A novel multiresponsive hydrogel has been synthesized by initiated chemical vapor deposition (iCVD). Hydrogels are known for their dynamic swelling response to aqueous environments. A chemical ...functionalization of the hydrogel surface was performed to add other stimuli-responsive functionalities and obtain a smart material that responds to two stimuli: light irradiation and exposure to aqueous environment. Modifying the hydrogel surface with solution-based methods is often problematic because of the damages caused by the permeation of solvents in the hydrogel. This issue is completely bypassed by the use of solvent-free techniques. Cross-linked polymers of 2-hydroxyethyl methacrylate (HEMA) were functionalized with azobenzene groups, as confirmed by IR spectroscopy and X-ray photoelectron spectroscopy (XPS). Through photoisomerization of the azobenzene, the polarity within the hydrogel is modified and as a consequence the affinity to water. Light irradiation modifies the degree of swelling within thin hydrogel films from 13% before exposure to UV light to 25% after exposure. The possibility of controlling the degree and rate of swelling by light irradiation was never reported before on these time scales and can have exceptional implications for light-induced drug delivery or light-controlled microfluidic systems. The light-responsive hydrogels showed also biocompatibility, which makes them suitable for a great variety of applications as biomaterials.
Photoemission spectroscopy is commonly applied to study the band structure of solids by measuring the kinetic energy versus angular distribution of the photoemitted electrons. Here, we apply this ...experimental technique to characterize discrete orbitals of large π-conjugated molecules. By measuring the photoemission intensity from a constant initial-state energy over a hemispherical region, we generate reciprocal space maps of the emitting orbital density. We demonstrate that the real-space electron distribution of molecular orbitals in both a crystalline pentacene film and a chemisorbed p-sexiphenyl monolayer can be obtained from a simple Fourier transform of the measurement data. The results are in good agreement with density functional calculations.
Imaging the wave functions of adsorbed molecules Lüftner, Daniel; Ules, Thomas; Reinisch, Eva Maria ...
Proceedings of the National Academy of Sciences - PNAS,
01/2014, Volume:
111, Issue:
2
Journal Article
Peer reviewed
Open access
The basis for a quantum-mechanical description of matter is electron wave functions. For atoms and molecules, their spatial distributions and phases are known as orbitals. Although orbitals are very ...powerful concepts, experimentally only the electron densities and -energy levels are directly observable. Regardless whether orbitals are observed in real space with scanning probe experiments, or in reciprocal space by photoemission, the phase information of the orbital is lost. Here, we show that the experimental momentum maps of angle-resolved photoemission from molecular orbitals can be transformed to real-space orbitals via an iterative procedure which also retrieves the lost phase information. This is demonstrated with images obtained of a number of orbitals of the molecules pentacene (C22H14) and perylene-3,4,9,10-tetracarboxylic dianhydride (C24H8O6), adsorbed on silver, which are in excellent agreement with ab initio calculations. The procedure requires no a priori knowledge of the orbitals and is shown to be simple and robust.
The most active and efficient catalysts for the electrochemical hydrogen evolution reaction (HER) rely on platinum, a fact that increases the cost of producing hydrogen and thereby limits the ...widespread adoption of this fuel. Here, a metal‐free organic electrocatalyst that mimics the platinum surface by implementing a high work function and incorporating hydrogen‐affine hydrogen bonds is introduced. These motifs, inspired from enzymology, are deployed here as selective reaction centres. It is shown that the keto‐amine hydrogen‐bond motif enhances the rate‐determining step in proton reduction to molecular hydrogen. The keto‐amine‐functionalized polymers reported herein evolve hydrogen at an overpotential of 190 mV. They share certain key properties with platinum: a similar work function and excellent electrochemical stability and chemical robustness. These properties allow the demonstration of one week of continuous HER operation without notable degradation nor delamination from the carrier electrode. Scaled continuous‐flow electrolysis is reported and 1 L net molecular hydrogen is produced within less than 9 h using 2.3 mg of polymer electrocatalyst.
Metal‐free polymers that show a selective electrocatalytic activity for hydrogen evolution are presented. The polymers are electrically conducting and functionalized with hydrogen‐bonds, which serve as catalytic sites and perform the hydrogen evolution on par with best‐in‐class noble metals.
