Second-Order Nonlinear Optical Poly(organophosphazenes): Synthesis and Nonlinear Optical Characterization

The synthesis and second-order nonlinear optical properties of a series of mixed-substituent poly(organophosphazenes) that possess covalently attached donor-acceptor-substituted, conjugated moieties are reported. The mixed-substituent polymers have the general structure [NP(OCH₂CF₃),(OR)_y]_n, where OR = 0(CH₂CH₂O)_kC₆H₄(CH=CH)_mC₆H₄NO₂, where k = 1–3 and m = 1-3,O(CH₂CH₂O)₃C₆H₄CH=CHC₆H₄-CN, OCH₂CH₂N(CH3)C6H₄NO2, and OCH₂CH₂N(CH₂CH₃)C₆H₄N=NC₆H₄NO₂ and x + y = 100%. Model compound studies were carried out with the cyclic trimers N₃P₃(OC₆H₅)(OR) and N₃P₃(OCH₂CF₃)5(OR), where OR = O(CH₂CH2O)3C₆H₄CH=CHC₆H₄NO₂. Molecular structural characterization for the high polymers was achieved by ¹H and ³¹P NMR, infrared, and UV-visible spectroscopy, gel permeation chromatography, and thermal analysis. The second-order nonlinear optical properties of polymer films were evaluated by second-harmonic generation. Alignment of the nonlinear optical groups was achieved by application of an electric field by a corona discharge. The second-order nonlinear coefficients, d(33, of the polymer films were in the range d₃₃ = 4.1–34 pm/V and were determined by a Maker fringe analysis of the data. The second-harmonic generation behavior decayed following removal of the electric field. The variation in the d₃₃ values was correlated to the molecular structure and the percent incorporation of the chromophores.