Pentacene-based polycyclic aromatic hydrocarbon dyads with cofacial solid-state π-stacking

Article abstract of DOI:10.1002/chem.200902179

An efficient, stepwise route to unsymmetrically 6, 13-disubstituted pentacenes for the formation of pentacene-based polycyclic aromatic hydrocarbons (PAH) dyads has been reported. PAH are a class of compounds that has received attention from the fields of organic chemistry, materials chemistry, theoretical chemistry, cancer research, environmental science, and astronomy. The stability of PAH materials was assessed by thermal gravimetric analysis (TGA) and differential scanning calorimetry. UV/Vis spectroscopy (CH₂CL ₂) was used to explore the electronic outcome of varying the PAH partner connected to the pentacene moiety. X-ray crystallography reveals that with increasing size of the pendent PAH group, cofacial packing is maximized between nearest neighbors. This fact should provide for a larger transfer integral, and, thus, enhanced conduction and improved charge-carrier mobility.

Full text of DOI:10.1002/chem.200902179

DOI: 10.1002/chem.200902179
Pentacene-Based Polycyclic Aromatic Hydrocarbon Dyads with Cofacial
Solid-State p-Stacking
Dan Lehnherr,[a] Adrian H. Murray,[a] Robert McDonald,[b] Michael J. Ferguson,[b] and
Rik R. Tykwinski*[a, c]
Polycyclic aromatic hydrocarbons (PAHs) are a class of
compounds that has received attention from the fields of or-
ganic chemistry, materials chemistry, theoretical chemistry,
cancer research, environmental science, and astronomy.[1] In
particular, PAHs play a significant role as materials for or-
ganic electronics applications, and their properties have
been tuned for applications such as light-emitting diodes,
field effect transistors, photovoltaic cells, and sensors.[2]
Short linearly-fused acenes such as naphthalene, anthracene,
and tetracene (naphthacene) are reasonably stable and their
semiconducting properties have been systematically studied.
On the other hand, larger acenes such as hexa-, and hepta-
cene necessitate functionalization to obtain stable materi-
als[3] and little has been reported concerning their semicon-
ductive performance. When appropriately functionalized,
pentacene seems to strike a balance between these two ex-
tremes: it shows intriguing charge-transport abilities, while
still offering the prospect for stable, processable materials.
For this and other reasons, pentacene derivatives continue
to be a rich source of discovery in the field of organic semi-
conductors.[2]
near-IR. Unfortunately, functionalized pentacenes such as
6,13-bis(triisopropylsilylethynyl)pentacene (1) typically have
a very narrow absorption region in the ultraviolet (centered
around ca. 310 nm) in addition to absorption bands in the
visible region of 525–660 nm; they are quite transparent in
the region of 350–525 nm.[5] Thus, a homologous series of
pentacene–PAH dyads (2a–f, Scheme 1) has been realized
in which the absorption is tuned in the visible region of the
spectrum.[6] The design of these targets is based on three
key factors. First, a PAH should be attached to the penta-
cene through an ethynyl linker at the 13-position to provide
extended conjugation and enhance absorption in the 300–
475 nm range.[7] Second, the increased p-surface provided by
the PAH moiety should offer improved p-stacking interac-
tions in the solid-state. Finally, a triisopropylsilylethynyl
group should be appended to the 6-position to maintain the
solubility of the product. The results of this study are com-
municated herein, including the synthesis of dyads 2a–f and
a description of their absorption, emission, and electrochem-
ical properties. Furthermore, motivated by the model for the
solid-state packing developed by Anthony and co-workers
for 6,13-bisfunctionalized pentacenes,[5] the X-ray crystallo-
graphic data of dyads 2b–d and 2 f have been analyzed.[8]
The two-step synthesis of pentacene-based dyads 2a–f is
shown in Scheme 1. Deprotonation of the corresponding ter-
minal acetylene with HexLi gave acetylides 3a–e, which
were then added to a solution of 4[8e,9] in THF to afford
diols 5a–e in good yields (83–99%). For the synthesis of
diol 5 f, anthraceneacetylide 3 f was formed by the reaction
of 9-(trimethylsilylethynyl)anthracene with MeLi to avoid
isolation of 9-ethynylanthracene. Finally, the SnII-mediated
reduction of 5a–f afforded conjugated pentacene dyads 2a–f
in good yields of 83–98%.
