Photoinduced Energy vs Electron Transfer Between Fe(II) and Ru(II

Article pubs.acs.org/JPCC

Photoinduced Energy vs Electron Transfer Between Fe(II) and Ru(II) Bisterpyridyl Complexes in Solution and Grafted on a Gold Surface Sébastien Liatard, Jérôme Chauvin,* Damien Jouvenot, Frédérique Loiseau, and Alain Deronzier Université Joseph Fourier Grenoble 1/CNRS, Département de Chimie Moléculaire, UMR-5250, Laboratoire de Chimie Inorganique Rédox, Institut de Chimie Moléculaire de Grenoble FR-CNRS-2607, BP-53, 38041 Grenoble Cedex 9, France S Supporting Information *

ABSTRACT: Quantitative oxidation of [Fe(ttpy)2]2+ (1) (ttpy = 4′-(4-methylphenyl)2,2′:6′,2″-terpyridine) has been performed in CH3CN by continuous photolysis experiments in the presence of [Ru(ttpy)2]2+ (2) and a large excess of a diazonium salt (ArN2+) as sacrificial oxidant. The reaction occurs with a low quantum yield (ϕ = 10−3). The photooxidation process is fully characterized and shows that (2) acts as an antenna and transfers the excitation energy to (1). The latter complex in its excited state is oxidized by ArN2+. Thiolated derivatives of both complexes, i.e., [Fe(ttpySH)2]2+ (3) (ttpySH = 4′(4-(2-mercaptoethoxy)phenyl)-2,2′:6′,2″-terpyridine) and [Ru(ttpySH)2]2+ (4), have been self-assembled on gold. The reactivity of the [Ru(ttpy)2]2+ core is modified between solution and the self-assembled monolayer (SAM) structure. Whereas in solution under irradiation (2) is not oxidized by ArN2+, a cathodic photocurrent generation is obtained from a SAM of (4) with ArN2+ in solution. A bilayer assembly combining (4) as a photoactive unit and (3) as an electron transport relay has been obtained on a gold electrode by a two-step process: A self-assembled monolayer (SAM) of (3) is immobilized on gold, followed by the covalent attachment of (4) via an electroinduced S−S bond formation. A stable cathodic photocurrent is observed under visible irradiation when the potential applied to the electrode is fixed at 0.4 V vs Ag/AgNO3 10−2 M. The system achieves the uphill transport of electron of 0.8 eV.

1. INTRODUCTION Construction of well-organized systems mimicking the functions performed by Nature such as photoinduced electron/energy transfer is an intense field of research due to their relevance not only to solar energy conversion1−3 but also for the development of molecular components for nanoelectronic devices.4−6 In natural systems, the active photosensitizer (P), electron donor (D), and acceptor (A) components are well organized in the thylakoid membrane, resulting in an efficient cooperative and unidirectional electron transfer reaction under solar illumination with limited back reactions. Following this precept, different techniques have been investigated in artificial systems to organize the active (D−P−A) triad molecules, notably using lipid bilayer membranes,7,8 Langmuir−Blodgett films,9−11 or the formation of self-assembled monolayers (SAM) on an electrode.12,13 More recently, the self-assembly approach, using coordination complexes containing terpyridine ligands substituted by thiol groups, has received significant attention. This approach couples the advantage of a covalent linkage on a conducting gold electrode with an easy synthetic procedure using stepwise coordination chemistry.14,15 However, this chemical approach is limited to metal centers for which coordination readily occurs under mild conditions, which is usually not the case for wellknown Ru(II) polypyridyl-based photosensitizers.16 To solve this problem, we recently proposed an alternative electrochemical method to build assemblies containing the Ru(II) © 2013 American Chemical Society

