Ruthenium Polypyridine Complexes Bearing Pyrroles and Ï-Extended

Review pubs.acs.org/Organometallics

Ruthenium Polypyridine Complexes Bearing Pyrroles and π‑Extended Analogues. Synthesis, Spectroelectronic, Electrochemical, and Photovoltaic Properties Marc Beley*,†,‡ and Philippe C. Gros*,†,‡ †

Université de Lorraine, SRSMC UMR7565, Photosens, F-54506 Vandoeuvre-Lès-Nancy, France CNRS, SRSMC UMR7565, Photosens, F-54506 Vandoeuvre-Lès-Nancy, France

‡

ABSTRACT: This personal account gives an overview of our research efforts dedicated to the chemistry of ruthenium polypyridine complexes, with a special focus on the effect of pyrrole substituents and their π-extended analogues. The synthesis of the complexes and their characterization will be presented, as well as their photovoltaic applications when applicable.

1. INTRODUCTION The conversion of solar energy into electrical or chemical energy is expected to play a key role in the replacement of fossil fuels. However, the high cost and complex production of silicon-based solar panels impose a restriction on their large-scale use. In this context dyesensitized solar cells (DSSCs) appear as robust, efficient, and cheap photovoltaic devices. Since the first works by Grätzel and O’Regan,1 this topic has been extensively investigated worldwide.2 The working principle of a DSSC is depicted in Figure 1. A transparent conducting oxide (TCO) is used as the substrate on glass for the photoelectrode. Nanocrystalline TiO2 is generally deposited as a semiconductor on the electrode. This wide band gap (3.2 eV) semiconductor has a good stability under irradiation in solution and a large surface area. However, sensitization of TiO2 is necessary to make it useful as a photoanode absorbing visible light of the solar spectrum. The photosensitizer attached at the semiconductor surface has to harvest the sunlight on a large domain (350−800 nm). The photoanode is in contact with a hole transport material (HTM), usually a redox electrolyte (D/D+) in an organic solvent. The photoanode is connected to a counter electrode, a thin layer of metal (Pt, Au, Al) deposited on another TCO glass. The different steps of the operating cycle of a DSSC are given as follows: (1) Excitation of S grafted on the electrode surface produces the excited state S* that injects an electron into the conduction band of TiO2. (2) The injected electron diffuses through the semiconductor and reaches the metal counter electrode trough the external circuit. (3) The oxidized photosensitizer S+ is reduced by the electron donor D, thus regenerating the ground state S and oxidizing D into D+. © XXXX American Chemical Society

Figure 1. Operating principle of a DSSC.

(4) D+ diffuses to the metallic cathode and is in turn reduced to D. Special Issue: Organometallic Electrochemistry Received: March 3, 2014

A

dx.doi.org/10.1021/om5002183 | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Review

Scheme 1. Preparation of Pyridine Precursorsa

a

Reagents and conditions: (i) BuLi−LiDMAE, toluene, −78 °C or (a) 0 °C, 1 h; (ii) C2Cl6, toluene, −78 °C; (iii) ClSnBu3, toluene, −78 °C.

moieties such as pyrrole, pyrrolidine, and their analogues to tune the photochemical and electrochemical properties of this chromophore family. This personal account will present our research efforts in the preparation of such complexes and their characterization and application as sensitizers in dye-sensitized solar cells.

