Going from green to red electroluminescence through ancillary ligand substitution in ruthenium(ii) tetrazole benzoic acid emitters

Journal ofMaterials Chemistry C

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aChemistry Department, University of Zan

znu.ac.irbLaser and Plasma Research Institute, Shah

† Electronic supplementary informa10.1039/c3tc31350f

Cite this: J. Mater. Chem. C, 2013, 1,6970

Received 13th July 2013Accepted 31st August 2013

DOI: 10.1039/c3tc31350f

www.rsc.org/MaterialsC

6970 | J. Mater. Chem. C, 2013, 1, 69

Going from green to red electroluminescence throughancillary ligand substitution in ruthenium(II) tetrazolebenzoic acid emitters†

Hashem Shahroosvand,*a Leyla Najafi,a Ahmad Sousaraei,a Ezeddin Mohajeranib

and Mohammad Janghourib

The synthesis, characterization, photoluminescence (PL) and electroluminescence (EL) properties of novel

ruthenium(II) emitters with 4-(1H-tetrazole-5-yl)benzoic acid (TzBA) as a novel basic ligand and 2,2-

bipyridine (bpy), 1,10-phenanthroline (phen) and pyridine tetrazole (pyTz) as ancillary ligands have been

reported. The EL results show that the luminescence of Ru(TzBA) depends on the ancillary ligands,

which can be used to tune the EL color of the Ru(TzBA) complexes from green to red emission. The role

of ancillary ligands in EL devices of the Ru(TzBA) complexes was investigated by DFT calculations. The

device [Ru(TzBA)(bpy)(pyTz)(SCN)] (5) has a luminance of 480 cd m�2 and a maximum efficiency of 1.2

cd A�1 at 16 V, which are the highest values among the five devices studied. The turn-on voltage of this

device is approximately 6 V. We suggest that an electroplex occured at the PVK–Ru complex interface

when pyTz was incorporated as an ancillary ligand into Ru(TzBA). OLED studies reveal that pyTz as an

ancillary ligand exhibits better current generating capacity than bpy and phen ligands, which are

introduced by an emitter (5).

1. Introduction

The interest in Ru(II) polypyridine complexes has exponentiallyincreased in the last two decades, as a result of their uniquephotophysical properties and the favorable combination ofseveral desirable features: light absorption in the visible spectralregion, low energy absorption and relatively long lifetimes,reversible electrochemical behavior, long-lived electronicallyexcited states, intense luminescence, high chemical and photo-stability from the lowest excited state 3MLCT, and appreciablephosphorescence quantum yields.1 As a consequence of thesefeatures, a huge number of Ru(II) polypyridine derivatives playingthe roles of chromophores, sensitizers, emitter or electron relays,as single molecules or as part of cluster architectures, have beenused in light harvesting systems, dye sensitized solar cells,switches and electroluminescent devices.2 The promisingperformances shown in these applications has motivatedconstant extensive research efforts, both in materials design andin device architecture.3 Many attempts have been made to designor modify the ligands of ruthenium complexes to improve theirmolecular recognition ability. A classical approach is to usemulti-chromophore systems, or properly tuning the ligands’

jan, Zanjan, Iran. E-mail: shahroos@

id Beheshti University, Tehran, Iran

tion (ESI) available. See DOI:

70–6980

electronic effects by the introduction of functional groups on theorganic part of the molecular fragment to improve emissionproperties of the light emitting diodes (LEDs). Moreover, elec-troluminescence from uorescent metal complexes is a prom-ising approach for low-cost and efficient future lighting. Amongmany organic and inorganic electroluminescence (EL) systems,EL based on Ru(bpy)3

2+ and its derivatives has been the moststudied due to their good stability and EL efficiency in differentmedia, favorable electrochemical properties, and compatibilitywith a wide range of analytes.4 With the main focus on efficiency,the vast majority of devices have utilized ruthenium complexeswhich typically emit in the red region, with emission spectrahaving a maximum wavelength between 600 and 650 nm (moredetail is presented in the ESI, S1†). In principle, new rutheniumorganic luminescence and the use of different p-conjugatedligands may overcome these drawbacks. Therefore, the range ofemission colors can be controlled through the introduction ofdifferent ligands into the ruthenium complexes. In this area,polypyridine ligands such as bpy and phen have a rigid frame-work and possess superb ability to coordinate a large number ofmetal ions, showing potential for technological applications dueto their high charge transfer mobility, strong absorption in theultraviolet spectral region, bright-light-emission and good elec-tro- and photoactive properties.5 As a new ancillary ligand, tet-razole derivatives have been extensively investigated due to theirmultiple coordinationmodes and the chemistry of the tetrazolateanion with d-metals.6

This journal is ª The Royal Society of Chemistry 2013

http://dx.doi.org/10.1039/c3tc31350f

http://pubs.rsc.org/en/journals/journal/TC

http://pubs.rsc.org/en/journals/journal/TC?issueid=TC001042

Paper Journal of Materials Chemistry C

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The preparation of ruthenium tetrazolate complexes,bearing both a metal and a ligand with particular opticalactivities, allows the design and preparation of hybridcompounds with applications for optoelectronic materials. Inthis context, the pyridine tetrazolate ligand has been found tobe useful as a new chelating ancillary ligand that allows thepreparation of light emitting materials with various improvedelectroluminescence properties. Despite these attractivefeatures, the development of tetrazole coordination chemistry isprimarily focused on rst row transition-metal chemistry, andhas, until now, been limited. However, the 5-substituted tetra-zole isosteric with the carboxylate group has been the subject ofvery limited study with metal ions, and few coordinationcomplexes with carboxylate–tetrazole ligands have been repor-ted.7 Based on this nding, we are condent that the relatedtetrazole chelating ligands and their functionalized analoguesshould be valuable in improving the basic design of Ru(II)emitters documented in the current literature. Thus, we foundthat the incorporation of these attractive ligands into oneemitting hybrid structure may provide some suggestions foroptimizing the color tunability in LED devices. Our recent workguided us to more investigations into unusual EL properties inRu tetrazolate complexes.8 Finally, in order to improve ourunderstanding about how substitution of the ancillary ligandinuences the EL properties, some Ru tetrazole complexes havebeen explored where the ancillary ligand has been tuned.

Fig. 1 The layer arrangement of the Ru(TzBA)-based LED device.

