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Luminescent 1D chain of platinum(II) terpyridyl units withp-dithiobenzoquinone organometallic linker: self-aggregationimparted from Pt · · · Pt/p–p interactions

Jamal Moussa,a Keith Man-Chung Wong,b Lise-Marie Chamoreau,a Hani Amouri*a andVivian Wing-Wah Yam*b

Received 12th June 2007, Accepted 13th June 2007First published as an Advance Article on the web 13th July 2007DOI: 10.1039/b708922h

Unlike p-dithiobenzoquinone (1), which is extremely reactive and has never been isolated, themetal-stabilised p-dithiobenzoquinone [Cp*Ir(g4-C6H4S2)] (2c) was prepared and used as an efficientorganometallic linker to construct novel supramolecular assemblies. Treatment of 2c with theelectrophilic Pt(II)(terpy) building blocks produced the supramolecular assembly{[Pt(terpy){Cp*Ir-p-(g4-C6H4S2)}Pt(terpy)][OTf]4}n (3), which was fully characterised and its molecularstructure was determined by X-ray crystallography. The structure of 3 revealed the presence of p–p andPt · · · Pt interactions among individual molecules describing a 1D chain. Complex 3 showed unusualUV/Vis absorption and luminescence behaviour at low temperature, imparted from self-aggregationmediated by p–p and Pt · · · Pt interactions.

Introduction

Supramolecular assemblies with Pt(II)(terpy) subunits have re-ceived considerable attention in recent years due to their pho-toluminescent properties.1–5 In fact such compounds exhibit bothd8–d8 metal–metal interactions and p–p stacking due to the planarterpyridyl ligand which has an extended p-conjugated system.These metal–metal and p–p interactions have been suggested toplay important roles in the photophysical excited states of thesecomplexes and a tuning of these interactions has been shown to in-duce a change in the nature and colour of the remarkable emissionproperties displayed by these Pt(terpy) systems.1–4 Interestingly,such assemblies are generally constructed using organic linkers.

On the other hand, in the course of our research to preparenew functional materials,6 we designed the organometallic linkers[Cp*M(g4-p-benzoquinone)],7 M = Rh, LRh (2a); M = Ir, LIr

(2b) and showed their ability to construct 1-D coordinationpolymers of formula {[Ag3(LM)2(CH3CN)2(OTf)][LM](OTf)2}n.8

These complexes exhibit Ag · · · Ag argentophilic interactions.Pursuing our research in this area,8,9 we recently designed a

novel organometallic linker based on a thioquinonoid framework.Thus the first metal-stabilised p-dithiobenzoquinone [Cp*Ir(g4-C6H4S2)] (2c) was reported and fully characterised.10 We antici-pated that the sulfur atoms in the thioquinonoid system should bemore nucleophilic than the related oxygen centres in the quinonoidstructure. It is worth mentioning that p-dithiobenzoquinone (1)is a reactive intermediate and has never been isolated, in fact theparent compound (1) (Fig. 1) has been generated and characterisedspectroscopically only at low temperature (10 K) in an argon

aLaboratoire de Chimie Inorganique et Materiaux Moleculaires, UMRCNRS 7071, Universite Pierre et Marie Curie-Paris 6, 4 place Jussieu,case 42, 75252, Paris Cedex 05, France. E-mail: amouri@ccr.jussieu.frbCentre for Carbon-Rich Molecular and Nano-Scale Metal-Based MaterialsResearch and Department of Chemistry, The University of Hong Kong,Pokfulam Road, Hong Kong (P.R. China). E-mail: wwyam@hku.hk

Fig. 1 Schematic drawings of 1, 2c and the related supramolecularassembly 3.

matrix11 because it is extremely reactive. Due to the stabilisingproperties of the “Cp*Ir” moiety,12 the elusive intermediate isisolated as a g4-metal complex (Fig. 1). Computational analysesusing DFT10a confirm a net transfer of about 0.8 units of electrondensity to the p* ligand LUMO by metal-to-ligand p-back-bonding. This additional electron density is largely localisedaround the thiocarbonyl bonds, resulting in a reduction in C=Sbond order and an increase in the nucleophilic character of thesulfur centres, thus rendering the complex [Cp*Ir(g4-C6H4S2)] (2c)an efficient organometallic linker.