According to this simplified mechanism, electric power can be generated without any irreversible chemical process. However, the excited state S* can decay by several recombination routes, such as back transfer from TiO2 photoinjected electrons to S+ and the recombination between the injected electron and oxidized redox species D+. These recombination mechanisms can be minimized by adequate association of dye/ mediator couples, cell design, and tuning of the nanocrystalline structure of the semiconductor. The free energy of the electron injection (ΔGINJ) is obtained using the equation ΔGINJ = e[ECB − ES*/S] (ECB = conduction band potential of TiO2 and ES*/S = excited state potential of the photosensitizer). The photosensitizer regeneration free energy (ΔGREG) is obtained using ΔGREG = e[ES/S+ − ED/D+], where ES/S+ and ED/D+ are the oxidation potentials of S and the mediator D, respectively. The maximum open circuit VOCM is determined by the difference of the Fermi level of each electrode according to VOCM = ED/D+ − EF, where EF is the Fermi level in the semiconductor. The parameters characterizing the overall performance of the solar cell are the cell efficiency (η) and the fill factor (FF), expressed as FF = (VmaxJmax)/(VOCMJSC) and η (%) = (VOCMJSC(FF))/PIN × 100, where JSC is the short-circuit current density (mA cm−2) and PIN the incident light power. Jmax and Vmax are the current and voltage values, respectively, where the maximum power output is given in the J−V curve. The incident photon to current efficiency (IPCE) spectrum is calculated using IPCE = [1240 × J (μA cm−2)]/[λ (nm) × P (Wm−2)], where J is the photocurrent density generated by monochromatic light with wavelength λ and intensity P. A number of alternative photosensitizers, redox mediators, and electrolyte systems have been explored.3 Generally, different families of dyes are involved in light collection, such as transition-metal complexes, in particular Ru polypyridyl compounds,4 organic dyes,5 and porphyrins and phthalocyanines.6 Carboxylic and phosphonic acids are the most successful functional groups to achieve the attachment of the dye to the semiconductor’s surface. Due to their withdrawing properties, these functionalities ensure efficient bonding of the sensitizer on the semiconductor surface and promote electronic coupling between the donor levels of the excited chromophore and the acceptor levels in the conduction band. Among the different photosensitizers, the family of transition-metal complexes has provided a relatively high efficiency. In particular, for ruthenium polypyridine complexes the 2,2′-bipyridine and 2,2′:6′,2″ -terpyridine ligands can be modified with different

2. RUTHENIUM COMPLEXES BEARING PYRROLES AND PYRROLIDINES 2.1. Synthesis of Ligands. In our first works,7,8 we focused on the preparation of ligands in which pyrrole or pyrrolidine was bound by its nitrogen at the 4-position of the pyridine ring in order to optimize the electron-donating effect to the ligand by conjugation of the nitrogen doublet. The strategy was to use 4-(1H-1-pyrrolyl)pyridine (1) or 4-pyrrolidinopyridine (2) as the starting material, which was subjected to selective metalation using the BuLi−LiDMAE (LiDMAE = Me2N(CH2)2OLi) reagent developed in our group.9 This methodology allowed the preparation of the useful 2-monochloro, 2,6-dichloro, and 2-tributyltin precursors 3−8 (Scheme 1). In the pyrrole series, the precursors 3 and 5 were used for the preparation of bipyridines pyrrbpy and pyrr2bpy and terpyridine pyrrtpy under palladium catalysis (Scheme 2).7 Pyrrolidine-based ligands pyrrldbpy, pyrrld2bpy, pyrrldtpy, and pyrrld2tpy were obtained by metal-catalyzed coupling of precursors 4, 6, and 8. In this case the symmetrical ligand pyrrld2bpy was prepared by a cross-coupling reaction of 2 with 8 instead of nickel-mediated homocoupling that led to the reduction of the carbon−chlorine bond in 2 (Scheme 3).8 To further evaluate the effect of electron delocalization, a π-extended ligand has been prepared by inserting styryl chains between the bipyridine and the pyrrole or pyrrolidine using a Knoevenagel reaction between dimethylbipyridine and the appropriate aldehydes (Scheme 4).10 2.2. Homoleptic Complexes Bearing Pyrrole, Pyrrolidine, Styrylpyrrole, and Styrylpyrrolidine. In order to examine the effect of the ligands on ruthenium(II) properties, the corresponding homoleptic complexes have been prepared by reactions of ligands in the appropriate stoichiometries (2 and 3 equiv for terpyridine and bipyridine, respectively) with RuCl3· xH2O. The complexes have been obtained in excellent yields and very short times under microwave irradiation (Schemes 5 and 6).7,8,10,11 All complexes were isolated as their PF6 salts obtained from anion metathesis using KPF6. B


