Electrochemical and spectroelectrochemical properties of new metal free, nickel(II), lead(II) and zinc(II) phthalocyanines

Synthetic Metals 217 (2016) 295–303

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Electrochemical and spectroelectrochemical properties of new metal free, nickel(II), lead(II) and zinc(II) phthalocyanines ba Sakaa , Rabia Zeynep Uslu Kobakb , Hakan Alpa , Gülbınar Sarkıa , Atıf Kocac , Ece Tug Halit Kantekina,* a b c

Department of Chemistry, Faculty of Science, Karadeniz Technical University, 61080 Trabzon, Turkey Department of Chemistry, Faculty of Science, Istanbul Technical University, Maslak, Istanbul, Turkey Department of Chemical Engineering, Engineering Faculty, Marmara University, 34722 Göztepe, Istanbul, Turkey

A R T I C L E I N F O

Article history: Received 21 December 2015 Received in revised form 25 March 2016 Accepted 6 April 2016 Available online xxx Keywords: Phthalocyanine Metal free Nickel Lead Zinc Electrochemistry Spectroelectrochemistry

A B S T R A C T

The new metal free, nickel(II), lead(II) and zinc(II) phthalocyanines containing 2-[2-(4-allyl-2methoxyphenoxy)ethoxy]ethoxy groups were synthesized and structurally characterized by using IR, 1 H NMR, 13C NMR, UV–vis and elemental analysis spectral data. Redox properties of the complexes were determined with voltammetric and in situ spectroelectrochemical measurements in different electrolytic systems, tetrabutylammonium perchlorate (TBAP) dissolved dichloromethane (DCM) and dimethylsulfoxide (DMSO). All complexes gave two reversible reduction couples in the cathodic sides of the voltammograms in TBAP/DCM. While two oxidation processes were observed for all complexes in DCM, these processes were recorded as broad and split waves. All of the redox processes of the complexes shifted to the negative potentials and behaved more reversible in TBAP/DMSO electrolyte. Due to the narrow anodic potential window of DMSO, only one oxidation process was recorded in DMSO, while third reduction processes could be observed at more negative potentials for all complexes. Changing of the metal center of the complexes caused to shifting of the redox processes due to the different effective nuclear charge on the metal ions of the complexes. It can be easily concluded that the results of the voltammetric and in situ spectroelectrochemical supported the proposed structure of the complexes. ã 2016 Published by Elsevier B.V.

1. Introduction Metallophthalocyanine (MPc) complexes have become very important during recent years due to their unique optical, electronic, catalytic and structural properties [1–3]. These unique properties lead to many applications in different scientific and technological areas such as photoconducting agents in photocopying devices, chemical sensors, catalysis, liquid crystals, photodynamic therapy of cancer, solar energy conversion, nonlinear optics, semiconductors, and optical data storage [4–12]. The applicability especially in some of these areas including electrocatalysis, electrochromism, and energy producing devices such as fuel cells and some optoelectronic devices is closely related to their unique electron transfer properties. Therefore, the identification of the redox properties of newly synthesized metallophthalocyanine (MPc) compounds has vital importance in determining the possibility of the usage in these technological

* Corresponding author. E-mail address: [email protected] (H. Kantekin). http://dx.doi.org/10.1016/j.synthmet.2016.04.004 0379-6779/ ã 2016 Published by Elsevier B.V.

applications. Some MPcs, especially iron, cobalt and manganese Pcs have been used in catalytic reactions because of their central metal based redox activity [13–18]. Owing to low solubility of phthalocyanines in common organic solvents and water, application of phthalocyanines is limited. For this reason, one of the main aims of research on the phthalocyanines is to increase their solubility in common organic solvents. Low solubility of phthalocyanines in common organic solvents can be overcome by introducing different kinds of substituents such as alkyl, alkoxy, phenoxy, macrocyclic groups [19,20] in common organic solvents and amino, sulfo or carboxyl groups in water [21,22]. In this work, we have synthesized metal free, nickel(II), lead(II) and zinc(II) phthalocyanines with 2-[2-(4-allyl-2-methoxyphenoxy)ethoxy]ethoxy functionalized groups. Thanks to these functionalized groups, metal free, nickel(II), lead(II) and zinc(II) phthalocyanines complexes 2–5 can readily dissolve in organic solvents [23]. In this paper, effects of the 2-[2-(4-allyl-2methoxyphenoxy)ethoxy]ethoxy substituents to the redox activity of the metal free, nickel(II), lead(II) and zinc(II) phthalocyanines complexes 2–5 were examined with different voltammetric

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techniques and in situ spectroelectrochemical measurements in different electrolytic systems.

