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Influence of the solvent, structure and substituents of ruthenium(II) polypyridyl complexes on their electrochemical and photo-physical properties
Inorganica Chimica Acta 440 (2016) 26–37
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Influence of the solvent, structure and substituents of ruthenium(II) polypyridyl complexes on their electrochemical and photo-physical properties Kévin Ruffray, Matthieu Autillo, Xavier Le Goff, Jérôme Maynadié ⇑, Daniel Meyer Institut de Chimie Séparative de Marcoule, UMR 5257 CEA/CNRS/UM/ENSCM, BP 17171, 30207 Bagnols-sur-Cèze CEDEX, France
a r t i c l e
i n f o
Article history: Received 3 September 2015 Received in revised form 20 October 2015 Accepted 22 October 2015 Available online 26 October 2015 Keywords: Ruthenium complexes Photo-physical properties Electron transfer Electrochemistry
a b s t r a c t This work highlights the relative importance between structural issues, longer range interactions and solvent effects for a series of ruthenium compounds on their electrochemical and photochemical properties. To study these effects, eight complexes of general formula [Ru(bpy)2(L)]2+ have been synthesized and characterized, where L represents substituted phenyl-imidazophenanthrolines (pip), dipyridophenazines (dppz) and pyrazinophenanthrolines (pzp). This experimental work supported by an important theoretical investigation has pointed out that the electrochemical and the photo-physical behavior of these types of molecular systems depends mainly of the close environment around the photosensitizer. A relation between the ditopic ligand structure (aromatic central part and/or the distal substituents) and the absorption properties have been observed, along with the lower reduction potential of dipyridophenazine and pyrazinophenanthroline moieties. This study points out the influence of the nature of the distal substituents, as well as the significant impact of the solvent nature and the interaction with water molecules, on the photo-physical properties of these ruthenium compounds. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Ruthenium polypyridyl compounds were first discovered almost 80 years ago [1], but they are still sources of new trails in coordination chemistry. The most simple and representative complex of this family, [Ru(bpy)3]2+, is useful in almost every field in photochemistry, due to its chemical stability and its unique photo-physical properties [2]. Polypyridyl ruthenium(II) complexes are employed in various research fields, including biodiagnostics [3], photovoltaics [4], organic LED [5]. The most recognized ones are the manufacturing of optical chemical sensors [6], and electron and energy-transfer molecular devices [7], primarily due to the metal-to-ligand charge transfer (MLCT) transition exhibited by these kind of complexes [8]. By irradiating these species at the appropriate wavelength, a long-lived excited state (near 600 ns to a few ls) [9] is produced. This excited species is well described as a ruthenium(III) ion and a one-electron reduced polypyridyl ligand [10]. This transient state could be engaged in electron transfer reactions resulting in a ruthenium(III) [11] or a ruthenium(I) [12] reactant, through an intermolecular pathway using a sacrificial electron acceptor or donor. ⇑ Corresponding author. Tel.: +33 4 66 79 13 80. E-mail address: [email protected] (J. Maynadié). http://dx.doi.org/10.1016/j.ica.2015.10.018 0020-1693/Ó 2015 Elsevier B.V. All rights reserved.
The ability to transfer the photo-excited electron to a second metal complex have promoted the [Ru(bpy)3]2+ analogues as very good compounds for photo-induced electron-transfer studies. To improve the efficiency of such systems, new complexes have been developed where the chromophore is covalently linked to an active metallic redox center. These bimetallic species are potentially able to increase the rate of light conversion into chemical energy by minimizing the diffusion phenomenon, usually encountered during intermolecular reactions. Studies are mainly focused on the influence of the size and the topology of the organic spacers on the electronic transfer between the chromophore and the active site [13–15]. In this field, catalytic enhancement has been highlighted on bimetallic polypyridyl Ru complexes based on substituted imidazophenanthroline [16–21]. This work is focused on a new synthetic pathway to obtain the ruthenium complexes [22], adding block by block the constituents to control the complexation of the different metal centers on the ditopic ligand. After studying a new metallo-assembly based on ruthenium and iron, the authors have opened a new discussion on the relation between the structure of the ditopic ligand and the catalytic activity of the polymetallic species [23]. In order to develop efficient heterometallic systems for charge separation, Ru-species incorporating substituted imidazophenanthroline ligands were studied and compared to other p-delocalized
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extended systems. The structure of the ditopic ligands can be divided into 3 parts: a phenanthroline moiety to coordinate the ruthenium, an aromatic core and distal coordinating groups. Phenylimidazole, quinoleine, and pyrazine moieties were examined as aromatic core, and carbonitrile, hydroxyl and/or carboxylic acid moieties as distal coordinating groups. While some of the studied Ru-complexes have already been singly reported for electronic transfer or DNA intercalation [24–30], it was necessary to acquire more fundamental information on the relation between their physicochemical properties and different structural aspects in the frame of photo-chemical energy storage. These data are also useful before moving towards the understanding of more complex heterometallic molecular systems. In this work, the ability of a photo-induced electron-transfer between the ruthenium center and the different ditopic ligands has been analyzed using various techniques including electrochemistry and spectrophotometry. A special attention was paid to longer range interactions such as the solvent and the intermolecular interactions on the photophysical properties of these systems. 2. Experimental 2.1. General 1
H NMR spectra were obtained using a Bruker AvanceIII 400 spectrometer, using TMS as the internal standard. Chemical shifts are expressed in ppm in relation to the deuterated solvent signal, and coupling constants are expressed in hertz. Elemental (C, H, N) analyses were performed on a Flash EA1112 (ThermoFinnigan 2003) instrument. Mass spectra were realized in Laboratoire de Mesures Physiques of the Max Mousseron technical platform, at Université de Montpellier. HR-ESI-TOF was recorded on an Alliance 2790 Waters spectrometer. UV–Vis absorption spectra were recorded on a Shimadzu UV-3600 UV–Vis-NIR spectrometer. IR spectra were recorded at solid state, using the ATR module of a Perkin–Elmer Spectrum100 FT-IR spectrometer. Electrochemical studies were realized using a Biologic SP300 apparatus. Electrochemical analysis cell includes a three-electrode assembly: a non-aqueous Ag/AgCl reference electrode, a platinum counter-electrode, and a glassy carbon working electrode. Electrochemical behaviors were studied in degassed acetonitrile, using tetrabutylammonium salts as support electrolytes. Complexes were studied at a concentration near 0.5 mM at 100 mV/s. Spectrofluorimetric properties were recorded on a Horiba FluoroMax-4 apparatus. The emission spectra were realized after excitation at 452 nm, and detected at 90° in relation to incident beam. Lifetime measurements were recorded after a 1.2 ns excitation at 452 nm, and detected at 90° in relation to incident beam. Caution! Perchlorate salts of metal complexes are potentially explosive. Only small quantities of material should be prepared and the samples should be handled with care. 2.2. Computational details Full geometry optimization and frequency computations were performed applying the DFT method with Becke’s three parameter hybrid functional [31] and Lee–Yang–Parr’s gradient-corrected correlation functional [32] (B3LYP). LANL2DZ basis set [33] were used for ligands and complexes (Ru, C, H, N, O), and assuming the singlet state for the ground state of all calculations. The structures of 3, 4, 5, 6, 7, 8, 9 and 10 were fully optimized. 160 singlet excitation energies were calculated with nonequilibrium TDDFT [34–36] method at the RB3LYP/LANL2DZ level considering solvent effect with polarizable continuum model (PCM) [37]. All computations were performed with Gaussian G09 quantum chemistry
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program-package [38]. UV–Vis spectrum was interpreted by using GaussSum 3.0 [39]. 2.3. Synthesis All experimental materials were of AR grade, and used as purchased without purification. Water was purified to Milli-Q grade, and all other solvents used as purchased without purification if not specified. cis-Ru(bpy)2Cl2 (1) [40], 1,10-phenanthroline-5, 6-dione [41] and [Ru(bpy)2(phendione)][X]2 (X = PF6 or ClO4) (2) [42] were synthesized as previously described. 2.3.1. 4-(1H-Imidazo[4,5-f][1,10]phenanthroline-2-yl)benzene-1,2diol bis(2,20 -bipyridine) ruthenium(II) hexafluorophosphate (3) Synthesized by adapting a literature procedure [43], using 2 and 3,4-dihydroxybenzaldehyde as starting reactants. Yield: 0.277 mg (66%). Anal. Calc. for C39H28F12N8O2P2Ru: C, 45.40; H, 2.74; N, 10.86. Found: C, 45.20; H, 2.67; N, 10.89%. 1H NMR (400 MHz, DMSO-d6, 25 °C), d (ppm) 14.06 (s, 1H, N-H), 9.08 (s, 2H, ligand), 8.87 (dd, J1 = 15.5 Hz, J2 = 8.2 Hz, 4H, bpy), 8.22 (td, J1 = 8.0 Hz, J2 = 1.4 Hz, 2H, bpy), 8.13 (td, J1 = 8.0 Hz, J2 = 1.4 Hz, 2H, bpy), 8.05 (d, J = 5.0 Hz, 2H, ligand), 7.92 (t, J = 5.0 Hz, 2H, ligand), 7.85 (d, J = 5.0 Hz, 2H, bpy), 7.76 (d, J = 2.1 Hz, 1H, ligand), 7.61 (m, 5H, bpy-ligand), 7.35 (t, J = 6.0 Hz, 2H, bpy), 6.99 (d, J = 8.2 Hz, 1H, ligand). UV kmax (MeCN)/nm 457, 433, 365, 321, 287, 240. IR (ATR, mmax/cm1) 1602, 1446, 1101, 831, 759, 724, 556. ESI-TOF (m/z, MeCN) 886.9 ([M-PF6]+, calc. 886.7), 741.0 ([M]+, calc. 741.8), 371.0 ([M]2+, calc. 370.9). 2.3.2. 4-(1H-Imidazo[4,5-f][1,10]phenanthrolin-2-yl)benzonitrile bis (2,20 -bipyridine) ruthenium(II) hexafluorophosphate (4) Synthesized by adapting a literature procedure [43], using 2 and 4-formylbenzonitrile as starting reactants. Yield: 0.175 mg (77%). Anal. Calc. for C40H27F12N9P2Ru: C, 46.89; H, 2.66; N, 12.30. Found: C, 46.41; H, 2.50; N, 12.14%. 1H NMR (400 MHz, DMSO-d6, 25 °C), d (ppm) 8.94 (dd, J1 = 8.2 Hz, J2 = 1.4 Hz, 2H, ligand), 8.88 (d, J = 8.2 Hz, 2H, bpy), 8.85 (d, J = 8.2 Hz, 2H, bpy), 8.53 (d, J = 8.6 Hz, 2H, ligand), 8.21 (td, J1 = 7.9 Hz, J2 = 1.2 Hz, 2H, bpy), 8.10 (td, J1 = 7.9 Hz, J2 = 1.2 Hz, 2H, bpy), 7.85 (m, 4H, bpy-ligand), 7.75 (m, 4H, ligand), 7.57 (m, 4H, bpy), 7.37 (td, J1 = 6.7 Hz, J2 = 1.0 Hz, 2H, bpy). UV kmax (MeCN)/nm 456, 429, 328, 287, 240. IR (ATR, mmax/cm1) 2225, 1604, 1464, 1446, 1423, 1098, 823, 757, 725, 554, 463, 419. ESI-TOF (m/z, MeCN) 734.2 ([M]+, calc. 734.8) 367.6 ([M]2+, calc. 367.4). 2.3.3. 4-(1H-Imidazo[4,5-f][1,10]phenanthrolin-2-yl)benzoic acid bis (2,20 -bipyridine) ruthenium(II) perchlorate (5) Synthesized by adapting a literature procedure [43], using the perchlorate salt of 2 and 4-carboxybenzaldehyde as starting reactants. Yield: 0.708 g (91%). Anal. Calc. for C40H28Cl2N8O10Ru: C, 50.43; H, 2.96; N, 11.76. Found: C, 50.24; H, 2.87; N, 11.62%. 1H NMR (400 MHz, DMSO-d6, 25 °C), d (ppm) 8.98 (d, J = 7.7 Hz, 2H, ligand), 8.87 (d, J = 8.0 Hz, 2H, bpy), 8.83 (d, J = 8.2 Hz, 2H, bpy), 8.41 (d, J = 8.2 Hz, 2H, ligand), 8.20 (t, J = 7.5 Hz, 2H, bpy), 8.09 (t, J = 7.7 Hz, 2H, bpy), 7.87 (m, 4H, bpy-ligand), 7.75 (m, 4H, ligand), 7.58 (m, 4H, bpy), 7.37 (t, J = 6.3 Hz, 2H, bpy). UV kmax (MeCN)/nm 456, 429, 326, 287, 240. IR (ATR, mmax/cm1) 1601, 1445, 1370, 1067, 806, 761, 722, 620, 421. ESI-TOF (m/z, MeCN) 753.14 ([M]+, calc. 753.79), 377.07 ([M]2+, calc. 376.90), 341.11 ([ligand-H]+, calc. 341.35). 2.3.4. 3-(1H-Imidazo[4,5-f][1,10]phenanthrolin-2-yl)benzoic acid bis (2,20 -bipyridine) ruthenium(II) hexafluorophosphate (6) Synthesized by adapting a literature procedure [43], using 2 and 3-carboxybenzaldehyde as starting reactants. Yield: 0.157 g (68%). Anal. Calc. for C40H28F12N8O2P2Ru: C, 46.03; H, 2.70; N, 10.74.
