LITERATURA ISLÁMICA
9. La Divina Comedia y sus Orígenes Islámicos
Christian Kuhnt,za Michael Karnahl,zb Stefanie Tschierlei,a Kristin Griebenow,c
Michael Schmitt,aBernhard Scha¨fer,bSven Krieck,bHelmar Go¨rls,bSven Rau,*d Benjamin Dietzek*ac
and Ju¨rgen Poppac
Received 31st July 2009, Accepted 13th November 2009
First published as an Advance Article on the web 16th December 2009 DOI: 10.1039/b915770k
Ru–dppz (dppz = dipyrido[3,2-a:20,3,30-c]phenazine) complexes play an important role as environmentally sensitive luminescence sensors and building blocks for larger supramolecular compounds. Their photophysical properties are known to be highly sensitive to intermolecular solvent–solute interactions and solvent bulk-properties. Here, the synthesis and characterisation of a novel Ru–dppz derivative is reported. The potential of drastically tuning the photophysical properties of such complexes is exemplified, by introducing very simple structural modifications, namely bromine, into the dppz-ligand scaffold. The photophysics i.e. nature of excited states and the excited-state relaxation pathway of the various complexes has been investigated by means of electrochemical measurements, steady-state emission experiments and femtosecond time-resolved spectroscopy. It could be shown that the location of bromine substitution influences the relative energy between a luminescent and a non-luminescent metal-to-ligand charge-transfer state and therefore quenches or facilitates transitions between both. Hence it is illustrated that the luminescent properties and the underlying ultrafast excited-state dynamics of the complexes can be controlled by structural variations, i.e. by intramolecular interactions as opposed to changes in the intermolecular interactions.
1. Introduction
Ruthenium polypyridine complexes are interesting building blocks for supramolecular architectures because they are chemically very stable, redox active and display a rich photo- chemistry.1–4 This combination of properties has rendered them an intensively investigated model system for artificial photosynthesis,5 dyes in dye sensitised solar cells,6,7 biomolecular probes8and light driven catalysis.9The sensitivity of their emission properties towards their surroundings can be utilised for the detection of diverse analytes such as metal ions,10–13 oxygen14 and biological macromolecules like DNA.15–23 Particularly, ruthenium complexes of dipyrido- [3,2-a:20,3,30-c]phenazine, dppz, play an important role for the latter application.
These complexes display an interesting photochemistry: The [(bpy)2Rudppz]2+ complex is non-luminescent in aqueous solution but displays a relatively strong emission upon addition of double stranded DNA. This so-called light-switch effect, which refers to an alteration of the luminescent properties by changes in the intermolecular i.e. solvent– complex interactions, may be explained by the existence of two states both associated with the dppz framework. The bright state is most often described as a3MLCT state residing on the phenanthroline part of the dppz whereas the dark state is associated with the phenazine part of dppz.20,23–27 The nature of this dark excited state is still subject to debate, descriptions put forward include a3ILCT (intra-ligand charge transfer) and a3pp* state.28–33A wide variety of spectroscopic techniques has been utilised to address the question how these two states interconvert and how to influence the luminescence and excited-state processes as well as the light-switch effect. These techniques include resonance Raman spectroscopy, pico- second time-resolved resonance Raman spectroscopy, femto- second transient absorption spectroscopy and temperature dependent time-resolved luminescence spectroscopy.23,25,28,29,34–36 Aside from these spectroscopic investigations, considering different chemical substitution patterns of the dppz with identical substituents should open an alternative route towards elucidating the photophysics of these complexes. In this context the structures I and II (see Scheme 1) of methyl- substituted dppz-complexes are known in the literature. A comparison of the respective luminescence properties shows that the lifetime of the excited state is influenced by the aInstitute of Physical Chemistry, Friedrich-Schiller-University Jena,
Helmholtzweg 4, 07743 Jena, Germany. E-mail: [email protected] bInstitute of Inorganic and Analytical Chemistry,
Friedrich-Schiller-University Jena, August-Bebel-Straße 2, 07743 Jena, Germany
cInstitute of Photonic Technology Jena e.V, Albert-Einstein-Straße 9, 07745 Jena, Germany
dDepartment of Chemistry and Pharmacy, Friedrich-Alexander-University Erlangen-Nu¨rnberg, Egerlandstraße 1, 91058 Erlangen, Germany. E-mail: [email protected]
wElectronic supplementary information (ESI) available: NMR spectra. CCDC reference number 617549. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b915770k z These authors contributed equally to the work.