Ideally, organic materials for solar cell applications would
absorb light throughout the solar spectrum.[4] This requires
materials with electronic absorption spectra covering a wide
range of energies across the UV/Vis spectrum and into the
[a] D. Lehnherr, A. H. Murray, Prof. Dr. R. R. Tykwinski
Department of Chemistry, University of Alberta
Edmonton, Alberta, T6G 2G2 (Canada)
[b] Dr. R. McDonald, Dr. M. J. Ferguson
X-Ray Crystallography Laboratory
Department of Chemistry, University of Alberta
Edmonton, Alberta, T6G 2G2 (Canada)
[c] Prof. Dr. R. R. Tykwinski
Dyads 2a–d are sufficiently soluble for purification by
column chromatography, while the less soluble pyrenyl 2e
and anthryl 2 f derivatives were purified by precipitation.
The stability of these materials was assessed by thermal
gravimetric analysis (TGA) and differential scanning calo-
rimetry (DSC, Table 1). Although TGA does not show any
New address: Institut fꢀr Organische Chemie
Friedrich-Alexander-Universitꢁt
Erlangen-Nꢀrnberg, Henkestrasse 42, 91054 Erlangen (Germany)
E-mail: rik.tykwinski@chemie.uni-erlangen.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902179.
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Chem. Eur. J. 2009, 15, 12580 – 12584

COMMUNICATION
Scheme 1. Synthesis of pentacene-based PAH dyads 2a–f.
Table 1. Thermal analysis of dyads 2a–f.
Compound
TGA
Td [8C]
DSC (decomposition)
onset [8C] peak [8C]
2a
2b
2c
2d
2e
2 f
435
435
420
425
420
390
174
190
206
218
248
270
180
197
210
221
256
271
significant weight loss (<5%) below 3908C for 2a–f, DSC
reveals decomposition occurs at much lower tempera-
tures.[10] In general, thermal stability increases as the PAH
substituent appended to the pentacene becomes larger. Fur-
thermore, the geometry of the PAH affects the stability as
well: a parallel arrangement of the acene (i.e., 1-naphthyl
2c and anthryl 2 f) provides improved stability in compari-
son to a non-parallel geometry (i.e., 2-naphthyl 2b and
phenanthryl 2d).[11]
UV/Vis spectroscopy (in CH2Cl2) was used to explore the
electronic outcome of varying the PAH partner connected
to the pentacene moiety (Figure 1, Table 2). In principle, the
ethynyl linker in 2a–f should facilitate electronic communi-
cation between the PAH pairs. Consistent with this expecta-
tion, a red shift is observed in lmax values for 2a–f as the
pendent PAH chromophore is formally increased in size.
The effect is, however, relatively small upon moving from
2a (lmax =652 nm) to 2 f (lmax =671 nm). More interesting
effects are observed in the mid-energy absorption range be-
tween 325–500 nm, as absorptions (Table 2, lmid) are sub-
stantially red-shifted with increasing size of the pendent
PAH group (Figure 1b). Thus, the structural evolution in
the acene series from phenyl 2a (lmid =360 nm) to naphthyl
2c (lmid =395 nm) to anthryl 2 f (lmid =465 nm) demon-
strates a red shift of over 100 nm for this absorption.
Figure 1. Solution-state UV/Vis absorption spectra for dyads 2a–f in
CH2Cl2. a) Low-energy region (500–725 nm) and b) mid-energy region
(325–500 nm) of spectra.
Solid-state UV/Vis absorption spectra for thin films of
pentacenes 2a–f feature red shifts of 19 to 41 nm for the
longest wavelength absorption lmax in comparison to solu-
tion-state data in CH2Cl2 (Table 2).[12] Pentacenes bearing
larger PAH moieties, such as pyrenyl 2e and anthryl 2 f,
show the largest red shifts, indicating more significant elec-
tronic coupling between molecules in the solid state. This
observation is qualitatively in agreement with the X-ray
crystallographic data currently available (vide infra).
Chem. Eur. J. 2009, 15, 12580 – 12584
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12581

R. R. Tykwinski et al.
the UV/Vis data (Egopt). This suggests that the energy of the
HOMO is raised while the LUMO energy is lowered as ben-
zannulation is increased along the series.