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Found: C, 45.64; H, 2.71; N, 10.63%. 1H NMR (400 MHz, DMSO-d6, 25 °C), d (ppm) 9.52 (s, 2H, ligand), 8.89 (d, J = 8.2 Hz, 2H, bpy), 8.85 (d, J = 8.2 Hz, 2H, bpy), 8.49 (d, J = 7.1 Hz, 1H, ligand), 8.22 (t, J = 7.7 Hz, 2H, bpy), 8.11 (t, J = 7.5 Hz, 2H, bpy), 7.99 (m, 3H, ligand), 7.87 (m, 4H, bpy-ligand), 7.58 (m, 5H, bpy-ligand), 7.38 (t, J = 6.3 Hz, 2H, bpy). UV kmax (MeCN)/nm 459, 432, 361, 318, 287, 238. IR (ATR, mmax/cm1) 1604, 1562, 1510, 1447, 1367, 837, 763, 724, 556. ESI-TOF (m/z, MeCN) 1061.94 ([M-H2O]+, calc. 1061.71), 359.32 ([ligand-H-H2O]+, calc. 359.37), 341.31 ([ligandH]+, calc. 341.35).
C, 42.63; H, 2.34; N, 14.51%. 1H NMR (400 MHz, DMSO-d6, 25 °C), d (ppm) 9.52 (d, J = 8.9 Hz, 2H, ligand), 8.87 (m, 4H, bpy), 8.38 (d, J = 4.8 Hz, 2H, ligand), 8.23 (t, J = 7.5 Hz, 2H, bpy), 8.14 (t, J = 8.8 Hz, 2H, bpy), 8.08 (dd, J1 = 8.1 Hz, J2 = 5.7 Hz, 2H, ligand), 7.80 (d, J = 5.1 Hz, 2H, bpy), 7.71 (d, J = 5.0 Hz, 2H, bpy), 7.60 (t, J = 6.5 Hz, 2H, bpy), 7.37 (t, J = 6.6 Hz, 2H, bpy). UV kmax (MeCN)/ nm 441, 330, 285, 265, 232. IR (ATR, mmax/cm1) 2211, 1605, 1555, 1446, 1374, 1127, 830, 761, 730, 556, 440. ESI-TOF (m/z, MeCN) 348.1 ([M]2+, calc. 347.9).
2.3.5. 2-Hydroxy-5-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl) benzoic acid bis(2,20 -bipyridine) ruthenium(II) hexafluorophosphate (7) Synthesized by adapting a literature procedure [43], using 2 and 5-formylsalicylic acid as starting reactants. Yield: 0.327 g (76%). Anal. Calc. for C40H28F12N8O3P2Ru: C, 45.34; H, 2.66; N, 10.57. Found: C, 45.48; H, 2.77; N, 10.45%. 1H NMR (400 MHz, DMSO-d6, 25 °C), d (ppm) 14.35 (s, 1H, N-H), 9.23 (d, J = 7.3 Hz, 1H, ligand), 9.12 (d, J = 8.4 Hz, 1H, ligand), 8.89 (d, J = 8.2 Hz, 2H, bpy), 8.85 (d, J = 8.2 Hz, 2H, bpy), 8.77 (s, 1H, ligand), 8.21 (m, 3H, bpyligand), 8.11 (t, J = 7.8 Hz, 2H, bpy), 8.03 (m, 2H, ligand), 7.90 (m, 4H, bpy-ligand), 7.61 (m, 4H, bpy), 7.36 (t, J = 7.2 Hz, 2H, bpy), 6.87 (d, J = 8.7 Hz, 1H, ligand). UV kmax (MeCN)/nm 460, 436, 369, 324, 287, 235. IR (ATR, mmax/cm1) 1622, 1602, 1447, 835, 761, 729, 699, 556. ESI-TOF (m/z, MeCN) 769.0 ([M]+, calc. 769.8), 385.0 ([M]2+, calc. 384.9).
3. Results and discussion
2.3.6. Dipyrido[3,2-a:20 ,30 -c]phenazine-11-carboxylic acid bis(2,20 -bipyridine) ruthenium(II) hexafluorophosphate (8) Synthesized by adapting a literature procedure [44], using 2 as starting reactant. Yield: 0.142 g (83%). Anal. Calc. for C39H26F12N8O2P2Ru: C, 45.49; H, 2.55; N, 10.88. Found: C, 45.40; H, 2.50; N, 11.15%. 1H NMR (400 MHz, DMSO-d6, 25 °C), d (ppm) 9.66 (td, J1 = 6.7 Hz, J2 = 1.3 Hz, 2H, ligand), 8.97 (s, 1H, ligand), 8.90 (d, J = 8.3 Hz, 2H, bpy), 8.87 (d, J = 8.2 Hz, 2H, bpy), 8.58 (dd, J1 = 6.8 Hz, J2 = 1.7 Hz, 1H, ligand), 8.23 (m, 5H, bpy-ligand), 8.15 (t, J = 7,9 Hz, 2H, bpy), 8.05 (m, 2H, ligand), 7.84 (d, J = 5.6 Hz, 2H, bpy), 7.78 (d, J = 5.6 Hz, 2H, bpy), 7.61 (td, J1 = 5.9 Hz, J2 = 1.1 Hz, 2H, bpy), 7.40 (td, J1 = 6.6 Hz, J2 = 1.8 Hz, 2H, bpy). UV kmax (MeCN)/nm 441, 362, 316, 284, 242. IR (ATR, mmax/cm1) 1622, 1602, 1447, 835, 761, 729, 699, 556. ESI-TOF (m/z, MeCN) 369.8 ([M]2+, calc. 369.9). 2.3.7. Dipyrido[3,2-a:20 ,30 -c]phenazine-11-carbonitrile bis(2,20 -bipyridine) ruthenium(II) hexafluorophosphate (9) Synthesized by adapting a literature procedure [44], using 2 and 3,4-diaminobenzonitrile as starting reactants. Yield: 0.199 g (90%). Anal. Calc. for C39H25F12N9P2Ru: C, 46.35; H, 2.49; N, 12.47. Found: C, 45.19; H, 2.41; N, 12.73%. 1H NMR (400 MHz, DMSO-d6, 25 °C), d (ppm) 9.64 (d, J = 7.9 Hz, 1H, ligand), 9.60 (d, J = 9.1 Hz, 1H, ligand), 9.19 (d, J = 1.2 Hz, 1H, ligand), 8.89 (t, J = 9.7 Hz, 4H, bpy), 8.68 (d, J = 8.8 Hz, 1H, ligand), 8.47 (dd, J1 = 8.8 Hz, J2 = 1.6 Hz, 1H, ligand), 8.28 (d, J = 5.1 Hz, 2H, ligand), 8.24 (t, J = 7.9 Hz, 2H, bpy), 8.15 (t, J = 7.5 Hz, 2H, bpy), 8.06 (m, 2H, ligand), 7.83 (d, J = 5.2 Hz, 2H, bpy), 7.77 (d, J = 5.6 Hz, 2H, bpy), 7.61 (t, J = 6,5 Hz, 2H, bpy), 7.40 (t, J = 6.5 Hz, 2H, bpy). UV kmax (MeCN)/nm 441, 362, 276, 246. IR (ATR, mmax/cm1) 2225, 1446, 1357, 1118, 833, 762, 729, 557, 417. ESI-TOF (m/z, MeCN) 360.6 ([M]2+, calc. 360.4). 2.3.8. Pyrazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile bis(2,20 -bipyridine) ruthenium(II) hexafluorophosphate (10) Synthesized by adapting a literature procedure [44], using 2 and diaminomaleonitrile as starting reactants. Yield: 0.157 g (73%). Anal. Calc. for C34H22F12N10P2Ru: C, 42.47; H, 2.31; N, 14.57. Found:
3.1. Synthesis Complexes of the general formula [M(bpy)2(L)]2+, where M is a metal center and L a poly-aromatic N-heterocyclic ligand, are generally prepared by synthesizing the ligand L, and then adding it to the metal center [17,43,45]. This commonly used synthetic pathway displays a large drawback, as the yields are sometimes very low [24]. The main explanation is based on the poor solubility of the ditopic ligand L in organic solvents since it is often large and aromatic. To circumvent this drawback, we have adopted the block by block building strategy. In the first step, we synthesized cis-Ru (bpy)2Cl2 (1) [40]. Then, by adding 1,10-phenantroline-5,6-dione [41], we obtained [Ru(bpy)2(1,10-phenanthroline-5,6-dione)]2+ complex (2) following a well-known protocol [42]. The central aromatic core is obtained by coupling the complex 2 with the appropriate aldehyde [43] or diamine [44], as depicted on Fig. 1. By comparison to the commonly used synthetic route, this pathway allows a 50% improvement of the total yield starting from complex 1. Reaction of 2 with different aldehydes following a Steck-Day reaction [46], in a buffer solution of acetic acid with an excess of ammonium, during 3 h at reflux, leads to a first family of 5-member ring complexes: phenylimidazophenanthrolines (pip, 3 to 7). For these complexes, the phenylimidazole moiety is easy to functionalize. Hydroxyl, carbonitrile and carboxylic acid substituents were chosen to study their influence on the photo-physical behavior of the complexes, depending on their electron-withdrawing or donor characters. As complex 5 is difficult to be obtained from the usual hexafluorophosphate complex 2, the perchlorate form of 2 was used to synthesize it. Condensation of complex 2 with diamines reagents leads to the second family of 6member ring complexes: dipyridophenazines (dppz, 8 and 9) and pyrazinophenanthrolines (pzp, 10). All the resulting complexes were fully characterized by 1H NMR, high resolution mass spectrometry, IR and UV–Vis spectroscopy. A typical 1H NMR spectrum of the studied complexes is given in Fig. 2. Attribution of the signals was achieved using a COSY 1 H–1H experiment (see ESI). The bipyridine protons of all synthesized complexes are not affected by the general structure of the ditopic ligand (protons H6 to H13 on top of Fig. 2). In all cases, the phenanthroline proton H3 of the ditopic ligand is highly deshielded compared to complex 2 due to the vicinity of the nitrogen-containing 5 or 6-member ring. Its signal can be found around 9 ppm for pip-based complexes and 9.5 ppm for dppz and pzp-based complexes. All other signals observed between 7 and 10 ppm are related to the ditopic ligand. The complex 6 displays a particular 1H NMR spectrum in DMSO-d6 at 298 K (on bottom of Fig. 2). The resonances of the protons located around 9.4 ppm and 8.5 ppm are broadened. The same phenomenon is observed in CD3CN precluding any peculiar interaction of the Ru-complex with the deuterated solvent. Evolution of the NMR spectrum was studied at various temperatures between 298 K and 358 K in DMSO-d6 (Fig. 3). An increase of the
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Fig. 1. Synthetic routes to polypyridyl ruthenium species 3 to 10.
