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Synthetic Metals 161 (2011) 680–691

Contents lists available at ScienceDirect

Synthetic Metals

journa l homepage: www.e lsev ier .com/ locate /synmet

odulating spectroelectrochemical properties of [Ni(salen)] polymeric films atolecular level

oão Tedima,1, Sónia Patrícioa, Joana Fonsecaa, Alexandre L. Magalhãesa, Cosme Mourab,. Robert Hillmanc, Cristina Freirea,∗

REQUIMTE/Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, 4169-007 Porto, PortugalCIQ/Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, 4169-007 Porto, PortugalDepartment of Chemistry, University of Leicester, Leicester LE1 7 RH, UK

r t i c l e i n f o

rticle history:eceived 24 January 2010eceived in revised form 4 January 2011ccepted 13 January 2011vailable online 12 February 2011

eywords:ickel salen complexes

a b s t r a c t

Electroactive polymer films based on [Ni(salen)]-type complexes were fabricated and their electronicproperties characterized using in situ UV–visible spectroelectrochemistry. The extent of � electronicdelocalisation and electronic asymmetry were manipulated by introduction of different conjugated iminebridges. Measured electronic spectra were interpreted in terms of polaronic states in the band gapand metal-oxidized ligand charge transfer bands. Density functional theory (DFT) calculations for themonomers showed that the HOMO orbital (which governs oxidation potential) is ligand-dominated, andthat substituents with greater electronic delocalisation in the diimine bridge decrease the HOMO–LUMO

lectropolymerisationonductive materialspectroelectrochemistryensity functional calculations

energy gap. Replacement of methyl by methoxyl substituents in the aldehyde moiety increases the cal-culated dipole moment. Substitution-driven variations in EHOMO–ELUMO for the monomers were reflectedin the corresponding polymer band gaps, demonstrating that monomer electronic properties can beused predictively in the manipulation of polymer electronic properties. An important strategic aspectis the correlation of DFT predictions with the observed electronic properties of monomeric and poly-meric materials; the extent to which such modelling can be used to optimise synthetic effort isdiscussed.

. Introduction

The facility to synthesise materials with useful electronic andptical properties and subsequently to vary these via the exter-al stimulus of an applied potential is an extremely attractivettribute of conductive polymeric materials. Here we show, forhe case of a series of substituted [Ni(salen)] species, how onean use DFT to predict monomer electronic properties which cann turn be related to the electronic properties of their polymericnalogs. We then go on to show how the optical properties of

he polymers can be manipulated electrochemically, via the cre-tion or removal of controlled populations of charge carriers, thexistence and nature of which can be rationalised in terms of con-entional models. This approach will allow efficient utilization

∗ Corresponding author at: REQUIMTE/Departamento de Química e Bioquímica,aculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto,ortugal. Tel.: +351 22 0402590; fax: +351 22 0402659.

E-mail address: acfreire@fc.up.pt (C. Freire).1 Present address: CICECO, Departamento de Engenharia Cerâmica e do Vidro,niversidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro,ortugal.

379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2011.01.014

© 2011 Elsevier B.V. All rights reserved.

of future synthetic effort in the design and optimization of newmaterials.

The development of molecular based materials with good elec-tronic conductivity is currently one of the primary strategies toobtain systems and devices with high performance in fields suchas energy, electronics or medicine. Conducting polymers based onaromatic heterocyclic monomers are among the most successfulsystems. Generically, these monomers [1] are oxidisable at rela-tively low anodic potentials and they polymerise by coupling ofcharged radical intermediates. Despite the simplicity of prepara-tion, the opportunities for substitution chemistry and (in manycases) acceptable stability, polymers based on carbon backbonesare not intrinsically conductors: they are semiconductors, requiringan external driving force to generate charge carriers.

In the case of “traditional” poly-heterocyclic organic conductingpolymers, the “dopant” species are inserted to satisfy electroneu-trality; their location in the material is determined by a well-

defined combination of electrostatic forces and steric effects. As aconsequence, nominally identical materials prepared by differentworkers may differ; this has generated a rather confused literature.In contrast, the incorporation of metal centres in polymeric net-works involves rather more specific interaction (bonding) with the

c Metals 161 (2011) 680–691 681

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olymer. While this may be synthetically more demanding [2], itffers the promise of greater reproducibility of materials properties.straightforward way of accomplishing this is to use monomeric

pecies containing the metal, for example in the form of a transitionetal complex, to assemble the supramolecular structures.The rich diversity of electronic states and chemical proper-

ies associated with transition metals makes them interestingdopants” (in the terminology commonly used in this field) forncorporation in conductive organic polymers. In these systems theonducting polymer enhances communication between the metalentres, [3,4] leading to materials with novel – and potentiallyxploitable – physical and chemical properties.

Here, we focus on a class of metal complexes widely studiedn monomeric form, comprising complexes of transition metalsnotably Ni, Cu and Pd) with salen type ligands, here designateds [M(salen)] complexes. Polymers based on [Ni(salen)] complexesere first reported by Goldsby [5–7], following which we and other

roups have studied this type of polymeric system using diverselectrochemical and spectroscopic techniques [8–23]. The mainutcomes of this work were establishment of (electro)chemicaltability, i.e. durability and reproducibility, of poly[Ni(salen)] films14–19]. Subsequently, the aim has been to improve the perfor-

ance of these supramolecular systems, via changes in structuret the molecular level (monomer structure), that allow refinementf their fabrication and properties in two specific areas, namely ionecognition/sensing systems [22–26] and optical applications. It ishe latter that we focus upon here, through an exploration of theacility to manipulate spectral characteristics electrochemically.

Our initial work on [Ni(salen)]-type materials focused on elec-rochemical exploration of electron transfer processes underlyingheir redox chemistry (ligand vs metal oxidation) and EQCM/probeeam deflection measurements of the resultant coupled ion trans-ers [14–16,18]. The (electro)chemical responses and stability ofhese materials prompted synthetic introduction of aldehyde andridge substituents [19] into the salen ligand. In the latter instance,ediated charge transfer to solution species was shown to be

fficient [17] and crown ether functionalities were shown toave potential for analytical applications [23–26]. However, theomplexities of the polymer–analyte solution interaction haverompted a return to a more fundamental examination of filmtructure. In the case of the heavy elements – Ni and crown ether-omplexed species such as Ba – this has been accomplished usingXAFS [25]. The missing component – which we address here –s electronic structure. This defines, and can be used to rationalise,he energies at which electrochemical transformations occur and isentral to the interaction of the polymer with the ambient mediumn any application.

