Resonance Raman and lifetime studies on regioselectively deuteriated ruthenium(ii) polypyridyl complexes

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Resonance Raman and lifetime studies on regioselectively deuteriatedruthenium(II) polypyridyl complexes†

Wesley R. Browne,‡a William Henry,a Paolo Passaniti,b Maria Teresa Gandolfi,b Roberto Ballardini,b

Christine M. O’Connor,c Clare Brady,d Colin G. Coates,e Johannes G. Vosa and John J. McGarvey*d

Received 18th September 2006, Accepted 5th January 2007First published as an Advance Article on the web 9th February 2007DOI: 10.1039/b613471h

Two series of ruthenium(II) polypyridyl complexes [Ru(bipy)2(phpytr)]+ and [Ru(bipy)2(phpztr)]+

(where Hphpytr = 2-(5-phenyl-1H-[1,2,4]triazol-3-yl)-pyridine and Hphpztr =2-(5-phenyl-1H-[1,2,4]triazol-3-yl)-pyrazine) are examined by electrochemistry, UV/Vis, emission,resonance Raman, transient resonance Raman and transient absorption spectroscopy, in order toobtain a more comprehensive understanding of their excited state electronic properties. Theinterpretation of the results obtained is facilitated by the availability of several isotopologues of each ofthe complexes examined. For the pyridine-1,2,4-triazolato based complex the lowest emissive excitedstate is exclusively bipy based, however, for the pyrazine based complexes excited state localisation onparticular ligands shows considerable solvent and pH dependency.

Introduction

Ru(II) polypyridyl complexes continue to receive considerableinterest in applications as diverse as models for natural pho-tosynthetic processes and solar energy conversion1 to oxygensensing,2 in particular with respect to their photophysical andphotochemical properties.3 The nature of the lowest excited statesof these complexes is of particular concern,4 with many studiesbeing concerned with the extent of localisation/delocalisation ofthe lowest lying 3MLCT (metal to ligand charge transfer) excitedstate/s in both homo- and heteroleptic complexes. Of the manytechniques that have been applied to such investigations, perhapsresonance Raman (rR), electronic absorption and luminescencespectroscopy have been employed the most widely in the studyof the photophysical properties of homo- and heteroleptic Ru(II)polypyridyl complexes and their lowest excited electronic states.5–8

Woodruff,23 Krausz,9 Yersin,4 Kincaid,8 and coworkers havecarried out detailed investigations of the paradigm complex[Ru(bipy)3]2+ and its selectively deuteriated isotopologues,10,23 andafter considerable debate over several decades it is now widelyaccepted that the lowest emissive 3MLCT state is localised on asingle bipy ligand on the vibrational timescale but ‘hops’ between

aNational Centre for Sensor Research, School of Chemical Sciences, DublinCity University, Dublin 9, Ireland. E-mail: [email protected]; Fax: +3531 7005503; Tel: +353 1 700 5307bDipartimento di Chimica “G.Ciamician”, University of Bologna Via Selmi2, 40126, Bologna, ItalycSchool of Chemistry, Dublin Institute of Technology, Kevin Street, Dublin8, IrelanddSchool of Chemistry and Chemical Engineering, Queen’s University, Belfast,BT9 5AG, Northern Ireland. E-mail: [email protected] Technology, 7 Millennium Way, Springvale Business Park, Belfast,BT12 7AL, Northern Ireland† This paper was published as part of the special issue to commemoratethe 70th birthday of Vincenzo Balzani.‡ Present address: Stratingh Institute, Organic and Molecular InorganicChemistry, University of Groningen, Nijenborgh 4, 9747AG, Groningen,The Netherlands. Email: [email protected]

ligands over the lifetime of the excited state. In contrast, forheteroleptic Ru(II) complexes such as [Ru(bipy)2(pypz)]2+ (wherepypz is 2-(2-pyridyl)-pyrazine), it has been demonstrated that thelowest emissive excited state is essentially localised on the pyrazinemoiety.8b These studies have provided an invaluable set of data bywhich the rR spectra of many bipy and pyrazine Ru(II) complexesmay be interpreted.6

In our earlier reports, we have employed rR and transientresonance Raman (TR2) to investigate the excited state propertiesof Ru(II) polypyridyl complexes containing 1,2,4-triazole basedligands.11,12 Two classes of complexes within this group (Fig. 1)show markedly disparate excited state properties despite showingsimilar ground state chemistry (e.g., acid/base chemistry, redoxpotentials etc.). In the case of the pyridyl-1,2,4-triazole complexes,e.g. 1, only bipy based vibrational features are observed in bothrR and TR2 spectra, confirming the assignment of the lowestMLCT bands (both singlet and triplet) to bipy centred electronicstates. The bipy based excited states are considerably lower inenergy than those centred on the electron rich 1,2,4-triazole basedligand.11d For the pyrazyl-triazole based complexes (i.e., 2) a morecomplicated situation was encountered. At first sight it might beexpected that, as for the pyridyl-1,2,4-triazole based analogues,the electron rich nature of the triazole ring would result in adestabilisation of the pyrazine-1,2,4-triazole based excited states

Fig. 1 Structural formulae of the compounds examined.

