indian Journal of Chemistry Vo l. 42A, April 2003, pp. 782-788

Comparison of solid state voltammetry and membrane voltammetry: The reduction of 4,4',4",4'" -(2IH,23H-porphine-5, I 0, 15,20-tetrayl)

tetrakis[l-octylpyridinium] tetrabromide imrnobilised as microcrystalline solid and accumulated into a mesoporous Ti02 phytate membrane

Frank Marken *i, Susan M Parkhouse, Lesley A Hoe, Katy J McKenzie & Roger J Mortimer

Department of Chemistry. ~oughborough Universit y , Loughborough. Leicestershire, LE II 3TU, UK.

and

Steve] Vickers & Natalie M Rowley

School of Chemical Sciences. The Uni versity of Birmingham. Edgbaston, Birmingham BI5 2TT. UK.

Received II No vell/ber 2002

The tetra-ca ti onic free base porphyrin. 4.4',4",4'''-(2 1 H,23 H-porphine-5 , I 0, 15,20-tetray l) tetrak isll -octy lpy ridiniumj4+ (PTT04+) shares the properties of both a long alky l chai n containing molecule with hydrophobic shell and a highly charged water so luble molecule with hydrophilic core. As a result. PTT04+ is only very sparingly water soluble and tends to aggregate on suitable surfaces. It is shown here th at the electrochemical reduction of PTT04

+ in aqueous med ia can be studied either by 'so lid state vo ltammetry' or by ' membrane voltam metry ' methodology with complemen tary results. When illllnobili sed by adhering the microc rystalline powder to the surface of a basa l plane pyro ly tic graph ite electrode. PTT04

+

exhi bits two separate one electron-one proton reduction responses, which are both coupled to chemica l follow-up processes (assumed here is a H-shift reacti on). Both reducti on responses become reversible at suffi ciently high scan rates. In contrast. when accumulated into a Ti02 phytate membrane, PTT04+ exhibits a chemically reversible 2 electron-2 proton reducti on response. Upon increasing of the concentrati on of PTT04+ in the membrane or upon increasing the thickness of the membrane more complex vo lt3mmetric responses are detected. High loca l 'concentrati on' conditi ons during solid state voltammetry experiments may be regarded as a limiting case of high concen tration membrane vo ltammetry at very thin membranes.

Mesoporous membranes and hybrid organic-inorganic membranes I are of considerab le interest for the development of thin layer devices, e.g. electrochromic displays", energy storage devices", and sensors4

. The conventional approach to thin film deposition of these membranes is based on either plasma technology or so l-gel chemistri. Alternatively, elegant layer-by­layer deposition procedures have been reported, in which nanoparticles and polymeric molecules are app lied to a surface alternately with the building blocks bound together predon~inantl y by elec trostatic forceso. We have recently shown that the layer-by­layer approach (or 'directed assembly' ?) may also be employed with smaller binder molecules such as phyt ic ac id (see Structure I). Phytic ac id is a naturall y occurring cyc lic hexaphosphate, which readil y binds to ox ide surfaces such as iron ox ide8 or titanium ox ide'). Nanoporous oxide films by directed assembly of nanoparticu late oxides and phytic acid exhibit surface properties which are dominated by the

t E-mai l: f.marken@lboro.3c.uk

adsorbed surface layer of phytic ac id (P KAI <3.5, pKA2 4.6 to 10 [ref. 10]). That is, posi ti ve ly charged molecules such as redox proteins can be accumulated and protected in the nanoporous s tructure~. The resulting device may have applications, for example, as sensors or in biocatalysis. Transport and redox processes within the mesoporous membrane in volving bulk diffusion, surface diffusion , and elec tron exchange are of key importance and are studi ed here for a model system.

In thi s study, a dye molecule based on the free base porphyrin 4,4',4/1,4/1'-(21 H,23H-porphine-5, 10, 15,20-tetrayl) tetrak is[ l-octy lpyridinium]4+ (PTTU1+, see Structure II ) is accumulated into the mesoporous f i 1m formed from titanium ox ide particles (ca. 6 nm tn diameter) and a phytic acid (see Structure I ) 'molecular binder' . The solubility of PTT04+ in aqueous so lutions is relati vely low and strongly dependent on the type and concentration of elec trolyte present. However, the accumulation of PTTO~+ into the Ti02 phytate film allows strong vo ltammetric responses to be detected and redox properti es to be