This journal iscthe Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 1357–1368 | 1357
position at which the methyl substituent is introduced as I displays 820 ns37and II 300 ns.29Furthermore, the light-switch effect is disabled in compound I, which has been explained by a pure steric protection of the phenazine nitrogen atoms.29,37 Other possibilities to control the excited-state properties of dppz-complexes with the help of substitutions have been examined by George and coworkers who used rhenium as the metal center. They showed by means of time-resolved infrared spectroscopy that the destination of photoinitiated charge- transfer of [Re(Cl)3(py)(11,12-X2dppz)]+-complexes—with X = H, F, methyl, CO2ethyl—depends on X.38–40
Here, to the best of our knowledge, we present the first detailed ultrafast time-resolved comparative study on the influence of substitution at both dppz compartments on the photophysics and photochemistry of its ruthenium complexes. We chose bromine substitutions which we introduced at the 2,7-position (L2) and at the 11,12-position (L3) of dppz (see Scheme 2). Since bromine substitution in the 2,7-position has no steric effect on the phenazine nitrogens substitution- induced electronic interactions can be deciphered without contributions from steric interactions. Furthermore, from a previous study on related dibromophenantroline complexes it is known that bromine affects the photophysics of such systems.41,42 The substitution pattern with bromine in the 11,12-position is known from the literature.43The emission properties of the corresponding ruthenium complex (Ru3) i.e. [(tbbpy)2Ru(dppz-Br2)](PF6)2 (tbbpy = 4,40-di-tert-butyl- 2,20-bipyridine, dppz-11,12-Br
2 = 11,12-dibromo-dipyrido- [3,2-a:20,3,30-c]phenazine) showed a lack of emission in polar aprotic environment and emission in less polar solvents.43 However, the ultrafast excited-state processes that underlie this behaviour remained to be investigated, which constitutes one focus in the work at hand. For comparative reasons the unsubstituted ruthenium dppz complex (Ru1) was included in this study. The photophysical properties of the three complexes Ru1–Ru3, i.e. [(tbbpy)2Ru(dppzR2)](PF6)2(dppz = dipyrido- [3,2-a:20,30-c]phenazine, with R representing the bromine/hydrogen substituents at different positions), were investigated using an array of techniques including electrochemistry, resonance Raman (rR) spectroscopy, ultrafast transient absorption and time-resolved luminescence spectroscopy. This approach allowed us to characterise the location of the excited state using rR spectroscopy and to unravel the ultrafast dynamics of subsequently formed states.
Experimental
Synthesis
If not stated otherwise all solvents were of HPLC grade and used as purchased. 1,10-Phenanthroline-monohydrate,
o-phenylenediamine, bromine, NH4PF6 and all other materials were of commercial grade and used without further purification. [(tbbpy)2RuCl2], 4,40-di-tert-butyl-2,20-bipyridine (tbbpy), 3,8-dibromo-1,10-phenanthroline (phenBr2), 1,10- phenanthroline-5,6-dione (phenO2), dipyridophenazine (dppz), 11,12-dibromodipyridophenazine (dppz-11,12-Br2) and Ru3 [(tbbpy)2Ru(dppz-11,12-Br2)](PF6)2were prepared according to literature methods.41,43–49
All dipyridophenazine type ligands dppzR2 (L1–L3) (with R = H or Br at different positions) were complexed with [(tbbpy)2RuCl2] (where tbbpy = 4,40-di-tert-butyl-2,20- bipyridine) by application of a microwave-assisted method for preparation of heteroleptic ruthenium complexes (see Scheme 2).48 All complexes contain tbbpy to increase the solubility in less polar organic solvents. The respective complexes of the type [(tbbpy)2Ru(dppzR2)](PF6)2 were isolated in good yields between 61 and 84% and in short reaction times of approximately 3 h. All compounds were structurally characterised using two dimensional 1H- and 13C-NMR methods, electrospray ionisation mass spectroscopy (ESI), elemental analyses and IR-spectroscopy.
