Photochromism of dinaphthylethene derivatives. Stability of the closed-ring forms

Stability of various types of closed-ring forms of dinaphthylperfluorocyclopentenes was investigated from both experimental as well as theoretical points of view. The stability was dependent on the energy differences between the open-ring and the closed-ring forms. When the energy difference was small, the closed-ring form became stabler.     

In-situ scanning tunneling microscopy of carbon monoxide adsorbed on Au(111) electrode

In-situ scanning tunneling microscopy (STM) coupled with cyclic voltammetry was used to examine the adsorption of carbon monoxide (CO) molecules on an ordered Au(111) electrode in 0.1 M HClO4. Molecular resolution STM revealed the formation of several commensurate CO adlattices, but the (9 x radical 3) structure eventually prevailed with time. The CO adlayer was completely electrooxidized to CO2 at 0.9 V versus RHE in CO-free 0.1 M HClO(4), as indicated by a broad and irreversible anodic peak which appeared at this potential in a positive potential sweep from 0.05 to 1.6 V. A maximal coverage of 0.3 was estimated for CO admolecules from the amount of charge involved in this feature. Real-time in-situ STM imaging allowed direct visualization of the adsorption process of CO on Au(111) at 0.1 V, showing the lifting of (radical 3 x 22) reconstruction of Au(111) and the formation of ordered CO adlattices. The (9 x radical 3) structure observed in CO-saturated perchloric acid has a coverage of 0.28, which is approximately equal to that determined from coulometry. Switching the potential from 0.1 to -0.1 V restored the reconstructed Au(111) with no change in the (9 x radical 3)-CO adlattice. However, the reconstructed Au(111) featured a pairwise corrugation pattern with two nearest pairs separated by 74 +/- 1 A, corresponding to a 14% increase from the ideal value of 65.6 A known for the ( radical 3 x 22) reconstruction. Molecular resolution STM further revealed that protrusions resulting from CO admolecules in the (9 x radical 3) structure exhibited distinctly different corrugation heights, suggesting that the CO molecules resided at different sites on Au(111). This ordered structure predominated in the potential range between 0.1 and 0.7 V; however, it was converted into new structures of (7 x radical 7) and ( radical 43 x 2 radical 13) on the unreconstructed Au(111) when the potential was held at 0.8 V for ca. 60 min. The coverage of CO adlayer decreased accordingly from 0.28 to 0.13 before it was completely removed from the Au(111) surface at more positive potentials.     

Electrochemical redox control of ferrocene using a supramolecular assembly of ferrocene-linked C(60) derivative and metallooctaethylporphyrin array on a Au(111) electrode

Supramolecular assembled layers of ferrocene-linked C(60) derivative (C(60)Fc) and various metal ions coordinated to octaethylporphyrin (MOEP) were formed on the surface of a Au(111) single-crystal electrode by immersing the Au substrate successively into a benzene solution containing MOEP and one containing C(60)Fc molecules. The MOEPs used were zinc(II) (ZnOEP), cobalt(II) (CoOEP), copper(II) (CuOEP), and iron(III) chloride (FeClOEP) of OEP (2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine). The molecules of C(60)Fc directly attached to the Au(111) electrode showed poorly defined electrochemical redox response, whereas a clear electrochemical redox reaction of the ferrocene group in the C(60)Fc molecule was observed at 0.78 V versus reversible hydrogen electrode on ZnOEP, CoOEP, and CuOEP adlayers, but not on the FeClOEP adlayer. Adlattices of the underlying layer and the top layer of C(60)Fc were determined by in situ scanning tunneling microscopy. Adlayer structures of MOEP were independent of the central metal ion; that is, MOEP molecules were arranged hexagonally with two different orientations. Highly ordered C(60)Fc arrays were formed with 1:1 composition on the ZnOEP-, CoOEP-, and CuOEP-modified Au(111) surface, whereas a disordered structure of C(60)Fc was found on the FeClOEP-modified Au(111) surface. The presence of Cl ligand was found to prevent the formation of supramolecularly assembled layers with C(60)Fc molecules, resulting in an ill-defined unclear electrochemical response of the Fc group. The well-defined electrochemical response of the Fc group in C(60)Fc was clearly due to the control of orientation of C(60)Fc molecules.     

Role of the anion in the underpotential deposition of cadmium on a Rh(111) electrode: probed by voltammetry and in situ scanning tunneling microscopy

In situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV) were employed to examine the underpotential deposition (UPD) of cadmium on a rhodium(111) electrode in sulfuric and hydrochloric acids. The (bi)sulfate and chloride anions in the electrolytes played a main role in controlling the number and arrangement of Cd adatoms. Deposition of Cd along with hydrogen adsorption occurred near 0.1 V (vs reversible hydrogen electrode) in either 0.05 M H2SO4 or 0.1 M HCl containing 1 mM Cd(ClO4)2. These coupled processes resulted in an erroneous coverage of Cd adatoms. The process of Cd deposition shifted positively to 0.3 V and thus separated from that of hydrogen in 0.05 M H2SO4 containing 0.5 M Cd2+. The amount of charge (80 microC/cm2) for Cd deposition in 0.5 M Cd2+ implied a coverage of 0.17 for the Cd adatoms, which agreed with in situ STM results. Regardless of [Cd2+], in situ STM imaging revealed a highly ordered Rh(111)-(6 x 6)-6Cd + HSO4- or SO42- structure in sulfuric acid,. In hydrochloric acid, in situ STM discerned a (2 x 2)-Cd + Cl structure at potentials where Cd deposition commenced. STM atomic resolution showed roughly one-quarter of a monolayer of Cd adatoms were deposited, ca. 50% more than in sulfuric acid. Dynamic in situ STM imaging showed potential dependent, reversible transformations between the (6 x 6) Cd adlattices and (square root 3 x square root 7)-(bi)sulfate structure, and between (2 x 2) and (square root 7 x square root 7)R19.1 degrees -Cl structures. The fact that different Cd structures observed in H2SO4 and HCl entailed the involvement of anions in Cd deposition, i.e. (bi)sulfate and chloride anions were codeposited with Cd adatoms on Rh(111).     

In situ scanning tunneling microscopy of molecular assemblies of cobalt(II)- and copper(II)-coordinated tetraphenyl porphine and phthalocyanine on Au(100)

Molecules of copper(II) and cobalt(II) 5,10,15,20-tetraphenyl-21H,23H-porphine (CuTPP and CoTPP) and cobalt(II) phthalocyanine (CoPc) are spontaneously adsorbed onto reconstructed Au(100) substrate from a benzene solution containing each individual complex. In situ scanning tunneling microscopy (STM) was used to examine the real-space arrangement and the internal molecular structure of each of the individual molecules in 0.1 M HClO4 under potential control. The adsorption of CuTPP and CoTPP produced the same highly ordered square array with an intermolecular spacing of 1.44 nm on a reconstructed Au(100) surface. These molecular superlattices and the underlying reconstructed Au(100) predominated between 0 and 0.9 V, but lifting of the reconstructed Au(100) surface and elimination of the ordered adlayers occurred at more positive potentials. Molecular resolution STM revealed propeller-shaped admolecule with its center imaged as a protrusion for Co(II) and a depression for Cu(II). In contrast, the spontaneous adsorption of CoPc molecules resulted in a rapid phase transition from the reconstructed Au(100) surface to the (1 x 1) phase, coupled with the production of locally ordered, square-shaped arrays with an intermolecular distance of 1.65 nm. This molecular adlayer and the Au(100)-(1 x 1) remained unchanged when the potential was modulated between 0 and 1.0 V. These results indicate that the subtle variation in the molecular structure of adsorbate influenced not only its spatial arrangement but also the structure of the underlying Au(100) substrate.     

Effect of adlayer structure on the host-guest recognition between calcium and crown-ether-substituted phthalocyanine arrays on Au single-crystal surfaces.

Adlayers of 15-crown-5-ether-substituted cobalt(II) phthalocyanine (CoCRPc) were prepared by immersion of either Au(111) or Au(100) substrate into benzene-ethanol (9:1 v/v) mixed solutions containing CoCRPc. In situ STM imaging was carried out after transferring the CoCRPc-modified Au crystals into aqueous HClO(4) solution. The packing arrangement of the CoCRPc array on Au(111) was determined to be p(8 x 4 radical 3R - 30 degrees ), and the internal structure was clearly observed by high-resolution STM. Two adlayer structures of CoCRPc, (8 x 9) and (4 radical 5 x 4 radical 5)R26.7 degrees, were found on the Au(100)-(1 x 1) terrace. In the presence of 1 mM Ca(2+), two Ca(2+) ions were trapped in two diagonally located 15-crown-5-ether moieties of each CoCRPc molecule on Au(111), whereas encapsulation of Ca(2+) ions was not seen in the CoCRPc arrays on the Au(100)-(1 x 1) surface. The present study demonstrates that the relationship between crown moieties of CRPc and the underlying Au lattice is important in the trapping of Ca(2+) ions in crown rings.     

Two-dimensional supramolecular organization of copper octaethylporphyrin and cobalt phthalocyanine on Au(111): molecular assembly control at an electrochemical interface

Mixed adlayers of 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine copper(II) (CuOEP) and cobalt(II) phthalocyanine (CoPc) were prepared by immersing Au(111) substrate in a benzene solution containing CuOEP and CoPc molecules, and they were investigated in 0.1 M HClO(4) by cyclic voltammetry (CV) and in-situ scanning tunneling microscopy (STM). The composition of the mixed adlayer consisting of CuOEP and CoPc molecules was found to vary depending on the immersion time. CoPc molecules displaced CuOEP molecules during the modification process with increasing immersion time, and the CuOEP molecules were completely replaced with CoPc molecules in the mixed solution after a long modification time. The two-component adlayer consisting of CuOEP and CoPc, which has a structure with the constituent molecules arranged alternately, was found to form either a p(9 x 3(square root)7R - 40.9 degrees) or a p(9 x 3(square root)7R - 19.1 degrees) structure, each involving two molecules on the Au(111) surface. The surface mobility and the molecular reorganization of CuOEP and CoPc were accelerated by modulation of the electrode potential. Different surface structures were produced at different electrode potentials, and hence potential modulation should allow a precisely controllable phase separation to take place in aqueous HClO(4).