2. Experimental2.1 Materials and methods

All chemicals and solvents were purchased from Merck andAldrich and used without further purication. IR spectra wererecorded on a Perkin-Elmer 597 spectrometer. 1H NMR spectrawere recorded with a Bruker 250 MHz spectrometer. Theruthenium content of the nal material was determined byinductively coupled plasma atomic emission spectroscopy(ICP-AES), model Perkin Elmer 1100DV. Electrochemicalmeasurements were carried out in THF using a Model 273Apotentiostat. A conventional three-electrode congurationconsisting of a glassy carbon working electrode, and Pt wires asboth the counter and reference electrodes was used. The sup-porting electrolyte was 0.1 M [Bu4N]PF6. Ferrocene was added asan internal standard aer each set of measurements, and allpotentials reported are quoted with reference to the ferrocene/ferrocenium (Fc/Fc+) couple at a scan rate of 100 mV s�1. Theoxidation (Eox) and reduction (Ered) potentials were used todetermine the HOMO and LUMO energy levels using theequations EHOMO ¼�(Eox + 4.8) eV and ELUMO¼�(Ered + 4.8) eV,which were calculated using the internal standard ferrocenevalue of �4.8 eV with respect to the vacuum.9 The PL spectra ofthe ruthenium compounds and PVK (polyvinyl carbazole):PBD(2-(4-biphenyl)-5-(4-t-butyl-phenyl)-1,3,4-oxadi-azole) weremeasured in 1,2-dimethylformamide solution. The PL spectrawere recorded with an Ocean Optic spectrometer USB2000 with405 nm irradiation. Full geometric optimization and thecalculation of the energetics for all of the structural variableswere carried out using the B3LYP/DFT method. All calculations


and optimizations were performed using the Gaussian 03package.10 Initial calculations were performed on the donor andacceptor moieties of the novel ruthenium complexes to deter-mine the HOMO and LUMO levels of the moieties indepen-dently. Aer the initial calculations, donor and acceptormoieties were paired, and the HOMO and LUMO levels werecalculated and compared. The molecular orbital densities wereviewed using Gauss View.11

2.2 Preparation of EL devices and testing

The structure of the fabricated devices were as follows:ITO/PEDOT:PSS (90 nm)/PVK:PBD (70 nm)/Al (200 nm) and,

ITO/PEDOT:PSS (90 nm)/PVK:PBD:ruthenium complex (70 nm)/Al (200 nm), as shown in Fig. 1.

PVK as a hole-transporting material and PBD as an electron-transporting material were doped with yttrium compounds. Glasssubstrates, coated with ITO (sheet resistance of 70 U m�2), wereused as the conducting anode. The ratio of ruthenium complexesfor each type was 8 wt% in PVK:PBD (100 : 40). Poly(3,4-ethylenedi-oxythiophene) : poly(styrenesulfonate) (PEDOT:PSS) was used as ahole injection and transporting layer. All polymeric layers weresuccessively deposited onto the ITO-coated glass by using a spin-coating process from solution. A metallic cathode of Al wasdeposited on the emissive layer at 8 � 10�5 mbar by thermalevaporation. The PEDOT:PSS was dissolved in DMF, spin-coatedonto ITO and was placed in an oven at 120 �C for 2 h aerdeposition. PVK, PBD and ruthenium complexes at a ratio of100 : 40 : 8 were blended in DMF, and then spin-coated and bakedat 80 �C for 1 h. The thickness of the polymeric thin lm wasdetermined by a Dektak 8000. The EL intensity and spectra weremeasured with an Ocean Optic USB2000 under ambient condi-tions. In addition, a Keithley 2400 sourcemeter was used tomeasure the electrical characteristics of the devices.

2.3 Synthesis of ligands and complexes

Caution: azide and tetrazolate compounds are potentially explo-sive. In this laboratory, the reactions described here were run usinga few grams and no problems were encountered. However, cautionshould be exercised when handling or heating compounds of thiskind.

J. Mater. Chem. C, 2013, 1, 6970–6980 | 6971


Scheme 1 Synthesis procedure of pyTz, TzBA and TzBE.

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4-(1H-tetrazole-5-yl)benzoic acid (TzBA) was prepared accord-ing to the literature method with some changes.12 A mixture of 4-carboxy benzonitrile (2.05 mL, 20 mmol), 40mL of water, sodiumazide (1.43 g, 22 mmol), and zinc bromide (4.5 g, 20 mmol) wereplaced in a round-bottomed ask with three vertical necks. Thereaction was reuxed in a hood, but le open to the atmosphere,for 24 h with vigorous stirring. Aer cooling to room tempera-ture, HCl (3 N, 30mL) and ethyl acetate (100mL) were added, andvigorous stirring was continued until no solid was present andthe aqueous layer had a pH of 1. The combined organic layers

Scheme 2 Schematic representation of the different synthetic approaches to Ru(T

6972 | J. Mater. Chem. C, 2013, 1, 6970–6980

were evaporated, 200 mL of 0.25 N NaOH was added, and themixture was stirred for 1 h, until the original precipitate wasdissolved and a suspension of zinc hydroxide was formed. Thesuspension was ltered and the solid was washed with 20 mL of1 N NaOH. To the ltrate was added 40 mL of 3 N HCl withvigorous stirring, causing the tetrazole to precipitate. The tetra-zole was ltered and washed with 2� 20mL of 3 NHCl and driedin a drying oven to yield the TzBA as a white or slightly coloredpowder. The preparation of pyTz is similar to that of TzBA, exceptthat 4-carboxy benzonitrile was replaced by 2-cyano pyridine.

zBA) complexes 1–5.



Fig. 2 Absorption spectra of Ru(TzBA) complexes 1–5 in 10�5 M DMF solution.


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To prepare methyl-4-(1H-tetrazole-5-yl)benzoate (TzBE),TzBA ligand (1 mmol, 0.19 g) was dissolved in a mixture ofH2SO4 (1 mL, 3 N) and 150 mL of methanol. The mixture washeated at 120 �C in an oil bath under reux for 72 h. Subse-quently, the needle crystals formed were ltered and washedwith water and acetone. Scheme 1 shows the synthesis proce-dure of the pyTz, TzBA and TzBE ligands.

