Synthesis and Studies of Dendritic Poly (Ether Imine) Boronates and Cholesteryl-Functionalized Mesogens

Abstract

Synthesis and Studies of Dendritic Poly(Ether Imine) Boronates and Cholesteryl-Functionalized Mesogens SYNOPSIS Dendrimers are hyperbranched synthetic macromolecules having branches-upon-branches structures, high molecular weights, globular shapes and monodispersities. Dendrimers possess a large number of modifiable functional groups at their peripheries. Initial efforts were largely concerned with the synthesis, design and development of new dendrimers. Exploring the chemical, biological and material applicability of these macromolecules are relevant to current interests, as a result of the unique structural features of dendrimers. Incorporation of transition metals and organic moieties at the peripheries of the dendrimers was studied to determine their efficacies in catalysis. Evolution of dendritic effects was observed in few instances, that were non-linear in nature. On the other hand, dendritic peripheries were also utilized to study mesogenic properties in liquid crystals. Chapter 1 of the Thesis gives an overview of the types of dendrimers, its structural features and their application in catalysis and as liquid crystalline materials. Chapter 2 describes the synthesis of a new type of poly(ethyl ether imine) dendrimer, having nitrogen as a branching unit, ethylene moiety as the spacer and an oxygen as the connecting linker. Synthesis, characterization, and studies of the photophysical properties of these dendrimers are described in this chapter. The molecular structure of second generation dendrimer is shown in Figure 1. Synthesis of this dendrimer was initiated using 2,2'-oxy-bis(ethan-1¬amine) as the core. The reaction sequence of two alternate nucleophilic substitutions and two alternate reductions, involving ethyl bromoacetate and bromoacetonitrile as monomers was employed in the synthesis of the dendrimer. The formation of dendrimers having ether linkage and tertiary amines as branching unit was established by spectroscopies and mass spectrometry. A number of functional groups, such as, acid, alcohol, amine, ester and nitrile are present at the peripheries of each generation the dendrimers that open up the possibilities for further studies. Carboxylic acid terminated poly(ethyl ether imine) dendrimers are substituted iminodiacetic acids, belonging to the class of polyaminocarboxylic acid. Methyl iminodiacetic acid boronates with NB coordination have emerged as an excellent substitute for unstable boronic acids. Upon increasing the steric bulk on the nitrogen moiety, the hydrolytic stability of the boronates to a base-catalyzed hydrolysis is increased. Combining the structure of carboxylic acid terminated dendrimer and the stability of the dendritic boronates, such dendritic iminodiacetic acids were reacted with arylboronic acids to prepare bis-and tetrakis-boronates (Figure 2). Kinetic hydrolytic studies of boronates were conducted to assess the stabilities of the newly synthesized dendritic boronates. From the studies it was observed that the tetrakis-boronate was ~20 times more stable in comparison with dimeric and monomeric boronates (Figure 3). Subsequent to synthesis and hydrolytic stability studies, C-C bond-forming Suzuki-Miyaura cross-coupling reactions were conducted. A comparison of the reactivities among monomeric, dimeric and tetrameric arylboronates in C-C bond-forming reactions showed a higher reactivity of monomeric and dimeric boronates, than the tetrameric aryl boronate to construct ter-and tetra-aryl in one-pot iterative manner (Figure 4). Chapter 3 of this Thesis describes the synthesis and characterization of dendritic boronates and studies of their hydrolytic stability in Suzuki-Miyaura cross¬coupling reactions to construct ter-and tetraaryls. Figure 4. Synthesis of (a) ter-(6) and (b) tetra-aryls (7) by following one-pot iterative cross-coupling reactions. Step-wise iterative synthesis of dendrimer allows a uniform branching throughout the structure. The first and second generation poly(ether imine) dendrimer series, having hydroxyl groups at their peripheries were chosen for further modification. A versatile mesogenic group, namely, cholesterol was covalently attached at the peripheries of the dendrimers with succinic moiety as linker, so as to install 4 and 8 cholesteryl moieties at the peripheries of the dendrimers (Figure 5), that were characterized by H, C NMR spectroscopies and elemental analysis, so as to confirm their structural homogeneities. Figure 5. Molecular structures of the first and second generation dendritic mesogens. Subsequent to synthesis and characterization, liquid crystalline properties of all the dendritic mesogens was assessed through differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD) studies. In POM study, broken fan or leaf like texture revealed the lamellar arrangement, whereas homeotropic appearance of texture on surfactant (cetyltrimethylammonium bromide) coated substrate indicated the lamellar nature of G1-Et-(OCS)4, G1-Pr-(OCS)4 and G2-Pr-(OCS)8 (Figure 6). From DSC studies, the change in enthalpy was found to increase with increase in generation and change in enthalpy per mesogenic unit was found to be ~ -1 1-2 kJ mol, which indicated the mesophase arrangement to be lamellar. Decrease in the length of spacer dendritic backbone and increase in the generation increased the isotropization temperature of the dendritic liquid crystals. Variable temperature XRD studies were undertaken to characterize the mesophase property. Two sharp peaks in small angle region and a diffused halo in wide angle region in XRD pattern of the material suggested the smectic A (SmA) liquid crystalline arrangement of G1-Et-(OCS)4, G1-Pr-(OCS)4 and G2-Pr-(OCS)8 (Figure 7).

