Detection of different oxidation states of individual manganese porphyrins during their reaction with oxygen at a solid/liquid interface

Abstract

[4][5][6][7] . The majority of these studies use ensemble techniques in solution, such as electrochemistry and absorption, electron paramagnetic resonance and nuclear magnetic resonance spectroscopy. The scanning tunneling microscope (STM) has proven to be a promising tool to study reactions at the molecular level [8][9][10][11][12] , and initial experiments have been reported in which the reactive properties of metal porphyrins were studied on a surface at the single molecule level by STM in ultrahigh vacuum (UHV) [13][14][15][16][17] and under ambient conditions 18,19 .Here we report detailed STM studies of the complex redox chemistry of reactions between 10,15,R,R,porphyrin manganese(III) chloride (Mn1Cl, Fig. 2a) and different oxygen donors at a solid/liquid interface. Although the dynamic nature of the molecules at this interface complicates the use of techniques that are typically being used under UHV conditions, such as X-ray 3 Photoelectron Spectroscopy (XPS), STM can provide extremely high spatial resolution and insight into changes in the electronic properties of molecules with a liquid medium over them.These liquid conditions make the system far more comparable with processes taking place in biological systems than the UHV conditions, where solvent-mediated factors such as the diffusion and concentration of reactants are fully absent. It is in principle possible with STM to monitor single molecules at the highest detail while they are involved in multistep chemical reactions with STM, and to image and identify reactants, intermediates and products in the most direct way, i.e. by imaging. This approach allows the study of reaction dynamics in realspace and real-time, which may provide unique information about reaction mechanisms that remains hidden in ensemble measurements at the macroscopic scale. STM may reveal variations in reactivity of single molecules, the relation of these variations to molecular adsorption geometry, and cooperativity effects at the nanometre scale.An overview of the possible reaction pathways involving manganese porphyrins and oxygen, based on the literature 1-3,18,20-29 and the observations described here, is depicted in Figure 1 In a first experiment we deposited a droplet of a ~10 −5 M solution of Mn1Cl in 1-octanoic acid at the basal plane of a freshly cleaved highly oriented pyrolytic graphite (HOPG) sample under ambient conditions. Related alkyl-functionalised free base porphyrin molecules are known to form well-ordered monolayers on the same substrate 30,31 . STM revealed the 5 instantaneous self-assembly of the molecules of Mn1Cl in extended lamellar arrays, in which the porphyrin planes are adsorbed parallel to the surface and the alkyl chains interdigitated (Fig. 2B). The orientation of the lamellae and the alkyl chains with respect to the main crystallographic directions of the graphite surface is identical to that observed for the corresponding free base derivative. 30 When the surface was scanned under ambient conditions at a negative bias voltage of −800 mV and ...

Thermodynamic synthesis of solution processable ladder polymers

“…20,60 The highly ordered self-assembly was likely a result of the strong π–π interaction between the HOPG substrate and the aromatic ladder-type backbone. 61,62 These rods under STM showed alternating sections of high and low signal with a periodic length at around 1.27 nm. According to the single crystal structure of 3 and DFT calculations on model oligomers resembling P2 , the feature length of each repeating unit on P2 was close to 1.26 nm, matching well with the experimental results.…”

Cobalt‐Porphyrin‐Catalyzed Oxygen Reduction Reaction: A Scanning Tunneling Microscopy Study

“…showed that the catalytic activity of FePc for surface oxidation can be reproducibly switched on or quenched by controlling the FePc local adsorption site, and the FePc–(η 2 ‐O 2 )–Ag coordination complex was imaged by using ultrahigh vacuum (UHV) STM . Furthermore, different oxidation states and oxidation products of individual MnP derivatives have also been investigated by using STM. However, despite the studies of the MP‐catalyzed oxidation reaction in UHV or ambient environment,– there are few in situ electrochemical studies about the ORR mechanism at the single‐molecule scale.…”

Effects of Alkyl Chain Length and Hydrogen Bonds on the Cooperative Self‐Assembly of 2‐Thienyl‐Type Diarylethenes at a Liquid/Highly Oriented Pyrolytic Graphite (HOPG) Interface

“…[9][10][11][12][13][14][15] To efficiently design and predict the formationo ft hermodynamically stable molecular ordering formed through self-assembly processes, interplays of various interactions,s uch as molecule-substrate interactions,i ntermoleculari nteraction, [16][17][18][19][20] solvent effects, [21][22][23][24][25] and the balance of enthalpy and entropyt erms, [26][27][28][29] must be taken into account. Various systems employing these interactions, such as chiral assemblies, [21,[30][31][32][33] post-assembly manipulation, [34][35][36][37] and porous networks, [2,17,38] have recentlyb een intensively investigated by many groups.…”