Fine Tuning of Solid-State Properties of Septithiophenes by Tailoring the Substituents

Abstract

A series of five R,ω-substituted septithiophenes, which differ in the geometry of the peripheral branched alkyl substitutents, is presented. The position of the branching point has a substantial effect on the solubility and melting points of the oligomers. In contrast, organic field-effect transistors (OFETs) that have been prepared from those materials via vapor deposition show all mobilities in the same range (0.18-0.018 cm 2 V -1 s -1 ). The relative unsusceptibility of the mobilities on the molecular structure is attributed to the fact that all septithiophenes form smectic-like structures at room temperature under solid-state conditions, as revealed by temperature-dependent X-ray diffraction. Furthermore, four of the five oligomers exhibit thermotropic liquid crystalline smectic C phases. Strong interactions between the thiophene cores are assumed as driving forces for those structural features. Thus, tailoring the peripheral substituents makes it possible to fine-tune the thermal and solubility properties and, to a certain extent, the ordering under solid-state conditions. The ordering and the electrical properties are mainly dominated by the length of the core of the oligothiophenes.

Effect of Double Branching in α,ω‐Substituted Oligothiophenes on Thermal Solid‐State Properties

“…The low enthalpy of around 8 kJ mol -1 (Figure 1 and Figure S1 in the Supporting Information) is in accordance with the fluidity of the texture upon shearing. [21] On the other hand, various columnar phases might show similar textures, as described in the literature. In the vicinity of the low-temperature transition at 65°C (Figure 4e), straight dark lines with an angle of around 80°appear across the fans until the crystalline phase is formed below around 45°C (Figure 4f ).…”

“…[27] For shorter oligomers (quaterto sexithiophene) and substituents (n-decyl) the values are still in the range of 50 to 80 kJ mol -1 and 80 to 200 J mol -1 K -1 for the enthalpy and entropy, respectively. [21] Hence, the much bulkier double-branched substituents in the present contribution consistently cause even lower values. [27] Interestingly, a small second branch, through the introduction of a 2-ethylhexyl group, reduces the enthalpy further to 20 kJ mol -1 and the entropy to 37 J mol -1 K -1 .…”

“…[21,22] Thiophene was lithiated and alkylated with a bromoalkene and in a hydrosilylation step treated with dichloro(methyl)silane to give 1TnSi (Scheme 2). At first, the various substitution patterns were prepared at a single thiophene unit (Scheme 2) followed by the construction of the oligomers (Scheme 3), similarly to a procedure previously described for oligomers with mono-branched substituents.…”

“…In turn, the weakening might have a strong effect the substituents causes surprisingly low overall transition enthalpies and entropies for the oligomers of 10-20 kJ mol -1 and 15-40 J mol -1 K -1 , respectively, whereas the pre-oligomers with interrupted conjugation display values in the range of 15-40 kJ mol -1 and 40-100 J mol -1 K -1 , respectively. [19][20][21][22] The present contribution is a logical extension of the previous reports, with further branching points introduced into the substituents. on solid-state structures and thus electronic properties in (thinfilm) applications.…”

“…We have reported on the effect of varying branching topology on the solid-state properties of long oligothiophenes (septithiophenes and longer) with single-branched end-groups. [19][20][21][22] The present contribution is a logical extension of the previous reports, with further branching points introduced into the substituents. In total, five new substituents were synthesized and introduced into septi-, noni-and undecithiophenes with systematic variation of the linkers between the branching points leading to dendrimer-like structures (Scheme 1).…”