MARKEN el 01.: COMPARISON OF SOLID STATE VOLTAMMETRY & MEMBRANE VOLTAMMETRY 783

Structure I- Phyti c ac id lIIyo- inositol hexak is (d ihydrogen phosphate)

R

R

R

R = -GN- Octy l

Structu re 11--4.4'.4",4'''-(2 1 H,23 H-porphine-5, I 0, 15,20-tetrayl) tetrakis[ l -octy lpyridi niumJ (PTf04+)

studied as if under solution conditions. The e lectrochemi ca ll y dri ven redox reactio ns in the mesoporous membrane are shown to be sensiti ve to changes in the p H, the e lectro lyte concentration, and the concentrati on of PTT04+.

T he results fro m membrane vo ltammetry are compared with the vo ltammetri c characteri sti cs of surface immobilised microcrys talline solid II. The so lid state vo ltammetric characte ri sti cs fo r PTT04+ immobilised by adhering microcrysta ls at the surface of a basal pl ane pyrolyt ic graph ite e lectrode are reported . C he mical processes whi ch are fas t in the membrane environment are s low in the so lid state and can be resolved vo ltammetrically.

Materials and Methods III ST ru menla I i Oil

Electrochem ical ex periments were conducted wi th an Auto lab PGST A T20 sys tem (EcoChemi e, The

etherl ands) contro lled by a PC and connec ted to a conventional three-elec trode g lass cell. ITO working e lectrodes (tin -doped ind ium ox ide on g lass , Image Optics Components Ltd., Bas il don, ES5ex) were cut into 4 mm wide rectangul ar pl ates and cleaned by soni cat ing in ethano l, rins ing with distilled water, and furnace treatment (Elite tube furnace, mode l TSH

12/65/550) a l 500°C in air fo r 60 min to remove carbon impu rities. These e lectrodes are res isti ve w ith

typ ica ll y 20 O hm cm- I and were surface modifi ed as described be low. For so lid state vo ltammetry experiments a 4.9 mm di amete r basal plane py ro lyti c graph i te (' Pyrocarbon', Le Carbone) e lectrode was empl oyed . A Saturated Calomel Electrode (SCE)

served as reference and a Pt gauze e lectrode was used as the auxiliary e lectrode.

For FEGSEM (Leo 1530 Fie ld Emi ss ion G un Scanning Electron Microscope) imaging a thin gold coating of ca. 5 nm was applied to the sample surface . NMR spectra were recorded on a Bruker AC300 spectrometer. Mass spectra were obtained with a Bruker BiFlex IV syste m. Rate constants data fo r the case o f a finite di ffusion space were es timated by digital simulation employ ing the commercia l software package Dig iSim ™ (BAS).

Chemical reagents KOH (Fisher), HC I (Aldrich), phyti c ac id (A ldrich,

myo-inosito l hexakis(dihydrogen phosphate, sodiu m salt), KH 2P04 , and K2HP04 (Fisons) were obtained commercia lly and used without further puri fication. Filtered and demineralised water was taken from an Elgastat water purificatio n sys te m (Elga, Hi gh Wycombe, Bucks, UK) with at leas t 18 MOhm cm res isti vity. Titani a sol (TKS-202, anatase, typica lly 6 nm diameter, 30 to 37%, ac id ified with nitric acid) was avail able from T ayca Corp., Osaka, Japan.

Synthesis of 4,<4"4"'-(21 H,23H-porp hille-5, 10,15,20-tetray/) tetrakis[ 1 -octylpyridillilllll j tetrabrom.ide (P7T04

+ Br-.J) The sy nthes is of PTT04+Br-4 fo llowed the

procedure fo r the sy nthes is of the tetrachloride PTT04+Cr4 (CAS registry number 12489 1-59-4 [ref. 12]). Reac tion of tetrapyridy l porphine 13

·I.J with

I-oc ty lbromide in DMF at 90°C over 2 days gave the titl ed compo und :