Table 2. UV/Vis absorption and emission properties for dyads 2a–f.
[a]
[b]
[d]
Compound lmid
lmax
lmax
lmax, em
Stokes FF
shift (in
(in
(in
(thin
(in
CH2Cl2) CH2Cl2) film)
[nm]
CH2Cl2) [nm][c] CH2Cl2)
[nm]
Solid-state packing of the PAH dyads has been investigat-
ed by using X-ray crystallography. Suitable single crystals of
the 1-naphthyl (2c),[15] phenanthryl (2d),[16] and anthryl
(2 f)[17] derivatives are obtained relatively easily by slow
evaporation from solutions in CH2Cl2 layered with acetone.
On the other hand, crystals of the 2-naphthyl derivative
2b[18] are obtained by slow evaporation of a solution in
CDCl3. Under analogous conditions, the phenyl derivative
2a and the pyrenyl derivative 2e afford fibrous bundles un-
suitable for X-ray crystallographic analysis. The derivatives
2b–d and 2 f crystallize with the PAH moiety coplanar or
nearly coplanar with the pentacene moiety giving centro-
symmetric or pseudo-centrosymmetric face-to-face dimers.
In three cases (2b, 2d, 2 f), long-range face-to-face p-stack-
ing interactions are also observed.
Unlike for 2b, 2d, and 2 f, the crystallographic analysis of
1-naphthyl derivative 2c (Figure 2a) shows no long-range p-
stacking. The unit cell of 2c contains two crystallographical-
ly independent molecules, and the naphthyl and pentacenyl
moieties are nearly coplanar in both cases, with torsion
angles between the acene moieties of 10–118.[19] The solid-
state packing of 2c shows cofacial p-stacking interactions
between the naphthyl and pentacenyl moieties of neighbor-
ing molecules with intermolecular C···C distances as small as
3.4 ꢃ. There is, however, no long-range cofacial p-stacking;
rather a herringbone arrangement is observed that features
[nm]
[nm]
2a
2b
2c
2d
2e
2 f
360
381
395
398
440
465
652
657
658
660
671
671
671
679
679
686
712
712
661
667
668
670
681
688
9
0.13
0.12
0.12
0.11
0.07
0.006
10
10
10
10
17
[a] Wavelength of most intense absorption in the range of 325–500 nm.
[b] Lowest energy absorption maxima. [c] Based on solution-state meas-
urements. [d] Measured by using lexc =551 nm (see ref. [13]).
Solution-state emission in CH2Cl2 of pentacenes 2a–f has
been explored with an excitation wavelength (lexc) of
551 nm (Table 2).[12,13] Dyads 2a–e have an emission quan-
tum yield (FF), of about 0.12 and a Stokes shift of ca.
10 nm. The anthryl derivative 2 f, however, shows both a sig-
nificantly lower FF =0.006 and a larger Stokes shift of
17 nm, and this behavior is analogous to the essentially non-
fluorescent conjugated pentacenes dimers recently repor-
ted.[6e] Finally, the emission energy and peak shape for all
derivatives are largely unaffected by the choice of excitation
wavelength (i.e., lexc =551 nm, the strongest mid-energy
band, or strongest high energy band at ca. 310 nm).[12]
Cyclic voltammetry provides insight into how the PAH
partner appended to the pentacene framework influences
the HOMO and LUMO energies. It also allows for a com-
parison of band gaps obtained from solution-state UV/Vis
data via the absorption edge (Table 3).[14] Pentacene deriva-
tives 2a–d show two reduction and one oxidation potentials,
all three of which are reversible. In addition to two reversi-
ble reduction waves, pentacenes 2e and 2 f also show two
oxidation waves, although only the first is reversible. While
both the reduction and the oxidation potentials decrease
slightly as the size of the pendent PAH increased, there is
surprisingly little variation in either the first oxidation or re-
duction potentials. The second reduction potential shows a
marginal dependence on structure, deceasing from À1.90 V
for 2a to À1.77 V for 2 f. Overall, the trend for the electro-
chemical HOMO–LUMO gap determined by the CV data
(Egelectro) is reasonably consistent with that determined from
À
intermolecular edge-to-face interactions with a C H···C(p)
distance of 2.9 ꢃ (H16A and C4B’, see Figure 2a). This is
comparable with the edge-to-face interaction in one of the
solid-state morphologies of pristine pentacene with
a
[20]
À
C H···C(p) distance of 2.92 ꢃ.
