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Journal of Electroanalytical Chemistry 585 (2005) 113–119

ElectroanalyticalChemistry

Spectroelectrochemical analysis of the thionine transfer coupled toa photoinduced chemical reaction across the

water|1,2-dichloroethane interface

R.A. Iglesias *, M.F. Mora, A.M. Baruzzi

Instituto de Investigaciones en Fisicoquımica de Cordoba (INFIQC – CONICET), Departamento de Fisicoquımica, Facultad de Ciencias

Quımicas, Universidad Nacional de Cordoba, Pabellon Argentina, Ala 1, Piso 2, Ciudad Universitaria, Cordoba CP 5016, Argentina

Received 24 May 2005; received in revised form 3 August 2005; accepted 5 August 2005Available online 13 September 2005

Abstract

Thionine (Thio+) is a dye used in photogalvanic and dye-sensitized solar cells to convert light into chemical energy. The photo-physical properties of this cell are strongly affected by aggregate formation and polymerization of thionine because the monomer isthe photoactive species. In this paper, we report the electrochemical interfacial transfer of thionine from water to 1,2 DCE and thefurther photochemical reaction upon illumination of this organic phase. Cyclic voltammetry coupled to interfacial absorbance mea-surements allowed us to perform a kinetic analysis. Values of eThio = (2.5 ± 0.1)104 M�1 cm�1, eproduct = (1.4 ± 0.1)10 4 M�1 cm�1

and a kinetic constant k = 0.28 ± 0.05 s�1 for the chemical reaction, were calculated by simulation of the interfacial absorbance pro-files. NMR and UV–vis spectra of the phases at different pH values contributed to identify the nature of the reacting species of thechemical reaction and showed the important influence of pH on the kinetic of the process. The effect of the organic supporting elec-trolyte on the kinetics of the reaction was also analyzed.� 2005 Elsevier B.V. All rights reserved.

Keywords: Photoinduced reaction; Liquid interface; Thionine; Numerical simulation; Interfacial absorbance; Dye; Solarcell

1. Introduction

Solar cells based on dyes involve a sequence of reac-tions which include light absorption, redox reaction witha mediator in homogeneous phase, conversion of bulkspecies to surface species and charge transfer reactionsat electrodes regenerating the dye [1]. These are the ba-sics steps in a solar cell but there are also others sidereactions that may occur decreasing the efficiency ofthe cell, like the back thermal homogeneous electron

0022-0728/$ - see front matter � 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.jelechem.2005.08.002

* Corresponding author. Tel.: +54 351 4334169/80; fax: +54 3514334188.

E-mail addresses: riglesias@mail.fcq.unc.edu.ar (R.A. Iglesias),abaruzzi@mail.fcq.unc.edu.ar (A.M. Baruzzi).

transfer and the disproportionation reaction of the re-duced dye.

Many attempts have been done to decrease theseundesired processes. Some experiments indicate thatstorage of photochemical energy can be achieved bydividing the photoproducts between two phases in aninhomogeneous system [2]. The presence of an interfaceseparating both half reactions would inhibit the reversereaction between mediator and dye, increasing the cellefficiency. The electron transfer between excited ruthe-nium complex (in water) and different quenchers (in anorganic phase) was studied by Marecek et al. [3] andThomson et al. [4].

Not only photochemical electron transfer at the inter-face between two immiscible electrolytes (ITIES) butalso photoinduced ion transfers, e.g., photoinduced

114 R.A. Iglesias et al. / Journal of Electroanalytical Chemistry 585 (2005) 113–119

transfer of tetraphenylborate anion across the water|1,2-dichloroethane interface [5] and photochemical reac-tions coupled to ion transfer [6,7] have been considered.These authors showed the photoelectrochemical effectdue to the transfer across the interface of photochemi-cally generated charged particles (ions or radicals).The product of an electron phototransfer in the organicphase is a charged particle that diffuses to the interfaceand is transferred to the aqueous phase with potentialgradient.

Electrochemical studies about chemical reactionscoupled to ion transfer at ITIES, on the other hand,have also been reported [8,9].

Thionine (Thio+) is one of the dyes used in photogal-vanic cells [1,10]; in this case, the reaction between thio-nine and iron is used to convert light into chemicalenergy.

