End Group Effects on the Structure and Spectroscopy of Oligoazines
William B. Euler, Meng Cheng, Chao Zhao, Chemistry of Materials, 1999, 11, 3702-3708.
Abstract
Oligoazines with three different types of end groups, ketone, hydrazone,
and 2-pyridine, are studied in solution. When the end groups are ketone
or 2-pyridine, the 1H and 13C NMR spectra
are simple and show that the oligomers maintain 2-fold symmetry up to the longest
chain lengths investigated (five azine units for the 2-pyridine end groups).
In contrast, when the end groups are hydrazones, the 1H NMR
spectra show evidence for as many as five conformers present in solution
for each oligomer while the 13C NMR spectra are still simple,
indicative of a single compound. Semiempirical AM1 calculations suggest that all of the oligoazines
should be severely twisted but give no insight
about the origin of the large number of conformers found for the materials
with hydrazone end groups. The end groups have a significant effect on
the UV spectra. All the oligoazines show two absorptions close in energy,
but the relative intensities change depending upon the end group. For ketone
end groups the higher energy transition is stronger, for 2-pyridine end
groups the lower energy transition dominates, and for the hydrazone end
groups the two transitions are about the same intensity. Deconvolution
of the UV spectra allowed a more accurate assessment of the profile of
each absorption peak. Use of the fitting parameters for the lowest energy
azine 
-
transition
removed the effect of the end groups and gave a consistent estimate of
the band gap in a poly(methylazine) of 3.31 eV.