Organometallics

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Scheme 2. Preparation of Pyrrole-Based Ligandsa

wave, the pyrrole moiety was found to be oxidized irreversibly to a radical cation at 1.2 V/SCE for Ru(pyrrtpy), while for the other complexes this irreversible oxidation process coalesces clearly with that of Ru in the process. It is important to note the absence of pyrrolidine oxidation before those of RuII into RuIII. This behavior was firmly demonstrated by performing an exhaustive oxidation of Ru(pyrrld2bpy) at 0.5 V/SCE. After an exchange of 1.2 F/mol, the solution was blue-green (λabs(max) 763 nm), which is characteristic of a RuIII complex. After reduction at −0.2 V/SCE the electronic spectrum is in agreement with that of the starting RuII complex (λabs(max) 520 nm). This can be attributed to the significant electron-donating effect of the pyrrolidine group, which dramatically induces a decrease of the electronic density in this substituent on the ligand. The homoleptic complexes have two or three reversible or quasi reversible reduction processes localized on the ligand. For each compound the potential of the first process is collected in Table 1. The electron resides in the π-antibonding orbital which is the LUMO in the complex. The N-pyrrole moiety induced an increase of the reduction potential values in comparison with the parent complex due to partial conjugation of pyrrole π electrons with the bipyridine ligand. However, with the complexes bearing the N-pyrrolidine moiety the reduction is more difficult due to its donor effect. For some complexes no reduction was detected. This may be attributed to the formation of a nonconducting adsorption layer at the electrode surface. For Ru(stypyrr2bpy) and Ru(stypyrrld2bpy) complexes the cyclic voltammetry showed two oxidation waves. For Ru(stypyrr2bpy), in addition to the reversible RuII/RuIII wave at 1.13 V/SCE, the pyrrole was found to be oxidized irreversibly at 1.0 V/SCE. For Ru(stypyrrld2bpy) the reversible oxidation of RuII is easier (1.02 V/SCE). However, the pyrrolidine moiety was found to be oxidized irreversibly at 0.70 V/SCE. In the extended ligand stypyrrld2bpy the donor effect of the pyrrolidine on the metal is attenuated by the styryl spacer. This behavior

a

Reagents and conditions: (i) 2-(tributylstannyl)pyridine (1.1 equiv), [PdCl2(PPh3)2] (5 mol %), PPh3 (10 mol %), degassed xylene, reflux, 24 h; (ii) NiCl2 (1.1 equiv), PPh3 (4.4 equiv), Zn (1.1 equiv), degassed DMF, 50 °C, 1.5 h.

2.2.1. Characterization. The complexes have been characterized by UV−vis and electrochemistry (Table 1). The nature of the ligand around the RuII cation plays a crucial role in the electrochemical and photophysical behaviors of homoleptic complexes. In this study, we show clearly the destabilization of the HOMO metal orbital by modification of the 2,2′-bipyridine or of the 2,2′:6′,2″-terpyridine with electron-donor group such as N-pyrrole or N-pyrrolidine. All complexes displayed a reversible RuII to RuIII oxidation wave. When the N-heterocycle was directly bound to polypyridine ligands, the oxidation potentials were found at lower values, in comparison with the parent complexes [Ru(bpy)3]2+ and [Ru(tpy)2]2+. In the pyrrole series the oxidation potential decreased by about 0.15 V. On the other hand, in the pyrrolidine series the potential decreased dramatically by about 1.0 V when the oligopyridine ligand bore two pyrrolidine moieties. In addition to the reversible RuII/RuIII Scheme 3. Preparation of Pyrrolidine-Based Ligandsa

a

Reagents and conditions: (i) 2-PyrSnBu3 (1.1 equiv), PdCl2(PPh3) (5%), PPh3 (10%), xylene, Ar, reflux, 18 h; (ii) 8 (1.1 equiv), PdCl2(PPh3)2 (5%), PPh3 (10%), xylene, Ar. C


Organometallics

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Scheme 4. Synthesis of π-Extended Ligands

Scheme 5. Synthesis of Homoleptic Complexes from Bipyridine Ligands

Scheme 6. Synthesis of Homoleptic Complexes from Terpyridine Ligands

is completely different from that described above for Ru(pyrrld2bpy), where the pendant heterocycle is directly attached to the bipyridine. The electronic effect of the N-pyrrole and N-pyrrolidine moieties was also clearly evidenced by bands in the electronic spectra in the visible domain that are attributed to a singlet

multiplicity MLCT transition, in which electrons from metal d orbitals are promoted into the unfilled ligand π*. Electron-donor groups destabilize the HOMO (πt2g) metal orbital more than the LUMO (π*) ligand orbital. Consequently, both absorption and emission maxima are shifted to wavelengths longer than those of [Ru(bpy)3]2+and [Ru(tpy)2]2. Indeed, complexes bearing the D