Pursuing this strategy, in this work we show the effects ofntroducing highly conjugated substituents into the [Ni(salen)]tructure, with a view to determining polymer spectroelec-rochemical properties via enhanced �-electron delocalisation,cheme 1. This is accomplished via cyclic voltammetric and initu UV–visible spectroscopic measurements, supported by densityunctional theory (DFT) calculations. The theoretical calculationsuide and allow optimization of the efficiency of synthetic effortnd the experimental data provide insight into how increasing elec-ronic conjugation in the monomer allows one to influence polymerlectrochemical and electronic (band gap) responses.

. Experimental

.1. Chemicals

Solvents for complex synthesis were from Merck, nickel(II)-cetate-4-hydrate from Riedel-de-Haen, and diaminomaleoni-

Scheme 1. Structures of [Ni(salen)] complexes.

trile from Fluka. 2-Hydroxy-3-methoxybenzaldehyde, 2-hydroxy-3-methylbenzaldehyde, and 4,5-dichloro-1,2-phenylenediaminewere obtained from Aldrich and used as received. Tetrabutylam-monium perchlorate (TBAP) (Fluka, puriss. Electrochemical grade)and acetonitrile (Romil, HPLC grade) were used as received. Warn-ing! perchlorate salts are hazardous because of the possibility ofexplosion.

2.2. Synthesis of ligands and complexes

The salen ligands were prepared by the standard method ofrefluxing an ethanolic solution containing stoichiometric amountsof the aldehyde and the diamines. Similarly, the respective com-plexes were prepared by the usual method of refluxing an ethanolicsolution of the salen ligand and the metallic acetate in stoi-chiometric quantities [27–29]. The exception was the complex[Ni(3-MeOsaldiCN)] (complex (3)) which was prepared by a pro-cedure described in the literature [30].

[Ni(3-Mesalophen-4,5-Cl2)]·H2O – complex (1): 1H NMR(300 MHz, DMSO), d/ppm: 8.93 (s, 2H, –CHN), 8.45 (s, 2H, Harom),7.23–7.41 (m, 4H, Harom), 6.56–6.60 (t, 2H, Harom), 2.14 (s, 6H,–CH3). Elemental analyses: Found: C: 54.2; H: 3.4; N: 6.0%; Calcfor NiC22H16N2O2Cl2·H2O: C: 54.2; H: 3.7; N: 5.7%.

682 J. Tedim et al. / Synthetic Metals 161 (2011) 680–691

Table 1Computed dipole moments and orbital energies of the frontier molecular orbitals for the [Ni(salen)] complexes.

Complex Dipole moment [D] % Niin HOMOa EHOMO [eV] ELUMO [eV] �ELUMO–HOMO [eV]

1 0.1 23.9 −5.3 −2.2 3.0

(H6f

C6C

2

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2

Pe((PslCivFt3gi

Hmtcdt(aagrTs

2 2.3 17.03 7.1 11.5

a % Niin HOMO = (˙c2Ni

/˙c2HOMO)× 100.

[Ni(3-MeOsalophen-4,5-Cl2)]·H2O – complex (2): 1H NMR300 MHz, DMSO), d/ppm: 8.51 (s, 2H, –CHN), 7.16–7.20 (s, 2H,arom), 6.64–6.96 (m, 4H, Harom), 6.46–6.60 (t, 2H, Harom), 3.76 (s,H, –CH3). Elemental analyses: Found: C: 50.3; H: 3.1; N: 5.3%; Calcor NiC22H16N2O4Cl2·H2O: C: 50.8; H: 3.5; N: 5.4%.

[Ni(3-MeOsaldiCN)]·H2O – complex (3): 1H NMR (300 MHz,DCl3), d/ppm: 8.08 (s, 2H, –CHN), 7.01–6.74 (m, 6H, Harom), 3.07 (s,H, –OCH3). Elemental analyses: Found: C: 53.7; H: 3.1; N: 12.3%;alc for NiC20H14N4O2·H2O: C: 53.3; H: 3.6; N: 12.4%.

.3. Computational details

The geometries of all complexes were fully optimised withoutny constraints at the DFT/B3LYP level [31,32] using the GAUS-IAN03 package program [33]. The 6-31G(d,p) basis set was used for, C, N, O, Cl and Ni atoms. The electron density and dipole momentsere calculated using the same basis set at the DFT/B3LYP level. In

ddition, the integral equation formalism of the polarizable contin-um model IEFPCM [34,35] was also considered at the same levelf calculation to take into account the electrostatic influence of theolvent (dichloromethane for complex 1 and acetonitrile for com-lexes 2 and 3). HOMO and LUMO orbitals were generated by theolekel software [36].

.4. Physical measurements

Electrochemical studies were performed using an AutolabGSTAT 20 potentiostat/galvanostat. A closed standard three-lectrode cell was used with a Pt disk working electrodearea 0.0314 cm2), a Pt wire counter electrode, and an Ag/AgCl1 mol dm−3 NaCl) reference electrode (Metrohm ref. 60724140).rior to use, the working electrode was polished with an aqueoususpension of 0.3 �m alumina (Beuhler) on a Master-Tex (Beuh-er) polishing pad, and then washed with deionised water andH3CN. All the solutions were deaerated with Ar. Under the exper-

mental conditions used (0.1 mol dm−3 TBAP/CH3CN; scan rate,= 0.05 V s−1, E1/2 for the Fc+/Fc couple is 0.48 V and �Ep = 0.085 V.urthermore, separate impedance data for the solutions show thathe solution resistance for the electrode configuration used is ca.00 � which, even at the typical peak current of ca. 20 �A, onlyenerates a maximum ohmic (“iR”) drop of ca. 6 mV; this does notntroduce significant additional uncertainty in the data.

The spectroscopic instrumentation used was a Hewlett PackardP8451A spectrophotometer. Spectroscopic measurements wereade in situ in transmission mode, with the electrode under poten-

ial control. The working electrode was indium tin oxide-coatedonducting glass (ITO, Balzers) and its area (typically 2.1 cm2) wasefined by a silicone sealant (Dow Corning 3145RTV). All poten-ials were measured and are quoted with respect to an Ag/AgCl0.1 mol dm−3 NaCl) reference electrode; the counter electrode wasPt gauze. The spectrometer was programmed to acquire spectra

t 0.5 s intervals in the wavelength range 280–1100 nm. A back-round spectrum (0.1 mol dm−3 TBAP/CH3CN or CH2Cl2) and aeference spectrum (0.5–1 mmol dm−3 of [M(salen)], 0.1 mol dm−3

BAP/CH3CN or CH2Cl2) were collected before the electropolymeri-ation.

