Triple Phase Boundary Photovoltammetry

DOI:10.1002/cphc.200000094

Triple Phase Boundary Photovoltammetry:Resolving Rhodamine B Reactivity in

4-(3-Phenylpropyl)-Pyridine Microdroplets

Andrew M.Collins,[a]Xiaohang Zhang,[a]Jonathan J.Scragg,[a]Gary J.Blanchard,[b]and Frank Marken*[a]

1.Introduction

The aim of this study is to demonstrate and investigate photo-electrochemical processes at a liquid j liquid j solid electrode interface.Photoelectrochemical processes at liquid j liquid in-terfaces are important and may be regarded as key steps in biomimetic models for photosynthesis.[1–3]Liquid j liquid inter-faces provide a unique environment for photoelectrochemical processes to occur due to the change of solvent properties across the interface.Photoexcitable dyes may be dissolved in the organic phase[4,5]or adsorbed on the liquid j liquid inter-face,[6,7]which leads to the possibility of photoelectrochemical-ly driven ion transfer at the interface.The ion-transfer step is coupled to a polarisation effect which is part of the natural light energy harvesting process.[8]In contrast to work on pho-tochemical processes at conventional liquid j liquid interfaces, there has been relatively little attention to photoelectrochemi-cal processes at liquid j liquid j solid triple phase boundary sys-tems[9]apart from pioneering work by Wadhawan et al.[10,11] Due to coupled ion-and electron-transfer processes,these triple phase boundary as well as thin film liquid j liquid devi-ces[12]are of considerable interest for the study of photoelec-trochemical processes in microphase environments,and in future they could lead to new insights and approaches in light harvesting.

Electrochemical reactions in microdroplet deposits at elec-trode surfaces have been studied extensively for organic drop-lets immersed in aqueous media[13,14]and for aqueous micro-droplets immersed in organic media.[15,16]In contrast to work with thin-film deposits,which create a continuous organic–aqueous liquid–liquid interface,[17]microdroplet deposits are characterised by an extended liquid j liquid j electrode triple phase boundary zone and the absence of intentionally added electrolyte in the organic phase.[18,19]The size of the droplets (typically1to10m m)is important in controlling reactivity[20] and in particular random arrays of microdroplets formed at basal plane pyrolytic graphite electrodes have been employed successfully in several previous studies.[21]A range of experi-mental approaches to enhancing triple phase boundary pro-cesses has been proposed including use of mesoporous hosts,[22]punctured droplets,[23]arrays of droplets,[24]microflui-dic two-phase systems,[25]and ultrasonic mobilisation.[26]How-ever,in this study,processes at basal plane pyrolytic graphite surface are studied to provide a first screening tool for poten-tial photoelectrochemical activity for triple phase boundary systems.

Rhodamine B(see Figure1A)is a well-known xanthene dye molecule[27]with applications as a laser dye,[28]in flow injection analysis with chemiluminescence detection,[29]for hot electron-induced chemiluminescence,[30]as fluoresecent chemosen-sors,[31]as a structural probe for DNA and proteins,[32,33]as a sensitizer for photoinjection of electrons at semiconductor sur-[a]A.M.Collins,X.Zhang,J.J.Scragg,Dr.F.Marken

Department of Chemistry,University of Bath

Claverton Down,Bath BA27AY(UK)

Fax:(+44)1225-386231

E-mail:f.marken@https://www.360docs.net/doc/962682319.html,

[b]Prof.G.J.Blanchard

Michigan State University,Department of Chemistry

East Lansing,Michigan48824-1322(USA)

Rhodamine B base is employed as a photoactive redox system in a liquid j liquid j electrode triple phase boundary electro-chemical experiment.Microdroplets of the water-immiscible 3-(4-phenylpropyl)-pyridine(PPP)containing rhodamine B are deposited onto a basal plane pyrolytic graphite electrode sur-face which is then immersed into an aqueous solution contain-ing0.5m phosphate buffer solution(pH12).In cyclic voltam-metry experiments,it is shown that dark reduction of rhodami-ne B to leuco-rhodamine B in PPP occurs in two steps—both one electron processes—and the re-oxidation occurs in a single two-electron process.Voltammetric signals are consis-tent with Na+transfer(for the first reduction step)and proton transfer(for the second reduction step)coupled to the elec-tron transfer.A proton-driven disproportionation reaction(with k dis=3molà1dm3sà1at pH12)is observed for the one-electron reduced intermediate.In the presence of light,a new photoca-talysed oxidation process for the two-electron conversion of leuco-rhodamine B to rhodamine B is observed.Photoexcita-tion of rhodamine B is shown to trigger an effective photo-comproportionation mechanism.Quantitative insights into the dark and photomechanisms are obtained by comparison of rhodamine B and rhodamine B octadecylester and by applying an approximate numerical simulation procedure based on Digi-Sim.

