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A. Bottom-Up Fabrication of Metal Complex Oligomer and Polymer Wires

A. Bottom-Up Fabrication of Metal Complex Oligomer and Polymer Wires

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Bottom-Up Fabrication of Redox-Conducting Wires

(a)



N



N

N



(b)



N



N



391



N



N



N

N



N



N



N



N



N



N

N



S



N



N



N



2



N



L2



L1

N



N



N

N Fe N

N

N

N



N

N



N



tpy-AB-SS-AB-tpy



N

N



N



L

L



N



L



N



N



N



Fe



N



N



N



N



N



N



N



N



N



NN



N



S



S



Fe



N

N Fe N

N

N

N



N

N



N



N



N



NN



NN



S



S



N



L3

N



(c)

N



N



N



N



N



N



L

L

Fe

N

N



N



N



N



N



L



N



N



N



N



N

N



N



N



N



N

N

N

Fe

N

N

N



N

N

N

Fe

N

N

N



N



Fe(BF4)2

N



S



N



N



N

N



tpy-AB-SS-AB-tpy



Fe



L3

N



S



N



N



S



N



N

N



N



N



Fe

N



N



N



N

N



N

NN

S



FIGURE 2. Chemical structures of the ligands and stepwise coordination methods to

prepare linear and branched oligomer and polymer wires on an Au/mica or Au/ITO

plate. (Reprinted with permission from Ref. 13.)



moieties (L3: 1,3,5-C6H3(terpyridine)3) (Fig. 2). This type of wire can form a

branched structure such as that of a dendrite.

A typical method for fabricating multiple complex layers is illustrated in

Figure 2.11,12 First, an Au/mica or Au/ITO plate is immersed in a chloroform

solution of tpy-AB-SS-AB-tpy (tpy52,2u:6u,2v -terpyridyl), providing Au-S-ABtpy SAM on the plate. In the case of connecting the Fe(II) ion, the tpy-terminated

plate is immersed in 0.1 M Fe(BF4)2 aq or (NH4)2Fe(SO4)2 aq to form a metal

complex. Subsequently, the metal-terminated surface is immersed in a chloroform solution of the ligand L1 or L2 to form a bis(tpy)iron structure (Fig. 2b). The

latter two processes are repeated for the preparation of multilayered bis(tpy)iron

(II) complex films with linear structures. When L3 is used instead of L1 or L2, the

resulting molecular wires have a dendritic structure (Fig. 2c).



392



Redox and Photo Functions of Metal Complex Wires



It is also possible to prepare the film with bis(tpy)cobalt complex oligomers

and polymers. In addition to the process of bis(tpy)iron(II) complex films, a process

of electrochemical oxidation from CoII to CoIII is required after immersing the plate

in a chloroform solution of L1, in which the plate is held at 0.3 V (vs. Ag/Ag1) to

obtain multilayered bis(tpy)cobalt complex films.11 Hereafter, the film for the nth

complexation cycles using metal ion and bridging ligand Lx is abbreviated as [nMLx].

The films with linear oligomers can be quantitatively fabricated by

stepwise coordination unless steric hindrance occurs. In [nFeL1] and [nFeL2]

(n 5 1À10), a quantitative increase in molecular surface coverage Γ with

increasing n was confirmed by an increase in an absorption peak at 592 nm

ascribed to the MLCT transition in the ultraviolet-visible (UV-VIS) spectra,

and by the amount of charge for the reversible redox reaction of the FeIII/FeII

couple, which appeared at 0.67 V vs. ferrocenium/ferrocene (Fc1/Fc) in the

cyclic voltammograms (CV) in Bu4NClO4ÀCH2Cl2 (Figs. 3 and 4). In contrast,



(a)

0.3



5



Abs.



4

0.2

3

2



0.1



1

0

250



(b)



350



450

Wavelength/nm



550



650



0.1

0.08



Abs.



0.06

0.04

0.02

0



0



1



2

3

4

The number of complex layers



5



FIGURE 3. (a) UV-VIS absorption spectra of [nFeL1] (n51À5), and (b) plots of the

absorption peak at 592 nm vs. the number of complex layers. (Reprinted with permission from Ref. 13.)



Bottom-Up Fabrication of Redox-Conducting Wires



Current density/μA cm–2



(a)



60



Peak current/μA cm–2



8



6

4

2



30



0



–30



–60



(b)



10



393



0



0.3



0.6

Potential vs. Fc+/Fc



0.9



80



60



40



20



0



0



2



4

6

8

The number of complex layers



10



12



FIGURE 4. (a) Cyclic voltammograms of [nFeL1] (n 5 2, 4, 6, 8, and 10) in 0.1 M

Bu4NClO4ÀCH2Cl2 at a scan rate of 0.1 V s21, and (b) plots of the anodic peak current

vs. the number of complex layers. (Reprinted with permission from Ref. 13.)



in the case of the film with a dendritic structure, the increase in molecular

surface coverage Γ should ideally follow the 2nÀ1 relationship with increasing

n. The ideal increase in surface coverage, however, stopped beyond four

complex layers (Fig. 5) because of collisions between the edge of the molecular

wires and the Au electrode surface, as expected from the MM1 calculation

(Fig. 6). Once the n value exceeded 4, the increase in Γ slowed and approached

the relationship for that of linear polymers, [nFeL2] (Fig. 5), which is reasonable because for both linear and branched wires only vertical space is available

for additional molecular growth.