Fig. 2. 1H NMR spectrum of complex 5 in DMSO-d6 at 298 K and signals attribution (top) and 1H NMR spectrum of complex 6 in DMSO-d6 at 298 K (bottom).
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Fig. 3. Evolution of the 1H NMR signals of complex 6 in DMSO-d6 in function of the temperature.
temperature induces an improvement of the resolution of the spectrum. Some resonances have split and assignment can be achieved using a 2D COSY 1H–1H NMR experiment (Fig. 4). The most deshielded and broadened signals observed on the spectrum at 298 K are attributed to the protons H1 and H5 located near the imidazole fragment. This phenomenon can be explained by the interaction of a water molecule in the cavity formed by these two protons, the imidazole and the carboxylic acid groups (Fig. 5a). At high temperature, the exchange rate of the water molecule between the complex and the medium is enhanced, resulting in a better resolution of the 1H NMR spectrum of the complex 6. This phenomenon is not observed for complex 7 although it presents a similar structure to complex 6. As represented on Fig. 5b, the phenol group in complex 7 interacts with the carboxylic acid, leading to a conformation avoiding peculiar interaction with a water molecule.
The mass spectra of the ruthenium complexes are similar to one another and exhibit the expected molecular ions (see ESI), except for complex 6 that will be discussed later. The observed peaks fit to the simulated isotopic patterns. Complex 5 presents a typical spectrum (Fig. 6), where the highest signal at 377.07 refers to the molecular ion [Ru(bpy)2(L)]2+, while a second signal at 753.14 corresponds to the deprotonated [Ru(bpy)2(L)]2+ ion (except for the dppz and pzp-based complexes that cannot be easily deprotonated). Two weak signals at 341.11 and 853.10 are also observed and can be attributed respectively to the protonated ditopic ligand [LH]+ and the [Ru(bpy)2(L)][ClO4]+ ion. The complex 6 presents a specific behavior. Although its purity was established by NMR analysis (see full spectrum in ESI), the mass spectrum of this complex presents a lot of peaks provided by fragmentation/recombination mechanisms (see Fig. 7 and ESI). Considering their isotopic patterns, some of the signals at 341.31, 415.21, 663.45 and
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5
1
11
12
10 2
13 8 4
7
6
15
9
3
14
Fig. 4. COSY 1H–1H NMR spectrum of complex 6 in DMSO-d6 at 358 K.
(a)
(b)
Fig. 5. Proposed structures of complexes 6 (a) and 7 (b) in wet solvents.
963.46 can be attributed to organic fragments. The peaks at 359.32 and 1061.94 on the mass spectrum correspond respectively to the protonated [LH-H2O]+ and to the {[RuIII(bpy)2(L-H2O)][PF6]2}2+ 2 ions. The peak at 359.32 confirms a possible interaction between the ditopic ligand and a water molecule. The peak at 1061.94 highlights a specific behavior of complex 6 which is able to dimerize in presence of water molecules as represented on the Fig. 7. This result is in agreement with the behavior observed in NMR and previously described. 3.2. Electrochemistry Cyclic voltammograms were recorded at room temperature, in dry and degassed acetonitrile in the presence of 0.02 M of tetrabutylammonium hexafluorophosphate (or perchlorate for complex 5) as electrolyte support. Measurements were made with a glassy carbon working electrode, at 100 mV/s. The results for the synthesized compounds are compiled in Table 1, and they are compared
to [Ru(bpy)3]2+. As examples, voltammograms of the pip-based complex 4 and the dppz-based complex 9 are presented in Fig. 8. Voltammograms of the other complexes are reported in ESI. A quasi-reversible process corresponding to RuIII/RuII couple stands between 1030 and 1125 mV versus non aqueous Ag/AgCl, a typical trend for this kind of compounds [47–49]. All the studied complexes present a RuIII/RuII couple oxidizing at slightly higher potentials than [Ru(bpy)3]2+, a tendency usually observed for all compounds containing higher p-acceptor ligands than bipyridines. Close to the RuIII/RuII signal, various irreversible waves were observed, attributed to the electroactive part of the ditopic ligand. Between 0.5 V and 2.5 V, complex processes, located on the bipyridines and the ditopic ligands, appear as several reversible and irreversible waves. Complexes of the pip ligands display similar electrochemical behaviors. In addition to the quasi-reversible process assigned to the ruthenium couple, more complex processes attributable to oxidation of the organic part of the ditopic ligand can be observed.
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Fig. 6. ESI-TOF mass spectrum of complex 5 (left) and enlargement of the principal signal (right).
First, the enlargement of the RuIII/RuII wave (at 1053 mV for complex 4) is due to a second process taking place at such potential (Fig. 8). Studies made on similar compounds have showed that this high potential process corresponds to the oxidation of the imidazole part of the ditopic ligand [16]. Other oxidation processes can be observed at lower potentials than ruthenium oxidation for all pip-based complexes. By acquisition of the oxidation part of the voltammogram (see ESI), the first processes (113 mV and 475 mV for complex 4) disappear. They can be associated to reoxidation of electro-generated species. However, the last irreversible process (at 709 mV for complex 4) is always observed and is attributed to the oxidation of the distal phenyl group. This is confirmed by some studies made on similar compounds, without this phenyl fragment, which do not show any process at such potential [48]. The potential of this process is mostly linked to the nature of the substituents: a cathodic shift is observed for electron-donating hydroxyl substituents (389 mV for complex 3), while higher potentials are obtained for complexes 4 to 7, between 657 and 799 mV, due to the with-drawing effect of carbonyl and carbonitrile fragments.
In the negative part of the voltammogram, by comparing the results obtained for complexes 3 to 7 with those obtained for the complex 2, it can be observed the presence of two waves between 1690 and 2011 mV, attributed to the reduction of bipyridine fragments (1713 and 1933 mV for complex 4). The reversible wave observed at higher negative potential (2350 mV for complex 4) corresponds to the phenanthroline part of the pip ligand. Another process takes place at less negative potentials (-1470 mV for complex 4), and might be associated with localized reduction processes on the ditopic ligand, as it possesses the most stable LUMO [48]. The weak wave at 519 mV for complex 4 is only observable after a first oxidation cycle. No more precision can be brought on the exact nature of these irreversible phenomena present for all 5-member ring complexes. Cyclic voltammograms of dppz and pzp complexes share some features with the pip analogs. In the positive part of the voltammogram of complexes 8 to 10, the reversible wave around 1V is also attributed to the RuIII/RuII couple. No additional signal close to the RuIII/RuII couple from the ditopic ligand is observed (Fig. 8). The second irreversible oxidation wave, only observable for
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33
Fig. 7. ESI-TOF mass spectrum of complex 6 and supposed structure of the dimer ion.
Table 1 Redox potentials of [Ru(bpy)3]2+ and complexes 2 to 10 in acetonitrile vs. non aqueous Ag/AgCl. Complex
Positive E1/2 (mV)
Negative E1/2 (mV)
[Ru(bpy)3]2+ 2 3 4 5 6 7 8 9 10
1010 1125 1030; 1053; 1036; 1052; 1098; 1044; 1048; 1091
1620; 1825; 2075 430; 1130; 1765; 1995 457; 956; 1720; 1950 519; 1470; 1713; 1933 1524; 1690; 1924 361; 1521; 1716; 1845 616; 913; 1725; 1941 1303; 1762; 1967 1026; 1704; 1827; 2011 452; 868; 1726; 1851; 1981
389 709 690 971; 799 657 410 719
complexes 8 (410 mV) and 9 (719 mV) after a first reduction cycle (see ESI), might be related to the distal phenyl group, as this process is missing for complex 10. As for pip ligands family, explanation of the negative part of the voltammogram is complicated due to the combination of several redox processes. As previously mentioned, by comparison with [Ru(bpy)3]2+, the waves observed between 1762 and 2011 mV can be attributed to bipyridines reduction processes, and the wave around 2400 mV to the reduction of the phenanthroline part. Two other waves appear at less negative potentials (1026 and 1704 mV for 9 as example). Those reduction processes are attributed to the first and second reduction of the phenazine fragment [47]. The influence of the distal substituents is obvious, as a cathodic shift is observed according
to their electron-withdrawing power. The two carbonitrile groups of complex 10 make the fragment easily reducible (452 and 868 mV), while the alone carbonyl group of complex 8 makes the second reduction take place at the same potential as the first bipyridine reduction (1762 mV). 3.3. Spectroscopic properties UV–Vis spectroscopy was conducted at room temperature in acetonitrile and the results are reported in Table 2. TD-DFT modelling has been performed to simulate the absorption spectra in order to determine the character of the different electronic transitions. The calculated spectra match qualitatively with the experimental one (Fig. 9 for complexes 5 and 8 and in ESI for other complexes). UV–Vis absorption bands are easily assigned by comparison with [Ru(bpy)3]2+ [50]. The intense band around 285 nm is usually attributed to a bipyridines p ? p⁄ electronic transition. The region between 300 and 400 nm has been deconvoluted and consists of different electronic transitions of medium intensity, corresponding to a mix between ligand-to-ligand charge transfer (LLCT), metal-to-ligand charge transfer (MLCT) and metal–ligand-toligand charge transfer (MLLCT) transitions. The two intense bands located between 410 and 470 nm are attributed to two MLCT transitions, generating the well-known luminescent properties of the ruthenium polypyridyl compounds. This transition corresponds to the electron transfer from the HOMO localized on the metal center to the LUMO shared between the surrounding ligands
34
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Fig. 8. Cyclic voltammograms (2nd cycle) of complexes 4 (top) and 9 (bottom) in acetonitrile at room temperature. v = 100 mV/s. [NBu4][PF6] = 0.02 M.