−5.2 −2.2 2.9−5.5 −3.1 2.4

2.5. Procedures

Films were deposited by cycling the potential (between 0.0 and1.3 V at 0.1 V s−1) of the working electrode immersed in CH3CN(complexes (2) and (3)) or CH2Cl2 (complex (1)) solution, con-taining 0.5–1 mmol dm−3 of nickel monomer and 0.1 mol dm−3

TBAP. Film thickness was varied via the number of potential cycles(1–150). After film deposition, the modified electrode was thor-oughly rinsed with dry solvent and transferred to 0.1 mol dm−3

TBAP/CH3CN and cycled between 0.0 and 1.3 V at a scan rate0.1 V s−1. Coulometric assay in monomer-free solution for each filmyielded the resultant electroactive surface coverage, � /mol cm−2

(cited in terms of monomeric units), using n values (doping level)calculated from comparison of coulometric data for film depositionand cycling [14]. The voltammograms used in the calculation of theelectroactive surface concentration were performed at 0.01 V s−1

to ensure complete redox conversion. Dynamic studies were thenperformed at shorter effective timescale by varying the scan rate inthe range of 0.01–0.5 V s−1.

For spectroscopic characterisation, the films were preparedwith 5 deposition cycles (between 0 and 1.3 V, at 0.020 V s−1)using solutions containing 0.5–1 mmol dm−3 of nickel monomerand 0.1 mol dm−3 TBAP. After electropolymerisation, the modifiedelectrodes were washed with CH3CN, immersed in backgroundelectrolyte, and cycled as described above.

3. Results and discussion

3.1. DFT

DFT calculations were performed with the molecular systems,the precursor monomers (see Scheme 1), embedded in a polar-izable continuum which simulates the electrostatic effect of thesolvent (here, dichloromethane for monomer (1) and acetonitrilefor monomers (2) and (3)). The calculations were used to provideinformation about their electronic structure: the relevant param-eters are summarized in Table 1. The results show that all threecomplexes are tetra-coordinate, i.e. there is no axial coordinationof solvent, a prediction confirmed by UV–visible spectroscopy (seebelow). The highest occupied molecular orbitals (HOMO) obtainedfor the optimised geometries of all complexes are depicted in Fig. 1.For all complexes, the HOMO is largely confined to the salen-typeligand. The contribution to the HOMO of the nickel centre variesfrom 23.9% (complex (1)) down to 11.5% (complex (3)), indicatingthat the primary role of the metallic centre is a structural bridgebetween the aldehyde moieties.

Substitution of the dichlorophenylenediimine bridge (com-plexes (1) and (2)) by dicyanoethylenediimine (complex (3)) hasa more pronounced effect on electronic properties than does thereplacement of substituent in 3-position of the aldehyde moiety (cf.,the methyl group in complex (1) and methoxyl group in complexes(2) and (3)). The change in the diimine bridge leads to considerable

delocalisation of � electronic density over the complex, with theHOMO orbital acquiring a significant salicylidene ring character.Moreover, this structural modification also decreases significantlythe energy between the frontier HOMO and LUMO orbitals from 3.0and 2.9 eV for complexes (1) and (2) to 2.4 eV in complex (3) at the

J. Tedim et al. / Synthetic Metals 161 (2011) 680–691 683

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ea

ismseatcdats

3c

rcdrchao(af

opf

ig. 1. HOMO orbitals produced by MOLEKEL program package for the optimised [NA3) complex (3).

xpense of more negative values for both the energies of HOMOnd LUMO in the latter complex (see Table 1).

The calculated dipole moments (see Table 1 and Fig. 1) decreasen the order (3) > (2) > (1). However, all the dipole moments have theame direction, from the imine bridge (negative pole) towards theethyl/methoxyl groups in the aldehyde moiety (positive pole),

uggesting that the upper (lower) part of complex acts as thelectron acceptor (donor). Comparison between complexes (1)nd (2) suggests that the methyl group acts as a weaker elec-ron donor than the methoxyl group, as expected. Concurrently,omparison between complexes (2) and (3), indicates that theicyanoethylenediimine bridge may act as a stronger electroncceptor than the dichlorophenylenediimine bridge. Combininghese, complex (3) contains the strongest electron acceptor and thetrongest donor groups, leading to the largest dipole moment.

.2. Polymer film deposition and cyclic voltammetricharacterisation

The monomeric complexes show similar voltammetricesponses, typical of chemically irreversible processes: theurrent response during the first cycle is quite different from thaturing subsequent cycles. Fig. 2(A1) depicts the voltammetricesponse of complex (2) as an example. In the first anodic half-ycle there are waves at 0.90 V and 1.08 V, and in the cathodicalf-cycle two incompletely resolved features at 0.89 V and 0.55 Vnd a well defined wave at 0.67 V. Analogously, complex (1) showsne anodic (1.10 V) and two cathodic (0.93 V and 0.41 V) featuresFig. S1A, Supplementary material), and complex (3) shows twonodic (1.02 V and 1.11 V) and two cathodic (1.02 V and 0.75 V)

eatures (see Fig. 2(A2)).

In the first cycle, the first anodic process is assigned to monomerxidation and can thus be used to probe the HOMO energetics. Theeak potential data show that it is easiest to remove an electronrom complex (2) and hardest to remove an electron from com-

n)] complexes obtained at the B3LYP level: (Al) complex (1); (A2) complex (2) and

plex (1). The DFT results show that the energy of HOMO orbitals(bearing in mind that the energy and voltage scales are related by aminus sign) is more negative in the following order: (2) < (1) < (3).Since the DFT result is a thermodynamic one but the experimentaldata can also be influenced by kinetic factors, we suggest that thereversal of order for complexes (1) and (3) may be associated withkinetic differences. A more subtle distinction, which may accountin part for this reversal, relates to the differing “solvent” dielec-tric media imposed in the two cases. (Since solubility constraintsrequired the two materials to be studied experimentally in differ-ent solvents, the calculations were run in appropriately differentsimulated media.). We note that the observed variations with sub-stitution (at fixed metal ion) are consistent with ligand dominanceof the HOMO, as predicted by DFT.

In subsequent potential cycles, the new anodic and cathodicwaves are attributed to redox switching of deposited film. Themagnitudes of these features increase with further cycling, reflect-ing accumulation of an electroactive polymer on the electrodesurface. Fig. 2(B1 and B2) shows typical voltammetric responsesof deposited poly[2] (which responds similarly to poly[1]) andpoly[3] films upon redox cycling in monomer-free supporting elec-trolyte. As commonly observed for electropolymerized films, theresponse on the first cycle is slightly different. This effect, whichdiminishes rapidly with cycling, is generally attributed to eitheroxidation of residual monomeric/oligomeric species trapped in thepolymeric matrix [5] or to electrochemical modification of the film[10].

The voltammetric response of poly[2] (Fig. 2(B1)) shows threeelectrochemical processes, whereas poly[3] (Fig. 2(B2)) and poly[1](Fig. S1B, Supplementary material) show two reversible processes.