2,7-Dibromodipyridophenazine (dppz-2,7-Br2) L2.A mixture of 3,8-dibromo-1,10-phenanthroline-5,6-dione (0.20 g, 0.54 mmol) and o-phenylenediamine (98%, 0.06 g, 0.55 mmol) was refluxed in 75 ml dry ethanol for 7 h. After that time the dark yellow suspension was filtered, washed well with water, methanol and diethyl ether. The precipitate was dried in vacuo. Yield = 76% (0.18 g); M (C18H8N4Br2) = 440.1 g mol1; anal. calcd for C18H8N4Br2*1H2O: C = 47.19, H = 2.20, N = 12.23; found: C = 47.31, H = 1.98, N = 12.08; MS (DEI): m/z = 220 (50%, M2+/2), 280 (30%, M+–2Br), 359 (40%, M+–Br), 440 (100% M+); 1H-NMR (400 MHz, CDCl3/CF3COOD, 300 K): d = 10.18 (d, 2H, J = 2.0 Hz, 3 + 6), 9.33 (d, 2H, J = 2.0 Hz, 1 + 8), 8.53 (dd, 2H, J = 6.8 Hz and J = 3.6 Hz, 10 + 13), 8.17 (dd, 2H, J = 6.8 Hz and J = 3.6 Hz, 11 + 12) ppm;13C-NMR (100 MHz, CDCl3/ CF3COOD, 300 K): d = 124.54, 130.03, 130.72, 133.79, 137.80, 137.92, 141.74, 143.68, 149.79 ppm.
[(tbbpy)2Ru(dppz)](PF6)2 Ru1. [(tbbpy)2RuCl2] (0.128 g, 0.18 mmol) and an equimolar amount of dipyridophenazine (0.05 g, 0.18 mmol) were suspended in DMF–H2O (90 ml/12 ml) and heated at reflux for 3 h using microwave irradiation (150 W). Then the cooled reaction mixture was filtered, followed by evaporation of the solvent on the rotary evaporator. After that the reddish brown complex was dissolved in a mixture of ethanol and water. The addition of an excess of NH4PF6to the red solution leads to a precipitate, which was filtered, washed with water and diethylether and dried in vacuo. Further Scheme 1 Constituent parts of dppz (phen = 1,10-phenanthroline and phz = phenazine) and the structures of I and II. The two conceptually different compartments phz and phen of the dppz-moiety are highlighted.