Table 2 Absorption wavelength (nm) of [Ru(bpy)3]2+ and complexes 2 to 10 in acetonitrile at room temperature. Complex
LLCT
LLCT/MLLCT/MLCT
MLCT
MLCT
[Ru(bpy)3]2+ 2 3 4 5 6 7 8 9 10
287 285 286 287 286 286 287 284 277 263; 288
– 305; 327; 325; 327; 324; 326; 316: 363; 346
424; 413; 430; 427; 428; 431; 428; 424; 437; 423;
– – – – – – – 546 551 524
350 369 345; 374 346 361 370 356; 375 387
453 440 464 461 460 462 463 459 468 458
consisting of overlapping Ru(dp) ? ditopic ligand (p⁄) and Ru (dp) ? bpy(p⁄) transitions. The appearance of broad MLCT bands results of the lowered symmetry that removes the degeneracy of the p⁄ levels. Complexes of the dppz and pzp ligands display another weak band after 500 nm, confirmed by theoretical
investigations, already reported [2] for [Ru(bpy)3]2+ at low temperature and corresponding to a MLCT transition. Spectrofluorimetry experiments were performed at room temperature, in degassed acetonitrile. All compounds were excited at 452 nm to record their emission spectra and excited state lifetime (pulse = 1.2 ns). All data are available in Table 3. The studied compounds show a broad emission band between 617 and 646 nm (Fig. 10). No trend stands out of any influence of the ligand structure on the emission wavelength. Excited state lifetime measured for [Ru(bpy)3]2+ matches with the values found in the literature [51]. Lifetimes of both [Ru (bpy)2(phen)]2+ [9] and complex 2 show that replacing one of the bipyridine ligand by a phenanthroline fragment decrease the lifetime of the complex. This effect is improved by the presence of the two carbonyl groups of the ortho-quinone ligand. For all complexes, except for complexes 5 and 9 which present particular behaviors, shorter lifetimes than [Ru(bpy)2(phen)]2+ were measured. The extension of the ditopic ligand contributes to decrease the excited lifetime of the ruthenium species. The
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Fig. 9. Experimental absorption spectra of complexes 5 and 8 in acetonitrile at room temperature (o), and the corresponding TD-DFT simulated spectra (full line).
Table 3 Spectroscopic data about emission of [Ru(bpy)3]2+ and complexes 2 to 10 in acetonitrile at room temperature. Complex
kem (nm)
s (ns)
[Ru(bpy)3]2+ [Ru(bpy)2(phen)]2+ [9] 2 3 4 5 6 7 8 9 10
611 610 675 642 627 617 628 627 632 640 646
871 798 473 787 534 1138 714 612 650 <20 538
aromatic core of the ditopic ligand (5-member or 6-member rings) does not have any significant effect on the lifetime. This trend is confirmed by the comparison of the excited state lifetimes of pip compounds (3, 4, 6 and 7) and the dppz and pzp ones (8 and 10), which are close to one another. However, a few trends appear related to the pending substituents on the distal part of the ligand. Insertion of vibrator fragments, as carbonitrile or carboxylic acid group, lowers slightly the excited state lifetime of the complexes. For complexes 5 and 9, an unusual behavior is observed. Complex 5 shows the higher lifetime, whereas complex 9 presents no luminescence in acetonitrile as already described [29] but also in degassed ethanol, DMF, pyridine or DMSO.
The excited state lifetime and emission wavelengths of complexes 4, 5 and 6 in selected degassed solvents are summarized in Table 4. No relationships can be evidenced between the photophysical properties of the complexes and basic physicochemical properties of the solvents (dielectric constant and dipole moment). Nevertheless, some measurements of the excited state lifetime of the complexes allow considering an interaction between the molecular structures of the solvents and the ditopic ligands. For complexes 4 and 5, in solvents sharing a vibrating group able to interact with the complex, the lifetime drops. The carbonitrilebased complex 4 quasi doubles its lifetime in ethanol (s = 941 ns) compared to acetonitrile (s = 534 ns), while the lifetime of the carboxylic-based complex 5 is strongly decreased in ethanol (s = 701 ns, compared to 1138 ns in acetonitrile). In a solvent sharing no interacting moiety with complexes 4 and 5 (i.e. DMF, pyridine and DMSO), values of their excited states lifetimes are closer. Some studies already exist about the solvent effects [51], but it is the first time that a specific intermolecular interaction between solvents and substituents based on similar moieties in their structure is exposed. An important impact on the excited state lifetime is observed for complex 6 when moving from dried to wet DMF. In wet solvents, as previously explained, the water molecules act as conformation blockers through the formation of a dimer, constraining the carbonyl orientation in a defined position and avoiding the desexcitation by vibrations. In dried solvents, no water molecule is present in the structure decreasing the excited state lifetime of
Fig. 10. Emission spectra of complex 4 after excitation at 452 nm during 1.2 ns in degassed acetonitrile at room temperature (left) and fluorescence decay at 627 nm (right).
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Table 4 Excited-state lifetimes (ns) of complexes 4 to 6 in different solvents at room temperature (emission wavelengths (nm) in brackets). Complex
Acetonitrile
Ethanol
Pyridine
DMSO
DMF
Dried DMF
4
534 (627) 1138 (617) 714 (628)
941 (623) 701 (611) 648 (613)
1224 (622) 1083 (628) 801 (637)
1104 (642) 985 (639) 961 (639)
882 (640) 875 (641) 794 (636)
822 (637) 769 (640) 464 (638)
5 6
Table 5 Calculated energy differences between complex 6 without water and with water in two different conformations.
DG (kcal/mol)
Structure
0.00
1.2
0.8
the complex 6 by vibrations. The conformation blocker behavior of the water molecules counteracts the solvent-ligand vibrational interaction previously described. It results in a slight decay of the excited state lifetime of the complex 6 in ethanol, which should have dropped due to the vibrational interaction between the distal carboxylic acid of the ligand and the solvent.
3.4. DFT calculations We have performed DFT modelling to describe in a more accurate way the observed physicochemical properties. All the
computed molecular orbitals are presented in the Supporting information. For complex 6, to confirm the ability of the ditopic ligand to interact with a water molecule, the energy for two potentials interactions of the complex with a water molecule were calculated and compared with the water-free compound (Table 5). The comparison of the calculated structures and conformations shows an energy difference in the range of ±2 kcal/mol, in agreement with the NMR temperature dependent experiments (Fig. 3) and confirming the interaction of a water molecule with the complex 6. Moreover, a stable structure integrating a water molecule trapped between the imidazole and carbonyl fragments of the ditopic ligand can be calculated. Association of two blocks supports the formation of a dimer through hydrogen bounds, confirming the results obtained by mass spectrometry. Concerning the computed molecular orbitals, the complex 3 displays a very unusual behavior compared to the other complexes. Its HOMO is highly localized on the ditopic ligand, while the other complexes HOMOs are mostly localized on the ruthenium center (Table 6). The presence of two electron-donating hydroxyl groups on the distal part of the ligand can explain the unique composition of the complex 3 HOMO. For the pip family complexes 4 to 7, the complexes can be separated in two groups: compounds integrating linear ligands (4 and 5), and bent ones (6 and 7). For complexes 4 and 5, LUMO and LUMO+1 are delocalized on the bipyridines and the whole ditopic ligand; for complexes 6 and 7, the same orbitals are localized also on the bipyridine and only on the phenanthroline fragment. This significant difference between 5-member ring linear and bent complexes, based on their structure, is illustrated on Table 6. This full delocalization, highlighted only for linear pip-based complexes, matches with the high excited state lifetimes observed for both complexes 4 (s = 941 ns in ethanol) and 5 (s = 1138 ns in acetonitrile), when they are measured in the appropriate solvent. Complexes of the dppz and pzp ligands (8 to 10) exhibit a different behavior. The LUMOs of the complexes are only localized on the ditopic ligands, compared to the behavior previously described for the 5-member ring complexes 3 to 7 (see ESI). This phenomenon is in agreement with the electrochemical results which have demonstrated that the 6-member ring phenazine moiety is the most easily reducible fragment. Complex 10 has the same behavior as 8 and 9, but its LUMO+1 is also fully localized on the ditopic ligand due to the presence of the two electronwithdrawing carbonitrile groups on the distal part of the ligand. This particular behavior of the 6-member ring complexes could lead to a better efficiency of the electron transfer towards a second redox active metal localized on the distal coordinating groups.
Table 6 Representations of HOMO, LUMO and LUMO+1 of complexes 3 to 6. Orbitals LUMO+1
LUMO
HOMO
Complex 3
Complex 4
Complex 5
Complex 6
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4. Conclusion
Appendix A. Supplementary material
We have studied and compared the physical–chemical behavior of a series of eight ruthenium(II) polypyridyl complexes, representative of the variety of structures and pending substituents usually encountered in the electron transfer studies. We have highlighted a slight effect of the aromatic core (5 or 6-member ring) on the electrochemical behavior and the UV–Vis absorption spectra. Complexes of the 6-member ring family (dppz and pzp) are more easily reducible than pip-based complexes and they show an additional electronic transition band around 500 nm attributed to an MLCT transition. For all the studied complexes, the general composition of the ditopic ligands has a huge impact on the electronic delocalization and on the lifetime of the excited states. For pipbased complexes, high excited state lifetime and LUMO’s delocalization on the whole ditopic ligand are only obtained with linear structures. DFT calculations have supported that the electronic transfer is spread over the bipyridines and the ditopic ligand, resulting in a potential efficiency loss. For dppz and pzp-based complexes, lower excited state lifetime were measured but according to the DFT modelling, the electronic transfer takes place only towards the ditopic ligand. The solvent effect on the luminescence properties is strongly related to the nature of the distal pending substituents: if their structures present interacting fragments, non-radiative desexcitations are enhanced through a vibrational coupling that decreases the excited state lifetime. The presence of supramolecular interactions due to the structure of the ditopic ligand, in complex 6, shows an additional dynamic effect on the photo-physical properties. All the results obtained in this study highlight not only the impact of the structure–properties relationship of the ruthenium complexes, but also the effect of the specific interactions between the solvent and ruthenium compounds through a vibrational coupling, on the electrochemical and photo-physical properties of the Ru-systems. Based on this work, complexes 5 and 8 present potential photo-physical properties for an efficient charge separation. In acetonitrile, those two complexes, containing a carboxylic acid group in distal position, exhibit high excited state lifetimes and delocalized LUMO on the ditopic ligand (which is more easily reduced than the bipyridines). Moreover, the photo-physical features of the studied Ru-complexes in various solvents allow envisaging electron-transfer studies in other media than acetonitrile which is generally used.
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2015.10.018.
Acknowledgments The authors thank Sandra Maynadié for carrying out the high temperature and 2D NMR experiments, and the Laboratoire de Mesures Physiques of the Max Mousseron technical platform, at Université de Montpellier for carrying out the mass spectrometry. We thank RSTB project of CEA/DEN/DISN for financial supporting.