Heteroleptic Ru(II) complexes were synthesized by standardprocedures, as shown in Scheme 2. In a typical procedure, forcomplex 1, to a 100 mL round-bottomed ask with three verticalnecks and a condenser combined with a gas outlet under argongas was introduced 2.2 equiv. of phen and 1 equiv. of tri-chlororuthenium(III). The necessary amount of degassed ethanolto dissolve the solids was added, and themixture was reuxed for1 h. To this intermediate complex was added 2.2 equiv. of TzBE inthe minimum volume of dry and degassed ethanol. The mixturewas reuxed for 4 h. For complexes 2, 4 and 5, aer cooling, anexcess of KSCN was added to the dark brown mixture (10 timesthe stoichiometric amount) and the mixture was reuxed for 4 h.The reaction mixture was allowed to cool to room temperaturebefore being ltered through a sintered glass crucible. The neblack powder in the suspension was recovered by centrifugationat a speed of 5000 rpm for 15 min. Subsequently, the productswere washedwith water andmethanol three times and then driedin air at room temperature. Finally, to convert the ester group ofthe complex to an acidic group, to the complex was added 20 mLof triethylamine (TEA) in 10 mL of water. The mixture wasreuxed for 24 h. Subsequently, the reaction mixture was allowedto cool to room temperature before being ltered through asintered glass crucible. The black powder in the suspension wasrecovered by centrifugation at a speed of 5000 rpm for 30 min.The products were washed with water, acetone and ether threetimes and dried in air at room temperature. The samples werepuried on an alumina column using methanol–acetonitrile asthe eluent. Complexes 2–5 were prepared using the same proce-dure except using different molar ratios of ligands.

Fig. 3 PL spectra of Ru(TzBA) complexes 1–5 in 10�5 M DMF solution (lexc ¼405 nm).

3. Results and discussion3.1 Characterization of complexes

Carboxylate and tetrazolate are multifunctional ligands in thatthey have the potential to coordinate via the nitrogen of tetrazoleand the oxygen atom of the carboxylic group.13 The UV-vis spectraof the ligands and complexes 1–5 were measured in DMF solu-tion at room temperature. They show that the Tz carboxylic acidand pyTz ligands exhibit two absorption bands in the range of200–300 nm. In the UV-vis spectrum of phen, two absorptionbands were also observed at 225 and 260 nm. These bands areattributed to the p–p* transition of the aromatic rings.

Aer coordination the absorption bands underwent a slightred shi, owing to the formation of the rigid conjugated systemconsisting of pyridine and tetrazole rings aer coordination.Electronic spectra of samples 1–5 are shown in Fig. 2. Thecomplexes show two absorption bands: one sharp band centeredat 280 nm due to the spin-allowed p* transitions of the pyTz,TzBA and polypyridine ligands, and a second broad band inregion 400–550 nm corresponding to both spin-allowed and spin-


forbidden metal-to-ligand charge transfer (MLCT) transitions.The intensity of the spin-forbidden bands arises from the strongspin–orbit coupling of Ru(II) which causes mixing with higher-lying spin-allowed transitions. The shoulder in the region 600–800 nm is indicative of strongly overlapped transitions todifferent MLCT states, with the state, or degenerate states, athigher energy localized on the polypyridine ligand. These bandsare typical of low-spin hexa-coordinated octahedral Ru(II) species.

The emission spectra of the Ru(TzBA) complexes weremeasured in DMF solution at room temperature by exciting theelectrons with a wavelength of 405 nm. According to the spectraas shown in Fig. 3, we can see that the complexes show extensiveuorescence in the region from 450 nm to 750 nm.

The different emission intensities in the visible region indi-cated that TzBA, pyTz and polypyridine ligands were good organic

J. Mater. Chem. C, 2013, 1, 6970–6980 | 6973


Fig. 5 A simplified pictorial representation of the different photophysicalprocesses in Ru(TzBA) 5. kr and k 0

r are radiative decay constants. (Common groundstate (S0),

1MLCT and 3MLCT states are shown for the sake of convenience only.)


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ligands which were benecial for energy transfer from Ru(II) tothese ligands, and emitted in the characteristic PL emission.

Generally, if the energy gap between the ligand triplet stateand the emitting level of the Ru(II) ion is less than the optimumenergy gap required for efficient energy transfer, back energytransfer may also take place from the ruthenium ion to the tripletlevel of the sensitizing ligand, resulting in the quenching ofruthenium-centered luminescence.14 If the energy of the tripletlevel of the ligand is too high, it will not be able to overlap withthe emitting level of the ruthenium(II) ion and therefore is inef-fective for sensitizing the ruthenium luminescence.15 The PLspectra of ruthenium(II)–polypyridyl complexes in the literatureare largely attributed to the 3MLCT and 3p–p* states which arepopulated via an ultrafast intersystem crossing (ISC) processfrom directly photoexcited 1MLCT states.16 Particularly, there is adegree of mixing between these two emissive states in theruthenium polypyridyl complexes which has not been extensivelystudied with other ligand combinations. Generally, the transitionarises from the promotion of an electron from a lled t2g orbitalon the metal to a vacant p*-orbital on the neutral, aromatic bpy/phen-ligand. The ligand-centered 3(p–p*) transition typicallyinvolves the movement of electrons between lled and vacant p-orbitals on the bpy/phen-ligands.17 Fig. 4 shows the proposed3MLCT electron transfer in Ru(TzBA) 5.

To understand the PL properties of mixed systems, it isimportant to observe pure 3MLCT and 3(p–p*) systems rst. Thus,the ruthenium(II) complex containing only bpy ligand and the bpyligand were carried out (ESI, S2†). The Ru(bpy)3

2+ complex and thebpy ligand exhibit only 3MLCT and 3(p–p*) transitions, respec-tively. These spectral differences make it possible to assess thecontribution of each transition type to the light emission inmixed-state systems. By comparing the PL properties of the pure systemswith those of the mixed-state systems, it becomes clear that asimple correlation between the two does not exist. However, thereare similarities in wavelength and band broadening between theemission band of [Ru(TzBA)(bpy)(pyTz)(TzBA)SCN] at 500 nm andthe emission band of bpy ligand at 520 nm. So, the emission peakat 500 nm in Ru(TzBA) 5 could be assigned to the 3(p–p*)bpytransition. Moreover, the 3(p–p*) emission band in rutheniumcomplexes (containing the bpy ligand) tends to be lower in energythan in the pure bpy ligand. By comparing the PL emissions ofRu(bpy)3

2+ and Ru(TzBA) complexes with the proposed electrontransfer mechanism, the emission peak at 650 nm could be

Fig. 4 Electron transfer in Ru(TzBA) 5.

6974 | J. Mater. Chem. C, 2013, 1, 6970–6980

assigned to the 3MLCT / S0 transition. The new red shiedemission band in the 725 nm region is attributed to the 3(p–p*)excited states of the pyTz and TzBA ligands. The ligand localizedexcited states are lower in energy than the 3MLCT and 3bpy states.Fig. 5 shows a simplied pictorial representation of the differentphotophysical processes in Ru(TzBA) 5. Interestingly, in our earlierreport on the PL of Ru(Tz) complexes, there were no bands above600 nm.8 This means that the presence of the electron with-drawing groups such as carboxylic acid on the 4-position of thephenyl ring of Tz affected the PL emission.