Elemental Analysis C88H90N8O16: calcd. C 69.73; H 5.98; N 7.39, found: C 69.47; H 5.84; N 7.71.

2. Experimental

2.3.2. General procedure of metal phthalocyanines (3–5) To give the metal phthalocyanines, the mixture of phthalonitrile compound 1 (0.2 g, 0.53 mmol), the related anhydrous metal salt NiCl2 (33.7 mg, 0.26 mmol) for compound 3, PbCl2 (72.32 mg, 0.26 mmol) for compound 4, Zn(CH3COO)2 (0.045 mg, 0.26 mmol) for compound 5 and four drops of DBU was heated at 160  C in dry npentanol (3 mL) in a sealead tube, and stirred for 24 h. At the end of the reaction, green product was precipitated by addition of ethanol (20 mL) and filtered off. Along 2 h, the green solid product was refluxed with ethanol (30 mL), filtered off again and washed with hot ethanol, distilled water and diethyl ether. After drying under vacuum, the product was chromatographed on basic alumina column with chloroform–methanol (99:1) for compound 3, (95:5) for compound 4, (99:1) for compound 5 solvent system as eluent.

2.1. Materials 4-Nitrophthalonitrile were prepared according to the literature [24], all reagents and solvents were of reagent grade quality and were obtained from commercial suppliers. All solvents were dried and purified as described by Perrin and Armarego [25]. 2.2. Equipment The IR spectra were recorded on a Perkin Elmer 1600 FT-IR spectrophotometer, using KBr pellets. 1H and 13C NMR spectra were recorded on a Bruker Avance III 400 MHz spectrometers in CDCl3 and chemical shifts were reported (d) relative to Me4Si as internal standard. Optical spectra in the UV–vis region were recorded with a Perkin Elmer Lambda 25 spectrophotometer. Melting points were measured on an electrothermal apparatus and are uncorrected. GC Agilent Technologies 7820A equipment (30 m
0.32 mm
0.50 mm DB Wax capillary column, FID detector) was used GC measurements. Elementel analysis were recorded on Costech ECS 4010 Spectrometer. 2.3. Synthesis 2.3.1. Metal free phthalocyanine (2) A mixture of 4-[2-(2-(4-allyl-2-methoxyphenoxy)ethoxy)ethoxy]phthalonitrile 1 (0.2 g, 0.53 mmol), four drops of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), and 3 mL of n-pentanol was taken in a reaction tube. The mixture was heated in the sealed glass tube for 24 h under dry inert atmosphere at 160  C. The reaction mixture was precipitated by adding aqueous acetic acid. The precipitate was filtered, washed with hot ethanol, and acetonitrile. After drying under vacuum, the product was chromatographed on a basic alumina column with chloroform–methanol (99:1) solvent system as eluent. Yield: 100 mg (47%). FT-IR nmax/cm 1 (KBr pellet): 3291 (N H), 3074–3007 (Ar H), 2931–2837 (Aliph. CH), 1613, 1504, 1417, 1339, 1321, 1264, 1214, 1150, 1120, 1091, 1008, 919, 818, 742. 1H NMR. (CDCl3), (d:ppm): 8.17–8.09 (m, 12H, Ar H), 7.50–7.01 (m, 12H, ArH), 5.90 (m, 4H,  CH = ), 5.26 (m, 8H, ¼CH2), 4.32 (t, 16H, O CH2), 3.83 (s, 12H,  OCH3), 3.29 (d, 8H, CH2), 2.02 (m, 16H, CH2O), 4.58 (s, 2H, NH). 13C NMR. (CDCl3), (d:ppm): 159.92, 151,67, 149.68, 146, 76, 143.36, 137.47, 133.39, 123.32, 122.14, 121.32, 120.56, 119.50, 110.40, 110.19, 103.48, 107.27, 70.10, 69.39, 68.89, 68.50, 68.00, 58.82, 55.76. UV–vis (CHCl3): lmax, nm (log e): 702 (5.00), 672 (4.96), 609 (4.43), 342 (4.81).

O

O OCH3

OR

RO N

RO

CN

R=

N

N CN 1

(i)

N

M

N

N

N N

OR RO M= 2H, Ni(II), Pt(II), Zn(II) (2-5) Fig. 1. The synthesis of new Metal free, Ni(II), Pb(II) and Zn(II) phthalocyanines. Reagent and condition: (i) n-pentanol, DBU, 160  C, as metal salts NiCl2, PbCl2 and Zn (CH3COO)2 with 47%,47%, 45% and 40% yield.