386 | Photochem. Photobiol. Sci., 2007, 6, 386–396 This journal is © The Royal Society of Chemistry and Owner Societies 2007

and hence the lowest energy excited electronic states would bepredominantly bipy based. However, the p-acceptor properties ofthe pyrazine ring were found to be sufficient to counterbalancethe destabilising effect of the 1,2,4-triazole ring.11d Unusually,these counteracting effects result in a situation where the pyrazineand bipy based MLCT excited states are close in energy and thecomplexes exhibit quite unusual excited state properties, includinga pH dependence for the localisation of the lowest emissive 3MLCTexcited states and the observation of temperature dependent dualemission.11d

In the present study, two series of selectively deuteriatedRu(II) complexes have been examined in order to obtain amore comprehensive understanding of the electronic propertiesof these complexes. Selective deuteriation together with TR2,absorption and emission spectroscopy are employed in probingthe excited state structure of the complexes 3 [Ru(bipy)2(phpytr)]+

and 4 [Ru(bipy)2(phpztr)]+ (where Hphpytr = 2-(5-phenyl-1H-[1,2,4]triazol-3-yl)-pyridine and Hphpztr = 2-(5-phenyl-1H-[1,2,4]triazol-3-yl)-pyrazine). The structural formulae of the com-plexes investigated are shown in Fig. 1.13 The interpretation ofthe results obtained is facilitated by the availability of severalisotopologues of each of the complexes examined.14

Experimental

Materials

All solvents employed were of HPLC grade or better andused as received unless stated otherwise. For all spectroscopicmeasurements Uvasol (Merck) grade solvents were employed. Allreagents employed in synthetic procedures were of reagent grade orbetter. cis-[Ru(bipy)2Cl2]·2H2O,15 cis-[Ru([D8]-bipy)2Cl2]·2H2O15

and tetraethylammonium perchlorate (TEAP)16 were prepared bypreviously reported procedures. The ligands [D8]-bipy, [H8]-phpztr,[D3]-Hphpztr, [D5]-Hphpztr and [D8]-Hphpztr, [H9]-Hphpytr and[D9]-Hphpytr (where Hphpztr = 2-(5′-phenyl-4′H-[1,2,4]triazol-3′-yl)-pyrazine and Hphpytr = 2-(5′-phenyl-4′H-[1,2,4]triazol-3′-yl)-pyridine) were available from earlier studies.17 The preparationof [Ru(bipy)2(phpztr)]PF6 (4a) is described elsewhere.18 All com-plexes reported in this contribution were prepared by this syntheticmethod. The spectroscopic and analytical data for the compoundsare given below. Only the N2 isomers are examined.

[Ru([D8]-bipy)2(phpztr)]PF6·4H2O (4b). 0.80 g (0.8 mmol) ofHphpztr were added slowly to 230 mg (0.5 mmol) of cis-[Ru([D8]-bipy)2Cl2] in 80 ml ethanol–water 2 : 1 for 4 h. The ethanolwas subsequently removed in vacuo, the solution was made basicby addition of 1 drop of concentrated ammonia solution andthe product precipitated with concentrated aqueous ammoniumhexafluorophosphate solution. The precipitate was collected undervacuum and recrystallised from 30 ml acetone–water 5 : 1 with2 drops of concentrated ammonia solution. The N2 isomer wasisolated by chromatography with acetonitrile as eluent. Yield ofN2 isomer 180 mg (0.2 mmol, 40%). Elemental analysis (calculatedfor RuPF6N9C32H8D16·4H2O): C 44.15 (44.24%), H 2.68 (3.23%),N 14.37 (14.52%). 1H NMR in [D3]-acetonitrile: d ppm 9.29 (1H,d), 8.255 (1H, d), 7.96 (2H, d), 7.61 (1H, d), 7.38 (2H, dd), 7.33(1H, dd). ESI-MS (calcd for RuN9C32H8D16) 652.3 (652.0) m/z.

[Ru(bipy)2([D3]-phpztr)]PF6·3H2O (4c). This was prepared inthe same manner as 4b. Elemental analysis (calculated forRuPF6N9C32H21D3·3H2O): C 45.23 (45.88%), H 2.93 (3.23%), N14.79 (15.05%). 1H NMR in [D3]-acetonitrile: d ppm 8.47 (4H, m),8.04 (4H, m), 7.96 (3H, m), 7.88 (1H, d), 7.80 (2H, dd), 7.4 (7H,m). ESI-MS (calcd for RuN9C32H21D3) 639.3 (639.0) m/z.

[Ru(bipy)2([D5]-Hphpztr)](PF6)2·H2O (4d). This was preparedin the same manner as 4b. Elemental analysis (calculated forRuP2F12N9C32H20D5·H2O): C 41.54 (40.46%), H 2.72 (2.74%), N13.15 (13.28%). 1H NMR in [D3]-acetonitrile: d ppm 9.29 (1H, d),8.47 (4H, m), 8.255 (1H, d), 8.04 (4H, m), 7.96 (1H, d), 7.88 (1H,d), 7.80 (2H, dd), 7.61 (1H, d), 7.4 (4H, m). ESI-MS (calcd forRuN9C32H19D5) 641.4 (641.5) m/z.

[Ru(bipy)2([D8]-phpztr)]PF6·3H2O (4e). This was prepared inthe same manner as 4b. Elemental analysis (calculated forRuPF6N9C32H16D8·3H2O): C 45.56 (45.61%), H 2.89 (3.21%), N14.66 (14.96%). 1H NMR in [D3]-acetonitrile: d ppm 8.47 (4H, m),8.04 (4H, m), 7.96 (1H, d), 7.88 (1H, d), 7.80 (2H, dd), 7.4 (4H,m). ESI-MS (calcd for RuN9C32H16D8) 644.3 (644.0) m/z.

[Ru([D8]-bipy)2([D3]-phpztr)]PF6·3H2O (4f). This was pre-pared in the same manner as 4b. Elemental analysis (calculatedfor RuPF6N9C32H5D19·4H2O): C 43.27 (44.09%), H 2.83 (3.21%),N 14.20 (14.47%). 1H NMR in [D3]-acetonitrile: d ppm 7.96 (2H,d), 7.38 (2H, dd), 7.33 (1H, dd). ESI-MS (calcd for RuN9C32H5D19)655.4 (655.0) m/z.

[Ru([D8]-bipy)2([D8-]-phpztr)]PF6 (4g). This was prepared inthe same manner as 4b. ESI-MS (calcd for RuN9C32D24) 660.4(660.5) m/z.