Organometallics

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Table 1. Electronic and Photophysical Properties of Homoleptic Complexes L in RuLn2+ bpy pyrrbpy pyrr2bpy stypyrr2bpy pyrrldbpy pyrrld2bpy stypyrrld2bpy tpy pyrrtpy prrldtpy pyrrld2tpy

λabs(max) (nm)a

ε (103 M−1 cm−1)

λem(max) (nm)b

451 465 480 374 490 481 520 438 483 (sh) 474 490 501 493

13.8 8.6 22.2 115.0 52.0 13.5 13.2 120.0 108.7 10.4 38.5 15.0 10.8

615 630 650 689

0.059 0.022 0.044 0.016

691 710 725

0.021 noe

629 no

8000 M−1 cm−1). The molar extinction coefficient of the Ru(DTP2Me)(dcbpy)(NCS)2 complex is also higher in the MLCT region as a result of greater π delocalization. The complexes Ru(DTP1-Me)(dcbpy) and Ru(DTP1Hex)(dcbpy) showed two oxidation waves: the first one was for the RuIII/RuII couple, appearing at 0.93 V/SCE for both complexes. The inductive effects from the methyl and hexyl groups, respectively, are thus similar, in agreement with absorption spectra. This oxidation wave was reversible for Ru(DTP1-Me)(dcbpy) and irreversible for Ru(DTP1-Hex)(dcbpy). At higher potentials, a second oxidation process appears (1.00 V/SCE). Such an irreversible wave is attributed to the formation of the thiophene radical cation. Tris-heteroleptic complexes Ru(DTP1-Hex)(dcbpy)(NCS)2 and Ru(DTP2Me)(dcbpy)(NCS)2 also showed two oxidation waves: the first wave, which is irreversible, corresponds to the RuII/RuIII couple and the second wave, also irreversible, is attributable to the thiophene radical cation. The oxidation potential of the

Figure 12. Absorption spectra of DTP-based homoleptic complexes in acetonitrile.

visible region were found to be red-shifted and have lower intensity in the UV region. The RuDTP1 series was emitting in fluid solution. When the ligand was excited, two distinct emission bands were observed, one centered on the 480−550 nm region and one centered at around 680 nm originating from the typical 3MLCT phosphorescence. Surprisingly, at room temperature, for the RuDTP2 series no MLCT type emission was observed, the only emission being that of the LC type in the 510−580 nm region, as confirmed by energy similar to that in the RuDTP1 series. 3.2. Heteroleptic Complexes. 3.2.1. Synthesis. Bisheteroleptic and tris-heteroleptic complexes were prepared according to Scheme 11.26 3.2.2. Characterization. The characterization data are gathered in Table 5. Spectra of the bis-heteroleptic complexes Ru(DTP1-Me)(dcbpy) and Ru(DTP1-Hex)(dcbpy) featured L


Organometallics

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Scheme 11. Synthesis of DTP-Based Bis-Heteroleptic and Tris-Heteroleptic Complexes

Table 5. Electronic and Photophysical Properties of DTP-Based Bis-Heteroleptic Complexes dye Ru(DTP1-Me)(dcbpy)

Ru(DTP1-Hex)(dcbpy)

Ru(DTP1-Hex)(dcbpy)(NCS)2

Ru(DTP2-Me)(dcbpy)(NCS)2

λabs(max) (nm)a (ε (M−1cm−1))

λem(max) (nm) (λexcit (nm))b

461 (38900) 400 (49900) 311 (109000) 479 (38200) 406 (49100) 308 (96500) 534 (14700) 409 (30300) 314 (67000) 550 (19200) 468 (29800) 320 (23900)

674 (500) 447 (320)

66

0.93 (rev)

1.03

−1.26

−0.85

1.88

677 (500) 446 (310)

90

0.93 (irr)

1.00

−1.25

−0.86

1.87

783 (530) 420 (320)

40

0.80 (irr)

1.00

n.d.h

−0.99

1.79

435 (320)g

60

0.70 (irr)

0.85

n.d.h

τ (ns) Eox(RuII/RuIII)c Epa(L/L+)d Epc(L−/L)

