Dielectric and Absorbate Effects on the Optical Properties of Phosphazenes
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largest terms for Yxxxx have the form:
, revealing that
the critical terms are the electronic transition energies, and state and transition dipole strengths. Organic molecules, the high hyperpolarizabilities result from high transition dipole strengths originating from it -->7t* transitions. Previous calculations on cyclic 769 Mat. Res. Soc. Symp. Proc. Vol. 328. @1994 Materials Research Society
phosphazenes have shown that the dominant optical characteristics are n-->Tc* electronic transitions, which have notoriously weak transition strengths, and a large AEtrans [6]. Applying the perturbative method to the linear phosphonitrilic system of F3 P(NPF 2 )NH, we found the largest five contributors to the hyperpolarizability were: Transition 35 -->43 37 --> 43 38 -->41 39 -->41 39 -- > 42
it-it* it-it*
7t'-7t(L)'* 7t'-it(L)'* it'-it(L)'*
5"
Ei-Eo
1.014 0.721 0.872 0.774 0.663
1.380 3.631 3.518 4.723 4.233
16.214 14.792 12.714 12.000 13.420
These characteristics would indicate that the phosphonitrilic systems represent special cases of conjugated molecules as their it orbital interactions are localized and have high state dipole strengths, but low transition strengths. In this sense our goals are simple, perturb the electronic structure from its normal gas phase patterns. At the atomistic level, solvent dielectric interactions can alter gas phase bond dipoles and increase delocalization. Absorbate will break gas phase symmetry conditions which would reduce transition dipole strengths, and through direct atomic site interaction change site-to-site charge differences. In this paper, we use a combination of experimental and theoretical methods to probe the absorbate and acidity effects on the nonlinear optical properties of phosphonitrilic molecules. EXPERIMENTAL AND THEORETICAL METHODS ComputationalModel Semiempirical valence electron molecular orbital methods were used to evaluate the electronic structure of derivatized phosphonitrilic compounds. All molecular structures were fully force optimized with no geometric constraints other than obvious symmetries. Structural parameters for initial geometries were taken from available x-ray crystal structure determinations, whenever possible. Previous studies have shown that the MNDO and PM3 Hamiltonians to be highly accurate for the prediction of proton affinities (7.2 kcal/mole mean absolute error with systematic tendencies for chemical classes of molecules) [7-8]. Hyperpolarizabilities were calculated by finite field methods using the PM3 Hamiltonian in MOPAC 6.0 (QCPE #455). Sample Preparationand Characterization. Derivatized phosphazene trimers and tetramers were prepared from synthetic methods in open literature [9]. Reaction products were characterized using 3 1P NMR spectra obtained at 121.1 Hz with a Varian VXR-300 spectrometer and referenced to 85 % H 3P0 4 . Measured NMR spectra were consistent with published results [8]. Third-order susceptibilities were determined from concentrated phosphazene solutions (10% wt.) by DFWM measurements [10]. For acidic c
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