Synthesis and structure of 4,4′-bis(2,3,4,5,6-pentafluorostyryl)stilbene, a self-assembling J aggregate based on aryl-fluoroaryl interactions

Article abstract of DOI:10.1039/b100002k

The synthesis and characterisation of 4,4′-bis(2,3,4,5,6-pentafluorostyryl)stilbene is described; this molecule forms a J aggregate by self-assembly based on aryl-fluoroaryl interactions.

Full text of DOI:10.1039/b100002k

Synthesis and structure of 4,4A-bis(2,3,4,5,6-pentafluorostyryl)stilbene, a
self-assembling J aggregate based on aryl–fluoroaryl interactions
W. James Feast,*ab P. Wilfried Lövenich,ab Horst Puschmannb and Carlo Talianic
a IRC in Polymer Science and Technology, Department of Chemistry, University of Durham, South Road, Durham,
UK DH1 3LE
b Department of Chemistry, University of Durham, South Road, Durham, UK DH1 3LE
c
Instituto di Spettroscopia Molecolare C.N.R., Via Gobetti 1010, I-40129 Bologna, Italy
Received (in Cambridge, UK) 2nd January 2001, Accepted 6th February 2001
First published as an Advance Article on the web 20th February 2001
The synthesis and characterisation of 4,4A-bis(2,3,4,5,6-
pentafluorostyryl)stilbene is described; this molecule forms a
J aggregate by self-assembly based on aryl–fluoroaryl
interactions.
The molecular organisation of conjugated organic materials is
important in relation to their potential for application as the
active components of electronic and optoelectronic devices;1 in
particular, the structures of aggregates of organic materials can
have profound effects on their optical properties.2 Detailed
studies of crystalline oligomers often aid understanding of the
behaviour of polymeric materials; for example, distyrylbenzene
(DSB) has been studied as a model for poly(phenylene
vinylene) (PPV)3 and self-assembling systems are particularly
attractive since they have considerable processing advantages in
device construction. Hydrogen bonding,4 p-stacking5 and
arene–perfluoroarene interactions6 have been used as organisa-
tional motifs in the design and synthesis of self-organising
systems. The latter phenomenon was described by Patrick and
Prosser in 1960 who demonstrated that benzene (mp 5.5 °C) and
hexafluorobenzene (mp 4 °C) form a stable 1+1 complex (mp 24
°C),7 consisting of a face-to-face stack of alternating benzene
and hexafluorobenzene molecules. The origin for this stable
arrangement has generally been thought to lie in the quad-
Fig. 1 a) ‘Brickwall’ motif, b) compound 1 side view, c) compound 1 top
view, d) compound 1 edge on view.
rupolar interaction between the two molecules, they both have
large molecular quadrupole moments but of opposite sign;8
however recent work suggests that the effect can be ascribed
largely to van der Waals interactions.9 Coates and Grubbs et al.
have demonstrated that these non-covalent interactions can be
used in substituted stilbenes and DSBs for self-assembly into
structures aligned for photochemically induced [2+2] reactions
in the solid state, while Bazan et al. have investigated the effect
of different fluorine substitution patterns on the molecular
packing of DSBs.10
We became interested in the possibility of making self-
organising J aggregates of large aromatic chromophores
suitable for optoelectronic applications using the directing
effect of phenyl–pentafluorophenyl interactions. For this pur-
pose the oligo-phenylenevinylene, distyrylstilbene, was the
molecule of choice. J aggregates are characterised by a narrow
and intense absorption band that shows a bathochromic shift
relative to the single isolated molecule. This is a result of
coherent excitonic coupling of the molecules in the solid state.
Transitions to and from the bottom of the excitonic band are
consequently allowed and fluorescence, as well as absorption, is
expected to be intense and narrow. Jelley was one of the first to
observe this strong absorption for the case of cyanine dyes
(hence the name).11 One possible arrangement for J aggregates
is the simple ‘brickwall motif’ in which each ‘brick’, or
molecule, is situated over the gap between the bricks in the row
below (see Fig. 1a).12 Here we report the synthesis and
molecular packing of 4,4A-bis(2,3,4,5,6-pentafluorostyryl)stil-
bene 1, which is accessible by the route summarised in Scheme
1 and can be processed by sublimation or from solution. The
synthesis involves McMurry coupling of p-tolylaldehyde 2 to
Scheme 1 Route for the synthesis of 4,4A-bis(2,3,4,5,6-pentafluorostyryl)-
stilbene.
DOI: 10.1039/b100002k
Chem. Commun., 2001, 505–506
This journal is © The Royal Society of Chemistry 2001
505