bisterpyridine unit. We notably obtained a heterobimetallic assembly of bis(4′-(4-(2-mercaptoethoxy)phenyl)-2,2′:6′,2″terpyridine) (tpySH) complexes of Fe(II) (3) and Ru(II) (4) (Scheme 1), by an easy process resulting from the electroinduced S−S coupling chemistry between a SAM of (3) on gold, bearing thiol extremities exposed to the solution, and (4) added to the medium.17 Ru(II) complexes containing tridentate ligands of the 2,2′:6′,2″-terpyridine (tpy) type result in a more linear and organized system than the renown Ru(II) analog with bidentate ligand ([Ru(bpy)3]2+ (bpy) = 2,2′ bipyridine). However, due to the short lifetime of their excited state in solution (τ = 250 ps for [Ru(tpy)2]2+ at 298 K18), they are much less investigated as photosensitizers. Hence, in this article, before studying the photoinduced electron transfer phenomena occurring in the [(4)−(3)/Au] heterometallic assembly on gold (Scheme 1), we report on the photoredox behavior in solution of a mixture of [Fe(ttpy)2]2+ (1) (ttpy = 4′-(4-methylphenyl)-2,2′:6′,2″terpyridine) and [Ru(ttpy)2]2+ (2) in the presence of a diazonium salt (ArN2+) as a sacrificial electron acceptor. Results show the low efficiency of the photoinduced oxidation of (1) in solution by ArN2+, and the kinetics is drastically accelerated using (2) as an external photosensitizer. Furthermore, it Received: June 5, 2013 Revised: September 12, 2013 Published: September 16, 2013 20431

dx.doi.org/10.1021/jp4055704 | J. Phys. Chem. C 2013, 117, 20431−20439

The Journal of Physical Chemistry C

Article

Scheme 1. Molecular Structure of the Monometallic Complexes (1−4) and the Heterobimetallic [(4)−(3)/Au] Assembly on Gold

−2 V was then run in a 0.1 M solution of TBAPF6 in acetonitrile with gold as a working electrode prior to the grafting. This reduction cycle improved the successive grafting of the thiol derivatives presumably since it cleans the electrode by desorbing impurities and modifies the state of the surface. The gold electrode was then immersed in a 5 × 10−4 M solution of (3) or (4) for 15−20 h. Before any electrochemical or photoelectrochemical experiments, the modified electrode was rinsed with acetonitrile and transferred to a solution of 0.1 M TBAPF6 in acetonitrile. The surface coverage Γ has been determined according to eq 1

appears that the reactivity of the [Ru(tpy)2]2+ core is modified between the solution and the SAM structure. Whereas under irradiation (2) in solution is not oxidized by ArN2+, a significant cathodic photocurrent response is obtained for a SAM of (4) on gold in the presence of ArN2+. The results demonstrate also an efficient photoinduced electron transfer process within the [(4)−(3)/Au] molecular construction, opening an easy route for the development of triads on gold.

2. EXPERIMENTAL SECTION Materials and General. Acetonitrile (Rathburn, HPLC grade), tetra-n-butylammonium hexafluorophosphate (TBAPF6, Aldrich), and 4-bromobenzenediazonium tetrafluoroborate (ArN 2 +, BF 4 −, Accros) were used as received. Tri(pbromophenyl)amine (tBrTPA) 19 and iron(II) 20 and ruthenium(II)21 bis(4′-(4-methylphenyl)-2,2′:6′,2″-terpyridine) hexafluorophosphate, respectively, denoted (1) and (2) were prepared according to the literature. Iron(II) and ruthenium(II) bis(4′-(4-(2-mercapto-ethoxy)phenyl)2,2′:6′,2″-terpyridine) hexafluorophosphate (3) and (4) were synthesized following already reported procedures.17 Electrochemistry. All electrochemical measurements were run under argon in a dry glovebox at room temperature. Cyclic voltammetry (CV) (Epa, anodic peak potential; Epc, cathodic peak potential; E1/2 = (Epa + Epc)/2; ΔEp = Epa − Epc) was performed using a Biologic SP300 potentiostat. A standard three-electrode electrochemical cell was used. Potentials were referred to a Ag/AgNO3 10 mM reference electrode in CH3CN + 0.1 M TBAPF6. Formation of SAM on Gold. Gold disc working electrodes (CH Instruments diameter = 3 mm) were polished thoroughly with diamond paste. A blank reduction cycle from 0 V down to

Γ=

Q nFA

(1)

where Q is the charge required to oxidize the metallic center determined from the area under the oxidation peak of M2+/3+ (for the bilayer assembly only the Fe2+/3+ oxidation peak was taken into account), n the number of electrons transferred (n = 1), F the Faraday’s constant, and A the area of the electrode (A = 0.071 cm2). Reproducibility is estimated to ±10−10 mol/cm2. Absorbance and Emission. Absorption and emission spectra were obtained using, respectively, a Cary 300 UV− visible spectrophotometer (Varian) and a Cary Eclipse fluorimeter. Emission lifetime measurements were performed after irradiation at λ = 400 nm using the technical support of the Nanobio platform at the University of Grenoble.20 Transient absorption was acquired using a LP920K system from Edinburgh Instruments. Excitation was carried out from the third-harmonic (355 nm) of a Brilliant-Quantel Nd:YAG Laser at 6 Hz. A Xe900 pulsed Xenon Lamp is used as the probe source. The photons were dispersed using a monochromator, transcripted by a R928 (Hamamatsu) photo20432



