STRUCTURAL AND CONFORMATIONAL STUDIES OF QUATERTHIOPHENES HAVING EXTRA BULKY TERMINAL GROUPS

An integrated set of experiments including crystallographic analysis a nd spectroscopy have been carried out to investigate structural and co nformational features of tetramer oligothiophene compounds (quaterthio phenes), These quaterthiophenes are characterized by the presence of e xtra bulky terminal groups N,N-bis(trimethylsilyl)aminomethyl or -tetr amethyl-1-aza-2,5-disila-1-cyclopentyl)alkyl, where the alkyl is methy l or ethyl. These bulky groups produce variation in molecular conforma tion and morphology, The X-ray analysis shows that two types of confor mers are presently stable in the crystals: one is all-anti and the oth er is syn-anti-syn with respect to the molecular long axis. The molecu lar conformations have been further studied in solutions by spectrosco pic methods, especially temperature-dependent ones. Of these, the UV-v is spectroscopy gives important information about the relative stabili ty between the conformers. A clearly resolved isosbestic point means t hat two interconvertible conformers are present as the major species i n the solutions. Numerical analysis of the spectra indicates that the free energy difference between these two is similar to 0.8-1.2 kcal/mo l and that an absorption edge of the less stable conformer is signific antly blue-shifted relative to the other. The former conformer is thou ght to be generated on account of the destruction of pi-conjugation ca used by distortion around the bonds connecting the thiophene rings. Th is associates the conformers with rotamers, The interconversion rate p rocesses between these rotamers have been studied by the dynamic H-1 N MR spectroscopy. The line-width analysis of the coalesced spectral lin es that are observed for the coupled protons implies that the free ene rgy of activation for the interconversion is around 20 kcal/mol. These spectroscopic data are compared with the results of the quantum chemi cal calculations.