Morphology of Thin Films of Aromatic Ellagic Acid and Its Hydrogen Bonding Interactions

Abstract

Ellagic acid (EA), an antioxidant from fruits or other plants, has recently evoked interest in the field of organic electronics because of its weak electron donor properties. In this work, the preparation of uniaxial π-stacked EA films by thermal evaporation on different surfaces is reported for the first time. The (102) lattice plane of the π-electron system was confirmed as the contact plane for one monolayer equivalent on Ag(111) by lowelectron energy diffraction. X-ray and atomic force microscopy measurements revealed nanocrystalline grains with an average inplane size of 50 nm and considerably smaller average out-of-plane crystallite sizes (16−25 nm) in films of 16−75 nm thickness. The influence of different substrates was minor compared to the effect of the film thickness. An increase in the in-plane density of grains at larger film thicknesses was deduced from the trend in their uniaxial optical properties. Weak and strong intermolecular H-bonding interactions were identified in the EA crystal lattice, while a surplus of weak H-bonding was observed for the nanocrystallites in thin films, as compared to bulk EA. Finally, EA was coevaporated with the semiconducting thiophene molecule DCV4T-Et 2 to demonstrate principle interactions with a guest molecule by H-bonding analysis. Our results illustrate the feasibility of applying EA films as alignment layers for templating other semiconducting organic films used in organic electronic devices.

Structural Templating of an Organic Solar Cell Absorber by Ellagic Acid To Tune Its Aggregation, Molecular Orientation, and Optical Properties

“…Specular X-ray diffraction (XRD) measurements to explicitly probe q z were not feasible for these 5 nm thin films as it would require extremely long signal integration times. However, for thicker EA films (30 nm), these reference measurements were performed and the π-stacking texture was ascertained . Consequently, the same texture can be assumed for the 5 nm thin films based on the presented GIWAXS data.…”

“…Ellagic acid (2,3,7,8-tetrahydroxychromeno­[5,4,3- cde ]­chromene-5,10-dione, Alfa Aesar, A15722) was used in solar cells as received. For other studies, it was purified once by vacuum-gradient sublimation by the Surface Science Group of Friedrich-Schiller University, Jena, Germany . All layer stacks were deposited via thermal evaporation in a UHV chamber (CreaPhys GmbH, Germany) at a base pressure of 10 –7 –10 –8 mbar using a deposition rate of 0.1–0.3 Å/s.…”

“…Alignment of the sample was performed at GISAXS conditions at a detector–sample distance of 1060 mm. GIWAXS data analysis was done with a self-developed python script to transform the detector data to a spherical projection ( q xy , q z ) , and to evaluate line cuts I ( q ) and I (χ). For all of the layer stacks, in-plane isotropy of the active layer was assumed based on the agreement of ellipsometric measurements of the samples at various in-plane orientation angles (Figure S12).…”

“…EA is a polyphenol present in numerous fruits like pomegranates with a delocalized π system and additional H-bonding functionality (Figure a), crystallizing in a monoclinic space group . It is well known in food research for its anticancer and antioxidant effects , but has also been investigated as an organic electrode material and a sensing material recently. , In a previous study, we reported on EA thin films prepared by physical vapor deposition with thicknesses between one monolayer equivalent (MLE) and 75 nm . These films were shown to grow as crystalline clusters with a π-stacking texture on different substrates (Ag(111), Au, Si, C 60 ).…”

“…32,33 In a previous study, we reported on EA thin films prepared by physical vapor deposition with thicknesses between one monolayer equivalent (MLE) and 75 nm. 34 These films were shown to grow as crystalline clusters with a π-stacking texture on different substrates (Ag(111), Au, Si, C 60 ). The measured lattice parameters were in good agreement with the monoclinic crystal structure (P2 1 /c: a = 3.74 Å, b = 11.44 Å, c = 12.52 Å, β = 90.2°) given in the literature for unhydrated EA.…”