pH-Responsive Lanthanide Complexes Based on Reversible Ligation of a Diphenylphosphinamide

Ln-dpp-DO3A-based complexes [dpp is a pendant diphenylphosphinamide moiety and DO3A is 1,4,7-tris(carboxymethyl)-1,4,7,10- tetraazacyclododecane] exhibit pH-responsive reversible ligation of the phosphinamide for both the gadolinium(III) and europium(III) analogues. pKa values were 8.1 (±0.1) and 7.8 (±0.1) for Gd.1 and Gd.2, respectively. The relaxivities (20 MHz, 298 K) of the gadolinium(III) analogues were r1 = 7.9 mM−1 s−1 (Gd.1) and r1 = 8.2 mM−1 s−1 (Gd.2) in acidic media, corresponding to a hydration state q = 2; in basic media, deprotonation and coordination of the phosphinamide occurs, with r1 = 5.4 mM−1 s−1 (Gd.1) and r1 = 4.4 mM−1 s−1 (Gd.2) corresponding to q = 1. Sensitized luminescent emission was observed from the europium(III) analogues following excitation at λex = 270 nm. The hydration state values of the europium(III) analogues were consistent with those of the gadolinium(III) complexes, i.e., q = 1 and 2 in basic and acidic media, respectively. The ratio of the emission intensities of the ΔJ = 1 and 2 bands enables concentration independent reporting of the pH. Excited-state pKa values were 8.3 (±0.1) and 8.5 (±0.1) for Eu.1 and Eu.2, respectively.