Gelation And Liquid Crystalline Behaviour Of Dihydrazide Derivatives And Bi-1, 3, 4-oxadiazole Derivatives

Self-assembled systems, such as supramolecular gels and liquid crystals (LCs), are fascinating organized soft materials that can respond to external stimuli such as temperature, electrical pulses, light and chemicals. There are potential applications in template synthesis, controlled release, separations, and biomimetics. Supermolecule structures are the result of not only additive but also cooperative interactions, including hydrogen bonding,π-πstacking, donor–acceptor interactions, hydrophobic forces, metal coordination and van der Waals interactions. Among the noncovalent interactions, hydrogen bonds are one of the most important interactions in the self-assembly of molecules because of their strength, directionality, reversibility, and selectivity. The donor–acceptor interactions take place mainly between heterocyclic electron donors (porphyrins, oxadiazole etc.) and aromatic electron acceptors (phenyl, C60 etc.). The variation of the donor or acceptor strength and the substitution scheme considerably influences the electronic behavior of the compound and therefore its optical properties like absorption and fluorescence.In this context, as part of our continuing research in supramolecular self-assembly, we report the preparation and self-assembly behaviour of five new series of compounds derived from dihydrazide (BPH-n and BFH-n) and symmetric bi-1,3,4-oxadiazole core (BBOXD-n, BBOXD-Bn and BBOXD-Tn). Furthermore, the photophysical properties have been investigated. These discoveries of the new materials are examples of the continuing effort to explore the limits of molecular structures compatibility with self-assembly behaviour.1. synthesis and self-assemble behaviors of dihydrazide derivatives.Two novel classes of dumbbell-shape dihydrazide derivatives have been synthesized, namely 1,4-bis[(3,4-bisalkoxyphenyl)hydrozide] phenylene (BPH-n ) and Oxalyl N',N'-bis(3,4-dialkoxybenzoyl)-hydrazide (BFH-n). 1H NMR, FT-IR and Elemental Analysis were employed to confirm their molecular structures.(1) Length of terminal alkyl chains has a critical effect on liquid crystallinity. BPH-4 is nonmesomorphic. Higher homologues BPH-n (n=6, 8, 10) exhibited thermotropic hexagonal column (Colh) mesophase, which are stable at room temperature as revealed by DSC, POM, and XRD studies. BFH-4 only showed monotropic rectangular phase during cooling from its isotropic phase. BFH-n (n=6, 8, 10) exhibited enantiotropic columnar mesophases and the symmetry of the mesophase changes from rectangular to hexagonal on increasing the temperature. The rectangular columnar mesophases of BFH-n (n = 6, 8, 10) remained stable down to 10℃during cooling and the subsequent recrystallisation from the Colr phase of BFH-n (n = 6, 8, 10) was observed on the second heating runs. Furthermore, the average number of molecules packing in a column slice was estimated to be three, based on their X-ray diffraction results.(2) Temperature-dependent 1H NMR spectroscopic experiments were performed for BPH-6 in 20% DMSO-d6/CDCl3 to confirm the primary involvement of N-H protons in intermolecular hydrogen bonding. Interestingly, there are two different hydrogen-bonding modes in BFH-n. The NH-1 protons were favorable to form an intramolecular hydrogen bonding with C=O groups of adjacent molecules. However, the NH-2 protons were involved in intramolecular hydrogen bonding with C=O groups, forming two six-membered rings. Intermolecular or intramolecular hydrogen bonding between–C=O and -N-H groups in crystalline and liquid crystalline phases was further confirmed by temperature-dependent FT-IR spectroscopy.(3) BPH-n have shown great ability to gel a variety of organic solvents to form stable organogels with the critical gelation concentration as low as 8.7×10-4 mol L-1 (0.06 wt %). The formed gel has a high gel-sol transition temperature (Tgel) at low gelation concentration. Aggregation-induced emission (AIE) has been observed after gelation though conventional chromophore units not incorporated in BPH-n. The fluorescence quantumyields of xerogel are 2 orders higher than that of dilute solution. It was attributed to the combination of the partial double-bond properties of C-N bonds in the hydrazide group, the restricted intramolecular rotational motions in aggregate state, and J-aggregation by hydrogenbonding interactions between the hydrazide groups. In addition, the BPH-n (n=6, 8, 10) exhibited thermotropic hexagonal column (Colh) mesophase, which are stable at room temperature as revealed by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and X-ray diffraction (XRD) studies.(4) BPH-n formed the organogels in toluene and DMF. Xerogel of BPH-n (n=6, 8, 10) from toluene consists of flat ribbons with the width of 0.2-1.5μm, while BPH-10 xerogel from DMF showed left- and right-handed helical micrometer-ribbons with non-uniform helical pitch. Polarity of the solvent has a key effect on the aggregation morphology of the organogels.2. synthesis and self-assemble behaviors of oxadiazole derivatives. Three novel classes of bi-1,3,4-oxadiazole derivatives have been synthesized, namely 5, 5'-bis(phenyl 4-alkoxybenzoate)- 2, 2'-bi-1,3,4-oxadiazole (BBOXD-n), 5,5'-bis[phenyl bis(3,4-alkoxybenzoate)]- 2,2'-bi-1,3,4-oxadiazole (BBOXD-Bn) and 5,5'-bis[phenyl tris(3,4-alkoxybenzoate)]-2,2'-bi-1,3,4-oxadiazole (BBOXD-Tn) respectively. 1H NMR, FT-IR and Elemental Analysis were employed to confirm their molecular structures.(1) The phase behaviors were strongly affected by the length of the flexible terminal chains. All BBOXD-n exhibited remarkably stable SmC phases by virtue of the high transition enthalpies of SmC–I. In addition, BBOXD-6 and BBOXD-10 showed an enantiotropic nemetic phase with enthalpies of the N–I transition up to 5.16 kJ mol-1. As confirmed by wide-angle X-ray diffraction analysis and MM2, molecules of BBOXD-n showed high-angle tilting (55–57o) within their smectic C phases. The hexacatenar derivatives (BBOXD-Tn) are non-mesomorphic, while the tetracatenar ones (BBOXD-Bn) showed a classic progression from smectic C phases (for BBOXD-B6 and BBOXD-B10) to a hexagonal columnar (Colh) phase (for BBOXD-Bn (n=12,14,16) with the increase in length of the terminal chains. Molecules of BBOXD-Bn (n=6, 10) also tilt about 55-56o from the layer normal within their SmC phase.(2) BBOXD-n exhibited fluorescence emission in the apolar solvent. It was found that the polarity of the organic solvent greatly affected the fluorescence quantum yieldΦF. Interestingly, the fluorescence quantum yield of film is up to 94.3%. We evaluated BBOXD-6 as electron-transport materials in single layer organic light-emitting diode (OLED) by using blends of MEH-PPV and BBOXD-6 as the emissive material. OLED exhibited strongly light emission changes from red to blue on increasing the concentration of BBOXD-6. The maximum brightness and current efficiency of OLED with concentration 75% are 60260 cd/m2 and 4.14 cd/A, which is 204 times and 30 times than those with MEH-PPV as emissive layer.