The structure of 2-naphthylpentacene 2b also contains
two crystallographically independent molecules in the unit
cell (Figure 2b). In molecule A, the naphthyl and pentacen-
yl moieties are coplanar (torsion angle of 0.88), whereas for
molecule B this angle is 8.28.[19] The two crystallographically
independent molecules of 2b form a pseudo-centrosymmet-
ric dimeric pair, and neighboring pairs are then stacked in a
1D columnar arrangement that provides long-range order.
The intermolecular separation between neighboring mole-
cules A and B, both within a dimeric pair and between di-
meric pairs, is estimated to be 3.4 ꢃ.[21]
Table 3. Summary of redox and band gap data for dyads 2a–f.[a]
Phenanthryl derivative 2d crystallizes with a negligible
torsion angle of 1.58 between the two PAH moieties in each
dyad (Figure 2c).[19] Adjacent molecules form centrosym-
metric dimeric pairs (e.g., bottom two molecules in Fig-
ure 2c) separated by an interplanar distance of 3.25 ꢃ. Ad-
jacent dimers then pack in a 1D slipped-stack arrangement
with an interplanar distance of 3.32 ꢃ (top two molecules in
Figure 2c).[22]
Finally, the solid-state packing of anthryl derivative 2 f
(Figure 2d) mirrors that of 2d. The PAH moieties are essen-
tially coplanar within each molecule (torsion angle of
1.28),[19] and neighboring molecules form a centrosymmetric
electro
opt[b]
Compound
Ered1
[V]
Ered2
[V]
Eox1
[V]
Eg
Eg
[eV]
[eV]
2a
2b
2c
2d
2e
2 f[c]
À1.44
À1.42
À1.42
À1.40
À1.40
À1.38
À1.90
À1.86
À1.86
À1.83
À1.80
À1.77
0.39
0.38
0.39
0.38
0.34
0.33
1.83
1.80
1.81
1.79
1.74
1.71
1.81
1.79
1.79
1.78
1.74
1.74
[a] Cyclic voltammetry was performed in benzene/MeCN (3:1 v/v) solu-
tions containing 0.1m nBu4NPF6 as electrolyte at
a scan rate of
150 mVsÀ1. Potentials are referenced to the ferrocenium/ferrocene (Fc+/
Fc) couple used as an internal standard. [b] See ref. [14]. [c] Measured at
a scan rate of 200 mVsÀ1
.
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Chem. Eur. J. 2009, 15, 12580 – 12584

Pentacene-Based Polycyclic Aromatic Hydrocarbon Dyads
COMMUNICATION
3.51 ꢃ between each pair (top two molecules in Fig-
ure 2d).[22]
In summary, we demonstrate an efficient, stepwise route
to unsymmetrically 6,13-disubstituted pentacenes for the for-
mation of pentacene-based PAH dyads. The formal benzan-
nulation of phenylpentacene 2a provides pentacenes 2b–f,
which exhibit intermolecular cofacial p-stacking between
the pentacene and the PAH moiety. The choice of the PAH
moiety, as well as the point of attachment to the 6,13-di-
AHCTUNGTREGeNNNU thynylated pentacene group, offers the ability to manipu-
late solid-state packing as well as the electronic properties
as documented by both cyclic voltammetry and UV/Vis ab-
sorption/emission experiments. X-ray crystallography nicely
shows that with increasing size of the pendent PAH group
in 2b–d and 2 f, cofacial packing is maximized between near-
est neighbors. This fact should ultimately provide for a
larger transfer integral, and, thus, enhanced conduction and
improved charge-carrier mobility.[23]
Acknowledgements
This work has been generously supported by the University of Alberta,
the Natural Sciences and Engineering Research Council of Canada
(NSERC) through the Discovery grant program, and the Canadian Foun-
dation for Innovation (CFI). We thank Dr. Khalid Azyat for kindly do-
nating the pyrene starting material. D.L. thanks NSERC (PGS-D), the
Alberta Ingenuity Fund (AIF), the Alberta Heritage Fund, the Killam
Trusts, and the University of Alberta for scholarship support. A.H.M.
thanks NSERC (PGS-D), AIF, and the University of Alberta for scholar-
ship support.
