Mechano‐Responsive, Thermo‐Reversible, Luminescent Organogels Derived from a Long‐Chained, Naturally Occurring Fatty Acid

Abstract

The gelating ability of an α-diketo derivative of oleic acid, 9,10-dioxooctadecanoic acid (DODA), is investigated. DODA can gelate aromatic liquids and many other organic liquids. By contrast, none of the liquids examined can be gelated by the methyl ester of DODA. DODA is a more efficient gelator than stearic acid and the monoketo derivative due to its more extensive intermolecular dipole-dipole interactions. Formation of organogels of DODA can be induced by both thermal and mechanical stimuli, during which the luminescent and mechanical properties can be modulated significantly. The emission from DODA in 1-octanol exhibits a large, reversible, hypsochromic shift (≈25 nm) between its thermally cycled gel and sol states. The emission changes have been exploited to probe the kinetics of the aggregation and deaggregation processes. DODA is the simplest gelator of which we are aware that exhibits a reversible shift in the emission. Although the self-assembled fibrillar networks of the DODA gels in 1-octanol, benzonitrile, or silicone oil are crystalline, isothermal mechanical cycling between the gel and the sol states is rapid and can be repeated several times (i.e., they are thixotropic). The single-crystal structure of DODA indicates that extended intermolecular dipole-dipole interactions are crucial to the thermal and mechanical formation of DODA gels and the consequential changes in emissive and mechanical properties. From analyses of structural information, gelator packing, and morphology differences, we hypothesize that the mechanical destruction and reformation of the gel networks involves interconversion between the 3D networks and 1D fiber bundles. The thermal processes allow the fibrillar 3D networks and their 0D components (i.e., isolated molecules or small aggregates of DODA) to be interconverted. These results describe a facile approach to the design of mechano-responsive, thermo-reversible gels with control over their emission wavelengths.

Nonconventional macromolecular luminogens with aggregation‐induced emission characteristics

“…Figure lists some of the most common moieties for the construction of organic fluorophores, like polycyclic aromatic hydrocarbons (PAH), aromatic amines, BODIPY, and fluorescein. With regard to the nonconventional luminogens, so far, more and more examples including molecular and macromolecular, synthetic and natural occurring compounds, such as PAMAM, PEI, starch, cellulose, bovine serum albumin (BSA), PAN, PNHSMA, glucose, poly(acryl acid) (PAA), perfluorosulfonate ionomers (PFSIs), and supramolecular systems are found . For each system, people try to identify the exact contributing moieties.…”

“…Among these emerging luminophors, nonconjugated polymers with chain flexibility and structure tunability have aroused great interests because of their significant fundamental importance and promising technical applications . Generally, these polymers involve electron‐rich heteroatoms, such as nitrogen (N), oxygen (O), sulfur (S), phosphorus (P), and/or unsaturated C = C and C≡N subgroups. Apart from the most intensively investigated archetypal poly(amidoamine)s (PAMAM), other macromolecular nonconventional luminogens, such as poly(amino ester)s (PAE), poly(ether amide)s (PEA), polyacrylonitrile (PAN), poly( N ‐vinylpyrrolidone) (PVP), polyethylenimines (PEI), polyureas (PU), sulfonated acetone–formaldehyde (SAF) condensates and derivatives, poly( N ‐hydroxysuccinimide methacrylate) (PNHSMA), polysiloxanes, and even supramolecular assemblies have been becoming increasingly eye‐catching owing to their unique intrinsic emissions.…”

Luminescent Behavior of Gels and Sols Comprised of Molecular Gelators

“…Although numerous examples of fluorescent LMOGs have been reported, few examples of phosphorescent LMOGs and gels are known. An interesting example of a phosphorescent LMOG producing gels was found by Zhang et al using an organic molecule with an α-diketo group at the 9,10 positions along an 18-carbon fatty acid chain, DODA [44]. Details are discussed in Sections 3 and 5.…”

“…Fluorescent and phosphorescent LMOGs producing gels with AIEE properties need not contain π-conjugated backbones, metals, or heavy atoms. For example, Zhang et al exploited the known ability of α-diketo groups to both fluoresce and phosphoresce in solution at room temperature [79,80] to construct a structurally simple LMOG, DODA [44]. The synthesis of DODA involved oxidation by potassium permanganate at the 9 and 10 positions of methyl oleate to yield 9,10-dioxooctadecanoic acid methyl ester, followed by hydrolysis ( Figure 17).…”

X-Ray Crystal Structures and Organogelator Properties of (R)-9-Hydroxystearic Acid

“…The structure of (R) -12-HSA, both as pure solid and in its organogels, has been the subject of intense structural research. However, its full understanding is hampered by the lack of a single crystal structure, owing to the difficulties in achieving diffraction quality single crystals of molecular gelators of the class of long-chain fatty acids [20,21,22,23]. Structures of two polymorphs of rac -12-HSA have been reported [24,25] and their comparison shows in both cases the presence of centrosymmetric carboxylic dimers, made by pairing of the enantiomers.…”