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Influence of the solvent, structure and substituents of ruthenium(II) polypyridyl complexes on their electrochemical and photo-physical properties Kévin Ruffray, Matthieu Autillo, Xavier Le Goff, Jérôme Maynadié ⇑, Daniel Meyer Institut de Chimie Séparative de Marcoule, UMR 5257 CEA/CNRS/UM/ENSCM, BP 17171, 30207 Bagnols-sur-Cèze CEDEX, France
a r t i c l e
i n f o
Article history: Received 3 September 2015 Received in revised form 20 October 2015 Accepted 22 October 2015 Available online 26 October 2015 Keywords: Ruthenium complexes Photo-physical properties Electron transfer Electrochemistry
a b s t r a c t This work highlights the relative importance between structural issues, longer range interactions and solvent effects for a series of ruthenium compounds on their electrochemical and photochemical properties. To study these effects, eight complexes of general formula [Ru(bpy)2(L)]2+ have been synthesized and characterized, where L represents substituted phenyl-imidazophenanthrolines (pip), dipyridophenazines (dppz) and pyrazinophenanthrolines (pzp). This experimental work supported by an important theoretical investigation has pointed out that the electrochemical and the photo-physical behavior of these types of molecular systems depends mainly of the close environment around the photosensitizer. A relation between the ditopic ligand structure (aromatic central part and/or the distal substituents) and the absorption properties have been observed, along with the lower reduction potential of dipyridophenazine and pyrazinophenanthroline moieties. This study points out the influence of the nature of the distal substituents, as well as the significant impact of the solvent nature and the interaction with water molecules, on the photo-physical properties of these ruthenium compounds. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Ruthenium polypyridyl compounds were first discovered almost 80 years ago [1], but they are still sources of new trails in coordination chemistry. The most simple and representative complex of this family, [Ru(bpy)3]2+, is useful in almost every field in photochemistry, due to its chemical stability and its unique photo-physical properties [2]. Polypyridyl ruthenium(II) complexes are employed in various research fields, including biodiagnostics [3], photovoltaics [4], organic LED [5]. The most recognized ones are the manufacturing of optical chemical sensors [6], and electron and energy-transfer molecular devices [7], primarily due to the metal-to-ligand charge transfer (MLCT) transition exhibited by these kind of complexes [8]. By irradiating these species at the appropriate wavelength, a long-lived excited state (near 600 ns to a few ls) [9] is produced. This excited species is well described as a ruthenium(III) ion and a one-electron reduced polypyridyl ligand [10]. This transient state could be engaged in electron transfer reactions resulting in a ruthenium(III) [11] or a ruthenium(I) [12] reactant, through an intermolecular pathway using a sacrificial electron acceptor or donor. ⇑ Corresponding author. Tel.: +33 4 66 79 13 80. E-mail address: [email protected] (J. Maynadié). http://dx.doi.org/10.1016/j.ica.2015.10.018 0020-1693/Ó 2015 Elsevier B.V. All rights reserved.
The ability to transfer the photo-excited electron to a second metal complex have promoted the [Ru(bpy)3]2+ analogues as very good compounds for photo-induced electron-transfer studies. To improve the efficiency of such systems, new complexes have been developed where the chromophore is covalently linked to an active metallic redox center. These bimetallic species are potentially able to increase the rate of light conversion into chemical energy by minimizing the diffusion phenomenon, usually encountered during intermolecular reactions. Studies are mainly focused on the influence of the size and the topology of the organic spacers on the electronic transfer between the chromophore and the active site [13–15]. In this field, catalytic enhancement has been highlighted on bimetallic polypyridyl Ru complexes based on substituted imidazophenanthroline [16–21]. This work is focused on a new synthetic pathway to obtain the ruthenium complexes [22], adding block by block the constituents to control the complexation of the different metal centers on the ditopic ligand. After studying a new metallo-assembly based on ruthenium and iron, the authors have opened a new discussion on the relation between the structure of the ditopic ligand and the catalytic activity of the polymetallic species [23]. In order to develop efficient heterometallic systems for charge separation, Ru-species incorporating substituted imidazophenanthroline ligands were studied and compared to other p-delocalized
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extended systems. The structure of the ditopic ligands can be divided into 3 parts: a phenanthroline moiety to coordinate the ruthenium, an aromatic core and distal coordinating groups. Phenylimidazole, quinoleine, and pyrazine moieties were examined as aromatic core, and carbonitrile, hydroxyl and/or carboxylic acid moieties as distal coordinating groups. While some of the studied Ru-complexes have already been singly reported for electronic transfer or DNA intercalation [24–30], it was necessary to acquire more fundamental information on the relation between their physicochemical properties and different structural aspects in the frame of photo-chemical energy storage. These data are also useful before moving towards the understanding of more complex heterometallic molecular systems. In this work, the ability of a photo-induced electron-transfer between the ruthenium center and the different ditopic ligands has been analyzed using various techniques including electrochemistry and spectrophotometry. A special attention was paid to longer range interactions such as the solvent and the intermolecular interactions on the photophysical properties of these systems. 2. Experimental 2.1. General 1
H NMR spectra were obtained using a Bruker AvanceIII 400 spectrometer, using TMS as the internal standard. Chemical shifts are expressed in ppm in relation to the deuterated solvent signal, and coupling constants are expressed in hertz. Elemental (C, H, N) analyses were performed on a Flash EA1112 (ThermoFinnigan 2003) instrument. Mass spectra were realized in Laboratoire de Mesures Physiques of the Max Mousseron technical platform, at Université de Montpellier. HR-ESI-TOF was recorded on an Alliance 2790 Waters spectrometer. UV–Vis absorption spectra were recorded on a Shimadzu UV-3600 UV–Vis-NIR spectrometer. IR spectra were recorded at solid state, using the ATR module of a Perkin–Elmer Spectrum100 FT-IR spectrometer. Electrochemical studies were realized using a Biologic SP300 apparatus. Electrochemical analysis cell includes a three-electrode assembly: a non-aqueous Ag/AgCl reference electrode, a platinum counter-electrode, and a glassy carbon working electrode. Electrochemical behaviors were studied in degassed acetonitrile, using tetrabutylammonium salts as support electrolytes. Complexes were studied at a concentration near 0.5 mM at 100 mV/s. Spectrofluorimetric properties were recorded on a Horiba FluoroMax-4 apparatus. The emission spectra were realized after excitation at 452 nm, and detected at 90° in relation to incident beam. Lifetime measurements were recorded after a 1.2 ns excitation at 452 nm, and detected at 90° in relation to incident beam. Caution! Perchlorate salts of metal complexes are potentially explosive. Only small quantities of material should be prepared and the samples should be handled with care. 2.2. Computational details Full geometry optimization and frequency computations were performed applying the DFT method with Becke’s three parameter hybrid functional [31] and Lee–Yang–Parr’s gradient-corrected correlation functional [32] (B3LYP). LANL2DZ basis set [33] were used for ligands and complexes (Ru, C, H, N, O), and assuming the singlet state for the ground state of all calculations. The structures of 3, 4, 5, 6, 7, 8, 9 and 10 were fully optimized. 160 singlet excitation energies were calculated with nonequilibrium TDDFT [34–36] method at the RB3LYP/LANL2DZ level considering solvent effect with polarizable continuum model (PCM) [37]. All computations were performed with Gaussian G09 quantum chemistry
27
program-package [38]. UV–Vis spectrum was interpreted by using GaussSum 3.0 [39]. 2.3. Synthesis All experimental materials were of AR grade, and used as purchased without purification. Water was purified to Milli-Q grade, and all other solvents used as purchased without purification if not specified. cis-Ru(bpy)2Cl2 (1) [40], 1,10-phenanthroline-5, 6-dione [41] and [Ru(bpy)2(phendione)][X]2 (X = PF6 or ClO4) (2) [42] were synthesized as previously described. 2.3.1. 4-(1H-Imidazo[4,5-f][1,10]phenanthroline-2-yl)benzene-1,2diol bis(2,20 -bipyridine) ruthenium(II) hexafluorophosphate (3) Synthesized by adapting a literature procedure [43], using 2 and 3,4-dihydroxybenzaldehyde as starting reactants. Yield: 0.277 mg (66%). Anal. Calc. for C39H28F12N8O2P2Ru: C, 45.40; H, 2.74; N, 10.86. Found: C, 45.20; H, 2.67; N, 10.89%. 1H NMR (400 MHz, DMSO-d6, 25 °C), d (ppm) 14.06 (s, 1H, N-H), 9.08 (s, 2H, ligand), 8.87 (dd, J1 = 15.5 Hz, J2 = 8.2 Hz, 4H, bpy), 8.22 (td, J1 = 8.0 Hz, J2 = 1.4 Hz, 2H, bpy), 8.13 (td, J1 = 8.0 Hz, J2 = 1.4 Hz, 2H, bpy), 8.05 (d, J = 5.0 Hz, 2H, ligand), 7.92 (t, J = 5.0 Hz, 2H, ligand), 7.85 (d, J = 5.0 Hz, 2H, bpy), 7.76 (d, J = 2.1 Hz, 1H, ligand), 7.61 (m, 5H, bpy-ligand), 7.35 (t, J = 6.0 Hz, 2H, bpy), 6.99 (d, J = 8.2 Hz, 1H, ligand). UV kmax (MeCN)/nm 457, 433, 365, 321, 287, 240. IR (ATR, mmax/cm1) 1602, 1446, 1101, 831, 759, 724, 556. ESI-TOF (m/z, MeCN) 886.9 ([M-PF6]+, calc. 886.7), 741.0 ([M]+, calc. 741.8), 371.0 ([M]2+, calc. 370.9). 2.3.2. 4-(1H-Imidazo[4,5-f][1,10]phenanthrolin-2-yl)benzonitrile bis (2,20 -bipyridine) ruthenium(II) hexafluorophosphate (4) Synthesized by adapting a literature procedure [43], using 2 and 4-formylbenzonitrile as starting reactants. Yield: 0.175 mg (77%). Anal. Calc. for C40H27F12N9P2Ru: C, 46.89; H, 2.66; N, 12.30. Found: C, 46.41; H, 2.50; N, 12.14%. 1H NMR (400 MHz, DMSO-d6, 25 °C), d (ppm) 8.94 (dd, J1 = 8.2 Hz, J2 = 1.4 Hz, 2H, ligand), 8.88 (d, J = 8.2 Hz, 2H, bpy), 8.85 (d, J = 8.2 Hz, 2H, bpy), 8.53 (d, J = 8.6 Hz, 2H, ligand), 8.21 (td, J1 = 7.9 Hz, J2 = 1.2 Hz, 2H, bpy), 8.10 (td, J1 = 7.9 Hz, J2 = 1.2 Hz, 2H, bpy), 7.85 (m, 4H, bpy-ligand), 7.75 (m, 4H, ligand), 7.57 (m, 4H, bpy), 7.37 (td, J1 = 6.7 Hz, J2 = 1.0 Hz, 2H, bpy). UV kmax (MeCN)/nm 456, 429, 328, 287, 240. IR (ATR, mmax/cm1) 2225, 1604, 1464, 1446, 1423, 1098, 823, 757, 725, 554, 463, 419. ESI-TOF (m/z, MeCN) 734.2 ([M]+, calc. 734.8) 367.6 ([M]2+, calc. 367.4). 2.3.3. 4-(1H-Imidazo[4,5-f][1,10]phenanthrolin-2-yl)benzoic acid bis (2,20 -bipyridine) ruthenium(II) perchlorate (5) Synthesized by adapting a literature procedure [43], using the perchlorate salt of 2 and 4-carboxybenzaldehyde as starting reactants. Yield: 0.708 g (91%). Anal. Calc. for C40H28Cl2N8O10Ru: C, 50.43; H, 2.96; N, 11.76. Found: C, 50.24; H, 2.87; N, 11.62%. 1H NMR (400 MHz, DMSO-d6, 25 °C), d (ppm) 8.98 (d, J = 7.7 Hz, 2H, ligand), 8.87 (d, J = 8.0 Hz, 2H, bpy), 8.83 (d, J = 8.2 Hz, 2H, bpy), 8.41 (d, J = 8.2 Hz, 2H, ligand), 8.20 (t, J = 7.5 Hz, 2H, bpy), 8.09 (t, J = 7.7 Hz, 2H, bpy), 7.87 (m, 4H, bpy-ligand), 7.75 (m, 4H, ligand), 7.58 (m, 4H, bpy), 7.37 (t, J = 6.3 Hz, 2H, bpy). UV kmax (MeCN)/nm 456, 429, 326, 287, 240. IR (ATR, mmax/cm1) 1601, 1445, 1370, 1067, 806, 761, 722, 620, 421. ESI-TOF (m/z, MeCN) 753.14 ([M]+, calc. 753.79), 377.07 ([M]2+, calc. 376.90), 341.11 ([ligand-H]+, calc. 341.35). 2.3.4. 3-(1H-Imidazo[4,5-f][1,10]phenanthrolin-2-yl)benzoic acid bis (2,20 -bipyridine) ruthenium(II) hexafluorophosphate (6) Synthesized by adapting a literature procedure [43], using 2 and 3-carboxybenzaldehyde as starting reactants. Yield: 0.157 g (68%). Anal. Calc. for C40H28F12N8O2P2Ru: C, 46.03; H, 2.70; N, 10.74.