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1
give (E)-4,4A-dimethylstilbene 3. The methyl groups were
brominated with NBS and the bromine substituted with
triphenylphosphine yielding the bis(phosphonium) salt 5. A
Wittig reaction with two equivalents of pentafluorobenzalde-
hyde afforded 1, which was purified by sublimation.
conjugation. H NMR spectroscopy shows a new signal at d
4.88 for cyclobutane hydrogens and the 19F NMR spectrum
shows three new resonances at d 2142.11, 2156.11 and
2162.98 with concomitant reduction in the intensity of the
initial signals at d 2143.81, 2157.47, 2163.75.
1
The H NMR spectrum shows three resonances for olefinic
The work reported here provides strong evidence for the
value and power of fluoroaryl–aryl face-to-face interactions as
a design motif for self-assembling systems. The electronic and
optical properties of 1 in the solid state are the subject of on-
going investigations.
We thank the European Commission HPRT Network LAM-
INATE (Contract No HPRN-CT-2000-00135) for financial
support and Professor T. B. Marder (Durham) for his interest
and helpful comments.
protons; a singlet at d 7.17 and two doublets at d 7.00 and 7.43
with coupling constants of 16.5 Hz typical for a trans vinylene
configuration. The 19F NMR spectrum confirms that only one
isomer was formed since only one set of three resonances was
observed (d 2143.81, 2157.47, 2163.75).13 Identical crystals
of 1 were grown by sublimation and from solution in stilbene at
170 °C indicating that there is one predominant form of
aggregation. The crystal structure† shows that all double bonds
are trans. The centre of each molecule is an inversion centre so
that only half the molecule is crystallographically unique. The
fluorinated and the non-fluorinated rings are twisted very
slightly with respect to each other (2.2°) and the two central
rings are twisted with an angle of 3°. This almost planar
arrangement allows strong p–p interactions.
Notes and references
† Crystallographic data for 1. C30H14F10, Mr = 564, space group P1, a =
¯
6.0624, b = 7.4468, c = 13.0565 Å, a = 78.442, b = 82.972, g = 85.693°,
U = 572.378 Å3, T = 100 K, Z = 1, m(Mo-Ka) = 0.152 mm–1, 2604
reflections measured, 2604 unique (Rint = 0.0797) which were used in all
calculations and 1486 greater than 2s(I). The final R(F) was 0.0853
(I > 2s(I) data) and the wR(F2) was 0.1885 (all data). The relatively large R
factor is due to the small crystals available, 0.20 3 0.10 3 0.05 mm3. CCDC
156709. See http://www.rsc.org/suppdata/cc/b1/b100002k/ for crystallo-
graphic files in .cif formats.
The molecules aggregate in a simple brickwall motif in which
each molecule overlaps with two halves of neighbouring
molecules in the row below and above it (Fig. 1a and b).
Thereby, each central aromatic ring is sandwiched between two
terminal fluorinated aromatic rings of neighbouring molecules
and vice versa. The distance between the mean planes of all the
atoms in the molecules is 3.41Å. However, the situation is
further complicated by the fact that there is some lateral
slippage between the aromatic rings which results in successive
rows of the ‘brickwall’ being slightly offset with respect to each
other; analogous structures have been reported and described as
‘molecular staircases’.14 This slippage is not equally distributed
between all aromatic rings but the molecules are paired in the
sense that one ring overlaps more with its fluorinated counter-
part above than the one below or vice versa (Fig. 1c). As a
consequence of this the two olefinic carbons of the central
molecule in Fig. 1c are closer to their counterparts below (3.659
Å) than the one above (3.869 Å).
In addition to the face-to-face p-stacking the molecular
packing is associated with intermolecular C–F…H–C inter-
actions.13 Four such interactions are observed on each ring
resulting in 16 per molecule. H…F distances derived from this
X-ray study are 2.390, 2.467, 2.480 and 2.595 Å, derived using
the neutron normalised C–H distance of 1.083 Å. The
corresponding C–H…F angles are 159.4°, 157.6°, 132.1° and
122.9°. The distance between the terminal fluorine atoms in the
4 positions of edge-to-edge neighbours is short (2.676 Å).
The packing of 1 is analogous to that of 2,3,4,5,6-penta-
fluorostilbene (PFS); the densities are very similar (1.635
g cm23 for 1; 1.641 g cm23 for PFS). Molecules of PFS in
crystals are also paired, but the pairing is less obvious than in the
present case since the distance between olefinic carbons of
neighbouring PFS molecules is quoted as 3.700 Å for one side
and 3.707 Å for the other. Grubbs and Coates have shown
photochemically induced [2+2] reactions in crystals of PFS.10
We expected a similar topochemical reaction for the olefinic
bonds in 1 which are separated by 3.659 Å and observed that
UV irradiation of a thin film of 1 resulted in a colour change
from yellow to almost colourless consistent with the loss of
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13 NMR data: 1H NMR d (300 MHz; TCE-d2) 7.00 (2H, d, J = 16.5 Hz,
olef.), 7.17 (2H, s, olef.), 7.43 (2H, d, J = 16.5 Hz, olef.), 7.57 (8H, s,
Ar-H); 19F NMR d (188 MHz, C7D8) 2143.81 (4F, m), 2157.47 (2F, t,
J = 21 Hz), 2163.75 (4F, m).
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