Cite this: RSC Advances, 2013, 3, 2174

Received 14th November 2012,Accepted 6th December 2012

Ternary phthalocyanato Hf(IV) and Zr(IV)polyoxometalate complexes3

DOI: 10.1039/c2ra22903j

www.rsc.org/advances

Ivana Radivojevic,a Kemakorn Ithisuphalap,a Benjamin P. Burton-Pye,a Raihan Saleh,a

Lynn C. Francesconia and Charles Michael Drain*ab

Ternary phthalocyanine–metal–polyoxometalate (Pc–M–POM)

complexes were synthesized and characterized. Group (IV) Hf or

Zr ions reside outside the plane of the macrocycle and are

coordinated to both the phthalocyanine and the lacunary

polyoxometalate. The metal ions mediate the electronic commu-

nication between the Pc and the POM.

Phthalocyanines (Pcs) are attractive molecules for use in a diversenumber of materials due to their various photonic and chemicalproperties, and stability.1–3 Pcs are used in display technologies,recording media and solar energy conversion because of theirlarge extinction coefficients in the red region of the visiblespectrum,4 e . 105 M21 cm21. Polyoxometalates (POMs) arenanometer sized metal oxide clusters that have distinct photo-chemical properties and can undergo multiple redox reactionswhile maintaining their structural integrity.5–7 A POM can be anefficient photocatalyst with UV light,8–11 and serve as a good modelof oxide surfaces.5,6 There are reports on coordination,12–14

covalent,15 electrostatic,16–18 tethered,19 and other materials20,21

containing POMs and metalloporphyrins, whereas only electro-static layer-by-layer films between POM and Pc are found in theliterature.22–25

Herein, we report the first synthesis of supramolecularphthalocyanine–metal–polyoxometalate ternary complexes, (Pc)–M–(PW11O39)[TBA]5, where M = Zr or Hf (Fig. 1). Oxophilic metalions such as Hf(IV) or Zr(IV) are 7 to 8 coordinate and have largeionic radii so reside outside of the plane of the Pc macrocycle,which enables these metal ions to be coordinated to both themacrocycle core and to the lacunary site of a Keggin POM,13

PW11O3927. The group IV metal ions in the Pc–M–POM couple the

photonic properties of the antenna chromophore to the POM. Inaddition to the photosensitization of the POM for photocatalysis,the Pc–M–POM may display nonlinear optical properties similar to

the porphyrin–M–POM.26 These ternary structures also represent amodel of binding Hf and Zr Pc dyes to metal oxide surfaces suchas TiO2 and ZnO in dye sensitized solar cells.27

(Pc)Hf(PW11O39)[TBA]5 and (Pc)Zr(PW11O39)[TBA]5

The facile synthesis of the ternary complexes is similar to thatreported for ternary porphyrin complexes, (Por)–M–(PW11O39)[TBA]5, by our laboratory.13 The (Pc)Hf(OAc)2 and(Pc)Zr(OAc)2 were synthesized according to the reported litera-ture.27–30 The addition of a stoichiometric amount of POM inacetonitrile to solutions of (Pc)Hf(OAc)2 or (Pc)Zr(OAc)2 in 1 : 1CH2Cl2 : CH3OH yields Pc–M–POM complexes by the displace-ment of the acetate ligands for the lacunary oxygens ofH3PW11O39[TBA]4. An equivalent of [TBA]Br must be added tothe mixture to yield the neutral (Pc)Hf(PW11O39)[TBA]5 or(Pc)Zr(PW11O39)[TBA]5 complex, since the overall charge of theternary complex is 25. The reaction was buffered by triethylaminesince the acidic protons from the POM can lead to thedemetalation during the synthesis of the corresponding por-phyrin.13 Multinuclear NMR, electronic spectra, mass spectro-metry, and luminescence data all are consistent with theformation of the ternary complexes.{

In contrast to the corresponding ternary porphyrin complexes,(Pc)Hf(PW11O39)52 and (Pc)Zr(PW11O39)52 are more stable in thepresence of excess POM. The addition of up to two equivalents ofPOM does not result in the demetalation of the phthalocyanine

aHunter College and Graduate Center of the City University of New York, 695 Park

Avenue, New York 10065, USA. E-mail: cdrain@hunter.cuny.edu; Fax: 212-772-5332;

Tel: 212-650-3791bRockefeller University, 1230 York Avenue, New York 10065, USA

3 Electronic supplementary information (ESI) available: Experimental details, UV-Vis,fluorescence, NMR, mass spectra. See DOI: 10.1039/c2ra22903j

Fig. 1 The (Pc)Hf(OAc)2 or (Pc)Zr(OAc)2 is mixed with the lacunary Keggin POM,H3PW11O39[TBA]4, to yield the ternary complex, (Pc)–M–(PW11O39)[TBA]5, M = Zr orHf. The a and b positions of the Pc are indicated.

