Determination of Ground- and Excited-State Isomerization Barriers for the Oligothiophene 3′,4′-Dibutyl-2,2′:5′,2″-terthiophene

We report on the crystal structure and rotational isomerization of a poly(alkylthiophene) oligomer, 3′,4′- dibutyl-2,2′:5′,2″-terthiophene. The X-ray crystal structure of this polythiophene oligomer shows an all-anti configuration with an ~ 33° mean dihedral angle between adjacent thiophene rings. Measurements of the torsional barrier to rotation between thiophene rings have been performed for both the ground and first excited singlet states of this oligomer. The S₀ barrier to rotation is measured to be 19.7 kcal/mol using ¹H NMR and the S₁ barrier to rotation is determined to be 4.2 kcal/mol using fluorescence lifetime measurements. We discuss the significance of these results in the context of understanding structure/property relationships in poly(alkylthiophenes).