Influence of PNIPAm on log Kf of a copolymerized 2,2′-bipyridine: revised bifunctional ligand design for ratiometric metal-ion sensing

Abstract

Here we describe the synthesis of a model compound (1) based upon a previously reported bifunctional 2,2′-bipyridine (2). Ligand pK_a and thermodynamic stability constants were investigated by potentiometric titrations for 1 in order to assess the metal-binding capabilities of 2 following subsequent incorporation within a temperature-responsive polymer that functions as a fluorescent metal-ion indicator. While the log K_Cu1 measured here was found to be 8.86 ± 0.05 at 25 °C, this value was previously seen to fall 2.8 orders of magnitude following copolymerization of 2 with poly(N-isopropylacrylamide) (PNIPAm). This drop in affinity was attributed to stabilization of the neutral ligand by the polymer environment and elevated temperatures at which metal-binding experiments were performed. ΔH (−54.4 kJ mol⁻¹) and ΔS (−12.8 J K⁻¹ mol⁻¹) were therefore determined through variable temperature titrations in order to establish the temperature dependence of log K_Cu1. Doing so enabled elucidation of the overall effect that the polymer environment exerts on thermodynamic stability of copolymerized 2. Specifically, the polymer indicator was found to decrease the thermodynamic stability by 2.2 orders of magnitude, whereas elevated temperatures account for the additional 0.6 order of magnitude drop observed. This finding has implications regarding the design of future bifunctional ligands for ratiometric sensing within our temperature-responsive polymer indicator.