Synthesis, characterisation andelectrochemical investigation ofphthalocyanines with pendant4-{2-[2-(4-tert-butylphenoxy)ethoxy]ethoxy} substituents_Irfan Acar,a Saim Topc�u,b Zekeriya Bıyıklıo�gluc,*and Halit Kantekinc

aDepartment of Woodworking Industry Engineering, Faculty of Technology, KaradenizTechnical University, 61830, Trabzon, TurkeybDepartment of Chemistry, Faculty of Arts and Sciences, Giresun University, Giresun, TurkeycDepartment of Chemistry, Faculty of Sciences, Karadeniz Technical University, 61080,Trabzon, TurkeyEmail: zekeriya_61@yahoo.com

Received: 31 March 2012; Accepted: 17 August 2012

In this study, a novel metal-free phthalocyanine and three metallophthalocyanines carrying four 2-[2-(4-tert-butylphenoxy)ethoxy groups on the periphery were prepared by cyclotetramerisation of a dinitrile derivative inthe presence of the corresponding divalent metal salts [zinc(II), cobalt(II), copper(II)]. These newphthalocyanine compounds have been characterised by infrared, 1H and 13C nuclear magnetic resonanceand electrospray mass spectroscopies and elemental analysis. The electrochemical properties of the metal-free, zinc(II) and cobalt(II) phthalocyanines were investigated by cyclic voltammetry and differential pulsevoltammetry methods. The cobalt complex showed a metal-based reduction process, while the metal-freeand zinc(II) phthalocyanines showed ligand-based electron transfer processes. It has been found that theabsorption spectra substantially depend on concentration. It has been shown that these changes areattributable to the association of the phthalocyanine molecule. The number of molecules in the associatesand the equilibrium constants for this association are determined.

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IntroductionPhthalocyanines are aromatic macrocyclic molecules whosefeatures make them suitable for applications such asphotovoltaic optics, non-linear optics, solar cells, chemicalsensors, molecular electronics [1,2], medicine [3] andphotocatalysis [4]. In recent years, a new development hasbeen the application of phthalocyanines in the photody-namic therapy (PDT) of cancer [5].

Applications of phthalocyanines are restricted owing totheir insolubility in common solvents and water [6].Peripheral substitution with bulky groups or long alkyl,alkoxy or alkylthio chains leads to phthalocyanine productsthat are soluble in apolar solvents. Sulfo or quaternaryammonium groups enhance solubility in aqueous mediaover a wide pH range [7–13]. The synthesis of solublephthalocyanines ensures excellent properties, as deter-mined by optical, electronic, redox and magnetic investiga-tion of their chemical and physical characteristics [14–16].

In recent years, microwave-assisted synthesis of phthalo-cyanines has not only shortened chemical reaction times fromhours to minutes but also increased the yields and improvedreproducibility [17–26]. Therefore,many academic and indus-trial research groups are already using microwave-assistedorganic synthesis for rapid optimisation of reactions, for theefficient synthesis ofnewchemical entitiesand fordiscoveringand probing new chemical reactivity [27].

The redox properties of phthalocyanines are criticallyrelated to most of their industrial applications. The redox

process depends on various factors such as the type ofmetal, solvent, axial ligands and substituents. Cyclicvoltammetry (CV) is the most widely used method fordetermining electrochemical properties in solution. Theelectrochemical properties of metallophthalocyanines havebeen intensively studied [28,29]. Many phthalocyanines areknown to dimerise and further aggregate in aqueoussolutions or even non-aqueous solutions. They can formdifferent assemblages by non-covalent interactions and p–pinteractions, and conformationally flexible peripheral sub-stituents containing hetero atoms can play a vital role in theformation of different architectural ensembles.

The aim of the authors’ ongoing research is to synthes-ise soluble metal-free and metallophthalocyanine com-plexes to be used in certain application areas, as Zn–phthalocyanine complexes show good photophysical andphotochemical properties that are very useful for PDTstudies. Recently, it was reported that metal-free andmetallophthalocyanine (Zn, Co, Ni, Cu) derivatives func-tionalised with substituents such as 2-[2-(1-naphthyloxy)ethoxy]ethanol, 2-[2-(2-naphthyloxy)ethoxy]ethanol [10,30]and 2-[2-(dimethylamino)ethoxy]ethanol [31] groups inperipheral and non-peripheral positions on the phthalocy-anine ring had promising PDT and electrochemicalproperties. The present paper reports on the synthesis,characterisation and electrochemical properties of metal-free and metallophthalocyanine complexes substitutedwith 4-{2-[2-(4-tert-butylphenoxy)ethoxy]ethoxy} groups inperipheral positions.

