Structure–property relationships of symmetrical and asymmetrical azobenzene derivatives as gelators and their self-assemblies

Abstract

Two different series of symmetrical and asymmetrical azobenzenes containing terminal cholesteryl/adamantyl derivatives (SAC/SAA and AAC) with varying spacer lengths (alkyl chains) have been developed. The gelation and aggregation of these derivatives were studied relative to structural motifs, spacer lengths, solvent affinity, temperatures and light conditions. Among these derivatives, the cholesteryl derivatives that have short alkyl chains (<3) act as efficient gelators in a variety of solvents. However, the cholesteryl derivatives with longer alkyl chains (11 spacer) and adamantyl derivatives did not possess this ability. Self-assembled fibrous structures were constructed by gelators with short alkyl chains (<3), while flower-like structures were constructed by gelators with moderately longer alkyl chains (3-6) at their respective critical gelation concentrations (CGCs) according to SEM (Scanning Electron Microscopy) and TEM (Transmission Electron Microscopy) analyses. In some cases, a partial/weak gel was observed in different solvents, which exhibited uniform spherical nanoparticles at CGCs. These nanoparticles were further entangled to form interconnected fibrous structures when the concentration was increased above the CGC (according to the SEM and TEM analyses). Secondary forces (van der Waals/H-bonding) and π-π stacking played important roles in the aggregation of both series in the solvents according to variable temperature (1)H-NMR analysis. The reversibility of sol-gel transitions by light was studied with respect to solvent affinity. This study revealed that reversible transitions were only observed in the non-polar solvents, as supported by the FTIR analysis of the gelators in the various solvents. The thermal and mesomorphic behaviors of the gelators by DSC (Differential Scanning Calorimetry) and POM (Polarized Optical Microscopy) analyses revealed that the chiral nematic (N*) and cholesteric mesophase (Ch*) were exhibited by only the short and longer alkyl chain cholesteryl derivatives, respectively. However, the cholesteryl derivative without a spacer (AAC0) did not exhibit any liquid crystalline phase but acted as an efficient gelator relative to the other gelators in this study.

Nanobelts of hexagonal columnar crystal lattice through ionic self-assembly

“…Considering the longer molecular structure here (0.9 nm) than Ad (0.60 nm) and FcM (0.70 nm) groups, the adjacent Azo groups might not be packed in the same plane like the two others. The π-π stacking of Azo groups could adopt a face-to-face packing, resulting in a small D value [17]. The structure directing role of AOT here was also certified by a control experiment.…”

The effect of meta versus para substitution on the aggregation of bis-cholesteryl appended 2,6-disubstituted pyridine-based gelators

“…A densely packed fibrous morphology was observed for the xerogel of PyMC in xylene, where gelation occurred at room temperature, indicating a strong interaction between the solvent and a gelator molecule, which is reasonable since the intermolecular interactions are more extensive in non-polar solvents than in polar solvents. 13 Helical structures, however, could not be seen in xylene. The morphological behaviors of xerogels were investigated by SEM analysis, and the results were depicted in Fig.…”

“…The structures of the LMWGs have been regulated successfully by changing the components of L or A, and the resulting gelators are able to gel a range of solvents including protic/aprotic and polar/nonpolar solvents. 11,12 Very recently we reported 13 the aggregation behavior of azobenzenecholesteryl-based A(LS) 2 -type gelators with respect to varying spacer length [the linker (L) between the azobenzene (A) and the cholesteryl unit (S)], in which we found that the azobenzene derivatives with either no alkyl or a short alkyl chain acted as more efficient gelators compared to the long chain azobenzene derivatives. Moreover, in our previous reports, 14 clear helicity was seen through TEM only in the gelator not possessing an alkyl chain between the core (A) and the terminal cholesteryl unit (S).…”

Synthesis, characterization and spectroscopic studies of luminescentl-valine modified alkynyl-based cyclometalated gold(iii) complexes with gelation properties driven by π–π stacking, hydrogen bonding and hydrophobic–hydrophobic interactions

“…† Interestingly, while complexes 5, 8 and 9 exhibit fibrous structures that are commonly seen in the literature, complex 7 exhibits unusual globular-shaped aggregates in the TEM images. 56 The lack of fibrous network may account for the lower stability of the gel, as reflected by the low sol-gel transition temperature as well as the long time required for gel formation, probably resulting from the relatively short single alkoxy chain. 5(c) and (d), respectively.…”

“…5(c)). 56 The lack of fibrous network may account for the lower stability of the gel, as reflected by the low sol-gel transition temperature as well as the long time required for gel formation, probably resulting from the relatively short single alkoxy chain. In contrast, complex 8, with a longer alkoxy chain, could exhibit a stronger hydrophobic-hydrophobic interaction, tends to form an extensive fibrous network and gives a higher sol-gel transition temperature.…”