An Alternative Route to Highly Luminescent Platinum(II) Complexes: Cyclometalation with N^∧C^∧N-Coordinating Dipyridylbenzene Ligands

The remarkable luminescence properties of the platinum(II) complex of 1,3-di(2-pyridyl)benzene, acting as a terdentate N∧C∧N-coordinating ligand cyclometalated at C2 of the benzene ring ([PtL1Cl]), have been investigated, together with those of two new 5-substituted analogues [PtL2Cl] and [PtL3Cl] {HL2 = methyl-3,5-di(2-pyridyl)benzoate; HL3 = 3,5-di(2-pyridyl)toluene}. All three complexes are intense emitters in degassed solution at 298 K (λmax 480−580 nm; φlum = 0.60, 0.58, and 0.68 in CH2Cl2), displaying highly structured emission spectra in dilute solution, with lifetimes in the microsecond range (7.2, 8.0, and 7.8 μs). On the basis of the very small Stokes shift, the highly structured profiles, and the relatively long lifetimes, the emission is attributed to an excited state of primarily 3π−π* character. At concentrations >1 × 10-5 M, structureless excimer emission centered at ca. 700 nm is observed. The X-ray crystal structure of [PtL2Cl] is also reported.