Biomedical Engineering Reference
In-Depth Information
in
1c
and
1d
resulted in a dramatic change in the phase properties giving
ribbon-like LCs dispersed in isotropic solution, which persists even at a
very high concentration (e.g., 30%).
Interestingly, the introduction of stereogenic centers (
3c
and
3d
) in the
alkyl chains resulted in chiral chromonic phases that are due to the forma-
tion of a helical twist in the column of molecules. h e achiral analogue
3a
and
3b
showed no characteristics of chiral phases; only schlieren textures
of N phases were observed.
h e ef ects of aromatic ring structures were investigated by compar-
ing the properties of
1
,
4
and
6
. Compound
1a
and
6
showed a dramatic
dif erence in solubility as well as electronic absorption properties. Dye
6
was only soluble in acids, such as HCOOH, CF
3
COOH and concentrated
H
2
SO
4
, but not in water. As a result of more extended molecular core,
the λ
max
of absorption of dye
6
is 830 nm, whereas λ
max
for
1a
is 530 nm.
Interestingly, both
1a
and
6
form H-aggregates and chromonic N phases
despite the dif erence in solubilities.
Changing from
1a
to
4
lowered the solubility of the latter in aque-
ous solutions, presumably a decrease in the charge-to-surface area ratio.
Whereas,
4a
was soluble in hot water forming a N phase,
1d
is very much
soluble in water and formed chromonic M ribbons at high concentrations.
h ere are some studies (single crystal X-ray and polarized absorption
spectra) that have been reported for
4b
at room temperature. Single crystal
analysis showed that in the compound
4b,
the perylenemonoimide rings
stacked to each other, which is driven by strong π-intercations between the
rings. To minimize repulsion between the quaternized ammonium ions,
the ionic perylenemonoimides stacked anti-parallel to each other along
a column. h is crystal packing is stabilized by electrostatic attractions
between ammonium ions and sulphonate ions. It is true that molecular
packing of
4b
will be dif erent from that in the chromonic N phase because
of geometry that controls molecular packing in crystals and the amount of
solvent molecules present.
Phthalocyanine and porphyrin derivatives (Table 9.1) have also
been shown to form chromonic mesophases. For example, copper(II)
and nickel(II) complexes of 2,3,9,10,16,17,23,24-octa(dodecyloxy)
phthalocyanine and copper(II) complex of 2,9,16,23-tetra(decylthio)
phthalocyanine (Figure 9.3) possess both thermotropic and lyotropic
mesomorphism with apolar organic solvents, which can be attributed to
chromonic type. h ese materials are very attractive for electronic appli-
cations because of their high mobilities for photoinduced charge car-
riers. h e existence of Col
N-I
phase transition shown by these lyotropic
systems form the basis for their uses to study the radiation-induced
conductivity-dependence on the phase state of discotic mesogens. h e
