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;N;N)]-26,28-dihydroxycalix[4]arene, HC;N;N = 6-phenyl-2,2'-bipyridine), Pt2L(mu-dppCn)2 (dppCn = bis(diphenylphosphino)-methane (2, n = 1), -ethane (3, n = 2), -propane (4, n = 3), and Pt2L(PPh3)22 (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;N;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 sigma-donating ancillary phosphine ligands affords binuclear platinum(II) 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...Pt) and ligand-ligand (pi-pi) distances, thereby leading to a switch of 3MMLCT and excimeric 3(pipi*) excited states to a common 3MLCT excited state.