethane. Considering
GPC Analysis
GPC analysis does not only reveal product mixtures but can
be a monitor for successful cyclization. Although the actual mass
difference is only 12 mass units (see Figure 3), PS-calibrated
GPC interprets this difference as 1.0 × 103 g mol-1 (12a > 3a,
R ) C6H13) and 0.8 × 103 g mol-1 (12b > 3b, R ) C16H33),
respectively (for elugrams of 12a and 12b, see the Supporting
Information). Table 1 summarizes all GPC data for the
compounds 3, 10, and 12 and points out two apparent tendencies
for both spoked wheels 3a and 3b and their respective
precursors. (1) The more the molecules resemble flat rigid disks,
the smaller is the molecular weight overestimation by GPC
(10 > 12 > 3). (2) With increasing length (and number) of the
alkyl chains attached to the rigid backbone, the relative
overestimation of the molecular weight by GPC decreases (12a
> 12b, 3a > 3b). Because of the branched structure of 10, the
overestimation factor (1.4) is considerably lower than for linear
rigid systems.38,54 This trend becomes even more pronounced
after the 6-fold coupling of 10 to the hub 4. Cyclization reduces
the hydrodynamic radius because spoke/rim moieties of 12 are
forced into a coplanar conformation and lose their rotational
degrees of freedom around the spoke axes. As a result, the loss
of 12 acetylene protons is strongly overestimated by GPC.
Because the hydrodynamic radii of the spoked wheel and the
open precursors are, roughly speaking, determined by their
diameter, the higher molecular weight of 3b as compared to 3a
(and also 12b vs 12a) is not completely reflected by a similar
higher molecular weight determination by GPC. An error
cancelation finally leads to the situation that the GPC analysis
the size of the molecules (3a, C774H858; 3b, C1254H1818), the 13
C
spectra are surprisingly concise, which is again a consequence
of the high symmetry of the spoked wheel scaffold (see the
Supporting Information).
MALDI MS Measurements
For the prototype wheel 2, we drew on MALDI-TOF MS as
a powerful method to prove complete cyclization and the
molecular uniformity of the sample.18,23 Accordingly, for each
pair of precursor/spoked wheel (3 and 12), a mass difference
of approximately 12 mass units could be resolved, which
corresponds to the loss of 12 acetylene protons in the course of
cyclization (Figure 3). Alkyl fragmentation occurs particularly
in the spectra of 12a (left, gray curve) and 12b (right, gray
curve), whereas 3a (left, black curve) and 3b (right, black curve)
seemed to be more resistant to the conditions of the measure-
ment and, in addition, hardly formed any matrix adducts (matrix
material: dithranol). The loss of alkyl side chains and the
(54) Xu, Z.; Kahr, M.; Walker, K. L.; Wilkins, C. L.; Moore, J. S. J. Am.
Chem. Soc. 1994, 116, 4537–4550.
(55) A similar behavior is also observed for the prototype spoked
wheel 2.
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J. AM. CHEM. SOC. VOL. 132, NO. 4, 2010 1415

A R T I C L E S
Mo¨ssinger et al.
Figure 3. MALDI-MS spectra of the spoked wheels (black) and the respective noncyclic precursors (gray). Both for 3a/12a (left) and for 3b/12b (right),
a difference of approximately 12 m/z was found. The fragmentation peaks can most probably be attributed to the loss of C5H10 and C15H30 units due to
McLafferty rearrangement. Matrix material: dithranol (226.23 g mol-1).
Figure 5. Quantitative UV-vis absorption spectra (continuous lines) and
normalized emission spectra (dashed lines, λexc ) 365 nm) of the spoked
wheel (3a, black), the noncyclic precursor (12a, red), and the linear model
rod (14, blue, Scheme 4) in CHCl3. Note that 14 constitutes a suitable model
chromophore both for the absorption and for the emission of the spoked
wheel. See Table 2 for the molar absorption coefficients and fluorescence
quantum yields.
Figure 4. Hydrodynamic radius (Rh) distributions obtained from dynamic
light scattering (DLS) experiments on samples of 3a (continuous line; 1.0
× 10-4 mol L-1) and 3b (dashed line; 0.6 × 10-4 mol L-1) in chloroform
(black), toluene (blue), and cyclohexane (red). For all solvents investigated,
only monomer-sized diffusing particles can be observed.
formation of sample-matrix clusters are known phenomena,
which particularly occur at medium to high laser powers.56
solutions we can be sure to determine the optical properties of
individual molecules without any (intermolecular) cooperative
effects.
