During the first heating (v = 2 °C min21), the sample
transformed at 50–52 °C to a more birefringent shearable
texture which cleared at 128–130 °C. Subsequent slow cooling
(v = 2 °C min21) from the isotropic melt led, after a
considerable supercooling (ca. 10 °C), to the growth of a typical
texture (broken focal-conic) for a columnar mesophase. Further
cooling to room temperature produced, after an annealing of 24
h at 25 °C, a new one columnar texture.
Table 1 X-Ray data for molecule 1 at different temperatures
aLattice
Observed
Miller
T/°C
const./Å
spacing/Å
ind. (hkl)
Phase
Colh
25
a = 38.9
a = 36.6
33.7
19.7
17.5
12.6
4.5
31.7
4.5
3.6
(100)
(110)
(200)
(210)
b
The thermal behavior of 1 was examined by DSC (Mettler
Toledo S). First, the sample was homogenized by heating to the
100
(100)
Colho
b
isotropic. An exothermic peak at 139 °C (DH = 21.7 J g21
)
c
followed by a second transition at 233 °C, constituting of a
small peak (DH = 22.7 J g21) and a slope change in the heat-
flow curve, was obtained on cooling from the liquid state. Both
transitions reappeared in the second heating experiment.
However a new endothermic peak, which had not been detected
on the previous cooling, was registered at 51 °C (DH = 0.4 J
g21). These results agree with the microscope observation and
suggest the presence of two mesophases: the first from 233 to
51 °C and the second from 51 °C to the clearing point at 139 °C.
The two phases were stable during consecutive heating and
cooling processes. The small broad transition located around 20
°C in the second heating run does not seem to correspond to any
process observed, neither by polarized optical microscopy nor
by X-rays. To ensure that no polymerization nor degradation of
the acrylic system had taken place, the sample was analyzed by
1H NMR spectroscopy before and after the heating cycle. The
same spectra were obtained in both cases. In addition, the
sample was heated at 150 °C for half an hour and it remained
unchanged from spectroscopic data. No evidence for polymeri-
zation or degradation of the sample was detected.
The nature of these two liquid crystalline phases was
examined from their X-rays at different temperatures. The
sample was first led to its liquid state in order to homogenize it.
Then, several X-rays were registered cooling the porphyrin
from the liquid state, showing at any temperature from 120 to 25
°C, the same diffraction pattern, which corresponds to an
hexagonal columnar mesophase. The inter-columnar distance
was 36.6 Å (calculated from the (100) diffraction ring). In
addition, the broad halo located at 4.5 Å can be attributed to the
disordered alkoxy chains and the sharp ring that appeared at 3.6
Å can be attributed to the intermolecular periodicity along the
axis of the column. We conclude that the compound shows an
ordered hexagonal columnar mesophase Colho.
a Calculated from (100) diffraction ring. b Alkoxy chains. c Intracolumnar
periodicity.
temperatures and before the liquid state. However, the observed
behavior for molecule 1 seems to be different from the re-
entrant one, since the more disordered phase is observed only at
low temperatures, and at high temperatures does not appear
anymore. A similar phenomena to ours, has been described for
some truxene derivatives,15 in which an ordered hexagonal
columnar phase appeared at higher temperatures than dis-
ordered rectangular and nematic mesophases. This unusual
behavior could be a result of the competition between
intermolecular forces. As the temperature is raised it causes
some of the weaker bonds to be broken first and the
supramolecular organization resulting from the new balance of
intermolecular interactions may result in more ordered pack-
ing.15
The study of the mesomorphic properties of porphyrin
analogues of 1 with side chains of different length, together with
their corresponding metalloporphyrins, is now in progress.
Financial support from the Comisión Interministerial de
Ciencia y Tecnología, (Project no. PB96-1496) of Spain is
acknowledged. Mercè Castella gratefully thanks the Comisió
Interdepartamental de Recerca i Innovació Tecnològica (Gen-
eralitat de Catalunya) for a doctoral fellowship.
Notes and references
† All compounds had spectroscopic data (IR, 1H NMR, 13C NMR)
consistent with the assigned structure.
However, when the sample was annealed at 25 °C for 24 h, a
new diffraction pattern appeared, whose peaks are consistent
with a second two-dimensional hexagonal lattice Colh (Table 1)
in which the inter-columnar distance is now 38.9 Å (calculated
from the (100) diffraction ring). At this point, the sample was
again heated, so that the Colh remained invariable until 51 °C,
but at higher temperatures the above mentioned Colho re-
appeared until its isotropization. These results agree with those
obtained by DSC and polarized optical microscopy.
The dimension of the molecular structure of compound 1 was
calculated by the CS Chem3D 5.0 program, placing all the alkyl
chains in their total elongated conformation and it was found to
be 48.1 Å. Taking into account that the inter-columnar distance,
estimated by X-rays, is 38.9 Å and 36.6 Å, respectively, for the
two observed liquid crystal phases, we assumed that the
interdigitation between the aliphatic chains within adjacent
columns as well as their conformational disorder causes a
decrease of 20–25% in the chain length in both mesophases.
The asymmetrically substituted porphyrin 1 showed inter-
esting mesomorphic behavior, with two hexagonal columnar
mesophases from 233°C to its clearing point at 139 °C. It is
unusual to find the most ordered mesophase at higher
temperatures. However, this is clearly shown by the appearance
of an intra-columnar periodicity of 3.6 Å, as a result of the p–p
stacking, in the X-ray analyses when the sample was heated to
51 °C. Similar findings have been reported elsewhere.14,15 Thus
the re-entrancy phenomena is well known, even for a discotic
molecule,14 in which a less ordered phase appears both at low
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