purification was obtained by recrystallisation from acetone/ water. Yield = 83% (0.18 g); M (C54H58N8RuP2F12) = 1210.08 g mol1; anal. calcd for C
54H58N8RuP2F12: C = 53.59, H = 4.83, N = 9.26; found: C = 53.69, H = 4.71, N = 9.18; MS (ESI in methanol): m/z = 919 (5%) [M–2PF6–H+]+, 1065 (100%) [M–PF6]+;1H-NMR (400 MHz, CD3CN, 300 K): d = 9.65 (dd, 2H, J = 8.0 Hz and J = 1.2 Hz, 3 + 6), 8.52 (d, 2H, J = 2.0 Hz, H III), 8.49 (d, 2H, J= 1.6 Hz, H III0), 8.47 (dd, 2H, J = 6.4 Hz and J = 3.2 Hz, 10 + 13), 8.14 (dd, 2H, 11 + 12), 8.12 (dd, 2H, 1 + 8), 7.91 (dd, 2H, J = 8.4 Hz and J = 5,6 Hz, 2 + 7), 7.69 (d, 2H, J = 6.0 Hz, H VI), 7.60 (d, 2H, J = 6.0 Hz, H VI0), 7.48 (dd, 2H, J= 6.0 Hz and J = 2.0 Hz, H V), 7.24 (dd, 2H, J = 6.0 Hz and J = 2.0 Hz, H V0), 1.45 (s, 18H, tert-butyl), 1.35 (s, 18H, tert-butyl0) ppm;13C-NMR (100 MHz, CD 3CN 300 K): d = 30.43, 30.52, 36.28, 36.38, 122.51, 122.59, 125.49, 125.66, 128.35, 130.63, 131.79, 133.50, 134.13, 141.03, 143.78, 151.67, 152.06, 152.40, 154.41, 157.81, 158.04, 163.66, 163.77 ppm; IR (KBr, cm1): n(C–CH3) = 2963 cm1(s), n(CQC–) = 1616 cm1 (s), n(CQN–) = 1483 cm1 (s), n(CQC–) = 1415 cm1 (s), n(PF6) = 840 cm1 (s). Crystal data of Ru1:w [C54H58N8Ru]2+ 2[PF6] * 1.25 C2H3N * 0.25 C4H10O Mr = 1279.94 gmol1, red-brown prism, size 0.07 0.06 0.03 mm3, triclinic, space group P1, a = 16.6339(2), b = 17.8896(4), c = 23.0113(5) A˚, a = 99.969(1), b = 102.875(1), g = 112.505(1)1, V = 5907.7(2) A˚3, T=90 1C, Z = 4, rc= 1.439 gcm3, m(Mo-Ka) = 4.04 cm1, F(000) = 2632, 41167 reflections in h(19/21), k(19/23), l(27/29), measured in the range 4.131 r Y r 27.461, completeness Ymax= 98.2%, 26567 independent reflections, Rint = 0.0549, 16487 reflections with Fo > 4s(Fo), 1455 parameters, 0 restraints, R1obs= 0.0770, wR2obs= 0.1741, R1all = 0.1382, wR2all = 0.2103, GOOF = 1.033, largest difference peak and hole: 1.269/0.752 e A˚3.47
[(tbbpy)2Ru(dppz-2,7-Br2)](PF6)2Ru2.Starting from 2,7-di- bromodipyridophenazine (dppz-2,7-Br2, 0.04 g, 0.09 mmol) and [(tbbpy)2RuCl2] the same standard procedure as for the synthesis of [(tbbpy)2Ru(dppz)](PF6)2 (see above) was used. Yield = 78% (0.09 g); M (C54H56N8Br2RuP2F12) = 1367.86 g mol1; anal. calcd for C54H56N8Br2RuP2F12*1H2O: C = 46.80, H = 4.22, N = 8.09; found: C = 46.69, H = 4.11, N = 7.37; MS (ESI in methanol): m/z = 1078 (10%) [M–2PF6]+, 1223 (100%) [M–PF6]+; 1H-NMR (400 MHz, CD3CN, 300 K): d = 9.64 (d, 2H, J = 2.0 Hz, 3 + 6), 8.52 (d, 2H, J = 2.0 Hz, H III), 8.49 (d, 2H, J = 1.6 Hz, H III0), 8.46 (dd, 2H, J = 6.8 Hz and J = 3.6 Hz, 10 + 13), 8.18 (dd, 2H, J = 6.8 Hz and J = 3.6 Hz, 11 + 12), 8.09 (dd, 2H, J= 1.6 Hz, 1 + 8), 7.67 (d, 2H, J = 6.0 Hz, H VI0), 7.64 (d, 2H, J = 6.0 Hz, H VI), 7.48 (dd, 2H, J = 6.0 Hz and J = 2.0 Hz, H V), 7.29 (dd, 2H, J = 6.0 Hz and J = 2.0 Hz, H V0), 1.50 (s, 18H, tert-butyl), 1.37 (s, 18H, tert-butyl0) ppm; 13C-NMR (100 MHz, CD 3CN 300 K): d = 31.07, 31.11, 31.50, 36.95, 37.03, 123.39, 124.86, 126.03, 126.34, 131.32, 132.85, 134.74, 137.03, 140.71, 144.61, 150.54, 152.84, 153.50, 155.84, 158.31, 158.66, 164.67, 164.74 ppm; IR (KBr, cm1): n(C–CH3) = 2959 cm1 (s), n(CQC–) = 1615 cm1 (s), n(CQN–) = 1482 cm1 (s), n(CQC–) = 1414 cm1(s), n(PF6) = 836 cm1(s).
