Side chain disorder and phase transitions in alkyl-substituted, conjugated oligomers. relation to side-chain melting in P3ATs

Certain 4,4'-alkyl substituted 2,2'-bithiazole and bithiazole-thiophene oligomers display an endothermic transition in their DSC trace below their respective melting points. Variable-temperature FTIR, MAS-H-1 NMR, UV-vis spectra, and XRD all indicate that the thermal transition is due to a crystal-crystal phase transition that we have labeled alpha -> beta. FTIR shows a stepwise increase in the concentration of gauche defects at the alpha -> beta transition temperature, but MAS NMR spectra show little increase in the side chain motion until the mp is reached. UV-vis spectra demonstrate that the conjugated main chains remain essentially planar through the alpha -> beta transition, and significant deviations from planarity occur only at higher temperatures, but well below the mp. The close similarity of this behavior to the phase transitions in long chain n-paraffins and the "side-chain melting" phenomenon in poly(3-alkylthiophenes), P3ATs, suggests that the latter may actually be more accurately described as a crystal-crystal phase transition of the crystalline fraction, driven by side chain disorder.