It is useful to note that the nonlinear relationship betweenthese data suggests that it may not be feasible to predict theluminescent properties of a mixed-state system through simpleextrapolation from the pure excited-state materials, and that amore convoluted interplay between these states exists. Theemission quantum yield (Ø) was calculated for each complexaccording to the reference literature.18 The emission quantumyields of the Ru(TzBA) complexes 1–5 were found to be 0.002,0.004, 0.005, 0.007 and 0.009, respectively. It is reported in theliterature that a carboxyl moiety forms a hydrogen bondedadduct with the surrounding polar solvent molecules, whichfavours a non-radiative loss of photoexcitation energy.19

Thus, the lower emission quantum yield of Ru(TzBA)complexes is attributed to non-radiative decay through the H-bonding network of the carboxyl moiety. However, all compoundsexhibit a quantum yield (Ø) in the range between 0.002 and 0.009.The highest photochemically generated quantum yield achievedwas Ø ¼ 0.009 for Ru(TzBA) 5. The obtained quantum yield ofsample 5 is comparable to reported Ru tetrazole complexes forfuture PL and EL studies (ESI, S3†). Such values are lower thanthat found for Ru(bpy)3

2+, which shows a PL efficiency of 0.06under the same experimental conditions.

As outlined above, in mixed-ligand complexes of this type thelocation of the emitting triplet state of the ligands is animportant factor. Particularly, for heteroleptic tetrazole-con-taining complexes such as Ru(TzBA) derivates the situation ismore complicated than for the ruthenium polypyridinecomplexes.




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To our knowledge, there has been no detailed discussion onthe energy transfer mechanism for this initial step in excited-state evolution in this class of molecules. So, more photo-physical studies are required to improve the understanding ofthe relationship between the population of 3MLCT of theligands, and radiative and non-radiative processes. Thesestudies should involve femtosecond time resolved transientabsorption spectroscopy.

At temperatures above 77 K, the luminescence lifetimeexperiments suggested the presence of a non-emissive, rapidlyrelaxing triplet state at least 3000 cm�1 higher in energy thanthe 3MLCT state.20 Correlations of the activated crossover to thisnon-emissive state with the photochemical substitutionallability of the complex led to the assignment of this state as atriplet ligand eld state of bpy (Fig. 5). In the particular casewhere the 1MLCT state intersystem crosses to the 3MLCT stateexclusively, the population of the 3(p–p*) states of the ligandsdepends on the relative values of kr and k 0

r (kr and k 0r ¼ radiative

decay constants). Of course, the relative magnitude of kr and k 0r

depends on the energy gap between the 3MLCT and 3(p–p*)states. From the PL spectra, the triplet level states of TzBA andpyTz are lower than that of the bpy ligand. Finally, the higheremission efficiency at 500 nm than 725 nm might be due to thehigher population of 3TzBA and 3pyTz than 3bpy, which indi-cates that kr < k 0

r. Since the focus of this article is on examiningthe effects of different ligands on OLED properties, it is worthpresenting in future studies the luminescence decay for severalkinetic cases, where forward and reverse energy transfer arecomparable relative to the relaxation of either of the states.

FT-IR spectroscopy has been shown to be a powerful tool forextracting structural information from molecules. The mainabsorption peaks in the IR spectra of the ve Ru(TzBA)complexes are listed in Table 1.

Generally, in the IR spectra of phen, bpy, pyTz and TzBA,strong bands were observed in the frequency region between1400 and 1650 cm�1, one band occurring at 1505 cm�1, thesecond appearing at 1590 cm�1 and the third band at 1423cm�1. The region of particular interest is between 1800 and1000 cm�1, as the various C–O stretching bands which arefound here indicate the types of C–O bonding which are presentin the molecule. This region is complicated, with vibrations ofthe bpy framework, carboxylic acid and carboxylate groups allcontributing to the spectra. There are a number of bands atlower energy (to 1000 cm�1) in the dyes, which contain both C–Cand C–N stretching and C–H deformation character. The NCS-group has two characteristic modes, n(NC) and n(CS), which arefrequently considered as diagnostic with respect to the coordi-nation mode of the ambidentate NCS ligand. The IR spectra of

Table 1 The main absorption peaks in the IR spectra of complexes 1–5 (cm�1)

No. n(C]S) ns(COO�) nas(COO

�) Dn

1 — 1427 1634 2072 2101 1426 1627 2013 — 1476 1633 1574 2100 1427 1628 2015 2108 1445 1633 188


Ru(TzBA) complexes 2, 4 and 5 show an intense absorbance at2120 cm�1 for n(NC) and 780 cm�1 for n(CS) due to the N-coordinated NCS ligand. The absence of IR bands at about1700 cm�1 in the FT-IR spectra indicates that there are no acidicC]O bonds present. The other intense band in the spectra at1380 cm�1 is assigned to the symmetric stretch of –CO2

�, sinceit is not assignable to the polypyridine ligands.21

Further structural information on the complexes wasobtained from 1H NMR spectroscopy. The 1H NMR spectrum ofthe TzBA ligand shows two doublet peaks at 7.8 and 8.2 ppmdue to the hydrogen of phenyl, and one broad peak at 10.8 ppmdue to the carboxylic acid moiety of TzBA. The pyTz ligand alsoshows four groups of peaks of pyridine of the pyTz and Tzmoiety and both TzBA and pyTz ligands show one single sharppeak at 6.6 ppm due to N–H of the Tz moiety. The phen ligandalso shows two doublet peaks at 8.06 and 9.05 ppm and twotriplet peaks at 7.60 and 7.80 ppm. The downeld-shiedproton resonance peaks can be assigned to the protons that areclose to the nitrogen of the pyridine units, whereas the high-eld proton resonances are assigned to the protons that are inthe vicinity of non-nitrogen atoms. This assignment is based onthe assumption that deshielding of the protons will be due to aninduced magnetic eld created by the ring current circulationon pyridine aromatic moieties. This shielding is only signicantat short distances and therefore only affects protons that areclose to the pyridine moieties.22 The 1H NMR spectra of allcomplexes show all the peaks of pyTz, TzBA, phen and bpy inregion 7.2–9.5 ppm, indicating the presence of the ligands inthe complexes. The absence of any peak in the region of 5–7ppm due to the N–H moiety of Tz indicates that pyTz and TzBAwere coordinated to the Ru metal through the nitrogen atom ofTz. Also, the presence of one peak at around 11 ppm is assignedto the hydrogen of the carboxylic acid of the TzBA ligand. Thesignal integration for complex 1 reveals the incorporation of twophen and two TzBA units. For complex 2, two phen units andone TzBA unit were found by signal integration. Signal inte-gration for complex 3 reveals the incorporation of two bpy andtwo TzBA units. For complex 4, one phen, one TzBA and onepyTz unit were found by signal integration. The signal integra-tion for complex 5 reveals the incorporation of one bpy, onepyTz and one TzBA unit. The difference between the chemicalshi of the free ligands and complexes indicates that thecoordination of the ligands to metal occurred. CHN and ICPanalysis were carried out to determine the presence of ligandsand metal in the complexes. CHN analysis of the compoundswas obtained as follows: Anal. Calc. for 1, (C40H26N12O4Ru): C,57.189; H, 3. 119; N, 20. 024. Found: C, 57.196; H, 3. 148; N, 20.108%. Anal. calc. for 2, (C33H21N9O2Ru): C, 53.507; H, 2.854; N,17.017. Found: C, 53.588; H, 2.890; N, 17.0178%. Anal. calc. for3, (C36H26N12O4Ru): C, 54.614; H, 3.307; N, 21.229. Found: C,54.598; H, 3.289; N, 21.238%. Anal. calc. for 4,(C27H17N12O2SRu): C, 48.071; H, 2. 537; N, 24.913. Found: C,48.112; H, 3.380; N, 25.001%. Anal. calc. for 5,(C25H17N12O2SRu): C, 46.152; H, 2.631; N, 25.832. Found: C,46.189; H, 2.638; N, 25.846%. Ru was analyzed on a PLASMA-SPEC (I) ICP atomic emission spectrometer. A few mg of thecomplexes (0.02 g) were added to HNO3 (68%) and nally

