Conductive and optically non-linear polymeric langmuir-blodgett films of poly(3-dodecylthiophene)

Monolayer film formation at the air/water interface was investigated for both electrochemically and chemically prepared poly(3-dodecylthiophene) using surface pressure-molecular area isotherms. Only the electrochemically prepared polymer formed a stable monolayer, which was successfully transferred using the horizontal lifting method. The transferred Langmuir-Blodgett films were characterized by u.v.-visible spectroscopy, quartz crystal microbalance measurements, electrical conductivity measurements and femtosecond degenerate four-wave mixing studies of the third-order optical non-linearity. The third-order optical susceptibility of undoped poly(3-dodecylthiophene) was found to be χ⁽³⁾ ∼ 10^-9, esu, large enough to allow the first reported observation of a degenerate four-wave mixing signal from ultrathin Langmuir-Blodgett films. In situ iodine-doping studies of u.v.-visible absorption, electrical conductivity and third-order non-linear optical susceptibility were carried out. Upon doping, the conductivity increased by more than eight orders of magnitude and the χ⁽³⁾ value decreased to within ten percent of the original value.