The redox processes of poly[1] and poly[2] occur at less positivepotentials than those of poly[3]. In other words, the relative orderof HOMO energies for the monomeric complexes is retained in thepolymer. The similarity of responses for poly[1] and poly[2] and dis-tinctiveness (sharper peaks) of poly[3] suggest that film electronic

684 J. Tedim et al. / Synthetic Metals 161 (2011) 680–691

-30

-20

-10

0

10

20

1.20.90.60.30.0

-30

-15

0

15

1.20.90.60.30.0

-8

-4

0

4

8

-8

-4

0

4

8

**

*

**

E / V (vs. Ag/AgCl)E / V (vs. Ag/AgCl)

*

*

*

*

**

B2B1

A2

i / μ

A

i/μA

i / μ

A

i / μ

A

A1

0.48 V

0.81 V

1.00 V

0.71 V

0.27 V

*

*

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0.92 V

0.89 V

0.75 V

*

*

*

*

F (panelc nel Bl)v

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w

a function of potential scan rate (v/V s−1) for films of varying cov-erage. The diagnostic is the slope of a plot of log(ip) vs log(v). Rapidcharge transport (on the timescale set by the scan rate) is char-acterized by a slope of unity, along with small (ideally zero) and

0

50

100

150

200

250

300

Slope = 4.60x10-9

Slope = 7.35x10-10

Slope = 6.14x10-10

Γ / n

mo

l cm

-2

poly[1]

poly[2]

poly[3]

ig. 2. Cyclic voltammograms showing the electropolymerisation of complexes 2onditions). Redox switching of the resultant films of poly[2] (F = 37 nmol dm−3; pa= 0.10 V s−1. Asterisks denote features referred to in the main text.

tructure is more sensitive to the changes at the imine bridge thant the 3-position of the aldehyde.

A first identification of the redox active site (ligand vs metal) wasased upon comparison of the anodic charges during polymerisa-ion (qpol) and redox switching (qred) described in more detail inur previous work [14], qpol/qred = (2 + 2y + x)/(2y + x), where y andare the numbers of electrons transferred per phenyl and nickel

tom centre, respectively. While y may assume a fractional value, xan only be an integer. Applying this constraint to the data, x = 0 forll the films; this is the signature of a ligand-based redox process.e find y = 0.45, 0.38 and 0.40 for poly[1], [2] and [3], respectively,

o the doping level (charge per salen monomer, n = 2y) is n = 0.89,.78 and 0.80 for poly[1], poly[2] and poly[3]. These values are ofimilar magnitude to those obtained for analogous poly[M(salen)]lms, M = Ni(II) and Cu(II) for which a ligand-based redox pro-ess was identified [13–15,20,22,23]. We address this issue moreirectly below, on the basis of the associated UV–visible spectro-copic responses.

However, this is not an unexpected result because the H2salenre indeed non-innocent ligands, since they show redox chem-stry in CH3CN and undergo polymerisation under essentially theame conditions as the complexes (not shown), but leading to elec-rode passivation. This probably happens because the ligands have

trans-conformation, while the metal-complexed ligands havecis-conformation and planar geometry that facilitates electron

elocalisation and thence high conductivity. Ligand-based redoxhemistry pertains for metal complexes in which the metal oxi-ation potential M(II)/M(III) is higher; this is the case for Ni(II),

d(II) and Cu(II). On the contrary, when the metal oxidises at aess positive potential than the ligand, oxidation is expected to be

etal-based; this is the case for Co(II) and Fe(II) salen complexes.The increase in surface coverage of electroactive material, � ,

ith the number of polymerisation scans is fastest for poly[2], at

Al) and 3 (panel A2) at a Pt disk electrode (see text for solution composition andand poly[3] (F = 4.4 nmol dm−3; panel B2) in 0.1 mol dm−3 TBAP/CH3CN; scan rate,

ca. 4.6 nmol cm−2 cycle−1 (see Fig. 3). Poly[1] and poly[3] have filmdeposition rates of 0.61 and 0.74 nmol cm−2 cycle−1, respectively.While increases in the apparent value of � are initially linear withnumber of deposition cycles, the rate of increase ultimately slows,due to decreasing polymerisation efficiency and/or charge trans-port limitations across the growing film.

The latter issue was explored by voltammetric measurements as

160140120100806040200

Number of deposition cycles

Fig. 3. Plot of the apparent surface coverage, � , with the number of scans employedin the polymerisation of the different complexes.

J. Tedim et al. / Synthetic Metals 161 (2011) 680–691 685

-6.0

-4.0

4.0

6.0

0.0-0.4-0.8-1.2-1.6-2.0

-6.0

-4.0

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0.30

0.60

0.90

1.20

-0.4-0.8-1.2-1.6-2.0

0.30

0.60

0.90

1.20

log(ipa)

log(ipc)

Slope = 1.2

log(ipa)

log(ipc)

Slope = 0.92

Slope = 1.1

Slope = 0.68

Slope = 0.80

log

ior

log

|i|

21

EpaEpc

log νlog ν

logio

r lo

g |i|

E / V

E / V

B1

A2A1

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EpaEpc

B2

F panelF

stgcrreromoi

irrp(uot

TV

ig. 4. Plots of Epa and Epc vs log v (panel 1) and of log(ipa) and log(ipc) vs log v (ig. S2 (Supplementary material).

can rate-independent peak potential difference (�Ep = Epa−Epc);his signature also corresponds to constant redox charge, whichives the total coverage via the Faraday constant. Conversely, slowharge transport will result in a less than linear increase in peak cur-ent and thereby a decrease in total charge passed (both of whicheflect incomplete redox conversion and thus a coulometric under-stimate of coverage) and an increase in �Ep with potential scanate. In the case that the limitation is diffusional in nature, the slopef a plot of log(ip) vs log(v) is 0.5. This latter behaviour is com-only attributed to slow counter ion diffusion, although strictly

ne cannot separate the coupled transport of electrons and counterons.