Unfortunately, as the monomer of thionine is thephotoactive species, the photophysical properties of thiscell are strongly affected by polymerization and aggre-gate formation which may occur in addition to the unde-sired side processes previously mentioned.

Different organized media such as micelles and micro-emulsions have shown to improve considerably the out-put voltage of the classical aqueous thionine–iron cell[10]; the surfactant media contributes on one side to in-hibit the back homogeneous redox reaction and on theother to avoid thionine agregation and polymerization[10]; these aspects can also be improved by photoinduc-ing reactions of thionine in an organic solvent. Some pa-pers report the photophysical properties of this dyemodified with substituents like macromolecules [11] giv-ing a hydrophobic environment to the dye which resultsin an excited state with longer lifetime and changes inthe efficiency of the cell.

In the present paper, we report the potential con-trolled interfacial transfer of thionine from water to1,2 DCE and the further photochemical reaction uponillumination of the organic phase. We present voltam-metric experiments coupled to absorbance measure-ments (Voltabsortometry) corresponding to thioninetransfer across the water|1,2-dichloroethane (1,2 DCE)interface at several pH values. Simulation of the absor-bance profiles, performed by the explicit differencemethod as proposed in a previous paper [12] allowedus to extract kinetic and thermodynamic parameters ofthe chemical reaction. The effect of the organic support-ing electrolyte on the kinetics of the reaction was alsoanalyzed.

The particular features of the experimental settingused in this work (i.e., separation of products and reac-tants between two phases, potential control of thehomogeneous chemical reaction and inhibition of theabove mentioned processes, like aggregates formationand polymerization) make it suitable for studying thiskind system.

2. Experimental

Cyclic voltammetry and potential pulse experimentswere carried out in a four-electrode system using a con-ventional glass cell of 0.15 cm2 of interfacial area. Twoplatinum wires were used as counter-electrodes and thereference electrodes were Ag|AgCl|Cl� (0.01 M). Thereference electrode in contact with the organic solutionwas immersed in an aqueous solution of 1.0 · 10�2 Mtetrapentylammonium chloride (TPnA+Cl�, NBS). Thepotential values reported are the applied potentialswhich include the transfer of the reference ion TPnA+.Positive currents correspond to cation transfer fromthe aqueous to the organic phase or anions from the or-ganic to the aqueous phase; the opposite is valid for neg-ative currents.

The aqueous base electrolyte was 1.0 · 10�2 M KCl(Merck p.a.), in ultrapure water (Milli Q-Millipore).The organic base electrolytes were 1.0 · 10�2 M tetra-pentylammonium tetraphenylborate (TPnA+TPB�) in1,2-dichloroethane (1,2 DCE), Dorwil, p.a. TPnATPBwas prepared as described in [13]. 3,7-Diaminophenothi-azine chloride (Thionine, Thio+Cl�) solutions were pre-pared with different supporting electrolytes (HCl, NaCl,NaOH or NaH2PO4) depending on the pH required, asindicated in the text for each particular case. It is impor-tant to keep in mind that in all cases the chlorideconcentration was constant and equal to 1.0 · 10�2 M.

The voltammetric and voltabsortometric experimentswere performed using a home made potentiostat withpositive feedback for IR drop compensation. Experi-mental setup and cell for the spectroelectrochemicalexperiments were also homemade, guarantying a totalinternal reflection mode of a green solid state laser(532 nm and 1.5 mW filtered through two neutral filters3 OD). The laser is used to analyze changes of absorbingspecies in the organic interfacial region. Besides, whitelight impinges on the liquid interface from a 100 Wtungsten bulb without any filtering and it is used to in-duce the photochemical reaction of the dye.

A HiTEK waveform-generator was used at the inputof the potentiostat. The UV–vis spectra of both organicand aqueous solutions were recorded using a ShimadzuUV-1601 spectrophotometer. NMR 1H spectra were ta-ken with a Bruker AC-200 spectrometer in deuteratedacidic and alkaline methanolic solutions of thionine.

