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B. Electron Transport Behavior of the Molecular Wires on the Electrode

B. Electron Transport Behavior of the Molecular Wires on the Electrode

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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).



Bottom-Up Fabrication of Redox-Conducting Wires



(a)



397



(b)



60×60 nm



60×60 nm



Å



Å



10

5.0

5



2.5



0

50



100 150 200 250 300 350 400 450 500 550 Å



0

50



(c)



100 150 200 250 300 350 400 450 500 550



Å



(d)



60 ×60 nm



120 ×120 nm

Å



Å



15

50



10



25



5

0

0



200



400



600



800



Å



0

0



50



100



150



200



250



300



Å



FIGURE 8. STM images of (a) [2FeL1], (b) [2FeL3], and (c,d) [1FeL2nFeL3] (n53, 4,

t1510 s). (Reprinted with permission from Ref. 13.)



This difference strongly supports the dominance of the through-bond

electron transport pathway. This occurs because if the simple diffusion

process containing through-space electron transfer is dominant, the electron transport kinetics of the branched oligomer film is similar to, or faster

than, that of the linear oligomer films, as the density of redox sites in the

branched oligomer film is similar to, or higher than, that in the linear

oligomer films.

An explanation of the characteristic i-t behaviors requires the application

of simple kinetic determination based on the molecular-level sequential redox



Current density/μA cm–2



(a)



Redox and Photo Functions of Metal Complex Wires

–6



(b) –3

Current density/μA cm–2



398



–4.5

2



4



6



8



–3



–1.5



0



0



0.03



0.06



0.09



Time/s



2



3



4



–2



–1



0



0



0.05



0.1



0.15



Time/s



FIGURE 9. Current-time plots after the potential step from 0.96 to 0.36 V vs. Fc1/Fc

to reduce the FeIII complex moieties in (a) [nFeL2] (n52, 4, 6, and 8) and in (b) [nFeL3]

(n52, 3, and 4) in 1 M Bu4NClO4ÀCH2Cl2 (solid lines). The numbers in the figure refer

to n. Simulated curves in dotted lines are obtained with k15240 s21, k2 (L2)51.7 3 1013

cm2 mol21 s21, and Cdl510 μC cm22 for [2FeL2]; k15240 s21, k2 (L2)51.7 3 1013 cm2

mol21 s21, and Cdl512 μC cm22 for [4FeL2]; k15210 s21, k2 (L2)51.6 3 1013 cm2 mol21

s21, and Cdl527 μC cm22 for [6FeL2]; and k15210 s21, k2 (L2)51.2 3 1013 cm2 mol21

s21 and Cdl540 μC cm22 for [8FeL2] for A, and obtained with k15270 s21, k2 (L2)54.9

3 1013 cm2 mol21 s21 and Cdl519 μC cm22 for [2FeL3]; k15270 s21, k2 (L3)55.0 3 1012

cm2 mol21 s21, and Cdl519 μC cm22 for [3FeL3]; and k15260 s21, k2 (L3)54.4 3 1012

cm2 mol21 s21, and Cdl527 μC cm22 for [4FeL3]. (Reprinted with permission from

Ref. 13.)



conduction along molecular wires, as displayed in Figure 10. The concept of

this mechanism is as follows. When an oxidized complex, Ox, in molecular

wires is reduced to form a reduced complex, Red, with application of a sufficient overpotential to prevent back electron transfer in PSCA, the electron

transfer kinetics in the case of the nth complex sequence can be written as

follows:

Ox1 ỵ e -Red1 ;



rate constant : k1



aị



Red1 ỵ Ox2 -Ox1 ỵ Red2 ;



rate constant : k2



bị



Red2 ỵ Ox3 -Ox2 ỵ Red3 ;



rate constant : k3



cị





Redn1 ỵ Oxn -Oxn1 ỵ Redn ; rate constant : kn



ðdÞ



ð2Þ



where Redi and Oxi are reduced and oxidized forms, respectively, in the ith

layer or generation in the film, and



Bottom-Up Fabrication of Redox-Conducting Wires



399



(a)

[O4]







[O3]

1-G , 2-G , 3-G…

[O2]



k2



[O1]



k1



(b)

N



N



N

N



N



N



N



N

N

N

Fe

N

N

N



N

N

N

Fe N

N

N



N



N



[O4]

N

N



[O3]



N



k2

N

Fe

N

N

N

N



[O2]







