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Journal of Electroanalytical Chemistry 589 (2006) 232–236

ElectroanalyticalChemistry

Photo-induced electron transfer and electrocatalytic properties ofnovel charge-transfer compound (TMB)3HPMo12O40

Ying Wu a,b,*, Junwei Zheng a, Ling Xu b, Zhenping Wang a, Dijiang Wen b

a Institute of Chemistry and Chemical Engineering, Suzhou University, Suzhou, Jiangsu 216006, Chinab Institute of Material Engineering, Suzhou University, Suzhou 215006, China

Received 7 July 2005; received in revised form 21 January 2006; accepted 20 February 2006Available online 30 March 2006

Abstract

A novel charge-transfer compound (TMB)3PMo12O40 (TMB-PMo12, TMB = 3,3 0,5,5 0-tetramethylbenzidine) was synthesized. Thereis a strong electronic interaction between organic donor TMB and PMo12O3�

40 heteropoly anion. Irradiation of ultraviolet light can causethe intramolecular electron-transfer between TMB and heteropoly anion in the complex. Electrochemical behavior of the TMB-PMo12

modified electrode in H2SO4 solution showed well-defined redox couples corresponding to the redox reactions of the incorporated TMBand heteropoly anion. The TMB-PMo12 modified electrode exhibits excellent catalytic activity for the reduction of IO�3 , where the incor-porated TMB may play an important role to activate the catalytic activity of the heteropoly anion.� 2006 Elsevier B.V. All rights reserved.

Keywords: Charge-transfer; Heteropoly acid; 3,30,5,50-Tetramethylbenzidine; Electronic absorption; Electrochemistry

1. Introduction

Heteropoly acids possess good redox properties andstructural versatility and have been widely applied in manyfields, such as medicine [1,2], catalysis [3–5], and materialssciences [6]. They are also excellent inorganic buildingblocks for advanced materials. Recently, heteropoly acidsincorporated with various organic moieties have receivedincreasing attention, because such novel functional charge-transfer hybrid molecules exhibit some interesting proper-ties, such as photochromism, magnetism, photocatalysisand third order optical nonlinearities as compared to theirbases [7–13]. To develop rational methods for the modifica-tion and functionalization of heteropoly acid, systems canprovide the means to chemical, structural and electronic ver-satility and exploit more fully desirable attributes [14].

In this paper, we reported the synthesis and chargetransfer properties of a novel inorganic-organic hybridcomplex, which consists of molybdophosphoric acid with

0022-0728/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.jelechem.2006.02.013

* Corresponding author. Tel.: +86 512 65606675; fax: +86 51267613888/65224783.

E-mail address: yingwu@suda.edu.cn (Y. Wu).

keggin structure, H3PMo12O40 (PMo12) and 3,3 0,5,5 0-tetra-methylbenzidine (TMB). The reduced TMB is colorless,which has p–p and p–p conjugation structure, leading tothe molecule favorable to transfer its electrons to otherelectron-acceptor form charge-transfer compounds [15,16].The present study shows that there is strong electronicinteraction between TMB and heteropoly anion PMo12.The results prove that photo-induced intramolecular elec-tron transfer occurs in the hybrid complex, where PMo12

and TMB in the hybrid compound can actually serve asthe electron acceptor and donor, respectively. The hybridcomplex also exhibits good electrocatalytic activity forthe reduction of iodate.

2. Experimental

2.1. Chemicals

TMB was purchased from Suzhou New District BECFine Chemicals Co. Ltd. and used without further purifica-tion. H3PMO12O40 Æ nH2O was prepared by the literaturemethod [17]. All other chemicals used were of reagent

Fig. 1. ESR spectrum of the TMB-PMo12 hybrid complex at roomtemperature.

400 500 600 700 800 9000.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75 without irradiation after UV irradiation for 5 min after UV irradiation for 20 min

A

Wavelength / nm

Fig. 2. Solid electronic-spectra of TMB-PMo12.