Aggregation Studies (DLS)
Optical Properties
In preparation for the photophysical studies, we aimed to
ensure that the spoked wheels 3a and 3b do not form aggregates
(e.g., columnar stacks) in solution. On this account, we
performed dynamic light scattering (DLS) investigations of
Absorption measurements of both spoked wheels in chloro-
form reveal structured absorption bands stretching from 270 to
about 410 nm with maxima at 366 nm (3a, Figure 5, black
curve; 3b, see the Supporting Information). As expected, we
found nearly identical absorption and emission spectra for the
spoked wheels 3a and 3b as well as for the noncyclic precursors
12a and 12b, respectively (see Figure 5 and the Supporting
Information). Obviously, the alkyl chain length does not
influence the rigid backbone conformation in such a manner
that the relative absorption of different chromophores within
the molecule changes. As already illustrated for the prototype,23
3a (Figure 5, black curve) and 3b can also be distinguished
from their noncyclic precursors 12a (red curve) and 12b by their
absorption spectra. For 2 and its precursor 1, we explained these
findings by the presence of different absorbing chromophores
in 1 and 2, but identical fluorophores.23 The new, more
symmetrical wheels 3a and 3b consist of identical chromophores
both in the spokes and in the rim segments and therefore
encourage a closer look at the optical properties.
dilute solutions of 3a (continuous line; 1.0 × 10-4 mol L-1
)
and 3b (dashed line; 0.6 × 10-4 mol L-1) in chloroform and
toluene at 20 °C (Figure 4). The chloroform solutions (black
curves) of both spoked wheels contained only one scattering
(and diffusing) species with a hydrodynamic radius distribution
from roughly 1 to 10 nm and maxima at 2.7 and 3.5 nm,
respectively. In contrast to the hexyl-decorated 3a, the hexadecyl
derivative 3b is even well soluble in cyclohexane. Here again,
as it holds for the chlorinated and the aromatic solvent, the only
scattering species detected by DLS was nonaggregated 3b
(Figure 4, dashed red curve). As intended, the solubilizing
groups orthogonal to the plane of the wheel are sufficient to
prevent stacking and aggregation in solution. Hence, in dilute
(56) Ho¨ger, S.; Spickermann, J.; Morrison, D. L.; Dziezok, P.; Ra¨der, H. J.
Macromolecules 1997, 30, 3110–3111.
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1416 J. AM. CHEM. SOC. VOL. 132, NO. 4, 2010

Large All-Hydrocarbon Spoked Wheels of High Symmetry
A R T I C L E S
Scheme 4. Local Assignment and Syntheses of the Linear Model Chromophores 14 and 16
Table 2. Molar Absorption Coefficients (in CHCl3) and
spoke. Because of the high symmetry of 3a and 3b, the rim
segments and spokes consist of nearly identical linear chromophores
and can be modeled by the same model compound 14
(Scheme 4).
Fluorescence Quantum Yields (in THF) of the Compounds
Discussed ((5% Accuracy)
compound
λmax/nm
ελ/L mol-1 cm-1
ΦPL (in THF)
3a
12a
366
338
366
365
323
1.06 × 106
0.81 × 106
0.60 × 106
0.11 × 106
0.44 × 105
0.51
0.58
As a matter of fact, 14’s absorption and emission spectra
(Figure 5, blue curves) are comparable to those of the spoked
wheel (black curve), which implies that absorption and emission
of 3a predominantly occur from the linear segments (6 spokes
+ 6 rim segments). The respective absorption coefficients
support this finding (Table 2). The molar absorption coefficient
of 3a at 366 nm is about 10 times as large as that of the model
chromophore 14 (also Figure 5). Hence, spoked wheels 3a and
3b can be regarded as covalently bound ensembles of linear
chromophores of various orientations.
14
16
0.56
Again, upon excitation (λexc ) 365 nm) both the spoked wheels
and their precursors exhibit almost congruent emission spectra
(Figure 5, right) with maxima at 409 nm and stretched vibronic
progression features. Because of the constrained geometry of the
molecules, we find small Stokes shifts of about 43 nm (23 250
cm-1) resulting in a spectral overlap of absorption and emission.
