Effects of Odd–Even Side Chain Length of Alkyl-Substituted Diphenylbithiophenes on First Monolayer Thin Film Packing Structure

Hylke B Akkerman,Stefan C B Mannsfeld,Luc Burnier,Michael F Toney,Paulette Clancy,Xueliang Liu,Arjan P Zoombelt,Alán Aspuru-Guzik,Sule Atahan-Evrenk,Jonathan D Saathoff,Eric Verploegen,Zhenan Bao,Ananth P Kaushik,Sanghyun Hong

doi:10.1021/ja400015e

Abstract

Because of their preferential two-dimensional layer-by-layer growth in thin films, 5,5'bis(4-alkylphenyl)-2,2'-bithiophenes (P2TPs) are model compounds for studying the effects of systematic chemical structure variations on thin-film structure and morphology, which in turn, impact the charge transport in organic field-effect transistors. For the first time, we observed, by grazing incidence X-ray diffraction (GIXD), a strong change in molecular tilt angle in a monolayer of P2TP, depending on whether the alkyl chain on the P2TP molecules was of odd or even length. The monolayers were deposited on densely packed ultrasmooth self-assembled alkane silane modified SiO2 surfaces. Our work shows that a subtle change in molecular structure can have a significant impact on the molecular packing structure in thin film, which in turn, will have a strong impact on charge transport of organic semiconductors. This was verified by quantum-chemical calculations that predict a corresponding odd-even effect in the strength of the intermolecular electronic coupling.

Highlights

The influence of substrate temperature on the apparent grain size of P2TPs The effect of substrate temperature on grain size was determined by thermally evaporating a half covered monolayer of C8-P2TP-C8 on ODTS at substrate temperatures ranging from room temperature to 115 °C
To see the effect of side chain length on the grain size, we evaporated the series of P2TP molecules on ODTS at 60 °C with a film thickness of 40 nm (Figure S5)
Grazing incidence X-ray diffraction The motif by which the molecules pack inside the CN-P2TP-CNunit cells was extracted by crystallographic refinement of the diffraction intensity obtained from the Bragg rods

Summary

Current address

The alignment of the two CN-P2TP-CN molecules in the determined monolayer unit cell was extracted from the GIXD peak intensities by numerical optimization of the crystallographic residual that fits the corrected theoretical diffraction intensities along the Bragg rods to the experimentally obtained intensity data. We determined that the ODTS molecules could be fixed in place during the simulation without loss of accuracy of the surface topography and subsequent morphology of P2TP molecules This simplification is possible because the movement of individual SAM molecules is limited at this high density. Once the minimized configuration for the two molecules in the unit cell was obtained, a larger lattice consisting of several unit cells with this molecular configuration was constructed to represent the experimental set-up of a layer of P2TP molecules deposited on the ODTS surface. The system was equilibrated as given in the section describing the simulation details below

Intermolecular Potential Models

Simulation Details

Findings

Transfer characteristics FETs