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Found: C, 45.64; H, 2.71; N, 10.63%. 1H NMR (400 MHz, DMSO-d6, 25 °C), d (ppm) 9.52 (s, 2H, ligand), 8.89 (d, J = 8.2 Hz, 2H, bpy), 8.85 (d, J = 8.2 Hz, 2H, bpy), 8.49 (d, J = 7.1 Hz, 1H, ligand), 8.22 (t, J = 7.7 Hz, 2H, bpy), 8.11 (t, J = 7.5 Hz, 2H, bpy), 7.99 (m, 3H, ligand), 7.87 (m, 4H, bpy-ligand), 7.58 (m, 5H, bpy-ligand), 7.38 (t, J = 6.3 Hz, 2H, bpy). UV kmax (MeCN)/nm 459, 432, 361, 318, 287, 238. IR (ATR, mmax/cm1) 1604, 1562, 1510, 1447, 1367, 837, 763, 724, 556. ESI-TOF (m/z, MeCN) 1061.94 ([M-H2O]+, calc. 1061.71), 359.32 ([ligand-H-H2O]+, calc. 359.37), 341.31 ([ligandH]+, calc. 341.35).
C, 42.63; H, 2.34; N, 14.51%. 1H NMR (400 MHz, DMSO-d6, 25 °C), d (ppm) 9.52 (d, J = 8.9 Hz, 2H, ligand), 8.87 (m, 4H, bpy), 8.38 (d, J = 4.8 Hz, 2H, ligand), 8.23 (t, J = 7.5 Hz, 2H, bpy), 8.14 (t, J = 8.8 Hz, 2H, bpy), 8.08 (dd, J1 = 8.1 Hz, J2 = 5.7 Hz, 2H, ligand), 7.80 (d, J = 5.1 Hz, 2H, bpy), 7.71 (d, J = 5.0 Hz, 2H, bpy), 7.60 (t, J = 6.5 Hz, 2H, bpy), 7.37 (t, J = 6.6 Hz, 2H, bpy). UV kmax (MeCN)/ nm 441, 330, 285, 265, 232. IR (ATR, mmax/cm1) 2211, 1605, 1555, 1446, 1374, 1127, 830, 761, 730, 556, 440. ESI-TOF (m/z, MeCN) 348.1 ([M]2+, calc. 347.9).
2.3.5. 2-Hydroxy-5-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl) benzoic acid bis(2,20 -bipyridine) ruthenium(II) hexafluorophosphate (7) Synthesized by adapting a literature procedure [43], using 2 and 5-formylsalicylic acid as starting reactants. Yield: 0.327 g (76%). Anal. Calc. for C40H28F12N8O3P2Ru: C, 45.34; H, 2.66; N, 10.57. Found: C, 45.48; H, 2.77; N, 10.45%. 1H NMR (400 MHz, DMSO-d6, 25 °C), d (ppm) 14.35 (s, 1H, N-H), 9.23 (d, J = 7.3 Hz, 1H, ligand), 9.12 (d, J = 8.4 Hz, 1H, ligand), 8.89 (d, J = 8.2 Hz, 2H, bpy), 8.85 (d, J = 8.2 Hz, 2H, bpy), 8.77 (s, 1H, ligand), 8.21 (m, 3H, bpyligand), 8.11 (t, J = 7.8 Hz, 2H, bpy), 8.03 (m, 2H, ligand), 7.90 (m, 4H, bpy-ligand), 7.61 (m, 4H, bpy), 7.36 (t, J = 7.2 Hz, 2H, bpy), 6.87 (d, J = 8.7 Hz, 1H, ligand). UV kmax (MeCN)/nm 460, 436, 369, 324, 287, 235. IR (ATR, mmax/cm1) 1622, 1602, 1447, 835, 761, 729, 699, 556. ESI-TOF (m/z, MeCN) 769.0 ([M]+, calc. 769.8), 385.0 ([M]2+, calc. 384.9).
3. Results and discussion
2.3.6. Dipyrido[3,2-a:20 ,30 -c]phenazine-11-carboxylic acid bis(2,20 -bipyridine) ruthenium(II) hexafluorophosphate (8) Synthesized by adapting a literature procedure [44], using 2 as starting reactant. Yield: 0.142 g (83%). Anal. Calc. for C39H26F12N8O2P2Ru: C, 45.49; H, 2.55; N, 10.88. Found: C, 45.40; H, 2.50; N, 11.15%. 1H NMR (400 MHz, DMSO-d6, 25 °C), d (ppm) 9.66 (td, J1 = 6.7 Hz, J2 = 1.3 Hz, 2H, ligand), 8.97 (s, 1H, ligand), 8.90 (d, J = 8.3 Hz, 2H, bpy), 8.87 (d, J = 8.2 Hz, 2H, bpy), 8.58 (dd, J1 = 6.8 Hz, J2 = 1.7 Hz, 1H, ligand), 8.23 (m, 5H, bpy-ligand), 8.15 (t, J = 7,9 Hz, 2H, bpy), 8.05 (m, 2H, ligand), 7.84 (d, J = 5.6 Hz, 2H, bpy), 7.78 (d, J = 5.6 Hz, 2H, bpy), 7.61 (td, J1 = 5.9 Hz, J2 = 1.1 Hz, 2H, bpy), 7.40 (td, J1 = 6.6 Hz, J2 = 1.8 Hz, 2H, bpy). UV kmax (MeCN)/nm 441, 362, 316, 284, 242. IR (ATR, mmax/cm1) 1622, 1602, 1447, 835, 761, 729, 699, 556. ESI-TOF (m/z, MeCN) 369.8 ([M]2+, calc. 369.9). 2.3.7. Dipyrido[3,2-a:20 ,30 -c]phenazine-11-carbonitrile bis(2,20 -bipyridine) ruthenium(II) hexafluorophosphate (9) Synthesized by adapting a literature procedure [44], using 2 and 3,4-diaminobenzonitrile as starting reactants. Yield: 0.199 g (90%). Anal. Calc. for C39H25F12N9P2Ru: C, 46.35; H, 2.49; N, 12.47. Found: C, 45.19; H, 2.41; N, 12.73%. 1H NMR (400 MHz, DMSO-d6, 25 °C), d (ppm) 9.64 (d, J = 7.9 Hz, 1H, ligand), 9.60 (d, J = 9.1 Hz, 1H, ligand), 9.19 (d, J = 1.2 Hz, 1H, ligand), 8.89 (t, J = 9.7 Hz, 4H, bpy), 8.68 (d, J = 8.8 Hz, 1H, ligand), 8.47 (dd, J1 = 8.8 Hz, J2 = 1.6 Hz, 1H, ligand), 8.28 (d, J = 5.1 Hz, 2H, ligand), 8.24 (t, J = 7.9 Hz, 2H, bpy), 8.15 (t, J = 7.5 Hz, 2H, bpy), 8.06 (m, 2H, ligand), 7.83 (d, J = 5.2 Hz, 2H, bpy), 7.77 (d, J = 5.6 Hz, 2H, bpy), 7.61 (t, J = 6,5 Hz, 2H, bpy), 7.40 (t, J = 6.5 Hz, 2H, bpy). UV kmax (MeCN)/nm 441, 362, 276, 246. IR (ATR, mmax/cm1) 2225, 1446, 1357, 1118, 833, 762, 729, 557, 417. ESI-TOF (m/z, MeCN) 360.6 ([M]2+, calc. 360.4). 2.3.8. Pyrazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile bis(2,20 -bipyridine) ruthenium(II) hexafluorophosphate (10) Synthesized by adapting a literature procedure [44], using 2 and diaminomaleonitrile as starting reactants. Yield: 0.157 g (73%). Anal. Calc. for C34H22F12N10P2Ru: C, 42.47; H, 2.31; N, 14.57. Found:
3.1. Synthesis Complexes of the general formula [M(bpy)2(L)]2+, where M is a metal center and L a poly-aromatic N-heterocyclic ligand, are generally prepared by synthesizing the ligand L, and then adding it to the metal center [17,43,45]. This commonly used synthetic pathway displays a large drawback, as the yields are sometimes very low [24]. The main explanation is based on the poor solubility of the ditopic ligand L in organic solvents since it is often large and aromatic. To circumvent this drawback, we have adopted the block by block building strategy. In the first step, we synthesized cis-Ru (bpy)2Cl2 (1) [40]. Then, by adding 1,10-phenantroline-5,6-dione [41], we obtained [Ru(bpy)2(1,10-phenanthroline-5,6-dione)]2+ complex (2) following a well-known protocol [42]. The central aromatic core is obtained by coupling the complex 2 with the appropriate aldehyde [43] or diamine [44], as depicted on Fig. 1. By comparison to the commonly used synthetic route, this pathway allows a 50% improvement of the total yield starting from complex 1. Reaction of 2 with different aldehydes following a Steck-Day reaction [46], in a buffer solution of acetic acid with an excess of ammonium, during 3 h at reflux, leads to a first family of 5-member ring complexes: phenylimidazophenanthrolines (pip, 3 to 7). For these complexes, the phenylimidazole moiety is easy to functionalize. Hydroxyl, carbonitrile and carboxylic acid substituents were chosen to study their influence on the photo-physical behavior of the complexes, depending on their electron-withdrawing or donor characters. As complex 5 is difficult to be obtained from the usual hexafluorophosphate complex 2, the perchlorate form of 2 was used to synthesize it. Condensation of complex 2 with diamines reagents leads to the second family of 6member ring complexes: dipyridophenazines (dppz, 8 and 9) and pyrazinophenanthrolines (pzp, 10). All the resulting complexes were fully characterized by 1H NMR, high resolution mass spectrometry, IR and UV–Vis spectroscopy. A typical 1H NMR spectrum of the studied complexes is given in Fig. 2. Attribution of the signals was achieved using a COSY 1 H–1H experiment (see ESI). The bipyridine protons of all synthesized complexes are not affected by the general structure of the ditopic ligand (protons H6 to H13 on top of Fig. 2). In all cases, the phenanthroline proton H3 of the ditopic ligand is highly deshielded compared to complex 2 due to the vicinity of the nitrogen-containing 5 or 6-member ring. Its signal can be found around 9 ppm for pip-based complexes and 9.5 ppm for dppz and pzp-based complexes. All other signals observed between 7 and 10 ppm are related to the ditopic ligand. The complex 6 displays a particular 1H NMR spectrum in DMSO-d6 at 298 K (on bottom of Fig. 2). The resonances of the protons located around 9.4 ppm and 8.5 ppm are broadened. The same phenomenon is observed in CD3CN precluding any peculiar interaction of the Ru-complex with the deuterated solvent. Evolution of the NMR spectrum was studied at various temperatures between 298 K and 358 K in DMSO-d6 (Fig. 3). An increase of the
K. Ruffray et al. / Inorganica Chimica Acta 440 (2016) 26–37
Fig. 1. Synthetic routes to polypyridyl ruthenium species 3 to 10.