Molecular models indicate that the six phenylene units surrounding
the central benzene hub are considerably twisted out of the
molecular plane, leading to a “turbine-like” conformation of the
spokes. Hence, the conjugation between two opposite spokes must
be interrupted. If we further presume a perturbation of conjugation
by the meta-linkage at the corners,57 the wheel’s most extended
chromophore is formed by either one linear rim segment or a single
Apart from identical emission spectra,58 the characteristic
absorption band of 14 (λmax ) 365 nm and shoulder around
390 nm) is also present in the spectrum of the noncyclic
precursor 12a (Figure 5, red curve). Because the rim segments
are still “cut in half”, this absorption must originate from the
spokes. However, 12’s main band (λmax ) 338 nm) must be
contributed by less conjugated, but more numerous chro-
mophores and can hence be assigned to the half-rim segments
(represented by 16). This hypothesis can be verified by taking
(57) Boldi, A. M.; Diederich, F. Angew. Chem., Int. Ed. Engl. 1994, 33,
(58) 12’s emission exclusively emanates from the spokes, even if excited
468–471.
at 325 nm.
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A R T I C L E S
Mo¨ssinger et al.
Figure 7. Fluorescence intensity decay curves for the model compound
14, wheels 3a, 3b, and 2, and their corresponding noncyclic precursors 12a,
12b, and 1, respectively. The fluorescence lifetime for each sample is given
in parentheses. The lifetime depends solely on the length of the emissive
chromophore.
Figure 6. Comparison of the noncyclic precursor’s absorption spectrum
(12a, red) and the sum spectrum (black) composed of the model compounds
14 (blue, 6-fold contribution) and 16 (green, 12-fold contribution). Apart
from small differences in extinction, the spectrum of 12a can be reproduced
by a weighted combination of the model compound spectra (14 and 16).
See Table 2 for the molar absorption coefficients.
ping decay curves. For sake of comparison, the fluorescence
decay curve of the model compound 14 is also presented in
Figure 7. Once again, we find that 14 is an accurate representa-
tion of the chromophoric segment in 3a, as it exhibits identical
decay kinetics. The alkyl chain lengths do not affect the emission
properties of these rigid molecular wheels or their noncyclic
counterparts, and consequently the fluorescence lifetimes of the
hexyl and hexadecyl analogues are identical. The molecules with
shorter conjugated chromophoric segments (2 and 1) quite
expectedly show a 2.5-fold slower fluorescence decay,59,60 due
to a reduction in oscillator strength arising from fewer electrons
participating in conjugation.
Because of a highly regular and well-defined molecular
architecture, these multichromophoric wheels also make inter-
esting candidates for the investigation of intramolecular excita-
tionenergytransfermechanismsrelevantforlightharvesting.15,61,62
In such systems, one of the fundamental challenges is to
understand the strength of interchromophoric interactions.63,64
Fluorescence depolarization measurements offer a unique way
to study the dynamics of the photoexcitation energy transfer
between different chromophoric segments within the same
molecule.15,65,66 As the excitation moves from one chromophore
to the other, the transition dipole reorients itself, leading to a
depolarization of the emission. Experimentally, fluorescence
depolarization can be monitored by the change in polarization
anisotropy, r, determined by measuring the emission polarization
parallel and orthogonal to the excitation polarization [r ) (I| -
the absorption coefficients into account (Table 2). 12a’s
absorption spectrum can be approximately composed of the
absorption spectra of the model compounds, because 12a should
contain six spoke chromophores 14 and 12 half rim segment
chromophores 16. Figure 6 demonstrates the good agreement
of the spectrum obtained by summation of the constituent
absorptions with the actual absorption spectrum of 12a.
The absorption coefficients reveal some further details. As
listed in Table 2, the absorption of precursor 12a is dominated
by the half rim segments (main band, λmax ) 338 nm). However,
the band originating from the spokes’ absorption (λmax ) 366
nm) can be compared to the main bands of the spoked wheel
3a and the rod model 14, which appear at similar wavelengths.
Remarkably, we find a nearly additive relation between 3a, 12a,
and 14. The absorption coefficient of the precursor 12a (366
nm, 6 chromophores) is almost 6 times as large as that of the
model 14 (1 chromophore) and constitutes about 57% of the
spoked wheel’s absorption coefficient (3a, 12 chromophores).
