Structural, photophysical, and electrophosphorescent properties of platinum(II) complexes supported by tetradentate N2O2 chelates

We present an examination of the structural and photophysical characteristics of [Pt(N2O2)] complexes bearing bis(phenoxy)diimine auxiliaries (diimine = 4,7-Ph(2)phen (1) and 4,4'-tBu(2)bpy (2)) that are tetradentate relatives of the quinolinolato (q) ligand. These neutral derivatives display high thermal stability (> 400degreesC in N-2). While the crystal lattice in 1 consists of (head-to-tail)-interacting dimers, molecules of 2 are arranged into infinitely stacked planar sheets with possible pi-pi interactions but no close (PtPt)-Pt-... contacts. Complexes 1 and 2 exhibit moderately intense low-energy UV/Nis absorptions around A=400-500nm that undergo negative solvatochromic shifts. Both derivatives are highly luminescent in solution at 298 K with emission lifetimes in the mus range, and mixed (3)[1 --> pi *(diimine)] (1 = lone pair/phenoxide) and (3)[Pt(d) --> pi*(diimine)] charge-transfer states are tentatively assigned. The excited-state properties of 2 are also investigated by time-resolved absorption spectroscopy and by quenching experiments with pyridinium acceptors to estimate the excited-state redox potential. These emitters have been employed as electrophosphorescent dopants in multilayer OLEDs. Differences between the brightness, color, and overall performance of devices incorporating 1 and 2 are attributed to the influence of the diimine substituents.