Two different polymorphic forms of the titled compound, which has the molecular conformation of complete S-syn-anti-syn in oligothiophene backbone, have been grown on glass substrate by vacuum deposition. The two phases are the single-crystal phase (Type I) and the new high-temperature phase (Type II) observed only when the thin film process in the physical vapor transport (vacuum evaporation) is carried out. The ratio of the two phases can be controlled with the substrate temperature and deposition rate. The spacing of Type II by X-ray diffraction measurement is shorter than that of Type I, indicating that the long axis of the molecule in Type II is more inclined against the substrate than those in Type I. Infrared and Raman spectra indicated that Type II is attributed to the conformational polymorphism: conversion from S-syn-anti-syn to S-all-anti. Therefore, the polymorphs originate from the different molecular packing involving the conformational change of the molecule. This unique property is attributed to the extra bulky terminal groups of the compounds. The origin of the transformation from Type I to Type II is that the vapor phase conversion caused by reduction of the activation energy of rotational isomerization barrier. However, in spite of the extra bulky terminal groups, the mentioned polymorphism is not observed in the titled compound analogue, which has S-all-anti conformation. The origin is discussed with the difference of rotational isomerization barrier from syn to anti conformation.
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