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macrocycle. NMR and crystal structure data of the tetraphenylpor-phyrin–M–POM complexes show that the POM pushes up againstthe orthogonal phenyl groups enough to distort the otherwiseplanar porphyrin and considerably red-shift the electronicspectra.13 The small red shifts in the UV-visible absorption spectraof the phthalocyanine ternary complexes compared to the starting(Pc)–M–(OAc)2 indicate that they are much less distorted than thecorresponding porphyrin ternary complexes. Since the Pc bears noexocyclic substituents, there are less steric interactions.

NMR

The 1H NMR spectra for the (Pc)Hf(PW11O39)[TBA]5 complex(Fig. 2) is somewhat better resolved than the spectra for(Pc)Zr(PW11O39)[TBA]5. Compared to Zr, Hf has a greater ionicradius and does not sit as deeply into the POM and protrudesfurther out from the Pc. The steric interactions between the Pc andthe POM are less for the (Pc)–Hf–(POM) than for the (Pc)–Zr–(POM). The off-center lacunary site of the POM results in the Pctilted to one side (Fig. 1), so the reduced symmetry creates twodifferent halves of the molecule. The chemical shift of the Ha andHb protons depends on their proximity to the POM. The resolutionis diminished because of the rotation of the Pc relative to thePOM. Three multiplets centered at 9.53 ppm integrated one to onewith two multiplets centered at 8.21 ppm. The NMR spectra showa small excess of TBA, but integration verifies at least 40 H for –CH2– groups and 60 H for the –CH3 group for all five TBA counterions for the ternary complex (Fig. S5, ESI3). The same spectralcharacteristics were observed for (Pc)Zr(PW11O39)[TBA]5, withbroad multiplets centred at 9.49 ppm and 8.11 ppm (Fig. S6,ESI3). Both ternary complexes show different resonances in thearomatic region compared to the starting (Pc)Hf(OAc)2 or(Pc)Zr(OAc)2 complexes, which exhibit two broad singlets inte-grated to eight H each at 9.48 and 8.23 ppm for (Pc)Hf(OAc)2 and9.42 and 8.20 ppm for (Pc)Zr(OAc)2. Consistent with the smallchanges in the UV-visible spectra, the small differences inchemical shifts between the Pc–M–POM complexes and startingmetal acetate complexes do not indicate a distortion of the Pcmacrocycle. The 31P NMR spectra of both ternary complexes showa single resonance at 215.68 ppm and 215.46 ppm for the Hf andZr ternary complexes, respectively (Fig. S10 and S11, ESI3),

indicating the purity of the complexes. The smaller peak observedat 215.77 ppm for (Pc)Zr(PW11O39)[TBA]5 is consistent with ourearlier observation with Por–M–POM complexes and may arisefrom an exchange of TBA cations.13

UV-visible spectroscopy

Phthalocyanines exhibit a strong Q-band typically centered in the600–700 nm region, and a broad Soret band of smaller intensityaround 350 nm. The UV-visible spectra for the starting(Pc)Hf(OAc)2 and (Pc)Zr(OAc)2 complexes are similar. The Soretbands are centered at 334 nm and 336 nm, respectively. Thehafnium complex displays Q-bands at 614 nm and 684 nm, andthe zirconium complex has Q-bands at 615 nm and 685 nm. Theformation of the ternary complexes was monitored by the UV-visible spectra of the phthalocyanine. The Q-bands display a smallred shift as the reaction progresses. The Q-bands are located at616 nm and 685 nm for the (Pc)Hf(POM) ternary complex, and forthe (Pc)Zr(POM) complex they are at 618 nm and 687 nm (Fig. S12and S15, ESI3). The small red-shifts also indicate a minimal or nostructural distortion in the planar phthalocyanine macrocycle afterthe complexation with the POM. Generally, exchanging the ligandsbound to group (IV) metallophthalocyanines does not significantlyinfluence the absorption spectra, and the reported values are inaccordance with our data.30 This is in contrast with the Por–M–POM complexes wherein the orthogonal phenyl groups distort themacrocycle to a significant extent as indicated by the 15 nm redshift of the electronic spectra compared to the starting diacetatecomplexes.13

