Structural and Conformational Studies of Quaterthiophenes Having Extra Bulky Terminal Groups

An integrated set of experiments including crystallographic analysis and spectroscopy have been carried out to investigate structural and conformational features of tetramer oligothiophene compounds (quaterthiophenes). These quaterthiophenes are characterized by the presence of extra bulky terminal groups N,N-bis(trimethylsilyl)aminomethyl or (2,2,5,5-tetramethyl-1-aza-2,5-disila-1-cyclopentyl)alkyl, where the alkyl is methyl or ethyl. These bulky groups produce variation in molecular conformation and morphology. The X-ray analysis shows that two types of conformers are presently stable in the crystals: one is all-anti and the other is syn-anti-syn with respect to the molecular long axis. The molecular conformations have been further studied in solutions by spectroscopic methods, especially temperature-dependent ones. Of these, the UV-vis spectroscopy gives important information about the relative stability between the conformers. A clearly resolved isosbestic point means that two interconvertible conformers are present as the major species in the solutions. Numerical analysis of the spectra indicates that the free energy difference between these two is ∼0.8-1.2 kcal/mol and that an absorption edge of the less stable conformer is significantly blue-shifted relative to the other. The former conformer is thought to be generated on account of the destruction of π-conjugation caused by distortion around the bonds connecting the thiophene rings. This associates the conformers with rotamers. The interconversion rate processes between these rotamers have been studied by the dynamic ¹H NMR spectroscopy. The line-width analysis of the coalesced spectral lines that are observed for the coupled protons implies that the free energy of activation for the interconversion is around 20 kcal/mol. These spectroscopic data are compared with the results of the quantum chemical calculations.