Luminescence Color Tuning of PtII Complexes and a Kinetic Study of Trimer Formation in the Photoexcited State

We investigated the luminescence properties and color tuning of [Pt(dpb)Cl] (dpbH = 1,3-di(2-pyridyl) benzene) and its analogues. An almost blue emission was obtained for the complex [Pt(Fmdpb)CN] (FmdpbH = 4-fluoro-1,3-di(4-methyl-2-pyridyl) benzene), modified by the introduction of -F and -CH3 groups to the dpb ligand and the substitution of -Cl by -CN. As the concentration of the solution was increased, the color of the emission varied from blue to white to orange. The color change resulted from a monomer-excimer equilibrium in the excited state. A broad emission spectrum around 620 nm was clearly detected along with a structured monomer emission around 500 nm. Upon further increases in concentration, another broad peak appeared in the longer wavelength region of the spectrum. We assigned the near-infrared band to the emission from an excited trimer generated by the reaction of the excimer with the ground-state monomer. The emission lifetimes of the monomer, dimer, and trimer were evaluated as tau(M) = 12.8 mu s, tau(D) = 2.13 mu s, and tau(T) = 0.68 mu s, respectively, which were sufficiently long to allow association with another Pt-II complex and dissociation into a lower order aggregate. Based on equilibrium constants determined from a kinetic study, the formation of the excimer and the excited trimer were concluded to be exothermic processes, with Delta G(D)* = -24.5 kJ mol(-1) and Delta G(T)* = -20.4 kJ mol(-1) respectively, at 300 K.