Inorganica Chimica Acta 363 (2010) 4381–4386

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Inorganica Chimica Acta

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Synthesis, structure, and characterization of two polyoxometalate–photosensitizerhybrid materials

Jie Song, Zhen Luo, Haiming Zhu, Zhuangqun Huang, Tianquan Lian, Alexey L. Kaledin,Djamaladdin G. Musaev, Sheri Lense, Kenneth I. Hardcastle, Craig L. Hill ⇑Department of Chemistry, Emory University, Atlanta, GA 30322, USA

a r t i c l e i n f o a b s t r a c t

Article history:Available online 17 July 2010

This article is dedicated to Achim Müller forhis many years of spectacular, creative andinsightful inorganic cluster chemistry andthe sizable impact of this collective researchon other areas of science.

Keyword:Electronic interactionPhotosensitizerPolyoxometalate

0020-1693/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.ica.2010.07.022

⇑ Corresponding author.E-mail address: chill@emory.edu (C.L. Hill).

Two hybrid materials based on the tris(bipyridine)ruthenium(II), [Ru(bpy)3]2+ and Keggin-type polyoxo-metalates, [PW11O39]7� and [PW12O40]3�, namely, [Ru(bpy)3][K5PW11O39] (1) and [Ru(bpy)3][KPW12O40](2) were synthesized. X-ray crystallographic study of the red-colored complex, 2, shows that it crystal-lizes in the orthorhombic space group Pbcn and the polyanions are associated with the [Ru(bpy)3]2+ coun-terions by Coulombic forces and supramolecular interactions. The molecular complex is furtherconnected and forms a three-dimensional framework through C–H� � �OPOM and other weak interactions.These complexes were further characterized by FT-IR, UV–Vis, 1H and 31P NMR, luminescent spectra andcomputational studies. Significantly, these combined spectroscopic studies show that these polyoxomet-alate–dye hybrids have strong electronic interactions between the cationic dye and polyanion units.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

It is well known that photosensitizers (henceforth ‘‘dyes”) playthe key role in light harvesting and light-to-current conversion inthe dye-sensitized solar cells (DSSCs). Grätzel long ago demon-strated that photoanodes comprising [Ru(bpy)3]2+ bonded to TiO2

not only generate photocurrent with visible light when interfacedwith the iodine/iodide couple but also operate for thousands ofhours with minimal degradation in performance [1,2]. The counte-rions of cationic dyes affect the photoinjection quantum yields, inpart because they impact charge separation and recombinationrates and efficiencies [3,4]. Polyoxometalates (POMs) are a wideclass of negatively charged later transition metal oxide clusterswith variable molecular components, structures, and redox proper-ties [5,6]. Recently, particular d-electron-transition metal-substi-tuted POMs were reported to be stable and fast water oxidationcatalysts [7–11]. In the proposed catalytic cycle of water oxidation,the tris(bipyridine)ruthenium(II) chloride dye is used to induce theelectron transfer process from the water molecule to the POM cat-alyst. The process initiated from excited dye produced either bychemical oxidants or by visible light demonstrates that the elec-tron transfer between the dye and POM molecule is possible underselected experimental conditions. Interest in the synthesis ofPOM–dye hybrid materials has increased in recent years due to

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their potential applications [4,12–15], but few studies on the de-tails of the POM–dye structure associated with the electron trans-fer process among these systems have been reported [16–25].

Here we report the synthesis of hybrid POM–tris(bipyri-dine)ruthenium dye complexes, [Ru(bpy)3][K5PW11O39] (1) and[Ru(bpy)3][KPW12O40] (2), but significantly that spectroscopicmeasurements combined with theoretical calculations indicatestrong interactions between the cationic and anionic moieties inthese hybrid materials.