faces,[34]and as a fluorescent pH sensor.[35]The excitation of

rhodamine B into a highly fluorescent excited state with %2ns lifetime [36]occurs with an absorption maximum near 550nm and an emission maximum in the vicinity of 580nm.Excitation into a triplet excited state has been reported for rhodamine B in aqueous solution [37]and this excited triplet state has been shown to be involved in electron-transfer reactions.[38]Most rhodamine derivatives exhibit a low efficiency for intersystem crossing.[39]The effect of adding base on enhancing the availa-bility and lifetime of the rhodamine triplet excited state has been pointed out.[40]The triplet excited state is associated with photochemical processes and therefore important in this study.

The electrochemical properties of rhodamine B have been reported.[41–43]In liquid j liquid electrochemical experiments,the potential-dependent adsorption of amphoteric rhodami-ne B derivatives at the liquid j liquid interface between di-chloroethane and water has been shown.[44]There are several previous reports on the photoelectrochemical activity of rho-damine B.A light-harvesting system based on ZnO semicon-ductor,rhodamine B sensitiser,and a quinol supersensitiser has been proposed [45]based on earlier work on the photoelectro-chemistry of rhodamine B on tin oxide and gold surfaces.[46]The first report on the photoelectrochemistry of rhodamine B by Krüger and Memming [47]described electron transfer at con-ducting SnO 2film electrodes.The reduction of rhodamine B was shown to first lead to a radical anion,which then forms a complex with rhodamine B.Disproportionation of the one-electron reduced rhodamine B into rhodamine B and leuco-rhodamine B was expected (based on experiments with pyro-nine)but not observed,possibly due to the association of the one-electron reduced rhodamine B with rhodamine B in aque-ous solution at pH 12.Herein,rhodamine B is dissolved into a water-immiscible organic solvent,4-(3-phenylpropyl)-pyridine (PPP),and then exposed to aqueous buffer media at pH 12,and under these biphasic conditions present in the organic phase the disproportionation process is shown to become im-portant.In the presence of light,a highly effective photocata-lytic comproportionation occurs.Photovoltammetry data anal-ysis is demonstrated based on the DigiSim software package.

Experimental Section

Chemicals and Reagents:Rhodamine B base (Aldrich 97%),4-(3-phenylpropyl)-pyridine (PPP ,Aldrich 97%;dielectric constant ca.e =6.6;[48]after contact to water PPP contains ca.55mol %

water [49]

),rhodamine B octadecyl ester perchlorate (Aldrich !98%),tetra-N -butylammonium hexafluor-ophosphate (Aldrich !99%),phos-phoric acid (85%.wt),and NaOH (Aldrich !97%)were obtained commercially and used without further purification.Demineralised and filtered water was taken from a Thermo Scientific water purifica-tion system (Barnstead Nanopure)

with not less than 18.2MOhm cm resistivity.Experiments were con-ducted at 20?28C.

Instrumentation:An Ivium Compactstat Potentiostat (release 1.725)was used in electrochemical experiments.A 4.9mm diameter basal plane pyrolytic graphite electrode (bppg)mounted in Teflon was used as the working electrode with a platinum counter and a satu-rated calomel reference electrode (SCE,REF401,Radiometer).Pho-toelectrochemical measurements were conducted on basal plane pyrolytic graphite electrodes.The photocurrent responses were measured under potentiostatic control using a three-electrode cell with gold counter and SCE reference electrodes.

A Dolan–Jenner high-intensity illuminator (MI-150,halogen bulb)was employed as the light source and a simple chopping mecha-nism based on a rotating sector blade was employed.Photocurrent spectra were obtained using monochromatic light of variable wavelength provided by a tungsten lamp and grating monochro-mator.The incident light was chopped at 6Hz,and a lock-in ampli-fier (Stanford Research Systems)was used to detect the photocur-rent.UV/Vis measurements were conducted using a Perkin–Elmer Lambda 35spectrometer.

