Synthesis and Characterization of a Series of Bis-homoleptic

Aug 1, 2017 - School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K.. ‡ Sun Yat-sen University, Guangzhou 510275, P.R. C...
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Synthesis and Characterization of a Series of Bis-homoleptic Cycloruthenates with Terdentate Ligands as a Family of Panchromatic Dyes Thomas W. Rees,† JinFeng Liao,†,‡ Alessandro Sinopoli,§ Louise Male,† Giuseppe Calogero,∥ Basile F.E. Curchod,⊥ and Etienne Baranoff*,† †

School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K. Sun Yat-sen University, Guangzhou 510275, P.R. China § Qatar Environment & Energy Institute (QEERI), Hamad bin Khalifa University (HBKU), Doha Qatar ∥ CNR-IPCF, Istituto per i Processi Chimico-Fisici, Messina 98158, Italy ⊥ Centre for Computational Chemistry, School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K. ‡

S Supporting Information *

ABSTRACT: A series of six homoleptic bis-cyclometalated ruthenium complexes, Ru(N^N^C)2, is reported where N^N^C is a 6-(2,4-difluoro-3-R3-phenyl)-4-R2-4′-R1-2,2′-bipyridine with R3 = −H or −CF3 and R2 and R1 = −COOEt or −CF3. An effective synthesis of the ligands and the complexes is described. The UV− visible absorption studies demonstrate that these complexes are panchromatic dyes absorbing up to 900 nm. Importantly, the onset of absorption depends only on the substitution on the metalated phenyl, whereas the intensity of absorption throughout the spectra is a function of substituents on both the phenyl and the bipyridine moieties. The same trend is observed in electrochemistry as the redox gap depends only on the substitution on the metalated phenyl, whereas the oxidation and reduction potentials are a function of substituents on both the phenyl and the bipyridine moieties. Preliminary tests as sensitizer for dye-sensitized solar cells demonstrate that the number of anchoring groups on the dye has a major influence on the device efficiency.

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INTRODUCTION Organometallic complexes based on 2,2′-bipyridine (bpy) and 2,2′;6′,2′′-terpyridine (tpy) are some of the most well-known in the literature. In particular, [Ru(bpy)3]2+ and [Ru(tpy)2]2+ have been thoroughly investigated due to their strong absorption of light in the UV and visible region, intense emission and interesting redox properties.1,2 Ruthenium(II) polypyridyl complexes and their derivatives are particularly suited to a role as dyes in photovoltaics due to their Ru(II)/(III) reversible redox couple and broad absorption profiles.3−5 Derivatives of [Ru(bpy)3]2+ and [Ru(tpy)2]2+ polypyridyl complexes of other metals such as osmium have also been found to be of particular interest in supramolecular chemistry. Alteration of a bpy or tpy ligand by replacing a pyridyl with a phenyl moiety gives a ligand that is isoelectronic and capable of cyclometalation. This modification results in complexes with generally lower oxidation potentials, red-shifted absorption, and more metal-to-ligand charge-transfer (MLCT) character in the excited state of the complex. One of the most well-known complexes of this type is iridium tris(2-phenylpyridine), Ir(ppy)3. Due to its intense and highly efficient green phosphorescence, Ir(ppy)3 and its derivatives are of great © 2017 American Chemical Society

interest for use in emissive devices such as organic lightemitting diodes (OLEDs).6 Cyclometalated complexes of ruthenium are also well-known in the literature. Notable examples include potential anticancer agents7−11 and highly efficient dye molecules for photovoltaics.12−17 Although there are numerous literature examples of cyclometalated complexes of transition metals using N^N^C or N^C^N cyclometalated terdentate ligands,18−33 there are no reported bis-homoleptic terdentate examples of ruthenium and only a few examples of rhodium and iridium.34−36 This is therefore a relatively unknown area of chemical space to explore. Herein, we report a series of Ru(N^N^C)2 complexes (Figure 1) where N^N^C represents a cyclometalated 6phenyl-2,2′-bipyridine terdentate ligand. Our design is based on a 6-(2,4-difluorophenyl)-2,2′-bipyridine core ligand substituted on positions 4 and 4′ of the bipyridine with −COOEt or −CF3 and on position 3 of the phenyl with −H or −CF3. In the literature, there is a broad interest in the effect of fluorinated Received: June 2, 2017 Published: August 1, 2017 9903

DOI: 10.1021/acs.inorgchem.7b01412 Inorg. Chem. 2017, 56, 9903−9912

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Inorganic Chemistry

Figure 1. Chemical structure of bis-homoleptic bis-cycloruthenated complexes 1a−1f.

