π-π interactions in organometallic systems. Crystal structures and spectroscopic properties of luminescent mono-, bi-, and trinuclear trans-cyclometalated platinum(II) complexes derived from 2,6-diphenylpyridine

A series of mono- and multinuclear dicyclometalated platinum(II) complexes, namely, [Pt-(C^NC)L¹] [HC^NCH = 2,6-diphenylpyridine; L¹ = 4-tert-butylpyridine (1), 1-methyl-4,4′-bipyridinium (MQ⁺) hexafluorophosphate (2(PF₆)), 2,6-dimethylphenylisocyanide (4), tricyclohexylphosphine (5), triphenylphosphine (7)], [Pt₂(C^NC)₂(μ-L²)] [L² = pyrazine (pyr; 3), bis(dicyclohexylphosphino)methane (dcpm; 6), bis(diphenylphosphino)methane (dppm; 8)], and [Pt₃(C^NC)₃(μ₃-dpmp)] [dpmp = bis(diphenylphosphinomethyl)phenylphosphine (9)], were synthesized from [Pt(C^NC)dmso] (dmso = dimethyl sulfoxide). The X-ray crystal structures of 1,3·CHCl₃, 4, 6·CHCl₃·CH₃OH·4H₂O, 7, 8 (yellow form), and 8·CHCl₃ (orange form) have been determined. For complex 1, the Pt(C^NC) units are oriented into pairs in a head-to-tail fashion with an interplanar separation of 3.40 Å. For complex 4, dimeric π-π overlap between Pt(C^NC) and phenyl moieties in a head-to-tail manner is observed, with interplanar stacking at 3.39 Å. There are two crystal forms for complex 8. Only one intramolecular Pt(C^NC)-phenyl π-π interaction (separation 3.381 Å) is apparent in the yellow form. For the orange form (8·CHCl₃), there are two intramolecular Pt(C^NC)-phenyl π-π contacts (separations 3.115 and 3.287 Å). For 3·CHCl₃, the dihedral angles between the pyrazine and the two Pt(C^NC) planes are 124.0° and 130.5°, which allows the possibility for partial orbital interaction between the (5d)Pt and pπ*(pyrazine) orbitals. Complexes 1 and 4-9 display vibronically structured emission (λ_max of most intense band = 508-526 nm) in 77 K methanol/ethanol (1:5, v/v) glass, which are assigned to ³IL excited states. For 1, 4-6, 8, and 9, the 77 K structureless emission at λ_max 596-636 nm in methanol/ethanol glasses is assigned as ππ* excimeric in nature. These excimeric emissions are concentration-dependent for 1 and 4-6 and concentration-independent for 8 and 9. The 298 K solid emission spectra of 1, 4-6, 8, and 9 are characterized by a broad, unstructured band at λ_max 566-633 nm, which red-shift in energy upon cooling to 77 K. These bands are comparable in energy to the 77 K glassy emissions and are similarly assigned. Complexes 2 and 3 show an intense absorption at λ_max 410 (shoulder) and 456 nm, which are ascribed to the MLCT (5d)Pt → π*(1-methyl-4,4′-bipyridinium/pyrazine) transition, respectively. Likewise, the solid-state emissions of 2 and 3 at λ_max 686 and 658 nm, respectively, are assigned as MLCT. The vapochromism of complex 8 has been investigated.