J O U R N A L O F
Liquid-crystalline phenanthrolines
Sandrine J. P. Bousquet and Duncan W. Bruce*
School of Chemistry, University of Exeter, Stocker Road, Exeter, UK EX4 4QD.
E-mail: d.bruce@exeter.ac.uk; Fax: z44 1392 263434; Tel: z44 1392 263489
C H E M I S T R Y
Received 6th April 2001, Accepted 10th May 2001
First published as an Advance Article on the web 22nd May 2001
The synthesis and mesomorphism of the first liquid-
crystalline phenanthrolines are reported.
bipyridine chemistry is facilitated by the availability of various
isomers, both symmetric and unsymmetric, of dimethyl
derivatives which are readily accessed on a good scale using
laboratory procedures. However, while certain substituted
phenanthrolines are commercially available, none are available
with a 3,8-substitution pattern which would be ideal for the
realisation of calamitic mesogens. Nor are they easily made,
and the classical Skraup syntheses do not cope readily with
such a substitution pattern.
Like bipyridines, phenanthrolines are ligands to a number of
photoactive complexes11 and the photoactivity of these
complexes has, however, found an outlet in, for example, the
fast-growing field of dendrimer chemistry and in the construc-
tion of three-dimensional arrays. This has led to development
of the chemistry of the bipy and phen ligands. Thus, we have
used the reports by Siegel12 and by Yamamoto13 of improved
syntheses for 3-bromo (2) and 3,8-dibromophenanthrolines (3)
as the basis for the construction of new mesomorphic materials.
The bromophenanthrolines are obtained in a direct bromi-
nation12 which leads to moderate yields of the 3-bromophe-
nanthroline (38%, separated by column chromatography) and
quite good yields13 of the 3,8-dibromophenanthroline (60%).
With the advent of many different protocols for performing
cross-coupling reactions, having the bromo compounds in
hand it is therefore a rather straightforward task to produce a
range of substituted phenanthrolines. In order to demonstrate
this concept, we now report the synthesis and mesomorphic
properties of three derivatives which we believe represent the
first examples of mesomorphic phenanthrolines.
The bromophenanthrolines were obtained as described
previously and were then coupled (Scheme 1) with either
acetylenes or phenylboronic acids, using Pd-catalysed proce-
dures, to give a range of new mesogens. Some of these were
then further reacted in a step-wise fashion to give mono- and
di-methyl derivatives (8 and 9). The integrity and purity of the
new compounds was established by NMR spectroscopy and by
CHN analysis, while liquid crystal properties were determined
by polarised optical microscopy and differential scanning
calorimetry. Thermal data are collected in Table 1.
First, none of the 3-monosubstituted phenanthrolines (4, 5)
was mesomorphic and the compounds simply melted on
heating. However, when symmetric, 3,8-disubstituted materials
were made, the situation changed and materials with both
smectic and nematic phases were found. Thus, the bis(acetyl-
ide) 6a showed a wide-range nematic phase, while extension of
the chain introduced a SmC phase. Neither of the two 3,8-
diphenyl derivatives showed a nematic phase with 7a showing a
SmA phase and 7b and SmC phase, with a monotropic crystal
smectic phase observed on cooling. Note that in all these cases,
the melting points were above 100 uC, while the clearing points
approached 300 uC, often with decomposition.
In studies of metallomesogens, there are reported many metal–
ligand combinations; in some of these the ligand is meso-
morphic, while in others it is not.1 In the latter case
coordination induces mesomorphism, often for structural
reasons and the metal can be defined in a particular roˆle.
However, in the former case, we are presented with another
opportunity, namely to study the effect of the metal on the
mesomorphism of the ligand.
The first person to do this systematically with calamitic
mesogens was Ghedini who took a range of mesomorphic
azobenzene mesogens and complexed them via orthometalla-
tion to Pd(II).2 In so doing, he created ‘H-shaped’ mesogens
which had mesophases which were different from those shown
by the ligand, although they were found at considerably higher
temperatures. Since this work, published in the early 1980s,
many groups have worked with this motif and a range of
properties has been identified, including ferroelectric response.1
Complexation of metals has the possibility to influence the
properties of ligands in a number of ways. For example, we
have shown how very high polarisability3 and birefringence4
are a consequence of complexation, while we and others have
found that certain metal complexes are highly dichroic.5
In order that such studies advance, it is necessary continually
to identify new ligand types, both mesomorphic and non-
mesomorphic, which can bind to metals and generate
mesomorphic complexes. The choice of ligand will determine
the metals which can be complexed and in turn, these will
determine which if any specific properties may be introduced.
One of the ligands with the greatest range of coordination
chemistry is 2,2’-bipyridine and we6 and Ziessel7 have shown
that by appropriate 5,5’-substitution, it is possible to generate
mesomorphic analogues. Subsequently, we used these to
generate mesomorphic complexes.8 One of the interesting
features of 2,2’-bipyridine is that it exists in a transoid state
when uncomplexed and so has no net dipole. However, on
complexation, the conformation changes to cisoid in order to
facilitate N,N-coordination (although N,C-coordination via
orthometallation is known) conferring on the ligand a lateral
dipole of the order of some 3.5 D (calculated for the gas-phase
structure).9
By comparison 1,10-phenanthroline, which has a similarly
wide coordination chemistry, has its conformation fixed by the
exocyclic double bond, is ready set up for N,N-coordination
and has a permanent lateral dipole. However, while Ziessel has
recently reported the use of substituted non-mesogenic
phenanthrolines to generate mesomorphic copper(I) com-
plexes,10 there are, to our knowledge, no reported examples
of uncomplexed, mesomorphic phenanthrolines. Undoubtedly,
one of the reasons for this is the relative unavailability of
suitably substituted phenantholines from which to start. Much
These temperatures are clearly high and so it was of interest
to seek strategies to see whether they could be reduced. Two
DOI: 10.1039/b103150n
J. Mater. Chem., 2001, 11, 1769–1771
This journal is # The Royal Society of Chemistry 2001
1769
Bousquet