In this paper we report the synthesis and crystal structureof a rare 1D chain {[Pt(terpy){Cp*Ir-p-(g4-C6H4S2)}Pt(terpy)]-[OTf]4}n (3) assembled via the organometallic linker 2c and usingPt(terpy) units as building blocks. In addition the photolumine-scent properties of 2c and 3 are also reported.

Results and discussion

Synthesis, characterisation and X-ray molecular structure of{[Pt(terpy){Cp*Ir-p-(g4-C6H4S2)}Pt(terpy)][OTf]4}n (3)

Treatment of [Cp*Ir-p-(g4-C6H4S2)] (2c) with two equivalents of[Pt(terpy)Cl]Cl in CH3CN in the presence of AgOTf providedan orange solution, from which an orange complex was isolatedin 82% yield and was identified as a 1D supramolecular assem-bly, {[Pt(terpy){Cp*Ir-p-(g4-C6H4S2)}Pt(terpy)][OTf]4}n (3). The

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1H NMR of 3 recorded in CD3CN shows the presence of a singletat d 7.30 ppm attributed to the metallated p-dithiobenzoquinoneand a singlet at d 1.92 ppm assigned to the Cp*Ir moiety. Furtherwe note the presence of three multiplets in the range d 7.30–8.90 ppm assigned to the “(terpy)Pt” fragment. In addition, theX-ray molecular structure of 3 was determined (vide infra, Table 1).

Complex 3 crystallises in the triclinic unit cell, space groupP1 with Z = 2. A view of the cationic part of the complexand selected bond distances and angles are shown in Fig. 2a.The structure of 3 shows indeed that the p-dithiobenzoquinoneiridium complex bridges two “Pt(terpy)” fragments through thetwo sulfur centres with two equal Pt–S bond distances of 2.330 A.In this supramolecular system, “Pt–Ir–Pt”, the “Pt(terpy)” unitscan be described as two large wings of a butterfly where themain body is defined by the p-dithiobenzoquinone complex,[Cp*Ir-p-(g4-C6H4S2)]. The coordination geometry around each

Fig. 2 (a) View of the cationic part of complex 3. Selected bond distances(A) and angles (deg) for 3: Ir1 · · · C16 = 2.346(6), Ir1 · · · C17 = 2.230(3),Ir1 · · · C18 = 2.252(6), Ir1 · · · C19 = 2.343(10), Ir1 · · · C20 = 2.244(2),Ir1 · · · C21 = 2.248(10), C16S1 = 1.763(12), C19S2 = 1.765(13). (b) 1 Dsupramolecular chain formed through p–p and Pt · · · Pt contacts betweenindividual molecules.

Table 1 Crystal data and structure refinement details for 3

Empirical formula C54H47F12IrN8O12Pt2S6

M 2002.74Crystal system TriclinicSpace group P1a/A 12.836(2)b/A 16.1552(10)c/A 17.537(2)a/deg 69.378(6)b/deg 89.134(9)c /deg 75.192(8)V/A3 3279.3(7)Z 2qcalcd/g cm−3 2.028T/K 250 (2)k/A 0.71073l,/cm−1 65.69R1, wR2 (I > 2r(I))a 0.0596, 0.1437R1, wR2 (all data)a 0.1421, 0.1715

a R1 = (R‖F o| − |F c‖)/R |F o|; wR2 = [R [w(F o2 − F c

2)2]/R [w(F o2)2]]1/2.