In a 250 ml round botto med fl ask equipped with a reflu x condenser and N2 inlet was added 5, I 0, I 5,20-p-( CsH4)porphH2 (0.50 g, 0 .8 1 mmo l, I eqv) and 1-bromooctane ( 10 ml , 6 1 mmo l, 75 eqv) in 100 ml o f dry DM F. It was then heated to 90°C and left fo r 48 hours. A fter cooling to ambient temperature, the solvent was re moved ill vacuo until a dark red prec ipitate appeared. T hi s was fi ltered, pre-dried ill vacuo, and then dri ed overni ght in a vacuum des iccato r. Yi e ld: 0 .36 g (32%). I H NMR (DMSO 300 MH z): 89.6 1 (8 H, d , J = 9 Hz, Ha and Ha'), 9.25 (8 H, s, ~-pyrro l e H of rings A,B,C and D), 9.00 (8 H. d , J = 7 Hz, H ~ and HW ), 4.98 (8 H, t, N+-C H2 H), 1.6 1- 1.24 (48 H, um, (C H2)7 H of a lkyl chains), 0 .93 ( 12 H, t, C H3 H of a lky l cha ins), -3. 1 (2 H, s. central NH of porphyrins ring). M ALDI MS : mlz 1072 M+-4 Br- 985 M+-4Br-- I(CsH I7) 846tvr-4 Br--2(CsH17 ) 73 1 M+-4 Br- -3(CsH I7)' 620 M+-4 Br- -4(CsHI7)'

784 INDIAN J CHEM , SEC A . A PRIL 2003

DireCled asselllb/v oI TiO ] phyla /ejl/Ill s

The 'directed assembly' procedure for the forma ti on of Ti02 phytate f ilms was based on altern an! 30 s immersion o f the elec trode surface into Ti02 sol (ca. 4% in water) and into aqueous 40 mM phyti c acid (pH 3) w ith intermittent rinsing steps with distilled water. By repeating thi s sequence a well­defined nanoporous fi lm o f Ti02 phytate is formed layer by layer. Figure I shows a typical elec tron micrograph of a 30 layer deposit. The thi ckness can be es timated as approximately 800 nm, which suggests an average growth o f 27 nm per layer.

Results and Discussion VO /wlllili el l'\' oI III i c l'Oc rvsra/l ill e TPPOI+ Br-" illllllo /)ilised 011 10 basal p/a ll e pyro/y lic g raphile

e1ecfrodes

Compared to the highly water-so luble tetra-methy l derivative of the free base porphineI 5

.1

<J, the tetra-oetyl deri vati ve PTTO~+ is only sparingly so luble, in parti cul ar in the presence of high concentrations of elec troly te. The type of electrol y te and the solution pH affect the solubility. A lthough the low solubility of PTTO~+ limits poss ibilities fo r so lution phase studi es in aqueou. elec troly te so lution, the electrochemical characteri stics of PTT04+ can be readil y ex plored in the so lid state. In order to do thi s, microcrystalline materia l is immobi li sed at the electrode surface pri or to Immersion into the electro ly te solution. In recent work by Scho lz and

" (1'1· . h coworkers- .- mIcrocrys tals of the tetrap eny l derivative of the free base porphyrin have been

Fig. I·- FEGSEM image of a 30 layer T iO:, phylalC depos il formed by al lernaling ex posure of an ITO eleclrode surfacc lO

Ti02 sol and phylic acid So lul ion

studied in aqueous electrolyte and demonstrated to undergo electrochemically driven meta l inserti on processes.

A basal plane pyro ly ti c graphite electrode modified w ith microcrys talline PTf04+Br-4 and immersed in aqueous 0.2 M phosphate buffer (pH 5) allows a well ­defined reduction response to be observed at a potential of -0.37 V vs. SCE (see peak a in Fig. 2). At

MARKE eta!.: COMPARISON OF SOLID STATE VOLTAMMETRY & MEMBRANE VOLTAMMETRY 785

sufficientl y fast scan rates (> 0. 1 Y S- I) thi s reduction response is fully electrochemicall y reversible (see Fig. 2A, scan rate I Y S- I ) and no additional signals are detected. A t scan rates slower than 0.1 Y S- I a chemical foll ow-up process causes new peaks (not shown, consistent with peaks y' and 8' in Fig. 2B observed at higher scan rates after the second

reduction) to appear and the ox idation peak a ' to di sappear. Anal ys is of the shift in the peak posi tion of

peak a with p H indicates uptake of one proton for every electron transferred to the molecule (a shift of ca. 60 mY per pH unit over a limited range from pH 5 to p H 7). Based on these results the reduction process assoc iated with peak a at sufficientl y high scan rates is tentatively attributed to a one proton - one electron reduction process (Eq. I ).