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diluted in water to a 1 : 10 ratio for measurement. Theconcentrations found for all complexes were estimated to beabout 4.2–4.5 ppm. The cyclic voltammograms of complexes 1–5show the features characteristic of ruthenium–polypyridinecomplexes, with a pseudo reversible metal based oxidation(RuIII/RuII) at positive potential and two pseudo reversiblereductions at negative potentials. The rst reduction wave isdue to the reduction of the carboxyl ligand, and is followed at amore negative potential by successive one-electron reductionsof the bpy, phen and pyTz ligands. The presence of the pyTzligand on the complex does affect the redox potential values.Our results show that the pyTz ligand makes oxidation of themetal and reduction of the ligand easier. The LUMO and HOMOof the ground state of emitters 1–5 are about 5–6.2 and 2.5–3.8 Vversus NHE, respectively.

3.2 DFT calculations

Fig. 6 shows the four highest and four lowest molecular orbitalenergy levels of the novel Ru(TzBA) complexes. Note that theLUMO energy levels of the Ru(TzBA) complexes containing bpyand phen ligands (1–3) are relatively higher than those of theRu(TzBA) complexes containing the pyTz ligand (4 and 5), withan estimated value of 1.49 eV, and that their HOMO energylevels are relatively lower than those of the Ru(TzBA) complexescontaining the pyTz ligand (4 and 5) with a difference of

Fig. 6 Diagram of the four highest occupied and four lowest unoccupiedmolecular orbital levels of the novel Ru(TzBA) complexes.

6976 | J. Mater. Chem. C, 2013, 1, 6970–6980

approximately 1.40 eV. The presence of the pyTz liganddecreases the HOMO–LUMO gap. For the acceptor moiety,carboxylic acid is the main functional group, and it is a goodanchoring functional group for dye sensitized solar cell appli-cation.23 The presence of the ancillary pyTz ligand decreasedtheir LUMO energy levels considerably relative to [Ru(SCN)-(phen)(pyTz)(TzBA)] and [Ru(SCN)(bpy)(pyTz)(TzBA)] complexesby approximately 1.59 eV. The relative positions of the LUMOlevels for the donor and acceptor moieties are very important forcharge transport to be effective.24 Comparing the LUMO levelsof the [Ru(phen)2(TzBA)2], [Ru(SCN)(phen)2(TzBA)], and [Ru-(bpy)2(TzBA)2] complexes with those of the [Ru(SCN)-(phen)(pyTz)(TzBA)], Ru(SCN)(bpy)(pyTz)(TzBA)] complexeswould show that they have higher LUMO levels. The donor andacceptor moieties cited above have a LUMO donor–LUMOacceptor difference that ranges from approximately 1.6–2.4 eV,which is sufficient for charge transport. Fig. 7 shows the HOMOand LUMO orbital spatial orientation of the Ru(TzBA)complexes 1–5 synthesized by our group, suggesting that thecharge transport is related to the spatial distribution of thefrontier orbitals. The HOMO should be localized on the donormoiety and the LUMO on the acceptor moiety.25 The HOMO ofcomplex 5 is localized on the donor moiety and the LUMO ismostly in the acceptor region. The presence of rutheniummetalmight be responsible for the delocalization of both the HOMOand LUMO, suggesting the presence of a metal-to-ligand chargetransfer (MLCT) or metal-centered transitions from both themetal and the ligand p orbitals in the LUMO.26 Another note-worthy feature in Fig. 6 and 7 is the effect of the attached pyTzligand on the donor moiety. The pyTz ligands seem to localizethe HOMO on their acceptor moiety. There is also a trend in thecombination of donor moieties with the acceptor moieties: if

Fig. 7 Molecular orbital spatial orientation for Ru(TzBA) complexes.



Fig. 9 Schematic showing the energy levels of the device and the dynamicprocesses of EL emission in Ru(TzBA) 1–5 .


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both have small HOMO–LUMO gaps, the donor–acceptor pairswould be more likely to have small HOMO–LUMO gaps.

3.3 EL characterization

An emissive layer without ruthenium complex was fabricated torecord the PVK:PBD EL spectra and to nd a relationshipbetween the EL spectra of ruthenium compounds and PVK:PBDin order to separate it from the emission of the Ru(TzBA) andcomplexes (Fig. 8). In the EL spectra, ruthenium complexesshow a long red shi unlike the PVK:PBD EL spectra, demon-strating that effective energy transfer is taking place in theemissive layer. As the driving voltage is biased to the electrodes,the electrons and holes begin to inject into the emitter. Someelectrons and holes under electromagnetic forces form excitons,which emit light with their annulation between the HOMO andLUMO energy levels of the layers. The formation of Forstertransfer is possible in Ru(TzBA) complexes 1–3 since there isoverlap between the absorption spectra of the emitters and theemission spectrum of PVK:PBD. Moreover, the arrangement ofthe layers causing the Forster transfer of energy resulted fromthe PVK:PBD host to the ruthenium complexes (Fig. 9).

