Amphiphilic Phenylene-Ethynylene Oligomers in Langmuir-Blodgett Films. Self-Assembling Multilayers for Electroluminescent Devices

New rigid amphiphilic molecules based on a p-phenylene-ethynylene unit with hydrophilic side chains were synthesized by a step by step method up to the heptamer. The two most interesting materials, the pentamer and the heptamer, are amphiphilic enough in nature to produce stable Langmuir films on hydrophilic substrates such as hydrophilic glass, ITO, or hydrophilic silicon. A transfer ratio of 1, observed only by lifting, suggests a Z-type deposited film. The multilayer deposition can be carried out up to 36 layers. The films were analyzed by X-ray reflectivity and are revealed as well structured with a layering period of 3.7 nm. This suggests a rearrangement in a Y-type bilayer occurring after transfer deposition from the water surface. Using AFM, the surfaces of films deposited on glass or Si are shown to exhibit steps of 3.6-3.7 nm height or multiples, which are coherent with a self-rearrangement of the single deposited layer to a double layer during the drying process. The heptamer and pentamer show high photoluminescence and large Stokes shifts with emission peaks at 516 and 504 nm. LED properties are demonstrated using the ITO/oPEn/LiF/Al sandwich yielding photon emission at 516 nm for the heptamer. The luminescencevoltage characteristics of two diodes using 22 and 36 LB layers show threshold voltage at 4.5 and 6 V respectively and in those conditions the electroluminescence yield is close to 10^-3%. It is concluded that the electroluminescence in a LB film of molecules aligned parallel to the substrate is interesting because it confirms the possibility of tailoring conduction and emission properties of devices using a layer by layer deposition technique.