Langmuir-Blodgett films: molecular engineering of non-centrosymmetric structures for second-order nonlinear optical applications

Optical second-harmonic generation (SHG) is conditional upon the structure being non-centrosymmetric and consequently, the Langmuir-Blodgett (LB) technique is of interest because it permits control at the molecular level. However, the criteria for alignment at the air/water interface, i.e. a hydrophilic head and a hydrophobic tail, tend to impede the required packing arrangement within the multilayer. Most show inversion symmetry with the interfaces being alternately hydrophobic (tail-to-tail) and hydrophilic (head-to-head). This has been overcome by interleaving the layers with inactive spacers and, in such films, the long-range structural order is controlled by utilising compatible component molecules, interdigitating arrangements ('molecular zips') and interlayer hydrogen-bonding. Furthermore, the employment of optically nonlinear chromophores with two hydrophobic end groups eliminates the need for inactive spacers. The molecules form stable non-centrosymmetric structures because, unlike above, the interfaces are invariably hydrophobic. In this review, the molecular requirements for LB deposition are discussed together with the film-forming behaviour and properties of a variety of optically nonlinear materials.