Scheme 1. Synthesis of Phenylbipyridine Ligandsa

a

Reagents and conditions: (i) mCPBA (1 equiv), 2a: CH2Cl2 rt, 16 h. 2b: CHCl3, reflux, 16 h; (ii) POCl3, reflux, 2 h; (iii) Pd(OAc)2, PtBu3-HBF, K2CO3, toluene, reflux, 24 h; (iv) 2,4-difluorophenylboronic acid (7a, 7c, and 7e) or 2,4-dilfluoro-3-(trifluoromethyl)phenylboronic acid (7b, 7d, and 7f), Pd(PPh3)4, Na2CO3, THF/H2O, 70 °C, 16 h.

ligands on the electronic properties of metal complexes.37−40 Our particular interest in these substituents is multifaceted. First, the use of electron-withdrawing groups is expected to increase the oxidation potential of the complexes and, hence, offset the decrease of oxidation potential expected from two cyclometalations. This would lead to more air-stable complexes, easier to manipulate, hence facilitating the development of a new synthetic method. Second, we have a long-standing interest in the effect of substituents on the optoelectronic properties of photoactive transition metal complexes. Our recent work on this topic has focused on phosphorescent cyclometalated iridium complexes,41 in which case electronwithdrawing substituents have been widely used to induce a blue shift of the emission. However, insights are limited because information is obtained mostly from the emissive triplet state, whereas the absorption spectra are made of largely overlapping bands very much restricted to the 350−400 nm range.

Expecting Ru(C^N^N)2 complexes to absorb across the visible spectrum with more defined absorption bands, we anticipate that these complexes will act as a suitable platform to study the effect of substituents. Third, the use of ester substituents would allow for the preparation of the carboxylic acid derivative, a widely used anchoring group for dyes for dye-sensitized solar cells42 and to control lipophilicity of biologically relevant photoactive drugs.43 Since the first report of the black dye N749,4 ruthenium complexes with terdentate ligands have attracted attention due to their ability to harvest photons in the near-infrared.44 The use of thiocyanate ligands affords an effective tuning of the energy of the highest occupied molecular orbital (HOMO), yet the ligand itself has two disadvantages. First, the ligand is ambidentate; coordination of the sulfur instead of the nitrogen causes linkage isomerism. Second, the ligand is monodentate and can more easily disassociate than a polydentate ligand. 9904

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would mean introducing the variation on the first rather than last step, greatly reducing the efficiency of the synthesis by effectively doubling the number of steps required to access the two ligands. In order to solve this two-fold problem and achieve an efficient synthesis of these ligands, as well as avoid toxic stannanes, we turned to the cross-coupling method developed by the Fagnou group.54 In this method, pyridyl oxides are coupled at the 2-position with aryl halides, and in the product, the original oxidized nitrogen remains oxidized. Applying this method to 5 and 6 then allows access to 3c, which can then be chlorinated in the same way as previously described to yield 4b. With the three chlorides 4a−c in hand, Suzuki coupling with the relevant phenylboronic acids gave the ligands 7a−f in yields from 55 to 93%. The next challenge was to synthesize the complexes. As this is a new family of compounds, no specific literature exists; however, synthetic routes to terdentate cyclometalates in the literature include a broad range of solvents and conditions (Table 1). Generally, polar solvents and bases to aid

Overall, thiocyanate ligands are expected to reduce the stability and therefore the longevity of the sensitizer.45 Utilization of a terpyridine ligand as in N749 not only allows an increased range of absorption but also a greater stability compared to ligands of a lower denticity. However, the use of two terpyridine ligands has an overall negative effect. The excited states become too low for injection of electrons to be efficient; the distorted octahedral geometry causes a decrease in the gap between the MLCT and metal-centered (MC) states, allowing decay of photoexcited states,46 and the increased symmetry causes a narrowing of the absorption spectrum.47 The van Koten group demonstrated the first use of terdentate cyclometalating ligands in a ruthenium dye. Although the efficiencies of the devices were only in the region of 2%, it nevertheless revealed an effective method to overcome the deficiencies present in a bis-terpyridine complex.11 The use of terdentate cyclometalated ligands not only gives the advantage of stability over thiocyanate ligands but also allows independent tuning of the HOMO and π*-based LUMO.48 A further improvement is that the HOMO of the dye is expanded over the metal and anionic section of the ligand, while the lowest unoccupied molecular orbital (LUMO) remains on the polypyridyl ligands. This gives an array of metal-to-ligand and ligand-to-ligand transitions resulting in a broad absorption of incident photons.47