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Table 1. UV/Visible Absorption Data for (1)−(4) and Emission Data for (4) In Deoxygenated CH3CN absorption λabs max/nm

complexes (4) (3) (2) (1) a

M M M M

= = = =

Ru Fe Ru23 Fe20

233 230 284 285

(25100); (36700); (68000); (68200);

282 283 310 320

−1

(ε/M

(37500); (41400); (76000); (62800);

307 322 490 567

emission at 77 Ka

emission at 298 K −1

cm ) (48200); 493 (20100) (49500); 569 (20200) (28000) (25900)

λem max/nm

τ/ns

λem max/nm

656

0.7

640

640

0.95

628

τ/μs 12 9.1

In rigid butyronitrile.

Table 2. Redox Potentials of (1)−(4) in Deoxygenated CH3CN + 0.1 M TBAPF6a complexes (4) (3) (2) (1)

M M M M

= = = =

Ru Fe Ru23c Fe20

MIII/MII

thiol oxidation

E1/2 (ΔEp)

Epa

0.93 0.76 0.95 0.76

V (51 mV) V (58 mV) V V (60 mV)

0.73 V 0.76 Vb

ligand-based reductions E1/2 (ΔEp) −1.54 −1.53 −1.54 −1.54

V (64 mV); −1.81 V (184 mV) V (92 mV); −1.70 V (73 mV); −1.87 (100 mV) V V (60 mV); −1.65 V (60 mV); −2.30 (90 mV)

Potentials are reported vs Ag/AgNO3 10−2 M. bThe potential cannot be accurately determined due to the proximity of the FeIII/FeII redox system. The potentials have been converted from SSCE to Ag/AgNO3 10−2 M by subtracting 302 mV.

a c

multiplicator, and recorded on a TDS 3012C (Tektronix) oscilloscope. The sample (5 × 10−5 M) in CH3CN was degassed using argon prior to measurements. Quantum yield of formation of (1+) with (2)/ArN2+ was determined by actinometry after irradiation with a 250 W Xe lamp, using the quantum yield of the photoxidation of [Ru(bpy)3]2+ by ArN2+ as reference ϕ = 0.34.22 Samples were prepared with the same absorbance at 450 nm (Abs = 2). The variation of absorbance during irradiation was checked to be less than 0.1. Continuous irradiation of the solution was performed using a 250 W Xe lamp whose UV and IR irradiation were filtered with a large band pass (03MHG101 Melles Griot) (irradiation between 400 and 700 nm). The solution contained a mixture of complexes (5 × 10−5 M) in CH3CN under Argon with ArN2+ (15 mM). Under these conditions the concentration of ArN2+ can be considered as constant during experiments. Photocurrent Measurements. A standard three-electrode electrochemical cell in glass was used for photocurrent measurements. The modified electrodes were irradiated under argon in the visible region with a tungsten lamp (Oriel 66184) equipped with a constant current power supply (Oriel 68830) fixed at 180 W. A large band-pass (310−845 nm) Schott and Mainz filter was placed between the light source and the cell. The electrochemical cell was held 4 cm above the lamp. In these conditions, the radiant power landing on the surface of the electrode is 0.11 W cm−2. A CHI-620b potentiostat (CHInstruments) in chronoamperometry mode was used to monitor the evolution of current vs time. During the experiment, the light was alternatively switched on and off by placing a metallic plate over the lamp outlet.

