Biomedical Engineering Reference
In-Depth Information
h e ef ect of dif erent spacer lengths has been clearly demonstrated
(Table 9.4). Col
r
mesophase with a plane group
pm
was observed with
spacer length of C
11
,
while
Col
ob
had spacer length of C
8
. Compound
29c
with spacer length of C
6
was not LC. All these compounds exhibited glassy
transitions much below room temperature.
Similar compounds containing pyridinium
31a-e
instead of imidazo-
lium end groups have also been reported by Pal
et al.
(Figure 9.12) [76].
Synthesis of i ve compounds has been explored by varying spacer and
peripheral chain lengths. Increasing the spacer length connecting triphe-
nylene with pyridine moiety destabilized the mesophase while increasing
the number of carbon atoms on the peripheral chains of core-stabilized
columnar phase (Table 9.5).
h e ef ect of large substituents and of substituents carrying strong
dipole moments on the mesophase behavior and structure formation was
reported by Kettner and Wendorf [77]. Out of the nine compounds pre-
pared and studied, only compound
30
was ionic; it was prepared by reac-
tion of hydroxypenta(pentyloxy)triphenylene with corresponding acid
chloride which displayed monotropic behavior with isotropic tempera-
ture of 199 C and exhibited Col
h
phase on cooling at 168 C, which can be
supercooled to glassy state.
Sauer
et al
. [78] explored mesomorphic properties of compounds
prepared by addition of guanidinium chloride to triphenylene
32a-e
.
Displacement of Br in ω-bromo-substituted triphenylene by an amino
group was achieved by treatment of bromides with NaN
3
, followed by
reduction with LiAlH
4
; and i nally guanidinium moiety was introduced by
heating amines with tetramethylchloroformadinium chloride in the pres-
ence of base. Six compounds having the same spacer length but varying
Table 9.4
Phase transition behavior of unsymmetrical imidazole substituted
ionic triphenylenes.
Compound
X
Y
n
Phase transition behavior
29a
O
Br
5
Cr 67 Col 101 I
29b
O
I
5
Cr 64 Col
h
83 I
29c
COO
Br
5
Cr 71 I
29d
COO
Br
8
g -10 Col
ob
82 I
29e
COO
Br
11
g -20 Col
r
110 I