Procedures for Electrode Preparation and for Photoelectrochemical Measurements:Random arrays of microdroplets were formed by evaporation of a solution of rhodamine B (10mg)and PPP (80mg)in acetonitrile (10cm 3).This solution (typically 2m L)was pipetted onto a basal plane pyrolytic graphite electrode (4.9mm diameter).The acetonitrile solvent evaporated at room temperature under ambient conditions,leaving behind a random array of PPP micro-droplets (ca.16nL)containing about 0.28m rhodamine B.After ex-periments,the electrode was renewed by removing the top layer of graphite on a fine silicon carbide polishing paper.The modified electrode was placed into a custom-made flow cell (Teflon)with a gentle flow of the aqueous electrolyte (5cm 3min à1)to avoid ther-mal effects from light exposure.

2.Results and Discussion

2.1.Reduction and Re-oxidation of Rhodamine B Immoblised in PPP Microdroplets onto a Basal Plane Pyrolytic Graphite Electrode Surface I:Scan Rate and Concentration Effects

Before investigation of the photoelectrochemical properties of rhodamine B,the dark reactivity is studied.Rhodamine B is a well-known fluorescent dye with known redox activity (a chemically reversible two-electron reduction [47]).The aqueous solubility of rhodamine B can be considerable and therefore,in order to better immobilise rhodamine B in organic microdrop-lets of 3-(4-phenylpropyl)-pyridine (PPP)at the surface of

a

Figure 1.Molecular structure of:A)rhodamine B base and B)rhodamine octadecyl ester (as perchlorate salt).

Triple Phase Boundary Photovoltammetry

basal plane pyrolytic graphite electrode,the aqueous electro-lyte concentration was chosen to be 0.5m phosphate buffer at pH 12.Under these conditions,relatively stable voltammetric responses were observed with minimum loss of dye into the aqueous phase.

Figure 2A shows a set of typical voltammetric responses for the reduction and re-oxidation of rhodamine B immobilised in PPP microdroplets at a basal plane pyrolytic graphite elec-

trode.A reduction process (P1)is observed at a potential of à1.1V vs SCE.This reduction is associated with an oxidation peak at +0.4V vs SCE (see P3).Upon scanning the electrode potential more negative,a second reduction peak is observed at about à1.4V vs SCE.Changing the amount (and volume)of organic deposition solution affects the peak currents and also the peak shape.Figure 2B shows a plot of the peak currents increasing linearly with the amount of deposit.Inspection of Figure 2A also shows that the peaks broaden considerably,which is due to the formation of larger droplets and the result-ing decrease in the relative rate of transport.The effect of scan rate on the voltammetric signals is demonstrated in Figure 2C.For a sufficiently low volume of organic deposit (16nL),a linear correlation holds to approximately 0.3Vs à1(see Fig-ure 2D).For higher scan rates,the correlation becomes nonlin-ear due to incomplete electrolysis of the organic deposit.

The reduction process P1shows signs of reversibility at higher scan rates (see Figure 2C)and,indeed,when the experi-ment is repeated over a smaller potential window (see below),the process is fully reversible.Due to the triple phase boun-dary reaction conditions,the redox process must be associat-ed with a simultaneous ion-transfer process.The reversible response P1is insensitive to the aqueous solution pH (for pH >10,see below),which is indica-tive of Na +cation transfer.The facile transfer of Na +into PPP to form a solvated cation Na +(PPP)has been observed previously.[48]Under these conditions,the re-duction process P1is therefore tentatively attributed to a one-electron/one-cation process (Scheme 1).The peak current for the second reduction process P2is reduced (relative to P1)at lower scan rates,but at higher scan rates it reaches approximately the same peak current when compared to the reduction peak for process P1.The process can

be attributed to a further one-electron/one-proton reduction step (Scheme 2).The oxidation at about +1.1V vs SCE is associated with the recovery of the original rhodamine B material and integration of the reduction peaks P1and P2in comparison with the oxi-dation peak P3confirms that the reduction and oxidation charges are identical and consistent with an overall two-elec-tron process (Scheme 3).The mechanistic relationship between reduction processes P1and P2can be further investigated.The fact that process

P1Figure 2.A)Cyclic voltammograms (scan rate 0.1Vs à1)for the reduction and re-oxidation of 0.28m rhodamine B

immobilised in PPP [deposition volume:4(i),32(ii),and 64nL (iii)]in 0.5m phosphate buffer solution pH 12.