Time-Resolved Optical Investigations and Energy
Transfer
To unravel the interaction pathways between chromophores
within a wheel, we considered the transient fluorescence
dynamics. Fluorescence intensity decay curves of three different
molecular wheels (3a, 3b, and 2) and their broken-rim (non-
cyclic) precursors (12a, 12b, and 1, respectively) are shown in
Figure 7. All decay traces could be fitted by a single exponential
function. Compounds 3a and 3b are structurally similar except
for the length of the alkyl side chain. 2, on the other hand, is a
smaller molecule, and therefore has a shorter chromophore. We
find that the fluorescence lifetimes depend solely on the
conjugation length of the emissive chromophore.59 Because the
fluorophore in these molecular wheels is defined by the length
of the spoke, its degree of conjugation is not influenced by the
integrity of the outer rim. Consequently, the cyclic wheels and
the corresponding noncyclic precursors exhibit nearly overlap-
(60) The decrease in fluorescence lifetime with increasing conjugation
length is caused by several effects: (1) highly efficient intramolecular
energy transfer, (2) reduced vibronic coupling, (3) increased oscillator
strength, and (4) singlet state trapping defects.
(61) Ghiggino, K. P.; Reek, J. N. H.; Crossley, M. J.; Bosman, A. W.;
Schenning, A. P. H. J.; Meijer, E. W. J. Phys. Chem. B 2000, 104,
2596–2606.
(62) De Schryver, F. C.; Vosch, T.; Cotlet, M.; Van der Auweraer, M.;
Mu¨llen, K.; Hofkens, J. Acc. Chem. Res. 2005, 38, 514–522.
(63) Varnavski, O. P.; Ostrowski, J. C.; Sukhomlinova, L.; Twieg, R. J.;
Bazan, G. C.; Goodson, T. J. Am. Chem. Soc. 2002, 124, 1736–1743.
(64) Scholes, G. D. Annu. ReV. Phys. Chem. 2003, 54, 57–87.
(65) Varnavski, O.; Menkir, G.; Goodson, T.; Burn, P. L. Appl. Phys. Lett.
2000, 77, 1120–1122.
(59) Schindler, F.; Jacob, J.; Grimsdale, A. C.; Scherf, U.; Mu¨llen, K.;
Lupton, J. M.; Feldmann, J. Angew. Chem., Int. Ed. 2005, 44, 1520–
1525.
(66) Ruseckas, A.; Wood, P.; Samuel, I. D. W.; Webster, G. R.; Mitchell,
W. J.; Burn, P. L.; Sundstrom, V. Phys. ReV. B: Condens. Matter 2005,
72, 115214.
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Large All-Hydrocarbon Spoked Wheels of High Symmetry
A R T I C L E S
hexaphenylbenzene hub assume a “turbine-like” geometry, so
that the spokes (chromophores) are twisted out of the molecular
plane. Consequently, the π-conjugation is limited within an
individual spoke. Furthermore, the fact that the model compound
14 and spoked wheel 3a have identical fluorescence lifetimes
suggests that even in the excited state, the extent of conjugation
is limited only to individual spokes. The wheels therefore
resemble an ensemble of weakly interacting linear chro-
mophores. This is further substantiated by the fact that the molar
extinction coefficient, which is also a measure of the electronic
oscillator strength, scales almost linearly with the number of
chromophores (see Table 2).70
Scanning Tunneling Microscopy
Deposition of 3a and 3b from 1-phenyloctane onto highly
ordered pyrolytic graphite (HOPG) leads to the appearance of
close-packed hexagon-shaped bright objects, as revealed by
scanning tunneling microscopy (STM) at the solid/liquid
interface (Figures 9 and 10). The bright protrusions in the STM
images are attributed to the unsaturated backbones of the
molecules, whereas the dark areas inside the hexagons are empty
or occupied by disordered solvent molecules or alkyl side
chains.71 By comparison of the diameter of the hexagons with
a molecular model (vide infra), it can be concluded that the
hexagons correspond to individual molecules. High-resolution
images (Figures 9b and 10b) reveal the internal structure of the
molecules. The voids inside the molecules do not provide
enough space for adsorption of all side chains. We expect that
some or all side chains of the spokes and inner sides of the
rims are solvated as has been previously reported for other
alkylated molecules at solid/liquid interfaces.72
Despite the large number of hexadecyl chains orthogonal to
the plane of the spoked wheel, the 2D ordering of 3b at the
solid-liquid interface is surprisingly good (Figure 10). The
periodicity of 3b molecules (8.7 nm) is markedly greater as
compared to the periodicity of 3a molecules (7.0 nm), which is
attributed to the difference in the length of the alkyl chains. As
deduced from small-scale images such as Figure 10b, parallel
sides of adjacent molecules are linked by four interdigitated
alkyl chains. Based on the available experimental data, models
reflecting the molecular packing were built. Upon close inspec-
tion of these models (Figures 9c and 10c) and the experimental
data, it is obvious that the self-assembled monolayers formed
by 3a and 3b are chiral. The spokes of the molecules do not
point right to the center between the two nearest molecules. A
negative or positive angle between the orientation of the spokes
and the center between the two next-nearest neighbor molecules
determines the chirality of the domain. Consequently, domains
with opposite chirality were observed (not shown).23
Figure 8. Time-dependent polarization anisotropy of the model linear
chromophore 14 (9), wheel 3b (b), and noncyclic precursor 12b (O). While
the linear chromophore shows gradual depolarization of emission due to
molecular tumbling in solution, the wheel and its noncyclic precursor
undergo ultrafast depolarization mediated by rapid interchromophoric energy
transfer.
