The Formation of Luminescent Supramolecular Ternary Complexes in Water: Delayed Luminescence Sensing of Aromatic Carboxylates Using Coordinated Unsaturated Cationic Heptadentate Lanthanide Ion Complexes

Abstract

The synthesis of four lanthanide ion complexes Eu•1, Eu•2, Tb•1 and Tb•2, from the heptadentate tri-arm cyclen (1,4,7,10-tetraazacyclododecane) ligands 1 and 2 that were made in one-pot syntheses is described. These coordinatively unsaturated complexes have two labile metal-bound water molecules, as demonstrated by X-ray crystallography. This was also confirmed by evaluating their hydration state (q∼2) by measuring their lifetimes in D²O and H²O, respectively. The above complexes were all designed as being “photophysically silent” prior to the recognition of the anion, since they do not possess antenna that can participate in sensitisation of the Eu(III) or the Tb(III) excited state. However, the two water molecules can be displaced upon anion binding by the appropriate aromatic carboxylates to give ternary complexes in water, through either four- or six-member ring chelates (q∼0), or possibly via a monodentate binding. In the case of Tb•1 and Tb•2, large luminescence enhancements were observed upon the formation of such ternary complexes with N,N-dimethylaminobenzoic acid at ambient pH. Such binding and luminescent enhancements were also observed for Tb•1 in the presence of salicylic acid. On all occasions, the anion recognition “switched” the emission “on” over two logarithmic units. At higher concentrations, the emission is reduced possibly due to quenching. In the case of aspirin, the binding was too weak to be measured, indicating that Tb•1 selectively detects salicylic acid, the active form of aspirin in water. In the case of Eu•1 and Eu•2, the affinity of these complexes towards such aromatic carboxylates was too weak for efficient ternary complex formation.

Abstract

Various carboxylic-acid-based antennae tested.

Keywords:

• Lanthanide luminescence
• Lanthanide complexes
• Sensing
• Aspirin
• Salicylic acid

Acknowledgements

We thank Kinerton Ltd, Enterprise Ireland (Postgraduate Scholarships to A.J.H. and J.P.L.), National Pharmaceutical Biotechnology Center, Bio Research Ireland and Trinity College Dublin for financial support. We thank Dr Steven Faulkner, Dr Hazel M. Moncrieff and Dr Julie Tierney for their helpful discussions, and Dr John E. O'Brien for assisting with running NMR.

Notes

^†For such square antiprism geometry, two elements of chirality can be observed, which are associated with the sign of the torsion angles of the NCCO chelate (Δ or Λ) and the NCCN chelate rings (δ or λ) helicity of the N-alkylated pendent arms [80,81]. For Tb•1 (as shown in Fig. 3), these were determined to be an average of 19.2° and -58.5°, respectively. However, for Tb•1, the complex crystallised in a centrosymmetric space group containing both Δ and Λ conformations. Similar results were observed for Eu•1.

^‡There is also a possibility that at a very high concentration of 7 (OD ∼ 0.6 at its max.), some inner filter effects are observed. However, 8 and 9 (see below), which absorb at similar wavelengths, did not give rise to any sensitisation when interacting with Tb•1. Furthermore, using 10, this quenching was much less pronounced (see Fig. 11) for the same concentration range (see later).

^§In our earlier communication [70], we reported that we were unable to observe more than two emission bands (at 491 and 548 nm corresponding to J=6 and 5, respectively) when measuring these antennae. Though we did not conclude why this was the case, we have since then recorded all the above measurements on a new fluorimeter. As can be seen from Fig. 9, only the J = 2 transition is not observed. Hence, our original measurements were carried out on a machine that was not sensitive to these long-wavelength emission bands.

The Future of Supramolecular ChemistryThere is no doubt that Supramolecular Chemistry is a fast growing field of research. As the field grows, and more complex targets and applications are addressed, we must ensure that we do not neglect the fundamental science needed to achieve many of our principle aims. Recognition and signalling using chemosensors have become an ever more important part of supramolecular chemistry, particularly from a medical diagnostic point of view, since it enables the use of non-invasive, real-time monitoring of physiological species in vivo, something that we all will benefit from. In this paper, we try to address some of these fundamental aims such as recognition and signalling by developing “simple” self-assembly lanthanide-based sensors.Thorfinnur (Thorri) Gunnlaugsson was born in Iceland in 1967. He obtained his PhD with Professor A. P. de Silva at Queen's University Belfast, working on luminescent switches and sensors. He then moved to Durham University, England, as a postdoctoral fellow with Professor David Parker, working on developing luminescent lanthanide sensors, an area which still features strongly in his work. In 1998. he was appointed as the Kinerton Lecture in Medicinal Chemistry at Trinity College Dublin, and in 2000 as a lecturer in Organic Chemistry. In 2002 he was elected as Fellow of Trinity College Dublin. His main research interests are in the areas of Supramolecular and Medicinal Chemistry, and particularly in the fields of recognition and targeting.