[Ru(bipy)2(phpytr)]PF6·2H2O (3a). This was prepared inthe same manner as 4b. Elemental analysis (calculated forRuPF6N8C33H25·2H2O): C 49.52 (49.69%), H 3.12 (3.26%), N13.75 (14.05%). 1H NMR in [D3]-acetonitrile: d ppm 8.78 (1H,d), 8.75 (d, 1H), 8.66 (d, 2H), 8.20 (d, 1H), 8.14 (m, 4H), 8.04 (m,6 H), 7.94 (d, 1H), 7.72 (d, 1H), 7.57 (dd, 2H), 7.52 (dd, 1H), 7.46(dd, 1H), 7.31 (dd, 2H), 7.26 (m, 2H).

[Ru([D8]-bipy)2(phpytr)]PF6·2H2O (3b). This was prepared inthe same manner as 4b. Elemental analysis (calculated forRuPF6N8C33H9D16·2H2O): C 46.85 (47.65%), H 3.07 (3.25%), N13.13 (13.48%). 1H NMR (400 MHz) in [D3]-acetonitrile: d ppm8.16 (1H, d), 7.96 (2H, d), 7.91 (1H, dd), 7.52 (1H, d), 7.37 (2H,dd), 7.29 (1H, dd), 7.15 (1H, dd).

[Ru(bipy)2([D9]-phpytr)]PF6·2H2O (3c). This was prepared inthe same manner as 4b. Elemental analysis (calculated forRuPF6N8C33H16D9·2H2O): C 49.52 (49.69%), H 3.12 (3.26%), N13.75 (14.05%). 1H NMR in [D3]-acetonitrile: d ppm 8.78 (1H, d),8.75 (d, 1H), 8.66 (d, 2H), 8.20 (d, 1H), 8.14 (m, 3H), 8.04 (m,4H), 7.72 (d, 1H), 7.57 (dd, 2H), 7.46 (dd, 1H).

1H NMR spectra were recorded on a Bruker Avance 400 (400MHz) NMR spectrometer. All measurements were carried out in[D3]-acetonitrile. Peak positions are relative to residual solventpeaks.

Mass spectra were obtained using a Bruker-Esquire LC_00050electrospray ionization mass spectrometer at positive polarity withcap-exit voltage of 167 V. Spectra were recorded in the scan rangeof 50–2200 m/z with an acquisition time of between 300 and

This journal is © The Royal Society of Chemistry and Owner Societies 2007 Photochem. Photobiol. Sci., 2007, 6, 386–396 | 387

900 ls and a potential of between 30 and 70 V. Each spectrum wasrecorded by summation of 20 scans.

Elemental analysis was carried out at the Micro-analyticalLaboratory at University College Dublin.

Photophysical and electrochemical measurements

UV/Vis absorption spectra (accuracy ±2 nm) were recordedin 10 mm quartz cuvettes on a Shimadzu UV/Vis-NIR 3100spectrophotometer.

Emission spectra (accuracy ±2 nm) were recorded at 298 Kusing a LS50B luminescence spectrophotometer, equipped with ared sensitive Hamamatsu R928 PMT detector, interfaced with anElonex PC466 employing Perkin-Elmer FL WinLab custom builtsoftware. Emission spectra are uncorrected for photomultiplierresponse.

pH titrations were carried out in Britton–Robinson buffer(0.04 M H3BO3, 0.04 M H3PO4, 0.04 M CH3CO2H, pH wasadjusted using concentrated sulfuric acid or sodium hydroxidesolution) and followed by monitoring the pH dependence ofabsorption and emission spectra. pHi refers to the inflection pointof the emission titration curve. The appropriate isosbestic pointfrom the absorption spectra was used as the excitation wavelengthfor emission titrations.

Luminescence lifetime measurements were made using anEdinburgh Analytical Instruments time-correlated single-photoncounting apparatus (TCSPC) as described previously.19 Sampleswere deoxygenated by purging with argon gas for 20 min prior tomeasurements being carried out, followed by periodic deaerationto maintain oxygen exclusion; for selected samples freeze–pump–thaw degassing (three or more cycles) were employed to verifyefficiency of O2 exclusion. Emission lifetimes were calculatedusing a single exponential fitting function, involving a Levenberg–Marquardt algorithm with iterative reconvolution (Edinburghinstruments F900 software) and the estimated experimental erroris ±2.5%. The v2 and residual plots were used to judge the qualityof the fits.

Electrochemical measurements were carried out on a Model600A Electrochemical Workstation (CH Instruments). Typicalcomplex concentrations were 0.5–1 mM in anhydrous acetonitrilecontaining 0.1 M TEAP. A Teflon shrouded glassy carbonworking electrode, Pt wire auxiliary electrode and SCE referenceelectrode were employed. Solutions for reduction measurementswere deoxygenated by purging with N2 or Ar gas for 15 minprior to the measurement. Measurements were made in the rangeof −2.0 to 1.8 V (vs SCE electrode). Protonation of complexeswas achieved by addition of triflic acid (0.1 M in acetonitrile) tothe electrolyte solution. Cyclic voltammograms were obtained atsweep rates of 100 mV s−1. Redox potentials are ±10 mV.

Ground state rR spectra of the complexes were recorded at457.9 nm using an argon ion laser (Spectra Physics model 2050) asthe excitation source. The laser power at the sample was typically30–40 mW. The Raman backscatter was focused onto the entranceslit of a single stage spectrograph (JY Horiba HR640), whichwas coupled to a CCD detector (Andor Technology DV420-OE). Transient differential absorption spectra and TR2 spectra(recorded using the single colour pump–probe method20) werecarried out as reported previously.21

Results

The synthesis and structural characterisation of all ligands17 andcomplexes were carried out by previously reported methods.18

Heteroleptic complexes incorporating the 1,2,4-triazole moietydisplay complicated 1H NMR spectra with considerable overlapof 1H NMR absorptions. For these complexes deuteriation allowsfor a more detailed assignment of 1H NMR spectra as can be seenfrom Fig. 2.17 Absorptions assigned to the pyrazine-triazole ligandare found at 9.2, 8.15, 7.85, 7.45, 7.3 and 7.2 ppm.