© 2013 The Authors. Coloration Technology © 2013 Society of Dyers and Colourists, Color. Technol., 129, 1–8 1

doi: 10.1111/cote.12031

ExperimentalEquipment and materials2-[2-(4-tert-Butylphenoxy)ethoxy]ethanol 1 [32] and4-nitrophthalonitrile 2 [33] were prepared according tothe literature. 4-tert-Butylphenol and 2-(2-chloroethoxy)ethanol were purchased from Aldrich (USA). All reagentsand solvents were of reagent-grade quality and wereobtained from commercial suppliers. All solvents weredried and purified as described by Perrin et al. [34]. TheIR spectra were recorded on a 1600 Fourier Transform-infrared (FTIR) spectrophotometer (PerkinElmer, USA),using KBr pellets. 1H and 13C nuclear magnetic resonance(NMR) spectra were recorded on a Mercury 200 MHzspectrometer (Varian, USA) in CDCl3; chemical shifts (d)were reported relative to Me4Si as internal standard. Massspectra were measured on a Micromass Quatro LC/Ultimaliquid chromatograph with tandem mass spectrometrydetection (LC-MS/MS) (Waters, UK). The elemental anal-yses were performed on an ECS 4010 instrument (Costech,USA). Optical spectra in the UV-vis region were recordedwith a Lambda 25 spectrophotometer (PerkinElmer, USA).Optical spectra in the UV-vis region were recorded with aPG T80+ spectrophotometer (PG Instruments, UK). Allexperiments to determine the dimerisation constant wereperformed in triplicate.

The CV and differential pulse voltammetry (DPV)measurements were carried out with a BAS E2 Epsilonvoltammetric analyser (Bioanalytical Systems, USA). Athree-electrode system was used: a platinum wire counter-electrode, a double-junction Ag/AgCl reference electrodeand a 2 mm platinum disc electrode as the workingelectrode. The surface of the working electrode waspolished with an aqueous suspension of aluminium oxide(Al2O3) before each run. High-purity nitrogen was used fordeoxygenating the solution for at least 15 min prior to eachrun and to maintain a nitrogen blanket during the mea-surements. Ferrocene was used as the internal standard.

SynthesisSynthesis of 4-{2-[2-(4-tert-butylphenoxy)ethoxy]ethoxy}phthalonitrile (3)2-[2-(4-tert-Butylphenoxy)ethoxy]ethanol 1 (2 g, 8.40 9

10�3M) was dissolved in dry dimethylformamide (DMF)

(20 ml) under nitrogen, and 4-nitrophthalonitrile 2 (1.0 g,5.78 9 10�3

M) was added to the solution. After stirring for10 min, finely ground anhydrous potassium carbonate(K2CO3) (3.50 g, 25.36 9 10�3

M) was added portionwisewithin 2 h with efficient stirring. The reaction mixture wasstirred under nitrogen at 50 °C for 72 h. Then the solutionwas poured into ice-water (100 ml) and stirred for 1 day. Thecrude product was filtered, washed with water and dried invacuo over phosphorus pentoxide (P2O5). The crude productwas crystallised from ethanol. Yield 1.50 g (49%). IR (KBr)mmax (cm�1): 3071 (Ar-H), 2962–2871 (aliph. C–H), 2232(C≡N), 1599, 1563, 1513, 1487, 1364, 1310, 1292, 1250, 1212,1185, 1133, 1065, 953, 881, 831. 1H NMR (CDCl3) d (ppm):7.69 (t, 1H, Ar-H), 7.49 (d, 1H, Ar-H), 7.31 (m, 2H, Ar-H), 7.01(d, 1H, Ar-H), 6.85 (m, 2H, Ar-H), 4.12 (t, 2H, CH2–O), 3.98–3.87 (m, 4H, CH2–O), 3.69 (t, 2H, CH2–O), 1.29 (s, 9H, CH3).13C NMR (CDCl3) d (ppm): 162.59, 156.58, 143.86, 135.30,

126.20, 121.16, 120.05, 119.80, 115.66, 113.90, 112.50, 106.22,72.48, 70.06, 68.57, 67.31, 34.01, 31.45. MS (ES+) (m/z): 365[M + H]+. C22H24N2O3, calcd.: C 72.50, H 6.64, N 7.69;found: C 72.76, H 6.51, N 7.88.