Keywords: conjugation · hydrocarbons · oligomerization · pi
interactions · semiconductors
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Chem. Eur. J. 2009, 15, 12580 – 12584
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www.chemeurj.org
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[16] X-ray crystallographic data for 2d: C49H42Si, Mr =658.92; crystal di-
¯
mensions 0.27ꢆ0.20ꢆ0.17 mm; triclinic space group P1 (no. 2); a=
9.1190(10), b=12.8567(14), c=16.5030(18) ꢃ; a=70.9187(13); b=
83.3738(14), g=82.2715(15)8; V=1806.6(3) ꢃ3; Z=2; 1calcd
=
1.211 gcmÀ3
;
m=0.100 mmÀ1
;
l=0.71073 ꢃ; T=À1008C; 2qmax
=
52.748; total data collected=14497; R1 =0.0416 [5703 observed re-
2
2
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flections with [Fo ꢀ2s
ACHTUGNRTNE(NUGN Fo )]; wR2 =0.1147 for 451 variables and
2
2
7355 unique reflections with [Fo ꢀÀ3s
ACHTUNGRTEN(NUNG Fo )]; residual electron den-
sity=0.279 and À0.244 eꢃÀ3. See also ref. [15b].
[17] X-ray crystallographic data for 2 f: C49H42Si, Mr =658.92; crystal di-
¯
[8] For X-ray crystallographic structures of unsymmetrical pentacenes,
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mensions 0.27ꢆ0.11ꢆ0.08 mm; triclinic space group P1 (no. 2); a=
8.5889(11), b=14.8703(19), c=15.312(2) ꢃ; a=71.0863(16); b=
81.8475(17), g=78.8361(17)8; V=1808.4(4) ꢃ3; Z=2; 1calcd
=
1.210 gcmÀ3
;
m=0.100 mmÀ1
;
l=0.71073 ꢃ; T=À1008C; 2qmax
=
53.028; total data collected=14613; R1 =0.0482 [5103 observed re-
2
2
flections with [Fo ꢀ2s
ACHTUGNRTNE(NUGN Fo )]; wR2 =0.1324 for 451 variables and
2
2
7456 unique reflections with [Fo ꢀÀ3s
ACHTUNGRTEN(NUNG Fo )]; residual electron den-
sity=0.390 and À0.367 eꢃÀ3. See also ref. [15b].
[18] X-ray crystallographic data for 2b: C45H40Si, Mr =608.86; crystal di-
¯
mensions 0.66ꢆ0.18ꢆ0.08 mm; triclinic space group P1 (no. 2); a=
7.6451(14), b=13.410(3), c=34.704(7) ꢃ; a=88.638(2), b=
[9] A. Boudebous, E. C. Constable, C. E. Housecroft, M. Neuburger, S.
Shaffner, Acta Crystallogr. Sect. C 2006, 62, o243-o245.
[10] Under an inert atmosphere, thermal decomposition of 6,13-di-
84.211(2),
g=73.839(2)8;
V=3399.9(11) ꢃ3;
l=0.71073 ꢃ; T=À1008C; 2qmax
Z=4;
1calcd
=
1.189 gcmÀ3
;
m=0.100 mmÀ1
.
=
50.508; total data collected=24203; R1 =0.0732 [6935 observed re-
2
2
ethynylated pentacenes has been attributed to the Diels–Alder reac-
flections with [Fo ꢀ2s
ACHTUGNERTN(NUGN Fo )]; wR2 =0.2164 for 863 variables, 72 re-
2
2
tion between the alkyne and the pentacene chromophore, while
photochemical decomposition leads to [4+4] intermolecular dimeri-
zation similar to anthracene. See: a) M. M. Payne, S. A. Odom, S. R.
Parkin, J. E. Anthony, Org. Lett. 2004, 6, 3325–3328; b) P. Coppo,
S. G. Yeates, Adv. Mater. 2005, 17, 3001–3005; c) Y. Kim, J. E. Whit-
ten, T. M. Swager, J. Am. Chem. Soc. 2005, 127, 12122–12130;
d) S. H. Chan, H. K. Lee, Y. M. Wang, N. Y. Fan, X. M. Chen, Z. W.
