We are investigating a method for synthesizing oriented layered thin film structures that exhibit nonlinear optical properties. This method involves self-assembly of metal bisphosphonate multilayers on gold and silicon substrates. Multilayers are characterized via optical ellipsometry, diffuse reflectance and attenuated total reflectance FTIR, and grazing angle X-ray diffraction.
To incorporate nonlinear optical activity into self-assembled films, we use a,w-bisphosphonates containing aromatic π systems sandwiched between donor and acceptor groups. In order to maximize the overall polarizability of the film, the polarizable bisphosphonate molecules must preferentially bind in only one orientation. To accomplish this, one of the terminal phosphonate groups of the molecule is protected (in ester form). After binding the free phosphonic acid end, the terminal ester is converted to a phosphonic acid group by hydrolysis with bromotrimethylsilane (BTMS). Layering is continued by binding a metal layer and repeating the cycle. Films prepared via this route do exhibit second harmonic generation, but apparently incomplete deprotection of each layer leads to increasing disorder with increasing number of layers.