Charge transfer properties of imide-functionalized thiazole-based oligomers: Roles of oligomer length, thiophene donor, and fluorine-substitution

Abstract Three series of oligomers containing functionalized imide and thiophene rings, namely the acceptor type bisthiazole imide oligomer with different length (PBTzIn, n = 1–8), the donor-acceptor type derivatives by introducing three electron donating thiophene rings into BTzI (PBTzI3Tm, m = 1–4), and derivatives by substituting the two hydrogen of center thiophene in PBTzI3Tm with fluorine (PBTzI3T-2Fm, m = 1–4), were designed based on some experimentally synthesized oligomers to clarify the roles of oligomer length, thiophene donor, and fluorine-substitution on the charge transfer properties. The charge transfer properties of these imide-functionalized thiazole-based oligomers were systematically studied using density functional theory and traditional Marcus theory in term of geometric structures, frontier molecular orbitals, ionization potentials (IP), electron affinities (EA), reorganization energies, charge transfer integrals, crystal packing models, and charge transfer mobilities. The IP/EA decreased/increased with increasing oligomer length in PBTzIn, rendering long PBTzIn oligomers showing both p- and n-type characteristic of organic semiconductors. The orbital energy level and mobility show saturation characteristics for n = 6.PBTzI8 shows high and balanced hole/electron mobility of 1.089/0.249 cm2 V−1s−1 and thus is good ambipolar semiconductor. The introduced three thiophene rings in PBTzI3Tm (m = 1–4) enhances the backbone conjugate property and reduces the hole injection barrier. Substituting thiophene H atoms with F further enhances the interaction between the non-covalent bond N…S and S…F and thus increases the molecular planarity, rendering PBTzI3T-2Fm (m = 1–4) being better semiconductor. PBTzI3T3 is ambipolar organic semiconductor with hole/electron mobility of 11.760/1.396 cm2V−1s−1, while PBTzI3T-2F3 and PBTzI3T-2F4 show better ambipolar organic semiconductor performance with much larger hole/electron mobility of 16.325/3.674 cm2V−1s−1 and 60.019/5.086 cm2V−1s−1.