Abstract
Quinacridone (QA) has recently gained attention as an organic semiconductor with unexpectedly high performance in organic devices. The strong intermolecular connection via hydrogen bonds is expected to promote good structural order. When deposited on a substrate, another relevant factor comes into play, namely the 2D-chirality of the quinacridone molecules adsorbed on a surface. Scanning tunneling microscopy (STM) images of monolayer quinacridone on Ag(111) deposited at room temperature reveal the formation of quasi-one-dimensional rows of parallel quinacridone molecules. These rows are segmented into short stacks of a few molecules in which adjacent, flat-lying molecules of a single handedness are linked via hydrogen bonds. After annealing to a temperature of T = 550–570 K, which is close to the sublimation temperature of bulk quinacridone, the structure changes into a stacking of heterochiral quinacridone dimers with a markedly different intermolecular arrangement. Electron diffraction (LEED) and photoelectron emission microscopy (PEEM) data corroborate the STM findings. These results illustrate how the effects of hydrogen bonding and chirality can compete and give rise to very different (meta)stable structures of quinacridone on surfaces.
Highlights
An extended π-conjugated system is generally considered to be a prerequisite for high electron mobility and, a good performance in organic thin film devices
Scanning tunneling microscopy (STM) images of monolayer quinacridone on Ag(111) deposited at room temperature reveal the formation of quasi-one-dimensional rows of parallel quinacridone molecules
The recent work of Głowacki et al. Advances that this view may require revision: In their recent publications they illustrate the potential of quinacridone as functional material in organic field effect transistors (OFETs) and photovoltaics (PV).[1−3] The authors were able to achieve field effect mobilities larger than 0.1 cm2/(Vs) and photocurrents in the mA/cm[2] range under simulated solar illumination
Summary
An extended π-conjugated system is generally considered to be a prerequisite for high electron mobility and, a good performance in organic thin film devices. The fact that there are two antiphase domains with a relative shift of a/2 along the row direction and that no Moiré pattern was observed in the STM images is consistent with a simple commensurability of the structure along the a-direction; i.e., all the molecules within the stacks are located on equivalent adsorption sites on the Ag(111) surface. Note that the first-order superstructure spot (10)s along the a* direction (and all other odd multiples) are absent in the LEED pattern, such that only every other spot with a spacing of 2a* can be seen This is due to the fact that the superstructure unit cell is not a primitive one but contains a second molecule at a position a⃗/2 + εb⃗, i.e., halfway along the molecular rows and shifted by a fraction ε along the b-axis. This makes sense if one considers that the transition from the homochiral phase into the heterochiral phase upon annealing requires the breaking of the strong hydrogen bonds between molecules originally arranged in homochiral stacks
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