Electronic and structural properties of fluorene–thiophene copolymers as function of the composition ratio between the moieties: a theoretical study

Abstract

Through theoretical analysis, we study relevant properties of some molecular structures formed by oligothiophenes (T) and dioctylfluorenes (F) units, commonly employed in the fabrication of different kinds of optical and electronic devices. For so, we first consider F–(T)_n–F molecules with different numbers of thiophene rings (n). Among other characteristics, we calculate the dipole moment change between the ground and excited state (Δμ_ge), a quantity that greatly influences the exciton dissociation and charge carrier mobility. We show that the planarity of the ground state geometry correlates Δμ_ge to the exciton binding energy (E_b), with higher Δμ_ge's corresponding to lower E_b's when n > 3. We also unveil a relevant dependence of Δμ_ge with the odd–even parity of n and that Δμ_ge assumes higher values when the molecule is composed by bithiophene (instead of simple thiophenes) moieties in the syn-conformation (with the two heteroatoms pointing in the same direction). From molecules results, we then address larger systems, formed by different oligomers of F–T copolymers containing blocks of dioctylfluorenes and bithiophenes (T2). We systematic investigate their electronic and structural properties as function of the composition ratio between the T2 and F moieties. Similar to the molecules, we deduce that the magnitude of Δμ_ge is higher for the syn conformer of the T2 unit. Moreover, the highest values of Δμ_ge are achieved when the number of the T2 increases relative to a fixed number of the F units in the mer. Such behaviors are in agreement and actually can qualitative explain measurements in the literature on the quantum efficiency of charge carrier generation in F–T copolymers. The present findings can be helpful in designing novel materials with improved photoelectric responses.