Phase Stability Trend in Linear a-Polythiophene Oligomers

This work reports a thermodynamic study and structural analysis of a series of linear alpha-oligothiophenes to explore the effect of the successive increase of the number of thiophene rings on the thermodynamic properties related to solid liquid and solid gas equilibria. The compounds studied were alpha-quaterthiophene (alpha-4T), alpha-quinquethiophene (alpha-ST), and alpha-sexithiophene (alpha-6T). For each compound, the vapor pressures at different temperatures were measured using the Knudsen effusion method combined with a quartz crystal microbalance (KMI3Q). From the obtained results, the standard molar enthalpies, entropies and Gibbs energies of sublimation were determined. For alpha-4T and alpha-ST (alpha-6T decomposes), the fusion temperatures and the standard molar enthalpies of fusion were determined in a power compensated differential scanning calorimeter (DSC). Standard molar entropies of fusion were derived accordingly. The obtained results for sublimation and fusion were compared with available data for the analogous thiophene (T), 2,2'-bitiophene (alpha-2T) and 2,2':5',2 ''-terthiophene (alpha-3T) compounds. To support the energetic study, a structural analysis was performed, based on the available X-ray crystallographic data for solid phase, and computational chemistry calculations, using density functional theory (DFT) with the hybrid exchange correlation functional (B3LYP) at the 6-311++G(d,p) level of theory, for gas phase. Entropic and enthalpic differentiations along the linear a oligothiophenes series were observed, suggesting a subtle odd-even effect and also a trend change in the series, centered at alpha-4T, which is probably related with the change in the supramolecular structure.