Figure 6. POM textures of (a) G1-Et-(OCS)4 at 136 oC; (b) G2-Et-(OCS)8 at 129 oC; (c) G1-Pr-(OCS)4 at 92 oC; (d) G2-Pr-(OCS)8 at 118 oC and (e) transition temperatures for dendromesogens (DSC second heating cycle, heating-cooling rate = 10 oC min-1). Figure 7. Small angle XRD profiles of: (a) G1-Pr-(OCS)4 and (b) G2-Et-(OCS)8 at 60 o C (black), 150 oC (red) and 180 oC (green), (Insets: Lorentzean fit of wide angle peak). The second generation ethyl-linker dendrimer G2-Et-(OCS)8 exhibited a layered structure with a superimposed in-plane modulation (SmÃ), the length of which corresponded to a rectangular column width. Chapter 4 describes the synthesis, characterization and studies of mesophase property and fluorescence property of cholesterol functionalized homologous pairs of the PETIM dendritic liquid crystals. Peripheral functionalization of the dendrimers provides an easy access to dendritic liquid crystalline materials. The covalent functionalization was extended further with the dendrimers for both the series, so as to have 2, 4, 8 and 16 cholesteryl groups at the peripheries of 0, 1, 2 and 3 generation dendrimers, respectively, having succinic amide and phthalic ester functionalities for 1, 2 and 3 generation dendrimers with 4, 8 and 16 cholesteryl groups. Molecular structures of third generation dendrimers are shown in Figure 8. Figure 8. Molecular structures of third generation G3-Pr-(NHCS)16 and G3-Pr-(OCP)16. Subsequent to synthesis and characterization, mesophase property was studied through POM, DSC and XRD techniques. In POM study, a birefringent texture was observed in heating and cooling cycles. Leaflet, broken fan or bâtonnet like texture suggested the layered arrangement of the molecules (Figure 9). In DSC studiues, it was observed that the amide-linked dendrimers showed higher glass transition and isotropization temperatures than that of ester-linked dendrimers within the same generation irrespective of the back-bone of the dendrimer. Succinic moiety linked dendrimers showed lower glass transition temperature than that of phthalic moiety linked dendrimers and consequently, larger mesophase range. The change in enthalpy for isotropization was found to increase with increase in generation, whereas change in -1 enthalpy per mesogenic unit was 1-2 kJ mol, indicative of a layered arrangement in the mesophase.

Figure 9. POM textures (20x) of (a) G3-Pr-(NHCS)16 at 90 oC; (b) G3-Pr-(OCS)16 at 90 ooo C; (c) PG1-(NHCS)4 at 134 C; (d) G3-Pr-(OCP)16 at 98 C and (e) transition temperatures for dendromesogens (second cycle, heating-cooling rate = 10 oC min-1). Appearance of two sharp peaks in small angle region and a wide halo in wide angle region in XRD pattern supported lamellar mesophase property of the material (Figure 10). On decreasing the temperature, increase in the layer thickness also suggested the smectic A arrangement of the molecules except third generation phthalate derivative G3-Pr-(OCP)16, which showed rectangular columnar mesophase. For all the dendromesogens, the layer thickness increased with the increase in generation. Upon protonation, the first generation dendrimer showed a change in mesophase from simple smectic A to modulated smectic A with decrease in layer thickness. The change in liquid crystal property of the dendromesogens from lamellar to columnar mesophase by changing the linker of the mesogen is unknown so far in the dendrimer liquid crystals. Chapter 5 gives details of synthesis, characterization and mesophase property study of ester-and amide-linked dendritic liquid crystals. Overall, the Thesis establishes a synthetic methodology for the synthesis of a new homologous series of poly(ether imine) dendrimers with ethyl spacer; synthesis of dendritic boronates and their studies in cross-coupling reactions through in-situ slow release of boronic acid; hydrolytic stability study showed higher stability of dendritic boronates which was used in one-pot iterative cross-coupling reactions to construct ter-and tetra-aryls. decrease in linker length in dendrimer backbone modified the thermal, as well as, mesophase behavior of the dendritic liquid crystals;

change in the linker functionality from ester to amide changed the thermal behavior of dendritic liquid crystals; a switching of mesophase property from lamellar to columnar was observed by changing the rigidity of the linker from succinate to phthalate without changing the linker length.

The results of the above chapters are in different stages of publications: 1 Dendritic iminodiacetic acids and their boronates in Suzuki-Miyaura cross¬coupling reactions. Sharma, A.; Kumar, P.; Pal, R.; Jayaraman, N. Revised Manuscript submitted. 2 In-plane modulated smectic à vs smectic A lamellar structures in homologous pairs of dendritic liquid crystals. Kumar, P.; Rao, D. S. S.; Prasad, S. K.; Jayaraman, N. Revised Manuscript submitted. 3 Effect of protonation on dendritic liquid crystals of poly(ether imine) dendrimers: structure property relationship studies. Kumar, P.; Rao, D. S. S.; Prasad, S. K.; Jayaraman, N. Manuscript submitted. 4 Smectic to rectangular columnar switch from succinic to phthalic linker alteration in poly(ether imine) dendritic liquid crystals. Kumar, P.; Rao, D. S. S.; Prasad, S. K.; Jayaraman, N. Manuscript in preparation.