Fig. 2 1H NMR (400 MHz, CD3CN) spectra of 4a, 4e and 4b.

Electrochemical and photophysical properties

Electrochemical and electronic spectroscopic data for complexes 3and 4 (where H3/H4 are monoprotonated, at the 1,2,4-triazolatogroup, and H24 is doubly protonated with the second protonationat the non-coordinated nitrogen of the pyridine) are presentedin Table 1. All complexes undergo several reversible oxidationand reduction processes within the redox window investigated(between +2.0 and −2.0 V vs. SCE). An increase in the metal-basedoxidation potentials is observed upon protonation. The ligandbased reduction processes are as expected for Ru(II) polypyridylcomplexes and are not discussed further.4,5

Table 1 Electronic spectroscopic, electrochemical and acid/base data

BR bufferaqa kmax abs/nm Lum. 298 K (77 Kb) pKa

3 467 660 3.65e

H3 440 615 —4 452 705 3.10H4 447 695 —H24 545 — —

CH3CNc kmax abs/nm Lum. 298 K (77 Kb) E1/2d/V

3 — 680 0.81, −1.48, −1.72H3 — 610 1.144 454 665 (640) 0.93, −1.45, −1.65H4 436 674 1.09H24 547 — —

a In Britton–Robinson buffer. b In EtOH–MeOH 4 : 1 v/v. c Recorded inCH3CN; protonation with CF3SO3H. d Recorded in CH3CN–0.1 M TEAP;protonation with CF3SO3H (V vs SCE). e pHi = 1.7 (inflection point ofemission titration curve).


The absorption spectra of all complexes show intense bands inthe UV region (∼290 nm) and moderately intense bands in thevisible region (∼450 nm) which are typical for Ru(II) polypyridylcomplexes.4,5 The complexes are luminescent in acetonitrile solu-tion at 298 K and excitation spectra match the absorption spectraclosely. Deuteriation is seen to have little effect on the propertiesreported in Table 1.

Fig. 3 (a) Absorption and (b) emission spectra of 3a [Ru(bipy)2(phpytr)]+

(∼10−4 M) in Britton–Robinson buffer at 298 K. Arrows indicatedecreasing pH {8.75, 7.35, 6.15, 5.5, 4.45, 4.08, 2.92, 2.0, 1.65, 1.42, 1.24,1.04, 0.87, 0.38, 0.23}.

The spectroscopic, photophysical, and redox properties (videsupra) of all complexes are dependent on their protonation state.Titration in aqueous buffered solution (monitored by UV/Visspectroscopy) gives pKa values of the order of 3.65 and 3.10 for 3and 4, respectively, with sharp isosbestic points being observed(Fig. 3). On the basis of earlier acid/base studies on Ru(II)complexes containing triazolate ligands,11 the protonation can beassigned as protonation of the triazole rings. As with structurallyrelated complexes, protonation results in a blue shift in the UV/Visabsorption spectra of all complexes examined with the shift beingmost pronounced for the pyridyl-triazole based complexes. Theblue shift is seen for all solvent systems examined. For the pyrazine-triazole based complexes additional protonation states exist dueto protonation of the pyrazine group. However, such protonationoccurs only at negative pH values in aqueous solutions and in the

presence of strong acids in dry acetonitrile (Fig. 4).7 For this secondprotonation step considerable changes are observed in the UV/Visabsorption spectra of the pyrazine-triazole based complexes, withthe appearance of an absorption band at ∼550 nm. The nature ofthe 550 nm absorption band of H24 was investigated further by rRspectroscopy (Fig. 5).

Fig. 4 Absorption spectra of (4a) [Ru(bipy)2(phpztr)]+ (thick line), (H4a)[Ru(bipy)2(Hphpztr)]2+ (dashed line) and (H24a) [Ru(bipy)2(H2phpztr)]3+

(thin line) in acetonitrile at 298 K. Protonation with CF3CO2H.

Resonance Raman spectroscopy

The rR spectrum recorded for 3a shows scattering which can beattributed exclusively to the bipy ligands (confirmed by compari-son with 3b and 3c, n.b. the suffixes a, b, c etc. indicate differentisotopologues, see Table 2 and Experimental). By contrast, for4a, the rR spectra (Fig. 6) show vibrational features assignableto the phpztr− ligand in addition to the expected bipy features.Protonation of the triazole ring (H4a) results in only modestchanges to the rR spectrum. However, protonation of the pyrazinering (H24a) results in an almost complete disappearance of signalsattributable to the bipy ligands, with bands assignable to theH2phpztr+ ligand (on the basis of comparison with compounds4b–g) remaining. For 4a, it is apparent that, in contrast to 3a,the lowest 1MLCT states are not exclusively bipy based but that1MLCT states localised on the pyrazine moiety of the triazolebased ligand are close in energy. Furthermore, it is clear thatprotonation of the pyrazine ring has the direct effect of stabilisingthe pyrazine based MLCT states due to an increase in its p-acceptor strength and of destabilising the bipy based MLCTstates (Fig. 7). Similar spectroscopic features have been observedfor a related pyrazine bridged 1,2,4-triazole based binuclearruthenium complex.21 Importantly however, in the present case(H24a) features assignable to the bipy ligands are discernible inthe rR spectrum recorded at 514 nm indicating that not all bipy1MLCT states are destabilised (Fig. 6).