Figure 1. Absorption spectra of (4) (full line) and (3) (dotted line) at room temperature in CH3CN. The dashed line is the emission spectrum of (4) at 77 K in a rigid butyronitrile matrix.

transition and appears at wavelengths similar to those of (1) and (2). This absorption band is red-shifted for (3) compared to (4) in accordance with the lower oxidation potential of the Fe2+/3+ center compared to Ru2+/3+. In the UV region, absorption spectra are dominated by the strong π → π* transitions of the terpyridine ligands.23,24 Complex (4) is a poor emitter at room temperature in deoxygenated CH3CN but exhibits an emission spectrum centered at 640 nm with a lifetime of 12 μs at 77 K in a rigid matrix. This is typical of Ru(II) bisterpyridine complexes, in which a metal centered (MC) excited state which lies very close to the emitting MLCT level strongly contributes to deactivate it through a thermally activated surface-crossing process.24 As a consequence Ru(II) bisterpyridine complexes are usually poorly luminescent in solution at room temperature.25 Nevertheless, it has been proven to be an effective photosensitizer when grafted on a surface,26,27 electropolymerized on electrode,28 trapped in a zeolite,29 or linked to an enzyme.30 Complexes (1) and (3) are not luminescent due to an efficient deactivation within a few picoseconds of the MLCT state through a lower energy MC state.31 Cyclic voltammetry of (3) and (4) has already been reported.17 Data are summarized in Table 2. In the anodic part, (4) exhibits two successive oxidation processes (Figure 2). The first is irreversible and attributed to the oxidation of the thiol, forming disulfide bonds, sulfinate, or sulfonate, depending on

3. RESULTS AND DISCUSSION 3-1. Photophysical and Electrochemical Properties of (1−4). Complexes (1) and (2) have already been reported in the literature, and their main characteristics are summarized in Table 1 and Table 2 with corresponding references. Absorption spectra of (3) and (4) together with the emission spectrum of (4) at 77 K in a rigid butyronitrile matrix are given in Figure 1. The broad absorption band of (3) and (4) in the visible region is attributed to a metal to ligand charge transfer (MLCT) 20433



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They are close to those obtained for in situ assembled metal complexes on a free tpy-modified surface (Γ = 6 × 10−10 mol cm−2).32 It suggests a vertical edge-on orientation for (3) or (4) with one thiol attached to the surface and the second thiol projected toward the solution. The linear relationship between the current peak intensity and the potential scan rate is in agreement with an adsorbed monolayer on the electrode (Figure 3 inset). As previously reported, immobilization of a second layer on a SAM-modified electrode can be obtained by an electroinduced disulfide coupling reaction.17,33 Scheme 2 is an ideal representation to summarize the different steps leading to the formation of a bilayer assembly of (3) and (4) on gold [(4)−(3)/Au]. First, (3) is self-assembled on gold, and the modified electrode [(3)/Au] is then abundantly rinsed. Complex (4) (5 × 10−4 M) is added to the electrolyte, and iterative cyclic voltammetry from 0 to 1 V with [(3)/Au] as the working electrode induces the covalent attachment of (4) to the SAM by oxidative coupling between the pendant thiol of the SAM of (3) and the free thiol of (4) in solution (Figure 4B). Three

Figure 2. Cyclic voltammogram in deoxygenated CH3CN + 0.1 M TBAPF6 of (4) (5 × 10−4 M). Vitreous carbon working electrode (diameter = 2 mm), ν = 100 mV s−1.

the concentration of water or dioxygen in the medium (in our experimental conditions, the formation of the two latter compounds is however largely avoided). The second process is reversible and attributed to the oxidation of the metallic center. For (3) the two oxidation processes are superimposed. Comparison of the two potential values of the metal centered oxidation in (3) and (4) proves that Fe2+ can act as an efficient electron donor toward the Ru3+ species (ΔG = −0.170 eV). Monolayers of (3) [(3)/Au] or (4) [(4)/Au] can easily be obtained by a self-assembly method after immersion of a gold electrode in a solution of complex (10−4 M) in deoxygenated CH3CN for 15−20 h. Figure 3 shows the resulting cyclic

Figure 4. (A) Cyclic voltammogram of a SAM of (3) on gold (Γ = 4 × 10−10 mol cm−2) in CH3CN + 0.1 M TBAPF6. (B) Cyclic voltammogram of a SAM of (3) on gold in a solution of (4) (5 × 10−4 M) in CH3CN + 0.1 M TBAPF6. (C) Cyclic voltammogram of the resulting modified electrode after transfer to pure CH3CN + 0.1 M TBAPF6. ν = 100 mV/s. (Schematic representation: black circle = Fe2+, gray circle = Ru2+, both in thiolated bisterpyridine core).

