Jue Wei

Control of self-assembly of twin-tapered dihydrazide derivative: mesophase and fluorescence-enhanced organogels

A twin-tapered (six-chained) low-molecular-mass organic gelator bearing hydrazine linkage and end-capped by phenyl, 1,4-bis[(3,4,5-trihexyloxy phenyl)hydrazide]phenylene (TC6) was designed and synthesized. Quadruple hydrogen bonds between the dihydrazide units are the main driving force in forming supramolecular liquid crystals and organogels. The association constants (K) of the quadruple hydrogen bonds between the hydrazide units in chloroform are 7.13 × 103 M−1 and 1.53 × 103 M−1 on the basis of NH-1 and NH-2, respectively. Depending on the environmental conditions, compound TC6 can self-assemble into a hexagonal columnar mesophase in the condensed state, in which two molecules constitute the slice of the column, or into a smectic mesomorphic-like organization in the presence of apolar solvents such as 1,2-dichloroethane, or into a rectangular columnar mesomorphic-like organization in the presence of polar solvents such as ethanol, in which one molecule constitutes the slice of the column. Aggregation-induced emission enhancement (AIEE) has been observed after gelation though conventional chromophore units are not incorporated in TC6.

Direct Evidence for the Effect of Intermolecular Hydrogen Bonding on Organogels

In order to get direct evidence for the effect of intermolecular hydrogen bonding on the organogels, one amide group in N-(3, 4, 5-octyloxybenzoyl)-N′-(4′-aminobenzoyl)hydrazine(D8) was replaced by a Schiff base group, forming N-(3,4,5-octyloxybenzoyl)-N′-(4′-amidobenzoyl) acylhydrazone(T8SchA). D8 and T8SchA organogels in cyclohexane show the same hexagonal columnar structure. And the hydrogen bonding was demonstrated to be still interacting in the organogels. However, although the molecular geometry of D8 was well retained in T8SchA, the molecular dipole moment of T8SchA is bigger than that of D8 due to the reduction of the number of hydrogen bonds. Thus, the decreased gelling stability of T8SchA compared to that of D8 can only be attributed to the reduction of the number of intermolecular hydrogen bonds, which provides direct evidence that intermolecular hydrogen bonding plays an important role in stabilising organogels.

Exploring the difference in xerogels and organogels through in situ observation

Solvent–gelator interactions play a key role in mediating organogel formation and ultimately determine the physico-chemical properties of the organogels and xerogels. The ethanol organogels of 1,4-bis[(3,4,5-trihexyloxy phenyl)hydrazide]phenylene (TC6) were investigated in situ by FT-IR, Raman and fluorescence spectra, and XRD, and it was confirmed that the intermolecular interaction and aggregation structure of TC6 ethanol organogels were quite different from those of xerogels. Simultaneously, unprecedented phase transition from organogel to suspension upon heating was observed in ethanol organogel, and the suspension phase exhibited lytropic liquid crystalline behaviour with a rectangular columnar structure. This study may open the possibility to design new gelators with a new dimension of versatility.