I⊥)/(I| + 2I⊥)]. For an isotropic arrangement of linear dipoles,
the value of r is 0.4.67 However, in the solution phase,
polarization can be lost due to rotational diffusion of the emitting
molecule or energy transfer, the two mechanisms having very
different time scales. Figure 8 presents the time-dependence of
polarization anisotropy of the linear model compound 14, spoked
wheel 3b, and its noncyclic precursor 12b. The linear model
compound exhibits an initial anisotropy value of 0.39 that
decreases steadily due to rotational diffusion. The emission from
the bigger molecules, on the other hand, is completely depo-
larized right at the onset. Clearly, the loss of polarization, which
is a consequence of rapid interchromophoric energy transfer,
happens on time scales shorter than the resolution of our
measurement of about 2 ps. It is interesting to note that the
wheel and its noncyclic precursor exhibit an equally rapid
depolarization, within the accuracy of our measurements. This
suggests that resonant dipole-dipole coupling can occur
between different chromophores arranged around a central
benzene hub, quite similar to what is observed in certain star-
shaped molecules.63 It is possible that the wheels have an
additional energy transfer pathway through the rim. However,
the energy transfer around the central hub is so much faster
than our resolution limit that it is not possible to distinguish
between the cyclic wheel and its noncyclic precursor. It is
interesting to note that such a rapid interchromophoric energy
transfer in these molecules does not affect their fluorescence
quantum efficiencies. The photoluminscence quantum efficien-
cies for 3a (wheel), 12a (open precursor), and 14 (linear model
compound) are very similar (0.54 ( 0.04) (Table 2). Ultrafast
depolarization in multichromophoric, π-conjugated systems has
been associated with a coherent delocalization of the excited
state across strongly coupled chromophores.63,68,69 We, however,
believe that these molecular wheels represent a case of weakly
coupled chromophores. As discussed above, molecular modeling
suggests that in the ground state, the phenylene units of the
Atomic Force Microscopy
To probe the self-assembling properties of these molecules
beyond monolayer formation, atomic force microscopy (AFM)
studies were carried out. Figure 11 shows the typical assembly
formed by 3a on a graphite surface upon dropcasting a toluene
(70) Kopelman, R.; Shortreed, M.; Shi, Z.-Y.; Tan, W.; Xu, Z.; Moore,
J. S.; Bar-Haim, A.; Klafter, J. Phys. ReV. Lett. 1997, 78, 1239.
(71) In the constant current mode, the dark-white contrast refers to the
STM tip height: the brighter it is, the larger is the tip-substrate
separation.
(72) Tahara, K.; Furukawa, S.; Uji-i, H.; Uchino, T.; Ichikawa, T.; Zhang,
J.; Mamdouh, W.; Sonoda, M.; De Schryver, F. C.; De Feyter, S.;
Tobe, Y. J. Am. Chem. Soc. 2006, 128, 16613–16625.
(67) Lacowicz, J. R. Principles of Fluorescence Spectropscopy; Springer:
New York, 1999.
(68) Ranasinghe, M. I.; Wang, Y.; Goodson, T. J. Am. Chem. Soc. 2003,
125, 5258–5259.
(69) Lupton, J. M.; Samuel, I. D. W.; Burn, P. L.; Mukamel, S. J. Phys.
Chem. B 2002, 106, 7647–7653.