Figure 3. Cyclic voltammogram in CH3CN + 0.1 M TBAPF6 of a SAM of (4) on gold (Γ = 6 × 10−10 mol cm−2), ν = 50 mV/s. Inset: linear relation between the oxidation peak current for the Ru2+/3+ system and the scan rate.

voltammogram of [(4)/Au] obtained in pure electrolyte solution. The metal-centered oxidation and the two ligandcentered reductions appear at potentials similar to those of the free complexes in solution. The surface coverage Γ of (3) and (4) is estimated respectively at 4 ± 1 × 10−10 and 6 ± 1 × 10−10 mol cm−2 by integration of the Faradic current under the oxidation peak of the metal center (for this calculation the double-layer charge was not included).17 These values correspond to an average of five different modified electrodes.

repeated cycles induce the deposition of (4) in a quantity similar to (3) on the electrode. Figure 4C shows the anodic behavior of the resulting electrode after transfer into a pure electrolyte solution. The two redox systems ascribed to the metal centers Fe2+/3+ at 0.76 V and Ru2+/3+ at 0.93 V are well separated, and their relative intensities are in accordance with a 1:1 ratio. This immobilization technique has already been used for the generation of films of [Ru(tpy)2]2+ up to 30 nm height

Scheme 2. Proposed Oxidation Pathway for a SAM of (3) in a Solution of (4), Resulting in the Formation of a Disulfide Bond between the SAM and Thiol Groups of Molecules in Solution

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on gold.17 During the iterative cyclic voltammetry, the interlayer S−S linking reaction has been proved to be the predominant process toward the intralayer S−S bond formation between neighbors.34,35 The resulting electrode (denoted [(4)−(3)/Au]) corresponds mainly to the formation of dyads on gold. However, the surface can be inhomogeneous at the molecular level with a mixture of [(3)/Au] and [(4)− (4)−(3)/Au]. Previous experiments showed that the successive layers do not grow vertically to the surface but are tilted on the SAM.17 Comparison of a CV of [(4)−(3)/Au] (Figure 4C) with a CV of a SAM of (3) (Figure 4A) shows that the current intensity of the Fe(III)/Fe(II) system is of the same order of magnitude for both electrodes, suggesting that the desorption of (3) is limited during the bilayer construction. 3-2. Photoinduced Oxidation Process in Solution. Before studying the photoinduced electron transfer process occurring within the [(4)/Au] and [(4)−(3)/Au] modified electrodes, the photochemical reactivity of a mixture of related (1) and (2) parent complexes has been studied in CH3CN in the presence of the 4-bromobenzenediazonium cation (ArN2+) as a sacrificial electron acceptor. These complexes without thiol ends have been selected to avoid any possible photoinduced coupling reaction during the oxidation process. ArN2+ is wellknown as a very efficient quencher of the excited state of Ru(II) polypyridine complexes in solution, notably [Ru(bpy)3]2+* derivatives. Under continuous irradiation, a mixture of [Ru(bpy)3]2+ + ArN2+ in acetonitrile leads to the formation of [Ru(bpy)3]3+ with a quantum yield of 0.34 and a rate constant of 3.5 × 109 M−1 s−1.22,36,37 In addition, a mixture of [Ru(bpy)3]2+ + (1) with ArN2+ under irradiation has already proved the efficient generation of (1+) via the formation of the transient [Ru(bpy)3]3+ species.20 The continuous photolysis of an equimolar solution of (1) and (2) in the presence of a large excess of ArN2+ induced also the oxidation of the iron complex. Figure 5 shows the regular decrease of the MLCT band of (1) at 566 nm with the concomitant emergence of a shoulder

around 390 nm and a band centered at 670 nm typical of (1+).20 The quantitative transformation occurs within 90 min. As shown in the inset of Figure 5, light and ArN2+ are both necessary to induce the oxidation of (1), whereas irradiation without (2) leads slowly to (1+) (Figure SI1, Supporting Information). Prolonged irradiation of (1) and (2) with ArN2+ after the quantitative formation of (1+) does not show the formation of (2+). In addition, a mixture of (2) with ArN2+ in CH3CN is almost stable under irradiation in the same condition (only a small increase of absorbance in the UV is observed attributed to the decomposition of ArN2+) (Figure SI2, Supporting Information). These experiments prove that (2) is not oxidized under these conditions. In the mixture of (1) + (2) + ArN2+ under irradiation, the generation of (1+) would occur then from an electron transfer reaction between (1*) and ArN2+ instead of an oxidation of (1) by a transient Ru(III) species as it was previously reported when the [Ru(bpy)3]2+ is used as a photosensitizer. The photooxidation of (1) is however favored in the presence of (2) as the probable consequence of an energy transfer process between both metallic complexes.38−41 Complex (1) would serve as a trap for the excitation energy of (2*). Equations 2−5 summarize the successive reactions leading under visible irradiation to the formation of an FeIII center. hv

