As was discussed earlier, Byrne et al. conducted structural studies of btp ligands chelating various d-metals using btp ligand 16.113 The coordination, photophysical and electrochemical studies of Ru(II):btp complexes using ligands 16 and its hydrolysed analogue 32 was also carried out. When 32 was complexed with Ru(II) it gave rise to the formation of metallo-supramolecular gels. The crystal structure of the [Ru322](PF6)Cl complex gives an insight into the short range molecular arrangements comprising the metallo-supramolecular gel, Figure 1.23. The packing in the crystal structure displays intermolecular hydrogen bonding interactions between the carboxylic acid termini which hydrogen bond to water and ethanol molecules. There is also a non-classical hydrogen bonding interaction between one carboxylic acid group and a chloride ion which is itself involved in a hydrogen bonding interaction with the CH of one triazole ring. This example
Figure 1.23Btp ligands 16 and 32. X-ray crystal structure of (Ru322](PF6)Cl showing the various
supra-molecular interactions also observed in supramolecular gel formation. Figure reproduced from Ref.113 Copyright 2013: Royal Society of Chemistry.
demonstrated the solid state interactions likely to contribute to the formation of supramolecular gels. The use of X-ray diffraction analysis for gaining an insight into the short range order of gel systems developed over the course of this research thesis will contribute to our understanding of their structure on the molecular scale. Similar gel based systems will be discussed in more detail in Section 1.10.1.
Bradberry et al. went on to use 32 alongside the H2dpa derivative, 33, in reporting a
molecular logic gate mimic comprised of outputs arising from the use of Ln(III)- and ligand- centred emissions (Figure 1.24).70 This system represents one of only a few examples to date of the use of sensitising luminescent 4f ions as outputs in molecular logic. The encapsulation of complexes within a polymer organogel gave luminescence changes in response to the inputs [H+] and [F-]. The (0,0) state involved the green and red phosphorescence from Tb323 and Eu333, respectively. The (1,0) state was achieved by acidification which gave rise to an increase in the Eu(III) emission intensity through the PET effect of 33. The (0,1) state was brought about through addition of fluoride causing a decrease in both the Tb(III) and Eu(III) emission intensities through dissociation of the Tb323 complex and deprotonation of the already protonated Eu333, respectively. The (1,1) state was accessed by addition of both acid and fluoride resulting in an enhanced Eu(III) emission and loss of Tb(III) alongside an increase in the btp ligand 32 fluorescence.
In other work, the btp ligand 32 was further functionalised with a selection of amino acids, Gly, Ala, Phe, and Trp methyl esters, to give amide derivatives 34 which were then used in the study of the Ln(III) directed self-assembly of highly luminescent supramolecular bundles.112 This work was undertaken with a view to introducing biologically relevant and chiral moieties into such ligands, however, the complexes dissociated in aqueous media. These derivatives were shown to form stable, emissive Ln(III) complexes in organic solvents
Figure 1.24 Structures of ligands 32 and 33 and their complexes [Tb323] and [Eu333]. (Inset)
Coloured emission arising from complexes under UV irradiation at λ = 254 nm. Figure reproduced from Ref.70 Copyright 2015: Royal Society of Chemistry.
such as DMSO, possessing millisecond time range radiative excited states. Their photophysical properties in organic solutions were used to determine global stability constants of the self-assembly equilibria. Quantum yields for the Tb(III) complex emission of 46−70% in CH3CN solution were measured, reflecting the efficient energy sensitisation of the Tb(III) complexes by the btp ‘antenna’. By comparison, the Eu(III) complexes displayed less high quantum yields of 0.3−3%. It was concluded that the more efficient Tb(III) radiative state sensitisation was as a result of the better energy match with the btp triplet state. This was determined through low temperature phosphorescence emission measurements of the Gd(II) complex of 16. This article demonstrated that introducing functionality at the aryl terminal positions did little to perturb the self-assembly behaviour with Ln(III) ions as evidenced by the comparable speciation equilibria species formed, calculated global stability constants and quantum yields of luminescence. In the case of α-amino acids: Gly, Ala, Phe, derivatives. The Trp functionalised cases showed a less efficient quantum yield of luminescence in both Eu(III) and Tb(III) complexes.
By introducing methyl functionality closer to the btp chelating core, as seen for 35, in the form of methine chiral centres at the former methylene position (X), additional useable chiroptical spectroscopic properties were endowed on the btp system.156 Byrne et al. was able to monitor the self-assembly of ligand (S,S) and (R,R) entantiomers, 35S and 35R, respectively, with Eu(III) and Tb(III) using Circular Dichroism (CD) titrations upon addition of aliquots of Ln(III) metal solution to a solution of the ligand. Significant changes occurred in the CD spectra including an intense Cotton effect, giving rise to a bisignate CD couplet of the same sign as each respective ligand. Circular polarised luminescence (CPL) of the complexes was also measured with each pair of enantiomers displaying signals of equal magnitude and opposite sign for each band characteristic of a specific f-f electronic transition, thus demonstrating that the chirality of the ligands was transferred to the Ln(III) complexes. This was one of the first utilisations of CD titrations for probing the metal-directed self-
Figure 1.25 (A) X-ray crystal structures of 35S and (B) 35R enantiomers.156 Thermal ellipsoids
displayed at 50% probability with hydrogen atoms not involved in hydrogen bonding omitted for clarity.
assembly of supramolecular systems. Crystal structures of both enantiomers, 35S and 35R, were obtained which displayed interesting dimerisation of the ligands in the solid state through a self-templation effect, Figure 1.25. This phenomenom arises due to the acidic nature of the triazolyl CHs, which sees each triazolyl CH on each btp involved in a non- classical hydrogen bonding interaction with the pyridyl nitrogen of the other dimeric btp. As expected the opposite enantiomer displays mirroring behaviour.
Byrne et al. expanded on this work in achieving successful and high yielding synthesis of self-templated [2]catenanes 36a-c.102 The crystal structure demonstrates the self-assembly was made possible through a combination of non-classical btp hydrogen bonding interactions, as in the case of 35S and 35R, and also amide hydrogen bonding interactions, Figure 1.26. Details of the synthesis and properties of these, and analogous, mechanically interlocked systems will be discussed in more detail in Section 1.11. The structural nature of these [2]catenane hosts also exhibited a pronounced selectivity for the anion recognition
Figure 1.26 [2]Catenanes 36a-c and spacefill representation of crystal structure showing interlocked nature.102
of H2PO4− through the same interactions necessary for self-templation. With this in mind, a discussion of an interesting artefact of the anti-anti conformation of btp ligands and its dual ability to facilitate coordination with anions form the next section.