The microwave-assisted reactions were carried out using the Microwave Laboratory Systems MLS EM-2 microwave system. Elemental analyses were performed by the Micro- analytical Laboratory of the Friedrich-Schiller University Jena. 1H-NMR and 13C-NMR spectra were recorded at ambient temperature on a Bruker AC 200 or AC 400 MHz spectrometer (1H: 400.25 MHz,13C: 100.65 MHz). All spectra were referenced to TMS or deuterated solvent as an internal standard (measured values for d are given in ppm and for Jin Hz). The mass spectra were obtained using a SSQ 710, Finnigan MAT SSQ 710 instrument at the Friedrich Schiller University Jena. Electrospray ionisation-mass spectra (ESI) were recorded on a Finnigan MAT 95 XL. The positive ESI mass spectra were achieved with voltages of 3–4 kV applied to the electrospray needle. Infrared spectra were determined by using a Perkin-Elmer FTIR spectrometer System 2000 and KBr blanks of the corresponding samples.
Crystal structure determination
The crystal-structure analysesw were carried out on a Enraf Nonius Kappa CCD diffractometer, using graphite mono- chromated Mo-Ka radiation. The crystals are mounted in a stream of cold nitrogen, in a distance of 29 mm to the crystal detector. Data were corrected for Lorentz and polarisation effects but not for absorption.48,49,51,52 The structures were solved by direct methods (SHELXS53) and refined by full- matrix least squares techniques against Fo2(SHELXL-9754). The hydrogen atoms were included at calculated positions with fixed thermal parameters during the final stages of the refine- ment. All nonhydrogen atoms were refined anisotropically.54 The molecular illustrations were drawn using the program XP (SIEMENS Analytical X-ray Instruments, Inc.).50
Scheme 2 Synthesis and labelling of the presented ligand L1–L3 and their corresponding ruthenium complexes Ru1–Ru3.
Electrochemistry
Electrochemical data were obtained by cyclic voltammetry using a conventional single-compartment three-electrode cell arrangement in combination with a potentiostat ‘‘AUTOLABs, eco chemie’’. As working electrode a 0.196 cm2Pt disk, auxiliary electrode was used: glassy carbon and reference electrode Ag/AgCl (3 M KCl). The measure- ments were carried out in anhydrous and nitrogen purged ACN with 0.1 M tertbutylammonium tetrafluoroborate as supporting electrolyte. All potentials are referenced by the ferrocenium/ferrocene couple (E(Fc/Fc+) = 0.45 V). Photophysics
For the photophysical measurements the complexes were dissolved in aerated acetonitrile. Acetonitrile was chosen since a straight forward photophysical behaviour of Ru–dppz- complexes dissolved in this solvent as compared to alcohols or water was shown.23,28,55If not stated otherwise all experi- ments were performed at room temperature (22 1C in an air-conditioned laboratory). Absorption spectra were taken prior and subsequent to all measurements to ensure photo- chemical stability of the samples.