Emission spectroscopy at 298 K

As for the absorption spectra of the complexes, their emissionspectra also show pH dependence (Table 1). The emission spectraof the pyridine-triazole based complexes undergo a blue shiftfrom ∼680 nm to 610 nm in acetonitrile, coupled with a dramaticdecrease in the emission lifetime of the complexes (from ∼200 nsto <10 ns, Table 2). For the pyridine-triazole based complexes


Fig. 5 Resonance Raman spectra at kexc 514 nm of [Ru(bipy)2(phpytr)]+(3a) in conc. HCl(aq), [Ru(bipy)2(phpztr)]+ (4a) in H2O (pH 8), in H2O (pH 2.5)and in conc. HCl(aq) (prominent bipy features are indicated by dotted vertical lines).

Table 2 Luminescence lifetime data for [Ru(bipy)2(LL′)]+ in degassedacetonitrilea

Compound s/ns

1a [Ru(bipy)2(pytr)]+ 1451b [Ru(bipy)2([D5]-pytr)]+ 1471c [Ru([D8]-bipy)2(pytr)]+ 1931d [Ru([D8]-bipy)2([D5]-pytr)]+ 2102a [Ru(bipy)2(pztr)]+ 2502b [Ru(bipy)2([D4]-pztr)]+ 2602c [Ru([D8]-bipy)2(pztr)]+ 3002d [Ru([D8]-bipy)2([D4]-pztr)]+ 3104a [Ru(bipy)2(phpztr)]+ 2174e [Ru(bipy)2([D8]-phpztr)]+ 2154c [Ru(bipy)2([D3]-phpztr)]+ 2104d [Ru(bipy)2([D5]-phpztr)]+ 2124b [Ru([D8]-bipy)2(phpztr)]+ 2704f [Ru([D8]-bipy)2([D3]-phpztr)]+ 2694g [Ru([D8]-bipy)2([D8]-phpztr)]+ 274

a Experimental uncertainty: ±2.5%.

protonation results in an increase in emission energy and, contraryto expectations based on the energy gap law,22 a dramatic decreasein emission lifetime, due to lowering of the 3MLCT–3MC energygap.11

Protonation of the pyrazine based complex 4 to yield H4 resultsin a small red shift in the emission spectrum (∼10 nm, Table 1) inagreement with reported observations on similar pyrazine basedcomplexes,11 with a considerable increase in the emission lifetime(vide infra).

Transient absorption and resonance Raman spectroscopy

The transient differential absorption spectra of 3a and 4a inaqueous and non-aqueous environments show a strong depletion

Fig. 6 Resonance Raman spectra at kexc 514 nm of [Ru(bipy)2(phpztr)]+

(4a) and [Ru([D8]-bipy)2(phpztr)]+ (4b) in conc. HCl(aq) (features affectedby deuteriation of the bipy ligands are circled).

of ground state visible absorption upon excitation (Fig. 8 and 9),with a recovery time constant comparable to that obtained fromemission lifetime measurements. Importantly, under both acidicand basic conditions, the complexes show a strong excited stateabsorption at ∼355 nm allowing the excited state to be probed byTR2 spectroscopy at this wavelength.

Fig. 10 shows the excited state rR spectra of [Ru(bipy)2-(phpytr)]+ and [Ru([D8]-bipy)2(phpytr)]+. The spectra show thatthe excited state is localized on the bipy ligands with the markerbands for bipy•− (bipy anion radical) at 1211 and 1285 cm−1

showing the expected8 isotope dependence.For [Ru(bipy)2phpztr]+ and its isotopologues almost identical

TR2 spectra are obtained compared with 3a–c (see Fig. 11), withdeprotonated complexes showing excited state resonances typicalof bipy•−, thus confirming that the emitting state is bipy based.For the protonated complexes the ‘bipy marker bands’ at 1212 and1285 cm−1 are notably absent, with weaker bands observed at 1429and 1494 cm−1. The absence of bipy anion radical features in the


Fig. 7 (a) Absorption spectra of 4a and 4c in acetonitrile and in thepresence of 1–2 eq. of CF3CO2H. (b) Emission spectra of 4a and 4cin acetonitrile and in the presence of 1–2 eq. of CF3CO2H and excessCF3CO2H (∼500 eq.).

excited state spectrum cannot on its own confirm that the excitedstate is not bipy based as it is possible that upon protonationthe excited state absorption could move out of resonance withthe pump/probe laser (355 nm). However, transient absorptionspectroscopy (vide supra) confirms that the 355 nm excitationpulse is well within resonance of the excited state absorptionband both in the deprotonated and protonated complexes. Thissuggests that the excited state is pyrazine based. It should be noted,however, that the marker bands for a pyrazine*− based excited stateshow considerable overlap with neutral bipy modes, making theiridentification difficult.14b

Isotope effects on emission lifetimes

The effect of deuteriation on the emission lifetime of a seriesof related 1,2,4-triazole based complexes was examined. Theluminescence properties of 1/H1 and 2/H2 and their deuteriatedanalogues have been reported previously,11d however they havebeen re-measured here using time correlated single photon count-ing (TCSPC, Tables 3 and 4). The results obtained are discussed incomparison with results obtained previously by Keyes et al. usinglaser excitation/PMT detection based lifetime measurements.11d

In the deprotonated state none of the complexes exhibits anincrease in luminescence lifetime upon deuteriation of the triazolebased ligand, whilst deuteriation of the bipy ligands results inan increase of 20–25% in their emission lifetimes (Table 4). This

Fig. 8 Transient differential absorption spectra of 3a. Upper: inH2O–EtOH at 30 ns intervals and lower: in acidic and basic H2O at t =10 ns.

increase is similar in magnitude to the increase observed upondeuteriation of [Ru(bipy)3]2+.14