3. Results

3.1. Electrochemical analysis

Figs. 1(a)–(c) show the effect of illuminating the inter-face with white light on the voltammetric transfer of thi-onine across the water|1,2 DCE interface at differentsweep rates and pH 2.0.

0.2 0.3 0.4 0.5

-15

0

15

30a

I / µ

A

E / V0.2 0.3 0.4 0.5

b

E / V0.2 0.3 0.4 0.5

c

E / V

Fig. 1. Effect of light on the voltammetric profiles corresponding tothe transfer of Thio+ across the water|1,2 DCE at different sweep rates:(a) 0.20 V s�1; (b) 0.10 V s�1; (c) 0.05 V s�1. Non-irradiated interface(––––––) and irradiated interface (- - - - - - - -). Aqueous phase: Thio+Cl�

1 mM, pH 2.0. Organic phase: TPnATPB 10 mM in 1,2 DCE.

R.A. Iglesias et al. / Journal of Electroanalytical Chemistry 585 (2005) 113–119 115

When the interface is not irradiated a characteristicdiffusion controlled profile corresponding to a single di-rect cation transfer with the same value for the positiveand the negative current peaks is found. Upon illumina-tion of the interface during the cation transfer processthe positive current peak (p+) remains almost un-changed while the negative current peak decreases(p�). This effect depends significantly on sweep rateand intensity of light, being higher as the sweep rate de-creases and the intensity increases.

The interfacial absorbance profiles at 532 nm (corre-sponding to the wavelength of the laser that impingsthe interface), were also recorded during the polariza-tion of the interface at different sweep rates. As shownin Figs. 2(a)–(c), concomitant with the current, the inter-

0.25 0.50 0.75

0.0

0.1

0.2

0.3

0.4

0.5

E / VE / VE / V0.40 0.200.20 0.450.450.400.200.450.40

a

A /

a.u

.

t / V0.25 0.50 0.75

b

t / V0.25 0.50 0.75

c

t / V

Fig. 2. Effect of light on the interfacial absorbance profiles corre-sponding to the transfer of Thio+ across the water|1,2 DCE interface atdifferent sweep rates: (a) 0.20 V s�1; (b) 0.10 V s�1; (c) 0.05 V s�1.(––––––) non-irradiated interface and (- - - - - - - -) irradiated interface.Aqueous phase: Thio+Cl� 1 mM, pH 2.0. Organic phase: TPnATPB10 mM in 1,2 DCE.

facial absorbance profiles show a higher decrease in theabsorbance at low sweep rates upon illumination of theinterface. This fact and the decrease in the current valuesobserved during the backward sweep shown in Fig. 1indicates the occurrence of a photoinduced reaction inthe organic phase coupled to the Thio+ transfer fromthe aqueous to the organic phase. As the dye is trans-ferred it reacts in the organic phase decreasing in con-centration and therefore in absorbance if the productshave lower absortivity than the cation Thio+ at 532 nm.

Heterogeneous electron transfer between thionine inthe aqueous phase and TPB� in the organic phase cannot explain the small current observed under interfacialillumination after the positive transfer peak (Fig. 1); atthis potential values the amount of thionine in the aque-ous side of the interface is negligible and the absorbanceprofiles show a concomitant depletion of the thionine inthe organic side. These facts are a clear indication thatorganic Thio+ is consumed in this phase through ahomogeneous chemical reaction. Alternative experi-ments (not shown) have indicated that this photocurrentwould be mainly due to the assisted transfer of protonnecessary for this coupled reaction (see below the reduc-tion reaction of thionine). This photocurrent becomessmaller at higher pH values according to the interpreta-tion given before.

The reduction of thionine to the uncoloured leucothi-onine (HThio) in the presence of reducing agents and inacidic media is a very well-known process [1,10,14]

Thioþ þHþ þ 2e ! HThio

This reaction could explain the observed decreases incurrent and interfacial absorbance; therefore, electro-chemical experiments were performed in the range2.0 < pH < 10.0 considering the dependence of this reac-tion with H+ concentration. At all the pH values ana-lyzed the general effect of light and sweep rate shownin Fig. 1 was observed. Fig. 3 shows the dependence

2 4 6 8 10

0.36

0.39

0.42

0.45

EP+ /

V

pH

Fig. 3. Dependence of the EþP with the pH value of the aqueous phase.

