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

摘要

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 degrees 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 Delta H (-54.4 kJ mol(-1)) and Delta S (-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.