The Thermochromic Phase Transition of a Polydiacetylene Poly (ETCD) Studied by High Resolution Solid State 13C NMR.

The thermochromic phase transition of a polydiacetylene, poly (ETCD), whose substituent is -(CH sub 2) sub 4 OCONH-C sub 2 H sub5, has been studied by use of high resolution solid state sup (13)C NMR. The change in the electronic state of the conjugated backbone, which is responsible for the color change, is directly monitored through the change in the chemical shift observed for the -C == backbone carbon, which reflects the change in the delocalization of backbone pi-electrons. We demonstrate the absence of a stable butatrienic structure. The sidechain conformations have been delineated by the comparison of their chemical shifts with those observed for several model compounds and polymers whose conformations are known. The sidechains have more gauche character in the low temperature blue phase than in the high temperature red phase. The transition of the backbone electronic structure has been found to be related to the conformational transition of the sidechain alkyl group, which permits a retention of inter-sidechain hydrogen-bonding through the transition. The thermochromic phase transition may be caused by the strain placed on the backbone by the hydrogen- bonded sidechains. The difference in the motional states of both backbone and sidechains in each phase is also discussed.