Y. Wu et al. / Journal of Electroanalytical Chemistry 589 (2006) 232–236 233

grade. Solutions used in all experiments were prepared withMillipore water.

2.1.1. Synthesis of TMB – PMo12 hybrid compound

An ethanol solution containing 1.5 mmol TMB (10 mL)was added dropwise to 10 mL of 0.5 mmol PMo12 ethanolsolution. The mixture solution was heated to 60 �C on awater bath. Then, the solution was stirred at 60 �C for3 h. The color of the solution was gradually turned fromyellow-green to green, and to final black-green, and precip-itate appeared in the mixture. The precipitate was sepa-rated by filtration, washed with ethanol, and dried invacuum at room temperature. The final product was char-acterized by IR spectroscopy, ESR spectroscopy, and ther-mogravimetric analysis. The complex can be formulated as(TMB)3HPMo12O40 Æ 3H2O by the elemental analysis,found: C, 21.7; H, 2.61; N, 3.18; P, 1.18; Mo, 42.30%;calcd.: (C, 22.2; H, 2.65; N, 3.23; P, 1.19; Mo, 44.31%),

2.2. Apparatus and methods

Electrochemical measurements were carried out on aCHI 660 electrochemical station. Traditional three-electrode system was employed with saturated calomel elec-trode (SCE) as reference electrode and a Pt electrode ascounter electrode. The working electrode was prepared bythe following procedure. A glassy carbon electrode was pol-ished with 0.5 and 0.05 lm a-alumina powder, respectively.Then the electrode was sonicated in absolute alcohol, doublydistilled water successively, and dried at room temperature.Silica sol was prepared according to the literature protocol[18]. TMB-PMo12 hybrid (0.0015 g) was dissolved in 1–2drops of DMF, and mixed with 0.25 mL of silica sol and0.25 mL of H2O. The dispersion was sonicated for 10 min.After that, 8 lL of the suspension was dropped on the sur-face of a glassy carbon electrode and dried in air for 24 h.The thickness of the resulting film on the electrode was esti-mated to be ca. 0.1 lm.

Electronic absorption spectra were obtained on a Shi-madzu UV-240 spectrophotometer. The solid sample wasprepared as KBr pellets. Photochemical behavior wasdetermined using a 500 W high-pressure mercury lamp asthe light source. The distance between the lamp and thesample is 15 cm.

3. Results and discussion

3.1. Intramolecular charge transfer of the hybrid complex

The appearance of black green color of the solution inthe synthesis procedure and elemental analysis of thehybrid complex seem to be indicative of the formation ofthe ‘‘heteropoly blue’’, the reduced form of PMo12. To clar-ify this, ESR spectrum of the hybrid complex was mea-sured at room temperature; the result is shown in Fig. 1.A broad signal with g = 1.967, along with narrow signalcentered at g = 2.004, was observed. The former can be

ascribed to the existence of Mo(V). The peak-to-peakderivative width (DHpp) of narrow signal is about 15 G,which is a typical value for organic radical. Therefore, thissignal is tentatively assigned to the TMB�+ radical cation.The ESR result clearly verifies the occurrence of the chargetransfer of TMB to polyanion in the hybrid complex. Tofurther demonstrate the intramolecular charge transfer ofthe hybrid complex, photo-induced electron transfer andelectrochemical behavior of the complex were measuredand described next.

3.2. Electronic absorption spectra

The electronic spectra of the solid sample before andafter irradiation are shown in Fig. 2. The TMB-PMo12

hybrid molecule in solid state exhibited a strong absorptionband at 660 nm (Fig. 2 frame circle line). This band was notobserved for the solid sample of PMo12 or TMB alone. Theabsorption of the reduced PMo12 should be located at720 nm. Therefore, the band observed in Fig. 2 could comefrom the hybrid complex. A similar band was reported byMecheria et al. [19] in their study of TMB as a redox medi-ator for molecular recognition of NADH. This band wasattributed to the absorption of dimer TMB2þ