2.3.3. Synthesis of Nickel(II) phthalocyanine (3) Yield: 98 mg (47%). FT-IR nmax/cm 1 (KBr pellet): 3083 (Ar-H), 2977–2856 (Aliph. C H), 1699, 1680, 1656, 1608, 1578, 1556, 1523, 1448, 1355, 1286, 1242, 1188, 1066, 998, 938, 903, 856, 788, 675, 542. 1H NMR. (CDCl3), (d:ppm): 7.45-7.02 (m, 12H, Ar-H), 6.95–6.67 (m, 12H, Ar-H), 6.25 (m, 4H, CH = ), 5.02 (m, 8H, ¼CH2), 3.45 (m, 16H, O CH2), 3.39 (s, 12H, O CH3), 3.20 (d, 8H,  CH2), 1.31 (m, 16H, CH2 O). 13C NMR. (CDCl3), (d:ppm): 166.32, 150,89, 135.26, 134.88, 124,39, 124.23, 122.89, 122.64, 121.16, 120.85, 118.67, 118.22, 110.77, 110.34, 103.70, 111.34, 83.24, 78.60, 65.43, 62.76, 59.91, 55.82. UV–vis (CHCl3): lmax, nm (log e): 675 (5.12), 612 (4.48), 331 (4.76). Elemental Analysis C88H88N8O16Ni: calcd. C 67.21; H 5.64; N 7.12, found: C 66.94; H 5.27; N 7.08. 2.3.4. Synthesis of lead(II) phthalocyanine (4) Yield: 102 mg (45%). FT-IR nmax/cm 1 (KBr pellet): 3078 (Ar H), 2957–2876 (Aliph. CH), 1818, 1786, 1663, 1600, 1515, 1422, 1403, 1386, 1295, 1254, 1199, 1168, 1096, 1071, 956, 921, 856, 818, 734, 703, 645, 582. 1H NMR. (CDCl3), (d:ppm): 7.12–6.68 (m, 12H, Ar H), 6.56–6.27 (m, 12H, Ar H), 6.14 (m, 4H,  CH¼), 5.32 (m, 8H, ¼CH2), 3.81 (m, 16H, O CH2), 3.69 (s, 12H, O CH3), 3.50 (d, 8H, CH2), 1.42 (m, 16H, CH2 O). 13C NMR. (CDCl3), (d:ppm): 166.32, 155,12, 141.02, 138.72, 128,88, 125.61, 124.17, 122.34, 121.45, 120.61, 119.98, 118.45, 117.51, 110.67, 110.43, 110.30, 84.45, 78.78, 64.13, 60.45, 58.60, 54.01. UV–vis (CHCl3): lmax, nm (log e): 669 (5.16), 616 (4.56), 338 (4.18). Elemental Analysis C88H88N8O16Pb: calcd. C 61.42; H 5.15; N 6.51, found: C 62.01; H 5.19; N 7.00. 2.3.5. Synthesis of zinc(II) phthalocyanine (5) Yield: 110 mg (40%). mp > 300  C. FT-IR nmax/cm 1 (KBr pellet): 3063 (Ar H), 2961–2826 (Aliph. C H), 1768, 1716, 1637, 1603, 1505, 1446, 1417, 1336, 1265, 1216, 1149, 1119, 1086, 994, 911, 816, 742, 649, 597. 1H NMR. (CDCl3), (d:ppm): 7.25-6.99 (m, 12H, Ar H), 6.80-6.77 (m, 12H, Ar H), 6.01 (m, 4H, CH = ), 5.14 (m, 8H, ¼CH2), 3.75 (m, 16H, O CH2), 3.69 (s, 12H, O CH3), 3.42 (d, 8H,  CH2), 1.27 (m, 16H, CH2 O). 13C NMR. (CDCl3), (d:ppm): 164.17, 151,32, 137.41, 136.82, 125,28, 124.40, 122.39, 121.54, 121.36, 120.45, 119.03, 119.40, 119.30, 110.90, 110.36, 110.18, 73.14, 72.50, 62.03, 61.56, 58.82, 54.42. UV–vis (CHCl3): lmax, nm (log e): 678 (5.25), 610 (4.28), 332 (4.56). Elemental Analysis C88H88N8O16Zn: calcd. C 66.93; H 5.62; N 7.10, found: C 66.64; H 5.05; N 7.60. 2.4. Electrochemical and in situ spectroelectrochemical measurements The cyclic voltammetry (CV) and square wave voltammetry (SWV) measurements were carried out with Gamry Reference 600 potentiostat/galvanostat utilizing a three-electrode configuration at 25  C. The working electrode was a Pt disc with a surface

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297

Fig. 2. UV–vis Spectrum of metal free 2, Ni(II) 3, Pb(II) 4 and Zn(II) 5 phthalocyanines.