R ... (I )

A lthough the peak shape and the dependence of the

peak current on scan rate for process a suggest thin

layer behaviour (total charge 20 ~C), not all of the so lid material adhered to the electrode surface is reacti ng. Further strong reduction signals are detected at more negative potential (not considered here). Yo ltammetric signals are stab le over many potenti al cycles and the scan rate dependence of signals can be studied. An approx imately linear change of the peak current with scan rate is indicati ve of thin layer behaviour. Similar to cases observed for other types of inso luble redox-ac ti ve material 22-24, it is likely that the vo ltammetric respon ses detec ted here ori ginate from only a fraction of the solid (e.g. after formation of a thin layer by interfacia l swelling).

From the effect of the scan rate on the reversibi li ty of process a (Eq. I ) a first order chemical rate constant of approximately kl = I S- I can be es timated (by numerical simulati on). By scanning the potential to more negative potential s a second reducti on process (denoted ~, see Fig. 2B) is detected. The voltammogram shown in Fig. 2C shows that both

processes, a and ~ , become reversible at a scan rate higher than 5 Y S- I . The first order rate cons tant for the chemical reaction step after the second reduction step was estimated as approx imately k2 =' 20 S- I. A t lower scan rates a fast chemical process causes the

i] a' (A)

(B)

(e)

i.o i i

00 E I V vs. seE

Fig. 2- Voltallllllogralll s for Ihe rcducti on of microc ryslal line PTT04+Br-.j adhered to a basal plane pyroly tic graphilc eleclrode (4.9 111111 diameter) and illl illersed in aqueolls 0.2 M phosphate buffer (pH 5). (A) Voitaillmogram obtained with scan rate I V s 1

in a potential window from D.8 V to - 0.4 V VS. SCE. ( 13 ) Voitalllillogralil obtai ncd w ith a scan rate of I V S- I in a potent ial w indow from 0.8 V to - 1.0 V VS. SCE. (C) Volt<ll11mogral11 obtained wi th a scan rate of:; V S- I in a potentiJI w indow of 0.,' V to - 1.0 V vs. SCE.

fo rmati on of products as indicated by the peaks denoted y' and 8'. These voltammetric characteri stics

allow process ~ to be attributed to the second one proton - one electron reduction step (Eq. 2).

R R

-.I.,t + ll+

R R +---· t.!· ·II+

R R .. . (2)

The product from thi s reduction process has all four nitrogen si tes protonated and thi s kind of geometry has been reported to be unstablel5

, leading to a chemical fo llow-up reacti on step via H-shi ft (Eq. J).

R R

k1 R II

R R . .. (3)

The isomeric form with the meso-carbon posItion protonated is considerably more stable and therefore a shift in ox idat ion potential occurs to more positive potentials (processes y and 0). The slower chemical

786 INDIAN J C HEM, SEC A, APRI L 2003

fo llow-up reaction o f the product formed in the first reductio n step (Eq . I) could be due to a preceding di sproportionation step ultimate ly leading to the same product. However, the system is complex and a more deta iled mechani sti c interpretati on is currently un warranted .

Vollamll1etl~Y of TPP04+ Br-4 accurllulated into l1/esoporous TiOz phytate membrane electrodes

The layer-by- layer deposition procedure (or 'directed assembly,7]) allows very thin films of Ti02

nanoparticles to be fo rmed bound by a mo lecul ar binder, here phyti c ac id . The scanning e lectron mi crograph (Fig. I) shows a deposit w ith a thickness of approximately 800 nm formed by a lte rnant applicatio n of 30 layers Ti02 and phytic acid and therefore the film thickness is inc reased by approx imate ly 27 nm each depositio n cycle . The phyti c ac id surface modi f icati on of the Ti02 particles creates an overall negati ve surface charge, which a ll ows ca tions to be accumul ated . The accumulatio n of cati ons from low concentrati ons in aqueous media is parti cul arl y interesting in the case o f large and sensiti ve mo lecules such as redox prote ins9

. Here, the accumulati on of PTT0 4+ IS de mo nstrated and in ves tigated .

