Synthesis, Spectroscopic Characterization, and Electro-Optical Properties of Noncentrosymmetric Azobenzene/Zirconium Phosphonate Multilayer Films

Ultrathin noncentrosymmetric nonlinear optical films based on zirconium phosphonate (ZP) self-assembled multilayers that incorporate the asymmetric azobenzene chromophore [5-[4-[[4-[(6-hydroxyhexyl)sulfonyl]phenyl]azo]phenyl]pentoxy]phosphonic acid (HAPA) are synthesized and constructed. The ZP film structure and multilayer deposition chemistry are characterized by a combination of polarization/modulation Fourier transform infrared reflection absorption spectroscopy, surface plasmon resonance (SPR) measurements, and optical second harmonic generation (SHG). SPR measurements on the HAPA multilayer films yield an average monolayer thickness of 27 ± 0.5 Å. The resonant SHG at 365 nm from ultrathin HAPA ZP films on silica surfaces increases quadratically with the number of self-assembled HAPA monolayers, and an analysis of the polarization dependence of the surface SHG yields an orientation parameter D = 0.79 ± 0.03 corresponding to an average tilt angle of 27 ± 2° for the azobenzene chromophores with respect to the surface normal. An electro-optic coefficient r 33 for the HAPA monolayers of 11 pm/V at 632.8 nm is obtained from SPR modulation experiments of ZP films on gold substrates that have been incorporated into air-gap capacitors. SPR modulation experiments are then performed on a HAPA monolayer in an in situ electrochemical environment in order to determine that a modulation of ±25 mV corresponds to a change in the electric field strength of 1 × 10 4 V/cm within the ultrathin organic film at the electrode surface.