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

Abstract

We present an examination of the structural and photophysical characteristics of [Pt(N2O2)] complexes bearing bis(phenoxy)diimine auxiliaries (diimine = 4,7-Ph2phen (1) and 4,4′-tBu2bpy (2)) that are tetradentate relatives of the quinolinolato (q) ligand. These neutral derivatives display high thermal stability (> 400 °C in N2). 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 π-π interactions but no close Pt⋯Pt contacts. Complexes 1 and 2 exhibit moderately intense low-energy UV/Vis absorptions around λ=400-500 nm that undergo negative solvatochromic shifts. Both derivatives are highly luminescent in solution at 298 K with emission lifetimes in the ps range, and mixed 3[1→π*(diimine)] (l=lone pair/phenoxide) and 3[Pt(d)→π*(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.