B)Plot of the peak currents (for reduction peak P1and for oxidation peak P3,scan rate 0.1Vs à1

)versus the amount of rhodamine B deposited as 0.28m PPP solution.C)Cyclic voltammograms [scan rate:0.05(i),0.1(ii),0.3(iii),0.4(iv),and 0.5Vs à1(v)]for the reduction and re-oxidation of 0.28m rhodamine B immobilised in PPP (deposi-tion volume 16nL)in 0.5m phosphate buffer solution pH 12.D)Plot of the peak currents (for reduction peak P1

and for oxidation peak P3)versus scan rate (for 0.28m rhodamine B in 8nL

PPP).

Scheme 1.

F .Marken et al.

changes from a one-electron process at faster scan rates to a two-electron process at slow scan rate suggests a chemical follow-on reaction step which is likely to be disproportiona-tion.Figure 3A shows cyclic voltammograms recorded over a reduced potential window (only P1and P3visible),and the chemical reversibility of process P1can now be used as an in-dicator for the reaction progress.A Digisim simulation is em-ployed to quantitatively investigate the mechanism of the rho-

damine B reduction.Figure 3B shows simulated voltammo-grams which are in good agreement with the appearance of experimental data for P1.Even the shape observed for process P1is closely reproduced in the simulation.Only the broader shape (possibly due to mass-transport effects and the distribu-tion of droplet sizes)of process P3in the experimental data significantly deviates from the simulation data.The ratio of oxi-dation peak current to reduction peak current,I peak ox /I peak red ,for

process P1provides a sensitive measure for the determination of the chemical second-order rate constant for disproportiona-tion.The plot in Figure 3C shows good agreement for a disproportionation process (Scheme 4)with k dis =

3mol à1m 3s à1

(at pH 12).

In order to further confirm the disproportionation mecha-nism,the effect of rhodamine B concentration on the voltam-metric signals is investigated.Figure 3D shows a plot of ex-perimental versus simulated peak currents (for the reduction and oxidation peaks for process P1).The apparently increasing

reaction rate at higher rhodami-ne B concentration is confirmed in the behaviour of P1Ox and therefore the disproportionation mechanism is

confirmed.

Figure 3.A)Cyclic voltammograms [scan rate:0.005(i),0.01(ii),and 0.05Vs à1(iii)]for the reduction and re-oxida-tion of 0.28m rhodamine B immobilised in PPP (deposition volume 8nL)in 0.5m phosphate buffer solution

pH 12.B)DigiSim simulation for the reduction of rhodamine B 0.28m in a thin film without diffusion limitation (as-suming sufficiently high diffusion coefficients).The assumed simplified mechanism is given by A +e =B (P1),

B +B =

C +A,C =

D +e (P3),D =F +e (P3),where a disproportionation step is assumed with k dis =3mol à1m 3s à1.C)Plot of the ratio of peak currents I peak ox /I peak red for process P1versus the scan rate.Simulation data is shown as a

dashed line (a )and experimental data points as dots (*).D)Plot of the experimental (*)and simulated (c )peak currents for process 1versus rhodamine B concentration for the reduction of rhodamine B in PPP (deposition volume 16nL)immersed in 0.5m phosphate buffer solution pH

12.

Scheme

2.

Scheme 3.

Triple Phase Boundary Photovoltammetry

2.2.Reduction and Re-oxidation of Rhodamine B Immoblised in PPP Microdroplets onto a Basal Plane Pyrolytic Graphite Electrode Surface II:pH Effects

The mechanism of the rhodamine B reduction and re-oxidation at the triple phase boundary has been shown to be associated with a disproportionation reaction when the pH in the aque-ous phase is controlled at pH 12.Often the availability of pro-tons is linked to the rate of disproportionation processes and therefore the effect of the aqueous solution pH is investigated.Figure 4A shows cyclic voltammograms for the reduction and re-oxidation of rhodamine B in PPP immersed in phos-phate buffer media at pH 2,4,12,and 14.A clear shift of the peaks for processes P1and P3is observed for pH values lower than approximately ten (see Figure 4B).In the more alkaline pH range,both processes appear less sensitive to proton activ-ity.The reaction mechanism given in Schemes (1)–(4)reflects the case of weak pH dependence where the transfer of Na +is accompanying the transfer of electrons (in the rate-determin-ing step).Under more acidic conditions (lower pH values),pro-cesses P1and P2merge into a single two-electron/two-proton