Synthesis of metal-free phthalocyanine (4)A mixture of 4-{2-[2-(4-tert-butylphenoxy)ethoxy]ethoxy}phthalonitrile 3 (0.350 g, 96 9 10�3

M) and 96 9 10�3M

of 1.8-diazabicyclo[5.4.0]undec-7-ene (DBU) (0.3 ml) in2.5 ml of dry n-pentanol was heated and stirred at 160 °Cfor 18 h under nitrogen. The reaction mixture was thencooled and precipitated by adding ethanol. The precipi-tated green solid product was filtered off and then driedin vacuo over P2O5. The obtained green solid product waspurified by column chromatography on basic aluminiumoxide using chloroform (CHCl3): Methanol (CH3OH)(100:3) as the solvent system. Yield 0.13 g (37%). IR(KBr) mmax (cm�1): 3285 (N–H), 3032 (Ar-H), 2955–2862(aliph. C–H), 1607, 1508, 1474, 1237, 1182, 1111, 1092,1011, 938, 826, 744. 1H NMR (CDCl3) d (ppm): 7.68–7.59(m, 8H, Ar-H), 7.38–7.29 (m, 12H, Ar-H), 7.04 (m, 8H,Ar-H), 4.32 (m, 16H, CH2–O), 4.14 (m, 16H, CH2–O), 1.35(s, 36H, CH3), -5.65 (s, br, 2H, NH). UV-vis (CHCl3) kmax

(nm) (e, M�1 cm�1): 288 (51300), 342 (79400), 607(33100), 642 (52500), 667 (123000), 703 (141000). MS(ES+) (m/z): 1460 [M + H]+. C88H98N8O12, calcd.: C 72.40,H 6.77, N 7.68; found: C 72.66, H 6.98, N 7.36.

General procedures for metallophthalocyanine derivatives(5–7)A mixture of 4-{2-[2-(4-tert-butylphenoxy)ethoxy]ethoxy}phthalonitrile 3 (0.3 g, 0.82 9 10�3

M), an anhydrous metalsalt [Zn(CH3COO)2 (0.076 g), CoCl2 (0.054 g) or CuCl2(0.055 g)] and 2-(dimethylamino)ethanol (2.5 ml) was irra-diated in a microwave oven at 175 °C, 350 W for 8 min.After cooling to room temperature, the reaction mixture wasrefluxed with ethanol to precipitate the product which wasfiltered off and dried in vacuo over P2O5. Finally, puremetallophthalocyanines were obtained by column chroma-tography on basic aluminium oxide using CHCl3:CH3OH(100:3) as the solvent system.

Zinc(II) phthalocyanine (5)Yield 0.150 g (48%). IR (KBr) mmax (cm�1): 3060 (Ar-H),2961–2868 (aliph. C–H), 1607, 1506, 1487, 1394, 1363,1337, 1236, 1184, 1119, 1045, 947, 828, 747. 1H NMR(CDCl3) d (ppm): 7.55–7.46 (m, 8H, Ar-H), 7.38–7.28 (m,12H, Ar-H), 6.79 (m, 8H, Ar-H), 3.83 (m, 16H, CH2–O), 3.52(m, 16H, CH2–O), 1.30 (s, 36H, CH3). UV-vis (CHCl3) kmax

(nm) (e, M�1 cm�1): (e, dm3 mol�1 cm�1): 347 (45700), 615(15100), 682 (69200). MS (ES+) (m/z): 1523 [M]+.C88H96N8O12Zn, calcd.: C 69.39, H 6.35, N 7.36; found: C69.66, H 6.61, N 7.10.

Cobalt(II) phthalocyanine (6)Yield 0.150 g (48%). IR (KBr) mmax (cm

�1): 3072 (Ar-H), 2957–2866 (aliph. C–H), 1609, 1508, 1474, 1408, 1363, 1341, 1238,1182, 1121, 1094, 1062, 957, 751.UV-vis (CHCl3) kmax (nm) (e,M�1 cm�1): 289 (81300), 327 (63100), 615 (38900), 676(112000). MS (ES+) (m/z): 1517 [M + H]+. C88H96N8O12Co,calcd.: C 69.69, H 6.38, N 7.39; found: C 69.88, H 6.12, N 7.06.