2. Experimental

2.1. Material and instrumentation

K7[PW11O39] was synthesized according to the literature proce-dures [26], and all the other starting materials were obtained com-mercially and used as received. 1H and 31P NMR spectra wererecorded on a Varian INOVA 400 MHz instrument and chemicalshifts were measured in parts per million (ppm) relative to thedeuterated solvent used in the experiment. Infrared spectra (FT-IR) were obtained on a Nicolet 510 FT-IR spectrophotometer withKBr pellets. UV–Vis spectra were acquired using 8453 spectropho-tometer. Elemental analyses for C, H, and N were performed byAtlantic Microlab, Norcross, GA, and elemental analyses for allother elements were performed either by Columbia AnalyticalServices, Tucson, AZ or by Galbraith Laboratories Inc., Knoxville,TN. Mass spectra were recorded on a JEOL JMS-SX102/SX102A/E

Table 1Crystallographic parameters and refinement details for complex 2.

2

Empirical formula C30H24KN6O45.48PRuW12

Formula weight 3573.57T (K) 173(2) KCrystal system orthorhombicSpace group Pbcna (Å) 16.3036(17)b (Å) 28.330(3)c (Å) 16.4903(17)b (�) 90V (Å)3 7616.4(13)Z 4Dcalc (Mg/m3) 3.116Absorption coefficient (mm�1) 18.395Number of parameters/restraints 442, 0Measured reflections 61 348Independent reflections 7962R [I > 2r(I)] 0.0850Rw [I > 2r(I)] 0.2506Goodness-of-fit (GOF) on F2 1.016Largest difference peak and hole (e A�3) 2.733, �1.875

R = ||Fo| � |Fc||/|Fo|, Rw ¼ ½wðF2o � F2

c Þ2=wðF2

oÞ2�1=2.

4382 J. Song et al. / Inorganica Chimica Acta 363 (2010) 4381–4386

mass spectrometer (ESI). X-ray crystallography studies were car-ried out in the X-ray Crystallography Laboratory at Emory Univer-sity on a Bruker Smart Apex II CCD diffractometer. Steady-stateemission spectra of the samples were measured using a SPEX Flu-oroLog-3 self-contained and fully automated spectrofluorometer.

2.2. Syntheses

2.2.1. [Ru(bpy)3][K5PW11O39] (1)To a 10 mL solution of K7PW11O39 (0.49 g, 0.16 mmol) was added

[Ru(bpy)3]Cl2 (0.1 g, 0.16 mmol, in 10 mL of H2O). The resulted sus-pension was placed in dark and stirred for 12 h. Orange-red precip-itates were collected by centrifuge and washed by water(10 mL � 3), acetone (10 mL � 3), and diethyl ether (10 mL � 3) suc-cessively. Thus obtained product was dried in vacuo at room temper-ature overnight before use. The hybrid POM–dye complexes arelight-sensitive and thus covered with aluminum foil and stored inthe dark. Yield: 0.51 g (91%, based on dye) for (1). 1H NMR (d, d-DMSO): 8.845 (2H, d), 8.184 (2H, t), 7.746 (2H, d), 7.556 (2H, t)ppm. 31P NMR (d, d-DMSO): �8.679 ppm (in comparison toK7PW11O39: �7.782). FT-IR (KBr, cm�1): 3075.1 (w), 1632.8 (m),1602.6 (m), 1463.8 (m), 1445.4 (m), 1423.7 (m), 1077.0 (m),1039.4 (vs), 943.5 (vs), 852.9 (vs), 811.9 (vs), 758.4 (vs), 591.7 (w),511.6 (m); in comparison to H7PW11O39: 1631.7 (m), 1087.4 (m),1041.9 (m), 950.7 (vs), 901.2 (m), 857.0 (vs) 805.6 (vs), 729.6 (vs),591.0 (w), 509.0 (m). Anal. Calc. for C30H24K5N6O39PRuW11: C,10.47; H, 0.70; N, 2.44; K, 5.68; P, 0.90; Ru, 2.94; W, 58.75. Found:C, 10.02; H, 0.82; N, 2.36; K, 5.28; P, 0.86; Ru, 2.84; W, 58.15%.