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A R T I C L E S
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Figure 9. STM images and tentative molecular packing model of 3a self-assembled at the 1-phenyloctane/graphite interface. (a) 46.9 × 46.9 nm2, Iset ) 19
pA, Vbias ) -0.915 V. The p6 symmetry unit cell with parameter a ) 7.0 ( 0.4 nm is indicated in white. (b) High-resolution STM image 18.6 × 18.6 nm2,
Iset ) 19 pA, Vbias ) -1.154 V. The white lines indicate orientations of the main crystallographic directions of the underlying graphite surface. (c) Schematic
representation of the tentative packing model. Red bars highlight the orientation of equivalent spokes in different molecules. The angle between the orange
and the red bars demonstrates the chirality induced by lateral offset in the packing. Side chains other than peripheral hexyl groups have been omitted for
clarity.
Figure 10. STM images and tentative molecular packing model of 3b self-assembled at the 1-phenyloctane/graphite interface. (a) 116 × 116 nm2, Iset
)
30 pA, Vbias ) -0.918 V. The p6 symmetry unit cell with parameter a ) 8.7 ( 0.4 nm is indicated in white. (b) High-resolution STM image 27.7 × 27.7
nm2, Iset ) 30 pA, Vbias ) -0.913 V. White lines indicate orientations of the main crystallographic directions of the underlying graphite surface. (c) Schematic
representation of the tentative packing model. Side chains other than peripheral hexadecyl groups have been omitted for clarity.
Figure 11. Large-scale height (a) and high-resolution phase image (b) of the self-assembling structure of 3a deposited from toluene on graphite. Lamellae-
like features could be clearly seen in the high-resolution phase image, and their width was estimated at about 8 nm. The section profile along the white line
is shown in the inset.
solution. “Fiber”-like features 30 nm in width and 10-12 nm
in height were observed. These fibers can extend to hundreds
of nanometers, but are not always straight. High-resolution phase
images reveal that these fibers are composed of lamellae features
oriented perpendicular to the fiber axis. The width of the
lamellae was measured to be about 8 nm, roughly consistent
with the diameter of the spoked wheel. These images also
indicate that these so-called fibers are actually multilayers, with
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1420 J. AM. CHEM. SOC. VOL. 132, NO. 4, 2010

Large All-Hydrocarbon Spoked Wheels of High Symmetry
A R T I C L E S
is too small even if we assume that the alkyl chains do not
adsorb on the surface. One possibility is that not only the hub
but also the tert-butyl groups contribute to the phase contrast.
If so, the repeating period equals 1/ꢀ3 of the intermolecular
distance, which is expected to be 5.02 nm, and is in very good
agreement with our observation. This indicates that 3b forms
exactly the same packing on mica as on graphite, sketched in
Figure 15, which is quite surprising considering the significant
difference in properties of both substrates.
Conclusions
On the basis of the covalent-template approach, a rational
modular synthesis toward molecularly defined, rigid, highly
symmetrical 2D oligomers has been developed. Two 8 nm-sized
hydrocarbon spoked wheel structures, decorated with 48 alkyl
side chains of different length, were synthesized in very good
yields from simple prefabricated building blocks. Both com-
pounds could be efficiently produced in 50 mg quantities in a
single run over seven steps. Because of the high molecular
symmetry, the nanoscale materials could be fully characterized
despite their size and molecular weights around 10 and 17 kDa,
respectively. MALDI-MS investigations as well as concise high-
temperature 1H and 13C NMR spectra clearly prove the regular
cyclic structure. GPC measurements and comparison with linear
rigid compounds reveal an error-canceling effect of the molec-
ular shape and side chain length on the molecular weight
overestimation. While the hexyl substituents provide sufficient
solubility in THF, toluene, and chlorinated solvents, the hexa-
decyl-decorated wheel is even soluble in cyclohexane. DLS
investigations of dilute solutions of both compounds illustrate
that the solubilizing groups attached orthogonal to the molecular
plane are sufficient to prevent stacking and aggregation in the
solvents that we investigated. Hence, in dilute solutions, we can
be sure to determine the optical properties of individual
molecules without any cooperative effects.
Figure 12. Large-scale (a) and high-resolution (b) height image of 3b self-
assembled from toluene on graphite. (c) Corresponding phase image of
image (b).
each layer measuring around 6 nm in height. Considering all
of this evidence, we conclude that the lamellae-like features in
the fibers correspond to columns of stacked wheel molecules
oriented nearly perpendicular to the surface. Besides the fibers,
some amorphous features with much lower height were also
observed.
