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 th e heptamer. The two most interesting materials, the pentamer and the heptam er, are amphiphilic enough in nature to produce stable Langmuir films on hy drophilic substrates such as hydrophilic glass, ITO, or hydrophilic silicon . A transfer ratio of 1, observed only by lifting, suggests a Z-type deposi ted film. The. multilayer deposition can be carried out up to 36 layers. Th e films were analyzed by X-ray reflectivity and are revealed as well struct ured 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. U sing AFM, the surfaces of films deposited on glass or Si are shown to exhib it 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 dr ying process. The heptamer and pentamer show high photoluminescence and lar ge 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 luminescence-voltage characteristics of two d iodes using 22 and 36 LB layers show threshold voltage at 4.5 and 6 V respe ctively and in those conditions the electroluminescence yield is close to 1 0(-3)%. It is concluded that the electroluminescence in a LB film of molecu les aligned parallel to the substrate is interesting because it confirms th e possibility of tailoring conduction and emission properties of devices us ing a layer by layer deposition technique.