Sensitized terbium (III) macrocyclic-phthalimide complexes as luminescent pH switches

Abstract

Four new macrocyclic-phthalimide ligands were synthesised <i>via</i> the coupling of <i>N</i>-(3-bromopropyl)phthalimide either to cyclen (1,4,7,10-tetraazacyclododecane) itself or its carboxylate-functionalized analogues, and photophysical studies were carried out on their corresponding Tb(III) complexes in aqueous media as a function of pH. Luminescence intensities of <b>Tb·L^1a1a–Tb·L^3a3a</b> were in ‘<i>switched off</i>’ mode under acidic conditions (pH < 4), and were activated on progression to basic conditions as the phthalimido functions therein were hydrolysed to their corresponding phthalamates <b>Tb·L^1b1b1b–Tb·L^3b3b3b</b>. Emission of phthalamate-based macrocyclic Tb(III) complexes <b>Tb·L^1b1b1b–Tb·L^3b3b3b</b> was in ‘<i>switched on</i>’ mode between pH 4 and 11, exhibiting high quantum yields (<i>Φ</i>) and long lifetimes (<i>τ</i>) of the order of milliseconds at pH ∼ 6. Tb(III) emissions were found to decline with increasing number of chromophores. The values of <i>Φ</i> and <i>τ</i> were 46% and 2.4 ms respectively for <b>Tb·L^1b1b</b> at pH ∼ 6 when activated. This is the best pH-dependent sensor based on a Tb(III) complex reported to date, benefiting from the macrocyclic architecture of the ligand.