2.2.2. [Ru(bpy)3][KPW12O40] (2)The title crystalline 2 was prepared by a slow diffusion of

reactants by making three liquid layers: an aqueous solution(10 mL) containing H3PW12O40 (0.46 g, 0.16 mmol) and KCl(0.03 g, 0.32 mmol) on the bottom, a mixed solvent (2 mL, H2O/EtOH = 2/1; v/v) separating phase in the middle, and a [Ru(b-py)3]Cl2 (0.1 g, 0.16 mmol) solution in EtOH (5 mL) on top of themixed solvent layer. Orange-red crystals formed after 3 weeksand were removed by filteration, washed with water and air-dried.Yield: 0.52 g (94%, based on dye) for 2. 1H NMR (d, d-DMSO): 8.828(2H, d), 8.167 (2H, t), 7.732 (2H, d), 7.530 (2H, t) ppm. 31P NMR (d,d-DMSO): �12.798 ppm (in comparison to H3PW12O40: �12.762).FT-IR (KBr, cm�1): 3081.1 (w), 1704.4 (m), 1655.2 (m), 1603.4(m), 1464.0 (m), 1446.0 (m), 1079.2 (vs), 977.4 (vs), 897.6 (vs),814.1 (vs), 759.4 (vs), 729.3 (m), 596.0 (w), 520.2 (m); in compar-ison to H3PW12O40: 1616.8 (m), 1080.4 (vs), 984.5 (vs), 891.4 (vs),799.8 (vs), 595.1 (w), 524.0 (m). Anal. Calc. for C30H24KN6O40-

PRuW12: C, 10.34; H, 0.69; N, 2.41; P, 0.89; Ru, 2.90; W, 63.29.Found: C, 9.95; H, 0.79; N, 2.30; P, 0.81; Ru, 2.78; W, 59.49%.

For comparison, the spectra of the thoroughly studied parentphotosensitizer complex, [Ru(bpy)3]Cl2, are as follows: 1H NMR(d, d-DMSO): 8.844 (2H, d), 8.169 (2H, t), 7.727 (2H, d), 7.528(2H, t). FT-IR (KBr, cm�1): 3069.1 (w), 1654.9 (m), 1601.6 (m),1464.0 (m), 1443.7 (m), 1421.2 (m), 776.5 (vs), 730.6 (m). Both 1and 2 were subjected to ESI-MS and MALDI-MS but both tech-niques failed to yield informative values.

2.3. X-ray crystallography

A crystal of complex 2 with a size of 0.20 � 0.05 � 0.03 mm3

was mounted on a glass fiber capillary which was put on a BrukerSmart Apex II CCD diffractometer equipped with graphite mono-chromatic radiation and used for data collection. Data were col-lected at 173(2) K using Mo Ka radiation (k = 0.71073 Å). Thestructure was solved by direct methods (SHELXTL-97) and refinedby the full-matrix-block least-squares method on F2. All non-hydrogen atoms were refined with anisotropic displacement

parameters. Heavy atoms (W and Ru) were refined with aniso-tropic displacement parameters and other atoms (O, C, N and P)were refined isotropically. Hydrogen atoms were included at calcu-lated positions and refined with a riding model. A summary of thecrystal data and refinement results are listed in Table 1 and S3.

2.4. Static emission measurements

Solutions of [Ru(bpy)3]2+, [H3PW12O40], and their mixtures wereprepared and stored in the dark to avoid photodegradation. Thesolution of 5 lM [Ru(bpy)2]2+ and less than 30 lM [H3PW12O40]were used for all measurements due to the low solubility of the[Ru(bpy)3]2+/[H3PW12O40] mixture in the used solvents. All solu-tions were degassed with N2 before measurements. A 10 � 10mm quartz cuvette and 1.8-nm excitation and emission slits wereused. Samples were excited at 450 nm, and emission intensity datawere collected and averaged over 615–620 nm at 20 �C. Integrationtime was set at 0.05 s.