Table 1. Example Literature Methods for Synthesizing Terdentate Cyclometalates

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RESULTS AND DISCUSSION Synthesis. The synthesis of the complexes begins with accessing the ligands 7a−f. All six are phenylbipyridine ligands with variation introduced at the R1, R2, and R3 positions (Scheme 1). The strategy determined to be most efficient for all six was to proceed via the bipyridyl chlorides 4a−c. The synthesis of 4a and 4b was relatively simple and follows the method described by Kim et al.16 First, the symmetrical bipyridine (2a or 2b) was oxidized by reaction with metachloroperoxybenzoic acid. For 2a, this reaction occurred at room temperature in CH2Cl2. In the case of 2b, which has more electron-withdrawing trifluoromethyl groups, the reaction would not proceed at room temperature. Instead, reflux in CHCl3 was required to access 3b. Then, chlorination by reflux in phosphorousoxychloride followed to furnish 4a and 4c. Synthesis of 4b proved to be a more challenging procedure. First, was the issue of forming the carbon−carbon bond between the two pyridyl moieties; a standard Suzuki coupling method is not possible in this case as boronic acids at the 2pyridyl position undergo rapid protodeborylation.49 A classic coupling in this situation would be achieved via the Stille method, stannylation of the 2-pyridyl halide, and coupling with the complementary 2-pyridyl halide.50 Tin compounds are, however, notoriously toxic and if possible should be avoided.51 Another solution would be to use a modified Suzuki style crosscoupling method with a stabilized boronic acid analogue, such as the boronic esters of methyl iminodiacetic acid developed by the Burke group.49,52,53 Having accessed the asymmetric bipyridine, we would, however, be presented with a second problem. Oxidation of this asymmetric bipyridine would at best yield a mixture of bipyridyl-N-oxides and at worst furnish mostly the undesired product, oxidized on the nitrogen closest to the ester. Utilizing either Stille coupling or a stabilized boronic acid, we could instead approach the ligands 7c and 7d by forming the phenylpyridine moiety first. Oxidation followed by chlorination would then give a suitable halide for crosscoupling. As the ligands differ on the phenyl ring only, this

metal

solvent

Ir Ir Ru Ru Ru Ru Os

ethylene glycol glycerol n-butanol ethylene glycol MeOH/H2O CH3CN CH3CN

base

temp (°C)

duration (h)

ref

N-methylmorpholine N-ethylmorpholine NaOH NaOH

175 200 118 200 45 82 82

90 2 3 1 14 72 72

36 55 56 57 47 58 59

K2CO3

cyclometalation are used, and the ligands are added stepwise. We were initially able to access 1b using a method utilized by the Pfeffer group.58 First, the reaction of 2 equiv of ligand with ruthenium(II) para-cymene dichloride dimer, KOH, and KPF6 in MeCN gave the intermediate 8, which was then combined with another equivalent of ligand and refluxed in EtOH with Nethylmorpholine to furnish 1b in 5% overall yield (Scheme 2). In order to improve upon this method, we tried to devise a one-pot synthesis. To prevent trans-esterification and saponification, we would avoid alcohols and strong bases. Initially, we tried reflux of 4 equiv of ligand in MeCN with Nethylmorpholine as base for 144 h, which gave 1b in 23% yield. At this point, we identified the major side product of this reaction as the monocyclometalated intermediate 9, which was formed in a 27% yield. In order to optimize this reaction, it was repeated with triethylamine to determine if a stronger base would aid cyclometalation. The yield of both 1b and 9 was, however, greatly reduced to 2.7 and 1.4%, respectively, due to a large proportion of de-esterification. A third attempt using butyronitrile as a higher boiling point analogue of MeCN and the less basic N-ethylmorpholine gave 1b in only 11% yield but 9 in 72% yield. From this information, we hypothesized that the formation of 9 is favored at higher temperature as the activation energy barrier of its formation is more easily overcome; however, butyronitrile is not as efficient as MeCN at aiding the removal of the last remaining chloride. In the work by Koga et al. (the second entry in Table 1), a similar problem converting a bis-cyclometalated iridium chloride to the desired tris-cyclometalated product was overcome by heating in glycerol at 200 9905

