The first liquid-crystalline, expanded porphyrins

Article abstract of DOI:10.1039/b307901p

Aliphatic derivatives of hydrazinophyrin, a new type of tetrapyrrolic macrocycle prepared by the condensation of 2,5-diformylpyrroles with hydrazine, exhibit liquid crystalline properties as judged from polarising optical microscopy; these hydrazinophyrin

Full text of DOI:10.1039/b307901p

The first liquid-crystalline, expanded porphyrins†
Jonathan L. Sessler,*a Wyeth Callaway,a Stephen P. Dudek,a Richard W. Date,b Vincent Lyncha and
Duncan W. Bruce*b
a Department of Chemistry and Biochemistry, Institute for Cellular and Molecular Biology, The University
of Texas at Austin, Austin, TX 78712-0165, USA. E-mail: sessler@mail.utexas.edu; Fax: 1-512-471-7550
b Department of Chemistry, University of Exeter, Stocker Road, Exeter, UK EX4 4QD.
E-mail: d.bruce@exeter.ac.uk; Fax: 44 1392263434
Received (in Cambridge, UK) 8th July 2003, Accepted 26th August 2003
First published as an Advance Article on the web 5th September 2003
Aliphatic derivatives of hydrazinophyrin, a new type of
tetrapyrrolic macrocycle prepared by the condensation of
2,5-diformylpyrroles with hydrazine, exhibit liquid crystal-
line properties as judged from polarising optical microscopy;
these hydrazinophyrins appear to be the first mesogens
derived from an expanded porphyrin core.
Hydrazinophyrins are dark red in solution with high extinc-
tion coefficients typical of fully conjugated expanded porphyr-
ins. For example, an extinction coefficient of 3 3 105 cm21
M21 is seen for the 419 nm Soret-like absorption of 1a in
CH2Cl2. On the other hand, the free-base form of hydrazino-
phyrin contains a 32 p-electron periphery and is therefore best
viewed as being non-aromatic or, possibly anti-aromatic, as
judged by the small downfield shifts of the pyrrolic NH protons.
Techniques for calculating and mapping ring currents in
porphyrins and expanded porphyrins are now available13 and
could, in due course, be used to analyse the electronic nature of
1a in more detail.
Specific insights into the structure of hydrazinophyrin came
from an X-ray diffraction analysis of the free base of 1a.‡ The
resulting structure (Fig. 1) revealed the presence of two
molecules of methanol bound inside the central macrocyclic
cavity. As expected for a fully conjugated system, the
macrocycle is planar, with average torsional angles of 177.3 ±
1.3° about the four hydrazines. The hydrazine units in the
macrocycle are all in a trans configuration, while the system as
a whole possesses C2 symmetry. Proton NMR spectroscopy
indicates that 1a and its homologues retain a C2 symmetry
element in solution; in particular, two of the four imine signals
and one of the two pyrrolic NH signals are shifted upfield
Functionalised liquid crystals are key components in displays,
optoelectronic devices, and a wide variety of optical sensors.1
Although most of the liquid crystals in current use are calamitic,
considerable effort has been devoted to investigating the
properties of discotic mesogens. Two of the more popular cores
for discotic mesogens are the tetrapyrrolic macrocycles por-
phyrin2 and phthalocyanine,3 and many groups have used these
cores to prepare a variety of liquid crystals with columnar
mesophases. This success has led us to consider using expanded
porphyrins to produce liquid crystals. Expanded porphyrins are
oligopyrrolic macrocycles that have ring sizes larger than
porphyrin, incorporate more than four heterocyclic subunits, or
have a greater number of p-electrons within their (generally)
conjugated peripheries.4 Much of the interest in expanded
porphyrins comes from their unusual optical properties (e.g.,
red-shifted absorption bands). However, expanded porphyrins
have also been shown to coordinate cations of the transition
metal,5 lanthanide,6 and actinide7 series, as well as acting as
novel anion binding agents.4,8 Expanded porphyrins have thus
attracted attention in a range of application areas, that run the
gamut from optical sensors7,8 to pharmaceuticals.9 To the best
of our knowledge, however, expanded porphyrins have not yet
been incorporated into liquid crystals. As a first step towards
developing expanded porphyrin-based liquid crystals, we
present here the synthesis of a new tetrapyrrolic macrocycle,
‘hydrazinophyrin’ (1).
Hydrazinophyrins 1a–d were synthesised by the acid-