For wheel compound 3b, the AFM observations reveal
different characteristics. Instead of fibers, 3b forms layer-like
structures on the graphite surface (Figure 12). On the large scale,
this structure looks quite similar to that previously reported for
the prototype 2.23 The thickness of the layers, about 0.6 nm,
also corresponds well with that measured for 2. However, high-
resolution images (Figure 12b) reveal no clear ordering in the
layer, which indicates that, except for the monolayer in direct
contact with the graphite surface, the long-chain spoked wheels
(3b) align randomly in the layer with their molecular plane
parallel to the surface.
When mica is used as substrate, both 3a and 3b show
significantly different assembling characteristics as compared
to those observed on graphite, as summarized in Figures 13
and 14. In this case, 3a forms layered assemblies of different
height. The thicker layer measured 6.7 nm in height and
corresponds well with the diameter of the molecules, while the
thinner lower layer measures only 2.5 nm in height. Close
inspection reveals that the length of typical stacks of 3a is
limited to around 30 nm. The corresponding phase image reveals
no significant ordering in the thinner layer, which indicates that
the molecules in this layer are arranged randomly.
Figure 14 shows the self-assembled structure of 3b on mica:
layered structures with different heights are revealed. The thinner
layer appears very flat (height of about 0.6 nm), which
corresponds to a monolayer of molecules lying with their
molecular plane parallel to the surface. The rougher brighter
part measures 2.5 nm in height, similar to the amorphous layer
of 3a. Interestingly, the corresponding phase image shows stripe-
like features in the thinner layers. The orientation of these stripes
is indicated by the green lines in Figure 14b. In high-resolution
phase images, the ordering in the flat thin layer could be revealed
more clearly as shown in Figure 14d. To our surprise, the fast
Fourier transformation (FFT) of the phase image reveals a
hexagonal packing, and the repeating period is approximately
5.0 nm, only around one-half of the intermolecular distance
observed by STM on graphite (8.7 nm). Considering the
diameter of the wheel (only the rigid backbone) is around 6.4
nm (apex-to-apex distance), the repeating period observed here
Detailed UV-vis absorption and emission investigations and
comparisons with model chromophores give information about
the location and number of isolated linear chromophores within
the spoked wheels and their noncyclic precursors. Analogous
to linear oligomers and polymers, the fluorescence lifetimes of
the spoked wheels merely depend on the length of the longest
conjugated chromophore. Fluorescence depolarization measure-
ments reveal an ultrafast (<2 ps) decay of the polarization
anisotropy r, which is evidence of an intramolecular resonance
energy transfer between all linear chromophores located in the
spokes and the rim. Therefore, the spoked wheels can be
regarded as an ensemble of weakly interacting (identical) linear
chromophores, posing an intriguing 2D model system to study
molecular light-harvesting phenomena.
STM investigations illustrate that both wheels form periodic
2D monolayers at the solid/liquid interface. Remarkably, the
periodicity is efficiently controlled by the length of interdigi-
tating peripheral side chains, while the side chains (orthogonal
to the wheel’s plane) do apparently not perturb the adsorption.
This tight molecular packing results in periodicities of 7.0
nm (R ) C6H13) and 8.7 nm (R ) C16H33), respectively. A
comparison with packing models illustrates the layers’ p6
symmetry. Moreover, the offset caused by the chain interdigi-
tation leads to domains of opposite chirality. AFM studies reveal
a very different assembly behavior of the two compounds, on
both graphite and mica. The hexyl-decorated wheel shows a
stronger tendency to form stacks resulting in columnar super-
structures, whereas, most probably because of steric interactions,
9
J. AM. CHEM. SOC. VOL. 132, NO. 4, 2010 1421

A R T I C L E S
Mo¨ssinger et al.
Figure 13. (a) Large-scale height image of 3a self-assembled on mica surface. Two kinds of layered structures were observed with heights of 2.5 and 6.7
nm, respectively. The high-resolution phase image (b) clearly shows the inner structure of the thicker layer. (c) An enlargement of the area marked by the
rectangle in (b) (image size 120 × 120 nm2).
Figure 14. Large-scale (a) and high-resolution (c) height image of 3b self-assembled from toluene on mica. The thickness of the film equals 0.6 and 2.5
nm for the lower and higher part, respectively. In the flatter and thinner part, even at large scale the corresponding phase image (b) reveals periodical features
with different orientations (indicated with green lines). The high-resolution phase image (d) shows a hexagonal close packing of bright spots, which is
highlighted in the 2D FFT (inset).
the hexadecyl side chains seem to prevent both stacking and a
long-range ordering in the film (except for the layer in direct
contact with the substrate). Surprisingly, the AFM results
indicate that the hexadecyl-decorated wheel adopts the same
packing on mica as on graphite at the level of the layer in direct
contact with the substrate.
