Smart/intelligent contrast agent candidates for MRI based on Mn­(II) ion are rare, as it usually forms labile complexes with polyaminocarboxylate-type ligands. Here, we report the first example of a Mn­(II) complex that can be activated by changing the pH of its local environment. The PC2A-EA ligand with an ethylamine pendant arm was found to form a thermodynamically stable (log K MnL = 19.01, pMn = 9.27) and kinetically inert complex with Mn­(II) with respect to trans-chelation with a metal ion such as Cu­(II). The [MnH­(PCA2-EA)] complex displays a relatively slow water exchange rate ((4.0 ± 0.2) × 107 s–1), but the pH-dependent coordination of the ethylamine moiety occurs in the pH range of 6–8 (log K MnL H = 6.88), enabling the complex to exhibit pH-sensitive relaxivity in the biologically relevant pH range.

Toxicity concerns related to Gd(III)-based MRI agents prompted an intensive research towards their replacement by complexes of essential Mn(II) ion. Here, we report a macrocyclic chelate, [Mn(PC2A-BP)], which possesses high thermodynamic stability and kinetic inertness as well as remarkable relaxivity (r 1p = 23.5 mM-1 s-1 , 20 MHz, 37 °C) in the presence of human serum albumin allowing a significant MRI signal intensity increase in the vasculature even at low dose (25 mol/kg) of the complex.

A new potential Zn(II) responsive Mn(II)-based MRI CA candidate derived form pyclen-3,9-diacetate (3,9-PC2A) has been synthesized possessing a di-(2-picolyl)amine (DPA) moiety as an active arm. The PC2A-DPA ligand was found...

New “all-in-one” theranostic systems, combining a magnetic resonance imaging (MRI) contrast agent with a biphotonic photodynamic therapy (2P-PDT) photosensitiser generating cytotoxic singlet oxygen, were envisaged and synthesised. They are based...