E*ox(RuII*/ RuIII)e E0−0 (eV)f

a Measured in CH3CN/DMSO (4/1) at 25 °C. bPhotomultiplier-corrected emission maxima for the complexes in CH3CN/DMSO (4/1): A < 0.05 in the absence of O2. cFirst oxidation potential, standardized with Fc/Fc+ as the internal standard and converted into the SCE scale by adding 0.47 V (E1/2Fc/Fc+). Recorded in DMF using Bu4N+PF6− as supporting electrolyte at 100 mV s−1. dFirst reduction potential. eFirst oxidation potential at the excited state. fEnergy gap between vibrational levels (n = 0) at the ground and excited stated for the complexes in acidic form. gNo emission was detected upon excitation at 550 nm. hNot detected.

RuIII/RuII couple was found at lower values, i.e. at 0.70 V/SCE for Ru(DTP2-Me)(dcbpy)(NCS)2, as a consequence of the higher degree of conjugation in the corresponding ligand. In comparison with related bis-heteroleptic complexes, the RuII/RuIII redox couple in tris-heteroleptic complexes is shifted to slightly lower potentials. The RuII/RuIII oxidation potentials lie in the 0.7−0.93 V range. The regeneration of the dye ground state should be thermodynamically allowed for all of the dyes using iodine- or cobalt-based redox mediators. In addition, the calculation of the excited-state oxidation potential (E*1/2,ox(RuII*/RuIII)) for Ru(DTP1-Me)(dcbpy), Ru(DTP1-Hex)(dcbpy), and Ru(DTP1-Hex)(dcbpy)(NCS)2 (Table 5) indicated that their

excited states exhibit quite sufficient reducing abilities to allow the electron injection into the conduction band of TiO2 (Figure 14).23 3.2.3. Photovoltaic Properties. The sensitization by the dyes after chemisorption onto TiO2 films was monitored by UV−vis spectroscopy (Figure 15). A wide absorption of the visible part of the spectrum after adsorption is the sign of an efficient sensitization of the photoanode by all dyes. A wider spectral window was obtained with Ru(DTP2-Me)(dcbpy)(NCS)2, for which a high level of absorbance was observed until 600 nm. DSSCs were assembled, and mediators made of 1-propyl-3-methylimidazolium iodide (PMII, 0.6 M), LiI (0.1 M), and I2 (0.2 M) in methoxypropionitrile were used (I−/I3− couple). J/V curves obtained under the standard AM 1.5 G irradiation (100 mW.cm−2) and the action spectra (IPCE) are given in Figure 16. M


Organometallics

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Figure 15. Overlay absorption spectra of TiO2 photoanodes modified with bis- and tris-heteroleptic complexes.

Figure 13. Absorption spectra of bis- and tris-heteroleptic DTP-based complexes in acetonitrile/DMSO (4/1).

Table 6. NTO Isodensity Surfaces in Ru(DTP2-Me)(dcbpy)(NCS)2

Figure 14. Energy diagram showing excited- and ground-state oxidation potentials of (a) Ru(DTP1-Me)(dcbpy), (b) Ru(DTP1-Hex)(dcbpy), and (c) Ru(DTP1-Hex)(dcbpy)(NCS)2.

The best power conversion efficiency was obtained with Ru(DTP 2 -Me)(dcbpy)(NCS) 2 , with a photocurrent of 5.7 mA/cm2, a Voc value of 0.41 V, and an overall efficiency of 1.3%. Despite high ε values, efficient harvesting, and favorable thermodynamic parameters regarding injection and regeneration, the new dyes displayed unexpectedly poor performances. The IPCE spectra and the J/V curves (Figure 16) were in agreement. Ru(DTP2-Me)(dcbpy)(NCS)2 exhibited the best IPCE values (near 45%), maintained in the 450−600 nm range and slowly decreasing until 700 nm. Such poor performances are due to the limitation of charge injection efficiency, as evidenced by NTO calculations.28 oNTO can be seen as the orbital from which an electron is removed during transition, while vNTO is the orbital in which an electron is placed in the excited state. Thus, for a given transition, the location of vNTO on the dcbpy ligand will condition the injection process. The results obtained for Ru(DTP2-Me)(dcbpy)(NCS)2 are given in Table 6. Transitions at 638 and 656 nm showed appropriate location of holes and transfer of the excited state into the carboxylic groups, but their weight was weak and their role was negligible in the injection process. In contrast, for transitions at 516 and 594 nm with large weight, the created hole was found strongly delocalized

into the DTP ligand, as evidenced by oNTOs. While it was useful to prevent recombination of the oxidized Ru center with photoinjected electrons by maintaining the hole far from the semiconductor surface, the excited electron unfortunately showed a marked tendency to remain localized also on the DTP ligand, thus preventing efficient injection.