Acar et al. Synthesis and electrochemical properties of phthalocyanines

2 © 2013 The Authors. Coloration Technology © 2013 Society of Dyers and Colourists, Color. Technol., 129, 1–8

Copper(II) phthalocyanine (7)Yield 0.105 g (34%). IR (KBr) mmax (cm�1): 3056 (Ar-H),2959–2867 (aliph. C–H), 1603, 1508, 1476, 1404, 1363, 1236,1184, 1121, 1097, 1061, 950, 829, 748. UV-vis (CHCl3) kmax

(nm) (e, M�1 cm�1): 337 (36300), 614 (12000), 682 (34700).MS (ES+) (m/z): 1522 [M + H]+. C88H96N8O12Cu, calcd.: C69.48, H 6.36, N 7.37; found: C 69.66, H 6.61, N 7.20.

Results and DiscussionSynthesis and characterisationGenerally, common substituted phthalocyanines are syn-thesised by cyclotetramerisation of substituted phthalonit-rile. Starting with 4-{2-[2-(4-tert-butylphenoxy)ethoxy]ethoxy} 1 and 4-nitrophthalonitrile 2, the general syn-thetic route for the synthesis of new metal-free andmetallophthalocyanines is given in Schemes 1 and 2. Thesynthesis of phthalonitrile derivative 3 is based on thereaction of 4-{2-[2-(4-tert-butylphenoxy)ethoxy]ethoxy} 1with4-nitrophthalonitrile (in dry DMF), and in the presence ofdry K2CO3 as base, at 50 °C for 72 h. Cyclotetramerisationof the phthalonitrile derivative 3 to the metal-free phthalo-cyanine 4 was accomplished in n-pentanol in the presenceof a few drops of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)

as a strong base at 160 °C in a sealed tube. The metallopht-halocyanines 5–7 were obtained from the anhydrous metalsalts [Zn(CH3COO)2, CoCl2 and CuCl2] in 2-(dimethylamino)ethanol by microwave irradiation. The structures of thetarget compounds were confirmed using IR, 1H NMR,13C NMR and ES-MS mass spectroscopies and elementalanalysis. All the results were consistent with the predictedstructures given in the experimental section.

The IR spectrum of phthalonitrile compound 3 clearlyindicates the presence of a C≡N group by the intensestretching band at 2232 cm�1. The 1H NMR spectrum ofphthalonitrile compound 3 was recorded in CDCl3. In the1H NMR spectrum of compound 3, the OH group of 4-{2-[2-(4-tert-butylphenoxy)ethoxy]ethoxy} disappeared asexpected. Aromatic protons appear at 7.69 (t), 7.49 (d),7.31 (m), 7.01 (d) and 6.85 (m) ppm. The 13C NMR spectrumof compound 3 shows the presence of nitrile carbon atomsat d = 115.66 and 113.90 ppm. In the mass spectra ofphthalonitrile compound 3, the molecular ion peak wasobserved at m/z 365 [M + H]+.

The cyclotetramerisation of dinitriles 3 to phthalocya-nines 4–7 was confirmed by the disappearance of the sharpC≡N vibration at 2232 cm�1. The IR band characteristic ofthe metal-free phthalocyanine ring is N–H stretching at

N

NH N

N N

N

N

NH

R

R

R

R

OO CN

CN

O

OO O

OOHO

CN

CN

O2N

n-pentanolDBU

160 °C

R =

4

+

anhydrousDMF / K2CO3

50 °C

12

3

Scheme 1 The synthesis of the metal-free phthalocyanine 4

N

N N

N N

N

N

N

R

M

R

R

R

OO CN

CN

O

OO O

DMAE

175 °C350W

R =

5 : M = Zn6 : M = Co7 : M = Cu

Scheme 2 The synthesis of the metallophthalocyanines 5–7

Acar et al. Synthesis and electrochemical properties of phthalocyanines

© 2013 The Authors. Coloration Technology © 2013 Society of Dyers and Colourists, Color. Technol., 129, 1–8 3

3285 cm�1. The IR spectra of metal-free phthalocyanine 4and metallophthalocyanines 5–7 are very similar, except form (NH) vibrations of the inner phthalocyanine core in themetal-free molecule. These protons are also very wellcharacterised by the 1H NMR spectrum, which shows apeak at d = �5.65 ppm as a result of the 18 p-electronsystem of the phthalocyanine ring [35]. The 1H NMRspectrum of 4 indicated characteristic aromatic protons at7.68–7.59 (m, 8H, Ar-H), 7.38–7.29 (m, 12H, Ar-H) and7.04 (m, 8H, Ar-H) and aliphatic protons at 4.32 (m, 16H,CH2–O), 4.14 (m, 16H, CH2–O), 1.35 (s, 36H, CH3) ppm. Themass spectrum of 4 displayed the [M + H]+ parent ion peakat m/z = 1460, which confirms the same structure.