As an example, poly[2] films prepared with � < 40 nmol cm−2,llustrated in Fig. 4 for a film with � = 37 nmol cm−2 (voltammet-ic responses in Fig. S2, Supplementary material), show the fullange of behaviour as scan rate is varied. At low scan rates, theeak potentials are constant, but at high scan rates the anodic

cathodic) peak potentials shift to more positive (negative) val-es, i.e. �Ep increases. This transition, which may represent thenset of a charge transport limitation or the intervention of struc-ural factors (as discussed below), occurs at lower scan rate for

able 2alues of (dlog |i|/dlog �) for poly[Ni(salen)] films prepared with different numbers of pol

Film Deposition cycles � [nmol cm−2] Slopes (dlog |ip|/dlog �)

Anodic

Low � High �

Poly[1] 5 2.3 1.0 –150 91 0.98 –

Poly[2] 5 37 0.89 –150 282 1.1 0.92

Poly[3] 5 4.4 1.0 –150 60 1.0 0.73

2) for poly[2] films: (A) � = 37 nmol cm−2 and (B) � = 282 nmol cm−2. Data from

thicker films, illustrated in Fig. 4 for a film with 282 nmol cm−2

(voltammetric responses in Fig. S2, Supplementary material). Thesituation is quantified in Table 2 for the most relevant redox peaks,Epa and Epc in the range 0.60–0.90 V, for all films. Poly[1] films with2.3 < � /nmol cm−2 < 91 show rapid charge transport and completeredox site conversion. Thin poly[2] and poly[3] films show com-plete conversion at all scan rates explored, but thick films do so onlyat slow scan rate. For these films, increasing scan rate moves thesystem to a mixed regime, but not fully (in terms of film thicknessor scan rate) into the diffusion controlled regime.

As hinted at above, while physical interpretation of log(ip) vslog(v) plots with slope unity is unambiguous (redox conversion ofall sites, with no charge transport limitations or other complica-tions), departure from this situation is less straightforward. Theother frequently encountered situation is that of incomplete redoxconversion as a consequence of diffusionally limited charge trans-

port through the film, for which the signature is a log(ip) vs log(v)plot with slope one-half. Consequently, one interpretation of slopes0.5 < d[log(ip)]/d[log(v)] < 1.0 is a transition between the thin layer(diffusionless) and diffusional regimes. There are, however, alter-native explanations for slopes deviating from unity, based not on

ymerisation scans; data refer to processes at Ep,a and Ep,c = 0.60–0.90 V.

Cathodic

Break point [V s−1] Low � High � Break point [V s−1]

– 1.0 – –– 0.95 – –– 1.2 – –0.059 0.80 0.68 0.076– 1.1 – –0.082 0.92 0.68 0.084

686 J. Tedim et al. / Synthetic Metals 161 (2011) 680–691

Table 3Electronic bands (�max) and respective molar extinction coefficients (ε) for the Ni(II)complexes.

Complex �max/[nm] (ε [mol−1 dm3 cm−1])

1 386 (26,800), 454 (7200), 490 (9100)

ticIt>att

3

ibodh(stt3imedlp[tva

Umsvtwpices(

tamnismmp

1.0

2.0

3.0

4.0

5.0

1000800600400

A-A

TB

AP

λ / nm

A

Monomer

Polymer

B

a.u

.

2 255 (22,700), 300 (11,300), 380 (13,200), 495 (3900)3 290 (12,200), 314 (16,100), 388 (13,700), 437 (14,800), 562

(4600), 630 (3100)

ransport phenomena but rather on structural factors. One of theses the consequence of a fractal film structure [37]. The other is aonsequence of restricted pseudo-diffusion within mesopores [38].nterpreting the data of Table 2 in terms of the fractal model leadso three of the four sub-unity slopes being assigned dimensionality3, which is attributed “little physical meaning” [37]. Nonetheless,lthough these models may not describe our data quantitatively,hey do indicate that the possibility of structurally determinedransport should be considered.

.3. UV–visible transmission spectroscopy

UV–visible spectroscopic data for the monomers are presentedn Table 3. Typically, square planar Ni(II) salen complexes show aroad band of medium intensity (100 < ε < 1000 mol−1 dm3 cm−1)ccurring at � = 400–550 nm, which corresponds to three allowed–d transitions (dyz← {dz2; (dxy, dxz) and dx2–dy2}) thatave similar energies, and several bands of high intensityε > 1000 mol−1 dm3 cm−1) occurring at higher energies, corre-ponding to charge transfer and intra-ligand transitions [39]. Inhe case of the complexes under study, the bands observed inhe range 250–700 nm have extinction coefficients higher than000 mol−1 dm3 cm−1, which correspond to charge transfer and

ntra-ligand transitions. Consequently, the weaker d–d transitionsust lie under the envelope of these high intensity bands. Low

nergy charge transfer bands usually occur when there is a highegree of � electron delocalisation within the ligand. For salen

igands, this usually occurs when aromatic diimine bridges areresent, which is the case for the Ni complexes used in this work39,40]. This � electron delocalisation effect is particularly impor-ant for complex (3), since several charge transfer bands occur at �alues as low as 630 nm, when compared with the complexes (1)nd (2).

The polymerisation of all monomers was studied by in situV–visible spectroscopy. The spectra acquired during five poly-erisation cycles show similar behaviour: in the positive going

can there is a general increase in absorbance across the entireisible region for potentials higher than 0.5–0.6 V, correspondingo the beginning of film oxidation. In particular, at � = 500–650 nme clearly observe a new band whose intensity increases up to aotential of 1.3 V. At the end of each cycle, there is a net increase

n the absorbance associated with film deposition, thereby indi-ating the chromophoric nature of the deposited species onto thelectrode surface; Fig. 5(A) shows a representative example – thepectra acquired during the 5th polymerisation cycle of complex2).

After transfer from monomer solution to background elec-rolyte solution, the UV–visible spectra of the three polymers werecquired during an oxidation–reduction cycle. Comparing eachonomer spectrum with the corresponding polymer at the begin-

ing of the redox cycle (film in the reduced state), exemplifiedn Fig. 5(B) for poly[2], it is possible to see that both spectra are

imilar with the bands previously assigned to CT transitions in theonomer slightly shifted to lower energies in the case of the poly-ers; this is a consequence of increase in � delocalisation after

olymer formation. As no d–d bands could be observed in either

Fig. 5. (A) UV–visible spectra of poly[2] referenced to the TBAP/CH3CN, collectedduring the 5th cycle of polymerisation and B) spectra of monomer and polymer inthe reduced state, in background electrolyte.

monomer or polymer spectra, no direct information on the metalcoordination sphere after polymerisation could be obtained. Nev-ertheless, the similarity between the charge transfer bands in themonomer and corresponding polymer indirectly indicates that thecoordination sphere of the metal is retained upon polymerisation.As precedent for this, we note that direct proof of metal coordina-tion retention for a range of [M(salen)] based polymers (M = Ni, Cu)and their precursor monomers has recently been obtained usingEXAFS [25,26].