In Fig . 3 cyc li c vo ltammograms are shown fo r the

reduc ti on o f 13 11M PTT04+ di ssolved in aqueous 0 .025 M phosphate buffe r (PH 5). At a scan rate o f

0.02 V S- I a full y reversible reductio n-ox idatio n redox sys tem with 40 m V peak-to-peak separati on is detected . T he reversible potenti a l is centred at E II2 = Y2 (Ep'" + Ep'ed) = - 0.1 8 V vs. SCE. Thi s result is surpri sing when compared to the solid state reducti on of PTT0 4+ at p H 5 (Fig. 2), whi ch occurred at more negative potenti a ls. In fac t, the reversible vo ltammetric respo nse detec ted for PTT04+ in the

membrane is centred between processes a and ~ at

more negati ve potenti als and y and 8 at more positi ve potential s. It is therefore suggested that within the mesoporous membrane a much fas te r chemi cal reactio n step occurs and e lectrochemical and chemi cal processes merge into a sing le reversible 2 e lectron-2 proton process (Eq . 4).

.. . (4)

r, -,--,--.-,--,,-.

-0.4 0.2 E I V VS . SCE

Fig . 3- (A) Cycl ic vo ltammograms (scan rate i = 0.02 V s-' . ii = 0.05 V s-', iii = 0. 1 V s- ') obtained fo r the reduc ti on of PTT04

+

accumul ated into a 10 layer deposit o f Ti02 phytate at an ITO e leclrode (area 0 .2 cm2

) immersed in aq ueous 0.025 M phosphate buffe r sol ution (PH 5) containing 13 )lM P'TT04~ . Also show n are cycl ic voltammograms (sca n rate 0.02 V ~; -') obtained for the reducti on of PlT04

+ accumul ated into a 10 layer deposit of Ti 0 2

phytale at an ITO e lectrode (a rea 0. 2 cm2) immersed in aq ueous

0.025 M phosphate buffe r so lution (PH 5) comaining (8) 47 ~IM

and (C) 190 )lM PTT04+.

Based on the assumptio n of an ove ra ll two e lectron conversio n and complete thin film e lec tro lys is, the

charge under the vo ltammetri c peak, I 1.4 I1C, can be evaluated and then the concentratio n of PTT04+ in the film for a 270 nm thi ckness (a IO layer depos it), de te rmined . Under these conditi ons the concentrati o n o f PTT04+ within the film , II mM (assuming a ho mogeneous medium), is three orders o f magni tude

higher than that in soluti on, 13 11M. The Ti02 phytate film is highly effi c ient in accumul ating PTT04+ from the aqueous so lutio n phase. However, in spite of the high concentratio n the mo bility of PT r04+ in the film depos it is still suffi c ient to sus tain a fas t e lectrochemi cal process.

It is interes ting to ex plo re the effec t of chang ing the thi ckness of the Ti02 phy~te membrane o n the vo ltammetric charac teri st ics. In Fig . 4 vo ltammo­

grams are shown fo r the reduc ti on of aq ueous 13 11M PTT 0 4+ in aqueous 0 .025 M phosphate buffe r (p H 6) at an ITO e lec trode modi fied with I , 10, and 30 layers of deposit. It can be seen that the thi ckness or the deposit has a dramati c effect. In the presence of one layer Ti 0 2 phytate, the reduc tio n and re-ox ida ti o n ca n bare ly be detected but w ith increas ing thi ckness a

MARK EN e/ al. : COMPARI SON OF SOLID ST AT E VOLTAMM ETRY & MEMBR ANE VOLT AMM ETRY 787

I I 0.0 0.2

E / V vs. seE

Fig. 4-CycIi c vo ltam mograms (scan rate 0.02 V S- I) obtained for the reducti on of PTT04+ accumulated into a (i) I layer, (i i) 10 layer. and (iii) 30 layer deposit of Ti02 phytate at an ITO electrode (area 0.2 cm2

) immersed in aqueous 0.025 M phosphate buffer soluti on (p H 6) containing 13 11M PTT04+.

well-defined voltammetric response is detected . Perhaps surpri sing ly, at a thickness of 30 layers the voltammetri c response becomes more complex and parti ally irreversible. T hi s behaviour can be attributed to p H gradients developing w ithin the nanoporous membrane during the redox process (v ide infra). Protons are consumed during reduction and with an insuffi cient suppl y the pH within the membrane changes locally causing the redox process to change path way.

