Probing d8-d8 Interactions in luminescent mono- and binuclear cyclometalated platinum(II) complexes of 6-phenyl-2,2′-bipyridines

A series of luminescent mono- and binuclear cyclometalated platinum(II) complexes, namely [Pt(L1-6)Cl] (1a-6a; HL1-6 = 4-(aryl)-6-phenyl-2,2'-bipyridine; aryl = H (1), phenyl (2), 4-chlorophenyl (3), 4-tolyl (4), 4-methoxyphenyl (5), 3,4,5-trimethoxyphenyl (6)), [Pt(L-1)E](+) (E = py (7), PPh3 (8)), [Pt-2(L1-6)(2)(mu-dppm)](2+) (1b-6b, dppm = bis(diphenylpbosphino)methane), [Pt-2(L-1)(2)(mu-pz)](+) (9, Hpz = pyrazole), and [Pt-2(L-1)(2)(mu-dppcC(n))](2+) (dppC(n) = bis(diphenylphosphino)propane (10, n = 3) and -pentane (11, n = 5)), were synthesized in order to examine fluid- and solid-state oligomeric d(8)-d(8) and ligand-ligand interactions. The molecular structures of 4b(ClO4)(2) and 9(PF6) reveal intramolecular Pt-Pt distances of 3.245(1) and 3.612(2) Angstrom, respectively. While minimal metal-metal communication is expected for 9, weak pi-pi interactions are possible. All complexes described in this work are emissive in fluid solution at room temperature. Negligible changes in emission energy are detected by incorporating different aryl substituents into the 4-position of 6-phenyl-2,2'-bipyridine, and this indicates little electronic delocalization between them. Self-quenching of the (MLCT)-M-3 emission by the mononuclear derivatives are observed in CH2Cl2 at 298 K, and a red shia in the emission energy is exhibited by complex 7 in acetonitrile at 77 K. The fluid emissions of the mu-dppm species 1b-6b at lambda(max) 652-662 nm appear at substantially lower energies than their mononuclear counterparts and show dramatic solvatochromic effects. These emissions are ascribed to (3)[d sigma*, pi*] excited states. In contrast, the emission of 10 and 11, bearing long bridging diphosphine ligands, are attributed to (MLCT)-M-3 states of non-interacting [Pt(L-1)] moieties. Significantly, the luminescence of the mu-pyrazolate complex 9 displays transitional features which are reminiscent of both (3)[d sigma*, pi*] and (MLCT)-M-3 excited states. Hence a relationship is observed between emission energy, the nature of the lowest energy excited state, and metal-metal interactions. The excited-state redox potential [E(*Pt-2(2+)/Pt-2(+))] of 1b has been estimated by electrochemical studies (1.61 V vs NHE) and by quenching experiments with aromatic hydrocarbons (1.63 V vs NHE).