Order of Assembly, Not Thermodynamics, Programs the Zn2+–Terpyridine Coordination Stoichiometry on a Clay Nanosheet

Abstract

The emission properties of functional dyes and their metal complexation depend strongly on the surrounding environment. In homogeneous solution, metal–ligand complexation relaxes to the thermodynamically most stable product, so it is difficult to isolate a kinetically disfavored coordination mode selectively. In this work, we electrostatically immobilized a divalent cationic terpyridine derivative, tPy-A, on the surface of anionic synthetic saponite nanosheets (SSA) and compared its Zn2+ complexation and emission behavior with those in solution. Adsorption on SSA increased the fluorescence quantum yield of tPy-A from 0.009 in solution to 0.077, an 8.6-fold enhancement (surface-fixation induced emission, S-FIE), which was further enhanced by Zn2+ complexation. Two independent methods─a Job plot and a saturation-point analysis based on the method of continuous variation─showed that the tPy-A/Zn2+ = 2:1 complex dominant in solution changed to 1:1 on SSA. Furthermore, even when the ligand, metal, and concentration were identical, simply changing the order of adsorption and complexation switched the dominant complex species on the surface (Φf = 0.43 and 0.37), and the interconversion between them showed an activation barrier of 102 kJ mol–1. In other words, under the present conditions the assembly pathway kinetically determines which complex is initially formed, and the smooth surface then kinetically stabilizes that choice by suppressing the in-plane migration of tPy-A, so two distinct emissive complexes can be selectively prepared on a single surface. The immobilized 1:1 complex emits 48 times as strongly as the monomer in solution. The two-dimensional surface of clay nanosheets thus controls not only the emission but also the metal-coordination pathway of an adsorbed molecule, kinetically retaining a coordination mode that differs from the one favored in homogeneous solution.