Conformational Change of Alkyl Chains at Phase Transitions in Thin Films of an Asymmetric Benzothienothiophene Derivative

Abstract

Among many promising organic semiconducting materials, 2-decyl-7phenyl[1]benzothieno [3,2-b][1]benzothiophene (Ph-BTBT-C 10 ) shows outstanding device performances for organic field-effect transistors. This compound has a highly ordered liquid crystalline state, i.e., the smectic E (SmE) phase. Although the transition from the crystalline state to the SmE phase is believed to accompany melting of the alkyl chains, no spectroscopic evidence has been found so far. In this study, the conformational change of the decyl chains in Ph-BTBT-C 10 films across the phase transition is analyzed by temperature-dependent measurements in situ using infrared spectroscopy. The spectral analysis reveals that the polycrystalline film has latent conformational disorder (the gauche conformer), the rate of which becomes more pronounced with the heat treatment. As expected, melting of the decyl chains is observed above the transition temperature to the SmE phase. This study also highlights the discovery of some key bands sensitive to the phase transitions in liquid crystalline organic semiconductors.Letter pubs.acs.org/JPCL

Structural Rearrangement of Organic Semiconductor Molecules with an Asymmetric Shape in Thin Films

“…The ν a (CH 2 ) band of the films is commonly observed at approximately 2921 cm –1 , which is higher than the all-trans-specific position of ∼2916 cm –1 . According to our former study, in the case of a vapor-deposited film of Ph-BTBT-C 10 , the ν a (CH 2 ) band is apparently a single peak consisting of at least two components due to the all-trans and gauche conformers, and the apparent wavenumber position is determined by the rate of the two conformers . Thus, the spin-coated films in this study are also expected to have both gauche and trans-conformers.…”

“…Therefore, the band splitting of γ­(CH) Ph can be attributed to the Davydov splitting characteristic of the parallel packing in the bulk phase. This discussion is supported by the singlet band of a vapor-deposited film in which the molecules form the thin-film phase (Figure S2). Note that these two Davydov components cannot be resolved by measurements with a wavenumber resolution of 4 and 2 cm –1 (Figure S3), which are traditionally used for the structural analysis of solid samples.…”

“…To reveal the molecular origin for this, the aggregation structure of the decyl chains is also investigated. For this purpose, we focus on the CH 2 antisymmetric stretching vibration [ν a (CH 2 )] band of the decyl chain, which is sensitive to the chain conformation. ,, The position of the ν a (CH 2 ) band is known to correlate with the conformational order of alkyl chains. This band appears in the range of 2917–2914 cm –1 when the alkyl chain has an ordered (all-trans) conformation; , whereas it appears at 2928–2924 cm –1 for a disordered (gauche) conformation …”

“…The resulting films were stored at room temperature in a desiccator purged with nitrogen gas, and IR and X-ray diffraction (XRD) measurements were performed in ambient air. The nominal thickness of the as-spun film was calculated to be 9.17 nm by comparing its IR band intensity to that of a vapor-deposited film with a thickness of 100 nm . For comparison, a postannealed film was also prepared by heating the as-spun film at 391 K for 30 min.…”

“…This is surprising when considering that, as mentioned above, the crystal transition usually requires a long aging period of several months or more in the case of small-molecule semiconductors. In addition, high-resolution measurements by infrared (IR) Brewster-angle transmission (BAT) spectroscopy , have been performed to discuss the structural change of each chemical group in thin films during the conversion process.…”