2.5. Computational methods

Full geometry optimization and energy calculation of the stud-ied [Ru(bpy)3]2+, [Ru(bpy)3]3+, [PW12O40]3�, [PW12O40]4� and thecomplex {[Ru(bpy)3]2+� � �[PW12O40]3�} were performed in acetoni-trile (at 300 K and 1 atm). In these calculations the BP86 [27,28]density functional and polarizable continuum model (PCM, withUFF atomic radii for all atoms) [29] in conjunction with lanl2dz ba-sis sets [30–32] for all atoms were utilized. Basis sets of P and Nwere augmented with polarization d-functions with exponents of0.55 and 0.8, respectively. For Ru and W, the corresponding lanl2dzECP [30–32] were used. The calculations were carried out withGAUSSIAN-09 quantum chemistry software package [33].

3. Results and discussion

3.1. Syntheses

The two complexes were synthesized in one-step reaction withmolar ratio of 1:1 by simply mixing solutions of the reactants. Theresulting orange-red product could be purified by washing withpure water to remove the excess starting materials and byproduct,e.g. KCl or HCl (Scheme 1). Different photosensitizer–POM com-pounds could be produced conveniently by varying the reactant

Table 2Atomic distances (Å) and angles (�) of OPOM� � �Hpyr interactions inthe crystal structure of complex 2.

Bonding distance Angle

O(7)� � �H(14)C(14) 2.561 133.25O(8)� � �H(13)C(13) 2.899 98.67

J. Song et al. / Inorganica Chimica Acta 363 (2010) 4381–4386 4383

molar ratio, reaction temperature, concentration and other solu-tion variables.

Several crystal growth techniques including re-crystallization athigh or low temperatures and vapor diffusion were employed;however, the only single X-ray quality crystals of 2 were obtainedusing the liquid-to-liquid diffusion method.

O(10)� � �H(9)C(9) 2.691 121.30O(10)� � �H(10)C(10) 2.683 124.33O(11)� � �H(13)C(13) 2.634 115.23O(15)� � �H(2)C(2) 2.563 116.00O(15)� � �H(3)C(3) 2.623 112.76O(19)� � �H(3)C(3) 2.903 88.84O(19)� � �H(9)C(9) 2.963 121.34

3.2. Crystal structure

Complex 2 crystallizes in the orthorhombic space group Pbcn.The asymmetric unit is composed of half a Keggin anion, one potas-sium cation, and half a tris(bipyridine)ruthenium(II) cation (Fig. 1).The shortest distance between two Keggin anions is 2.946 Å andinvolves two terminal O(1) oxygen atoms. Each polyanion is sur-rounded by five adjacent [Ru(bpy)3]2+ units. Some polyanion oxy-gen atoms are within hydrogen bonding distance of some bpyhydrogen atoms (key O� � �H distances are given in Table 2 and oth-ers are in S3). According to the well-ordered packing of the anionand cation in the unit cell, all photosensitizer [Ru(bpy)3]2+ unitsare separated quite uniformly from one another by the polyoxoa-nions, and as a consequence, there is no expected p� � �p orC–H� � �p interactions between the rigid bipyridine rings.

The crystal packing of 2 exhibits interesting three-dimensionalstructures (Fig. 2). The crystal has large pores in all three crystallo-graphic directions, and big channels of electrostatically attracted[Ru(bpy)3]2+ and polyanion units exist along the c-axis. The micro-porous channels along the crystallographic c-axis exhibit longestand shortest dimensions of 10.067 and 9.700 Å, respectively. Suchporous structures are potentially attractive for applications in sen-sor, adsorption and catalytic technologies. Nanostructures or de-

Scheme

Fig. 1. ORTEP diagram of the asymmetric unit of complex 2 with the atomic numbering stungsten (black), nitrogen (blue), and oxygen (red) phosphorus (yellow). H atoms are omireader is referred to the web version of this article.)

vices that both detect and catalytically remove odorous and/ortoxic agents are of considerable current interest [34].

