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(L^1-6)Cl] (1a-6a; HL^1-6 = 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(L¹)E]⁺ (E = py (7), PPh₃ (8)), [Pt₂(L^1-6)₂(μ-dppm)]²⁺ (1b-6b, dppm = bis(diphenylphosphino)methane), [Pt₂(L¹)₂(μ-pz)]⁺ (9, Hpz = pyrazole), and [Pt₂(L¹)₂(μ-dppC_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 d⁸-d⁸ 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) Å, respectively. While minimal metal-metal communication is expected for 9, weak π-π 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 ³MLCT emission by the mononuclear derivatives are observed in CH₂Cl₂ at 298 K, and a red shift in the emission energy is exhibited by complex 7 in acetonitrile at 77 K. The fluid emissions of the μ-dppm species 1b-6b at λ_max 652-662 nm appear at substantially lower energies than their mononuclear counterparts and show dramatic solvatochromic effects. These emissions are ascribed to ³[dσ*, π*] excited states. In contrast, the emission of 10 and 11, bearing long bridging diphosphine ligands, are attributed to ³MLCT states of non-interacting [Pt(L¹)] moieties. Significantly, the luminescence of the μ-pyrazolate complex 9 displays transitional features which are reminiscent of both ³[dσ*, π*} and ³MLCT 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(*Pt₂²⁺/Pt₂⁺)] 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). © 1999 American Chemical Society.