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Bi-directional photocurrent generation dependent on the wavelength of irradiation of a mixed monolayer assembly

Type: Result | Advantage: None | Novelty: None | ConceptID: Res1

A mixed self-assembled monolayer consisting of a ruthenium tris-bipyridine complex linked to viologen and a palladium phthalocyanine derivative was fabricated on a gold electrode by means of pendant thiol groups.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

The direction of the photocurrent which is induced in the electrode when it is irradiated depends on the wavelength of the light used.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res1

The development of novel organic thin film devices based on the assembly of molecular components has been an exceptionally active field in recent times.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot1

Self-assembled monolayers (SAMs) of donor (D)–acceptor (A) pairs on electrodes have attracted particular attention for application as photoelectric conversion devices.^1–6

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot1

Photoinduced electron transfer between the donor and the acceptor is responsible for photocurrent generation.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

Implantation of light-harvesting molecules⁷ and fabrication of three-dimensional nanostructured assemblies by combining gold nanoparticles and thiol dyes^8–10 have proven to be successful avenues for improving photocurrent generation.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met1

Until now, however, the development of devices based on SAMs has been directed towards the improvement of photoelectric conversion efficiency.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met1

One of the noteworthy advantages of self-assembly is that different molecules can be implanted on the electrode at the same time.^1,7

Type: Method | Advantage: Yes | Novelty: Old | ConceptID: Met1

This may allow SAMs to be exploited for new applications in photonic fields.

Type: Method | Advantage: Yes | Novelty: Old | ConceptID: Met1

We have developed a novel photoelectric conversion device which uses a mixed SAM composed of a linked ruthenium bipyridine–viologen disulfide¹⁰ and a palladium phthalocyanine thiol derivative (Fig. 1).¹¹

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj2

The direction (cathodic or anodic) of the photocurrent induced in the monolayer can be reversed by altering the irradiation wavelength.

Type: Object | Advantage: Yes | Novelty: New | ConceptID: Obj2

The syntheses of the linked ruthenium bipyridine complex–viologen disulfide [RuVS]₂¹⁰ and the phthalocyanine disulfide [PcS]₂¹¹ have been described previously.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met2

The gold electrode was prepared by depositing a small amount of titanium and then gold on a glass plate by vacuum deposition.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

This gold electrode was immersed in a solution of [RuVS]₂ (5 × 10^–4 mol dm^–3) in MeCN–CH₂Cl₂ (1 ∶ 1 v/v) for 4 days.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

Then, the electrode was removed from the solution, rinsed with acetonitrile and dichloromethane, and then dried in air, to give an electrode modified with RuVS (denoted RuVS/Au).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

An electrode modified with [PcS]₂ (PcS/Au) was also prepared by the same procedure.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

The electrode modified with both compounds, (RuVS + PcS)/Au (Fig. 1), was prepared from a solution containing identical concentrations (5 × 10^–4 mol dm^–3) of [RuVS]₂ and [PcS]₂ by the same procedure as used for RuVS/Au.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp5

The surface coverage of RuVS in RuVS/Au (5.5 × 10^–11 mol cm^–2) was estimated by monitoring the reductive wave of the viologen moiety of RuVS, and the surface coverage of PcS/Au obtained from the absorption spectrum of PcS (1.6 × 10^–11 mol cm^–2).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

As to (RuVS + PcS)/Au, the surface coverage of each dye was evaluated from the reductive wave of the viologen moiety and from the adsorption ratio of RuVS and PcS on gold powder:^1,12 6.0 × 10^–12 mol cm^–2 for RuVS and 1.9 × 10^–12 mol cm^–2 for PcS.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp7

The reason for the lower coverage in the case of (RuVS + PcS)/Au is uncertain at present, but the photocurrent intensity correlates well with the surface coverage (vide infra).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

Photocurrent measurements on the modified electrodes were carried out using a three-electrode photoelectrochemical cell consisting of the modified electrode (working), an Ag/AgCl (sat.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp8

KCl) electrode (reference), and a platinum wire (counter) in an aqueous solution containing 0.1 mol dm^–3 NaClO₄ under a nitrogen atmosphere in the presence of methylviologen (MV²⁺; 5 × 10^–3 mol dm^–3) and triethanolamine (TEOA; 5 × 10^–2 mol dm^–3).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp8

The light from a 150 W tungsten lamp was passed through a monochromator and irradiated a 0.28 cm² area of the modified electrode.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp9

The photocurrent action spectra were measured by changing the excitation wavelength (Δλ = ±16 nm).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp10