Polymer oxidation leads to the appearance of new UV–visibleregion bands that show different dependences on applied potential,as can be seen in Figs. 6(A1), 7(A1) and S3(A1) (Supple-mentary material) where the absolute spectra (referenced tobackground electrolyte) for poly[2], poly[3] and poly[1] are pre-sented, respectively. These data provide authentic values for theabsorbance band maxima of the new electronic bands, but are notideal for visualizing their changes with potential. Consequently,the intricate pattern of behaviour is most readily appreciated bydepicting the spectra in difference form, using the responses atselected potentials as reference points. One caveat is that this dif-ferential representation has the potential drawback of introducingadditional errors in estimating peak wavelength values for thosefeatures in close proximity (as seen in the ca. 300–500 nm region).We estimate this (possible) error to be less than 20 nm, which cor-responds to ca. 0.1 eV (dependent on wavelength); accordingly, inthe tabulated data we indicate the wavelengths of “true” maxima

(undistorted by this complication) with the subscript max and thosethat may be subject to this modest distortion with a superscriptedasterisk. For the first part of film oxidation, we refer the spectra tothe spectrum of the reduced film. Part way through the oxidation

J. Tedim et al. / Synthetic Metals 161 (2011) 680–691 687

Table 4Electronic bands and their respective molar extinction coefficients (ε) for fully oxi-dized poly[Ni(salen)] films.

Film �max [nm]a ε×10−4 [M−1 cm−1] Energy [eV]

Poly[1] 326* 0.69 3.80366* 0.58 3.39428* 0.11 2.90534 0.73 2.32900 0.52 1.38

Poly[2] 329* 1.18 3.76369* 2.02 3.36435* 1.06 2.85634 2.28 1.96

1032 1.45 1.20Poly[3] 315* 2.79 3.9

386* 1.26 3.2425* 2.18 2.92485* 0.79 2.54585* 1.89 2.12660 4.81 1.88855 1.68 1.45

(ep

subpoviavlbo(saec

Fstrv��tp48baAt

Tvwbt

1050900750600450300

-0.40

0.00

0.40

0.80

-0.20

-0.10

0.00

0.10

0.200.00

0.40

0.80

1.20

λ / nm

A-A

po

lym

er

0.7

0 V

A1

A-A

po

lym

er

0.0

VA

A3

A2

Fig. 6. UV–visible spectra of a poly[2] film acquired during redox switching inbackground electrolyte (at 50 mV intervals): (Al) absolute spectra referenced to

−3

ences are slight variations in the � values as a consequence of

>1100 3.84 1.13

a Values marked * may be subject to uncertainty up to 20 nm (see main text).

at E = 0.7 V), when new bands start to appear, we shift the refer-nce point and refer all subsequent spectra to this new referenceoint.

The difference spectra of poly[1] and poly[2] films are quiteimilar in terms of �max values (see footnote to Table 4 regardingncertainties indicated in the previous paragraph) and electronicand variations with potential. Figs. 6(A2) and S3(A2) (see Sup-lementary materials) depict, respectively, the UV–visible spectraf representative poly[2] and poly[1] films. In the potential inter-al 0.0–0.7 V (Fig. 6(A2)), the onset of poly[2] oxidation at 0.2 Vs accompanied by a decrease in absorbance at � = 329 nm and theppearance and growth of new bands at �* = 369 nm, 435 nm, and aery large band with an anticipated maximum near the low energyimit of the spectrometer (�max∼1100 nm); these new growingands reach maximum absorbance at E≈0.7 V (hence our selectionf this as a reference point). In the potential interval 0.75–1.30 V,Fig. 6(A3)), the band at �* = 329 nm continues to decrease in inten-ity, the bands at �* = 369, 435 and �max = 1032 nm start to decrease,nd a new band at �max = 634 nm shows a sharp increase until thend of the anodic cycle. In the cathodic scan, these absorbancehanges are reversed.

Analogous data for a representative poly[3] film are shown inig. 7. Not surprisingly, since the electrochemical response of thisystem is different, so is the spectroscopic response. In the poten-ial range 0.00–0.60 V (Fig. 7(A2)) the absence of electrochemicaleaction leads to negligible spectral change. In the potential inter-al 0.65–0.80 V, (Fig. 7(A3)) there is a decrease in absorbance at* = 315 and 488 nm, and there are increases in the intensities at* = 386, 425 (sh), 585 and �max = 855 (sh) and at � values higherhan the low energy limit of the spectrometer (1100 nm). In theotential interval 0.85–1.30 V (Fig. 7(A4)) the bands at �* = 315 and88 nm continue to decrease in absorbance, those at � = 386, 425,55 (sh) and a band with � > 1100 nm start to decrease, and a newand at �max = 660 nm shows a sharp increase until the end of thenodic cycle. In the cathodic half-cycle, these changes are reversed.n interesting point is that this polymer clearly shows more bands

han the others; band assignment will be addressed below.The �max values of the observed bands are summarized in

able 4 and the potential dependences of their intensities (Abs

s E profiles) are plotted in Fig. 8. All the absorbance variationsith potential are chemically reversible, i.e. the initial state can

e restored upon returning the potential to its initial value andhe responses are reproducible upon subsequent cycles. However,

0.1 mol dm TBAP/CH3CN; (A2) spectra collected from 0.05 to 0.70 V with refer-ence to the neutral polymer and (A3) from 0.75 to 1.30 V, referenced to the polymerat 0.70 V.

for some bands, the intensity vs potential profiles show hystere-sis, indicative of either slow kinetics or slow charge transport; thisissue is explored below. This phenomenon cannot be attributedto charge trapping, since voltammetric data (see Fig. S1; support-ing information) show that the initial state is restored at the end ofthe cycle.

Another important aspect that can be highlighted from theplots in Fig. 8 is that three different patterns for band absorbancevariation can be distinguished for all the polymers: (i) bands(� = 366–435 nm) that start to increase at E≈0.35 V (poly[1] andpoly[2]) or 0.6 V (poly[3]) reach a maximum around E = 0.75–0.8 Vin the forward scan, and decrease thereafter until the positivepotential limit (1.3 V); (ii) a band at � = 530–650 nm, that is muchmore intense than the others, increases from E≈0.6–0.75 V untilE = 1.3 V (positive potential limit), with a continuous red shift and,finally, (iii) a band at �≈320 nm that decreases from E = 0.5 V andreaches a minimum near E = 0.8–0.9 V. Poly[1] and poly[2] showmore similar Abs vs E profiles (Fig. 8(A1) and (A2)), as a consequenceof the similarity of their electrochemical responses; the only differ-

max

modification of electronic structure caused by the different alde-hyde substituents. In comparison, poly[3] shows changes in Absvs E profiles shifted to higher potentials, analogous to the shiftin the underlying electrochemical processes. Additionally, poly[3]

688 J. Tedim et al. / Synthetic Metals 161 (2011) 680–691

1000800600400

-0.20

0.00

0.20

0.40

-0.01

0.00

0.01

0.02

1000800600400

-0.10

-0.05

0.00

0.05

0.00

0.20

0.40

0.60

0.80

1.00

λ / nm

A

λ / nm

A1

A-A

po

lym

er

0.6

0 V

A4A3

A-A

po

lym

er 0

.80 V

A-A

po

lym

er 0

V

A2

F groun −3

T e neut to tha

sssp

sittwrFt

A

wtFmoiacms

htttefist

ig. 7. UV–visible spectra of a poly[3] film acquired during redox switching in backBAP/CH3CN; (A2) spectra collected from 0.05 to 0.60 V and referenced to that of thhe polymer at 0.60 V and (A4) spectra collected from 0.85 to 1.30 V and referenced

hows more bands, notably at �* = 485 (which decreases in inten-ity, Fig. 8(A3)) and at 585 and �max 855 nm (with Abs vs E profilesimilar to the band at � = 366–435 nm in poly[1] and poly[2] (notlotted in Fig. 8(A3)).