The formations of these films were visually observed using secondary

electron microscopy (SEM) and scanning tunneling microscopy (STM). Figure 7

shows top views and cross-sectional views of a 47 complex layers film with a

linear structure, [47CoL1], on a Au electrode. The visualized thickness of

[47CoL1] in the cross-sectional views was B100 nm, in good agreement with



Current density/μA cm–2



(a)



Redox and Photo Functions of Metal Complex Wires

100



150



10

8

6

4

2



50



Current density/μA cm–2



394



0

Ϫ50

Ϫ100



[nFeL2]

0



0.3



0.6



3



50



2

1



0

Ϫ50

Ϫ100

Ϫ150



0.9



4



100



[nFeL3]

0



0.3



Surface coverage/10–10molcm–2



(b)



0.6



0.9



Potential vs. Fc+/Fc



Potential vs. Fc+/Fc

30

[nFeL3]

20



[nFeL2]

10



0



0



2



4



6



8



10



The number of complexation cycles



FIGURE 5. (a) Cyclic voltammograms of [nFeL2] (n 5 2, 4, 6, 8, 10) and [nFeL3]

(n 5 1À4) in 1 M Bu4NClO4ÀCH2Cl2 at a scan rate of 0.1 V s21, and (b) plots of the

coverage of redox-active sites, Γ (mol cm22) vs. n for [nFeL2] and [nFeL3]. The lines in

the figure show the relationship of Γ 5C 3 n for [nFeL2] and that of Γ 5C (2n À 1) for

[nFeL3]. (Reprinted with permission from Ref. 13.)



the ideal thickness of 94 nm (1 molecular unit length, 2 nm).11 STM observations distinguished the different structures of the films. In the film with a

linear structure, uniform 6 nm-o.d. circular domains were confirmed, where

distinct differences between each generation could not be obtained. This

observation indicates the uniform structure of the film. In contrast, for the film

with loosely distributed oligomer wires with dendritic structure composed of

Fe complex, the increase in generations was clearly reflected in the STM

image. In the case of [1FeL2nFeL3], the complex with L3 stacked onto the

sparse coverage of [1FeL2] provided an image of circular dots whose diameters

increased with the number of complexation cycles (Fig. 8). These results

strongly support the formation of ideal structures by the stepwise coordination

method on the surface.



Bottom-Up Fabrication of Redox-Conducting Wires

Top View



395



Side View



[3-G]



[4-G]



[5-G]



FIGURE 6. Molecular structures of [nFeL3] (n 5 3, 4, 5) acquired by MM1 calculation. (Reprinted with permission from Ref. 13.)



B. Electron Transport Behavior of the Molecular Wires

on the Electrode

We describe here that the redox oligomer wires fabricated with the

stepwise coordination method show characteristic electron transport behavior

distinct from conventional redox polymers. Redox polymers are representative

electron-conducting substances in which redox species are connected to form a

polymer wire.21À25 The electron transport was treated according to the concept

of redox conduction, based on the diffusional motion of collective electron

transfer pathways, composed of electron hopping terms and/or physical

diffusion.17,18,26À30 In the characterization of redox conduction, the Cottrell

equation can be applied to the initial currentÀtime curve after the potential

step in potential step chronoamperometry (PSCA), which causes the redox

reaction of the redox polymer lm:

i ẳ ne FACDapp =tị1=2



1ị



where ne, F, A, C, and Dapp refer to the number of electrons, the Faraday

constant, the electrode area, the concentration of redox sites in the film, and the



396



Redox and Photo Functions of Metal Complex Wires



(a)



(b)



(c)



(d)



FIGURE 7. SEM images of [47CoL1]: (a,b) top view; (c,d) cross-sectional view.

(Reprinted with permission from Ref. 13.)



apparent diffusion coefficient of collective electron transfer pathways in the

film, respectively.31À41

We first attempted to evaluate Dapp for the Fe(tpy)2 oligomer wires

prepared by stepwise formation on a gold electrode using PSCA, but in most

cases, the plots of i vs. t1/2 did not show any region of straight lines intersecting

the origin, indicating that electron transport analysis based on the diffusion

process is not applicable.42 Figure 9 shows the iÀt plots after the potential step

from 0.96 to 0.36 V vs. Fc1/Fc for reduction of the FeIII complex moieties in

linear Fe(tpy)2 oligomer wires, [nFeL2] (n 5 2, 4, 6, and 8), and in branched

oligomer wires [nFeL3] (n 5 2, 3, and 4), in 1 M Bu4NClO4ÀCH2Cl2. In all

cases, the curvatures for the FeIII/FeII couple showed a similar behavior. In

the initial period, a quasi-plateau region appeared, and a rapid decrease in

current followed. It is obvious that the branched wires [nFeL3] showed

much longer time-constant current flow behavior B0.12 s for [4FeL3];

Fig. 9b) than that of the linear wires [nFeL2] (B0.05 s for [8FeL2]; Fig. 9a).



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