Table 1 Voltammetric data of the complexes. All voltammetric data were given versus SCE. Complexes

Redox processes

a

E1/2 (V)

b

DEp (mV)

c

Ip,a/Ip,

d

DE1/2

c

H2Pc (in DCM)

[H2Pc1]1+/[H2Pc ]2+ [H2Pc2]/[H2Pc1]1+ [H2Pc2]/[H2Pc3]1 [H2Pc3]1/[H2Pc4]2

1.19 0.69 0.53 0.88

80 75 62 63

0.83 0.55 0.95 0.92

1.32

H2Pc (in DMSO)

[H2Pc2]/[H2Pc1]1+ [H2Pc2]/[H2Pc3]1 [H2Pc3]1/[H2Pc4]2 [H2Pc4]2/[H2Pc5]3

0.67 0.50 0.86 1.75

80 55 60 –

0.83 0.55 0.95 –

1.17

NiPc (in DCM)

[NiIIPc1]1+/[NiIIPc ]2+ [NiIIPc2]/[NiIIPc1]1+ [NiIIPc2]/[NiIIPc3]1 [NiIIPc3]1/[NiIIPc4]2

1.15 0.62 0.67 1.03

71 – 95 96

0.90 – 0.97 0.93

1.29

ZnPc (in DCM)

[ZnIIPc1]1+/[ZnIIPc ]2+ [ZnIIPc2]/[ZnIIPc1]1+ [ZnIIPc2]/[ZnIIPc3]1 [ZnIIPc3]1/ [ZnIIPc4]2

1.49 0.70 0.83 1.14

105 80 96 100

0.65 0.92 0.95 0.89

1.53

ZnPc (in DMSO)

[ZnIIPc2]/[ZnIIPc1]1+ [ZnIIPc2]/[ZnIIPc3]1 [ZnIIPc3]1/ [ZnIIPc4]2 [ZnIIPc4]2/ [ZnIIPc5]3

0.66 0.85 1.24

76 65 54

0.55 0.95 0.92

1.51

1.98

120

0.45

[PbIIPc1]1+/[PbIIPc ]2+ [PbIIPc2]/[PbIIPc1]1+ [PbIIPc2]/[PbIIPc3]1 [PbIIPc3]1/ [PbIIPc4]2

1.26 0.84 0.54 0.87

68 62 60 61

0.75 0.93 0.97 0.94

1.38

[PbIIPc2]/[PbIIPc1]1+ [PbIIPc2]/[PbIIPc3]1 [PbIIPc3]1/ [PbIIPc4]2 [PbIIPc4]2/ [PbIIPc5]3

0.68 0.64 0.91

64 57 59

0.71 0.98 0.97

1.32

1.73

71

0.44

PbPc (in DCM)

PbPc (in DMSO)

e

area of 0.071 cm2. A Pt wire served as the counter electrode. Saturated calomel electrode (SCE) was employed as the reference electrode and separated from the bulk of the solution by a double bridge. Electrochemical grade TBAP in extra pure dichloromethane (DCM) and dimethylsulfoxide (DMSO) was employed as the supporting electrolyte at a concentration of 0.10 mol dm3. Ferrocene was used as universal indicator and DEp’s of ferrocene were changed from 60 to 110 mV with increasing scan rates from 0.010 to 1.00 Vs1 in our system. UV–vis absorption spectra and chromaticity diagrams were measured with a OceanOptics QE65000 diode array spectrophotometer. In situ spectroelectrochemical measurements were carried out by utilizing a three-electrode configuration of thinlayer quartz thin-layer spectroelectrochemical cell at 25 C. The working electrode was a Pt gauze electrode. A Pt wire counter electrode separated by a glass bridge and a SCE reference electrode separated from the bulk of the solution by a double bridge were used. In situ electrocolorimetric measurements, under potentiostatic control, were obtained using an OceanOptics QE65000 diode array spectrophotometer at color measurement mode by utilizing a three-electrode configuration of thin-layer quartz spectroelectrochemical cell. The standard illuminant A with 2  observer at constant temperature in a light booth designed to exclude external light was used. Prior to each set of measurements, background color coordinates (x,y, and z values) were taken at open-circuit, using the electrolyte solution without the complexes under study. During the measurements, readings were taken as a function of time under kinetic control, however only the color coordinates at the beginning and final of each redox processes were reported. 3. Results and discussion 3.1. Synthesis and characterization

Ep value of aggregated species given in parentheses. a E1/2 values ((Epa + Epc)/2) were given versus SCE) at 0.100 Vs1 scan rate. b DEp = Epa  Epc. c Ip,a/Ip,c for reduction,Ip,c/Ip,a for oxidation processes. d DE1/2 = E1/2 (first oxidation)  E1/2 (first reduction).

