Chemistry Reference
In-Depth Information
6
Halogen-bonded Liquid Crystals
It was from this background in hydrogen bonding that our own work in halo-
gen bonding had its genesis. We became aware of the work published by the
Milan group [39] and considered whether we could use such a general ap-
proach to form new liquid-crystalline species. We had worked with stilbazoles
since the mid-1980s and they seemed the obvious choice of Lewis base, with
iodopentafluorobenzene a good starting point as a source of electron-poor
halide.
The first thing that became immediately apparent is that the synthetic
route had to be re-evaluated. In hydrogen-bonded systems, it is normally suf-
ficient to mix the two components in a common solvent and then remove it to
leave the pure complex. On occasions when this does not work (often due to
the high lattice energy/low solubility of one component), it is necessary only
to heat the mixture into the melt for a few minutes and then allow the whole
thing to cool down. In the case of the halogen-bonded materials, however, this
was often not the case and in many cases attempts to proceed in this way led
to materials that were clearly not single component in nature, as evidenced
by the observation of more than one melting event and of biphasic behaviour.
Clearly then, however stable a halogen bond might be, it would appear to be
much more labile than an analogous hydrogen bond. Therefore, the approach
adopted by us and followed since has been to try to grow single crystals of the
complexes and work exclusively with those, although it is apparent that this
is not always necessary. This presents obvious immediate advantages (lots of
crystal structures) and disadvantages (some complexes will simply not crys-
tallise). Thus, single crystals were obtained of complex
13
-
n
for
n
=4,6,8,10
and 12 and the mesomorphism was determined. X-Ray single crystal struc-
tures were also obtained for
n
= 8 and 10 [40]
1
.
The molecular structure of
13
-8 (Fig. 12) shows the presence of the halo-
gen bond (
d
N
···
I
= 2.811(4)
A
compared with the sum of the van der Waals'
radii of 3.53A)withaN
ˆ
I-C angle of 168.4
◦
.Thestructureof
13
-10 was
also determined, and this time the following parameters were observed:
d
N
···
I
= 2.789
A
;N
···
ˆ
I-C = 177.9
◦
.
Analysis of the packing of the complexes (e.g. Fig. 13) also showed the
absence of quadrupolar phenyl/perfluorophenyl interactions, an observation
···
1
Nguyen HL, Horton PN, Hursthouse MB, Bruce DW, unpublished work
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