2ND-ORDER NONLINEAR-OPTICAL PROPERTIES OF SUBSTITUTED ARYLPHOSPHINE OXIDES

We report computational and experimental studies of the second-order nonlinear optical properties of substituted arylphosphine oxides (4-X-C6H4)Ph(2)P=O (1), (4-X-C6H4)(2)PhP=O (2), (4-X-C6H4)(3)P=O (3) (where X = -NH2 (a) and -OH (b)), (4-NH2C6H4)(4'-NO2C6H4)PhP=O (4), and (4-NH2C6H4)(4'-CF3C6H4)(2)P=O (5) Computational results (AM1/finite field and AM1/sum-over-states) predict that 1, 2, 4, and 5 possess intrinsic molecular hyperpolarizabilities (parallel to beta parallel to) comparable to 4-nitroaniline(9). Molecular orbital analysis indicates that the hyperpolarizability arises not by charge transfer from the donor group (-NH2 or -OH) to the P=O moiety but instead by charge transfer from the donor group to the unsubstituted and/or acceptor-substituted aryl ring(s). This behavior is not well described by the ''two-level model'' of nonlinear optical response. The calculations indicate that the molecular hyperpolarizability and dipole moment are nearly orthogonal in 1 and 2, greatly reducing the component of the molecular hyperpolarizability in the direction of the dipole moment (beta(mu)). Experimental measurements of beta(mu) using the technique of electric-field-induced second-harmonic generation (EFISH) reveal modest values for compounds 1-3 (beta(mu) ranging from 1.0 x 10(-30) to 4.6 x 10(-30) cm(5) esu(-1)) in fairly good agreement with the calculations. EFISH measurements show phosphine oxides 4 and 5 have beta(mu) values (9.0 x 10(-30) and 10.1 x 10(-30) cm(5) esu(-1), respectively) twice as large as compounds 1-3 despite the fact that the intrinsic hyperpolarizabilities, parallel to beta parallel to, of 4 and 5 are smaller. The increase in beta(mu) arises because the dipole moment vector in 4 and 5 is more nearly collinear with the major components of the hyperpolarizability tenser. Compounds 1-3 form highly crystalline solid-state structures characterized by extensive intermolecular hydrogen bonding and high thermal and mechanical stability. Experimental powder SHG results reveal that monosubstituted (1a and 1b) and disubstituted (4) phosphine oxides are capable of second-harmonic generation (SHG) in the solid state (0.01, 1.0, and 1.0 x urea,respectively). X-ray crystal structures reveal that compounds 1a and 1b crystallize in the noncentrosymmetric space groups Cc and Pca2(1), respectively. Most importantly, the hyperpolarizability of phosphine oxides 1-3 and 5 arises in the absence of electronic absorptions in the visible spectral region; compounds 1-3 and 5 display excellent transparency (lambda(max), < 300 nm) in the visible and near-UV spectral regions.