The time-resolved transient absorption set-up is described in detail elsewhere.56 Briefly, the output of an amplified Ti:Sapphire laser (Libra, Coherent Inc.) was split in two beams, one of which was used to pump a non- collinear optical-parametric amplifier (TOPASwhite, LightConversion Ltd.). The TOPAS output pulses were spectrally tuned to 510 nm and served as pump-pulses in our pump–probe experiments. A fraction of the Ti:Sapphire output was employed to generate a supercontinuum by focusing the 800 nm fundamental into a Ti-sapphire plate. This supercontinuum served as a broad-band probe in the transient absorption experiments. In these experiments the probe pulses were focused into the sample by means of a 500 mm focal-length spherical mirror, while the reference pulses were biasing the sample and were directed onto the detector. Probe-pulses were spatially and temporally over- lapped at the sample position with the pump pulses, which were focused into the sample by means of a 1000 mm focal- length quartz lens. The different focus conditions were chosen to ensure homogeneous excitation of the sample over the entire probe volume. The energy of the pump pulses was chosen to be 0.5 mJ, while typical probe intensities fall into the range of hundreds of nJ. Probe- and reference intensities were detected on a double-stripe diode array and converted into differential absorption (DA) signals using a commercially available detection system (Pascher Instruments AB).
For a kinetic analysis the broad-band DA signals recorded as function of the delay time (t) and the probe wavelength (lpr) were chirp corrected and subsequently subjected to a global fit routine as e.g. described in57,58using a sum of exponential functions for fitting:
DAðt; lprÞ ¼ fðlprÞ þX n
i¼1
AiðlprÞ et=ti;
with j(lpr) being a constant offset to simulate pump-induced absorption changes on time-scales not captured by the
delay-time range accessible in our experiments (2 ns). The Ai(lpr) represent pre-exponential factors of the kinetic components ti. The plot of these pre-exponential factors Ai(lpr) corresponds to the so called decay-associated spectra (DAS) of the kinetic components ti. In order to avoid prominent contributions from coherent artefacts to the data analysis59,60 the pulse-overlap region was ignored in the data fitting procedure.
Fluorescence lifetimes were obtained by time-correlated single photon counting. As light source a Ti-Sapphire laser (Tsunami, Newport Spectra Physics GmbH) was used whose repetition rate was reduced by a pulse selector (Model 3980, Newport Spectra-Physics GmbH) to 800 kHz. The output was frequency doubled in a second harmonic generator (Newport Spectra-Physics GmbH) to create the 435 nm pump beam. The emission was recorded using a Becker & Hickel PMC-100-4 photon-counting module with 150 ps response-limited time resolution. To control the temperature of the sample an Oxford Instruments ITC 503 intelligent temperature monitor and control unit was used. Steady-state absorption spectra were recorded on a Jasco V-670 spectrophotometer, the fluorescence spectra were measured from dilute solutions using a Jasco FP-6200 spectrofluorimeter. Luminescence quantum yields were measured with diluted solutions (optical density o 0.05) using [Ru(bpy)3]Cl2in non-degassed water (F = 0.028) as reference (margin of error = 10%).61
The resonance Raman spectra were recorded in a conven- tional 901-scattering arrangement. Excitation light at 458, 476 and 488 nm was delivered by an argon ion laser (Coherent Innova 300C MotoFreD Ion Laser) and used for resonant excitation in the range of the Ru–MLCT absorption band. A rotating cell was employed to prevent heating of the samples. No changes in the absorption spectra of the samples could be observed after exposure to the laser light. The scattered light was collected with a lens (f1= 35 mm) and subsequently focused (f2= 50 mm) onto the entrance slit of an Acton SpectraPro 2758i spectrometer. The dispersed light was detected using a CCD-camera (Princeton Instruments Spec-10 400B/LN back-illuminated). To obtain the best signal-to-noise ratio the concentration of the sample was optimised in the mM range.