When the photocurrent action spectrum of RuVS/Au was measured at E = 0 mV, anodic photocurrents were observed in the 380–750 nm region.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

The action spectrum showed a broad band around 450 nm, almost overlapping with the absorption band of RuVS in acetonitrile.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

This suggests that the ruthenium complex is the photoactive species and the anodic photocurrent generation is mainly due to photoinduced intramolecular electron transfer from the photoexcited ruthenium complex to the viologen moiety.^1,4

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

The oxidized ruthenium complex is reduced by TEOA.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

On the other hand, cathodic photocurrents were observed in the case of PcS/Au at E = 0 mV.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

The action spectrum also showed broad bands around 700 nm, corresponding to the absorption bands of phthalocyanine in toluene.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

Thus, the phthalocyanine moiety is the photoactive site and intermolecular photoinduced electron-transfer from the photoexcited phthalocyanine to MV²⁺ in the bulk mainly induces the cathodic photocurrent generation in PcS/Au.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

A small amount of residual oxygen in the electrolyte solution may also contribute to some extent to the cathodic photocurrent generation.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

These photocurrent mechanisms are shown in Figure 2.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

At E = –200 mV, the photocurrent from (RuVS + PcS)/Au was observed in the cathodic direction when exciting in the 400–750 nm region and the photocurrent action spectrum showed broad peaks around 630 and 700 nm; thus, the contribution from RuVS is negligible.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res6

At E = +400 mV on the other hand, the photocurrent was observed only in the anodic direction when irradiating in the 400–750 nm region, with a relatively large broad peak around 450 nm, meaning that the contribution from PcS is negligible.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

These results show that RuVS functions individually for anodic photocurrent generation at E = –200 mV, while PcS acts similarly for cathodic photocurrent generation at E = +400 mV.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con3

At E = 0 mV, however, photocurrents were observed in both directions, depending on the irradiation wavelength, as shown in Fig. 3.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res8

The photocurrent action spectrum for (RuVS + PcS)/Au at E = 0 mV shows a broad anodic peak around 450 nm, corresponding with the absorption peak of RuVS, and two cathodic peaks around 630 and 700 nm, corresponding with the absorption peaks of PcS, with a cross-point (zero current) around 550 nm.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res8

The photocurrent measurements summarized above indicate that two kinds of photoinduced electron-transfer pathways operate almost independently in the (RuVS + PcS)/Au monolayer.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con4

Intramolecular photoinduced electron transfer occurs from the photoexcited ruthenium complex to viologen in RuVS and intermolecular photoinduced electron transfer from the photoexcited phthalocyanine to MV²⁺ (and residual oxygen) takes place in the bulk.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con4

The photocurrent generated by PcS/Au was compared under three different experimental conditions in terms of the sacrificial reagents: (1) [MV²⁺] = 5 × 10^–3 mol dm^–3 under aerobic conditions, (2) [MV²⁺] = 5 × 10^–3 mol dm^–3 under a nitrogen atmosphere, and (3) without MV²⁺ under aerobic conditions.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj3

The ratio of the photocurrent at 700 nm for the three conditions (1), (2), and (3) is 20 ∶ 4 ∶ 7.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs4

These results indicate that oxygen is a good electron acceptor for the photoexcited phthalocyanine and the presence of oxygen promotes electron transfer from the photoexcited phthalocyanine to MV²⁺.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

It is likely that oxygen accepts an electron from MV⁺, regenerating MV²⁺.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con5

In any case, one of these two pathways can be preferentially driven by irradiation at the appropriate wavelength, because the absorption bands of the ruthenium complex (around 450 nm; hν1) and phthalocyanine (600–700 nm; hν2) are widely separated from each other.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con6

At negative applied potentials, electron transfer from the electrode to the cation radical of phthalocyanine occurs easily in PcS (E_ox = +0.84 V vs.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs5

Ag/AgCl), while that from the viologen moiety to the electrode becomes difficult in RuVS (E_red = –0.39 V vs.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs5

Ag/AgCl).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs5

At positive potentials, electron-transfer from the one-electron reduced viologen moiety to the electrode becomes favorable in RuVS.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs6

Accordingly, the direction of the photocurrent in (RuVS + PcS)/Au at E = 0 mV reverses at ∼550 nm.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

The bi-directional photocurrent response reported here arises as a result of the implantation of two separate and individually functioning photoinduced electron-transfer systems on the electrode.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res10

This novel photocurrent behavior is promising for applications in molecular photonic devices and photoelectric logic devices.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con7