Charge-transfer processes at molecule–metal interfaces play a key role in tuning the charge injection properties in organic-based devices and thus, ultimately, the device performance. Here, the ...metal’s work function and the adsorbate’s electron affinity are the key factors that govern the electron transfer at the organic/metal interface. In our combined experimental and theoretical work, we demonstrate that the adsorbate’s orientation may also be decisive for the charge transfer. By thermal cycloreversion of diheptacene isomers, we manage to produce highly oriented monolayers of the rodlike, electron-acceptor molecule heptacene on a Cu(110) surface with molecules oriented either along or perpendicular to the close-packed metal rows. This is confirmed by scanning tunneling microscopy (STM) images as well as by angle-resolved ultraviolet photoemission spectroscopy (ARUPS). By utilizing photoemission tomography momentum maps, we show that the lowest unoccupied molecular orbital (LUMO) is fully occupied and also, the LUMO + 1 gets significantly filled when heptacene is oriented along the Cu rows. Conversely, for perpendicularly aligned heptacene, the molecular energy levels are shifted significantly toward the Fermi energy, preventing charge transfer to the LUMO + 1. These findings are fully confirmed by our density functional calculations and demonstrate the possibility to tune the charge transfer and level alignment at organic–metal interfaces through the adjustable molecular alignment.
Although geometric and electronic properties of any physical or chemical system are always mutually coupled by the rules of quantum mechanics, counterintuitive coincidences between the two are ...sometimes observed. The coadsorption of the organic molecules 3,4,9,10-perylene tetracarboxylic dianhydride and copper-II-phthalocyanine on Ag(111) represents such a case, since geometric and electronic structures appear to be decoupled: one molecule moves away from the substrate while its electronic structure indicates a stronger chemical interaction, and vice versa for the other. Our comprehensive experimental and ab-initio theoretical study reveals that, mediated by the metal surface, both species mutually amplify their charge-donating and -accepting characters, respectively. This resolves the apparent paradox, and demonstrates with exceptional clarity how geometric and electronic bonding parameters are intertwined at metal-organic interfaces.
Momentum-selective orbital hybridisation Yang, Xiaosheng; Jugovac, Matteo; Zamborlini, Giovanni ...
Nature communications,
09/2022, Volume:
13, Issue:
1
Journal Article
Peer reviewed
Open access
When a molecule interacts chemically with a metal surface, the orbitals of the molecule hybridise with metal states to form the new eigenstates of the coupled system. Spatial overlap and energy ...matching are determining parameters of the hybridisation. However, since every molecular orbital does not only have a characteristic spatial shape, but also a specific momentum distribution, one may additionally expect a momentum matching condition; after all, each hybridising wave function of the metal has a defined wave vector, too. Here, we report photoemission orbital tomography measurements of hybrid orbitals that emerge from molecular orbitals at a molecule-on-metal interface. We find that in the hybrid orbitals only those partial waves of the original orbital survive which match the metal band structure. Moreover, we find that the conversion of the metal's surface state into a hybrid interface state is also governed by momentum matching constraints. Our experiments demonstrate the possibility to measure hybridisation momentum-selectively, thereby enabling deep insights into the complicated interplay of bulk states, surface states, and molecular orbitals in the formation of the electronic interface structure at molecule-on-metal hybrid interfaces.
Hexacene, composed of six linearly fused benzene rings, is an organic semiconductor material with superior electronic properties. The fundamental understanding of the electronic and chemical ...properties is prerequisite to any possible application in devices. We investigate the orientation and interface properties of highly ordered hexacene monolayers on Ag(110) and Cu(110) with X-ray photoemission spectroscopy (XPS), photoemission orbital tomography (POT), X-ray absorption spectroscopy (XAS), low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and density functional theory (DFT). We find pronounced differences in the structural arrangement of the molecules and the electronic properties at the metal/organic interfaces for the two substrates. While on Cu(110) the molecules adsorb with their long molecular axis parallel to the high symmetry substrate direction, on Ag(110), hexacene adsorbs in an azimuthally slightly rotated geometry with respect to the metal rows of the substrate. In both cases, molecular planes are oriented parallel to the substrate. A pronounced charge transfer from both substrates to different molecular states affects the effective charge of different C atoms of the molecule. Through analysis of experimental and theoretical data, we found out that on Ag(110) the LUMO of the molecule is occupied through charge transfer from the metal, whereas on Cu(110) even the LUMO+1 receives a charge. Interface dipoles are determined to a large extent by the push-back effect, which are also found to differ significantly between 6A/Ag(110) and 6A/Cu(110).
The determination of reaction pathways and the identification of reaction intermediates are key issues in chemistry. Surface reactions are particularly challenging, since many methods of analytical ...chemistry are inapplicable at surfaces. Recently, atomic force microscopy has been employed to identify surface reaction intermediates. While providing an excellent insight into the molecular backbone structure, atomic force microscopy is less conclusive about the molecular periphery, where adsorbates tend to react with the substrate. Here we show that photoemission tomography is extremely sensitive to the character of the frontier orbitals. Specifically, hydrogen abstraction at the molecular periphery is easily detected, and the precise nature of the reaction intermediates can be determined. This is illustrated with the thermally induced reaction of dibromo-bianthracene to graphene which is shown to proceed via a fully hydrogenated bisanthene intermediate. We anticipate that photoemission tomography will become a powerful companion to other techniques in the study of surface reaction pathways.
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