As shown in Fig. 8, the range of EL emission colors can becontrolled through the introduction of different ancillaryligands into the ruthenium complexes. The most red shiedwavelength is from the Ru(TzBA)(L) complexes in which L is bpyor phen. Furthermore, the wavelength of the bands of theRu(pyTz)(TzBA)(L), in which L is bpy or phen, are similar. Thevariable which could affect the red or blue shi in wavelength ofRu(TzBA) complexes is the nature of the ancillary ligandscoordinated to Ru(II) ion, since the nature of the TzBA is similar.Therefore, pyTz is especially effective in tuning the color inRu(TzBA) complexes.

To achieve a good balance of holes and electrons, both hole-and electron-transporting functions (PVK:PBD) were

Fig. 8 EL spectra of PVK:PBD and Ru(TzBA) complexes 1–5 in a PVK:PBD blend.


incorporated into a single Ru(TzBA) emitter. It should be notedthat our results show that a high ratio of PBD leads to a lowerstability of the device due to the decrease in the ratio of PVK.

However, the ruthenium devices made without PBD in theblend layer are weakly electroluminescent, and PVK is thedominant emitter. Finally, an efficient ratio of 100 : 40 : 8 forPVK : PBD : Ru(TzBA) was suggested, which reduces the risk ofexcimer formation and increases the stability of the device. Theuse of a PEDOT:PSS interlayer also helps improve hole injectioninto the emissive layer, resulting in an increase in the currentand the probability of exciton formation.

Particularly, the role of the ancillary ligand is not only tosaturate the coordination number of the ruthenium ion but alsoto improve the volatility and stability of the ruthenium complex.The carrier-transporting and light emitting properties can beimproved by using tetrazole as the ancillary ligand, which hastwo more nitrogen atoms than phen or bpy. Hence, it is obviousthat the ancillary ligand plays an important role in rutheniumcomplex-based OLEDs. Because TzBA is incorporated to allcomplexes 1–5, the major difference in the structure of thecomplexes is limited to the ancillary ligand. Ru complexes withboth TzBA and phen/bpy (2, 3) as ancillary ligands showed ELpeaks centered at 625 nm, while ruthenium complexes withTzBA, phen/bpy and pyTz (complex 4, 5) showed EL peakscentered at 600 nm. As shown in Fig. 8, the emission of ruthe-nium complexes with TzBA and phen as ligands was moved to amuch lower wavelength of 525 nm. In our recent report,8

unusual EL (l ¼ 500 nm) has also been observed in Ru(Tz)-(phen), which indicated the role of the second ligand in thecolor tuning for applications in lighting and full color displays.The current density ( J) versus applied voltage (V) characteristicsof devices 1–5 are shown in Fig. 10. Device 5 gives the highest ELefficiency of all the devices. Detailed device characteristics for

J. Mater. Chem. C, 2013, 1, 6970–6980 | 6977


Fig. 10 Current density versus applied voltage for devices 1–5.

Fig. 11 The maximum efficiency (LE) versus applied voltage (V) characteristics ofRu devices 1–5.

Fig. 12 CIE coordinates of Ru(TzBA) 1–5.


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all Ru(TzBA) complexes 1–5 are shown in Table 2. One of theways to improve the EL of ruthenium complexes is to introducean ancillary ligand, such as Bphen, phen or bpy, into the metalcomplex.27

Here, pyTz is incorporated into Ru complexes as a newancillary ligand. In light of the large radius and the occurrenceof a stereochemically active lone pair of electrons in the ve-membered N-heterocycle-based ligand containing a pyridineligand, we anticipate that the coordination of pyTz to the Ru(II)center may lead to the formation of a new optical material witha structure different from that of the transition metal/pyTz.Fig. 11 shows the maximum efficiency (LE) versus appliedvoltage (V) characteristics of Ru devices 1–5. Device 5 also has aluminance of 480 cd m�2 and a maximum efficiency of 1.2 cdA�1 at 16 V. The order of maximum efficiency is 5 > 4 > 3 > 2 > 1.The turn-on voltages of devices 1–5 are approximately 3–7.5 V,respectively. To the best of our knowledge, there are only tworeports on the OLED performance of ruthenium tetrazolecomplexes (ESI, S3†). However, in our experiment, themaximum current density and luminous efficiency of Ru(TzBA)5 are better than those of both the ruthenium tetrazole derivatesand the known sample of [Ru(bpy)3](PF6)2 with the ITO/PEDOT:PSS/PVK:PBD/[Ru(bpy)3]

2+/Al device conguration.So, the obtained LED performance of sample 5 is acceptable

among reported Ru complexes for future investigations. Thehighest power efficiency was found to be about 0.1 lm W�1 fordevice 5. For device 5, the full width at half maximum (FWHM),

Table 2 Device characteristics of Ru(TzBA) complexes 1–5

No.ELmax

(nm) CIE (x, y)FWHM(nm)

CCT(K)

Maximum current density,at 20 V (mA cm�2)

1 525 (0.28, 0.57) 84 6506 1632 619 (0.59, 0.32) 101 2237 1933 602 (0.53, 0.39) 86 1809 2604 621 (0.63, 0.34) 178 2254 2625 596 (0.52, 34) 182 1943 275

6978 | J. Mater. Chem. C, 2013, 1, 6970–6980

color coordinates in the Commission Internationale del'Eclair-age (CIE 1931) chromaticity at 16 V and the correlated colortemperature (CCT) were 90 nm, (0.63, 0.36) and 3502 K,respectively (Table 2).

Fig. 12 shows the CIE coordinates of Ru(TzBA) 1–5. Fromthese results, the optical properties of the complexes are relatedto the structure of the ligands. So, we can control the emission

Turn-onvoltage (V)

Luminous efficiency(cd A�1) at 16 V

Luminance(cd m�2) at 16 V

Power efficiency(lm W�1)

7 0.87 365 0.717.5 0.98 380 0.793 1.07 412 0.905 1.12 230 0.916 1.2 480 0.1




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properties of OLED devices by the use of different ancillaryligands. What is the emission mechanism of the EL of Ru(pyTz)complexes 4 and 5 as a blend layer in PVK:PBD?

We suggest that the new red shied emission is from elec-troplex emission that occurred between PVK and the Rucomplexes 4 and 5.27 An electroplex is a particular emissivespecies that is different from an exciplex. It is a cross-recom-bination between electron transporting material and holetransporting material.

According to Kalinowski's model,28 if the difference betweenthe LUMO of A and the LUMO of D is high, an electroplex willoccur. As is shown in Fig. 9, PVK is good hole-transportingmaterial and holes can easily inject into PVK and transport in it.