process (see Figure 4B).It is interesting to also explore the rate of the disproportionation reaction as a function of pH.The plot in Figure 4C shows clear evidence for a much slower dis-proportionation in more alkaline environments.Therefore,it seems very likely that proton transfer from the aqueous phase into the organic PPP phase is accompanying the disproportio-nation process.This is indicated in Scheme 4by including the H +(PPP)proton,which is likely to transfer from the aqueous phase.The availability of H +(PPP)is probably facilitated by the carboxylate functional group in rhodamine B acting as the counter anion (see below).

2.3.Reduction and Re-oxidation of Rhodamine B

Octadecylester Immoblised in PPP Microdroplets onto a Basal Plane Pyrolytic Graphite Electrode Surface

It is interesting to compare the reactivity of derivatives of rho-damine B under the same reaction conditions,in particular with the carboxylic acid functionality protected.Rhodamine B octadecyl ester perchlorate (see structure in Figure 1B)was used to explore hydrophobicity effects and the role of the car-boxylate moiety.Figure 5A shows typical cyclic voltammo-grams for the reduction and re-oxidation of rhodamine B octa-decyl ester.Processes P1,P2,and P3can be identified,however,addition of NBu 4PF 6supporting electrolyte into the organic phase is observed to improve the voltammetric responses con-siderably in this case.This is likely to be linked to an im-proved ionic conductivity in the organic phase,which improves electron transfer in the organic solvent.

Figure 5B shows voltammo-grams for the reduction of rho-damine B octadecyl ester over two potential windows.The pro-cess P1can be seen to be fully electrochemically reversible and the ratio of reduction peaks for

processes P1and P2remains

close to one even at lower scan rates (see Figure 5C).This

find-Figure 4.A)Cyclic voltammograms (scan rate 0.1Vs à1)for the reduction and re-oxidation of 0.28m rhodamine B

immobilised in PPP (deposition volume 40nL)in 0.5m phosphate buffer solution:i)pH 2,ii)pH 4,iii)pH 12,and

iv)pH 14.B)Plot of the peak potentials for the reduction peak P1and the oxidation peak P3versus pH.C)Plot of

the ratio of peak currents I peak red (P2)/I peak red

(P1)for processes P1and P2versus solution

pH.

Scheme 4.

F .Marken et al.

ing suggests that the disproportionation process is ineffective

and the one-electron reduced radical intermediate for rhoda-mine B octadecyl ester is relatively stable under biphasic condi-tions.

The rhodamine B carboxylate functionality is a potent base

in organic solvent media and in this particular case it could fa-cilitate the transfer of protons from the aqueous phase into

the organic phase.For rhodamine B octadecyl ester without

this facilitated proton transfer,the disproportionation process

(see Scheme 4)may simply be too slow to be observed under

these conditions.

2.4.Reduction and Re-oxidation of Rhodamine B

Immoblised in PPP Microdroplets onto a Basal Plane

Pyrolytic Graphite Electrode Surface:

Photoelectrochemical Processes

Light-driven charge separation at the interface between two

immiscible liquids has been highlighted as a potentially impor-tant part of light harvesting processes.[50]

Here,rhodamine B is

immobilised within microdroplets of PPP and the photoelectro-chemical reactivity is investigated via cyclic voltammetry.The

rhodamine B system immobilized in PPP microdroplets and im-mersed in 0.5m phosphate buffer pH 12is highly sensitive to

light.When using a pulsed light source (0.3Hz chopping fre-quency with 50%duty cycle),a new peak for the oxidation of the two-electron reduced leuco-rhodamine B is observed with a peak at about à0.8V vs SCE (see Figure 6A).At the same time,the oxidation process P3disap-pears.Clearly,a light-driven pro-cess occurs which catalyses the oxidation of the two-electron re-duced leuco-rhodamine B back to rhodamine B.The new photo-oxidation peak occurs at a po-tential positive of that for pro-cess 1(positive of à1.0V vs SCE),which suggests that rhoda-mine B itself is involved in this photoelectrochemical mecha-nism.There are literature exam-ples of rhodamine B undergoing photoexcitation and abstracting a hydrogen atom from quinolic reagents,[45]and a similar process is proposed here (Scheme 5).Excitation of rhodamine B is

believed to trigger the formation of a photoexcited intermediate which then reacts with the two-electron reduced form of rhoda-mine B to give two molecules of the one-electron reduced form.