Pt centre is distorted square planar, formed by three nitrogenatoms of the terpy ligand and one sulfur centre from the bridgingorganometallic linker 2c. Due to this coordination mode, thearomatic ring of the organometallic linker (2c) is almost planar,with the thioquinone carbons bent out of the diene plane whileacting as a bidentate ligand, and connects two Pt(II) atoms suchthat every thioquinone sulfur is monodentate (hinge angle h =7.7◦ and the other hinge angle h = 5.6◦). These angles are smallerthan those reported for the free organometallic linker [Cp*Ir-p-(g4-C6H4S2)] (2c).10

The two Pt(terpy) wings in 3 are not linearly disposed butare puckered and lean backward with a Pt1 · · · Pt2 distance in[Pt(terpy){Cp*Ir-p-(g4-C6H4S2)}Pt(terpy)][OTf]4 (3) of 8.088 A.Analysis of the crystal packing of 3 reveals important informationsuch as that the molecules are stacked in a head-to-head and tail-to-tail fashion with two sets of p–p/Pt1 · · · Pt1 or p–p/Pt2 · · · Pt2interactions occurring at each side of one Pt(terpy) arm withthe adjacent molecule. For instance the first Pt(terpy) arm unitinteracts with the adjacent Pt(terpy) fragment through a p–pinteraction at d = 3.461 A, h = 25.07◦ while the Pt1 · · · Pt1 distanceis 4.037 A suggesting the absence of an interaction. The otherPt(terpy) arm forms a stronger p–p interaction with the adjacentPt(terpy) with d = 3.385 A h = 20.24◦ and exhibits a weakPt2 · · · Pt2 contact at 3.596 A. These Pt · · · Pt/p–p interactionsamong individual subunits of 3 construct a 1D supramolecularchain. Other platinum(II) complexes have also shown similarextended linear chain packing.1–4

To our knowledge this is the first example of a 1D supramolec-ular chain of Pt(terpy) subunits assembled by an organometalliclinker [Cp*Ir-p-(g4-C6H4S2)] (2c) through an unusual coordinationof C=S · · · Pt(terpy) interactions. We anticipated that such a newclass of compound might present interesting photoluminescentproperties due to this unusual coordination, which inducesPt · · · Pt/p–p interactions along the 1D chain (Fig. 2b).

Photophysical properties of 2c and 3

The electronic absorption spectrum of 2c in acetonitrile at roomtemperature showed an intense absorption band at 318 nm witha shoulder at 394 nm, assignable to [p → p*] intraligand (IL)transitions of the Cp*Ir(g4-dithiobenzoquinone) system. Intensehigh-energy absorption at 348 nm and low-energy absorptionbands at 378 and 396 nm were observed in the electronicabsorption spectrum of 3 under the same conditions. Theirphotophysical data are summarised in Table 2. With reference toprevious spectroscopic studies of [Pt(terpy)Cl]+,4a the high-energyabsorption is attributed to admixtures of [p → p*] intraligand (IL)transitions of terpyridine and the Cp*Ir(g4-dithiobenzoquinone)system, while the low-energy absorptions are tentatively assignedas admixtures of [dp(Pt) → p*(terpy)] metal-to-ligand chargetransfer (MLCT) and metal-perturbed [p → p*] IL transitionsof the Cp*Ir(g4-dithiobenzoquinone) system.

Excitation of 2c in fluid solution and in the solid state atroom temperature did not yield any emission, while an intenseemission at 467 nm was observed in the solid state at 77 K,ascribed to metal-perturbed [p → p*] 3IL emission of the Cp*Ir(g4-dithiobenzoquinone) system. Similar to the precursor complex,[Pt(terpy)Cl]+, excitation of 3 in acetonitrile solution at roomtemperature showed no emission, which is attributed to the

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Table 2 Photophysical data for 2c and 3

Medium Absorption Emissionb

Complex (T/K) kmax/nm (emax/dm3mol−1cm−1) kmax/nm (so/ls)

2c MeCN (298) 318 (17120), 394 sh (6445) Non-emissiveSolid (298) Non-emissiveSolid (77) 467

3 MeCN (298) 348 (24660), 378 (8265), 396 (7150) Non-emissiveSolid (298) 644 (0.71)Solid (77) 570 (1.84)Glass (77)a 478, 511, 511 (14.4)c