However, it is difficult for holes to inject into the Ru layerbecause of a barrier of 1.2 eV at the interface of PVK–Rucomplexes. Therefore, holes (electrons) will be blocked by theRu complexes (PVK) and accumulate at the PVK–Ru complexinterface. It is difficult for electrons to inject into the PVK layer.When electrons are injected into the Ru complex layer, there aretwo possibilities: either electrons leave the Ru complex layerand form excitons in PVK, or they recombine with the holes atthe interface and lead to electroplex emission. In this way, therecombination region usually occurs on both sides near thePVK–Ru complex interface.27

We note that the device structure has the potential forfurther modication. For example, the use of LiF cathodes,novel hole transport materials and p-extended ancillary ligandsthat result in increased efficiency are also applicable to thiswork.

4. Conclusion

The aim of this study is to look for emitters whose EL emissioncould be tuned by the use of different ancillary ligands. For thispurpose, the synthesis and characterization of ve new ruth-enium(II) emitters with 4-(1H-tetrazole-5-yl)benzoic acid (TzBA)as a new basic ligand and 2,2-bipyridine (bpy), 1,10-phenan-throline (phen) and pyridine tetrazole (pyTz) as ancillary ligandshave been reported. The results show that the luminescence ofthe Ru(TzBA) complex depends on the ancillary ligand, whichcan tune of the EL color of the Ru(TzBA) complexes. Therefore,the presence of both pyTz and bpy as ancillary ligands was agood composition for LED applications. To our knowledge, thisnew class of Ru tetrazole complexes has not been previouslyreported, and this nding opens the way to designing emittersfor light emitting devices by further modication of the ancil-lary ligand architecture, which will enhance the efficiency oflight emitting diodes. We continue to improve our performancevia a continuous development program.

References

1 (a) A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belserand A. V. Zelewsky, Coord. Chem. Rev., 1988, 84, 85; (b)V. Balzani and A. Juris, Coord. Chem. Rev., 2001, 211, 97; (c)I. Ubinstein and A. J. Bard, J. Am. Chem. Soc., 1980, 102,6641; (d) N. E. Tokel-Takvoryan, R. E. Hemingway and


A. J. Bard, J. Am. Chem. Soc., 1973, 95, 6582; (e) X. Hong,Z. Yingying, L. Dan, L. Mao, Z. Chengbo, Y. Bing andM. Yuguang, J. Phys. Chem. B, 2006, 110, 18718; (f)H. Rudmann, S. Shimada and M. F. Rubner, J. Am. Chem.Soc., 2002, 124, 4918.

2 E. Baranoff and J. Fossey, Annu. Rep. Prog. Chem., Sect. B,2012, 108, 71.

3 D. Pucci, A. Bellusci, A. Crispini, M. Ghedini, N. Godbert,E. Szerb and A. Talarico, J. Mater. Chem., 2009, 19, 7643.

4 (a) S. Bernhard, A. Jason, P. Houston, H. Abru~na,J. Ruglovksy, X. Gao and G. Malliaras, J. Am. Chem. Soc.,2002, 124, 13624; (b) A. Hector and A. Bard, J. Am. Chem.Soc., 1982, 104, 2641; (c) M. Richter, J. Am. Chem. Soc.,1982, 104, 2641.

5 (a) W. Browne, N. O'Boyle, W. Henry, A. Guckian, S. Horn,T. Fett, C. O'Connor, M. Duati, L. De Cola, C. Coates,K. Ronayne, J. McGarvey and J. Vos, J. Am. Chem. Soc.,2005, 127, 1229; (b) M. Klingele and S. Brooker, Coord.Chem. Rev., 2003, 241, 119; (c) S. Fanni, T. Keyes,C. O'Connor, H. Hughes, R. Wang and J. Vos, Coord. Chem.Rev., 2000, 208, 77; (d) S. Stagni, A. Palazzi, S. Zacchini,B. Ballarin, C. Bruno, M. Marcaccio, F. Paolucci,M. Monari, M. Carano and A. Bard, Inorg. Chem., 2006, 45,695.

6 (a) M. Zhang, Y. Zhao, H. Wu, S. Batten and S. Ng, DaltonTrans., 2006, 3170; (b) M. Dinca, A. Yu and J. Long, J. Am.Chem. Soc., 2006, 128, 8904; (c) O. El-Kadri, M. Heeg andC. Winter, Dalton Trans., 2006, 4506; (d) P. Lin, W. Clegg,R. Harrington and R. Henderson, Dalton Trans., 2005,2388; (e) A. Downward, P. Steel and J. Steenwijk, Aust. J.Chem., 1995, 48, 1625.

7 (a) P. Zhu and H. Li, J. Mol. Struct., 2011, 992, 106; (b)G. Wang and Y. Zhang, Inorg. Chim. Acta, 2011, 376, 212;(c) W. C. Song, J. g. Li, P. O. Song, Y. Tao, Q. Yu,X. N. Tong and X. H. Bu, Inorg. Chem., 2009, 48, 3792.

8 H. Shahroosvand, L. Naja, E. Mohajerani, M. Janghouri andM. Nasrollahzadeh, RSC Adv., 2013, 3, 6323.

9 (a) M. Thelakkat and H. Schmidt, Adv. Mater., 1998, 10, 219;(b) S. Ashraf, M. Shahid, E. Klemm, M. Al-Ibrahim andS. Sensfuss, Macromol. Rapid Commun., 2006, 27, 1454.

10 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,M. A. Robb, J. R. Cheeseman, J. A. Montgomery, T. Vreven,K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar,J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani,N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara,K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima,Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox,H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo,J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev,A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski,P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador,J. J. Dannen-berg, V. G. Zakrzewski, S. Dapprich,A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick,A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz,Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski,B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz,I. Komaromi, R. L. Martin, D. J. Fox, T. Keith,

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M. A. Al-Laham, C. Y. Peng, A. Nanayakkara,M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen,M. W. Wong, C. Gonzalez and J. A. Pople, Gaussian 03Revision B.04, Gaussian Inc., Wallingford, CT, 2004.

11 R. Dennington, T. Keith, J. Millam, K. Eppinnett,W. L. Hovell and R. Gilliland, Gauss View Version 3.08,Semichem, Inc., Shawnee Mission, KS, 2003.

12 Z. Demko and K. Sharpless, J. Org. Chem., 2001, 66, 7945.13 (a) Q. Li, G. Yang, Y. Ma, M. Li and Y. Zhou, Inorg. Chem.