Ru II → Ru II *

(2)

Ru II * + Fe II → Fe II * + Ru II

(3)

hv

Fe II → Fe II *

(4)

Fe II * + ArN+2 ⎯⎯⎯⎯⎯⎯⎯→ Fe III + ArH + N2

(5)

CH3CN

2+

To confirm the crucial role played by the [Fe(ttpy)2] group as an active actor in its photoinduced oxidation, we substituted it by tri(p-bromophenyl)amine (tBrTPA) and irradiated a mixture of (2) + ArN2+ + tBrTPA. The amine tBrTPA is a nonabsorbing visible light reversible electron donor, exhibiting an oxidation potential (0.79 V42) closed to that of (1). It is then a good substitute to (1) to study photoinduced electron transfer interaction in a related system without any energy transfer possibility. Moreover the radical cation (tBrTPA•+) is stable and can easily be detected by UV−visible absorption due to its strong absorbance centered at 702 nm.42 Irradiation of the mixture is stable and irradiation does not evidence the formation of tBrTPA•+ by UV−visible absorption (Figure SI3, Supporting Information). In contrast using [Ru(bpy)3]2+ instead of (2), the evolution of the UV−visible absorption spectra shows the fast and efficient oxidation of tBrTPA•+ by the transient [Ru(bpy)3]3+ species (Figure SI4, Supporting Information). The results proved that electron transfer reaction from (2*) is unlikely. In the mixture (1) + (2) + ArN2+ the oxidation of (1) under irradiation is due to an electron transfer reaction between (1*) and ArN2+. Fe(II) polypyridinyl complexes are usually disregarded as redox photosensitizers owing to their nonemissive and shortlived MC excited state which limits their implication in bimolecular photoredox reaction in solution. For instance, transient absorption experiments after excitation in the 1MLCT band of [Fe(bpy)3]2+ show that the ground state recovery occurs with a lifetime of 0.98 ns, while for terpyridine-based species, the lifetime can be slightly longer.43 Nanosecond timeresolved absorption spectra recorded for (1) upon irradiation at

Figure 5. Temporal UV−vis spectral changes of a mixture of [Ru(ttpy)2]2+ + [Fe(ttpy)2]2+ + ArN2+ in CH3CN under visible irradiation. Inset: Evolution of the absorbance at 566 nm of a mixture of [Ru(ttpy)2]2+ + [Fe(ttpy)2]2+ + ArN2+ in dark (curve a); [Ru(ttpy)2]2+ + [Fe(ttpy)2]2+ under visible irradiation (curve b); [Fe(ttpy)2]2+ + ArN2+ under visible irradiation (curve c); and [Ru(ttpy)2]2+ + [Fe(ttpy)2]2+ + ArN2+ under visible irradiation (curve d). 20435



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bond between the electrode and the aryl group,51 is suppressed at a bias above 0.4 V. To avoid this phenomenon all the photocurrent experiments with modified gold electrode are conducted at Eapp. = 0.4 V. At that potential only a negligible photocurrent of a few tens of nA cm−2 is observed with a naked electrode. Photocurrent generation of [(4)/Au] is shown in Figure 7. A stable cathodic photocurrent of 250 nA cm−2 from

355 nm show a bleaching between 450 and 650 nm due to the disappearance of the MLCT band of (1) (Figure 6). On that

Figure 6. Nanosecond transient absorption spectra of a deaerated solution of (1) (5 × 10−5 M) in CH3CN recorded after laser excitation at 355 nm, time increment 5 ns. Inset: decay time profil at λ = 570 nm.