The key attribute explored here is the extent to which spectro-copic changes can be controlled electrochemically, i.e. by chargenjection/removal. In this respect, the applied potential is simplyhe means by which film charge state is controlled. Thus the cen-ral issue is the extent to which the optical changes correlate notith potential but with charge, i.e. the charge (integrated cur-

ent) and absorbance responses (implicit in Fig. 4 and shown inig. 8, respectively) should be compared directly. Combination ofhe Beer–Lambert and Faraday laws leads to the relation [14,18,41]:

bs(�) = ε(�) q/nF

here q is the charge density (C cm−2), n is the number of elec-rons transferred per monomer unit (doping level; see above) andis the Faraday constant. This equation allows one to estimate theolar extinction coefficient (ε(�)/M−1 cm−1). In the simplest case

f two ideal interconverting species, whose optical properties arendependent of their populations, a plot of Abs(�) vs q will be linear;dditional species and/or variations in optical properties introduceurvature. The resulting plots are shown in Fig. 9 and estimatedolar extinction coefficients obtained from the slopes of the linear

ections, are summarized in Table 4.The ε-values are in the range 1000–2900 M−1 cm−1, much

igher than those expected for nickel d–d transitions and areypical of electronic transitions between states with large con-ributions from ligand orbitals, i.e. spin and symmetry allowed

ransitions [39]. As these electronic bands are associated with thelectroactivity of the polymers (they are not observed when thelms become electroinactive), they are assigned to electronic tran-itions between electronic states created during film oxidation, dueo delocalized charge carriers created in this process, thus con-

d electrolyte (at 50 mV intervals): (Al) absolute spectra referenced to 0.1 mol dmtral polymer; (A3) spectra collected from 0.65 V to 0.80 V and referenced to that oft of the polymer at 0.80 V.

firming that polymer oxidation involves ligand-based processes.Furthermore, the electronic band profiles of the polymers preparedin this work are similar to those of other poly[M(salen)] films, withM = Ni or Cu, we reported previously [13,14,18,20,42] and for whicha ligand-based EPR signal was observed. Furthermore, we showedby XAS that in poly[Ni(3-MeOsaltMe)] films, which have similarproperties to those prepared in this work, the nickel formal +2 oxi-dation state is essentially unaltered by the polymerisation, surfaceimmobilization and film oxidation [23,24].

In this context, the electronic bands in the oxidized polymerscan be assigned using the polaronic model developed for organicconductors with non-degenerate ground states [43–45], and usedpreviously for the poly[M(salen)] materials, with M = Ni or Cu[14,18,20].

The polaronic states (bonding and antibonding) formed withinthe band gap generate three new allowed electronic transitionswith energies below that of the intervalence band transi-tion, as depicted in Scheme 2: W1 from the valence band tothe bonding polaron level, W2 from the valence band to theanti-bonding polaron level and W3 from the bonding to theanti-bonding polaron level. The energies of these transitionsare related to each other and to the intervalence band (WI)as follows: W1 + W3 = W2 and W1 + W2 = WI (assuming, as com-monly done, symmetrical distribution of the gap-state energylevels). Further removal of electrons leads to generation of morepolarons or, if pairing of the charges is energetically favourableand kinetically achievable, to formation of bipolarons. Bipo-laronic states in the gap result in only two allowed subgaptransitions.

On the basis that electronic bands with the same Abs vs E pro-files are likely to be associated with the same charge carriers, we

propose the following band assignment for poly[1] and poly[2],respectively: (i) the electronic bands at �max≈326–329 nm (3.8 eV)are assigned to the intervalence band, WI, since they represent thelargest energy transition and decrease in intensity upon polymeroxidation; (ii) the bands at � = 366–369 nm (3.4 eV) are assigned to

J. Tedim et al. / Synthetic Metals 161 (2011) 680–691 689

Conduction band

Valence band

W1

W2

W3WI

Sp

tWtlapisi2W

aStwma[

fctFaat[EtEb

tai(sc(iiict(i

-0.30

0.00

0.30

0.60

-0.40

-0.20

0.00

0.20

0.40

1.51.20.90.60.30.0

-0.15

0.00

0.15

0.30

λ = 330 nm

λ = 369 nm

λ = 435 nm

λ = 634 nm

λ = 1032 nm

λ = 326 nm

λ = 366 nm

λ = 428 nm

λ = 534 nm

λ = 900 nm

λ = 315 nm

λ = 386 nm

λ = 425 nm

λ = 485 nm

λ = 660 nm

λ = 1100 nm

A3

A2

A-A

film

0V

A-A

film

0V

A-A

film

0V

E / V

A1

Fig. 8. Plots of the absorbance change vs E for selected electronic bands (identifiedby legend inset in figure) of polymer films: (Al) poly[l] (data from Fig. S3, Sup-plementary material), (A2) poly[2] (data from Fig. 6) and (A3) poly[3] (data fromFig. 7). Difference spectra acquired as described for Figs. 6 and 7. Arrows indicate

cheme 2. Schematic representation of electronic structure of oxidizedoly[Ni(salen)] films.

he transition between the valence and antibonding polaron level,2, (iii) the bands at � = 428–435 nm (2.9 eV) are assigned to the

ransition from the bonding polaron to the antibonding polaronevel, W3 and (iv) the bands at �≈900–1032 nm (1.4–1.2 eV) aressigned to the transition from the valence band to the bondingolaron level, W1. All the latter bands increase in intensity dur-

ng oxidation as a result of the increase in population of polaronictates in the band gap. As a check on these assignments, the exper-mental values for the respective transition energies satisfy, within0–25% experimental uncertainty, the relationships (see Scheme 2)1 + W3 = W2 and W1 + W2 = WI.The very intense band at � = 530–650 nm (2.3–1.9 eV) shows

different Abs vs E profile with respect to all the other bands.ince, during polymer oxidation, a new highly delocalized � sys-em is formed through the quinoid bond between two phenyl rings,e assign this band to a charge transfer transition between theetal and the newly created oxidized ligand electronic structure,

s previously noted for the analogous poly[M(salen)], M = Ni, Cu14,18,20].