The oxidation of this species occurs at the electrode surface as process P1(see Scheme 1).Further experiments were conducted as a function of aque-ous buffer pH.Figures 6A,B show the effect of the elevated proton concentration on the rate of the photoelectrochemical reaction.At a pH of about ten,the photo-oxidation responses are much less efficient (see Figure 6B).Under these conditions,the presence of protons can reverse the photo-comproportio-nation (Scheme 5)by causing enhanced dark disproportiona-tion (Scheme 4),which under these conditions corresponds to a “loss mechanism”.To further elucidate the mechanism for the photoelectro-chemical process,experiments were conducted as a function

of light intensity and as a function of scan rate (see Figur-es 6C–F).The light intensity has a considerable effect,as

shown in Figure 6C.By dimming the light source,the photo-oxidation response is clearly weakened.The peak (see Figure 6Cii)gradually shifts to more positive potentials,and under dark conditions,only process P3is observed (see Figure 6Ciii).This behaviour is a result of the availability of the photoexcited rho-damine B in the PPP phase.It is possible to model the forma-tion of the photoexcited state as a reversible pre-equilibrium where the steady-state concentration of the photoexcited state is given by the equilibrium constant multiplied by

the Figure 5.A)Cyclic voltammograms (scan rate 0.1Vs à1)for the reduction and re-oxidation of 32m m rhodamine B

octadecyl ester immobilised in PPP (deposition volume 62nL)in 0.5m phosphate buffer solution pH 12without (i)and with (ii)0.1m NBu 4PF 6added into the organic phase.B)Cyclic voltammograms (scan rate 0.1Vs à1)for the re-duction and re-oxidation of 32m m rhodamine B octadecyl ester immobilised in PPP (deposition volume 62nL,

with 0.1m NBu 4PF 6)in 0.5m phosphate buffer solution pH 12recorded over i)a small and ii)a wider potential

window.C)Cyclic voltammograms [scan rate:0.005(i),0.1(ii),and 0.5Vs à1(iii)]for the reduction and re-oxidation

of 32m m rhodamine B octadecyl ester immobilised in PPP (deposition volume 62nL,with 0.1m NBu 4PF 6)in 0.5m phosphate buffer solution pH 12.D)Plot of the peak current versus the deposition volume.E)Plot of the peak

current versus the scan rate.

Triple Phase Boundary Photovoltammetry

concentration of rhodamine B and by the apparent “concentra-tion of photons”.This simplistic model can be implemented in the DigiSim package together with the comproportionation step (Scheme 5).Results from the simulation attempt are shown in Figure 6E.The effect of the light intensity (or the “photon concentration”)is reproduced with a lower availability of the photoexcited intermediate causing the expected shift of the peak potential to the right.This effect is very well repro-duced in the simulation,however,the kinetic parameters are not fully determined.The simulation has been carried out with an assumed second-order rate constant for photo-compropor-tionation k com =k dis =3mol à1dm 3s à1(which is plausible given that both rate constants are second order and involve similarly sized molecules).This assumption results in the excited-state concentration to be approximately 1%of the initial bulk con-centration of rhodamine B.In reality this excited-sate concen-tration could be lower when k com is higher.Additional data are required to resolve this point.

Next,the effect of the scan rate on the photoelectrochemi-cal process was studied.Figure 6D shows voltammograms ob-tained at 0.001,0.01,and 0.05Vs à1.The effect of the scan rate on the shape of process 1under illumination is dramatic with increased scan rates causing a wider peak-to-peak separation.The shape of these voltammograms can again be reproduced by computer simulation with DigiSim and employing the same parameter set (see Figure 6F).In order to confirm the photoex-citation of rhodamine B as the key step,the action spectrum for the photo-comproportionation process is investigated.Fig-ure 7A shows the action spectrum obtained at 6Hz chopping rate.A clear peak at 560nm is a tell-tale sign for direct rhoda-mine B excitation.[34]UV/Vis spectra for rhodamine B in PPP before and after contact with phosphate buffer are shown in Figure 7B and the agreement with the action spectrum is ac-ceptable.The photo-comproportionation process in Scheme 5is thus confirmed.