N



1-G, 2-G, 3-G…

k2



NN







S







[O1]

k1



FIGURE 10. Images of the electron transfer mechanisms in (a) linear and (b) branched

molecular wires. (Reprinted with permission from Ref. 13.)



dẵOx1 =dt ẳ k1 ẵOx1 ỵ k2 ẵOx1 0 ẵOx1 ịẵOx2



aị



dẵOx2 =dt ẳ k2 ẵOx1 0 ẵOx1 ịẵOx2 ỵ ẵOx3 0 =ẵOx2 0 ịk2 ẵOx2 0

ẵOx2 ịẵOx3



bị



dẵOx3 =dt ẳ ẵOx3 0 =ẵOx2 0 ịk2 ẵOx2 0 ẵOx2 ịẵOx3

ỵẵOx4 0 =ẵOx2 0 ịk2 ẵOx3 0 ẵOx3 ịẵOx4



cị



3ị





dẵOxn1 =dt ẳ ẵOxn1 0 =ẵOxn2 0 ịk2 ẵOn2 0 ẵOxn2 ịẵOxn1

ỵẵOxn 0 =ẵOxn2 0 ịk2 ẵOxn1 0 ẵOxn1 ịẵOxn

dẵOxn =dt ẳ ẵOxn 0 =ẵOxn2 0 ịk2 ẵOxn1 0 ẵOxn1 ịẵOxn



dị

eị



where [Oxi]0 and [Oxi] are the initial and present two-dimensional concentrations, respectively, of the oxidized form of the redox moiety in the ith layer or

generation in mol cm22.



400



Redox and Photo Functions of Metal Complex Wires



The reaction kinetics is controlled by two factors: k1 (s21) for the electron

transfer between the nearest redox site and the electrode, and k2 (cm2 mol21

s21), for the electron transfer between the neighboring redox sites in a molecular wire. Here, the rate of electron transfer to the neighboring site in a

polymer wire is assumed to be constant in a primary approximation. In the case

of linear oligomer wires such as [nFeL1] and [nFeL2],

ẵRed1 ỵ ẵOx1 ẳ ẵRed2 ỵ ẵOx2 ẳ ẵRed3 ỵ ẵOx3

ẳ ẳ ẵRedn ỵ ẵOxn ẳ constant



4ị



and in the case of branched oligomer wires, [nFeL3],

ẵOx1 ỵ ẵRed1 ẳ ẵOx2 ỵ ẵRed2 ị=3 ẳ ẵOx3 ỵ ẵRed3 ị=7

ẳ ? ẳ ẵRedn ỵ ẵOxn ị=2n 1ị ẳ constant



5ị



The practical current can be observed as d[Ox1]/dt, which implies that the

constant current flows in the initial period when the electron transfer between

the neighboring redox sites is sufficiently fast compared to that between

the electrode and the first redox site. The numerical calculation involving the

double-layer capacitance, which decays exponentially with time, provided

the simulated curves with parameters k1 5 220 6 10 s21 and k2 (L2) 5 1.4 6

0.1 3 1013 cm2 mol21 s21 and Cdl525 6 15 μC cm22, satisfactorily reproducing

all of the experimental results of [nFeL2] (n 5 2, 4, 6, and 8), as shown in

Figure 9a. The similar calculation of [nFeL3] (n 5 2, 3, and 4) provided

simulated curves with parameters k15260 6 10 s21 and k2 (L3) 5 4.8 6 0.2 3

1012 cm2 mol21 s21 with the inclusion of Cdl of 22 6 4 μC cm22 (Fig. 9b). The

similarity of the k1 values between [nFeL2] and [nFeL3] is reasonable because

the first Fe complex layer is the same in both cases. The k2 value for the

branched oligomer wire is lower than that for the linear wire, since the bridging

ligand in the former is m-phenylene with a shorter π-conjugation than that in

the latter with p-phenylene.

In this section, we described the quantitative formation of redoxconducting metal complex oligomer films achieved by selecting the conditions of stepwise coordination reactions at the electrode surface, and the

characteristic through-bond electron transport of the molecular wires thus

formed. These results indicate that by repeating the connection of metal ions

and bridging ligands, we can synthesize polymer wires of various shapes

with a desired number of metal complex units. One important advantage of

this method is that we can easily prepare heterometal and heteroligand

complex polymer wires by placing different kinds of metal ions and different

kinds of bridging ligands at desired positions, after which we can accumulate

multiple molecular functionalities, connected with the through-bond redox



Photoelectric Conversion System Using Porphyrin and Wires



401



conduction, which are important in the construction of molecular devices.