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Inorganic Chemistry Scheme 2. Development of the Synthesis of 1ba

a

Reagents and conditions: (i) KOH, KPF6, MeCN, reflux, 72 h; (ii) N-ethylmorpholine, ethanol, reflux, 16 h; (iii) N-ethylmorpholine, MeCN, reflux, 144 h; (iv) NEt3, MeCN, reflux, 162 h; (v) N-ethylmorpholine, butyronitrile, reflux, 120 h; (vi) ethylene glycol, reflux, 2 h.

°C with K2CO3. In an analogous approach, 9 was refluxed in ethylene glycol for 2 h, and complete conversion to 1b was observed. Armed with these results, we were able to devise an optimized procedure. First, reflux of the ligand in butyronitrile with ruthenium(II) para-cymene dichloride dimer and Nethylmorpholine for 16 h followed by reflux in ethylene glycol for 2 h gave 1a−f in moderate to excellent yields (Scheme 3), 30−80%. Although inert atmosphere is necessary during the synthesis to avoid oxidation to Ru(III), the complexes can be

purified with chromatography in air and without protection from light, which demonstrates their stability in normal conditions. X-ray Crystal Structures. Single crystals of 1a, 1b, and 1d were obtained by slow diffusion of hexane into a dichloromethane solution of the complexes. Structures are shown in Figure 2, and selected bond lengths and angles are given in Table 2. All of the compounds are octahedral, which are distorted to a greater or lesser extent by the bite angle of the ligands and the relative bond strengths of the metal with the phenyl carbon and pyridyl nitrogens. The structure of 1a contains two crystallographically independent molecules, in which the ethyl group C(311)−C(312)/C(31′)−C(32′) is disordered over two positions with a refined occupancy ratio of 0.67(2):0.33(2). 1b also contains a disordered ethyl group C(109)−C(110)/C(09′)−C(10′) over two positions with the refined percentage occupancy ratio of 56(2):44(2). A molecule of dichloromethane solvent is present at 20% occupancy and is disordered over two positions. In the case of 1d, the ethyl groups are not disordered; however, the fluorine atoms F(10), F(11), and F(12)/F(10′), F(11′) and F(12′) are each disordered over two positions with a refined occupancy ratio of 0.681(17):0.319(17). Photophysical and Electrochemical Properties. Complexes 1a−f were studied by UV−vis absorption spectroscopy (Figure 3). All six complexes show very broad absorption from 250 to above 850 nm as well as very intense absorption with extinction coefficients mostly in excess of 10 000 M−1 cm−1. 1a and 1b show particularly strong absorption with extinction coefficients in excess of 20 000 M−1 cm−1. For all the complexes, we see two main series of peaks: the peaks attributed to π−π* transitions between 250 and 400 nm and

Scheme 3. Synthesis of Cycloruthenated Complexes 1a−fa

a Reagents and conditions: (i) [Ru(p-cymene)Cl2] (0.25 equiv), Nethylmorpholine, butyronitrile, reflux, 16 h; (ii) ethylene glycol, reflux, 2 h.

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Figure 2. X-ray crystal structures of 1a, 1b, and 1d with ellipsoids drawn at the 50% probability level. For 1a, only one of the crystallographically independent molecules is shown. For 1b, the minor component of the disordered ethyl group and the solvent molecule have both been omitted for clarity.

the peaks associated with MLCT transitions between 400 and 800 nm. As we move through the series from 1a to 1f and from the least to most electron-withdrawing ligand, we can see a clear trend that, relative to the π−π* transitions (which remain much the same), the intensity of the MLCT bands decreases. This is to be expected as the more electron-withdrawing the ligand is the more electron density lies on the ligands, and the absorption profile is more ligand centered in character. Theoretical calculations (DFT and LR-TDDFT using the PBE0 functional; see Supporting Information for full computational details) on the complete series provide similar trends between the different compounds (Figure 4), notably the