In the IR spectra of the metallophthalocyanines (5–7),cyclotetramerisation of 3 was confirmed by the disappear-ance of the sharp C≡N stretching vibration at 2232 cm�1.The 1H NMR spectrum of zinc(II) phthalocyanine com-pound 5 was almost identical to that of hydrogen phthalo-cyanine 4. 1H NMR measurements of the cobalt(II) andcopper(II) phthalocyanines 6 and 7 were precluded owingto their paramagnetic nature. In the mass spectrum ofcompounds 5, 6 and 7 the presence of molecular ion peaksat m/z = 1523 [M]+, 1517 [M + H]+ and 1522 [M + H]+,respectively, confirmed the proposed structures.

The metal-free and metallophthalocyanines display typ-ical electronic spectra with two strong absorption regions,one of them in the UV region at ca. 300–350 nm (B band)and the other in the visible region at 600–700 nm (Q band)[36]. The best indications for phthalocyanine systems aregiven by their UV-vis spectra in solution (Figure 1). In theUV-vis spectrum of metal-free derivative 4 (in CHCl3), splitQ bands appeared at 703, 667, 642 and 607 nm, while the Bband remained at 342 nm. The UV-vis spectra of metallo-phthalocyanines 5–7 (in CHCl3) exhibited an intense singleQ band absorption of p?p* transitions at ca. 682–676 nmand B bands in the UV region at ca. 347–289 nm [37,38].

Figure 2 shows electronic absorption spectra of metal-free phthalocyanine in the concentration range 4 9 10�6–

1.12 9 10�4 M in 1,2-dichlorobenzene (DCB). Beer’s lawwas obeyed for the band at 704 nm (monomer Q band) up toa concentration of 9 9 10�5 M, above which the resultsshowed considerable deviation from linearity (Figure 3).

Similar deviations were obtained for the spectra ofmetallophthalocyanines, except for the one Q band

centred at 674 and 680 nm for cobalt(II) phthalocyanineand zinc(II) phthalocyanine respectively. Increasing theconcentration of hydrogen phthalocyanine caused anincrease in intensity at 642 nm relative to 704 nm, withan isobestic point at 660 nm, indicating the formation ofaggregates. After plotting the dependence of the logarithmof absorption of the associate form vs. the logarithm ofabsorption of the monomer form, it is possible to deter-mine the number of molecules in the associate. The valueof AM (absorbance of monomer) for kmax of the monomerwas easily determined, but AAS (absorbance of associatespecies) was determined from 642 nm, where the contri-bution of monomer absorption is relatively small. Thiscontribution was taken into account by means of molarabsorptivity of the monomer at 642 nm. The plot of logA642 vs log A704 yields a straight line with a slope of2.2 � 0.2, confirming dimer formation in solution (Fig-ure 4).

Determination of the dimerisation equilibrium constantSupposing that the dimer is the main aggregate under theconditions given, the dimerisation equilibrium can bedescribed by:

2M $ D ð1Þ

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

250 350 450 550 650 750

Abs

orba

nce

Wavelength, nm

4 5 6 7

Figure 1 UV-vis spectra of 4, 5, 6 and 7 in CHCl3 (concentration10 9 10�6

M dm�3) [Colour figure can be viewed in the onlineissue, which is available at wileyonlinelibrary.com.]

0

10 000

20 000

30 000

40 000

50 000

500 550 600 650 700 750 800 850

ε , M

–1 c

m–1

λ , nmmax

Figure 2 Electronic absorption spectra of hydrogen phthalocyanine4 in DCB solution at concentrations ranging from 4 9 10�6 to1.12 9 10�4

M dm�3 [Colour figure can be viewed in the onlineissue, which is available at wileyonlinelibrary.com.]

0

0.4

0.8

1.2

1.6

2

0.0 10.0 20.0 30.0 40.0 50.0 60.0

Abs

orba

nce

C, µM

Figure 3 The negative deviation from Beer’s law of hydrogenphthalocyanine 4 at 704 nm in DCB [Colour figure can be viewed inthe online issue, which is available at wileyonlinelibrary.com.]

