Study of the thermochromic phase transition of polydiacetylene by solid state sup (13) C NMR.

The thermochromic phase transition of polydiacetylene (poly (ETCD) having the substituent R = (CH sub 2) sub 4 OCONHC sub 2 H sub 5) has been studied by solid state sup (13) C NMR for the first time. Clear evidence of a conformational change in the backbone has been obtained by chemical shift measurements. This structural change, which is responsible for the change in the electronic state, is a result of the thermally induced transition in the conformation and/or crystal-packing of the alkyl sidechain groups. The backbone structure has been found to be acetylenic both below and above the transition. No evidence for the disruption of hydrogen bonding between the sidechains has been observed. The transition is caused by a loss of backbone planarity at higher temperatures. The increased molecular motion of the mainchain and sidechains in the high-temperature phase, as determined by NMR, provides additional support for the above description of the thermochromic phase transition.