For the protonated complexes significant barriers to the ac-curate determination of emission lifetime data are present. Inparticular, for the pytr− based complexes, protonation resultsin a dramatic decrease (to <10 ns) in their emission lifetimes,while for the pyrazine based complexes the protonation of thefree pyrazine nitrogen occurs quite easily in the excited state andhence the “pH window” in which the complex remains singlyprotonated in the excited state is small (vide supra). An additionalfactor affecting measurement of the protonated complexes isthat of oxygen quenching. For the deprotonated complexes,degassing with an argon purge provided almost identical resultsin every case, compared with samples degassed by freeze–pump–thaw degassing cycles. In contrast, for the protonated complexesargon purge proved inadequate and all samples required morerigorous degassing by freeze–pump–thaw cycles to ensure thatreliable O2 free values were obtained. For the protonated pztr−

based complexes results obtained from measurements of boththe perprotio complex and the partially and fully deuteriatedcomplexes do not agree with previously reported values.11d Theearlier work suggested that the emission lifetime of the protonatedcomplexes doubled upon deuteriation. It is possible that the purgetechnique used previously11d did not completely remove oxygenfrom the samples in a consistent manner.

The effect of protonation on the emission spectrum of 4 iscomplicated by the second protonation step and by excited state


Fig. 9 Transient differential absorption spectra of 4a. Left: in CH3CN at10 ns with and without added HCl conc. and Right: in acidic and basicH2O at t = 10 ns.

acid–base behaviour.16 Addition of up to 12 mole equivalents oftriflic acid to an acetonitrile solution (under anhydrous conditions)results in a red shift in the emission spectrum and an increasein emission intensity and lifetime (Table 4). Further additionof acid results in a steady decrease in emission intensity due toprotonation of the pyrazine ring. The protonation of the pyrazinering is confirmed by the appearance of an absorption band at545 nm (Fig. 4). The shorter lifetime component of the emissiondecay of compound 4 can be attributed to the deprotonatedcomplex and it is the only component present when tributylamineis added in order to neutralize any acid impurities. Addition

Fig. 10 Transient resonance Raman spectra (kexc = 354.5 nm)of [Ru(bipy)2(phpytr)]+(3a), [Ru([D8]-bipy)2(phpytr)]+ (3b) and[Ru(bipy)2([D9]-phpytr)]+ (3c) in H2O (a–c) and in 0.1 M HCl (d–f),respectively.

Fig. 11 Transient resonance Raman spectra (kexc = 354.5 nm)of [Ru(bipy)2(phpztr)]+(4a), [Ru([D8]-bipy)2(phpztr)]+ (4b) and[Ru(bipy)2([D3]-phpztr)]+ (4c) in H2O (a–c) and in 0.1 M HCl (d–f),respectively.

of up to 12 equivalents of triflic acid results in an increase inthe contribution of the longer exponential decay, attributed tothe monoprotonated complex. In experiments with trifluoroaceticacid, with 50 equivalents of acid added, the intensity is reduceddue to the formation of H24 in which the pyrazine group isalso protonated. The emission decay lifetime of the complexesis similarly complicated by the second protonation step.

Table 3 Emission lifetime data for complexes 3a–c and 4a–g, and their protonated formsa

Compound s77K/ls Compound s77K/ls

3a [Ru(bipy)2(phpytr)]+ 4.9 H3a [Ru(bipy)2(Hphpytr)]2+ 5.23b [Ru([D8]-bipy)2(phpytr)]+ 6.4 H3b [Ru([D8]-bipy)2(Hphpytr)]2+ 6.53c [Ru(bipy)2([D9]-phpytr)]+ 5.1 H3c [Ru(bipy)2([D9]-Hphpytr)]2+ 5.14a [Ru(bipy)2(phpztr)]+ 4.3, 9.6 H4a [Ru(bipy)2(Hphpztr)]2+ 7.24b [Ru([D8]-bipy)2(phpztr)]+ 5.5, 11.6 H4b [Ru([D8]-bipy)2(Hphpztr)]2+ 8.14c [Ru(bipy)2([D3]-phpztr)]+ 4.1, 10.4 H4c [Ru(bipy)2([D3]-Hphpztr)]2+ 8.14d [Ru(bipy)2([D5]-phpztr)]+ 4.1, 8.9 H4d [Ru(bipy)2([D5]-Hphpztr)]2+ 7.34e [Ru(bipy)2([D8]-phpztr)]+ 4.5, 10.7 H4e [Ru(bipy)2([D8]-Hphpztr)]2+ 8.24f [Ru([D8]-bipy)2([D3]-phpztr)]+ N/A H4f [Ru([D8]-bipy)2(d3-Hphpztr)]2+ N/A4g [Ru([D8]-bipy)2([D8]-phpztr)]+ 4.4, 11 H4g [Ru([D8]-bipy)2([D8]-Hphpztr)]2+ 8.3

a In EtOH–MeOH 4 : 1 v/v with NaOH or CF3CO2H at 77 K (experimental uncertainty: ±2.5%).


Table 4 Effect of acid addition on the luminescence behaviour of 4a–ca

Compound Acid/base Lum. kmax /nm s/ns

4a [Ru(bipy)2(phpztr)]2+ — 668 182, 437 (93/7)b

TBAc 668 194TFAd (2 eq.) 685 580TFAd (12 eq.) 685 574

4b [Ru([D8]-bipy)2(phpztr)]2+ — 668 229, 670 (90/10)b

TFAd (1 eq.) 685 6594c [Ru(bipy)2([D3]-phpztr)]2+ TBAc 667 195

TFAd (2 eq.) 684 595TFAd (12 eq.) 681 693

a In CH3CN at 298 K, degassed by >3 freeze–pump–thaw cycles. b Percentage contribution in parentheses. c Tributylamine. d Triflic acid