catalysed condensation of 3,4-dialkoxy-2,5-diformylpyrroles10
with one equivalent of hydrazine (Scheme 1), and were fully
characterised by standard techniques. Yields for this key
macrocyclisation step ranged from 30 to 60%. In our hands,
only the ‘4 + 4’ condensation product was observed, although
other groups have obtained only the ‘2 + 2’ product under
related conditions.11,12
Scheme 1
† Electronic supplementary information (ESI) available: synthetic proce-
dures, hydrazinophyrin characterisation, and crystal structure data. See
http://www.rsc.org/suppdata/cc/b3/b307901p/
Fig. 1 Top and side views of the X-ray crystal structure of 1a. Two
molecules of methanol are bound in the cavity. Thermal elllipsoids are
scaled to the 50% probability level.
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relative to the others in CDCl3. These shifts in the NMR
spectrum lead us to suggest that two of the four pyrrolic protons
are involved in intramolecular hydrogen bonding involving
adjacent imine nitrogens. Consistent with this conclusion is the
finding that in DMSO-d6, the NMR spectrum shows degenerate
pyrrolic proton and imine proton signals, presumably because
this solvent serves to break up the internal hydrogen bonding
pattern.
The structure of 1a led us to consider that hydrazinophyrin
might represent a useful core for the generation of discotic
liquid crystals. To test this hypothesis, three different long-
chain 3,4-dialkoxy-2,5-diformylpyrroles were synthesised
using slightly modified literature procedures.11 These pyrroles
were then condensed with hydrazine to form the corresponding
hydrazinophyrins bearing eight alkoxy chains on the b-pyrrolic
positions containing six (1b), ten (1c), and fourteen (1d) carbon
atoms, respectively.
Compounds 1b to 1d were studied by polarising optical
microscopy. It was found that they were solid at room
temperature and rather immobile, although it was difficult to tell
whether they were crystalline. However, at around 50 °C they
became more mobile and were easy to spread around the slide
some ten degrees later. The lack of a defined event in the DSC
of these compounds suggests that the solid phase may then be a
glass. On heating above 120 °C, all compounds started slowly to
decompose and so it was not possible to define a meaningful
clearing point, although most were entirely isotropic/decom-
posed by around 160 °C. Once more, no event was observed by
DSC, supporting this assumption.
In terms of mesomorphism, it was apparent that once the
materials had softened, the compound was in the mesophase as
evidenced by the mobility and general birefringence observed
by microscopy. The decomposition of the materials at elevated
temperatures meant that textures could not be obtained on
cooling the isotropic phase as is commonly carried out.
Therefore the samples were heated to 90–100 °C, spread out and
annealed for periods of around 1 h. In this way, micrographs
such as that observed in Fig. 2 were obtained, showing clearly
that the material is in a columnar phase, although it is not
possible to determine the exact symmetry from this micro-
graph.
that the protonated forms of 1a are able to bind anions, such as
chloride, with high affinity in aprotic media. It may be possible
to incorporate liquid crystalline forms of hydrazinophyrin as an
integral component in anion-sensing detectors. Separately,
compounds 1b–d define a useful starting point for the
development of other types of expanded porphyrin-derived
liquid crystals.
This work was supported in part by the National Science
Foundation (grant CHE 0107732 to JLS) and by the Lev-
erhulme Trust (RWD).
Notes and references
‡ Crystal data for 1a: (thin needles, 0.41 3 0.11 3 0.09 mm): C17H22N6O5,
M = 390.41, monoclinic, a = 7.4409(1), b = 13.4669(2), c = 19.1745(4)
Å, a = 90, b = 90.1340(8), g = 90°, T 153 K, U = 1921.39(6) Å3, Z = 4,
m(Mo-Ka) = 0.71073 Å, 8262 reflections collected, 4363 independent
reflections (R(int) = 0.0264), R1 = 0.0654, 0.1008 (all data), wR(F2) =
0.1412, 0.1528 (all data). CCDC 216305. See http://www.rsc.org/suppdata/
cc/b3/b307901p/ for crystallographic data in CIF or other electronic
format.
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Fig. 2 Polarising photomicrograph of the columnar mesophase of 1b taken
at 95 °C on heating and after annealing for around 1 h.
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