2 (which is acharge-transfer complex of the oxidation product TMB2+

234 Y. Wu et al. / Journal of Electroanalytical Chemistry 589 (2006) 232–236

and the parent TMB) [19]. The absorption band of thereduced PMo12 may be superposed with the absorptionband of TMB2þ

2 . Accordingly, in the present case, the threeTMBs in the hybrid complex existed in different states.Two of them were reduced to form the dimer, whereasthe other one also in reduced form exists as isolated state.Although attempt to obtain the crystal of the hybrid com-plex was unsuccessful, the crystal structure of a similarcomplex, (DMAH)3PMo12, strongly confirms this argu-ment [20]. The band of TMB2þ

2 completely disappearedafter irradiation. Instead, two new absorption bandsappeared at 720 and 460 nm, respectively (see Fig. 1 solidcircle line and solid line). The absorption band at 720 nmarose from the transition of Mo(V)!Mo(VI) (IVCT).The band at 460 nm is pertained to the spectral absorptionof the oxidation product TMB2+ [19]. That is, further elec-tron transfer occurred between TMB and PMo12, resultingin the complete conversion of the TMB cation to dicationin the hybrid complex. The photo-induced electrons ofthe TMB, on the other hand, are trapped by heteropolyanion, in which more Mo(VI) is reduced to Mo(V). Thisresult means that hybrid compound is highly photosensi-tive in ultraviolet region; irradiation can result in a furtherintramolecular electron transfer in the complex.

3.3. Electrochemical behaviors

The cyclic voltammetric (CV) response of TMB-PMo12

modified GC electrode in a 0.5 mol/L H2SO4 solution isshown in Fig. 3. For comparison, the CVs of PMo12 andTMB modified electrodes are also presented. In the poten-tial range between 0.6 and �0.1 V, three pairs of redoxpeaks were observed for the PMo12 modified electrode.They can be attributed to three two-electron-transfer reac-tions of PMo12 [21]. TMB modified on the glassy carbonelectrode exhibited only a single pair of redox peaks, corre-sponding to a two-electron redox reaction of TMB toTMB2+ dication [19]. The CV of the hybrid complex exhib-its four reversible redox couples. Comparing with the CVsof PMo12 and TMB (Fig. 3 solid circle line), we can deduce

Fig. 3. CV curves of TMB-PMo12 (solid line); H3PMo12O40 (frame circleline); TMB (solid circle line) modified electrode in 0.5 mol/L H2SO4,100 mV/s.

that the pair with most positive potentials is from TMBand the rest three pairs are from the characteristic redoxcouples of PMo12. Nevertheless, the reduction potentialof TMB in the hybrid obviously negatively shifted from+0.53 V of the TMB modified electrode (Fig. 3 solid circleline) to +0.49 V of the hybrid complex modified electrode,and the oxidation potential of TMB positively shifted from+0.57 to +0.59 V. This suggests that an interaction couldtake place between TMB and heteropoly anion, and this,in turn, promoted the delocalization of the electronsobtained to whole hybrid molecule.

The Epa and Epc values were independent of the scanrate in the range 0.02–0.20 V/s (Fig. 4), each redox peakcurrent (ip) was proportional to the scan rate in the range0.02–0.20 V/s. This suggested that the electrode processesare the reversible or quasi-reversible, surface-confined elec-tron transfer process. After 20 cycles, only 5% decrease inthe currents was observed, indicating that the film on theelectrode surface had good stability.