Results and discussion
Synthesis and characterization
The different dppzR2-ligands were prepared by Schiff-base coupling between an orthophenylendiamine derivative and a potentially substituted 1,10-phenanthroline-5,6-dione in alcoholic solution. Thus, the bromine substituted dppz-2,7- Br2ligand for Ru2 was synthesised via a condensation reaction between 3,8-dibromophenanthroline-5,6-dione (phenBr2O2) and 1,2-diaminobenzene with a yield of more than 75%. The therefore required 3,8-dibromophenanthroline-5,6-dione was obtained from 3,8-dibromo-1,10-phenanthroline (phenBr2) which was easily oxidised to the corresponding phenBr2O2 in a yield of 67% using literature methods.41,44–47,49 The subsequent synthesis of the corresponding ruthenium complexes Ru1 and Ru2 was accomplished using microwave heating within 3 h and with good yields of above 80%.48
Structural characterisation was accomplished using various NMR techniques, MS and elemental analysis. Based on these investigations a conventional octahedral coordination was suggested. These hypotheses were supported by a solid state X-ray structure for Ru1 (Fig. 1).
The Ru–N bond lengths and angles of Ru1 are in the typical range for Ru polypyridine complexes. In the solid state structure of Ru1 the deviation from planarity within the dppz ligand is with 0.049 A˚, similar to the reported value of 0.038 A˚ for Ru3 which is negligible.43 All three complexes show a concentration-dependent behaviour in1H-NMR experiments. The signals corresponding to the dppz ligands are significantly shifted upon variation of the concentration (data presented in the ESIw). This behaviour is indicative of p–p-interactions in solution involving this ligand as seen by Kol and co-workers for similar ligands.62,63Significant interaction is also found in the solid state structure of Ru3.43Here two complex molecules are facing each other in an angle of nearly 1801 allowing an interaction of the dppz ligands. A different arrangement could be observed for Ru1. Here dppz–dppz interaction is accomplished with both complexes in a mutually perpendicular orientation. The arrangement of two neighbouring complexes of Ru1 is different to that of Ru3. The distance between both dppz planes for Ru1 is 3.24 A˚, which is even shorter than the
3.54 A˚ observed for Ru3 (Fig. 1). Comparing Ru1 and Ru3 no significant differences in bond lengths and angles around the ruthenium centre or within the dppz ligand can be observed. Electrochemistry
Cyclic voltammograms of Ru1–Ru3 are consistent with metal- based reversible oxidation and several ligand-based reduction steps,64which are presented in Table 1. In comparison to the unsubstituted dppz ligand in Ru1 (ERu(II/III)
ox = 0.85 V) the oxidation potentials of Ru(II)/(III) in Ru2 lead towards more positive values, whereas in Ru3 the oxidation is not affected by the substitution pattern. The first ligand-based reduction step of the phen moiety of dppz (Fig. 2) of Ru1 is located at1.34 V and the influence of the electron-withdrawing ability of the bromo substituents in 2,7-position results in a positive shift of 70 mV in Ru2. Fees et al. describes a cathodic shift of about 100 mV upon going from the unsubstituted dppz ligand to its dimethyl derivative.20 The second and third reductions at around 1.9 V and 2.1 V, respectively, are ascribed to the stepwise one-electron tbbpy-based reduction reactions occurring at potentials less negative than the second reduction of the dppz ligand. The latter takes place at2.48 to 2.50 V in Ru1 to Ru3 and is obviously not markedly affected either by the introduction of –I–substituents like Br or by the substitution pattern.
Steady-state spectroscopy
Fig. 3 comprises the absorption (A) and emission spectra (B) of Ru1–Ru3. In accordance with the literature24,34the absorption spectra show two distinct electronic transitions in the spectral range above 330 nm. The shortwave absorption band is attributed to a p–p* transition located on the dppz-ligand, while the longwave transition in the visible parts of the