Acar et al. Synthesis and electrochemical properties of phthalocyanines

4 © 2013 The Authors. Coloration Technology © 2013 Society of Dyers and Colourists, Color. Technol., 129, 1–8

K ¼ ½D�½M�2 ¼

½M�0 � ½M�2½M�2 ð2Þ

where [M]0 and [M] denote the initial and equilibriumconcentration of monomer, respectively, and [D] is theconcentration of dimer. The ratio of the monomer atequilibrium to total concentration a and the molar absorp-tion coefficient of the monomer eM are:

a ¼ ½M�½M�0

ð3Þ

eM ¼ A½M� � l ð4Þ

where A is the absorbance of the solution at the maximumabsorption wavelength of the monomer (kmax) and l is thelength of the optical path. Equation (4) is valid only formonomer existing in the solution. It is impossible tocalculate eM by Eqn 4 because [M] is unknown. It isconvenient to introduce the effective absorption coefficienteeff, which neglects partial association of the monomer andcan be expressed as:

eeff ¼ A½M�0 � l

ð5Þ

Equations 2 to 5 can be combined to give the equation[39,40]:

eeff ¼ eM � 2 � KeM

� e2eff � ½M�0 ð6Þ

Figure 5 shows that a straight line is obtained fromplotting eeff vs e2eff � ½M�0 for hydrogen phthalocyanine. Thedimerisation constant (K) can be evaluated from the com-bination of the slope and intercept values of the plot. Theplot of eeff vs e2eff � ½M�0 for cobalt(II) phthalocyanine yields aline (y = �0.487x + 42413, r2 = 0.980). However, exhaus-tive experiments could not yield a straight line for zinc(II)phthalocyanine owing to deviation from a simple mono-mer–dimer equilibrium established in the solution. Thedetermined equilibrium constants for association of dyeswere KD = 4.8 � 0.6 9 103 and KD = 1.1 � 0.5 9 104 forthe metal-free phthalocyanine and cobalt(II) phthalocyaninerespectively.

Electrochemical measurementsThe solution redox properties of the phthalocyanines werestudied by CV and DPV on a platinum working electrode inDCB containing 0.1 M of tetrabutylammonium hexafluoro-phosphate (TBAHFP). Table 1 lists the assignments of thecouples recorded and the estimated electrochemicalparameters, which include the half-wave potentials (E1/2),the ratio of anodic to cathodic peak currents (Ipa/Ipc),peak-to-peak potential separations (ΔEp) and the differencebetween the first reduction and oxidation processes(ΔE1/2). The potentials are given vs an Ag/AgCl referenceelectrode.

Reduction of an electroactive species is associated withthe energy level of its lowest unoccupied molecular orbital(LUMO), whereas oxidation of an electroactive speciesdepends on the energy level of the highest occupiedmolecular orbital (HOMO). Redox processes of phthalocya-nines occur via successive one-electron transfer between theworking electrode and the p-conjugated ring system of thephthalocyanine. A typical redox reaction yields a long-livedanion or cation radical in non-aqueous solutions. The redoxprocesses in metallophthalocyanines depend on the centralmetal ion, whether possessing an energy level between theHOMO and the LUMO of the phthalocyanine. Complexeswith Co, Fe and Mn, which possess this type of orbital,generally exhibit redox processes via the central metal ion.In contrast to Co and Fe complexes, Zn, Cu and Nicomplexes undergo reduction and oxidation to yield cationand anion radicals respectively [41–45]. The first reductionof cobalt(II) phthalocyanine is always on the Co(II) centre,and the first oxidation is dependent on the solvent used.

The cyclic voltammograms of metal-free phthalocyanine4 exhibited four redox processes in the �1.80 to +1.65 Vrange, as shown in Figure 6. It gives two reduction processes,labelled as R1 at �0.870 V and R2 at �1.155 V, and twooxidation processes, labelled as O1 at 0.978 V and O2 at1.468 V. The variations in the peak separations (ΔEp) withscan rate and Ep/log υ values are used to identify thereversibility of a redox process. The Ep/log υ value of the R1

couple is 40 mV; ΔEp of this couple is 90 mV at 0.025 V s�1

and increases with increasing scan rate. The anodic tocathodic peak ratio Ipa/Ipc of this peak is close to unity, whichindicates the quasi-reversible nature of the process. The

y = 2.160x + 1.018R 2 = 0.999

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

–0.3 –0.2 –0.1 0 0.1 0.2 0.3

–Log

AD

–Log AM

Figure 4 The log plot of absorbance of monomer to associate at642 nm in DCB

y = –0.217x + 44807

R2 = 0.998

30 000

35 000

40 000

45 000

50 000

0 10 000 20 000 30 000 40 000 50 000

ε eff

εeff2 ·[M ]0

Figure 5 The plot of eeff vs e2eff � ½M�0 at 704 nm [■ hydrogenphthalocyanine]