Fig. 2. 1H NMR spectrum of complex 5 in DMSO-d6 at 298 K and signals attribution (top) and 1H NMR spectrum of complex 6 in DMSO-d6 at 298 K (bottom).
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Fig. 3. Evolution of the 1H NMR signals of complex 6 in DMSO-d6 in function of the temperature.
temperature induces an improvement of the resolution of the spectrum. Some resonances have split and assignment can be achieved using a 2D COSY 1H–1H NMR experiment (Fig. 4). The most deshielded and broadened signals observed on the spectrum at 298 K are attributed to the protons H1 and H5 located near the imidazole fragment. This phenomenon can be explained by the interaction of a water molecule in the cavity formed by these two protons, the imidazole and the carboxylic acid groups (Fig. 5a). At high temperature, the exchange rate of the water molecule between the complex and the medium is enhanced, resulting in a better resolution of the 1H NMR spectrum of the complex 6. This phenomenon is not observed for complex 7 although it presents a similar structure to complex 6. As represented on Fig. 5b, the phenol group in complex 7 interacts with the carboxylic acid, leading to a conformation avoiding peculiar interaction with a water molecule.
The mass spectra of the ruthenium complexes are similar to one another and exhibit the expected molecular ions (see ESI), except for complex 6 that will be discussed later. The observed peaks fit to the simulated isotopic patterns. Complex 5 presents a typical spectrum (Fig. 6), where the highest signal at 377.07 refers to the molecular ion [Ru(bpy)2(L)]2+, while a second signal at 753.14 corresponds to the deprotonated [Ru(bpy)2(L)]2+ ion (except for the dppz and pzp-based complexes that cannot be easily deprotonated). Two weak signals at 341.11 and 853.10 are also observed and can be attributed respectively to the protonated ditopic ligand [LH]+ and the [Ru(bpy)2(L)][ClO4]+ ion. The complex 6 presents a specific behavior. Although its purity was established by NMR analysis (see full spectrum in ESI), the mass spectrum of this complex presents a lot of peaks provided by fragmentation/recombination mechanisms (see Fig. 7 and ESI). Considering their isotopic patterns, some of the signals at 341.31, 415.21, 663.45 and
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5
1
11
12
10 2
13 8 4
7
6
15
9
3
14
Fig. 4. COSY 1H–1H NMR spectrum of complex 6 in DMSO-d6 at 358 K.
(a)
(b)
Fig. 5. Proposed structures of complexes 6 (a) and 7 (b) in wet solvents.
963.46 can be attributed to organic fragments. The peaks at 359.32 and 1061.94 on the mass spectrum correspond respectively to the protonated [LH-H2O]+ and to the {[RuIII(bpy)2(L-H2O)][PF6]2}2+ 2 ions. The peak at 359.32 confirms a possible interaction between the ditopic ligand and a water molecule. The peak at 1061.94 highlights a specific behavior of complex 6 which is able to dimerize in presence of water molecules as represented on the Fig. 7. This result is in agreement with the behavior observed in NMR and previously described. 3.2. Electrochemistry Cyclic voltammograms were recorded at room temperature, in dry and degassed acetonitrile in the presence of 0.02 M of tetrabutylammonium hexafluorophosphate (or perchlorate for complex 5) as electrolyte support. Measurements were made with a glassy carbon working electrode, at 100 mV/s. The results for the synthesized compounds are compiled in Table 1, and they are compared
to [Ru(bpy)3]2+. As examples, voltammograms of the pip-based complex 4 and the dppz-based complex 9 are presented in Fig. 8. Voltammograms of the other complexes are reported in ESI. A quasi-reversible process corresponding to RuIII/RuII couple stands between 1030 and 1125 mV versus non aqueous Ag/AgCl, a typical trend for this kind of compounds [47–49]. All the studied complexes present a RuIII/RuII couple oxidizing at slightly higher potentials than [Ru(bpy)3]2+, a tendency usually observed for all compounds containing higher p-acceptor ligands than bipyridines. Close to the RuIII/RuII signal, various irreversible waves were observed, attributed to the electroactive part of the ditopic ligand. Between 0.5 V and 2.5 V, complex processes, located on the bipyridines and the ditopic ligands, appear as several reversible and irreversible waves. Complexes of the pip ligands display similar electrochemical behaviors. In addition to the quasi-reversible process assigned to the ruthenium couple, more complex processes attributable to oxidation of the organic part of the ditopic ligand can be observed.
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Fig. 6. ESI-TOF mass spectrum of complex 5 (left) and enlargement of the principal signal (right).
First, the enlargement of the RuIII/RuII wave (at 1053 mV for complex 4) is due to a second process taking place at such potential (Fig. 8). Studies made on similar compounds have showed that this high potential process corresponds to the oxidation of the imidazole part of the ditopic ligand [16]. Other oxidation processes can be observed at lower potentials than ruthenium oxidation for all pip-based complexes. By acquisition of the oxidation part of the voltammogram (see ESI), the first processes (113 mV and 475 mV for complex 4) disappear. They can be associated to reoxidation of electro-generated species. However, the last irreversible process (at 709 mV for complex 4) is always observed and is attributed to the oxidation of the distal phenyl group. This is confirmed by some studies made on similar compounds, without this phenyl fragment, which do not show any process at such potential [48]. The potential of this process is mostly linked to the nature of the substituents: a cathodic shift is observed for electron-donating hydroxyl substituents (389 mV for complex 3), while higher potentials are obtained for complexes 4 to 7, between 657 and 799 mV, due to the with-drawing effect of carbonyl and carbonitrile fragments.
In the negative part of the voltammogram, by comparing the results obtained for complexes 3 to 7 with those obtained for the complex 2, it can be observed the presence of two waves between 1690 and 2011 mV, attributed to the reduction of bipyridine fragments (1713 and 1933 mV for complex 4). The reversible wave observed at higher negative potential (2350 mV for complex 4) corresponds to the phenanthroline part of the pip ligand. Another process takes place at less negative potentials (-1470 mV for complex 4), and might be associated with localized reduction processes on the ditopic ligand, as it possesses the most stable LUMO [48]. The weak wave at 519 mV for complex 4 is only observable after a first oxidation cycle. No more precision can be brought on the exact nature of these irreversible phenomena present for all 5-member ring complexes. Cyclic voltammograms of dppz and pzp complexes share some features with the pip analogs. In the positive part of the voltammogram of complexes 8 to 10, the reversible wave around 1V is also attributed to the RuIII/RuII couple. No additional signal close to the RuIII/RuII couple from the ditopic ligand is observed (Fig. 8). The second irreversible oxidation wave, only observable for
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Fig. 7. ESI-TOF mass spectrum of complex 6 and supposed structure of the dimer ion.
Table 1 Redox potentials of [Ru(bpy)3]2+ and complexes 2 to 10 in acetonitrile vs. non aqueous Ag/AgCl. Complex
Positive E1/2 (mV)
Negative E1/2 (mV)
[Ru(bpy)3]2+ 2 3 4 5 6 7 8 9 10
1010 1125 1030; 1053; 1036; 1052; 1098; 1044; 1048; 1091
1620; 1825; 2075 430; 1130; 1765; 1995 457; 956; 1720; 1950 519; 1470; 1713; 1933 1524; 1690; 1924 361; 1521; 1716; 1845 616; 913; 1725; 1941 1303; 1762; 1967 1026; 1704; 1827; 2011 452; 868; 1726; 1851; 1981
389 709 690 971; 799 657 410 719
complexes 8 (410 mV) and 9 (719 mV) after a first reduction cycle (see ESI), might be related to the distal phenyl group, as this process is missing for complex 10. As for pip ligands family, explanation of the negative part of the voltammogram is complicated due to the combination of several redox processes. As previously mentioned, by comparison with [Ru(bpy)3]2+, the waves observed between 1762 and 2011 mV can be attributed to bipyridines reduction processes, and the wave around 2400 mV to the reduction of the phenanthroline part. Two other waves appear at less negative potentials (1026 and 1704 mV for 9 as example). Those reduction processes are attributed to the first and second reduction of the phenazine fragment [47]. The influence of the distal substituents is obvious, as a cathodic shift is observed according
to their electron-withdrawing power. The two carbonitrile groups of complex 10 make the fragment easily reducible (452 and 868 mV), while the alone carbonyl group of complex 8 makes the second reduction take place at the same potential as the first bipyridine reduction (1762 mV). 3.3. Spectroscopic properties UV–Vis spectroscopy was conducted at room temperature in acetonitrile and the results are reported in Table 2. TD-DFT modelling has been performed to simulate the absorption spectra in order to determine the character of the different electronic transitions. The calculated spectra match qualitatively with the experimental one (Fig. 9 for complexes 5 and 8 and in ESI for other complexes). UV–Vis absorption bands are easily assigned by comparison with [Ru(bpy)3]2+ [50]. The intense band around 285 nm is usually attributed to a bipyridines p ? p⁄ electronic transition. The region between 300 and 400 nm has been deconvoluted and consists of different electronic transitions of medium intensity, corresponding to a mix between ligand-to-ligand charge transfer (LLCT), metal-to-ligand charge transfer (MLCT) and metal–ligand-toligand charge transfer (MLLCT) transitions. The two intense bands located between 410 and 470 nm are attributed to two MLCT transitions, generating the well-known luminescent properties of the ruthenium polypyridyl compounds. This transition corresponds to the electron transfer from the HOMO localized on the metal center to the LUMO shared between the surrounding ligands
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Fig. 8. Cyclic voltammograms (2nd cycle) of complexes 4 (top) and 9 (bottom) in acetonitrile at room temperature. v = 100 mV/s. [NBu4][PF6] = 0.02 M.
