Mesophasic and thermal properties of H-bonded asymmetric hetero-dimers (with one H-bond):

where n = 12 and 16) with acidic H-donors (Am, Bm, and Cm, where m = 12 and 16);

(ii) H-bonded symmetric trimers (with two H-bonds) consisting of bis-pyridyl H-acceptors (I-II) with acidic H-donors (Am and Bm, where m = 12 and 16).

2.3. Results and Discussion

2.3.1. Mesophasic and thermal properties of H-bonded asymmetric hetero-dimers (with one H-bond):

2.3.1.1. Four- and five-ring systems (IIIn-Am, IIIn-Bm, and IVn-Am). In order to understand the influence of H-bonded sites (at the rigid cores) on mesomorphic, molecular stacking, and thermal properties, H-bonded four- and five-ring asymmetric hetero-dimers, i.e., IIIn-Am, IIIn-Bm, and IVn-Am (n, m = 12 and 16), were

Figure 2.2. POM textures at the cooling process: (a) the polar smectic phase with the spherulite texture of complex IV12-A12 at 96 °C; (b) the polar smectic phase with spherulite and non-specific grainy textures of complex IV12-B16 at 100 °C; (c) the polar smectic phase with spherulite and stripe textures of complex V12-A12 at 100 °C;

(d) the Col_r phase with dendritic- and mosaic-like textures of complex V12-B12 at 120 °C; (e) the polar smectic phase with the fan-like texture of complex V16-B16 at 140 °C; (f) the Col_r phase with dendritic- and mosaic-like textures of complex V12-C12 at 130 °C; (g) the smectic A phase with the fan-like texture of complex III16-B16 at 110 °C. (h) the smectic A phase with the fan-like texture of complex III16-A16 at 95 °.

investigated by POM and DSC measurements. In addition, an analogous fully covalent-bonded structure S12 (n = 12) with five rings (see Figure 2.1), which has been reported by Pelzl et al,^[2,4] was compared as well. Furthermore, their mesophasic textures, phase transition temperatures, enthalpy values are shown in Figure 2.2 (a, g,

and 16) possessed the smectic A (SmA) phase, which were verified by POM to show the enantiotropic fan-like texture. For instance, the fan-like texture of complexes III16-A16 and III16-B16 are demonstrated in Figs. 2h and 2g, respectively. However, complexes IVn-Am revealed a polar smectic (B2 or SmCP) phase[11,27a,52]

in both heating and cooling processes, and the POM texture of complex IV12-A12 is shown in Figure 2.2a.

Regarding H-bonded four- and five-ring asymmetric hetero-dimers IIIn-Am and IIIn-Bm with different bent-core lengths (4 and 5 rings) but the same near-central H-bonded sites at the rigid cores, complexes IIIn-Bm bearing longer bent-core lengths (5 rings) possessed higher phase transition temperatures and broader SmA phase ranges than analogous complexes IIIn-Am (4 rings). To compare the mesophasic type of five-ring complexes IIIn-Bm and IVn-Am with different H-bonded sites at the rigid cores, the SmCP and SmA mesophases were achieved for supramolecular mesogens with far- and near-central H-bonded sites, respectively. This phenomenon suggested that the polar smectic phase would be preferred if the H-bonded site was far away from the bent-core center in supramolecular design, which means the higher stability of the SmCP phase was induced by longer covalent-bonded bent cores of IVn in complexes IVn-Am. Moreover, the isotropization temperatures of complexes IIIn-Bm are higher than those of complexes IVn-Am due to the relatively higher isotropization temperatures of H-donors Bm with longer rigid cores in analogous Am and Bm (m = 12 and 16). In comparison with analogous compound S12, complexes IVn-Am show lower phase transition temperatures and similar mesophasic ranges.

Table 2.1. Phase Transition Temperatures and Enthalpies of H-Bonded Four- and Five-Ring Asymmetric Hetero-Dimers (with One H-Bond)

O O

Complex n m x Phase transition temperature/^oC [Enthalpy/kJ/g]

III12-A12 12 12 1 I 114.3 [20.5] SmA 71.2 [69.8] K transitions of compound S12 were obtained as I 119.0 [22.5] CmCP 109.0 [41.9] K.