Table 2. Selected Bond Distances (Å) and Angles (deg) for Complexes 1a, 1b, and 1d

1aa bond distance Ru−N(1) Ru−N(2) Ru−N(3) Ru−N(4) Ru−C(1) Ru−C(2) bond angle C(1)−Ru−N(2) C(2)−Ru−N(4) C(1)−Ru−C(2) N(2)−Ru−N(4) N(1)−Ru−N(3) a

1b

1d

1.980(4) 2.124(5) 1.987(4) 2.122(4) 2.039(5) 2.026(6)

1.983(5) 2.139(6) 1.982(5) 2.109(5) 2.015(8) 2.020(7)

1.979(4) 2.116(4) 1.987(4) 2.125(4) 2.026(5) 2.031(5)

157.3(2) 157.1(2) 91.7(2) 96.56(17) 177.87(18)

157.6(2) 157.6(2) 92.9(3) 96.75(19) 178.66(18)

157.88(18) 157.5(2) 88.40(19) 95.10(14) 178.13(15)

For one of the two crystallographically independent molecules.

Figure 4. Theoretical absorption spectra for compounds 1a−1f (LRTDDFT/PBE0 with implicit solvation). Insets show the assignment of selected transitions in terms of natural transition orbitals (singular value given in parentheses).

higher absorption of 1a and 1b. At this level of theory, the Kohn−Sham HOMO orbital is mostly composed of a 4d(Ru) orbital with small contributions from the phenyl moiety, whereas the LUMO mostly resides on the pyridine part of the ligands (see Supporting Information for graphical representation). The assignment of vertical transitions computed at the LR-TDDFT/PBE0 level of theory is greatly facilitated by the use of “natural transition orbitals” (NTOs), which offer a

Figure 3. UV−vis absorption spectra of 1a−f in CH2Cl2 at room temperature.

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Inorganic Chemistry Table 3. Photophysical and Electrochemical Data λmaxabs (nm) (ε, M−1 cm−1)a

complex 1a 1b 1c 1d 1e 1f

475 463 454 437 441 408

(28109), (25373), (18710), (15063), (15099), (10402),

617 600 618 600 617 468

Eox (V vs NHE)b

Ered (V vs NHE)b

Eox‑red (V vs NHE)b

0.70 0.90 0.75 0.94 0.79 0.99

−1.22 −1.13 −1.17 −1.06 −1.15 −1.04

1.92 2.03 1.92 2.00 1.94 2.03

(19252), 729sh (4030) (21639), 691sh (5010) (12711), 729sh (3665) (12219), 691sh (3592) (11418), 729sh (2571) (9455), 599 (9340), 691sh (2374)

a Data obtained from a CH2Cl2 solution at room temperature. bRedox data were obtained in a 0.1 M tetrabutylammonium hexafluorophosphate solution in CH2Cl2. Fc/Fc+ was used as an internal standard (Fc/Fc+ = 0.70 vs NHE).

Furthermore, Eox‑red, 1.92/2.03 eV, is very close to the energy of the absorption peak at 600/617 nm. Based on the theoretical calculations, this confirms that the oxidation occurs on the ruthenium ion and the reduction on the bipyridine part of the ligand. Dye-Sensitized Solar Cells. To perform a preliminary study of solar cells sensitized with these new complexes, we selected the ester complexes 1b and 1d and prepared the corresponding acid complexes 10 and 11 by hydrolysis with potassium hydroxide (Scheme 4). The choice was based on