General synthetic routes for the synthesis of new tetra peripherally metal free 2, nickel(II) 3, lead(II) 4 and zinc (II) 5 phthalocyanines bearing 2-[2-(4-Allyl-2-methoxyphenoxy)ethoxy]ethoxy groups are given in Fig. 1. The structure of the phthalocyanines 2-5 were verified by FTIR, 1H NMR, 13C NMR and UV–vis spectroscopic methods, as well as by elemental analysis. All the analytical spectral data are consistent with the predicted structures. The self-condensation of the dicyano compound 1 in a high-boiling solvent in the presence of a few drops 1,8-diazabicyclo

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Fig. 3. (a) CVs of H2Pc (5.0
105 mol dm3) recorded at various scan rates and (b) SWVs of H2Pc recorded at 0.100 Vs1 scan rate on a Pt working electrode in DCM/ TBAP.

Fig. 4. (a) CVs of NiPc (5.0
105 mol dm3) recorded at various scan rates and (b) SWVs of NiPc recorded at 0.100 Vs1 scan rate on a Pt working electrode in DCM/ TBAP.

[5.4.0] undec-7-ene (DBU) as a strong base at reflux temperature under a nitrogen atmosphere afforded the metal-free phthalocyanine 3. Conversion of 1 into metallophthalocyanine derivatives 3,4 and 5 were accomplished through the usual cyclotetramerization reactions in the presence of metal salts (NiCl2, PbCl2, Zn(CH3COO)2) and n-pentanol as solvent. Cyclotetramerization of the 4-[2-(2-(4Allyl-2-methoxyphenoxy)ethoxy)ethoxy]phthalonitrile 1 to phthalocyanines 2-5 was confirmed by the disappearance of the sharp  C¼N vibration at 2230 cm1. The sharp peak in the IR spectra for the C¼N vibration of phthalonitrile 1 at 2230 cm1 disappeared after conversion into metal-free phthalocyanine, indicative of metal-free phthalocyanine formation. IR bands

characteristic of the metal-free phthalocyanine ring is an N H stretching at 3291 cm1. The IR spectra of metal-free 2 and metallophthalocyanines 3, 4 and 5 are very similar, except these y (NH) vibrations of the inner phthalocyanine core in the metal-free molecule. The NH proton of metal-free phthalocyanine was also identified in the 1H NMR spectrum with a broad peak at d = 4.58 ppm, presenting the typical shielding of inner core protons, which is a common feature of the 1H NMR spectra of metal-free phthalocyanines [26,27]. The 1H NMR spectrum of metal-free phthalocyanine 2 is somewhat broader than the corresponding signals in the starting 4-[2-(2-(4-allyl-2-methoxyphenoxy)ethoxy)ethoxy]phthalonitrile 1. This broadening is likely to be due to chemical exchange caused by an aggregation–disaggregation equilibria and the fact that the product obtained in these reactions is a mixture of positional isomers which are expected to show chemical shifts that differ slightly from each other. The other resonances related to CH2¼, OCH2, OCH3, ¼CH2, CH2 and Ar–H protons in the 1H NMR spectra of 2-5 are very similar to that of the 4-[2-(2-(4-Allyl-2-methoxyphenoxy)ethoxy)ethoxy] phthalonitrile 1. After conversion of the 4-[2-(2-(4-Allyl-2-methoxyphenoxy) ethoxy)ethoxy]phthalonitrile 1 into metallophthalocyanines 3–5 the sharp peak in the IR spectra for the  C¼N vibration at 2230 cm1 completely disappeared. 1H NMR spectrum of metallophthalocyanines 3-5 indicated the characteristic chemical shifts and in accordance with proposed structures. The 1H NMR spectrum of 3–5 display the characteristic aromatic proton, CH2¼, OCH2, OCH3, ¼CH2, CH2 signals and confirm the proposed structures. UV-vis spectroscopy is one of the best spectroscopic technique for determination of the formation of phthalocyanines. This spectroscopy is a quite useful method for characterization of phthalocyanine compounds. Generally, two absorption bands are observed for phthalocyanine compounds in their electronic absorption spectra. One of them is observed at around 600– 750 nm due to the p-p* transitions from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the phthalocyanine ring and known as Q and the other one is observed in the ultraviolet region of spectrum at around 300–450 nm arising from deeper p-levels/LUMO and known as B or Soret band [28]. UV–vis spectra of metal-free phthalocyanine (Fig. 2) in CHCl3, the characteristic split Q band was exhibited with absorptions at 702 and 672 nm which can be attributed a1u ! eg transition [29]. A typical spectrum of the metal-free phthalocyanine 2 in CHCl3 was observed a B band region at 342 nm. The ground state electronic spectra of the compounds 3-5 showed characteristic absorptions in the Q band region at 675 nm for compound 3, 669 nm for compound 4, 678 for compound 5 in CHCl3. B band absorptions of the nickel(II) 3, lead(II) 4 and zinc(II) 5 phthalocyanines were observed at 331, 338, and 332 nm, respectively. In addition to these results, elemental analysis data of all compounds confirmed the proposed structures. 3.2. Voltammetric measurements In order to investigate redox activity of the complexes, CV and SWV responses of metal free, Ni(II), Zn(II), and Pb(II) phthalocyanine complexes were measured in DCM/TBAP and DMSO/TBAP electrolyte systems on a GCE working electrode. Analysis results of CV and SWVs responses are tabulated in Table 1. Basic voltammetric parameters, the assignments of the redox couples and estimated electrochemical parameters including the half-wave peak potentials (E1/2), ratio of anodic to cathodic peak currents (Ip, a/Ip,c), peak to peak potential separations (DEp), peak with (DEp), and difference between the first oxidation and reduction processes