An interesting phenomenon is observed when chronoam-perometry data are recorded at the midpoint potential for pro-cess P1(ca.à1.1V vs SCE).Figure 7C shows a typical trace for 3Hz chopped light.Upon illumination,the photo-oxidation is observed and in the dark the reduction occurs.An

alternating

Figure 6.A,B)Cyclic voltammograms (scan rate 0.01Vs à1)for the reduction and re-oxidation of 0.28m rhodamine B immobilised in PPP (deposition volume 42nL)and immersed into 0.5m phosphate buffer solution:A)pH 12and B)pH 10.Chopped light (0.3Hz)was applied.C)Cyclic voltammograms (scan rate 0.01Vs à1)for the reduction and re-oxidation of 0.28m rhodamine B base immobilised in PPP (deposition volume 40nL)in 0.5m phosphate buffer solution pH 12with i)full light intensity,ii)30%light intensity,and iii)in the dark.D)Cyclic voltammograms [scan rate:0.001(i),0.01(ii),and 0.05Vs à1(iii)]for the re-duction and re-oxidation of 0.28m rhodamine B base immobilised in PPP (deposition volume 40nL)in 0.5m phosphate buffer solution pH 12in the presence of light.E)DigiSim simulation for the reduction of 0.28m rhodamine B in a thin film without diffusion limitation (assuming sufficiently high diffusion coeffi-cients).The assumed simplified mechanism is given by A +e =B (P1),B +B =C +A,C =D +e (P3),D =F +e (P3),where a disproportionation step is assumed with k dis =3mol à1m 3s à1.In addition,the photochemical process is modeled as A +P =G (with very fast rate constants to provide a steady state equilibrium concentration of photoexcited rhodamine B with approximately [G]=k ex [P][A],see text)and an assumed comproportionation step G +A =B +B with

k com =3mol à1m 3s à1;the concentration of photons [P]was varied to produce traces (i),(ii),and (iii).F)DigiSim simulation for the reduction of 0.28m rhodamine B (as in E)for scan rates i)0.001,ii)0.01,and iii)0.05Vs à1

.

Scheme 5.

F .Marken et al.

current response with transient responses for both processes is observed.Note a very similar time constant for photo-compro-portionation and dark disproportionation,in agreement with similar mass-transport rates for both processes.

The rhodamine B octadecylester derivative did not show photoelectrochemical effects under the conditions employed in this study.For this compound,both the disproportionation and photo-comproportionation reactions appear ineffective,likely due to the missing carboxylate functionality which may be required as a facilitator for the proton transfer in this triple phase boundary photoelectrochemical process.Further work exploring the effect of carboxylates and organic solvent sys-tems is in progress.

3.Summary

1)It has been shown that rhodamine B in PPP immobilised at a graphite electrode surface and immersed in aqueous buffer solution gives highly effective triple phase boundary photoelectrochemical current responses when the buffer is adjusted to pH 12.

2)The dark reaction has been shown to be associated with a proton-driven disproportionation step (with k dis =3mol à1dm 3s à1at pH 12)resulting in the irreversible forma-tion of leuco-rhodamine B at potentials negative of about à1.1V vs SCE.

3)The photoexcitation of rhodamine B results in a compro-portionation process which leads to the corresponding photo-oxidation at potentials positive of à1.1V vs SCE.The availability of H +cations at the liquid j liquid interface and the competitions between H +and Na +transfer play an im-portant part in the overall mechanism which occurs in a triple phase boundary environment.

4)Photovoltammetry and data analysis via numerical simula-tion have been demonstrated as a powerful tool in mecha-nistic analysis for processes which occur in microphase en-vironments immobilised at electrode surfaces.

Acknowledgement

A.M.C.thanks the EPSRC and the NSF for a studentship entitled “Microphase Photo-Electrochemistry:Light Driven Liquid-Liquid Ion Transfer Processes and Two-Phase Micro-Photovoltaic Sys-tems”(EP/G002614/1).NSF support for this work was provided through Grant CHE-0822422.We thank Prof.L.M.Peter for help and discussion.

Keywords:DigiSim ·interfaces ·

ion

transfer

·

photoelectrochemistry ·rhodamine

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Received:February5,2010

Revised:February17,2010

Published online on April16,2010

F.Marken et al.