In protic solvents, a yet more complicated situation is observed.The increased rate of deprotonation/protonation of the complexesresults in an excited protonation occurring within the lifetime ofthe protonated complex. This results in a pH dependence of themeasured luminescence lifetime of the protonated species. Thesituation is complicated by the fact that whilst in the ground statethe protonation of the pyrazine ring occurs at negative pH values,in the excited state the protonation occurs at higher pH values.This is strongly indicative of the lowest excited state being largelypyrazine based in the protonated complex.24

Deuteriation of the pyrazine ring results in the exchange ofonly three C–H oscillators whilst deuteriation of the bipy ligandsresults in the exchange of sixteen. Unless the pyrazine C–H modesare several times more important in deactivating the excited state,a significant increase in emission lifetime upon deuteriation ofthat ligand might not be expected (Table 4). Indeed it has beenshown for triazine based complexes that deuteriation of the bipyligands will have no effect when the excited state is located on anon-bipy ligand.25

Temperature dependence of luminescence properties

Temperature dependent emission measurements have shown thatcomplex 4 is characterized by the presence of two emissive excitedstates between 77 and 200 K. These results are in agreement withthose found previously for [Ru(bipy)2(pztr)]+.11d Fig. 12 shows

Fig. 12 Temperature dependence of the emission spectrum of 4a, in EtOH: MeOH (4 : 1 v/v, with 1% Et3N) at 90 K and between 105 and 165 K(10 K increments). Spectral intensity adjusted for comparison. kexc 450 nm.

the change in relative intensities of the low and high energyemission bands with increasing temperature. As the temperatureis increased the relative contributions of the high and low energyemission changes. At 90 K the emission of the higher energycomponent dominates the spectrum. A bathochromic shift in thehigh energy emission is observed as the temperature passes theglass transition region (115–130 K for an EtOH–MeOH solventsystem). Above 130 K both high and low energy emission bandsare observed. At 170 K the lower energy emission is dominant,and by 200 K the high energy is no longer present. The excitationwavelength dependence of the emission spectrum is shown inFig. 13. As the excitation wavelength is changed to lower energy(from 420 to 520 nm) the spectral shape of the emission changesconsiderably to reveal the low energy emission band. Temperaturedependent emission studies of 3 show, as expected, a single emissiveprocess, characterised by a single vibrionic progression. For 3 amonoexponential emission decay is observed between 140 and200 K. In contrast, temperature dependent lifetime measurementsof 4 between 140 and 200 K confirm that the emission decay isbiexponential.

Fig. 13 Excitation wavelength dependence of the emission spectrum of4a in CH2Cl2–DMF 4 : 1 v/v at 90 K. From 420 to 520 nm in 10 nmincrements.

As discussed previously11 the observation of emission from twostates in a single complex is remarkable but may reflect the verylarge difference in the dipole moment of an excited 3MLCT statelocalised on the bipy ligands and that of an excited 3MLCT statelocalised on the pyrazine moiety.


Discussion

Over the last decade and a half, rR and excited state rR (esrR) havebeen employed extensively to probe the excited state propertiesof Ru(II) polypyridyl complexes containing 1,2,4-triazoles similarto the complexes reported in the present contribution.11 Thecombination of isotope labeling, electronic and rR spectroscopytogether with transient Raman and absorption spectroscopy helpto lay the basis for development of a more complete model of theexcited state structure of the pyridine- and pyrazine-1,2,4-triazolebased Ru(II)(bipy)2 complexes.

For 3, the electronic structure is analogous to that of theparadigm [Ru(bipy)3]2+ complex,3 in that the replacement of oneof the bipy ligands by the pyridine-triazole ligand results in onlya minor perturbation of the electronic structure. From both rRand TR2 spectral data, it is clear that the lowest energy 1MLCTand 3MLCT states are essentially localized on the bipy ligands.The absence of a significant contribution of the pyridine ring ofthe 1,2,4-triazole based ligand to either the lowest 1MLCT or3MLCT states was inferred from the absence of any influenceof deuteriation of that ligand on either the rR or TR2 spectra.The 1,2,4-triazole group does however have an indirect effect onthe bipy based MLCT states. Protonation of the triazole ligandresults in both a decrease in the r-donor strength of the 1,2,4-triazole moiety, thereby stabilizing the metal based ground state,and an increase in the p-acceptor properties of the pyridine moietyof that ligand, which destabilizes the bipy based excited states.Overall, protonation of 3 results in an increase in the energy gapbetween the ground state and the lowest 3MLCT state, reflectedin blue shifted MLCT emission. In this situation, although theenergy gap law22 would predict an increase in emission lifetime,the opposite is observed. Analogous behaviour has been observedpreviously for this class of complexes and can be accounted for bythe presence of a low lying 3MC state, which provides a channel forrapid deactivation of the 3MLCT state.11d In addition to loweringthe ground state, protonation also stabilises this 3MC state to someextent and hence counteracts the effect of the increased ground–3MLCT energy gap. From the TR2 spectra of 3a–c it is apparentthat the lowest 3MLCT excited state is bipy based both in theprotonated and deprotonated forms. The increase in emissionlifetime observed upon deuteriation of the bipy ligands and theabsence of such an increase upon deuteriation of the 1,2,4-triazoleligands, both at 298 and 77 K, support this assignment.

For the pyrazine-triazole based complexes (e.g. 4) a moreintricate excited state pattern and pH dependence are apparent.As for 3, protonation of the 1,2,4-triazole moiety of 4 leads toa stabilization of the metal based ground state and increase inthe p-acceptor strength of the pyrazine moiety. However, for 4,a second protonation step is possible, i.e. of the pyrazine ring,and results in considerable changes to the UV/Vis absorptionspectra, including a pronounced blue shift in the bipy based1MLCT absorption bands and the appearance of a pyrazinebased 1MLCT absorption band at ∼550 nm. Indeed both theabsorption spectrum and the rR spectra of H24 show strikingsimilarity to the corresponding spectra of the related dinu-clear ruthenium(II) complex, [(Ru(bipy)2)2(bis-(5′-methyl-1,2,4-triazolato)-2,5-l-pyrazine)]2+.21 It should be noted that the twoprotonation steps are observed in all solvents examined, howeverin non-aqueous solvents the formation of H24 is achieved by

addition of several equivalents of a strong acid whilst in thepresence of even small amounts of water a large excess of acidis required (due to the buffering capacity of water at low pH).

