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

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.