Figure 7. Cathodic photocurrent response of [(4)/Au] (Γ = 6 × 10−10 mol cm−2) (dashed line) and [(4)−(3)/Au] (Γ = 4 × 10−10 mol cm−2) (full line) in the presence of 15 mM ArN2+ in CH3CN + 0.1 M TBAPF6.

time scale we were not able to detect positive absorption characteristics that can be assigned to the reduced form of the terpyridine ligand. We presume that the MLCT excited state was rapidly deactivated by a low-lying ligand field state. The ground state completely regenerates in 35 ns after the laser pulse, and excited state lifetime is estimated to 9.5 ns at 298 K. This excited state lifetime appears then long enough to allow a photoinduced electron transfer reaction with ArN2+. To confirm the hypothesis, similar experiments have been conducted with a mixture of (1) (5 × 10−5 M) and ArN2+ (1.5 × 10−4 M) under argon in CH3CN (Figure SI5, Supporting Information). The excited state litetime of (1) in this case is reduced to 8.5 ns. These values fall near the limit of our nanosecond pump−probe setup, and for that reason a Stern−Volmer experiment was not undertaken. Few examples of photoinduced electron transfer reactions induced by Fe(II) terpyridine-based complexes are described in the literature. Among them the Fe(II) core is mainly grafted on surface or immobilized on clay.44−46 Examples of photooxidation in water or organic media are however rare.47,48 The photooxidation quantum yield of (1) by ArN2+ using (2) as an external photosensitizer was estimated to ϕ ∼ 10−3. For comparison the photooxidation quantum yield of a [Fe(bpy)3]2+ core by the [Ru(bpy)3]2+/ArN2+ system is equal to 0.22.49 In this latter case, as recalled earlier in the text, the photooxidation proceeds via the generation of the transient [Ru(bpy)3]3+ which oxidizes in turn the [Fe(bpy)3]2+ species. 3-3. Photocurrent Generation in [(4)/Au] and [(4)−(3)/ Au]. Cathodic photocurrent generation was performed under argon in CH3CN + 0.1 M TBAPF6, using ArN2+ as a sacrificial oxidant. Utilizing this quencher, appreciable photocurrent has already been obtained from a photocathode using [Ru(bpy)3]3+-functionalized polypyrrole films. It was observed that the intensity of the photocurrent drastically decreases with the film thickness,50 and in this context the utilization of a monolayer as photoelectrode could be an advantage. ArN2+ exhibits an irreversible reduction peak at Epc = −0.4 V vs Ag/ AgNO3 10−2 M. A control experiment carried out with a clean gold electrode immersed in a solution of ArN2+ (15 mM) in CH3CN + 0.1 M TBAPF6 showed that below 0.4 V a cathodic current is observed due to the direct reduction of ArN2+ at the electrode. This reduction, which leads to the formation of a

the gold electrode to the electrolyte appears immediately upon visible irradiation. The generation of a photocurrent is proof of an effective photoinduced electron transfer reaction at the junction between the electrode and ArN2+ in solution. The photocurrent drops immediately when the illumination is switched off. The magnitude of the photocurrent is of the same order of SAM obtained with porphyrin dyes52,53 and shows that (4) can be used as an efficient redox photosensitizer when immobilized into a monolayer on gold. The photoreactivity of the [(Ru(tpy)2)]2+ core is modified between the solution and the self-assembled structure. Upon irradiation, the UV−visible spectrum of a solution of (2) in the presence of ArN2+ is stable and does not evidence the photo-oxidation of the complex. When the complex is grafted on gold, irradiation of the electrode with ArN2+ in solution generates an efficient electron transfer reaction. Based on the Rehm−Weller equation, the redox potentials of the excited state of (4) are estimated at −1.01 and +0.4 V for the oxidation and reduction process, respectively.54−56 Taking into account the potential applied to the electrode and the Epc value of the sacrificial electron acceptor, the photocurrent generation presumably proceeds by the mechanism depicted in Scheme 3, where the first electron transfer reaction occurs between the adsorbed (4*) layer and ArN2+ in the solution to generate (4+). The latter species is then reduced by the bias. The light to current power conversion of the SAM estimated to 1.8 × 10−4 % remains low. With the aim to optimize the electron transfer reaction between the photosensitizer grafted on electrode and the final electron acceptor in the solution, (3) has been incorporated into the assembly as an electron relay since its oxidation occurs at an intermediate potential between the gold electrode at 0.4 V and the reduction potential of the (4+) transient species (Table 2). Irradiation of [(4)−(3)/Au] leads to a photocurrent (Figure 7) that appears perfectly stable during an alternating illumination/dark cycle. However, incorporation of (3) into the assembly decreases the magnitude of the photocurrent from 250 nA cm−2 for [(4)/Au] to 100 nA cm−2 for [(4)−(3)/Au] (Figure 5). It shows that despite the energy transfer process 20436