2.3.1.2. Six-ring systems (IIIn-Cm, IVn-Bm, and Vn-Am). Three series of comparable six-ring asymmetric hetero-dimers, i.e., IIIn-Cm, IVn-Bm, and Vn-Am (n, m = 12 and 16), were investigated for the influence of different H-bonded sites (at

Table 2.2. Phase Transition Temperatures and Enthalpies of H-Bonded Six-Ring

Complex n m Phase transition temperature/^oC [Enthalpy/kJ/g]

III12-C12 12 12 I 147.2 [20.3] N 120.1 [2.7] K crystalline state. The phase transitions were measured by DSC at the 2nd cooling scan with a cooling rate of 5 °C/min.

in Table 2.2 and Figure 2.3b. With respect to the mesophasic types, the enantiotropic nematic phase was obtained in complexes IIIn-Cm (n, m = 12 and 16) with near-central H-bonded sites (at the rigid cores) to indicate their loose molecular stackings, but analogous complexes IVn-Bm and Vn-Am (n, m = 12 and 16) exhibited SmCP phases owing to their far-central H-bonded sites, whose trends are

the same as five-ring asymmetric hetero-dimers IVn-Am with H-bonded sites far away from the bent-core centers. The mesophasic textures were examined by POM experiments, for instance, complex IV12-B16 revealed spherulite and non-specific grainy textures in Figure 2.2b, and complex IV12-A12 exhibited spherulite and stripe textures in Figure 2.2c, which suggested the polar smectic (SmCP) phase.

Comparing the phase transition temperatures of six-ring asymmetric hetero-dimers IIIn-Cm, IVn-Bm, and Vn-Am (Figure 2.3b), complexes IIIn-Cm revealed the highest isotropization temperatures due to the relatively much higher isotropization temperatures of ingredients in H-donors Cm with the longest rigid cores (see Supporting Information Figure S2.1 and Table S2.1) in analogous Am, Bm, and Cm (m = 12 and 16). Similar phenomena were also displayed in isotropization temperatures of five-ring complexes IIIn-Bm and IVn-An.

Meanwhile, the SmCP phase ranges of Vn-Am are slightly wider than those of IVn-Bm, which might be due to the higher stability of the SmCP phase caused by longer covalent-bonded bent cores of Vn in complexes Vn-Am.

2.3.1.3. Seven- and eight-ring systems (IVn-Cm, Vn-Bm, and Vn-Cm).

Three series of H-bonded seven- and eight-ring asymmetric hetero-dimers, i.e., IVn-Cm, Vn-Bm, and Vn-Cm (n, m = 12 and 16), were investigated for the influence of different H-bonded sites (at the rigid cores) and ring numbers on their mesophasic types and phase transition temperatures as shown in Table 2.3 and Figure 2.3c. Due to the large variation of solubilities in H-donors Cm and H-acceptors IVn, phase separation occurred in the preparation of complexes IVn-Cm, so these complexes could not be compared in this study. In regard to the mesophasic types, the nematic phase was observed in both series of complexes Vn-Bm and Vn-Cm. Besides, the rectangular columnar (Colr or B1) and SmCP

Table 2.3. Phase Transition Temperatures and Enthalpies of H-Bonded Seven- and Eight-Ring Asymmetric Hetero-Dimers (with One H-Bond)

O

Complex n m Phase transition temperature/^oC [Enthalpy/kJ/g]

IV12-C12 12 12 Phase separation measured by DSC at the 2nd cooling scan with a cooling rate of 5 °C/min.

the rigid cores), where complex V12-B12 demonstrated the Col_r phase due to its shorter flexible chain length (n, m = 12). However, analogous complexes Vn-Cm (n, m = 12 and 16) with eight rings (at the rigid cores) exhibited only the Col_r phase owing to their longer rigid cores in contrast to complexes Vn-Bm (with 7 rings). The mesophasic textures were evidenced by POM experiments, for example, complex V16-B16 displayed the fan-like texture in Figure 2.2e as the evidence of the polar smectic (SmCP) phase, and complexes V12-B12 and V12-C12 exhibited dendritic-like and mosaic-like textures in Figures 2.2d and 2.2f, which were the

symbolic textures of the Col_r phase. In general, complexes Vn-Cm (with 8 rings) had higher transition temperatures and wider nematic phase ranges than Vn-Bm (with 7 rings), where the wider nematic phase ranges in complexes Vn-Cm is due to their higher length ratios of rigid cores (with 8 rings) to flexible chains.

Overall, comparing all H-bonded asymmetric hetero-dimers (with one H-bond), complexes IVn-Am, IVn-Bm, Vn-Am, and Vn-Bm (n, m = 12 and 16, except Vn-Cm), and symmetric trimers: (d) five- and seven-ring systems (I-Am and II-Am).

2.3.2. Mesophasic and thermal properties of H-bonded symmetric trimers (I-Am, I-Bm, II-Am, and II-Bm with two H-bonds)

Four series of analogous symmetric trimers, i.e., I-Am, I-Bm, II-Am, and II-Bm (m = 12 and 16) with 5, 7, and 9 rings, were investigated for the influence of different ring numbers and H-bonded sites (at the rigid cores) on their mesophasic types and phase transition temperatures as shown in Table 2.4 and Figure 2.3d.