compact description of the donating and receiving orbitals in an electronic transition. LR-TDDFT/PBE0 indicates that the bands between 400 and 800 nm indeed exhibit a strong MLCT character, where the electron mostly departs from a mostly pure 4d(Ru) orbital with a weak contribution of the phenyl moiety (Figure 4). Looking at the theoretical spectra for 1a and 1b, we observe a trend between the two main MLCT bands in the range of 400−600 nm similar to what is observed experimentallythe two bands for 1a are both red-shifted with respect to the ones of 1b, and 1b’s low-energy band is stronger than 1a, whereas the opposite is true for the higher-energy one. The MLCT region can be further split into three main areas. First, between 400 and 500 nm, we see a single peak for 1a, which is blue-shifted and broadened as we go through the series until in 1f it splits into two distinct peaks. This trend would indicate that the bipyridyl portion of the ligand is strongly involved in this transition. Investigation of the NTOs of 1a confirms this assignment for the receiving orbital in the brightest transition to S13 (Figure 4), with the donating orbital also displaying a weak contribution from the central pyridine moiety. The second section in the MLCT region lies between 500 and 650 nm. We see that as we go through the group from the 2,4-difluorophenyl to the more electron-withdrawing 2,4difluoro-3-(trifluoromethyl)phenyl cyclometalating group, the peak is blue-shifted. The pattern repeats through the series as the ligands alternate between the two cyclometalating moieties, the peak shifts from 617 to 600 nm and back again, independently of the substitution on the bipyridine part of the ligand (Table 3). The final part of the MLCT region is the shoulder observed for all six complexes between 650 and 800 nm. Theoretical calculations indicate that this band is due to weakly allowed MLCT transitions, even if we cannot rule out the presence of spin-forbidden MLCT transitions (calculations do not include spin−orbit coupling). The complexes were also analyzed by cyclic voltammetry, and the resulting data are presented in Table 3. We can see that the main effect on the oxidation potential is the change in cyclometalating moiety. As the phenyl group alternates from less to more electron-withdrawing, the oxidation potential changes from 0.63−0.72 to 0.83−0.92 V vs NHE. This trend among the oxidation potential is almost perfectly reproduced by the trend among the Kohn−Sham HOMO energies (see Figure S3 in Supporting Information). We also see that as we go through the group from least to most electron-withdrawing ligand, as ester moieties are replaced by −CF3, there is an overall increase in oxidation potential. The same trend is observed with reduction potentials as the more electron-withdrawing the substituents on the ligand, the less negative is Ered. However, within the uncertainty of the measurements, the redox gap Eox‑red = Eox − Ered depends only on the presence/absence of −CF3 on the metalated phenyl.

Scheme 4. Synthesis of 10 and 11a

a

Reagents and conditions: (i) KOH, acetone, 60 °C, 16 h; (ii) HCl, rt.

their oxidation potential, about 0.9 V vs NHE, which appeared similar to efficient ruthenium dyes using terdentate ligands, between 0.8 and 1 V vs NHE.14,16,44,60,61 This gave us one complex with four acid moieties, 10, and one complex with only two acid moieties, 11. Photovoltaic measurements were performed on an AM 1.5 solar simulator (100 mW cm−2). The incident light was calibrated by using a Si photodiode reference. Photocurrent− voltage (JV) curves were obtained by applying an external bias to the cells and measuring the generated photocurrent with a Keithley digital meter. The main photovoltaic parameters for the constructed DSSC devices, using 10 and 11, are listed in Table 4 with N719 as a benchmark comparison. The overall conversion efficiencies η were derived from the equation η = JscVocFF/Is, where Jsc is the short-circuit current density, Voc is Table 4. Photovoltaic Parameters of Dyes 10, 11, and N719a dye

Voc (V)

Jsc (mA cm−2)

FF (%)

η (%)

10 11 N719

0.2 0.59 0.63

0.35 11.2 18.4

0.33 0.61 0.58

0.024 4.3 6.84

a

TiO2-based DSSCs; the electrolyte contained 0.1 M LiI and 0.05 M I2, in acetonitrile/valeronitrile (1:1, v/v) mixture together with 0.6 M N-methyl-N-butyl imidazolium (BMII) and 0.5 M tert-butylpyridine. 9908

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Inorganic Chemistry the open-circuit voltage, FF is the fill factor, and Is is the intensity of the incident light. Figure 5 shows the JV and

groups could facilitate the interaction between the oxidized dyes and the iodide, enhancing the regeneration constant.37 Further investigation will include the entire set of carboxylated complexes and shed light on the charge-transfer kinetics inside the cells.

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CONCLUSIONS A new family of bis-homoleptic cycloruthenated complexes was synthesized. A synthetic strategy was devised to efficiently access phenylbipyridines containing an asymmetrical bipyridine moiety, avoiding the use of toxic stannanes. A rational methodology was also developed to access bis-homoleptic ruthenium complexes, which was optimized to give moderate to excellent yields depending on the ligand. Selected complexes were characterized by X-ray crystallography; the data from which show the distorted octahedral structure present in this type of complex. Analysis of the absorption profiles of 1a−f showed them to be extremely broad and intense. Cyclic voltammetry studies revealed the oxidation potentials to be between 0.63 and 0.90 V vs NHE. These properties of intense broad absorption and oxidation potentials around 0.75 V are also of interest as these compounds may have potential as dyes in photovoltaic devices and anticancer photoactive drugs.