The reflectance spectra of thin films of the ternary(Pc)M(PW11O39)[TBA]5 complexes on glass (Fig. 3) are similar tothe reflectance spectra observed for anatase TiO2 treated with(Pc)Hf(OAc)2 and (Pc)Zr(OAc)2, which binds the metallophthalo-cyanine to the oxide surface (see Fig. 4 and S17, ESI3).27

Fluorescence

Both starting materials and ternary complexes, excited in thephthalocyanine Q-band at 616 nm, exhibit similarly shapedemission spectra with one large peak close to 700 nm and weakerbands near 730 nm and 770 nm (Fig. S13, S16, ESI3). Thefluorescence intensity is ca. 50 times less for (Pc)–M–(POM)complexes vs. the diacetate compounds. We hypothesize that boththe electron transfer between the POM and the Pc24 and the

Fig. 2 500 MHz 1H NMR of (Pc)–Hf–(PW11O39)[TBA]5 in CD2Cl2; insert is themagnification of the aromatic region.

Fig. 3 The reflectance spectra of the two ternary complexes (Pc)–M–(PW11O39)[TBA]5 on glass.

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additional spin–orbit coupling, because of the heavy atoms,17 arelikely the major causes for the fluorescence quenching. Chargetransfer from the excited singlet state of the dye to the POM isobserved in other porphyrin–POM materials.13,15,17

(Pc)Zr and (Pc)Hf on TiO2

Both the ternary Pc–M–POM complexes and the tight binding of(Pc)M(OAc)2 to TiO2 nanoparticles are strong indications that theoxophylic group (IV) metallophthalocyanines bind to the semi-conductor by displacement of the acetate counter ions by theoxygens on the surface of the nanoparticle (Fig. 4).27,31 UV-visiblereflectance measurements of both systems show that the dyesremain bound to the POM or TiO2 surface (Fig. S17, ESI3).

Conclusion

The properties of these complexes exemplify how electronic andphotonic properties of two different molecules can be coupled intoan organic–inorganic Pc–M–POM hybrid material. The Pcchromophore in the (Pc)Hf(PW11O39)[TBA]5 and(Pc)Zr(PW11O39)[TBA]5 complexes may photosensitize the POMfor catalysis. Because POMs model defect sites of oxide surfaces,the ternary complexes serve as good models for attachingporphyrinoids to defect sites in oxide surfaces via group (IV)metal ions. Pc–Hf or Pc–Zr can be used as part of the active layerfor the sensitization of semiconductors such as TiO2 and ZnO indevices for solar energy conversion.27 As with the correspondingternary porphyrin materials,24 Pc–M–POM may display nonlinearoptical properties. These compounds add to the repertoire ofporphyrinoids for solar energy harvesting and many can be madecommercially viable.32–36

Acknowledgements

Supported by the U.S. National Science Foundation (NSF CHE-0554703, 0847997 to CMD and 0414218 and 0750118 to LCF);the Office of Science - U.S. Department of Energy (DOE) (AwardDE-SC0002456) and DE-FG02-09ER16097 (Heavy ElementChemistry) to LCF. Hunter College science infrastructure issupported by the NSF, the National Institutes of Health,including the RCMI program (G12-RR-003037 and MD007599)and the City University of New York.

References{ Experimental. Starting (Pc)Hf(OAc)2 and (Pc)Zr(OAc)2 were synthesizedaccording to previous studies.27–30 (Pc)Hf(PW11O39)[TBA]5 and(Pc)Zr(PW11O39)[TBA]5 were synthesized from the Keggin POMH3PW11O39[TBA]4 and the above mentioned acetate complexes. Forexample, 18 mg of (Pc)Hf(OAc)2 (0.022 mmol) was dissolved in 10 mL ofa 1 : 1 mixture of CH2Cl2 : CH3OH in a 18 6 150 mm test tube containinga magnetic stir bar at room temperature. 76 mg of H3PW11O39[TBA]4 POM(0.021 mmol) and 5 mg of [TBA]Br were dissolved separately in 5 mL ofacetonitrile containing 1% v/v triethylamine. The resulting clear POMsolution was added drop-wise over the course of 5 min to the stirringhafnium phthalocyanine solution, and the reaction was left to runovernight, whereupon a small amount of precipitate was observed at thebottom of the test tube. The reaction was monitored by UV-visiblespectroscopy. All compounds were characterized by 1H NMR, 31P NMR,MALDI-MS, UV-visible absorption and reflectance spectroscopy, andfluorescence spectroscopy.

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