Synthesis and photophysical studies of calix[4]arene-based binuclear platinum(II) complexes: Probing metal-metal and ligand-ligand interactions

A series of calix[4]arene-based binuclear platinum(II) complexes, Pt2LCl2 (1, L = 5,11,17,23-tetra-tert-butyl-25,27-di[methoxy(4-phenyl)-(C boolean AND N boolean AND N)]-26,28-dihydroxycalix[4]arene, HC boolean AND N boolean AND N = 6-phenyl-2,2'-bipyridine), [Pt2L(mu-dppC(n))]-(ClO4)(2) (dppC(n) = bis(diphenylphosphino)-methane (2, n = 1), -ethane (3, n = 2), -propane (4, n = 3), and [Pt2L(PPh3)(2)](ClO4)(2) (5), have been designed and synthesized in this work. Spectroscopic investigation demonstrates that p-tert-calix[4]arene is capable of assembling the two square-planar [(C boolean AND N boolean AND N)Pt(II)] units in a face-to-face manner, simultaneously suppressing intermolecular aggregation and increasing the solubility of the studied complexes. Facile replacement of the chloride ligand in 1 by the strongly a-donating ancillary phosphine ligands affords binuclear platinum(11) complexes with improved photophysical properties. All of the complexes are emissive both in the fluid/glassy solution and in the solid state, except for 1 in the solid state at room temperature. Moreover, the absorption and emission energies of the complexes are sensitive to the ancillary ligands. Varying the tethered phosphine auxiliaries from dppm (2) and dppe (3) to dppp (4) and PPh3 (5) modulates the intramolecular metal-metal (Pt center dot center dot center dot Pt) and ligand-ligand (pi-pi) distances, thereby leading to a switch of (MMLCT)-M-3 and excimeric (3)(pi pi*) excited states to a common (MLCT)-M-3 excited state.