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Fig. 5. (a) CVs of ZnPc (5.0
105 mol dm3) recorded at various scan rates and (b) SWVs of ZnPc recorded at 0.100 Vs1 scan rate on a Pt working electrode in DCM/ TBAP.

Fig. 6. (a) CVs of PbPc (5.0
105 mol dm3) recorded at various scan rates and (b) SWVs of PbPc recorded at 0.100 Vs1 scan rate on a Pt working electrode in DCM/ TBAP.

(DE1/2), are derived from the analyses of the complexes. All complexes give very similar redox responses as shown in Table 1. While all complexes gives two (in DCM) or three (in DMSO) chemically and electrochemically reversible reduction processes, broad and ill-defined oxidation processes are observed for all complexes. Fig. 3 illustrates CV and SWV responses of metal free phthalocyanine 2 recorded in DCM/TBAP electrolyte system. Metal free phthalocyanine 2 undergoes two reductions, IIIc/IIIa couple at 0.53 V (DEp = 62 mV and Ip,a/Ip,c = 0.95) and IVc/IVa couple at 0.88 V(DEp = 63 mV and Ip,a/Ip,c = 0.92). Both of the reduction

299

processes have chemically and electrochemically reversible characters with respect to DEp, Ip,a/Ip,c, and [Ip versus n1/2] data [30,31]. However oxidation processes of the complex (Ic/Ia and IIc/ IIa couples) have electrochemically and chemically quasi-reversible peak characters and they give broad and small peaks. This unusual behavior of the complex may be due to the aggregation of the complex. When compared with the similar papers in the literature, DE1/2 value (1.32 V) of metal free phthalocyanine 2 is smaller than HOMO-LUMO gap of the similar complexes [28,32]. Nickel(II) 3 (Fig. 4) and zinc(II) phthalocyanine 5 (Fig. 5) complexes give similar CV and SWV responses with metal free phthalocyanine 2 in DCM/TBAP electrolyte system. Both complexes undergo two reduction reactions and two oxidation reactions. While metal free phthalocyanine 2 gives reversible reductions and quasi-reversible oxidation processes, differently, reduction processes of nickel(II) 3 and zinc(II) phthalocyanines 5 get electrochemically irreversible peak characters with respect to large DEp values. Like metal free phthalocyanine 2, oxidation processes of nickel(II) phthalocyanine reflect broad and small waves (Ic/Ia and IIc/IIa couples). However oxidation processes of zinc(II) phthalocyanine are different than those of metal free phthalocyanine 2 and nickel(II) phthalocyanine 3. While oxidation couples of zinc(II) phthalocyanine 5 have similar peak currents and peak shapes with the reduction peaks, Ic/Ia couple of zinc(II) phthalocyanine 5 shifts to more positive potentials with respect to those of metal free phthalocyanine 2 and nickel(II) phthalocyanine 3. When easy of the reduction reactions were compared with each other, the order of “2 > 3 >5” was obtained. This order is directly related with the order of the effective nuclear charges of the ions in the Pc core of the complexes. Like metal free phthalocyanine 2, DE1/2 value (1.29 V) of nickel(II) phthalocyanine 3 complex is smaller than the similar complexes those reported in the literature [33]. However DE1/2 value (1.53 V) of zinc(II) phthalocyanine 5 is bigger than those of the other complexes. Lead(II) phthalocyanine 4 complex has different structure than other complexes. It is known that H+, Ni2+, and Zn2+ fit into the cavity of the Pc ring, thus these complexes have planar structure. But Pb2+ does not fit into the cavity of Pc and locate out of the planar Pc ring, so incorporating of Pb2+ into Pc core gives a distorted planar structure. PbPc complexes gave different electrochemical responses due to the distorted planar structure of the complexes. PbPc complexes are generally demetallized during the redox reactions due to the chemical instability of the complexes. Decree of the instabilities of these complexes changed with the substituents of the complexes. For instance it was introduced in our previous studies that while PbPcs substituted with [tetrakischloro-tetrakis-alkylthio] and [tetrakis chloro-tetrakis-alkylmalonyl] moieties demetallized before the first reduction reaction [34]. In another paper, tetra-(1,1-(dicarbpenthoxy)-2-(4-biphenyl)-ethyl), tetra-(1,1-(dicarbpenthoxy)-2-(1-naphthyl)-ethyl) and tetra-((1,1,2-(tricarbopentoxyethyl)) substituted PbPcs demetallized during the first reduction and oxidation reactions [35]. Differently in this paper, 4-[2-(2-(4-Allyl-2-methoxyphenoxy) ethoxy] substituted PbPc gives chemically and electrochemically reversible reduction processes and does not demetallize until the third reduction reaction in both of DCM/TBAP and DMSO/TBAP electrolyte systems. Fig. 6 shows CV and SWV responses of lead(II) phthalocyanine 4 recorded in DCM/TBAP electrolyte system. Lead(II) phthalocyanine 4 gives two reductions, IIIc/IIIa couple at 0.54 V (DEp = 60 mV and Ip,a/Ip,c = 0.97) and IVc/IVa couple at 0.87 V(DEp = 61 mV and Ip,a/Ip,c = 0.94) and two oxidation reactions, Ic/Ia couple at 0.84 V (DEp = 62 mV and Ip,a/Ip,c = 0.93) and IIc/IIa couple at 1.26 V(DEp = 68 mV and Ip,a/Ip,c = 0.75). As shown in Fig. 6 and Table 1, all redox processes are reversible chemically and electrochemically. These voltammetric data indicates stability of neutral and reduced forms of lead(II)

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Fig. 7. (a) CVs of PbPc (5.0
105 mol.dm3) recorded at various scan rates and (b) SWVs of PbPc recoded at 0.100 Vs1 scan rate on a Pt working electrode in DMSO/ TBAP.

phthalocyanine 4 complex with in the voltammetric measurement time scales. Like metal free phthalocyanine 2 and nickel(II) phthalocyanine 3 complexes, DE1/2 value (1.38 V) of lead(II) phthalocyanine 4 is smaller than those reported in the literature.

In order to illustrate the effects of the electrolyte system to the redox processes of the complexes, CV and SWV measurements of some complexes were measured in DMSO/TBAP electrolyte system instead of DCM/TBAP. As shown in Table 1, all complexes give three reduction and one oxidation reactions in DMSO/TBAP. Due to the large negative potential window of DMSO, third reduction processes were observed at more negative potentials for all complexes. Only one oxidation processes were recorded with in the narrow positive potential window of DMSO. When compared with the redox processes recorded in DCM, the redox processes of the complexes are generally more reversible chemically and electrochemically in DMSO. Another apparent effect of the DMSO to the redox reactions is the shifting of these processes to the more negative potentials due to the higher polarizing effect of DMSO with respect to DCM. CV and SWV responses of lead(II) phthalocyanine 4 recorded in DMSO/TBAP electrolyte system is discussed here as an example (Fig. 7). lead(II) phthalocyanine 4 gives one oxidation (IIc/IIa couple at 1.26 V with DEp = 68 mV and Ip,a/Ip,c = 0.75) and three reductions (IIIc/IIIa couple at 0.64 V with DEp = 57 mV and Ip,a/Ip,c = 0.98, IVc/IVa couple at 0.91 V with DEp = 59 mV and Ip,a/Ip,c = 0.97, and Vc/Va couple at 1.73 V with DEp = 71 mV and Ip,a/Ip,c = 0.44). When the voltammetric responses recorded in DMSO are compared with those recorded in DCM, it is apparent that the redox processes shifted toward the negative potentials. However each peak shifts in different extent. For instance, while the first reduction peak shift as 0.10 V and first oxidation couple shift about 0.16 V, which differs the DE1/2 value (1.32 V) of the complex in DMSO. With respect to DEp, Ip,a/Ip,c, and [Ip versus n1/2] data, All redox processes are electrochemically reversible and reversible character of these processes is higher than those recorded in DCM. Chemical reversibilities of these redox

Fig. 8. In-situ UV–vis spectral changes of ZnPc in DMSO/TBAP. (a) Eapp = 1.00 V. (b) Eapp = 1.40 V(inset: Eapp = 2.00 V). (c) Eapp = 1.00 V. (d) Chromaticity diagram (each symbol represents the color of electro-generated species; &: [ZnIIPc2], :[ZnIIPc3]1; 4: [ZnIIPc4]2; 5: [ZnIIPc5]3; $: [ZnIIPc1]+1).

