PREFACE
2.2. Results and Discussion
2.2.2. Thermal behavior
2.2.2.2. S-annulated perylene tetraesters
Stability of the compounds has been determined by TGA and it has been shown that these compounds were stable up to at least 280 oC and complete decomposition occurs at
≈550 oC (Figure 2.7).
Figure 2.7. TGA plots of compound 1a-d (heating rate of 10 oC/min, Nitrogen atmosphere).
the molecules known to enhance homeotropic (face-on) alignment over homogeneous alignment (edge-on) on polar surfaces (Figure 2.6c and d).24b In literature, it is also reported that discotic molecules where the core is connected to several flexible tails through ester groups have shown Colh phase with good homeotropic alignment.2b,17b,27 This is probably due to the intercolumnar dipole-dipole interaction exerted by the ester carbonyl group forcing the efficient correlation of columns over a wide thermal range.27c Further cooling of the mesophase results in a fibrous texture. This change in the optical texture is only due to the minor change in the alignment of the columns and not due to any mesophase transition as evidenced from the XRD studies. Finally the mesophase was crystallized at 58.6 oC (ΔH = 164.2 kJ/mol, Figure 2.8b).
Figure 2.8. (a) DSC thermograms of discotics 2a; (b) 2b; (c) 2c and (d) 2d showing the second heating (red trace) and the first cooling (blue trace) scans at a scanning rate of 5.0 ˚C min–1.
Figure 2.9. (a) Photomicrograph of compound 2b at 185 oC on cooling from isotropic liquid state (under crossed polarizer); (b) XRD profile depicting the intensity against 2θ obtained for the Colh phase of compound 2b at 100 oC.
Powder XRD measurements were done at temperatures 100 oC to elucidate mesophase exhibited by compound 2b. The data obtained on indexing the one-dimensional (1D) intensity vs. 2θ profile obtained for the Col phases are tabulated in Table 2.3. The XRD profile of the Col phase showed (Figure 2.9b) at 100 oC showed a sharp reflection corresponding to a Bragg spacing d100 at 17.33 Å and another corresponding to a Bragg spacing d110 at 10.10 Å. The d spacings corresponding to first two reflections are in the ratio of 1:0.58. Two diffused reflections were found at wide-angle region, one at 4.48 Å and the other at 3.54 Å. First diffused reflection corresponds to the ordering of alkyl chains, while the second diffused reflection corresponds to the ordered packing of rigid cores. All these features along with the optical textural patterns confirm that the Col phase is having a hexagonal symmetry. From the value of d100 one calculate the hexagonal cell parameter
‘a’ using the formula a = d100/Cos 30 or 2/√3 × d100; and in this case, the value of ‘a’ is found to be 20.01 Å. The value of lattice parameter ‘a’ calculated was found to be 27%
less, when compared to the diameter obtained from molecular modeling software (32.84 Å). The observed decrease in the value of lattice parameter can be ascribed to the flexible chain folding or their interdigitation in the neighboring columns.23a,b The columnar slice thickness is given by core-core stacking distance hc, which is 3.54 Å. This is also denoted as the lattice parameter ‘c’for theColh phase. The number of molecules ‘Z’ filling a unit cell of height hc = 3.54 Å can be calculated from the relation Z = δ × NA × Vunit cell
/M. For organic materials density δ is generally assumed as 1 g/cm3, molecular mass M for 2b is 907.26 g/mol and NA isAvogadro’s number. Volume of the hexagonal unit cell is given by the relation: Vunit cell =S × hc, where S is lattice area, which is calculated using the
(a) (b)
formula S = a2 × sin 60. After substituting all the values thus obtained (Table 2.3), the number of molecules filling the columnar slice i.e. Z is found to be 0.96. This means each stratum in the Col phase is made up of a single molecule, and packed in the columns with an intracolumnar distance 3.53 Å.
Table 2.3. Results of (hkl) indexation of XRD profiles of the compounds at a given temperature (T) of mesophasesa
Other homologues of this series i.e. compounds 2a, 2c and 2d exhibited enantiotropic hexagonal columnar phase with the characteristic optical texture and corresponding XRD profile (Figure 2.10a, 2.11a and c). The mesophase range decreased on going from lower homologue 2a to higher homologue 2d. Thus, the increase in the peripheral chain length leads to a lowering of clearing temperatures. TGA analysis showed that these compounds were stable up to ≈300 oC and complete decomposition occured at
≈550 oC (Figure 2.12).
Compounds (D/Å)
Phase
(T/oC) dobs (Å) dcal (Å) Miller indices (hkl)
lattice parameters (Å), lattice area S (Å2), molecular volume V (Å3)
2a (27.38 )
200
17.29 10.02 4.46 (ha) 3.55 (hc)
17.29 10.03
100 110 001
a = 19.97 c = 3.55 S = 345.1897 V = 1225.42 Z = 0.93
100
17.33 10.10 4.48 (ha) 3.54 (hc)
17.33 10.05
100 110 001
a = 20.01 c = 3.54 S = 346.7906 V =1227.64 Z = 0.93
2b
(32.84) 100
18.81 10.97 4.49 (ha) 3.53 (hc)
18.81 10.91
100 110 001
a = 21.72 c = 3.53 S = 408.5513 V = 1442.1861 Z = 0.96
2c
(37.38) 95
20.5 11.79 10.17 4.56 (ha) 3.54 (hc)
20.50 11.89 10.25
100 110 200 001
a = 23.67 c = 3.54 S = 485.2637 V = 1717.8355 Z = 1.02
2d
(41.85) 90
21.52 12.52 10.85 4.56 (ha) 3.49 (hc)
21.52 12.48 10.76
100 110 200 001
a = 24.85 c = 3.49 S = 617.4819 V = 1866.2942 Z = 0.99
aThe diameter (D) of the disc (estimated from Chem 3D Pro 8.0 molecular model software from Cambridge Soft). dobs: spacing observed; dcal: spacing calculated (deduced from the lattice parameters; a for Colh
phase; c is height of the unit cell). The spacings marked ha and hc correspond to diffuse reflections in the wide-angle region arising from correlations between the alkyl chains and core regions, respectively. Z indicates the number of molecules per columnar slice of thickness hc estimated from the lattice area S and the volume V.
Figure 2.10. (a) Photomicrograph of compound 2a at 232 oC on cooling from isotropic liquid state (under crossed polarizer); (b) XRD profile depicting the intensity against 2θ obtained for the Colh phase of compound 2a at 100 oC.
Figure 2.11. (a) Photomicrograph of compound 2c at 146 oC on cooling from isotropic liquid state (under crossed polarizer); (b) XRD profile depicting the intensity against 2θ obtained for the Colh phase of compound 2c at 95 oC; (c) Photomicrograph of compound 2d at 100 oC on cooling from isotropic liquid state (under crossed polarizer); (d) XRD profile depicting the intensity against 2θ obtained for the Colh phase of compound 2d at 90 oC.
(a) (b)
(c) (d)
(a) (b)
Figure 2.12. TGA plots of compound 2a-d (heating rate of 10 oC/min, Nitrogen atmosphere).