Aqueous phase: Thio+Cl� 1 mM. Organic phase: TPnATPB 10 mM in1,2 DCE.

116 R.A. Iglesias et al. / Journal of Electroanalytical Chemistry 585 (2005) 113–119

of the positive peak potential value ðEþP Þ with the pH; Eþ

P

remains approximately constant up to pH 8.0 and athigher pH values a shift to more positive potentials isfound.

As the pH was increased a change in the colour of theaqueous solution from blue to red and colouring of theorganic solution were observed. Due to the strong col-our of the organic phase at high pH values (pH > 8.0),it was not possible to measure the effect of light on thecation transfer under this experimental condition. Thischange of the chemical composition of the phases withpH made necessary to perform not only electrochemicalbut also chemical analysis to elucidate if the nature ofthe transferred species was changing with the protonconcentration and to analyze the chemical reaction fol-lowing the electrochemical transfer.

3.2. Chemical analysis

3.2.1. Aqueous solutions

Fig. 4(a) shows UV–vis spectra at different pH values;as the pH increases the absorbance peak at 597 nm,characteristic of thionine monomer at low pH values[15], decreases and a new peak appears at 510 nm indi-

4 6 8 10 120.0

0.1

0.2

0.3

0.4 b

AP

pH

300 400 500 6000.0

0.1

0.2

0.3

0.4 a

12.3

11.6

11.4

10.7

9.59.3

7.85.5

A

/ nm

Fig. 4. UV–vis analysis of a Thio+ aqueous solution at different pHvalues. [Thio+] = 1.5 · 10�5 M. (a) Absorbance spectra for Thio+

aqueous solution at pH values between 5.5 and 12.3. Arrows indicatethe effect of increasing pH. (b) Absorbance vaues at 597 nm.

cating a pH dependent interconversion of species. A va-lue of pKa � 8 for this acid–base reaction can be inferredfrom a change in the slope of a plot of the absorbance at597 nm vs. pH (Fig. 4(b)), and from a titration of anaqueous solution of thionine with NaOH. This valueof pKa is coincident with the electrochemical resultsshown in Fig. 3.

The characterization of the thionine species present atdifferent pH was established from the chemical shifts inthe 1H NMR spectral data performed in methanolicsolutions (Figs. 5(a) and (b)). The 1H NMR spectrumshown in Fig. 5(a) (acid solution, I) exhibited only threesignals corresponding to aromatic protons at d 8.05 (d,J = 9.5 Hz), 7.34 (dd, J = 9.5; 2.2 Hz) and 7.22 (dJ = 2.2 Hz), assigned to H-xx 0, H-yy 0 and H-zz 0, respec-tively. The 1H NMR spectrum in Fig. 5(b) (basic solu-tion, II) was closely related to that of I. The onlydifferences between the spectra of I and II were the high-field shifts for the three protons; this is an indicationthat in case II the species was shielded which is anevidence of its neutral nature. In acidic solutions thethionine presents a resonant positive charge as reportedin the literature [11], while a neutral imine (Fig. 5(c)) isidentified as the predominant specie at pH > 8. Consid-ering that the transfer of neutral compounds is notpossible to be observed at polarized interfaces, this

6.56.66.76.86.97.07.17.27.37.47.57.67.77.87.98.08.18.2(ppm)

Acid solution:

Basic solution:

Neutral imine

N

S NH2H2N+

N

SH2N NH2+

AB

C

A A ABBB

CCC

A

B

C

AB

C

N

S NHH2N

a

b

c

Fig. 5. 1H NMR spectra for Thio+ in deuterated methanolic solution:(a) 0.0015 g of Thio+Cl� in 0.5 ml of deuterated methanol; (b) 0.0015 gof Thio+Cl� in 0.5 ml of deuterated methanol: 40% NaOD (99:1);(c) chemical structures of thionine at acid and basic pH values.

