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Final Report Summary - TRAMLANES (Transition-metal / lanthanide dyads for two-photon cellular imaging)

Phosphorescent metal complexes offer major advantages over conventional fluorescent organic molecules as the basis of luminescent probes for cell imaging. The long luminescence lifetimes associated with triplet emission from complexes of e.g. Pt(II), Ru(II), Ir(III), Re(I) and lanthanides allow simple rejection of shortlived background autofluorescence which might otherwise interfere. In addition, variations in luminescence lifetimes of such complexes in different cellular regions, caused by the presence of different analytes such as O2, provide the basis of the recently-developed microsecond lifetime mapping techniques; phosphorescence lifetime imaging (PLIM) and time-resolved emission microscopy (TREM).

We developed a new rigid and conjugated ligand structure Ir•Lnx (where, x = 1 and 2 respectively) connecting phenanthroline and poly(amino-carboxylate) binding sites (see Chart 1) to provide mixed d/f complexes (transition metal + lanthanide). These show high potential for use in dual modal (PLIM and magnetic resonance) imaging, and well as for luminescence imaging using either luminescence component independently. Several features of these carefully-drsigned compounds make them valuable for use in these ways. In these complexes, a strongly phosphorescent Ir(III) unit connected to a water-stable Ln(III) unit via a fully conjugated and rigid connector. This results in both (i) long-lived luminescence which can be used in PLIM imaging under one-photon or two-photon excitation, and (ii) unusually long relaxivity from a single Gd(III) centre as a consequence of the rigid design. The combination of Ir(III) and Gd(III) components for dual-imaging purposes has been very little explored and this report is the first demonstration of PLIM using a complex that also has high relaxivity for MRI (the relaxivity of Ir•Gd is 11.9 mM− 1 s− 1 which is much more higher than the other reported values in current literature) purposes. The same ligand architecture also provides an effective through-bond coupling pathway for efficient Dexter Ir(III)→Eu(III) energy-transfer (EnT) in the isostructural Ir•Eu complex. Dual-luminescent d/f complexes are of interest for a range of applications from imaging to white-light emission and many of these applications hinge on the extent of d→f EnT which controls the balance of luminescence output from the two components.

Socio-economic impact: The value of these compounds is in healthcare: compounds of this nature that alow te bodym or tissue samples, to be imaged with high resolution can be used for diagnosis. MRI gives high anatomical resolution and deep tissue penetration, but lacks sensitivity. On the other hand, optical imaging gives high sensitivity but has limited tissue penetration. The combination of these two techniques will provide a more complete picture of the biological area of interest. Therefore, the new molecular architectures which demonstrates dual modal imaging approach could be promising probes for medicinal applications where the combination of two different imaging modes will provide two types of complementary imaging information from a single molecule.

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