4. ELECTROPOLYMERIZATION AND SOLID-STATE DSSC Drawn by our interest in solid-state solar cell development, we performed the electropolymerization of Ru(stypyrr2bpy) to obtain photosensitive metallopolymers.29 The film was N


Organometallics

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Figure 16. (left) J/V curves and (right) IPCE spectra for bis- and tris-heteroleptic complexes.

Figure 17. Repeated (20 scans) cyclic voltammograms (100 mV/s) of Ru(stypyrr2bpy) (0.2 mM) in CH3CN/0.1 M LiClO4 solution: (A) on a platinum electrode (10 mm wire); (B) on an ITO glass slide. Insets: corresponding cyclic voltammograms (100 mV/s) of polymer films in monomerfree CH3CN/0.1 M TBAP solution after 20 cycles.

Figure 18. (left) UV−vis absorption spectra of monomer (CH3CN solution) and film on an ITO glass slide (solid state). (right) Emission spectra of monomer (CH3CN solution, λexcit 490 nm) and film on an ITO glass slide (solid state, λexcit 475 nm).

of ΔEp characterizes a redox couple coated at the surface of the electrode. Such an increase of the oxidation potential in the film revealed a modification of the π-extended system. The coated ITO glass electrode was also characterized by solid-state UV−vis spectroscopy and the spectra compared with that of the parent complex in solution (Figure 18). The MLCT band was notably blue-shifted in comparison to the monomer. This important hypsochromic effect came with a decrease of the absorption in the ILCT region. The blue shift was also observed in the emission spectrum, the polymer being emissive at 670 nm (λexcit 475 nm) instead of 690 nm for the monomer (Figure 18).

generated on platinum and indium−tin oxide glass slides by repeated scan oxidation of Ru(stypyrr2bpy) in acetonitrile (0.1 M LiClO4 as supporting electrolyte). The repeated scans at 100 mV s−1 produced a fast intensity increase from 0 to 1.6 V (Figure 17). After several washings, the coated platinum electrode was characterized by cyclic voltammetry in complex-free solution. By scanning the potential of the modified electrode from 0 to 1.6 V, we observed a single reversible oxidation RuII/RuIII wave. E1/2,ox(RuII/RuIII) was found at 1.32 V/SCE with ΔEp = 20 mV instead of 1.13 V for the monomer (see Table 1). The low value O


Organometallics

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(65%) despite the absence of NCS ligands. This new family of dyes is thus a promising alternative to tris-heteroleptic complexes bearing NCS ligands with known instability. Bipyridine-bearing dithienylpyrrole moieties (DTP) have been used in tris-heteroleptic ruthenium complexes of the Ru(L)dcbpy(NCS)2 type that have been employed as TiO2 photosensitizers in DSSCs. Despite their intense absorption and wide aborption window, these dyes showed limited photovoltaic performance due to localization of the photoexcited electron on the DTP chromophoric ligand, as clearly demonstrated by transient spectroscopy. Taking into account the excellent lightharvesting ability of these DTP-based compounds, works are in progress to make them applicable in DSSCs. The electropolymerization of ruthenium complexes with pyrrolostyryl bipyridine ligands led to metallopolymers on Pt and ITO glass slides by repeated scans. The ruthenium-based ITO-coated electrodes displayed wide absorption domains, and the films were found to be stable under various conditions, making them promising for further use in light-harvesting and conversion devices. Our results show clearly that improvement in DSSC performance depends on developments in dye design in close relation with the optimization of the mediator or hole transport material.

Figure 19. Schematized principle of the light-initiated electropolymerization.

