Self-association of oligothiophenes in isotropic systems

Abstract

The self-association equilibrium constants, K_ass, for the dimerization of some small oligothiophenes in acetone, acetonitrile and chloroform were measured by ¹H NMR spectroscopy. The gas phase interaction energies for some oligothiophene dimers were determined by computational quantum chemistry. The ¹H NMR results indicate that K_ass generally increases with the chain length (the number of thienyl rings, n) and solvent polarity; however, K_ass for thiophene (n = 1) was found to be higher than for the bithiophenes (n = 2). The linear oligothiophenes 2,2′-bithiophene and 2,2′,5′,2′′-terthiophene were found to self-associate less than their corresponding nonlinear isomers 3,3′-bithiophene and 3,2′,5′,3′′-terthiophene in solution and in the gas phase. For α-quaterthiophene (n = 4) K_ass in solution was found to be smaller than expected. The non-linear dependence of the standard molar Gibbs energy of self-association, Δ_assG⁰_m, on the chain length in solution could be nicely reproduced and related to the conformational entropy change of dimerization. It was observed that the melting properties of oligothiophenes correlate well with their tendency to self-associate, with more self-association leading to increased liquid stability, and thus lower melting temperatures. These results highlight the relevance of self-association in isotropic systems for the correct molecular interpretation of phase equilibria.