Study of the correlation between the backbone conformation and the electronic structure of polydiacetylenes by solid state 13C n.m.r.

Abstract

The relationship between the conformation and the electronic structure of the backbones in polydiacetylene (PDA) has been investigated by high resolution, solid state 13C nuclear magnetic resonance (n.m.r.). We have studied four different PDAs: PTS in its blue phase, the red phase of PTCDU, PTCDU in the stress-induced blue phase, and the thermochromically related blue and red phases of PETCD and P4BCMU. We have not detected the presence of a stable butatrienic structure in any of these PDAs. Instead, we have found that there are two kinds of backbone conformations, or structures, one which is unique to the blue phase and the other unique to the red phase. For all the blue phase PDAs studied here, the C resonance appeared at c. 107 ppm, while for all the red phase PDAs the same resonance occurred at c. 103 ppm. The backbone structure of the blue phase was found to be more planar than that of the red phase based on the overall C chemical shift positions. These backbone structures appear to be general for PDAs. This study shows that the chemical shift position of C can be utilized as a fingerprint for the extent of planarity, or conjugation length, of any PDA system.