R.A. Iglesias et al. / Journal of Electroanalytical Chemistry 585 (2005) 113–119 117

result indicates that the voltammetrically transferredspecie at any pH value is the charged thionine, predom-inant at pH < 8. On the other hand, when both phasesare brought into contact at pH > 8 to perform the elec-trochemical experiment, the organic phase becomes col-oured due to the extraction of the neutral imine(Fig. 5(c)). This result also indicates that the shiftingin the peak potential at pH > 8 is due to a pH dependentprevious chemical reaction coupled to extraction of theimine to the aqueous phase to form Thio+.

3.2.2. Organic solutions

For the analysis of the chemical reaction occurringin the organic phase after the electrochemical transfer,UV–vis spectra of Thio+ in 1,2 DCE containingsupporting electrolyte were performed under differentexperimental conditions (Fig. 6). Due to the slightsolubility of the dye in 1,2 DCE, the organic solutionswere prepared by agitation of 1,2 DCE against acid orbasic aqueous solution of thionine (Figs. 6(a) and (b),respectively). The spectrum obtained from an acidicaqueous solution presents a maximum in absorbanceat 590 nm that decreases and almost vanishes whenthe solution is illuminated (Fig. 6(a)). On the opposite,in the case of alkaline solution the maximum in absor-bance appears at 527 nm and slightly decreases whenthe 1,2 DCE solution is illuminated (Fig. 6(b)). If bothspectra at alkaline pH are subtracted the resulting spec-trum resembles quite well that obtained in acidic solu-tion where Thio+ is the main species observed. Theseresults indicate on one side, that the pH value of theaqueous phase has an important influence in the wholeprocess as it determines the nature of the predominantaqueous species; on the other, they confirm thatcharged thionine is the reactant of the coupled chemi-

400 500 600 700

0.0

0.3

0.6

0.9

1.2

a

A

/ nm400 500 600 700

0.0

0.3

0.6

0.9

1.2 b

/ nmλλ

Fig. 6. Absorbance spectra for Thio+ in 1,2 DCE showing the effect ofillumination of the organic phase after extraction. Thio+ was extractedfrom (a) acid aqueous solution and (b) basic aqueous solution in a10 mM TPnTPB organic solution. (––––––) before illumination of thephase, (� � � � � � � � �) after illumination of the phase and (- - - - - - - -)difference spectrum at basic pH value.

cal reaction at all pH values. It is important to remarkthat these effects are only observed if the supportingelectrolyte is present, so these experiments are a quiteclear indication of the important participation of TPB�

in the homogeneous chemical reaction. The coupledchemical reaction is a redox reaction between TPB�

and Thio+, considering that the anion can be easilyoxidized, probably to form triphenyl boron as Geskeproposed many years ago [16]. Ion-pair formation be-tween TPB� and Thio+ could also be considered,although in fact in low-polarity solvents the tendencyof association of these two ions is very low as it waswell studied by Moon et al. [17].

3.3. Kinetic analysis

Numerical simulation of the absorbance profiles wasdone using the finite difference method to find the ki-netic parameters of the homogeneous reaction andthe absortivities of reactants and products. This calcu-lation was performed as indicated in a previous theo-retical report [12]. In that paper, a program to obtainvoltammetric and voltabsortometric profiles for differ-ent parametric conditions was presented. Charge (zi),partition coefficients, standard potential transfersðDw

o/oÞ, direct and reverse kinetic constants (k) of all

the reactions and organic molar absorptivities ðeoi Þ ofreactants and products can be controlled. The nextfour possible following chemical reactions coupled tothe heterogeneous transfer were considered in thatpublication:

Heterogeneous transfer

Possible coupled chemicalreactions

AzAðwÞ ¢AzA

ðoÞ:

(1) AzAðoÞ ¢

k1

k2Q

zQðoÞ

(2) AzAðoÞ ¢

k1

k2Q

zQðoÞ þ ZzZ

ðoÞ

(3) AzAðoÞ þXzX

ðoÞ ¢k1

k2Q

zQðoÞ

(4) AzAðoÞ þXzX

ðoÞ ¢k1

k2Q

zQðoÞ þ ZzZ

ðoÞ

where AzAðoÞ; XzX

ðoÞ; QzQðoÞ and ZzZ

ðoÞ are possible reactantsand products (neutral or charged).