Poly[3] shows similar Abs vs E profiles to the other polymersor the bands at � = 315, 386, 425 and 660 nm. The first three bandsan be assigned to the electronic transitions WI, W2 and W3, respec-ively, and the band at � = 660 nm can be assigned to the CT band.or the other observed bands that have no counterparts in poly[1]nd poly[2] we propose them to be due to bipolarons, since thessociated electrochemical response occurs at more positive poten-ials, where these charge carriers are more likely to be formed43–45]. We recall our previous work with poly[Ni(salen)] in whichPR was used to study the film oxidation at different positive poten-ials, where for the most positive potential a decrease in the radicalPR signal was observed, as a consequence of the formation ofipolarons[14].

The CT bands of the polymers follow a similar trend tohat observed for the monomers. More specifically, the energyssociated with this electronic transition in the films decreasesn the order poly[1] (� = 534 nm) > poly[2] (� = 634 nm) > poly[3]� = 660 nm). Comparing this with the monomer characteristics inolution, the CT band at lower energies decreases in the order:omplex (1) (� = 490 nm) > complex (2) (� = 495 nm) > complex (3)� = 630 nm). This shows that the metal - ligand charge transfern the oxidized polymer is of a “localised” nature, i.e. does notnvolve the delocalized �-system in the polymer, so the behaviours very similar to that of the corresponding monomer. However,

omparing the relative EHOMO–ELUMO values of the monomers withhe band gap energies of the corresponding resulting polymersintervalence band), we note that EHOMO–ELUMO values decreasen the order (1) > (2) > (3), and for the polymers, the band gap

scan direction.

varies as poly[1]≈poly[2] < poly[3]. Although there is an inver-sion in the ordering of energies for the complex EHOMO–ELUMOand corresponding film band gaps, the similarity between theelectronic structure of complexes 1 and 2 is retained in the cor-responding polymers; complex 3 and poly[3] show the majordifferences.

The significant outcome of the above is that the polymer struc-ture (most notably the band gap) is predictably determined by theelectronic properties and structure of the corresponding monomer.More importantly, these results demonstrate that spectroscopic(optical) properties of this type of supramolecular system can be

fine tuned at the molecular level by selection of the electronicdonor/acceptor substituents within the Ni structure and manip-ulated electrochemically.

690 J. Tedim et al. / Synthetic Metals 161 (2011) 680–691

8.06.04.02.00.0

-0.40

-0.30

-0.20

-0.10

0.00 8.06.04.02.00.0

-0.10

0.00

0.10

-0.24

-0.16

-0.08

0.00

-0.60

-0.40

-0.20

0.00

0.25

0.50

0.75

A-A

po

lym

er

0 V

A-A

po

lym

er

0 V

A3

λ = 435 nm

A-A

po

lym

er

0 V

Q / mC

A5

λ = 1032 nm

λ = 329 nm

A1

A-A

po

lym

er

0 V

λ = 634 nm

λ = 369 nm

A2

A-A

po

lym

er

0 V

A4

F � = 32(

4

sipio�sirac

pHotucasdcp

wb

Q / mC

ig. 9. Plots of the differential absorbance vs q for electronic bands of poly[2]: (Al)©) cathodic scan.

. Concluding remarks

The combination of DFT calculations with electrochemical andpectroscopic measurements is extremely effective at providingnsights into the nature, origins and manipulation of electronicroperties of structurally related poly[Ni(salen)] films. Here this

s demonstrated for the case of electroactive films derived byxidative polymerisation of Ni(salen)-type monomers bearing-conjugated diimine bridge and methyl/methoxyl aldehyde sub-

tituents. This includes substitutionally driven variations of (i) thendividual energies of the HOMO and LUMO (the source and sink,espectively, for electrons in monomeric complex redox chemistry)nd (ii) their energy difference, which defines the band gap that isentral to optical properties.

More specifically, DFT calculations for the monomeric com-lexes in acetonitrile correctly predict key features of theirOMO–LUMO energy gaps, notably their dependence on the degreef �-electron delocalisation and the electronic density asymme-ry. In the former case, the most sensitive change was obtainedpon incorporation of a conjugated diimine bridge. In the latterase, substituents in the 3-position in the aldehyde moiety playedn important role in terms of the electron density asymmetry. Weuggest that the facility of the calculations to describe �-electronelocalisation is the origin of the practically important fact that

alculated monomer properties are a good predictor of polymerroperties.

The potential of monomer oxidation was found to be correlatedith the energy of the HOMO orbital, which is mainly ligand-

ased (<25% Ni contribution). The voltammetric responses of the

9 (A2) � = 369, (A3) � = 435, (A4) � = 634 and (A5) � = 1017 nm; (�) anodic scan and

resulting modified electrodes were strongly dependent on the typeof diimine bridge. In particular, poly[1] and poly[2] show redoxprocesses at less positive potentials than poly[3]; this follows thetrend of the monomer HOMO energies. There are also differences interms of polymer growth rate and charge transport rates. Despitethese variations, the common feature for all the polymer films isligand-based redox chemistry, for which coulometric assay showeda doping level 0.78 < n < 0.89.

The spectroscopic responses can be largely explained on thebasis of the (bi)polaron model frequently used for conjugatedpolyaromatic materials. Within this, the presence of more thanone charge carrier (polarons and bipolarons) explains the unusu-ally high number of absorption bands. An additional band at� = 500–750 nm shows a completely different potential profilefrom the polaronic ones, and is associated with charge transferbetween the metal and the oxidized ligand. This “localised” transi-tion follows trends observed in the CT electronic transitions of themonomeric complexes in solution.

Finally, the ordering of EHOMO–ELUMO for the monomeric com-plexes was similar to that observed for the band gaps in thecorresponding polymers. Although the polymer can be viewed asa collection of monomeric units, the fact that it generates muchgreater charge delocalisation means that this is not a trivial result.Nonetheless, the important outcome is the implication that one

can predictively fine tune the electronic properties – notably theband gap – of [Ni(salen)] polymers based on substitutional molec-ular manipulation of the monomer, in a manner that is predictableand readily measurable. We suggest that this may be helpful inthe design of materials with improved properties, maximising the

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J. Tedim et al. / Syntheti

fficiency of synthetic effort in the preparation of monomeric mate-ials. Fine tuning of polymer properties is then possible via thepplication of the electrochemical potential.

cknowledgements

The work was partially funded by Fundacão para aiência e a Tecnologia (FCT), Portugal, through project RefTDC/QUI/67786/2006. JT, SP and JF thank FCT for PhD grants.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.synthmet.2011.01.014.

eferences

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