concentrations of 1-10 wt%, the transition temperature exceeds 150°C. The gels were observed to be thermoreversible, transparent, and stable over long periods.
Table 4.1: Gel formation of various imides
Figure 4.1: SEM of gel 4.1
Figure 4.2: SEM of gel 4.3 Figure 4.3: SEM of gel 4.4
The scanning electron micrograph (SEM) of each gel was recorded to predict the morphologies of xerogels (dried gels). The representative SEM images of some of the gel are shown in Figure 4.1-4.3. SEM analyses show that the gel prepared by compound 4.1 has short thick fibre-like structure varying from 10 to 40 m in length and from 0.5–1.0 m in width. The morphology of the other gels varies and they have network structures or plate-like structures of micron size. The gel of 4.3 has plate-shaped structures with 2–8 m length and 0.7–2.0 m width. On the other hand, the gel of 4.4 has interconnected fiber networks with width of about 0.25–0.40 m. The macroscopic properties of similar kind of organogelators have been investigated by various physico-chemical measurements such as optical microscopy, electron microscopy (e.g., SEM,
TEM), atomic force microscopy (AFM), X-ray diffraction, dynamic light scattering, small angle neutron scattering, etc. It is believed that the gelator molecules form some kind of 3D fibrous aggregates via spontaneous self-assembly process involving various intermolecular interactions and the bulk solvents are immobilized within such 3D network causing gelation. SEM of gels of pyridinedicarboxylic acid derivatives in aqueous DMSO show the presence of thin, straight fibres that are 10 m long and weakly interconnected.15 The morphological characteristics of the xerogels of the N,N'-bis-(pyridyl) urea-dicarboxylic acid gelators show a mixture of plate shaped and 1D slender fibers or a complicated 3D network of fibers of tape morphology and intertwined network of 1D fibers.14 In all the cases, the fibers are ~10 m long and the thickness varied from submicron to 10 m. The gelation properties of PBI dye, which is able to gelate a multitude of organic solvents at rather low concentration (~0.2 wt%), can be attributed to the presence of hydrogen bonds between the benzamide functional groups that enforce the strong - stacking interactions between PBI units leading to gelation.13cAFM of PBI gel shows the helical fibres with 3.1 nm of height, 8.0 nm of width and several micrometers in length.
Single crystal structure information on a gelator molecule is important to understand the supramolecular architecture of the meta-stable gel fiber in its native (gel) form.16 However, it is virtually impossible to determine the crystal structure of a gel fiber; Dastidar reported first example of crystal structure of a pyridyl urea based low molecular weight organic gelator molecule which was crystallized from its gelling solvents such as ethylene glycol and water.17 The supramolecular assembly of the gelator molecule and the interacting solvents in the crystal lattice displays microporous architecture with channels that contained both solvent molecules through hydrogen bonding interactions with the gelator molecules. We have also tried to crystallize the gelator molecules in a mixture of DMSO and water solvents but could not be achieved the suitable single crystals of any gelator molecule with both of its gelling solvents.
However, the compound 4.3 was crystallized in DMSO and the formation of a 1:1 solvate of 4.3 with DMSO was characterized by determining its crystal structure. The structure of the solvate is shown in Figure 4.4a. Generally, carboxylic acids are strongly hydrogen bonded amongst themselves, but it is very interesting to note that the solvate of 4.3 with DMSO is devoid of O–
H…O interactions amongst the carboxylic acid groups. However, it displays a hydrogen bond O2–H2…O4 interaction (dD···A 2.59Å, <D-H···A, 175.0º) between the oxygen atom of DMSO and the –OH of one of the carboxylic acid groups. This compound forms sheet-like structure in
which the DMSO molecules are between the two layers of the imides. The DMSO molecules themselves have weak S…O (dD···A 2.59Å, 3.31 Å) interactions (Figure 4.4b). The layers grow along the crystallographic a axis. This arrangement leads to a highly porous structure, which, possibly on addition of water, results in a strong hydrogen bonded network leading to formation of the gel. The mechanism of gelation of well known family of dipicolinic acid derivatives is proposed to arise from the base-assisted deprotonation of the carboxylic functionality, leading to the formation of the sodium chelate of the monodeprotonated ligand as the key gelling species.15 Electrostatic interactions and hydrogen bonds are thus believed to account to some extent for the formation of extended structures in which dipicolinic acid species self-assemble with the help of bridging water molecules. Xu et al reported naphthalene based -aminoacids and N- (fluorenylmethoxycarbonyl) amino acids hydrogelators for biomedical applications in which 3D fibril network is proposed to sustain by hydrogen bond network and - interactions.18
Figure 4.4: (a) ORTEP diagram of 1:1 DMSO solvate of 4.3 (20% thermal ellipsoid). (b) Weak interactions leading to sheet-like structure.
Most of the compounds such as 4.1,19a 4.219b and 4.319c,d are already reported in literature;
however, in this study we have shown their gel formation abilities.
A number of cyclic imide compounds having isoquinoline backbone are also synthesized and their solvent dependent fluorescence emission properties are studied as a part of this chapter. The
organic compounds having fluorophores that are sensitive to environment are of great importancein chemistry and biology.20 The fluorescence emission spectra of some heterocyclic compounds such as benzo[de]benzo[4,5]imidazo[2,1-a]isoquinoline (Scheme 4.2) are sensitive to environment21 and some of such heterocyclic compounds are used as organic photoconductive materials.22 So, the functionalization of such heterocyclic compounds is expected to form derivatives that may possess interesting optical properties.23 Heterocyclic compounds having more numbers of delocalized aromatic rings in conjugation to each other would lead to better as well as novel optical properties.22 With an interest to identify and characterize and also to understand optical properties of compounds bearing fluorophores that are sensitive to environment, we have synthesized a few heterocyclic compounds as shown in Scheme 4.2. We have studied fluorescence emission of these compounds and compared with some of their derivatives.
Scheme 4.2: Structures of heterocycles.
In this study we observed that the condensation reaction of 1,8-naphthalic anhydride with 4-nitro 1,2-diaminobenzene led to the formation of cyclic imide heterocyclic compound 4.7 in acetic acid solvent24 whereas the same reaction using N,N-dimethylformamide (DMF) solvent provided cyclic imide derivative 4.6 (Scheme 4.3).