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4. CONCLUDING REMARKS Our studies show first in solution that [Fe(ttpy)2]2+ (ttpy = 4′(4-methylphenyl)-2,2′:6′,2″-terpyridine) can be engaged in a bimolecular photoinduced electron transfer reaction with ArN2+ as a sacrificial electron acceptor. The kinetics and efficiency of the reaction are slow but can be markedly improved in the presence of [Ru(ttpy)2]2+. In that case, under irradiation, the [Ru(ttpy)2]2+ acts as an antenna to transfer the excitation energy to the [Fe(ttpy)2]2+. Second, if the [Ru(ttpy)2]2+ does not interact by electron transfer with ArN2+ in solution as a consequence of its short-lived excited state, the complex is able to photoinduce an electron transfer reaction when adsorbed on gold. This observation allows the consideration of many other systems with nonemissive and short-lived excited states for the study of photoinduced charge transfer in organized layers on conducting electrodes, as it has already been proved on TiO2 semiconducting materials.44 Finally, in heterobimetallic assembly of Fe(II) and Ru(II) bisterpyridine cores on gold, Fe(II) acts as an electron relay between the electrode and the Ru(III) transient species generated by irradiation of the assembly in the presence of ArN2+. This new approach to elaborate well-organized heterometallic assemblies will potentially allow the easy and rapid construction of photoactivable inorganic triads for further applications. In addition, the thiol extremity of the structure can also be used to anchor gold nanoparticles on the top of the assembly.

Scheme 3. Schematic Diagram Showing the Photoinduced Electron Transfer Process between a SAM of (4) on a Gold Surface and ArN2+ in Solution

between the Ru(II)* and Fe(II) centers within the [(4)−(3)/ Au] assembly the electron transfer reaction between Ru(II)* and ArN2+ remains largely promoted. The resulting Ru(III) species oxidizes the Fe(II) center, which is in a second step reduced by the bias. The mechanism of photocurrent generation is illustrated in Scheme 4. The energy transfer process is however not totally short circuited and contributes to reduce the magnitude of the photocurrent. It has already been shown in solution that this energy transfer process depends markedly on the nature of the bridging ligand between the [Ru(tpy)2]2+ and the [Fe(tpy)2]2+ subunits. In particular, in the case of a dyad built with a saturated linker, irradiation of the Ru(II) MLCT band leads to the formation of the triplet state of the [Fe(tpy)2]2+ subunit in less than quantitative yield.57 In the case of the (4)−(3) dyad grafted on an electrode, we presume that ArN2+ reacts preferentially with (4*) instead of (3*) since (4) is directly in contact with the solution. The oxidation of (3) is then mainly due to the generation of the (4+) species, in contrast to the mechanism of photo-oxidation in solution, where the process is mainly a consequence of the interaction between the Fe(II)* center and ArN2+. The formation of an ordered dyad [(4)−(3)/Au] on gold has then induced the vectorial electron transfer process from the electrode to the solution along the molecular assembly. The photoinduced process induces the uphill transport of electron of 0.8 eV.

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ASSOCIATED CONTENT

S Supporting Information *

Temporal UV−vis spectral changes under visible irradiation of a mixture of (1) + ArN2+; (2) + ArN2+; (2) + ArN2+ + tBrTPA; and [Ru(bpy)3]2+ + ArN2+ + tBrTPA. Nanosecond transient absorption spectra of (1) + ArN2+ in CH3CN. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (+33) 476514137. E-mail: jerome.chauvin@ujf-grenoble. fr. Notes

The authors declare no competing financial interest.

Scheme 4. Schematic Diagram Showing the Photoinduced Electron Transfer in a (4)−(3) Dyad on Gold with ArN2+ in Solution

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ACKNOWLEDGMENTS S.L. thanks the University Joseph Fourier of Grenoble for a PhD grant. F.L. thanks the CNRS for an “ATIP jeunes chercheurs” funding. The chemistry platform NanoBio campus in Grenoble is acknowledged for luminescence lifetime measurement facilities.

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Photoinduced Energy vs Electron Transfer Between Fe(II) and Ru(II

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