As reactions 3 and 4 involve the presence of two reac-tants, in our case Thio+ and TPB�, only these twomechanisms were considered and used for the calcula-tions. Actually, both mechanisms give the same behav-iour when the coupled chemical reaction is irreversible,as it is our case. Particularly, we used reaction 4 coupledto the heterogeneous transfer because it better representsa redox reaction.

Fig. 7 shows the dependence of the peak interfacialabsorbance value (Ap. See Fig. 2) with v�1/2 with andwithout illumination of the interface. The linear depen-dence observed under no light condition was expected

1 2 3 4 5

0.1

0.2

0.3

0.4

0.5

0.6

AP

ν-1/2 / (V.s-1)-1/2

Fig. 7. Dependence of Ap with v�1/2. Aqueous phase: Thio+Cl� 1 mM,pH 2.0. Organic phase: TPnATPB 10 mM in 1,2 DCE. Withoutinterfacial irradiation: (d) experimental data; (––––––) simulated data.With interfacial irradiation: (–) experimental data and (- - - - - - - -)simulated data.

118 R.A. Iglesias et al. / Journal of Electroanalytical Chemistry 585 (2005) 113–119

(Beer–Lambert Law), considering that absorbance is adirect measurement of concentration; the deviationobserved under illumination was obviously due to thedecrease in the thionine interfacial concentration afterthe reaction with the supporting electrolyte and itwas perfectly simulated. Unfortunately, this good fit-ting was obtained with several sets of parameters. Toavoid this drawback, the fitting of the whole profilesin the complete potential window was done for differ-ent sweep rates. The simulations performed in thisway allowed us not only to verify the agreementbetween a mechanism and the experimental resultsbut also to obtain a unique set of parameters for eachprofile. Fig. 8 shows examples at two sweep rates. The

0.00 0.25 0.50 0.75

0.0

0.1

0.2

0.3

0.4a

n t / V

A

0.00 0.25 0.50 0.75

0.0

0.1

0.2

0.3

0.4

b

n t / V

Fig. 8. Interfacial absorbance profiles corresponding to the transfer ofThio+ across the irradiated water|1,2 DCE interface. Aqueous phase:Thio+Cl� 1 mM, pH 2.0. Organic phase: TPnATPB 10 mM in 1,2DCE. (––––––) experimental profile and (- - - - - - - -) simulated profile:(a) v = 0.05 V s�1; (b) v = 0.075 V s�1.

values obtained as average of those resulting of com-plete profile simulation at each sweep rate are

eoThioþ ¼ ð2.5� 0.1Þ104 M�1 cm�1;

eoProduct ¼ ð1.40� 0.1Þ104 M�1 cm�1;

Dwo/

o0 ðThioþÞ ¼ ð0.05� 0.01Þ V;k1 ¼ ð0.28� 0.05Þ s�1.

The effect of pH and TPB� concentration on the ki-netic of the process was also considered. With this pur-pose the decrease in current of the negative sweep wasmeasured at pH = 2.1, 4.6, 6.9 (10 mM TPB� in the or-ganic phase) and also at 5, 10 and 30 mM TPB� (pH 2).The kinetic constant of the coupled chemical reactionshowed independence with the supporting electrolyteconcentration at least in the range analyzed indicatingthat not pseudo-order is implied in this constant. On theother side, at low pH values the homogeneous chemicalreaction becomes more important producing a higherdecrease in the negative current being this fact anotherindication of the participation of H+ in the reaction.

4. Conclusion

This paper shows clear evidences of the occurrenceof a light dependent reaction in the organic phase afterthe electrochemical transfer of thionine, and how akinetic analysis can be performed measuring the inter-facial absorbance during an experimental voltammo-gram. Global kinetic parameters were obtained byusing simulation of the whole voltammetric profiles;in this case it was not useful to build working curves,probably due to the number of experimental parame-ters involved.

It was shown also that the H+ concentration has animportant role in the yield of this reaction; not onlyincreasing the concentration of the charged thionine inaqueous phase but also having a remarkable effect onthe kinetic of the whole process.

Acknowledgements

Authors thank to Consejo Nacional de Investigaci-ones Cientıficas y Tecnicas (CONICET) and Secretarıade Ciencia y Tecnica (SeCyT) by the financial supportto make this work.

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