The excited state properties of 4 show marked differences tothose of 3. It is apparent that at room temperature the lowest3MLCT state is bipy based, with an increase in emission lifetimeobserved upon deuteriation of the bipy ligands (as for 3) and theobservation of the fingerprint features of the bipy anion radicalin the TR2 spectra. At 77 K, however, a more complex situationis encountered. In contrast to the room temperature lifetimes,which are strictly monoexponential, at 77 K a biexponentialemission decay is observed. The similarities in the magnitude oflifetimes of the contributing components precludes accurate fittingof the biexponential decay. However, in contrast to observationsat 298 K, no effect of ligand deuteriation is apparent at 77 K. Thetemperature dependent emission studies show that, as for 2,11d twoemissions are observed between 90 and 165 K (Fig. 12). The higherenergy component has a shorter and more temperature sensitiveemission lifetime than the lower energy component. The lattercomponent is the emission that persists to room temperature. Thewavelength dependence of the relative intensities of the low andhigh energy emission bands confirms that at <170 K, the twoemitting states are not in thermal equilibrium and that the lowerenergy emissive state is populated preferentially upon excitationinto the weaker 1MLCT absorption bands at the red end ofthe visible absorption spectrum. The temperature dependence ofeach emission signal and the confirmation that the lowest energy3MLCT state at room temperature is bipy based suggests that theassignment of the high and low energy emissions as being pyrazineand bipy based, respectively, is reasonable. However these statesare close in energy, albeit with a significant barrier to crossingbetween the pyrazine and bipy based 3MLCT states. Hence, it maybe concluded that, although at room temperature the bipy based3MLCT states have a significant Boltzman population, the statefrom which emission occurs cannot be identified unequivocally,since the pyrazine based 3MLCT states will almost certainly bepopulated also. Indeed, it should be noted that the absence offeatures attributable to a pyrazine based 3MLCT excited statein the TR2 spectra does not exclude its involvement as there isconsiderable overlap of the weak pyrazine anion radical and strongneutral bipy vibrational features.

The possibility of two protonation steps in 4 results in a morecomplicated excited state acid/base behaviour than that observedfor 3. In the case of pyrazine-triazole based complexes singleprotonation results in a blue shift in the main 1MLCT absorptionbands indicating that the majority of excited singlet states aredestabilized, however the weaker bands at lower energy persist.The emission spectrum of 4 undergoes a 9 nm red shift uponprotonation indicating that the emissive 3MLCT state is somewhatstabilized by protonation. The major 1MLCT absorption bandsare bipy based (from rR) and the weaker absorption bands to thered are predominantly, albeit not exclusively, pyrazine based. Thissuggests that the lowest excited state changes from being largelybipy based in 4 to pyrazine based in the protonated complex H4.

Measurement of the emission lifetime of the singly protonatedH4, can only be achieved under anhydrous conditions where therate of protonation to H24 and deprotonation to 4 is sufficientlyless than the emission lifetime. In contrast to the finding for thedeprotonated 4, deuteriation of the bipy ligands or the pyrazine


ligand results in a 15 and 12% increase, respectively, in the emissionlifetime of H4. However, it should be noted that the observeddecay rate may be more influenced by the rate of excited stateprotonation/deprotonation than for the deprotonated complexes.Hence these increases must be treated with caution.

In contrast to 4, for H4 features typical of the bipy anion radicalare absent in the excited state rR spectrum. It is possible that thiscould have been due to a shift in the excited state absorption outof resonance with the excitation wavelength (355 nm). However,transient absorption spectroscopy confirms that this is not thecase (Fig. 8 and 9). Hence, it may be concluded that the bipy based3MLCT states are not populated significantly and a pyrazine based3MLCT state can be expected.

In protic solvents, a still more complicated situation is observed.Protonation results in a dramatic decrease in the observed emissionlifetime while for the pyrazine based complexes the protonation ofthe free pyrazine nitrogen occurs quite easily in the excited state.Nevertheless at 77 K, changes in protonation state do not occurduring the lifetime of the excited state. In contrast to 4, for H4deuteriation of either the bipy ligands or the pyrazine moietyincreases emission lifetime equally whilst deuteriation of thephenyl ring has no significant effect. The absence of a deuteriationeffect at 298 K can therefore be ascribed to domination of theexcited state decay by interaction between strongly coupled states,in particular the 3MC state which is stabilized by protonation(Fig. 14) and is not thermally accessible at cryogenic temperatures.

Fig. 14 Schematic representation of the electronic excited state structureof 4, H4 and H24.

Conclusions

From the acid base and excited state properties of the relatedcomplexes 3 and 4 it is apparent that, when considering excitedstate localisation in heteroleptic complexes, the description ofexcited states as being ‘localised’ on particular ligands based onevidence from a single spectroscopic probe can be misleading.It is apparent from 4, in particular, that the old adage ‘absenceof evidence is not evidence of absence’ seems especially apt.For this complex, while the localisation of the emissive excitedstates on particular ligands may be well-founded, nevertheless theinteraction between states based on different ligands can result in

unexpected excited state behaviour, including dual emission in thecase of 4.

Acknowledgements

The authors thank Enterprise Ireland for financial support.Prof. L. De Cola is thanked for access to nano-second transientabsorption spectroscopy facilities.

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Documents

Resonance Raman and lifetime studies on regioselectively deuteriated ruthenium(ii) polypyridyl complexes