3.3. Spectroscopic studies of the hybrid complexes

3.3.1. UV–Vis and static emission spectraThe title complexes, 1 and 2, were suspended in various sol-

vents, followed by sonication and filtration. We used a mixed sol-vent for 2 because of its poor solubility in pure CH3CN. Fig. 3 showsthe UV–Vis spectra of 1, 2 and their components in different sol-vents. In pure DMSO, the absorption of each complex (1 or 2) is asum of the absorptions of [Ru(bpy)3]2+ and its POM counter ion([K5PW11O40]2� or [KPW12O40]2�), as shown in Fig. 3 for 2.

However, for 1 in CH3CN and for 2 in 9:1 CH3CN/DMSO, thespectra show absorption bands centered at 540, 470 and 365 nm,respectively. These are significantly different from that of [Ru(b-py)3]2+ suggesting the formation of strongly interacting complexes

1.

cheme (30% thermal ellipsoids). Potassium (green), ruthenium (pink), carbon (gray),tted for clarity. (For interpretation of the references to color in this figure legend, the

Fig. 2. Three-dimensional structure of 2: view of the ion channel of potassium in plane (0 0 1) or the z-axis.

Wavelength (nm)

300 400 500 600 700 800

Abs

. (O

D)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

375 450 525 600 675 7500.00

0.06

0.12

0.181 in CH3CN

2 in DMSO/CH3CN

2 in DMSO

[Ru(bpy)3]2+

[H3PW12O40]

Fig. 3. UV–Vis spectra of 1 in CH3CN (solid black line), 2 in CH3CN/DMSO (9:1 involume, dashed red line), 2 in DMSO (dotted blue curve), [Ru(bpy)3]2+ in CH3CN/DMSO (9:1 in volume, pink dash-dot-dash line), and [H3PW12O40] in DMSO (greendash-dot-dot-dash line). The inset shows the enlarged spectra. (For interpretationof the references to color in this figure legend, the reader is referred to the webversion of this article.)

[H3PW12O40] (µM)

0 10 20 30

I 0/I

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4 25%

50%

100%

Fig. 4. Stern–Volmer plots for samples in DMSO solutions containing 0% (greensquares), 50% (red triangles) and 75% (purple diamonds) of CH3CN in volume asindicated in the figure. All solutions contained 5 lM [Ru(bpy)3]2+ and were purgedwith N2. The emission intensities of [Ru(bpy)3]2+ with (I) and without (I0)[H3PW12O40] were collected, averaged at 615–620 nm after 450 nm excitation.(For interpretation of the references to color in this figure legend, the reader isreferred to the web version of this article.)

4384 J. Song et al. / Inorganica Chimica Acta 363 (2010) 4381–4386

between the anionic POM and the [Ru(bpy)3]2+ units. The nature ofthese new complexes and their absorption spectra are being inves-tigated by computation and time-resolved spectroscopy.

In addition, static emission quenching studies were carried outto verify the formation of ion pairs in DMSO/CH3CN solution (S1and S2). The steady emission of [Ru(bpy)3]2+ with (I) and without(I0) [H3PW12O40] was measured. Fig. 4 shows the resulting Stern–Volmer (SV) plots, I0/I versus [H3PW12O40]. As seen from thoseplots, the [Ru(bpy)3]2+ emission is quenched by the presence of[H3PW12O40]. In pure DMSO, the SV plot is roughly linear withinthe range of the used [H3PW12O40] concentrations, suggesting a

bimolecular emission quenching process that can be describedusing a typical linear SV equation:

I0=I ¼ 1þ kqs0½H3PW12O40� ð1Þ

where kq is the bimolecular quenching rate constant, and s0 is theintrinsic emission lifetime of the emitter, [Ru(bpy)3]2+.