As mentioned above, charge transfer readily occurs inthe hybrid complex under the synthesis conditions as wellas under irradiation. It is expected that the hybrid structureof the complex could result in new catalytic property. Theelectrochemical catalytical behaviors of the PMo12 modi-fied electrode and the hybrid complex modified electrodewere compared for the reduction of IO�3 , which wasselected as a model electrocatalytic reaction; the typicalresults are shown in Figs. 5 and 6, respectively. It can beclearly seen that in the presence of IO�3 , the cathodic cur-rent started to increase at 0.2 V (Fig. 5), which correspondsto the second redox pair of PMo12. Thus, only the secondtwo-electron reduction could show the catalytical activityfor the reduction of IO�3 . Similar result was reported byWang [22]. The TMB-PMo12 film modified electrode alsoshowed quite good electrocatalytic activity. As seen inFig. 6, in the presence of IO�3 in the solution, all the catho-dic currents increased dramatically, while the correspond-ing oxidation peaks decreased. In particular, the increasein the cathodic current was observed at the potential corre-sponding to the first reduction peak of PMo12 in the hybrid

Fig. 4. CV curves of the TMB-PMo12 modified GC electrode in differentscan rate U (mV/s): 20, 50, 100, 150, and 200.

Fig. 5. CV curves of the PMo12 modified electrode 50 mV/s (solid line) in0.5 mol Æ L�1 H2SO4; (solid circle line) in 0.5 mol Æ L�1 H2SO4 + 5 · 10�4

mol Æ L�1 KIO3.

Fig. 6. CV curves of the TMB-PMo12 modified electrode 50 mV/s (solidline) in 0.5 mol Æ L�1 H2SO4 + 1 · 10�4 mol Æ L�1 KIO3; (solid circle line)in 0.5 mol Æ L�1 H2SO4 + 5 · 10�4 mol Æ L�1 KIO3; (frame circle line) in0.5 mol Æ L�1 H2SO4 + 1 · 10�3 mol Æ L�1 KIO3.

Fig. 7. CV curves of the TMB-PMo12 modified electrode (a) in0.5 mol Æ L�1 H2SO4 solution, and 0.5 mol Æ L�1 H2SO4 + 7 · 10�4

mol Æ L�1 KIO3 solution (b) without and (c) with the UV irradiation.

Y. Wu et al. / Journal of Electroanalytical Chemistry 589 (2006) 232–236 235

complex. This demonstrates that the first two-electronreduction reaction of PMo12 in the hybrid complex can cat-alyze the reduction of IO�3 . The present results clearly dem-onstrate that the incorporation of TMB with PMo12 mayalter the catalytical property of the heteropoly anion. Inother words, the incorporated TMB with the delocalizedelectrons in the hybrid complex likely promote the firstreduction reaction of PMo12 to catalyze the reduction ofthe IO�3 anions. To further verify this point, the rate con-stants for the electrocatalytic reduction of IO�3 on thePMo12 and TMB-PMo12 modified GC electrodes weremeasured by chronoamperometry with different IO�3 con-centrations, according to the literature method [23]. Therate constants are 1.0 · 103 and 3.1 · 104 M�1 s�1 for thePMo12 and TMB-PMo12 modified GC electrodes,respectively.

The effect of the irradiation on the electrocatalyticreduction of IO�3 on the TMB-PMo12 modified GC elec-trodes was also investigated, the results are shown inFig. 7. It is evident that the catalytic current increased lar-gely under the irradiation of light (Fig. 7c), indicating that

the photo-induced intramolecular charge transfer in theTMB-PMo12 molecules has contribution to the overallelectrocatalytic reduction of IO�3 on the TMB-PMo12 mod-ified GC electrode.

4. Conclusion

Novel hybrid compound, TMB-PMo12, is photosensi-tive; irradiation with UV light can cause the intramolecularelectron transfer in the hybrid complex, where the photo-induced electrons are trapped by the heteropoly anion.There is a strong electronic interaction between the TMBand PMo12. The TMB-PMo12 modified electrode showedgood reversibility and stability. The modified electrodeexhibited excellent catalytical activity for the reduction ofIO�3 . The incorporated TMB may actually activate theearly-reduced form of the heteropoly anion to catalyze toreduction reaction of IO�3 on the electrode.

Acknowledgments

This work was supported by the National Natural Sci-ence Foundation of China (20275025) and the Natural Sci-ence Foundation of Jiangsu Committee of Education(05KJB150123).

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