Site-Isolation, Intramolecular Energy Transfer, and Crosslinking in Synthetic Dendritic Quinacridones

Abstract

Dendrimers incorporating a green emitter, quinacridone, for organic light emitting diodes (OLEDs) were synthesized and their photophysical and electrochemical properties were explored. Quinacridone dendrimers were synthesized for site isolation, intramolecular energy transfer, and photocrosslinking.Site-isolation of quinacridone at the core of a dendrimer was achieved by attaching bulky poly(aryl ether) dendrons to the quinacridone at the amino functional groups. Both benzyl- and t-butyl-terminated dendrimers were synthesized up to the third generation. These dendrimers showed enhanced solubility in organic solvents due to reduced aggregation and hydrogen bonding. Increased photoluminescence intensity was observed for the denderimers in the solid state indicating reduced self-quenching due to enhanced site-isolation. Preliminary incorporation of these dendrimers as dopants into OLEDs showed increased emission from the dendrimers as the doping percentage increases.When high-energy host absorbing groups, such as oligo(p-phenylene vinylene)s (oPPVs) were placed at the periphery of poly (aryl ether) dendrimers with quinacridone guest cores, intramolecular energy transfer occurs when the host periphery groups were excited. These dendrimers showed high efficiency energy transfer yields in both solution and the solid state, as well as an antennae effect which resulted in increased emission when the oPPVs were excited versus direct excitation of the quinacridone. For comparison, poly (methyl methacrylate) polymers with pendant oPPV groups were synthesized and combined both in solution and in thin films with the site-isolated dendrimers to investigate the architectural requirements for energy transfer. These mixtures showed no energy transfer in solution from the polymer to the dendrimers. However, in the solid state, energy transfer increaseed with decreasing generation due to the host/guest chromophores decreased separation.Finally, poly (aryl ether) dendrimers containing photocrosslinkable cinnamate groups at the periphery and quinacridone cores were synthesized. Thin films of the higher generation dendrimers were photopolymerized via ultraviolet irradiation. The films were resistant to solvent after the polymerization step indicating a stable crosslinked network. Standard photolithography was performed on the higher generation dendrimers to achieve feature sizes as small as 5 microns as observed by fluorescence and atomic force microscopy.