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© 2013 The Authors. Coloration Technology © 2013 Society of Dyers and Colourists, Color. Technol., 129, 1–8 5

linear variation in Ipc vs υ1/2 suggests a diffusion-controlled

electrode reaction. The other reduction and oxidationcouples have similar characteristics, given in Table 1.

The cyclic voltammogram and differential pulse voltam-mogram of zinc(II) phthalocyanine 5 are illustrated inFigure 7. Zinc(II) phthalocyanine gave very similar voltam-mograms to the metal-free phthalocyanine, except forpotential shifts corresponding to the polarising effect of thecentral metal ion. For zinc(II) phthalocyanine, two reductionand two oxidation processes are recorded (Table 1).

The peak current variation with scan rate of the firstreduction and oxidation processes are shown in theFigure 7 inset. The logarithmic plot of peak current vs scanrate for both peaks yields slope values close to 0.5. The peakcurrent ratios are close to unity, indicating that theelectrode reactions are diffusion controlled and reversiblefor the couples. As the second reduction and oxidationprocesses have larger ΔEp values, the second couples areless reversible than the first.

The cyclic voltammogram of cobalt(II) phthalocyanine 6exhibited four redox processes (Figure 8). By scanning thepotential to the negative side, three reversible reductionprocesses were recorded at �0.437, �1.205 and �1.504 V,and, by scanning anodically, one reversible oxidation at0.883 V. In Figure 9, the logarithm of the peak current vsthe logarithm of the scan rate of the peaks yields slope

values of 0.52, 0.54 and 0.35 for the cathodic reactions, andthe oxidation peak has a value of 0.62 (for clarity, only thefirst peaks are shown). The peak ratios and slope valuesindicate simple electron transfer reactions. There is noevidence for a pre/post chemical step or adsorption controlof the electrode reaction.

The separation between the first and second ring reduc-tions was found to be approximetaly 0.4 V for zinc(II)phthalocyanine, in accordance with literature valuesobtained for complexes that have a redox-inactive metalcentre. The differences in the electrochemical behaviour ofzinc(II) and metal-free phthalocyanines are the position ofthe redox potentials of the couples and the differencebetween the first oxidation and reduction processes (ΔE1/2)[41–45]. The introduction of a zinc ion into the phthalocy-anine ring can be considered as a perturbation of thefrontier molecular orbitals of the phthalocyanine molecule.The introduction of a metal ion results in an increase in thenegative charge of the ring system, and thus the systembecomes harder to reduce. Comparison of the first reductionpotential of the metal-free phtalocyanine and zinc deriva-tive clearly shows a more negative potential required for thereduction of the system.

The first and second reductions of the cobalt(II) phthalo-cyanine had a potential difference of approximately 0.768 V,

Table 1 Electrochemical data of the phthalocyaninesa

Compound Redox step Ep, V E1/2 ΔEp, mV Ipa/Ipc ΔE1/2, V

H2Pcb R1 �0.870 (�0.760) �0.815 110 0.92 1.727

R2 �1.155 (�1.046) �1.100 109 0.93O1 0.978 (0.846) 0.912 132 0.93O2 1.468 (1.402) 1.435 66 0.71

CoPcc R1 �0.437 (�0.302) �0.367 135 0.94 1.205R2 �1.205 (�1.153) �1.179 52 0.83R3 �1.504 (�1.407) �1.456 97 0.99O1 0.883 (0.792) 0.838 91 0.81

ZnPcd R1 �1.103 (�1.005) �1.054 98 0.92 1.718R2 �1.537 (�1.380) �1.458 157 0.65O1 0.693 (0.636) 0.664 57 0.83O2 1.395 (1.302) 1.348 93 1.09

a Scan rate 100 mV s�1; b Hydrogen phthalocyanine; c Cobalt(II) phthalocyanine; d Zinc(II) phthalocyanine

–1.0

–0.5

0.0

0.5

–2.0–1.0 –1.5–0.51.0 0.5 0.01.52.0

I, µA

E, V

Figure 6 Cyclic and differential pulse voltammograms of metal-freephthalocyanine at a 0.100 V s�1 scan rate on Pt in DCB/TBAHFP(hydrogen phthalocyanine concentration 1 9 10�4

M dm�3; pulsewidth 50 ms; pulse height 50 mV; step height 4 mV; step time200 ms) [Colour figure can be viewed in the online issue, which isavailable at wileyonlinelibrary.com.]

