Probing d(8)-d(8) 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) com plexes, namely [Pt(L1-6)Cl] (1a-6a; HL1-6 = 4-(aryl)-6-phenyl-2,2'-bipyridi ne; aryl = H (1), phenyl (2), 4-chlorophenyl (3), 4-tolyl (4), 4-methoxyphe nyl (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 o ligomeric d(8)-d(8) and ligand-ligand interactions. The molecular structure s 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 communica tion 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 differ ent aryl substituents into the 4-position of 6-phenyl-2,2'-bipyridine, and this indicates little electronic delocalization between them. Self-quenchin g 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 com plex 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. T hese emissions are ascribed to (3)[d sigma*, pi*] excited states. In contra st, the emission of 10 and 11, bearing long bridging diphosphine ligands, a re attributed to (MLCT)-M-3 states of non-interacting [Pt(L-1)] moieties. S ignificantly, the luminescence of the mu-pyrazolate complex 9 displays tran sitional features which are reminiscent of both (3)[d sigma*, pi*] and (MLC T)-M-3 excited states. Hence a relationship is observed between emission en ergy, the nature of the lowest energy excited state, and metal-metal intera ctions. 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).