Commun., 2008, 11, 795; (b) F. Chen, F. Zheng, G. Liu,A. Wu, M. Wang, S. Guo, M. Wua, Z. Liu, G. Guo andJ. Huang, Inorg. Chem. Commun., 2010, 13, 278; (c) P. Zhuand H. Li, J. Mol. Struct., 2011, 992, 106; (d) G. Wang,Y. Zhang and H. Li, Inorg. Chim. Acta, 2011, 376, 212; (e)Q. Li, G. Yang, X. Tang, Y. Ma, F. Zhou, W. Liu, J. Chenand H. Zhou, Inorg. Chem. Commun., 2010, 13, 254; (f)W. Song, J. Li, P. Song, Y. Tao, Q. Yu, X. Tong and X. Bu,Inorg. Chem., 2009, 48, 3792; (g) Q. Chen, Y. Li, F. Zheng,W. Zou, M. Wu, G. Guo, A. Wu and J. Huang, Inorg. Chem.Commun., 2008, 11, 969; (h) Q. Li, F. Zhou, C. Zhai,L. Shen, X. Tang, J. Yang, G. Yang, Z. Miao, J. Jin and W.Shen, Inorg. Chem. Commun., 2011, 14, 843; (i) G. Yang,Q. Li, Y. Zhou, G. Gu, Y. Ma and R. Yuan, Inorg. Chim.Acta, 2009, 362, 1234; (j) G. Yang, Y. Ma, Q. Li, Y. Zhou,G. Gu, Y. Wu and R. Yuan, J. Coord. Chem., 2009, 62, 1766;(k) X. Zhang, Q. Yu, H. Bian, S. Yan, D. Liao, W. Gu andH. Liang, Aust. J. Chem., 2008, 61, 303; (l) G. Yang,D. Chen, C. Zhai, X. Tang, Q. Li, F. Zhou, Z. Miao, J. Jinand H. Ding, Inorg. Chem. Commun., 2011, 14, 913; (m)G. Yang, Q. Li, Y. Zhou, G. Gu, Y. Ma and R. Yuan, Inorg.Chem. Commun., 2008, 11, 1239; (n) D. Fu, J. Ge, J. Dai,H. Ye and Z. Qu, Inorg. Chem. Commun., 2009, 12, 99; (o)M. Wu, G. Xu, F. Zheng, Z. Liu, S. Wang, G. Guo andJ. Huang, Inorg. Chem. Commun., 2011, 14, 333; (p)G. Yang, Q. Li, Y. Zhou, G. Gu, Y. Ma and R. Yuan, Inorg.Chim. Acta, 2009, 362, 1234.

14 (a) M. Irfanullah and K. Iikha, J. Fluoresc., 2011, 21, 673; (b)J. Bunzli and C. Piguet, Chem. Soc. Rev., 2005, 34, 1048; (c)M. Werts, Sci. Prog., 2005, 88, 101; (d) K. Binnemans, Chem.Rev., 2009, 109, 4283; (e) E. Brunet, J. Olga and J. Rodriguez-Ubis, Curr. Chem. Biol., 2007, 1, 11; (f) M. Latva, H. Takalo,V. Mukkala, C. Matachescu, J. Rodrıguez-Ubis andJ. Kankare, J. Lumin., 1997, 75, 149; (g) N. Sabatini andM. Guardigli, Coord. Chem. Rev., 1993, 123, 201; (h)I. Hemmila and V. Laitala, J. Fluoresc., 2005, 15, 529.

6980 | J. Mater. Chem. C, 2013, 1, 6970–6980

15 Y. Shiraishi, Y. Furubayashi, G. Nishimura and T. Hira, J.Lumin., 2007, 126, 68.

16 (a) N. H. Damrauer, G. Cerullo, A. Yeh, T. R. Boussie,C. V. Shank and J. K. McCusker, Science, 1997, 275, 54;(b) A. C. Bhasikuttan, M. Suzuki, S. Nakashima andT. Okada, J. Am. Chem. Soc., 2002, 124, 8398; (c)J. N. Demas and D. G. Taylor, Inorg. Chem., 1979, 18,3177; (d) J. N. Demas and A. W. Adamson, J. Am. Chem.Soc., 1971, 93, 1800.

17 (a) M. G. Colombo and H. U. Gudel, Inorg. Chem., 1993, 32,3081; (b) M. G. Colombo, A. Hauser and H. U. Gudel, Inorg.Chem., 1993, 32, 3088.

18 C. A. Parker, Measurement of Fluorescence Efficiency, ElsevierPublishing Co., New York, 1968, pp. 261–269.

19 D. A. Jose, P. Kar, D. Koley, B. Ganguly, W. Thiel,G. Ramakrishna, D. K. Palit, H. N. Ghosh and A. Das,Inorg. Chem., 2007, 46, 5576.

20 J. Van Houten and R. J. Watts, J. Am. Chem. Soc., 1976, 98,4853.

21 K. Nakamato, Application in Coordination, Organometallicand Bioinorganic, John Wiley & Sons, New York, NY, USA,1997.

22 F. Odobel, E. Blart, M. Lagree, M. Villieras, H. Goujtita, M. ElMar, S. Caramori and C. Bignozzi, J. Mater. Chem., 2003, 13,502.

23 M. Al-Ibrahim, A. Konkin, H. Roth, D. Egbe, E. Klemm,U. Zhokhavets, G. Gobsch and S. Sensfuss, Thin SolidFilms, 2005, 474, 201.

24 W. Campbell, A. Burrell, D. Officer and K. Jolley, Coord.Chem. Rev., 2004, 248, 1363.

25 J. Sauvage, J. Collin, J. Chambron, S. Guillerez, C. Coudret,V. Balzani, F. Barigelletti, L. De Cola and L. Flamigni,Chem. Rev., 1994, 94, 993.

26 C. Houarner-Rassin, E. Blart, P. Buvat and F. Odobel, J.Photochem. Photobiol., A, 2007, 186, 135.

27 (a) H. Shahroosvand, L. Naja, E. Mohajerani, A. Khabbaziand M. Nasrollahzadeh, J. Mater. Chem. C, 2013, 1, 1337;(b) H. Shahroosvand, F. Nasouti, E. Mohajerani andA. Khabbazi, Synthesis, Opt. Mater., 2012, 35, 79; (c)H. Shahroosvand, F. Nasouti, A. Sousaraei, E. Mohajeraniand A. Khabbazi, Phys. Chem. Chem. Phys., 2013, 15,9899; (d) H. Shahroosvand, F. Nasouti, E. Mohajeraniand A. Khabbazi, J. Lumin., 2013, 135, 339.

28 J. Kalinowski, M. Cocchi, P. Dimarco, W. Stampor, G. Giroand V. Fattori, J. Phys. D: Appl. Phys., 2000, 33, 2379.



Going from green to red electroluminescence through ancillary ligand substitution in ruthenium(ii) tetrazole benzoic acid emitters

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