The following section describes an example that displays photoelectric

conversion.



III. PHOTOELECTRIC CONVERSION SYSTEM

USING PORPHYRIN AND REDOX-CONDUCTING

METAL COMPLEX WIRES

Here, we introduce the application of molecular wires described in the

former section to a photoelectric conversion system. Construction of photoelectric conversion systems is a significant theme directly linked to environmental and energy issues. In nature, highly efficient photoelectric conversion

(B100%) is achieved in photosynthesis. This ultimate function is realized by

the appropriate arrangement of functional materials, which prevents back

electron transfers and generates efficient electron transfers. Considering the

photoelectric conversion system in nature, molecular assembly is potentially an

effective tool in the construction of an efficient photoelectric conversion system. Using this method of molecular assembly, the arrangement of photoreceptors, donors, and acceptors has recently been used for a photoelectric

conversion system. For example, Imahori et al. succeeded in the construction

of a photoelectric conversion system with high quantum efficiency and long

life, using donorÀphotosensitizerÀacceptor molecules.2,3 They selected a

ferrocene-porphyrin-fullerene triad for the donorÀphotosensitizerÀacceptor

molecule, constructing integrated artificial photoelectric conversion assemblies

on a gold electrode. This system established a cascade of photoinduced energy

transfer and multistep electron transfer, achieving a high quantum yield of

50%.2 They also reported an extremely long-lived, charge-separated state

using a ferrocene-zinc-porphyrin—freebase-porphyrin—fullerene tetrad, for

which the lifetime is 0.38 s, comparable to that for the bacterial photosynthetic

reaction center.3

While molecular assembly has proven to be effective for a photoelectric

conversion system, coordination reactions are possibly a simple approach for

connecting such functional molecules, as presented in the previous section. We

applied the stepwise coordination method to prepare a photoelectric conversion system. Since the molecular wire exhibits redox conduction through the

wire,11,13 efficient photo-electron transport through the redox sites in the wire is

also expected. In this section, we demonstrate the fabrication of a photoelectric

conversion system using ITO electrodes modified with M(tpy)2 (M 5 Co,

Fe, Zn) complex wires with a terminal porphyrin moiety as a photosensitizer.

The behavior of photo-electron transfer from porphyrin to ITO through the

molecular wire was investigated by changing the metal element in the M(tpy)2

moieties.14



402



Redox and Photo Functions of Metal Complex Wires



A. Bottom-Up Fabrication of the Porphyrin-Terminated

Redox-Conducting Metal Complex Film on ITO

We fabricated the modified ITO electrodes4 by a combination of SAM

formation with a terpyridine derivative and stepwise metal-terpyridine coordination reactions in a similar manner as that described in the previous

section (Fig. 11).11,13 A cleaned ITO was immersed in a 0.1 M solution of

4-[2,2u:6u,2v-terpyridin]-4u-yl-benzoic acid (tpy-BzA) in chloroform for 12 h to

anchor the carboxyl group to ITO. Subsequently, the modified ITO was

immersed in an aqueous solution of 0.1 M CoCl2, Fe(BF4)2 or Zn(BF4)2

for 2À3 h to form metal-terpyridine coordination reactions. Finally, the

metal-coordinated ITO was immersed in a 0.1 M acetonitrile solution of a

terpyridine-functionalized porphyrin, tpy-ZnTPP, providing the target

molecular wires, [M-ZnTPP], on electrodes (Fig. 11). In addition, a cleaned

ITO was immersed in a 0.1 M ethanol solution of carboxylate-functionalized

porphyrin, C10ZnTPP, to afford a modified ITO as a reference.



N



N



N



N

N



O



N



N



NH



Zn



O



N



N



N



O

H

R



tpy-ZnTPP



tpy-BzA



N



N

Zn



N



N

R



N



N



HN



O



R



Zn

N



N



R = O(CH2)10COOH



M-ZnTPP



C10ZnTPP



R

N



N

N



N



N



N



N



N



N



(ii)



(i)



N

M



M



N



N



(iii)

M = Fe2+, Co2+, Zn2+

O



O O



ITO



ITO



ITO



O



O O



ITO



FIGURE 11. Chemical structure of the ligands used in this chapter, and stepwise

coordination methods for the preparation of modified ITO electrodes: (i) immobilization of tpy-BzA, (ii) complexation with a metal ion, and (iii) complexation with

tpy-ZnTPP. (Reprinted with permission from Ref. 14.)



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