By varying the concentration of PTT04+ in the

aqueous buffer solution a simil ar deviation from ideal behav iour can be observed . In Fi g. 3 cyc li c vo ltammograms are shown for the reduction of PTT04+ accumul ated into a 10 layer deposit of Ti02

phytate in the presence of (A) 13 /lM, (B) 47 /lM , and (e) 190 /lM PTT04+. Increasing the concentration of PTT0 4+ in the aqueous buffer phase increases the concentration of PTT04+ in the mesoporous membrane. However, it can be seen that at higher concentrati on, vo ltammetri c responses become asy mmetri c and partra ll y irreversible. In a simil ar way, the vo ltammetric response for the case of 13 /lM PTT04+ becomes more asy mmetric at increased scan rates (see Fi g.3A). These observations arc consistent with effects introduced by local pH gradients, whi ch ari se due to the limited buffer capac ity of the membrane.

In the absence o f a sufficient concentration of protons, the vo ltammetri c response for the ox idati on of PTT04+ becomes less reversible and probably affected by further chemical reaction steps. In Fig. 5 are shown vo ltammograms fo r the reduction of 47 /lM PTTUI+ at a 10 layer depos it of Ti02 phytate on ITO immersed in aqueous 0 .025 M phosphate buffe r at pH I, 5, and 7. At p H 7 the voltammetric response is

ij -4

+-

I iii I I iii -0.3 0 .2

E / V vs. SeE

Fig. 5-Cycli c vo ltammograms (sca n rate 0.02 V S- I) obta ined for the reductio n of PTT04+ accumulated into a 10 layer deposit of Ti 0 2 phytate at an ITO e lectrode (area 0.2 cm2

) im mersed in aqueous 0.025 M phosphate buffer so lution (i = p H I, ii = p H 5. ii i = p H 7) containing 47 11M PTT04+.

~"j -2

..::=.=;;:=~ (A)

:::~~ (B)

r-'---:::=+==~::II' (C)

I f I -0.4 0.0 0.2

E / V vs. SeE

Fig. 6- Cyc li c vo ltammograms (scan ra te 0.02 V S- I) obta ined for the reduction of PTT04+ accumul ated in to a 10 layer deposit o f T iOz phytate at an ITO e lec trode (area 0.2 c m2

) and immersed in aqueous (A) 0 .025 M, (8 ) 0.05 M, and (C) 0. 1 M phosphate buffer so lutio n (PH 5) containing 13 11M PTT04+.

becoming more compl ex. At intermediate proton acti vities (ca. pH 3), PTT04

+ exhibits a considerable drop in aqueous solubility.

A surpri sing ly dramati c effect is detected when the concentration of the supporting electro lyte, here phosphate buffer at pH 5, is changed. Fi gure 6 shows cycli c vo ltammograms obtained for the reducti on of aqueous 13 /lM PTT04

+ in the presence of (A) 0.025 M , (B) 0 .05 M , and (e) 0 . 1 M phosph ate bu ffe r at p H 5. Doubling the phosphate buffe r concentration increases the amount of PTT04

+ in the Ti02 phytate me mbrane . However, a lso a change from a sy mmetric well -defined voltammetri c response to a partly irrevers ible response is observed (Fi g. 6 B). Further increase of the buffer concentration causes the reducti on response to shi ft to a more negati ve potenti al and to become more irreversibl e. Increasing the concentrati on of PTT04

+ in the membrane beyond approximately II mM (vide supra) appears to affect

788 INDI AN J CHEM, SEC A, APRI L 2003

the pathway of the overa ll reac tio n. In additi on to the effect of the concentrati on o f PTT04+ in the membrane, pH g radient effec ts and mo re general transport effects may contribute to the observed change in vo ltammetri c characteri sti cs .

It is inte resting to co nsider the transport processes within the T i02 phytate membrane as part o f the overa ll reac tion path way . Removal o f the Ti02

phytate coated e lec trode after accumul ati on o f PTT04+ into the membrane and re- immersio n of the e lectrode into a 0.025 M phosphate buffer solutio n (PH 5) conta ll1l11g no PTT04+, a llows the vo ltammetri c response for PTT04+ to be de tected . However, the respo nse decreases over consecuti ve po tenti al cycles as the PTT04+ concentratio n within the film is reduced . There fore PTT04+ is to some extent mobile within the essenti a lly electri ca lly insula ting Ti02 phytate film and diffu sion processes in the f ilm can be assumed to be important. The dramatic effect of increasing the buffer concentratio n (see Fig . 6) could , in part , be due to immo bili satio n o f PTT0 4

+ within the membrane. In contrast, when studi ed in so lie! state voltammetry experiments, a thin layer of PTT04+ immobilised o n the e lectrode surface appears to be e lectrochemically ac tive wi th a minimum of transport effects . As the concentratio n of PTT04+ 111 the membrane is increased , the vo ltammetric charac teri s tics change from those typica l fo r a soluti on phase process (fast chemi cal steps, reversible processes) towards those of the solid state case (s lower kineti cs for chemical reactio n stepsn, splitting of vo ltammetric sig nals). However, de tail s for both processes, the reduction o f PTT04+ in so li d state and when accumul ated into a Ti02 phytate membrane, are complex and currentl y not fully understood .

