Analyzing nanostructures in mesogenic host-guest systems for polarized phosphorescence

Feasibility of polarized phosphorescent organic light-emitting devices (OLEDs) had been previously demonstrated by combining a discotic Pt(II) complex with a glassy-nematic oligofluorene host to form a mesogenic host-guest phosphorescent emitting system. Previous photophysical studies suggested that in the host-guest film, the Pt(II) complex tended to aggregate into columnar stacks, exhibiting metal-metal-to-ligand charge transfer (MMLCT) emission. Both host molecules and guest aggregates in the host-guest films could be oriented by a conductive alignment layer, giving rise to polarized phosphorescence from the Pt(II) aggregates. Nevertheless, film morphologies and nanostructures of the mesogenic host-guest systems have remained to be elucidated. In this work, grazing incidence X-ray scattering (GIXS) was carried out to analyze nanostructures in both neat films of the discotic Pt(II) complex and mesogenic host-guest mixture films. In addition, confocal laser scanning microscopy (CLSM) was also utilized for visualization of the morphologies of mesogenic host-guest systems. The columnar axes of nanostructured Pt(II) stacks lying on the alignment-treated surfaces were found to be preferentially oriented perpendicular to the rubbing direction, which is responsible for the observed linearly polarized phosphorescence. (C) 2013 Elsevier B.V. All rights reserved.