Cai, H. N. C. Wong, Chem. Commun. 2005, 66–68; e) J. Chen, S.
Subramanian, S. R. Parkin, M. Siegler, K. Gallup, C. Haughn, D. C.
Martin, J. E. Anthony, J. Mater. Chem. 2008, 18, 1961–1969.
[11] For a study of substituent-size effects on solid-state packing and
electronic influence on stability, see: a) reference [5b,7b,10e];
b) B. H. Northrop, K. N. Houk, A. Maliakal, Photochem. Photobiol.
Sci. 2008, 7, 1463–1468.
straints, and 12272 unique reflections with [Fo ꢀÀ3s
ACHTUNGTRENN(UNG Fo )]; residual
electron density=0.514 and À0.429 eꢃÀ3. See also ref. [15b]. The
disordered isopropyl groups of molecule B were restrained to have
the same geometry as that of one of the ordered isopropyl groups of
molecule A by use of the SHELXL SAME instruction. Additional-
À
ly, the Si C distances of the disordered triisopropylsilyl group were
À
allowed to refine to a common value. Likewise, the C C distances
of the secondary carbon atoms of the minor component were al-
lowed to refine to a common value.
[19] Torsion angles were calculated from the angle between the least
squares plane generated from the carbon atoms of each acene.
[20] a) T. Siegrist, C. Kloc, J. H. Schçn, B. Batlogg, R. C. Haddon, S.
Berg, G. A. Thomas, Angew. Chem. 2001, 113, 1782–1786; Angew.
Chem. Int. Ed. 2001, 40, 1732–1736. For a discussion of the poly-
morphs of pentacene, see: b) C. C. Mattheus, G. A. de Wijs, R. A.
de Groot, T. T. M. Palstra, J. Am. Chem. Soc. 2003, 125, 6323–6330;
c) C. C. Mattheus, A. B. Dros, J. Baas, G. T. Oostergetel, A. Meet-
sma, J. L. de Boer, T. T. M. Palstra, Synth. Met. 2003, 138, 475–481.
[21] Neighboring molecules are not related by symmetry, and the least-
squares planes of the acenes in molecule A and B are not parallel,
thus requiring an alternative method of measuring the interplanar
distances. Distances were measured from every individual acene
carbon in molecule A to the neighboring least squares plane gener-
ated from the carbon atoms of both acenes in molecule B (and vice
versa) to obtain the average value reported.
[12] See the Supporting Information for spectra and details.
[13] Fluorescence quantum efficiencies were obtained by a comparison
to cresyl violet perchlorate in methanol, see: S. J. Isak, E. M. Eyring,
J. Phys. Chem. 1992, 96, 1738–1742.
[14] The wavelength used as the absorption edge corresponded to the
lowest energy absorption with
a
molar absorptivity eꢀ
1000 LmolÀ1 cmÀ1
.
[15] a) X-ray crystallographic data for 2c: C45H40Si, Mr =608.86; crystal
dimensions 0.46ꢆ0.40ꢆ0.24 mm; orthorombic space group Pna21
(no. 33); a=34.1011(13), b=13.6639(5), c=14.6209(6) ꢃ; V=
6812.7(5) ꢃ3; Z=8; 1calcd =1.187 gcmÀ3
;
m=0.100 mmÀ1
;
l=
0.71073 ꢃ; T=À1008C; 2qmax =55.088; total data collected=58526;
[22] Interplanar distances were calculated from the distance between the
least squares plane generated from the carbon atoms of covalently
bonded acenes (the pentacene and its covalently bonded PAH
moiety).
[23] J.-L. Brꢄdas, D. Beljonne, V. Coropceanu, J. Cornil, Chem. Rev.
2004, 104, 4971–5003.
2
2
R1 =0.0386 [13955 observed reflections with [Fo ꢀ2s
ACHTUGNERTN(NUGN Fo )]; wR2 =
2
0.1043 for 829 variables and 15673 unique reflections with [Fo
ꢀ
2
À3s
N
.
b) CCDC-736049 (2b) CCDC-736050 (2c), CCDC-736051 (2d),
CCDC-736052 (2 f) contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Received: August 5, 2009
Published online: October 21, 2009
12584
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ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 12580 – 12584