Conclusions The free-base porphyrin PTT04+ exhibits complex

e lectrochemi cal behaviour due to co upled e lec tron transfer and che mical reactio n steps. It has been shown in thi s pre:iminary s tudy that in spite of exhibiting only low so lubility in aqueous electro lyte so lutio n, PTT04+ may be inves tigated by ill/mobilisation of microcrystalline material o n a graphite surface or by accumulation into a mesoporous membrane. Furthe r work will be required for a better understanding o f the interplay of electron

transfer, chemi cal kineti cs in confined spaces. and local pH gradients in mesoporo Ll s membranes.

Acknowledgement FM thanks the Royal Socie ty fo r a Uni versity

Research Fe llo wship . Ni ck M ay is gratefully acknowledged for obtaining and interpreting MS data. Sl Y is grateful fo r fin ancia l support fro m EPS RC (Grants no . GR/N3408610 I and G R/N 34093/0 I).

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R. & Cabui I V. Chelll Mater. 13 (200 I ) 306 1.

2 Rosseinsky 0 R & Mortimer R J. Ad v Male r. 13 (200 I ) 7R3.

3 See for example Che G L. Lakshmi B B. Fisher ER & Martin C R, Nat l/ re. 393 ( 1998) 346.

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5 Brinker C J & Scherer G W. Sol-Gel Sciellce. (Academi c Press. New York). 1990.

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7 Mill ward R C. Madden C E. Sutherland I. Mort imer RJ. Fletcher S & Marken F. Chelll CO IIIII IIIII . (200 I ) 1094.

8 McKenzie K J. Marken F. Hyde M & Compton R G. Ne ll ' J Chelll , 26 (2002) 625.

9 McKenzie K J & Marken F. submitted. 10 Persson H, TUrk M, Nyman M & Sandberg A S . .I Agric

Food Chelll . 46 ( 1998) 3 194 and references cited therein . I I See fo r example Grygar T, Marken F. Schroder U & Scholz

F, Collec Czech Chelll COlli/ill/II . 67 (2002) 163 .

12 See for example Kana K. Takei M & Hashimoto S. J PllI's Chelll . 94 ( 1990) 2 18 I .

13 Kahn T A & Hriljac J A . /llorg Ch illi Acta 294 ( 1999) 179.

14 Hambright P. Adeyemo A. Shamim A & Lemelle S. Ill org SYllth. 23 ( 1988) 55 .

IS Kadish K M. Smith K M & Gull ard R. Eds. The POl17h\'ri ll /-Ialldbook. (Academic Press. New York). 2000. Vo l t) .

16 For a review see Davis 0 G in Dol phin D. Ed. The Porphyrills, (Academic Press. New York). 1t) 78. Vo l V. part C, p l41.

17 Neri B P & Wilson G S. /\ 1101 Chelll. 44 (1 972) 1003. 18 Neri B P & Wilson G S. Allal Chelll . 4S (1 97'3 ) 443. 19 Zhang J, Lever ABP & Pietro W J . .I Chelll Soc. Farada.'"

TrailS. 93 ( 1997) 3355. 20 Zhuang Q K & Scholz F, J Porph Phthal. 4 (2000) 202.

2 1 Zhuang Q K & Scholz F. Chelll J Chill Univer Chill . 22 (200 I ) 17 1.

22 Bond A M. Colton R. Marken F & Walter J N. Organollletallics, 13 ( 1994) 5 122.

23 Bond A M. Colton R. Dani els F. Fernando 0 R. Marken F, Nagaosa y , VanSteveninck R F M & Walte r J N. J Alii Chelll Soc, 11 5 ( 1993) 9556.

24 Bond A M. Marken F, Hill E, Compr.o n R G & HUgel H . .I Chelll Soc. Perkin TrailS 2 ( 1997) 173:5.