Electronic-State Changes of Ferrocene-Terminated Self-Assembled Monolayers Induced by Molecularly Thin Ionic Liquid Layers: A Combined Atomic Force Microscopy, X-ray Photoelectron Spectroscopy, and Ultraviolet Photoelectron Spectroscopy Study
Abstract:Ferrocene-terminated self-assembled monolayers (SAMs) have been studied in the last quarter-century to determine the relationship between their electrochemical properties and microscopic structures as a prototypical chemically modified electrode. Although electronic structures are directly related to the electrochemical properties, knowledge of such a relation concerning ferrocene-terminated SAMs is extremely limited. In this study, we deposited a small amount of ionic liquid onto three types of ferrocene-term… Show more
“…4 ) and is consistent with an increase in the Fe oxidation state to Fe(III). Also consistent with Fe(III) is the broadening and asymmetry of the Fe 2p 3/2 peaks (notable shoulder at 711.2 eV) owing to final state effects including multiplet splitting caused by the presence of an unpaired valence electron in the (low spin) paramagnetic Fe(III) 51 , 55 . Note that in TFSI − , the remaining Fe(II) features (707.4 eV) indicate residual Fc.…”
Section: Resultsmentioning
confidence: 78%
“…Based on the geometry of the cyclopentadienyl (Cp) ligands, Fc has the electronic configuration (a 1 ′) 2 (e 2 ′) 4 and (a 1g ) 2 (e 2g ) 4 for the eclipsed (D 5h ) and staggered (D 5d ) conformations, respectively 53 , 54 . The XPS Fe 2p (core level) and UPS valence spectra can be tracked together because the Fc HOMO is localised to the Fe heteroatom causing them to be strongly correlated 55 , 56 . Fig.…”
Section: Resultsmentioning
confidence: 99%
“…We add that the approach described here opens the door to other compelling opportunities including the modulation of the SAM structure, organic ions with strategically positioned heteroatoms 9 , 63 , use of ionic liquids as a coating or electrolyte 55 , or semiconducting electrodes 64 , 65 , which will alter the energetics of the electrode/monolayer/electrolyte interface. Our work shows a framework that can be extended to interrogate electrolyte effects and interfacial energetics in redox-active polymers 66 and in applications relating to functionalised electrodes for pseudocapacitive energy storage 67 .…”
Section: Discussionmentioning
confidence: 99%
“…The binding energies are referenced to the Au 4f 7/2 peak at 84.0 eV. Spectral fitting was done using Advantage 5.52 (Thermo Fisher) software using a Shirley background subtraction with Lorentzian and Gaussian ratio of 70:30 55 . Scofield sensitivity factors 70 were used for quantification.…”
Altering electrochemical interfaces by using electrolyte effects or so-called “electrolyte engineering” provides a versatile means to modulate the electrochemical response. However, the long-standing challenge is going “beyond cyclic voltammetry” where electrolyte effects are interrogated from the standpoint of the interfacial properties of the electrode/electrolyte interface. Here, we employ ferrocene-terminated self-assembled monolayers as a molecular probe and investigate how the anion-dictated electrochemical responses are translated in terms of the electronic and structural properties of the electrode/monolayer/electrolyte interface. We utilise a photoelectron-based spectroelectrochemical approach that is capable of capturing “snapshots” into (1) anion dependencies of the ferrocene/ferrocenium (Fc/Fc
+
) redox process including ion-pairing with counter anions (Fc
+
–anion) caused by differences in Fc
+
–anion interactions and steric constraints, and (2) interfacial energetics concerning the electrostatic potential across the electrode/monolayer/electrolyte interface. Our work can be extended to provide electrolyte-related structure-property relationships in redox-active polymers and functionalised electrodes for pseudocapacitive energy storage.