Table 2 Absorption wavelength (nm) of [Ru(bpy)3]2+ and complexes 2 to 10 in acetonitrile at room temperature. Complex
LLCT
LLCT/MLLCT/MLCT
MLCT
MLCT
[Ru(bpy)3]2+ 2 3 4 5 6 7 8 9 10
287 285 286 287 286 286 287 284 277 263; 288
– 305; 327; 325; 327; 324; 326; 316: 363; 346
424; 413; 430; 427; 428; 431; 428; 424; 437; 423;
– – – – – – – 546 551 524
350 369 345; 374 346 361 370 356; 375 387
453 440 464 461 460 462 463 459 468 458
consisting of overlapping Ru(dp) ? ditopic ligand (p⁄) and Ru (dp) ? bpy(p⁄) transitions. The appearance of broad MLCT bands results of the lowered symmetry that removes the degeneracy of the p⁄ levels. Complexes of the dppz and pzp ligands display another weak band after 500 nm, confirmed by theoretical
investigations, already reported [2] for [Ru(bpy)3]2+ at low temperature and corresponding to a MLCT transition. Spectrofluorimetry experiments were performed at room temperature, in degassed acetonitrile. All compounds were excited at 452 nm to record their emission spectra and excited state lifetime (pulse = 1.2 ns). All data are available in Table 3. The studied compounds show a broad emission band between 617 and 646 nm (Fig. 10). No trend stands out of any influence of the ligand structure on the emission wavelength. Excited state lifetime measured for [Ru(bpy)3]2+ matches with the values found in the literature [51]. Lifetimes of both [Ru (bpy)2(phen)]2+ [9] and complex 2 show that replacing one of the bipyridine ligand by a phenanthroline fragment decrease the lifetime of the complex. This effect is improved by the presence of the two carbonyl groups of the ortho-quinone ligand. For all complexes, except for complexes 5 and 9 which present particular behaviors, shorter lifetimes than [Ru(bpy)2(phen)]2+ were measured. The extension of the ditopic ligand contributes to decrease the excited lifetime of the ruthenium species. The
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Fig. 9. Experimental absorption spectra of complexes 5 and 8 in acetonitrile at room temperature (o), and the corresponding TD-DFT simulated spectra (full line).
Table 3 Spectroscopic data about emission of [Ru(bpy)3]2+ and complexes 2 to 10 in acetonitrile at room temperature. Complex
kem (nm)
s (ns)
[Ru(bpy)3]2+ [Ru(bpy)2(phen)]2+ [9] 2 3 4 5 6 7 8 9 10
611 610 675 642 627 617 628 627 632 640 646
871 798 473 787 534 1138 714 612 650 <20 538
aromatic core of the ditopic ligand (5-member or 6-member rings) does not have any significant effect on the lifetime. This trend is confirmed by the comparison of the excited state lifetimes of pip compounds (3, 4, 6 and 7) and the dppz and pzp ones (8 and 10), which are close to one another. However, a few trends appear related to the pending substituents on the distal part of the ligand. Insertion of vibrator fragments, as carbonitrile or carboxylic acid group, lowers slightly the excited state lifetime of the complexes. For complexes 5 and 9, an unusual behavior is observed. Complex 5 shows the higher lifetime, whereas complex 9 presents no luminescence in acetonitrile as already described [29] but also in degassed ethanol, DMF, pyridine or DMSO.
The excited state lifetime and emission wavelengths of complexes 4, 5 and 6 in selected degassed solvents are summarized in Table 4. No relationships can be evidenced between the photophysical properties of the complexes and basic physicochemical properties of the solvents (dielectric constant and dipole moment). Nevertheless, some measurements of the excited state lifetime of the complexes allow considering an interaction between the molecular structures of the solvents and the ditopic ligands. For complexes 4 and 5, in solvents sharing a vibrating group able to interact with the complex, the lifetime drops. The carbonitrilebased complex 4 quasi doubles its lifetime in ethanol (s = 941 ns) compared to acetonitrile (s = 534 ns), while the lifetime of the carboxylic-based complex 5 is strongly decreased in ethanol (s = 701 ns, compared to 1138 ns in acetonitrile). In a solvent sharing no interacting moiety with complexes 4 and 5 (i.e. DMF, pyridine and DMSO), values of their excited states lifetimes are closer. Some studies already exist about the solvent effects [51], but it is the first time that a specific intermolecular interaction between solvents and substituents based on similar moieties in their structure is exposed. An important impact on the excited state lifetime is observed for complex 6 when moving from dried to wet DMF. In wet solvents, as previously explained, the water molecules act as conformation blockers through the formation of a dimer, constraining the carbonyl orientation in a defined position and avoiding the desexcitation by vibrations. In dried solvents, no water molecule is present in the structure decreasing the excited state lifetime of
Fig. 10. Emission spectra of complex 4 after excitation at 452 nm during 1.2 ns in degassed acetonitrile at room temperature (left) and fluorescence decay at 627 nm (right).
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Table 4 Excited-state lifetimes (ns) of complexes 4 to 6 in different solvents at room temperature (emission wavelengths (nm) in brackets). Complex
Acetonitrile
Ethanol
Pyridine
DMSO
DMF
Dried DMF
4
534 (627) 1138 (617) 714 (628)
941 (623) 701 (611) 648 (613)
1224 (622) 1083 (628) 801 (637)
1104 (642) 985 (639) 961 (639)
882 (640) 875 (641) 794 (636)
822 (637) 769 (640) 464 (638)
5 6
Table 5 Calculated energy differences between complex 6 without water and with water in two different conformations.
DG (kcal/mol)
Structure
0.00
1.2
0.8
the complex 6 by vibrations. The conformation blocker behavior of the water molecules counteracts the solvent-ligand vibrational interaction previously described. It results in a slight decay of the excited state lifetime of the complex 6 in ethanol, which should have dropped due to the vibrational interaction between the distal carboxylic acid of the ligand and the solvent.
3.4. DFT calculations We have performed DFT modelling to describe in a more accurate way the observed physicochemical properties. All the
computed molecular orbitals are presented in the Supporting information. For complex 6, to confirm the ability of the ditopic ligand to interact with a water molecule, the energy for two potentials interactions of the complex with a water molecule were calculated and compared with the water-free compound (Table 5). The comparison of the calculated structures and conformations shows an energy difference in the range of ±2 kcal/mol, in agreement with the NMR temperature dependent experiments (Fig. 3) and confirming the interaction of a water molecule with the complex 6. Moreover, a stable structure integrating a water molecule trapped between the imidazole and carbonyl fragments of the ditopic ligand can be calculated. Association of two blocks supports the formation of a dimer through hydrogen bounds, confirming the results obtained by mass spectrometry. Concerning the computed molecular orbitals, the complex 3 displays a very unusual behavior compared to the other complexes. Its HOMO is highly localized on the ditopic ligand, while the other complexes HOMOs are mostly localized on the ruthenium center (Table 6). The presence of two electron-donating hydroxyl groups on the distal part of the ligand can explain the unique composition of the complex 3 HOMO. For the pip family complexes 4 to 7, the complexes can be separated in two groups: compounds integrating linear ligands (4 and 5), and bent ones (6 and 7). For complexes 4 and 5, LUMO and LUMO+1 are delocalized on the bipyridines and the whole ditopic ligand; for complexes 6 and 7, the same orbitals are localized also on the bipyridine and only on the phenanthroline fragment. This significant difference between 5-member ring linear and bent complexes, based on their structure, is illustrated on Table 6. This full delocalization, highlighted only for linear pip-based complexes, matches with the high excited state lifetimes observed for both complexes 4 (s = 941 ns in ethanol) and 5 (s = 1138 ns in acetonitrile), when they are measured in the appropriate solvent. Complexes of the dppz and pzp ligands (8 to 10) exhibit a different behavior. The LUMOs of the complexes are only localized on the ditopic ligands, compared to the behavior previously described for the 5-member ring complexes 3 to 7 (see ESI). This phenomenon is in agreement with the electrochemical results which have demonstrated that the 6-member ring phenazine moiety is the most easily reducible fragment. Complex 10 has the same behavior as 8 and 9, but its LUMO+1 is also fully localized on the ditopic ligand due to the presence of the two electronwithdrawing carbonitrile groups on the distal part of the ligand. This particular behavior of the 6-member ring complexes could lead to a better efficiency of the electron transfer towards a second redox active metal localized on the distal coordinating groups.
Table 6 Representations of HOMO, LUMO and LUMO+1 of complexes 3 to 6. Orbitals LUMO+1
LUMO
HOMO
Complex 3
Complex 4
Complex 5
Complex 6
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4. Conclusion
Appendix A. Supplementary material
We have studied and compared the physical–chemical behavior of a series of eight ruthenium(II) polypyridyl complexes, representative of the variety of structures and pending substituents usually encountered in the electron transfer studies. We have highlighted a slight effect of the aromatic core (5 or 6-member ring) on the electrochemical behavior and the UV–Vis absorption spectra. Complexes of the 6-member ring family (dppz and pzp) are more easily reducible than pip-based complexes and they show an additional electronic transition band around 500 nm attributed to an MLCT transition. For all the studied complexes, the general composition of the ditopic ligands has a huge impact on the electronic delocalization and on the lifetime of the excited states. For pipbased complexes, high excited state lifetime and LUMO’s delocalization on the whole ditopic ligand are only obtained with linear structures. DFT calculations have supported that the electronic transfer is spread over the bipyridines and the ditopic ligand, resulting in a potential efficiency loss. For dppz and pzp-based complexes, lower excited state lifetime were measured but according to the DFT modelling, the electronic transfer takes place only towards the ditopic ligand. The solvent effect on the luminescence properties is strongly related to the nature of the distal pending substituents: if their structures present interacting fragments, non-radiative desexcitations are enhanced through a vibrational coupling that decreases the excited state lifetime. The presence of supramolecular interactions due to the structure of the ditopic ligand, in complex 6, shows an additional dynamic effect on the photo-physical properties. All the results obtained in this study highlight not only the impact of the structure–properties relationship of the ruthenium complexes, but also the effect of the specific interactions between the solvent and ruthenium compounds through a vibrational coupling, on the electrochemical and photo-physical properties of the Ru-systems. Based on this work, complexes 5 and 8 present potential photo-physical properties for an efficient charge separation. In acetonitrile, those two complexes, containing a carboxylic acid group in distal position, exhibit high excited state lifetimes and delocalized LUMO on the ditopic ligand (which is more easily reduced than the bipyridines). Moreover, the photo-physical features of the studied Ru-complexes in various solvents allow envisaging electron-transfer studies in other media than acetonitrile which is generally used.
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2015.10.018.
Acknowledgments The authors thank Sandra Maynadié for carrying out the high temperature and 2D NMR experiments, and the Laboratoire de Mesures Physiques of the Max Mousseron technical platform, at Université de Montpellier for carrying out the mass spectrometry. We thank RSTB project of CEA/DEN/DISN for financial supporting.
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