Because the synthesis and purification of the compounds
evidently works without any limiting factors, upscaling efforts
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1422 J. AM. CHEM. SOC. VOL. 132, NO. 4, 2010

Large All-Hydrocarbon Spoked Wheels of High Symmetry
A R T I C L E S
(nD ) 1.4459; η ) 0.58 mPa s), toluene (nD ) 1.4961; η )
20
0.59 mPa s), cyclohexane (nD ) 1.4266; η )210 .00 mPa s).
20
Time-Resolved Fluorescence. Fluorescence lifetime and polar-
ization anisotropy measurements of these systems were carried out
in dilute solutions (0.3 mg L-1 concentration in CHCl3) using a
Hamamatsu streak camera with 2 ps time resolution under excitation
by a 80 MHz repetition rate, frequency-doubled Ti:Sapphire laser
at 370 nm.
Scanning Tunneling Microscopy. All experiments were carried
out at 20-24 °C. Experiments were performed using a PicoSPM
microscope (Agilent). Tips were mechanically cut from Pt-Ir wire
(80:20 alloy, diameter 0.25 mm). Prior to imaging, 3a and 3b were
dissolved in 1-phenyloctane, and a drop of the solution was applied
onto a freshly cleaved surface of graphite (grade ZYB, Advanced
Ceramics Inc., Cleveland, OH). The STM investigations were then
performed at the liquid/solid interface within 1.5 h from dropcasting
the solution. The graphite lattice was recorded by lowering the bias
immediately after obtaining images of the 2D structure. The drift
was corrected using this graphite lattice in the Scanning Probe
Image Processor (SPIP) software (Image Metrology ApS).
Atomic Force Microscopy. The samples were dissolved in
toluene at a concentration of less than 1 mg/g, and one droplet of
the solution was then deposited on freshly cleaved HOPG or mica.
After the solvent was completely evaporated (after at least several
hours), the sample was characterized by AFM operating in
intermittent contact mode. The AFM experiments were performed
with a Nanoscope IV instrument using a J scanner.
Figure 15. Schematic representation of the differences in repeating period
of 3b observed by STM and AFM. The yellow circles indicate those sites
that most probably show up in the AFM images.
are envisaged, an important requirement for investigations of
spoked wheels as rigid components in high-performance com-
posite materials. Our current efforts are focused on the func-
tionalization of the rim with branched side chains and dendrons
to obtain meltable and/or mesomorphic derivatives.
Acknowledgment. This Article is dedicated to Prof. Gerhard
Wegner on the occasion of his 70th birthday. Financial support by
the Deutsche Forschungsgemeinschaft (DFG), the SFB 624, the
VolkswagenStiftung, the K.U. Leuven (GOA), the Fund for
Scientific Research - Flanders (F.W.O.), and the Belgian Federal
Science Policy Office through IAP-6/27 is gratefully acknowledged.
J.M.L. is a David and Lucile Packard Foundation Fellow.
Experimental Section
Synthesis. The syntheses of 1, 2, 4, 6, and 8 have been described
previously,23,37,38 while the syntheses of all new compounds are
described in detail in the Supporting Information.
Dynamic Light Scattering. DLS measurements were carried
out at 20 °C on an ALV CGS-3 goniometer using a HeNe laser
(λ ) 632.8 nm) and an ALV LSE-5004 correlator. Sample solutions
(1 g L-1, 1.0 × 10-4 mol L-1, and 0.6 × 10-4 mol L-1, respectively)
were filtrated into a dust-free vial through a 0.45 µm hydrophobic
PTFE filter. The correlation curve was fitted in a data point interval
of 10-150 using the default DLS exponential g2(t) fit function with
a target PROB1 parameter of 0.5. The following solvent data (20
°C) were also used for the solutions by approximation:73 CHCl3
Supporting Information Available: Further details on the
recycling GPC separation, optical properties, and liquid crystal-
linity of 3b, as well as full 1H NMR, 13C NMR, and MS spectra
of 3a, 3b, 4, 9b, 10a, 12a, and 12b, plus the synthetic procedures
of all compounds not described previously by our groups. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(73) Weast, R. C. CRC Handbook of Chemistry and Physics, 63rd ed.;
CRC Press Inc.: Boca Raton, FL, 1982-1983.
JA909229Y
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