Light-Emitting Fluorene Photoreactive Liquid Crystals for Organic Electroluminescence

Abstract

Light-emitting liquid crystals for organic light-emitting diodes (OLEDs) require low-temperature liquid crystal phases for room-temperature processing and a range of molecular energies for electron and hole injection, as well as tunable color and color purity for multicolor OLEDs. We report a number of light-emitting polymerizable liquid crystals (reactive mesogens) based on 2,7-disubstituted-9,9-dialkylfluorene, whose energy levels can be tuned for optimized charge injection and light emission. As a consequence of these systematic property/structure investigations small molecule reactive mesogens have been synthezised, which exhibit low melting points, even below room temperature and nematic phases above room temperature as single components. Many of the molecules retain a supercooled nematic phase on cooling to room temperature. Simple binary eutectic mixtures of reactive mesogens with identical aromatic cores form light-emitting nematic phases at room temperature with a high clearing point to generate a high order parameter. The ionization potential of six-ring fluorene reactive mesogens can be tuned between 4.93 and 5.57 eV by chemical modification of the aromatic cores. Similarly the emission spectrum can be tuned from blue to green. A typical performance for an OLED using such liquid crystalline materials as a cross-linked polymer network is described.