Characterization of Slow Chromophore Reorientation in Nonlinear Optical Polymers by Two-Dimensional MAS NMR.

摘要

Two-dimensional 15N MAS NMR spectrometry was used to characterize slow reorientation of the chromophore at temperatures well below T sub g in the prototypical nonlinear optical (NLO) polymer 10% 4-nitroaniline-15N2/poly(methyl methacrylate). Qualitative analysis of 2D contour plots obtained for various mixing times at 289 K suggests that significant reorientation takes place between 0 ms and 200-300 ms, and little or no further change occurs up to 2000 ms. Straightforward statistical arguments were used to devise a simple model for quantitative analysis of integrated peak intensities in 2D MAS spectra. This analysis demonstrates that the chromophore undergoes random isotropic reorientation at the longest mixing times used and permits a calculation of the fraction of oriented chromophore remaining at any intermediate mixing time. The data at 298 K are well fit by an exponential autocorrelation function with a time constant of 91 ms. Limited data obtained at 323 K demonstrate that the chromophore reorientation is thermally activated and permit a rough estimate of the activation energy. The methods reported here should be readily applied to the characterization of chromophore dynamics at moderately long time scales for a variety of NLO polymer materials.