Due to the large variation of solubilities in H-donors Am, Bm, and H-acceptor II, phase separation occurred in the preparation of complexes II-Am and II-Bm, so these complexes could not be compared in this study. Regarding the mesophasic types, a tilt smectic phase was observed in both series of complexes I-Am and I-Bm (m = 12 and 16), where a SmX phase was obtained in complexes I-Bm with seven rings (at the rigid cores). The mesophasic textures were observed by POM experiments, for instance, complex I-A12 revealed the tilt smectic phase with spherulite and schlieren textures in Figure 2.4a, and complex I-B12 exhibited the tilt smectic (SmC) phase with the fan-like texture in Figure 2.4b, which were the evidence of the tilt smectic (SmC) phase. Besides, complex I-B12 demonstrated the undefined smectic phase with the arced fan-like texture in Figure 2.4c.

Figure 2.4. POM textures at the cooling process: (a) the till smectic phase with spherulite and schlieren texture of complex I-A12 at 100 °C; (b) the till smectic phase with the fan-like texture of complex I-B12 at 130 °C; (c) the undefined smectic phase with the arced fan-like texture of complex I-B12 at 80 °C.

To compare the phase transition temperatures of symmetric trimers I-Bm and I-Am (m = 12 and 16) (Figure 2.3c), complexes I-Bm (with 7 rings) possessed higher isotropization temperatures and wider SmC phase ranges than complexes I-Am (with 5 rings) due to the longer rigid core of I-Bm. Overall, in contrast to H-bonded asymmetric hetero-dimers (with one H-bond), all H-bonded symmetric trimers (I-Am and I-Bm with two H-bonds) do not show the SmCP phase due to higher flexibilities of two H-bonds in the supramolecular complexes.

Table 2.4. Phase Transition Temperatures and Enthalpies of H-Bonded Symmetric Trimers (with Two H-Bonds)

Complex m Phase transition temperature/^oC [Enthalpy/kJ/g]

I-A12 12 I 108.0

II-A12 12 Phase separation

II-A16 16 Phase separation

II-B12 12 Phase separation

II-B16 16 Phase separation

I = isoptropic state; SmC = normal tilt smectic phase without polar switching behavior; B1 = column rectangular (Colr) phase; SmX = undefinded smectic phase;

K = crystalline state. ^a means the temperature data is observed in POM only. The phase transitions were measured by DSC at the 2nd cooling scan with a cooling rate of 5 °C/min.

2.3.3. IR characterization

In order to prove the formation of supramolecules, (compared with the transition temperatures of individual components in Figure S2.1 and Table S2.1 of the Supporting Information) new transition temperatures and homogeneous phase transitions of H-bonded complexes would be observed in DSC and POM measurements, respectively. In addition, the existence of H-bonds in these H-bonded complexes can be characterized by IR spectra at various temperatures.

Therefore, two examples of asymmetric and symmetric H-bonded complexes are demonstrated as follows:

The IR spectra of H-bonded asymmetric complex V16-B16 (with one H-bond) and its constituents V16 (H-acceptor) and B16 (H-donor) were compared in Figure 5 to examine the H-bonds in crystalline and mesophasic states. In contrast to the O-H band of pure B16 (self-H-bonded dimeric acids) at 2546 cm^-1, and the weaker O-H bands observed at 2506 and 1920 cm^-1 in the H-bonded complex V16-B16 were indicative of hydrogen bonding between the pyridyl group of H-acceptor V16 and acidic group of H-donor B16. On the other hand, a C=O stretching vibration appeared at 1742 cm^-1 in complex V16-B16, which showed that the carbonyl group is in a less associated state than that in pure B16 with weaker C=O stretching vibration appeared at 1729 cm^-1 either in crystalline phase or mesophases (Figures 2.5a and 2.5b).^[53] Both results suggested that H-bonds formed between B16 and V16 in both solid and mesophasic states of complex V16-B16.

In addition, similar IR analysis of H-bonds in symmetric H-bonded complex I-B16 (with one H-bond) was inspected at various temperatures. With the IR evidence of weak O-H band at 2516 and 1914 cm^-1 and less association of C=O stretching vibration at 1740 cm^-1 as shown in Figs. 5c and 5d, it revealed the successful supramolecular framework of H-bonded complex I-B16 by complexation of H-donor

B16 and H-acceptor I in 2:1 molar ratio.

3200 3000 2800 2600 2400 2200 2000 1800 1600 1400

wavenumber (cm-1)

3200 3000 2800 2600 2400 2200 2000 1800 1600 1400

wavenumber (cm-1)

3200 3000 2800 2600 2400 2200 2000 1800 1600 1400

wavenumber (cm-1)

3200 3000 2800 2600 2400 2200 2000 1800 1600 1400

wavenumber (cm-1)

Figure 2.5. IR spectra of H-bonded asymmetric hetero-dimeric complex V16-B16 (a) at variable temperatures and (b) its composed moieties (at room temperature);

H-bonded symmetric trimeric complex I-B16 (c) at variable temperatures and (d)