The quenching efficiency increases with increasing CH3CN/DMSO ratio, suggesting that CH3CN enhances the interaction be-tween [Ru(bpy)3]2+ and [PW12O40]3�. In addition, the presence ofCH3CN causes the SV plots to deviate from linearity and show an

J. Song et al. / Inorganica Chimica Acta 363 (2010) 4381–4386 4385

upward curvature. This may be caused by the formation of non-emitting ion pairs as seen in the following equation:

I0=I ¼ ð1þ kqs0½H3PW12O40�Þð1þ Keq½H3PW12O40� ð2Þ

where Keq is the equilibrium constant of ion-pair formation. On theother hand, if emitting ion pairs form, the SV plot will show a down-ward curvature [35–37].

Therefore, our results imply that non-emitting or poorly-emit-ting ion pairs form in the solution, suggesting a fast photo-quench-ing process within the ion pair. Due to the lack of kinetic data,those plots were not able to be fit to the kinetic model outlinedabove. Kinetic studies by time-resolved fluorescence and transientspectroscopy to characterize the ion pairs are in progress.

3.3.2. FT-IR spectraThe FT-IR spectra are shown in Fig. 5, and the major vibration

modes are listed in Table 3. Both hybrid complexes exhibit severalabsorption peaks located in the range of 1423–1630 cm�1, whichcorrespond to the characteristic vibrations of the 2,20-bpy ligandof the photosensitizer cation [12]. For complex 1, the peaks at1039 and 1077 cm�1 are attributed to the P–O stretches and thepeak at 943 cm�1 is assigned to W@O stretches. In comparisonwith the lacunary complexes, K7PW11O39, the bands at 750, 812and 853 cm�1 could be assigned to the corner-shared (W–Oc–W)and edge shared (W–Ob–W) octahedra of the Keggin unit. For 2,the similar peak shifts are also observed (Fig. 5 and Table 3). Thehigher energies for the W@O and P–O stretches are consistent with

Fig. 5. FT-IR spectra of the dye

Table 3Major FT-IR peaks for K7PW11O39, H3PW12O40, [Ru(bpy)3]2+, and the two hybrid complexe

Complex 1 K7PW11O39 Dye

W–Oc–W 750, 812 730, 805W–Ob–W 853 857W@Od 943 950P–Oa 1039, 1077 1041, 10872,20-bpy 1423–1463, 1602–1632 142

Table 431P and 1H NMR for K7PW11O39, H3PW12O40, [Ru(bpy)3]2+, and the two hybrid complexes,

Dye K7PW11O39 Complex 1

31P �7.782 �8.6791H 8.844 (2H, d), 8.169 (2H, t), 7.727 (2H, d),

7.528 (2H, t)8.845 (2H, d), 8.1847.556 (2H, t)

a strong interaction existing between POM anion with the dye[Ru(bpy)3]2+ cation in the hybrids [38].

3.3.3. 31P and 1H NMR spectra31P and 1H NMR of the hybrid complexes were measured in

deuterated DMSO solvent (Table 4). The 31P NMR spectra clearlydemonstrate that the phosphorus peaks at �8.679 ppm for 1 and�12.798 ppm for 2 are moved to high field in comparison withthe chemical shifts of K7PW11O39 at �7.782 ppm and H3PW12O40

at �12.762 ppm, respectively. The 1H NMR spectra of both com-plexes also exhibit changes of chemical shift for protons on thebipyridine ligands.

3.4. Computational study

As demonstrated in our previous paper [35] the ground elec-tronic state of [Ru(bpy)3]2+ is a closed-shell singlet state. Its fewuppermost doubly occupied MOs (HOMOs) are primarily Rud-orbitals. The first three lowest-unoccupied MOs (LUMOs) arelocalized on the bpy ligands and represent p*-orbitals of the bpyrings. Removing an electron from [Ru(bpy)3]2+ to yield a doublet[Ru(bpy)3]3+ requires 6.1 eV.

