Enhanced Layered-Herringbone Packing due to Long Alkyl Chain Substitution in Solution-Processable Organic Semiconductors

Herein,
we report the stabilization and modulation of layered-herringbone (LHB) packing, which is known to afford high-performance organic
thin-film transistors, based on crystal structure analyses and calculations
of intermolecular interaction energies for alkyl-substituted organic
semiconductor (OSC) crystals. We systematically investigated the alkyl
chain-length dependence of the crystal structures, solvent solubilities,
and thermal characteristics for three series of symmetrically and
asymmetrically alkyl-substituted benzothieno­[3,2-b]­[1]­benzothiophenes (BTBTs). All the series exhibit LHB packing when
the BTBTs are substituted with relatively long alkyl chains (−CnH2n+1), i.e., n ≥ 4 for monoalkylated, n ≥
6 for dialkylated, and n ≥ 5 for phenyl-alkylated
BTBTs. LHB packing is also evident in the nonsubstituted and diethyl-substituted
BTBTs, although those substituted with short alkyl chains generally
did not feature LHB packing because of their lack of interchain ordering.
The density functional theory calculations of the intermolecular interactions
revealed that the BTBT cores inherently generate LHB packing, and
the stability is increasingly enhanced by the alignment of longer
alkyl chains. It was also found that the LHB packing is stabilized
by keeping the size ratios of the total intermolecular attractive
forces between the T-shaped and slipped parallel contacts at about 3:2 for all the LHB compounds, despite the slight
structural modifications generated by the substituents. We discuss
the effects of alkyl substitutions to modulate the LHB packing of
the BTBT cores and thus the two-dimensional carrier transport in layered
OSC crystals.