670 (3.7)d

743 (1.5)e

a EtOH–MeOH (4 : 1, v/v). b Corrected for instrumental response. c Excitation < 400 nm. d Excitation at 470 nm. The assignment of 670 nm as anotheremission maximum is based on the mono-exponential decay trace obtained upon monitoring the 670 nm emission. e Excitation at 550 nm.

quenching of 3MLCT emission of the platinum(II) terpyridyl coreby the presence of low-lying d–d excited states4a and possibly by thedithiobenzoquinone moiety. The solid-state emission spectra of 3,on the other hand, exhibit an intense emission band at 644 and570 nm at room temperature and 77 K, respectively. In view of thenon-emissive behaviour of the organometallic linker, Cp*Ir-p-(g4-C6H4S2), at room temperature and the occurrence of emission at77 K at a different energy (467 nm), together with the observationof emission energies similar to that of the 3MLCT emission of[Pt(terpy)Cl]+ in the solid state at room temperature and 77 K,4a,4b

the solid-state emission of 3 is suggested to be derived fromthe 3MLCT excited state of the platinum(II) terpyridyl moieties.Depending on the excitation energies, the emission spectra of 3in 77 K glass exhibit various emission profiles, suggestive of thepresence of different emissive states. Upon excitation at ca. 350–400 nm, vibronic-structured emission bands with fine structuresat 478, 511, 550 nm and a broad structureless emission band at ca.650 nm were observed (Fig. 3). The vibronic-structured emissionbands with vibrational progressional spacings of about 1350–1390 cm−1, typical of the C=C and C=N stretching frequencies ofthe tridentate ligand, are assigned as derived from the [p(terpy) →

Fig. 3 Corrected emission spectra of 3 in 77 K glass (EtOH–MeOH 4 :1 v/v) upon excitation at different wavelengths. Insets: (left) normalisedemission spectra of 3 in 77 K glass upon excitation at selected wavelengths,(right) normalised excitation spectra of 3 in 77 K glass upon monitoringat selected emission wavelengths.

p*(terpy)] 3IL excited state. It is interesting to note that theemission spectrum shows only the structureless emission bandat ∼ 650 nm upon excitation at wavelengths > 450 nm. Such abroad emission band was found to shift to the red upon loweringthe excitation energy from kexc = 470 to 550 nm (Fig. 3).

By scrutinising the exponential decay traces of the emission in-tensities and the excitation spectra monitored at various emissionwavelengths, the broad structureless emission band is suggested tobe composed of two emission bands with maxima at ca. 670 and743 nm. The red shift of the broad emission band upon excitationat various wavelengths is attributed to the relative compositionsof these two emission bands. As the relative emission intensitiesof the broad to the structured bands increase upon increasingcomplex concentrations of 3, as well as with reference to theprevious related low-temperature emission studies of [Pt(tpy)Cl]+

in glass,4a which also exhibited a midrange and a low-energyemission at ca. 600 and 720 nm in addition to the highly structuredemission, the two emissions of 3 at 670 and 743 nm are suggestedto originate from the excited states of the respective ’excimeric”intraligand transition and metal-metal-to-ligand charge transfer(MMLCT) transition arising from terpyridyl p–p and Pt · · · Ptinteractions. [An alternative designation of the transition as metal-metal-to-ligand-ligand charge transfer (MMLLCT) transition,which is similar to the MMLCT designation here, has beenused by others.13] A similar ’excimeric” intraligand origin hasbeen assigned to the broad emission band at ∼620 nm in low-temperature glass samples of platinum(II) terpyridyl carbenesystems, which also showed a growth of emission intensity withincreasing complex concentrations.4e

Interestingly, 3 in solution showed dramatic colour changesfrom yellow to magenta upon cooling. The temperature-dependentcolour changes have been monitored using variable-temperatureelectronic absorption spectroscopy in the temperature range of118–298 K (Fig. 4 (left)). At low temperature (118 K), a remarkablenew low-energy absorption band at 514 nm with a shoulder at472 nm appeared, which is responsible for the magenta colourin low-temperature glasses. Upon increasing the temperature, thelow-energy absorption band was found to exhibit a slight blueshift as well as a decrease in absorbance. Concomitant with thesechanges, the absorption bands at ca. 382 and 400 nm were found toincrease in intensity with a well-defined isosbestic point observedat 412 nm. The new low-energy absorption band that impartsthe magenta colour in 3 at low temperature is ascribed to the