The ground electronic state of the [PW12O40]3� polyoxoanion isalso a closed-shell singlet. Its first few LUMOs are W–O antibond-ing (mostly W d-orbitals) orbitals, while the about 40 uppermostdoubly occupied orbitals are 2p-lone pairs of the oxygen centers.The next set of doubly occupied orbitals of [PW12O40]3�, after the

and the hybrid complexes.

s, 1 and 2 (cm�1).

Complex 2 H3PW12O40

814 799897 891977 9841079 1080

1–1464, 1601–1654 1425–1464, 1603–1655

1 and 2 in d-DMSO (ppm).

H3PW12O40 Complex 2

�12.762 �12.798(2H, t), 7.746 (2H, d), 8.828 (2H, d), 8.167 (2H, t), 7.732 (2H, d),

7.530 (2H, t)

Fig. 6. Calculated structure and important geometry parameters (in Å) of the{[Ru(bpy)3]2+� � �[PW12O40]3�} complex.

4386 J. Song et al. / Inorganica Chimica Acta 363 (2010) 4381–4386

oxygen lone pairs, are the W–O bonding orbitals. Calculationsshow that the electron affinity of [PW12O40]3� is ca 4.6 eV.

The above presented thermodynamic data indicate that remov-ing one electron from [Ru(bpy)3]2+ and placing it on [PW12O40]3�

requires 1.5 eV. In other words, the electron transfer from [Ru(bpy)3]2+ to [PW12O40]3� cannot occur spontaneously at the groundstates of the fragments. Therefore, one may expect that the mixingof ground state [Ru(bpy)3]2+ with ground state [PW12O40]3� willlead to formation of a weakly bound {[Ru(bpy)3]2+� � �[PW12O40]3�}complex, which may have several isomers. In Fig. 6 we present theenergetically most stable isomer, where [Ru(bpy)3]2+ is coordi-nated to the face of +� � �[PW12O40]3�, which contains three W-centers, via three H-bonding (see Fig. 6). This coordination motifis consistent with the X-ray structure of this complex. Thecalculated fragmentation energy of the fully optimized{[Ru(bpy)3]2+� � �[PW12O40]3�} complex, in its ground singlet state,is found to be �5.5 kcal/mol.

However, charge transfer in the {[Ru(bpy)3]2+� � �[PW12O40]3�}complex, i.e. complex {[Ru(bpy)3]3+� � �[PW12O40]4�}, can beachieved by photoexcitation of [Ru(bpy)3]2+ from its S0 (groundstate) to S1 (the first excited state). Previously, we calculated[35] the S0 ? S1 vertical excitation energy in water to be�2.7 eV (459 nm) versus the measured value of 450 nm. Detailedstudies of charge transfer in {[Ru(bpy)3]2+� � �[PW12O40]3�} complexare in progress and will be reported elsewhere.

4. Conclusions

Two dye–POM complexes, [Ru(bpy)3][K5PW11O39] (1) and[Ru(bpy)3][KPW12O40] (2), have been synthesized and character-ized in both solid and solution state. The crystal structure of 2demonstrates an interesting 3D packing with ion-channel-likeporosity. All the spectroscopic and computational studies indicatethat a strong interaction exists between the cationic dye and poly-anion units. Future investigation will explore the electron transfercharacter including ET rate measurements between the cationicdye in its excited state and the polyanions.

Acknowledgments

This work was funded by the Division of Chemical Sciences,Geosciences, and Biosciences, Office of Basic Energy Sciences ofthe US Department of Energy through Grant DE-FG02-07ER15906. J.S. and C.L.H. thank Yu Hou and Rui Cao for construc-tive suggestions.

Appendix A. Supplementary material

CCDC 769589 contains the supplementary crystallographic datafor this paper. These data can be obtained free of charge from TheCambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this articlecan be found, in the online version, at doi:10.1016/j.ica.2010.07.022.

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