Enantiopure 2-(2-ethylhexyl)dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophenes: synthesis, single-crystal structure and a surprising lack of influence of stereoisomerism on thin-film structure and electronic properties

Abstract

Starting from a chiral resolution of 2-ethylhexanoic acid followed by conversions of functional groups without interfering with the enantiopurity, we have successfully introduced an enantiopure 2-ethylhexyl group on to dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) via a Negishi-coupling reaction to synthesize 2-(R)-(2-ethylhexyl)- and 2-(S)-(2-ethylhexyl)-DNTT (R- and S-EH-DNTT, respectively). Then, the crystallinities, thin-film structures, and the organic field-effect transistors (OFETs) based on R-, S- and racemic EH-DNTT (rac-EH-DNTT) were studied to elucidate the effect of stereoisomerism in the 2-ethylhexyl group. The crystal structures of the R- and S-EH-DNTTs are classified as herringbone packing and contain two crystallographically independent molecules connected by edge-to-face CH–π intermolecular interactions, and the molecules’ directly opposite directions avoid steric repulsion between the 2-ethylhexyl groups. Thin films of the EH-DNTTs fabricated using both the spin-coating and vacuum-deposition methods were revealed to have similar but slightly different packing structures to that in the single crystal. Intriguingly, the packing structures in the thin-film state depend on the deposition method, and not on the stereoisomers of EH-DNTT. Consistent with the packing structures in the thin-film state, the performance of OFETs based on the thin films of the R-, S- and rac-EH-DNTTs were affected by the deposition method, and not by the stereoisomerism. This means that the stereoisomerism in the alkyl side chain has a marginal effect on the packing structure and electronic properties in the thin-film state. This is endorsed by the theoretical calculations using the functional-group symmetry-adapted perturbation theory (F-SAPT), which indicated that the intermolecular interactions between the DNTT cores are dominant in the total intermolecular interaction energies, and implies that the crystallization process in the thin-film deposition could be governed by intermolecular interactions between the DNTT cores. We conclude that in 2-ethylhexyl-substituted organic semiconductors with a large and highly aggregative π-conjugated core, like EH-DNTT, the enantiopurity in the 2-ethylhexyl group does not significantly affect the thin-film structure and thus the performance of thin-film OFETs.