“…4 ) and is consistent with an increase in the Fe oxidation state to Fe(III). Also consistent with Fe(III) is the broadening and asymmetry of the Fe 2p 3/2 peaks (notable shoulder at 711.2 eV) owing to final state effects including multiplet splitting caused by the presence of an unpaired valence electron in the (low spin) paramagnetic Fe(III) 51 , 55 . Note that in TFSI − , the remaining Fe(II) features (707.4 eV) indicate residual Fc.…”
Section: Resultsmentioning
confidence: 78%
“…Based on the geometry of the cyclopentadienyl (Cp) ligands, Fc has the electronic configuration (a 1 ′) 2 (e 2 ′) 4 and (a 1g ) 2 (e 2g ) 4 for the eclipsed (D 5h ) and staggered (D 5d ) conformations, respectively 53 , 54 . The XPS Fe 2p (core level) and UPS valence spectra can be tracked together because the Fc HOMO is localised to the Fe heteroatom causing them to be strongly correlated 55 , 56 . Fig.…”
Section: Resultsmentioning
confidence: 99%
“…We add that the approach described here opens the door to other compelling opportunities including the modulation of the SAM structure, organic ions with strategically positioned heteroatoms 9 , 63 , use of ionic liquids as a coating or electrolyte 55 , or semiconducting electrodes 64 , 65 , which will alter the energetics of the electrode/monolayer/electrolyte interface. Our work shows a framework that can be extended to interrogate electrolyte effects and interfacial energetics in redox-active polymers 66 and in applications relating to functionalised electrodes for pseudocapacitive energy storage 67 .…”
Section: Discussionmentioning
confidence: 99%
“…The binding energies are referenced to the Au 4f 7/2 peak at 84.0 eV. Spectral fitting was done using Advantage 5.52 (Thermo Fisher) software using a Shirley background subtraction with Lorentzian and Gaussian ratio of 70:30 55 . Scofield sensitivity factors 70 were used for quantification.…”
Altering electrochemical interfaces by using electrolyte effects or so-called “electrolyte engineering” provides a versatile means to modulate the electrochemical response. However, the long-standing challenge is going “beyond cyclic voltammetry” where electrolyte effects are interrogated from the standpoint of the interfacial properties of the electrode/electrolyte interface. Here, we employ ferrocene-terminated self-assembled monolayers as a molecular probe and investigate how the anion-dictated electrochemical responses are translated in terms of the electronic and structural properties of the electrode/monolayer/electrolyte interface. We utilise a photoelectron-based spectroelectrochemical approach that is capable of capturing “snapshots” into (1) anion dependencies of the ferrocene/ferrocenium (Fc/Fc
+
) redox process including ion-pairing with counter anions (Fc
+
–anion) caused by differences in Fc
+
–anion interactions and steric constraints, and (2) interfacial energetics concerning the electrostatic potential across the electrode/monolayer/electrolyte interface. Our work can be extended to provide electrolyte-related structure-property relationships in redox-active polymers and functionalised electrodes for pseudocapacitive energy storage.
“…[65][66][67] Note that, for the monolayers of clean ferrocene, the Fe 2p spectrum does not show any satellite intensity. [65][66][67][68][69] In fact, this intensity depends on the ligands and it varies with their electronegativity. 70 Additionally, in the case of monolayers on metal substrates, the image-charge formed at the interface 71 may further screen the satellite intensities.…”
: We have investigated the radical functionalization of gold surfaces with a derivative of the perchlorotriphenylmethyl (PTM) radical, using two methods: by chemisorption from the radical solution and by on...
A series of cyclopentadienyl and indenyl molybdenum(II) compounds with intramolecularly coordinated pyridine arm, including scorpionate-like species bearing two irreversibly coordinated arms on indenyl core, was synthesized and characterized. All presented structural types were confirmed by X-ray diffraction analysis. Due to π-basicity of pyridine, the intramolecular interaction is considerably stronger than in case of analogous species bearing tertiary amines in the side chain. Although the starting compounds for syntheses are isostructural, the reaction outcomes differ considerably. The cyclopentadienyl precursor gives a pentacoordinated η 5 :κN-compound while the indenyl analogue produces a hexacoordinated species with unprecedented η 3 :κN-coordination mode of the indenyl ligand representing an unusual example of so-called indenyl effect. The unusually high stability of η 3 :κN-coordination compounds toward η 3 to η 5 haptotropic rearrangement was clarified by theoretical calculations. As the strong intramolecular interaction prevents rotation of indenyl, it cannot reach a conformation suitable for η 3 to η 5 rearrangement. As a result, the low hapticity is effectively locked.
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