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Fig. 4 (Left) Electronic absorption spectral changes of 3 (conc. = 9.5 ×10−5 M) in EtOH–MeOH mixture (4 : 1 v/v) at different temperatures.Insets: pictures showing the colour changes upon cooling. (Right)Corrected emission spectral changes of 3 (conc. = 9.5 × 10−5 M) inEtOH–MeOH mixture (4 : 1 v/v) at different temperatures upon excitationat 412 nm. Insets: pictures showing the luminescence enhancement uponcooling under UV irradiation.

MMLCT transition as a result of terpyridyl p–p and Pt · · · Ptinteractions arising from lattice contraction upon cooling. Theclose resemblance of this absorption band to the excitation bandmonitored at 755 nm emission in 77 K glass further supports itsassignment as a MMLCT transition.

the corresponding variable-temperature emission spectralchanges of 3 have been recorded (Fig. 4 (right)). Similar to thespectrum recorded in 77 K glass, both vibronic-structured andbroad emission bands were observed at 100 K. The intensitiesof all the emission bands were found to diminish and the broademission band was found to shift to the red upon increasing thetemperature. The almost complete disappearance of the vibronic-structured emission bands at 118 K and the continuous red shiftof the broad emission band to 715 nm at 118 K and to 785 nmat 148 K upon increasing temperature are indicative of the facileinter-state crossing or energy transfer down the energy cascadefrom the [p(terpy) → p*(terpy)] 3IL state to the ’excimeric” 3ILstate and to the 3MMLCT state as the temperature increases, givingrise to a change in the relative contribution of each emissioncomponent. The eventual disappearance of the lowest energy3MMLCT emission at 298 K is consistent with the facile quenchingby the d–d state and the loss of lattice contraction at roomtemperature.

Concluding remarks

In conclusion, we have reported the first example of a unique1D chain {[Pt(terpy){Cp*Ir-p-(g4-C6H4S2)}Pt(terpy)][OTf]4}n (3)assembled by the organometallic linker [Cp*Ir(g4-C6H4S2)] (2c)and exhibiting interesting p–p and Pt · · · Pt interactions amongindividual sub-units in the solid state. Complex 3 showed un-usual UV/Vis absorption and luminescence properties at lowtemperature, imparted from self-aggregation of the individualsub-units mediated by p–p and Pt · · · Pt interactions. Theseinteresting behaviours pave the way to the design of new photo-luminescent materials. Future efforts are now devoted to thepreparation of other supramolecular coordination polymers with

the organometallic linker 2c, but with different functionalised(terpy)Pt(II) cations as building blocks.

Experimental

All experimental manipulations were carried out under argon us-ing Schlenk tube techniques. All solvents were purified by standardtechniques. The 1H NMR spectra were recorded in CD3CN usinga Bruker Avance 400 NMR spectrometer at 400.13 MHz. Infra-red spectra were recorded from neat samples on a Bruker FT-IRspectrometer Tensor 27 equipped with an ATR Harricks. [Cp*Ir-p-(g4-C6H4S2)] (2c) and [(terpy)PtCl]Cl were prepared accordingto published procedures.10,14

Synthesis of [{Pt(terpy)}2{Cp*Ir-p-(g4-C6H4S2)}][OTf]4 (3)

