Hybrid Nanostructured Materials from Bile Acid Derived Supramolecular Gels

Abstract

Research activities towards the self-assembly of small organic molecules building blocks which lead to form supramolecular gel has increased extensively during the past two decades. The fundamental investigations of the morphological properties and the mechanical properties of these supramolecular gels are crucial for understanding gelation processes. Most supramolecular gelators were discovered by serendipity, but nowadays ratiional design of new gelators has become somewh at feasible. As a consequence, an increasing number of multi stimuli-responsive and functional molecular gels are reported, offering great prospects with myriads of applications includ ing drug delivery and smart materials as shown in scheme 1. Scheme 1 Part 2: Synthesis of semiconductor nanocrystals In the last two decades, the synthetic development of semiconductor col loidal nanocrystals has been extended from the adjustment of their size, shape, and composition of the particles at the molecular level. Such adjustments of nanocrystals at the molecula r level might open different fields of applications in materials and biological sciences. I n this chapter, the concept of the shape contr ol synthesis of colloidal nanocrystals with a narrow size distribution, and the synthesis of composition dependent alloy type mat erials are described (Scheme 2). Scheme 2 Chapter 2: Synthesis of luminescent semiconductor nanocrystals Part 1: Cadmium deoxycholate: a new and efficient precursor for high ly luminescent CdSe nanocrystals This part demonstrates the sy nthesis of Cadmium deoxycholate (CdDCh2), an efficient Cd-precursor for the synthesis of high quality, monodisperse, multi color emittting CdSe Scheme 3 nanocrystals, while maintaining their high photoluminescent quantum efficiency (Scheme 3). The high thermal stability of CdDCh2 (decomposition temperature: 332 °C) was utilized to achieve high injection and growth temperatures (∼300 °C) for the syntheesis of red emitting nanocrystals with a sharp f ull width at half maximum (FWHM) and multiple excitonic absorption features. We believe that CdDCh2 can be useful for the prreparation of other nanomaterials such as CdS, CdTe and CdSe@CdS core-shell QDs. Part 2: Ligand mediated exccited state carrier relaxation dynamics of Cd1-xZnxSe1-ySy NCs derived from bile salts Bile salts of Cadmium and Zinc provide a convenient and inexpensive single step synthetic route for highly photoluminescent and stable semiconductor nanocrystals (NCs). The high thermal stabilities of Cadmium and Zinc deoxycholates (CdDCh2 and ZnDCh2) allowed us to fine-tune the synthesis of the NCs at high temperatures while maintaining the monodispersity, crystallinity and reproducibility (Scheme 4). Organic capping agent induced lattice strain affects the excited Scheme 4 state relaxation processes of the NCs. The analysis of photoluminescence decay profiles revealed that the average lifettime decreased with the increasing lattice strain of the NCs. A kinetic stochastic model of photoexcited carrier relaxation dynamics of NCs was employed to estimate the values of the radiative recombination rates, the photoluminescence quenching rates and the non-radiative recombination rates of the NCs. These data showed that the non-radiative relaxation rates and the numbeer of surface trap states increased with the incrreasing lattice strain of the NCs. Such types of NCs can have great potential in nonlinear optics, photocatalysis and solar cells. Chapter 3: Synthesis of organic-inorganic hybrid materials Part 1: Hierarchical self-assembly of photoluminescent CdS nanoparticles into bile acid derived organogel: morphological and photophysical properties In this part a strategy towards integrating photoluminescent semiconductor nanoparticles into a bio-surfactant derived organoggel has been reported. A facially amphiphilic bile thiol was used for capping CdS nanoparticless (NPs) which were embedded in a gel derived from a new bile acid organogelator in order to furnish a soft hybrid material (Scheme 5). The presence of CdS NPs in a well-ordered 1D array on the organogel network was confirmed using microscopic Scheme 5 techniques. Photophysical stuudies of the gel–NP hybrid revealed resolved excitation and emission characteristics. Time resolved spectroscopic studies showed that the average lifetime value of the CdS NPs increased in the gel state compared to the sol phase. A kinetic model was utilized to obtain quantitative information about the different decay pathways of the photoexcited NPs in the sol and gel states. Part 2: A novel strategy towards designing a CdSe quantum dot–metallohydrogel composite material This section describes an efficiient method to disperse hydrophobic CdSe quaantum dots (QDs) in an aqueous phase using cetyltriimethylammonium bromide (CTAB) micelles without any surface ligand exchange. The water soluble QDs were then embedded in the 3D self-assembled fibrillar networks (SAFINs) of a hydrogel showing homogeneous dispersibility as eviidenced by Scheme 6 optical and electron microscopico techniques (Scheme 6). The photophyssical studies of the hydrogel–QD from composite are reported for the first time. These composite materials may have potential applications in biology, optoelectronics, sensors, non-linear optics and materials science. Part 3: Photophysical aspectts of self-assembled CdSe QD-organogel hyybrid and its thermoresponsive properties A luminescent hybrid gel was constructed by incorporating CdSe quantuum dots (QDs) in a facially amphiphilic bile acid derived dimeric urea organogel throough non-covalent interaction between ligands capped on QDs surface and hydrophobic pockets of the gel (Scheme 7). The optical transparency of the hybrid materials and the dirrectionalities of the QDs in the gel medium were confirmed by photophysical and microscopic studies. The detailed excited state dynamics of the QD–organogel hybrid has been reported for the first time with the help of lifetime analysis and a kinetic decay model, and thee data revealed that the average lifetime of the QDs decreased in the gel medium. The reversible thermoresponsive behavior of the QD doped organogel was investigated by steady-state fluorescence spectroscopy. W e believe that the results obtained herein provides a route to develop a thermoresponsive system for practical application, especially because of the spatial assembly between soft organic scaffolds and colloidal QDs. Scheme 7 Part 4: In-situ formation of luminescent CdSe QDs in a metallohydrogel: a strategy towards synthesis, isolation, storage and re-dispersion of the QDs A one step, in-situ, room temperature synthesis of yellow luminesce nt CdSe QD was achieved in a metallohydrog el derived from a facially amphiphilic bile salt, resulting in a QD-gel hybrid (Scheme 8). T he ordered self-assembly and homogeneous distribution of the CdSe QDs in the hydrogel network was observed from optical and electro n micrographs. The different excited state behav iors of the hybrid were revealed for the fir st time using time resolved spectroscopy. Ad ditionally, we described the successful isolation of the photoluminescent CdSe QDs from the gel followed by their re-dispersion in an organic solvent using suitable capping ligands. Scheme 8 Chapter 4: Facially a mphiphilic bile acid derived meta llohydrogel: an efficient template for th e enantioselective Diels-Alder reactio n An enantioselective Diels-Ald er reaction mediated by a facially amphiphilic bile acid derived metallogel scaffold has been a chieved (Scheme 9). Different hydrophobic domains present in Scheme 9 the gel appear to facilitate the enantioselective reaction. Various spectro scopic and electron microscopic techniques were employed to understand the possible reasons for the stereoselectivity in the gel. Subsequently, different counter anion s dependent rate accelerations and induced enantioselectivity in the ZnCh2 gel were studied in detail. These preliminary results of the non-covalent based supramolecular heterogeneous catalysis offer new possibilities for using metallogels as nanoreactors for different stereoselective reactions.