Other examples of soft materials reported by the Gunnlaugsson group

Moving on from the BTA motif, other supramolecular motifs have been used to generate gel materials within the Gunnlaugsson group. For example, a number of aryl-pyridyl urea derivatives, shown in Figure 1.36, were synthesised and some of these were found to form gels with antibacterial properties.15 _{The crystal structures of a number of these derivatives were also}

obtained, with these showing the expected hydrogen bonding and π-π interactions. Three of the derivatives were found to gelate in a variety of solvents at 1 wt% and were all stable to inversion. The gelation was found to be thermoreversible and were found to be fibrous in nature by SEM. Hydrogen bonding plays an important role in many antibacterial agents, as many of these agents work by inhibiting bacterial wall synthesis by forming hydrogen bond interactions with the peptides involved. As these derivatives contained a number of hydrogen bonding

Figure 1.36 (a) The aryl-pyridyl ureas, 49a-l, reported by Pandurangan and co-workers and (bottom) a) SEM of an organogel of the compound 49a, that formed after 24 h using a 1%wt concentration in THF, scale bar 10 µm; b) vial labelled 2 contains a gel formed at 1%wt in a toluene/THF/CHCl3 mixture and vial labelled 1 contains a

gel formed at 1%wt in MeOH; c) SEM of the organogel in a with AgNO3 in a THF/H2O mixture, scale bar 2 µm.

groups, there antibacterial properties against common strains were investigated. All the derivatives were found to possess antibacterial properties, although at various strengths. It was proposed that such soft materials would serve as useful antibacterial agents for use in coating implants to prevent bacterial infection.

Another report on the formation of gels involved the development of supramolecular metallogels based on a pyridine-2,6-dicarboxylic acid derivative as a type of polydentate host and lanthanides as guests to generate a soft material.16 Eu(III) and Tb(III) were used as the lanthanides and gels of each, along with a mixed metal gel, were generated. The Eu(III) gel emitted the characteristic red emission, while the Tb(III) had characteristic green emission. These highly luminescent gels could be mixed to give a variety of different colours depending

Figure 1.37 (a) Ligand 50 reported by Martinez-Calvo and co-workers, (b) the Eu(III) and Tb(III) gels in daylight and (c) under UV light and the (d) luminescence of the Eu(III) gel, mixed gel and Tb(III) gel on quartz plates and (e-g) SEM of the Eu(III) gel, Tb(III) gel and the mixed gel, scale bars 500 nm. Images reproduced from reference 16.

on the stoichiometric ratio of Eu(III) and Tb(III). The morphology was examined by SEM and showed the two gels to have similar cotton-like fibrous structures, while the Tb(III) gel had a higher density of fibres. The mixed gel was found to have a different morphology to that of the individual gels, as seen in Figure 1.37. All three gels formed were found to display self-healing properties, although to different extents and the mixed gel was found to be much softer and possess a smaller G’ than that of the ‘pure’ gels. It is presumed that there is some sort of self- recognition process occurring in the ‘pure’ gels that does not take place in the mixed gels and that this is the cause of the lower rheological values obtained for the mixed gel system.

Other gelators reported by the group include a healable lanthanide luminescent gel based on the 2,6-bis(1,2,3-triazol-4-yl)pyridine (btp) motif by McCarney,72 d-metal btp based gels by Byrne,111 and a most recently, the report of the combination of N-picolyl-1,8-naphthalimide derivatives and d-block metals to generate a series of metallogels by Lovitt.112

Another area of ongoing research within the Gunnlaugsson group is in the area of coordination polymers and porous materials. For example, Hawes reported the synthesis of two novel ditopic naphthalimide-based ligands and their subsequent coordination chemistry, Figure 1.38a&b.113 The preparation of three new coordination polymers was reported, with π-π interactions dominating the extended structures, resulting in a threefold interpenetrated network in the case of the Ag polymer, while robust and flexible intralayer interactions in the two- dimensional networks of the Cu and Cd polymers were observed. The Cu and Cd polymers were evaluated for CO2 uptake and were found to display modest guest uptake upon evacuation

and both materials could be reverted to their original phases after the uptake studies. 113

In another example from the group, Shanmugaraju reported the formation of two new coordination polymers from a naphthalimide-Tröger’s base (TB) derivative.114 The design of this ligand was influenced by a number of factors, for example, the fluorescent nature of the naphthalimide could be useful for sensing applications, the Tröger’s base motif could result in a cleft-shaped structure, while the flexible picolylamime used can result in unusual coordination topologies. This work resulted in the generation of a Co and Cd based coordination polymers using solvothermal methods, and the X-ray diffraction studies revealed that π-π interactions prevailed in the extended structures resulting in one-dimensional supramolecular polymeric networks, Figure 1.39.

Figure 1.38b (a) Interpenetration of the 51Ag complex with independent networks coloured separately, showing the π-π interactions between the three interpenetrated networks around a single channel and the overall two- dimensional structure of the assembly; (b) the extended structure of the 52Cu complex, showing the connectivity of a single layer and the interdigitation of the two adjacent layers with independent networks coloured separately; (c) the extended structure of the 53Cd complex showing interdigitation between two layers viewed parallel and perpendicular to the one-dimensional solvent channels, independent networks are coloured separately. Hydrogen atoms and lattice solvent are omitted from the bottom left and right images for clarity. Image reproduced from reference 114.

Other reports of coordination polymers synthesised within the group include the report by Dalton of eight new coordination complexes based on the coordination of two bipyridine ligands with Cd(II), Co(II) and Co(III)115 and the work of McCarney in which the synthesis of a three-dimensional MOF material derived from a btp-based ligand and Zn(II) was discussed.116 These reports are shown in Figure 1.40.

Figure 1.39 (a) The structure of the cobalt-TB coordination polymer and the extended structure for the complex, with the heteroatoms and pyridyl ring disorder omitted for clarity; (b) an example of the ligand geometry in the cadmium-TB coordination polymer and the coordination environment of the two cadmium ions and the extended structure of a single chain of the coordination polymer with the symmetry related ligand groups coloured together showing the two overlapping zig-zag chains present. Images reproduced from reference 114.

Figure 1.40 (a) One of the complexes reported by Dalton and co-workers with labelling scheme and showing hydrogen bonding interactions between the adjacent complexes through lattice water molecules. Selected hydrogen atoms are omitted for clarity. (b) Perspective view of the solvent channels in the crystal structure of the complex reported by McCarney and co-workers with the hydrogen atoms omitted for clarity Images reproduced from references 115 and 116.