Solution Chemistry Effects on the Nonlinear Optical Properties of Phosphazenes
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Mat. Res. Soc. Symp. Proc. Vol. 374 0 1995 Materials Research Society
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(c) Figure 1 - Proposed modes of altering the charge distribution along the P-N backbone. a) Ligand donation effects, b) Hydrogen bonding by backbone nitrogen, and c) Acid-base reaction by backbone nitrogen and solution. 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. Although previous work has used the PM3 Hamiltonian, in this work we used the AMI model which also has been shown to be quite accurate for proton affinities [6-7] and superior for predictions of electronic transition energies of methyl and simple amino derivatives. Estimated solution values were obtained with the COSMO (Conductor-like Screening Model) [8] module incorporated in MOPAC 93 using a solution e=78.4. Hyperpolarizabilities were calculated by the Time Dependent Hartree-Fock (TDHF) methodology [9,10] in MOPAC 93 [11]. Sample Pre_ arationand Characterization. Details of the synthetic procedures for preparation and characterization of derivatized phosphazene trimers and tetramers have been described previously in open literature [3,4] Reaction products were characterized using 3 1P NMR spectra and were consistent with previously published results. Third-order susceptibilities were determined from concentrated phosphazene solutions (10% wt.) by DFWM measurements [12]. For acidic conditions, the pH of these solutions was reduced to pH=1 by the addition of concentrated aqueous HC1. RESULTS AND DISCUSSION The aim of the current work is to enhance the nonlinear response of phosphonitrilic systems in a controlled fashion. In Table 1, we report experimental second hyperpolarizability yields for N 3P 3 (NHCH 3 )6 and N4 P4 (NHCH 3 )8 , as a 10% by wt. 218
solution in water and acidic conditions (Table 1). Previous work has noted that the nodal nature of the cyclic tetramer results in more localized it electron density, despite the larger P-N-P bond angles. Values for N 3P 3 (N(CH 3)2 )6 and the -N(CH 3 )2 and NH(CH 3 ) polymers are given to provide reference for a different ligand contribution for the cyclic timer case and polymer/model system comparisons, respectively. Notably, the X3 yield for N 3P 3 (NHCH 3 )6 in acidic conditions is over 3 times the original value, and the corresponding tetramer is over 6 times its free solution value. In order to better understand the X3 enhancement in acidic conditions, and the differences between the -N(CH 3 )2 and NH(CH 3) derivatives, we have performed a series of electronic structure investigations on phosphonitrilic molecules. The data in Tables
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