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EXPERIMENTAL SECTION

Materials, Synthesis, and Characterization. All reactions were performed under argon. Compounds 2a, 2b, 5, and 6 were synthesized using literature methods described in the Supporting Information. All commercial chemicals were used without further purification with the exclusion of phosphorousoxychloride, which was distilled before use. Column chromatography was performed on 40−60 μm mesh silica. 1H NMR, 19F NMR, and 13C NMR spectra were recorded in CDCl3 unless specified otherwise on a Bruker AVIII400 (400 and 100 MHz) or a BrukerAVIII300 (300 MHz). Chemical shifts are reported as δ values (ppm) referenced to the following solvent signals: CHCl3, δH 7.26, CDCl3, δC 77.0, CH3CN, δH 1.94, CD3CN, δC 118.3. Multiplicity of signals and coupling constants were obtained by processing on MestReNova. Mass spectra were recorded on a Micromass ZABspec spectrometer utilizing electrospray ionization with a MeOH or MeCN mobile phase. Infrared spectra were recorded neat as thin films on a PerkinElmer “Spectrum” spectrometer. The intensity of each band is described as s (strong), m (medium), or w (weak) where appropriate. UV−vis spectra were recorded on a Cary-5000 spectrometer in CH2Cl2 solution. Electrochemical data were obtained using a Metrohm Autolab Pgstat101 potentiostat with a glassy carbon disk as working electrode and Pt wire reference and counter electrodes. Cyclic voltammetry was performed in 0.1 M solutions of n-BuNPF6 in argon-degassed CH2Cl2. The ferrocene/ferrocenium couple (Fc/Fc+) was used as an internal standard. Crystallographic data were recorded at the EPSRC UK National Crystallography Service at the University of Southampton by Diamond Light Source on beamline I19. General Procedure for the Synthesis of Phenylbipyridine Ligands. The bipyridyl chloride (1.0 equiv), boronic acid (1.2 equiv), and Na2CO3 (1.2 equiv) were combined with THF and H2O (5:1). The solvent was degassed with argon for 20 min before tetrakis(triphenylphosphine)palladium(0) (5 mol %) was added. The mixture was then degassed with argon for a further 10 min before stirring under argon at 70 °C for 16 h. After being cooled to rt, the solvent was removed under reduced pressure, CH2Cl2 and H2O were added, and the organic phase was extracted. The aqueous phase was extracted with a further 2 portions of CH2Cl2. The organic phases were combined and dried (MgSO4), and the solvent was removed under reduced pressure. The resulting solid was purified by column chromatography on silica. General Procedure for the Synthesis of Cyclometalated Ruthenium Complexes. A two-neck round-bottom flask fitted with stirrer, condenser, and stopcock was charged with the ligand (4.0

Figure 5. Top: JV curves of DSSC constructed using complexes 10 (blue), 11 (red), and N719 (black). Bottom: IPCE traces for complex 11 and N719.

incident photon-to-current efficiency (IPCE) traces of the DSSCs based on the new dyes. Note that in this preliminary work, JV curves cannot be quantitatively compared to the IPCE traces because of the different measurement conditions. The IPCE trace for 11 showed a maximum of 25% at 500 nm, whereas the current generated by 10 was too low for recording an IPCE trace. In the same experimental condition and setup, N719 resulted in an IPCE value of 57%. The best result between the two tested dyes is obtained for complex 11 with an efficiency of 4.3%, compared to 0.024% for complex 10 and 6.84% for the benchmark dye N719 under the same conditions. Interestingly, although the complex 1d (ester version of 10) has less favorable absorption properties compared to those of 1b (ester version of 11), because of much lower absorption coefficient, about half of 1b, the device performances are significantly different with dye 11 being the one giving the highest efficiency. Being a preliminary test, the reasons for this discrepancy could not be fully understood, and detailed studies of the devices are outside of the scope of this paper. As a hypothesis, we attribute the higher efficiency for complex 11 to a better directionality of its anchoring groups, hence a higher injection rate, together with a higher dye loading related to its greater solubility with respect to the tetracarboxylated complex 10. Also, the replacement of two carboxylic groups with −CF3 9909