0.30

0.00

I, µA

2.0 1.5 1.0

E, V

0.5 0.0

1.2

0.8

I p

0.4

00.00 0.25 0.50 0.75 1.00

ϑ1/2

–0.5 –1.0 –1.5 –2.0

–0.30

–0.60

Figure 7 Cyclic and differential pulse voltammograms of zinc(II)phthalocyanine at a 0.100 V s�1 scan rate on Pt in DCB/TBAHFP[zinc(II) phthalocyanine concentration 1 9 10�4

M dm�3; pulsewidth 50 ms; pulse height 50 mV; step height 4 mV; step time200 ms]. Inset: the peak current variation of zinc(II) phthalocyaninewith scan rate (■ cathodic; ♦ anodic) [Colour figure can be viewed inthe online issue, which is available at wileyonlinelibrary.com.]

Acar et al. Synthesis and electrochemical properties of phthalocyanines

6 © 2013 The Authors. Coloration Technology © 2013 Society of Dyers and Colourists, Color. Technol., 129, 1–8

which was in aggrement with other cobalt(II) phthalocyaninecomplexes. The lower ΔE1/2 gap suggested that the firstreduction of the cobalt(II) phthalocyanine originated fromthe redox couple [Co(II)Pc(�2)]/[Co(I)Pc(�2)] in DCB [46].

ConclusionThe novel phthalonitrile derivative 3 substituted witha 2-[2-(4-tert-butylphenoxy)ethoxy]ethanol group wasprepared and used to synthesise metal-free phthalocyanine4 and zinc(II), cobalt(II) and copper(II) phthalocyaninecomplexes 5–7 for the first time. The new phthalonitrilecompound as well as the metal-free and metallophthalocy-anines have been characterised by UV-vis, IR, 1H NMR,13C NMR, ES-MS and elemental analysis. All these newphthalocyanine complexes showed excellent solubility inorganic solvents such as chloroform, tetrahydrofuran, DMF,ethyl acetate and DCB. The spectroscopic studies revealedthat considerable aggregation occurs with increasing con-centration of the phthalocyanine in DCB solution. Theassociates were found mainly in the form of dimers. Theorders of dimerisation constants are comparable withprevious studies in the literature. Electrochemical studiesof the metal-free phthalocyanine and its metal complexesshowed that the reduction potential of zinc(II) phthalocya-nine shifted to a more negative potential as a result ofcoordination to the zinc ion causing electron enrichment ofthe ring system. The electrochemical behaviour of cobalt(II)phthalocyanine differs from that of hydrogen and zinc(II)phthalocyanines in a metal-based reduction. All the pht-halocyanines exhibited reversible or quasi-reversible diffu-sion-controlled redox couples, as expected.

AcknowledgementsThis study was supported by the Research Fund ofKaradeniz Technical University (Project No. 8660) andGiresun University (Project No. 140411-22).

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–3.5

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–1.5

–0.5

0.5

1.5

–2.0–1.5–1.0–0.50.00.51.01.52.0

I, µA

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Figure 8 Cyclic and differential pulse voltammograms of cobalt(II)phthalocyanine at a 0.100 V s�1 scan rate on Pt in DCB/TBAHFP[cobalt(II) phthalocyanine concentration 1 9 10�4

M dm�3; pulsewidth 50 ms; pulse height 50 mV; step height 4 mV; step time200 ms) [Colour figure can be viewed in the online issue, which isavailable at wileyonlinelibrary.com.]

y = 0.523x + 0.645

R 2 = 0.992

y = 0.612x + 0.640

R 2 = 0.991–0.6

–0.2

0.2

0.6

1

–2 –1.5 –1 –0.5 0 0.5

Log

I p

Log υ

Figure 9 The log Ip variation with log υ of cobalt(II) phthalocyanine6 (♦ cathodic; ● anodic)

Acar et al. Synthesis and electrochemical properties of phthalocyanines

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Acar et al. Synthesis and electrochemical properties of phthalocyanines

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