The changes in the electronic spectra, especially blue shifts of the MLCT transition band (absorption and emission), signaled a deep change in the π-extended system upon polymerization. The formation of saturated chains between the metal centers via polymerization of the vinyl linkages was probably responsible for this conjugation cut.30 Indeed, the red shift of both the absorption and emission maxima should be expected as reported for ruthenium complexes bound by a π-conjugated system.31 The oxidative polymerization of vinyl links was probably enhanced by the electron-donor effect of pyrrole. The Ru(pyrrbpy)(dcbpy) complex was used as a dye to examine the concept of PEDOP-based DSSC.32 The principle was to deposit the PEDOP-conducting polymer on a sensitized TiO2 photoanode via a photoassisted electropolymerization33 initiated by the dye in its excited form (Figure 19). While the obtained photocurrents and efficiency were weak, the communication between the dye and PEDOP was clearly established by repeating the experiment with the dye [Ru(dnbpy)(dcpby)]2+ (dnbpy = 4,4′-dinonyl-2,2′-bipyridine), lacking the pyrrole groups.34 The cells based on PEDOP/ Ru(pyrrbpy)(dcbpy) generated, under the same conditions, nearly double the photocurrent delivered by the analogous PEDOP ([Ru(dnbpy)(dcpby)]2+).

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail for M.B.: [email protected]. *E-mail for P.C.G.: [email protected]. Notes

The authors declare no competing financial interest. Biographies

5. CONCLUSION AND OUTLOOK In this account were presented the efficient synthesis and the photophysical and electrochemical characterization of a series of pyrrole-based ruthenium dyes. The electronic effect of polypyridine ligands containing pyrrole and its analogues on the properties of ruthenium complexes is clearly shown. The DSSC efficiency depends strongly on both the dye properties (absorption domain, molar extinction, oxidation potentials) and transfer mediator. The TiO2 electrodes modified with bis-heteroleptic dyes bearing N-pyrrole exhibited extended absorption domains and higher absorbances in comparison to those of the known N3 dye with good IPCE (near 80%), providing a cobalt-based mediator was used. The design of π-extended ligands was found to be a valuable strategy: especially, the introduction of styryl chains between the bipyridine and pyrrole induced a very good light-harvesting ability with near 70% IPCE in the visible region. The π conjugation contributed to a dramatic improvement in comparison to a complex bearing a ligand with pyrrole directly bound to bipyridine, thanks to the better protective effect of the hindering styryl-pyrrolo groups against photoinjected electron recapture by the mediator. The homoleptic ruthenium complex Ru(pyrrbpyCOOH), bearing a dissymmetrical ligand, exhibited good IPCE values

Marc Beley is professor emeritus at the University of Lorraine in Nancy. He obtained his Ph.D. in 1978 in Strasbourg. His first research works focused on metal oxide electrodes for the electrocatalysis of oxygen reduction in fuel cells and in secondary batteries. Then, he spent one year as a postdoctoral fellow at the University of Dortmund with Prof. H. Rickert working on solid-state electrochemistry. After three years as a research engineer at the “Compagnie Générale d‘Electricité” he obtained a position as an assistant professor at Strasbourg, where he worked with Jean-Pierre Sauvage on the electrocatalytic reduction of CO2 and on long-range electronic coupling in ruthenium and osmium complexes. Appointed professor in 1995 at the University of Lorraine, he developed some projects in bioelectrochemistry and in the field of new chromophores for applications in solar energy: in particular, dyesensitized solar cells. P


Organometallics

Review

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Philippe C. Gros studied chemistry at the University of Lyon and obtained his Ph.D. in 1992. He then worked for two years as a postdoctoral fellow with the Isochem Company on phosgene derivative chemistry. In 1994 he entered the CNRS as a Researcher in the Laboratory of Prof. Paul Caubère at Lorraine University. He received his Habilitation (HDR) in 2000 and was appointed Research Director in 2006. His current research interests include the design of new metalating agents (organolithiums, ate complexes), structural and reaction mechanism investigation, transition-metal-catalyzed cross-couplings for ligand synthesis, and the building of photo- and electroactive organometallic materials for various applications, including solar energy.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the CNRS, French Ministry of Research, Université de Lorraine, and Région Lorraine for support. The authors are also indebted to all co-workers and technical staff who made this work possible. Prof. Carlo Bignozzi and Dr. Stefano Caramori in Ferrara, Italy, are also warmly thanked for their fruitful collaboration.

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REFERENCES

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