DOI: 10.1021/acs.inorgchem.7b01412 Inorg. Chem. 2017, 56, 9903−9912

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Inorganic Chemistry equiv) and [Ru(p-cymene)Cl2]2 (1.0 equiv). The apparatus was filled with argon before the addition of n-BuCN and N-ethylmorpholine. The resulting orange solution was degassed with argon for 20 min before heating and stirring at 120 °C, for 16 h under argon and reduced light. After being cooled to rt, solvent was removed under reduced pressure and the crude black solid was dried in vacuo. Ethylene glycol was then added and the resulting mixture degassed for 20 min with argon. The reaction mixture was then heated and stirred at 200 °C for 2 h under argon and reduced light. After being cooled to rt, the crude black mixture was combined with H2O and CH2Cl2. The organic phase was separated and the aqueous phase extracted with further portions of CH2Cl2. The organic phases were combined and dried (MgSO4), and solvent was removed under reduced pressure. The crude product was purified by column chromatography on silica. Dye-Sensitized Solar Cell Fabrication. The FTO glass, used for the preparation of transparent electrodes, was first cleaned with a detergent and then rinsed with water and ethanol. Clean conductive glass plates were then immersed in 40 mM TiCl4 aqueous solution at 70 °C for 30 min, washed with water and ethanol, and dried in oven at 80 °C for 30 min. The TiO2 layers were deposited on the FTO glass plates by screen-printing (frame with polyester fibers having 43.80 mesh/cm2). This procedure, involving two steps (coating and drying at 125 °C), was repeated three times. The TiO2-coated plates were gradually heated to 500 °C and sintered at this temperature for 30 min. After being cooled, the TiO2 film was further treated with 40 mM TiCl4 solution following the previously described procedure. Then a scattering layer of TiO2 large nanoparticles (particles size 150−200 nm) was deposited by screen-printing and sintered at 500 °C. Each anode was cut into rectangular pieces (2 cm × 1.5 cm) having a circular active area of 0.196 cm2. The anodes employed for IPCE experiments were prepared by screen-printing method, using a frame of polyester fibers with a mesh size of 77.48 in order to obtain a transparent ultrathin TiO2 film with an estimated thickness of about 4 μm and an active area of 1 cm2. Avoiding moisture absorption, all the prepared anodes were stored in oven at about 80 °C until their use. Before assembling the cells, anodes were immersed in the corresponding 1:1 (v/v) acetonitrile and tert-butanol dye baths (3 × 10−4 M) for 22 h. Counter electrodes consisted of predrilled FTO glass plates (2 cm × 2 cm) on which a Pt transparent layer was deposited by dropping H2PtCl6 solution (5 mM in isopropyl alcohol) and by heating the plates at 500 °C for 30 min. The anode and the counter electrode were assembled into a sandwich-type arrangement and sealed (using a thermopress) with a Surlyn hot melting gasket. The electrolyte was prepared by dissolving the redox couple, 0.1 M LiI and 0.05 M I2, in acetonitrile/valeronitrile (1:1, v:v) mixture together with 0.6 M N-methyl-N-butylimidazolium (BMII) and 0.5 M tertbutylpyridine. A drop of electrolyte solution was put on the hole in the back of the cathode. A drop of the electrolyte was introduced through the predrilled hole in the counter electrode via backfilling under vacuum and sealed afterward. An additive sealing with epoxy resins to avoid the leakage of electrolyte and ensure a longer temporal stability of the device was applied.

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Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Basile F.E. Curchod: 0000-0002-1705-473X Etienne Baranoff: 0000-0003-3780-0733 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Thanks to the School of Chemistry, University of Birmingham, for a studentship, and the EPSRC for underpinning funding. Thanks to the EPSRC U.K. National Crystallography Service62 at the University of Southampton for the collection of the crystallographic data for compound 1d and at the Diamond Light Source for compound 1b. We acknowledge support from the Centre for Chemical and Materials Analysis in the School of Chemistry at the University of Birmingham.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01412. Synthesis and characterization data for all compounds described in this work, including NMR, crystallographic, and electrochemical data; details of theoretical calculations (PDF) Accession Codes

CCDC 1552534−1552536 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The 9910

DOI: 10.1021/acs.inorgchem.7b01412 Inorg. Chem. 2017, 56, 9903−9912

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