To a suspension of [(terpy)PtCl]Cl (53 mg, 0.10 mmol) in CH3CN(10 mL) was added a solution of AgOTf (52 mg, 0.20 mmol)in CH3CN (10 mL) and the mixture was stirred for 4 hoursat room temperature. This mixture was then filtered throughCelite. To the pale orange filtrate (60 mL) was added [Cp*Ir-p-(g4-C6H4S2)] (25 mg, 0,06 mmol) in CH3CN (10 mL). After stirringfor one hour at room temperature, the solvent was removed undervacuum and the orange powder obtained was washed twice withacetone (5 mL) and CH2Cl2 (5 mL). 3 was obtained as an orangemicrocrystalline powder (80 mg, 0.04 mmol). Yield: 82%. Anal.Calcd. for C50H41N6IrPt2S6O12F12·2H2O (1955 g mol−1): C, 30.69;H, 2.32; N, 4.30; S, 9.83. Found: C, 30.95; H, 2.44; N, 4.45; S, 9.65.1H NMR (300 MHz, CD3CN) d 1.92 (s, 15 H, Cp*), 7.30 (s, 4 H,p-(g4-C6H4S2), 7.80 (ddd, 6 H, terpy), 8.40 (m, 12 H, terpy), 8.90(dd, 4 H, terpy). IR (KBr disk, cm−1): m(CF3SO3

−) 1030, 1263.

X-Ray structural determination of[{Pt(terpy)}2{Cp*Ir-p-(g4-C6H4S2)}][OTf]4 (3)

Orange crystals were grown by slow diffusion of diethyl ethervapor into a CH3CN solution of sample 3. A single crystal of3 was selected, mounted onto a glass fiber, and transferred ina cold nitrogen gas stream. Intensity data were collected with aBruker-Nonius Kappa-CCD with graphite-monochromated Mo-Ka radiation. Unit-cell parameter determination, data collectionstrategy and integration were carried out with the Nonius EVAL-14 suite of programs.15 The structure was solved by direct methodsusing the SHELXS-97 program16 and refined anisotropically byfull-matrix least-squares methods using the SHELXL-97 softwarepackage.17 Because of disorder and related instability during thelast refinement cycles, restrains were introduced for one of thetwo acetonitrile solvent molecules (atoms with isotropic thermalparameters) and to a -CF3 group of one triflate (geometricalrestrains).†

Electronic absorption and emission measurements

UV-visible spectra were obtained on a Varian Cary 50 UV-Visspectrophotometer. Steady state excitation and emission spectrain solid-state (298 K and 77 K), glass (77 K) and solution

† CCDC reference number 634318. For crystallographic data in CIF orother electronic format see DOI: 10.1039/b708922h

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at room temperature were recorded on a Spex Fluorolog-3Model FL3-211 fluorescence spectrofluorometer equipped witha R2658P PMT detector. Solid-state photophysical studies werecarried out with solid samples contained in a quartz tube insidea quartz-walled Dewar flask. Measurements of the solid-state orbutyronitrile glass sample at 77 K were similarly conducted withliquid nitrogen filled in the optical Dewar flask. All solutions forphotophysical studies were degassed on a high-vacuum line ina two-compartment cell consisting of a 10 ml Pyrex bulb and a1 cm path length quartz cuvette and sealed from the atmosphereby a Bibby Rotaflo HP6 Telflon stopper. The solutions were rig-orously degassed with at least four successive freeze–pump–thawcycles. Emission lifetime measurements were performed using aconventional laser system. The excitation source was the 355 nmoutput (third harmonic) of a Spectra-Physics Quanta-Ray Q-switched GCR-150-10 pulsed Nd-YAG laser. Luminescence decaysignals were recorded on a Tektronix model TDS620A digitaloscilloscope and analysed using a program for exponential fits. Thevariable-temperature electronic absorption and emission spectrawere obtained using a Varian Cary 50 UV-Vis spectrophotometerand a Spex Fluorolog-3 Model FL3-211, respectively, with anOxford Instrument OptistatDN cryostat to control the workingtemperature in the range of 100 K to 198 K.

Acknowledgements

This work was supported by the CNRS and by the UniversitePierre et Marie Curie-Paris 6 which we gratefully acknowl-edge. V.W.-W.Y. acknowledges support from the University Devel-opment Fund of The University of Hong Kong, and the CentralAllocation Vote (CAV) Grant from the Research Grants Councilof Hong Kong Special Administrative Region, P.R. China (ProjectNo. HKU 2/05C).

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3530 | Dalton Trans., 2007, 3526–3530 This journal is © The Royal Society of Chemistry 2007

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