Strategy for the Construction of Diverse Poly-NHC-Derived Assemblies and Their Photoinduced Transformations

A series of supramolecular assemblies of types [Ag-8(L)(4)](PF6)(8) and [Ag-4(L)(2)](PF6)(4), obtained from the tetraphenylethylene (TPE) bridged tetrakis(1,2,4-triazolium) salts H-4-L(PF6)(4) and Ag-I ions, is described. The assembly type obtained dependends on the N-wingtip substituents of H-4-L(PF6)(4). Changes in the lengths of the N4-wingtip substituents enables controlled formation of assemblies with either [Ag-4(L)(2)](PF6)(4) or [Ag-8(L)(4)](PF6)(8) stoichiometry. The molecular structures of selected [Ag-8(L)(4)](PF6)(8) and [Ag-4(L)(2)](PF6)(4) assemblies were determined by X-ray diffraction analyses. While H-4-L(PF6)(4) does not exhibit fluorescence in solution, their tetra-NHC (NHC=N-heterocyclic carbene) assemblies do upon NHC-metal coordination. Upon irradiation, all assemblies undergo a light-induced, supramolecule-to-supramolecule structural transformation by an oxidative photocyclization involving phenyl groups of the TPE core, resulting in a significant change of the luminescence properties.