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Fig. 9. In-situ UV–vis spectral changes of PbPc in DCM/TBAP. (a) Eapp = 0.75 V. (b) Eapp = 1.20 V. (c) Eapp = 1.00 V. (d) Chromaticity diagram (each symbol represents the color of electro-generated species; &: [PbIIPc2], :[PbIIPc3]1; 4: [PbIIPc4]2; $: [PbIIPc1]+1).

processes are higher in DMSO than those in DCM (except Vc/Va couple). 3.3. Spectroelectrochemical measurements Assignments of the redox processes of the complexes were performed by comparing the analyses of the voltammetric and in situ spectroelectrochemical measurements. It is known that 2H+ center of metal free phthalocyanine 2 is redox inactive with in the DCM/TBAP electrolyte system, thus all redox processes of this complex easily assigned to the electron release to or electron losing from the Pc ring of the complex [36–38]. Similarly both of nickel(II) phthalocyanine 3 and zinc(II) phthalocyanine 5 complexes have redox inactive Ni2+ and Zn2+ centers respectively, thus all redox processes should be Pc ring based reactions. Consequently, similar spectral changes characterizing ring-based reduction reactions were observed with these complexes in both of DCM/TBAP and DMSO/TBAP electrolyte systems. As an example, in situ spectroelectrochemical and in situ electrocolorimetric analyses of zinc(II) phthalocyanine 5 recorded in DMSO/TBAP were illustrated in Fig. 8. During the first reduction reaction (IIIc/IIIa) at 1.00 V constant potential, while the Q band at 684 nm decreases without a shift, new bands are observed at ligand to metal charge transfer region (LMCT) (580 and 964 nm) (Fig. 8a). These spectral changes are in harmony with MPcs having redox inactive metal centers. With respect to voltammetric analyses and in situ spectroelectrochemical measurements, the redox peak IIIc/IIIa is assigned to [ZnIIPc2]/[ZnIIPc3]1 process [39–42]. During this process, well-defined isosbestic points are observed at 661 and 713 nm in the spectra which indicates presence of one type reduced species during the first reduction

reaction. Fig. 8d shows in situ electrocolorimetric measurement results of zinc(II) phthalocyanine 5. As shown in the chromaticity diagram, the neutral [ZnIIPc2] has cyan color (x = 0.2871 and y = 0.3344). When cyan color of the complex is reduced, it changes to light blue (x = 0.2721 and y = 0.3115). Fig. 8b illustrates the spectral changes recorded during the second and third reduction reactions. Under 1.40 V potential application, decreasing the Q band without shifting and formation of a new band at 528 nm indicate ligand based character of the second reduction reaction and thus this process is easily assigned to [ZnIIPc3]1/[ZnIIPc4]2 process [39–41,43]. Well resolved isosbestic points at 650 and 715 nm illustrate the chemical stability of the dianionic [ZnIIPc4]2 species. Light blue color of [ZnIIPc3]1 species turns to blue color (x = 0.2879 and y = 0.2877) after the second reduction reaction. During the third reduction, all band decreases in the absorbance intensity due to the decomposition of the reduced species (Fig. 8b inset). Fig. 8c shows the spectral changes recorded during the oxidation reactions. Under 1.00 V potential application, the Q band decreases without shifting and two new bands starts to form at 510 and 743 nm. Then, due to the instability of the cationic [ZnIIPc1]1+ species, all bands decrease in intensity. Color of the neutral and electrogenerated ZnPc species are given in the chromaticity diagram in Fig. 8d. In our previous papers [34,35], we reported electrochemistry and spectroelectrochemistry of many PbPc complexes and it was represented that PbPc complexes can easily demetallized during the electron transfer reactions due to the distorted planar structure of the complexes, and demetallization reactions were easily followed from in situ spectroelectrochemical responses of the complexes. During the demetallization reaction, the Q band of the PbPcs turns to split Q band, which is a characteristic spectrum for

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the metal free phthalocyanines [34,35]. In order to perform assignments of the redox reactions of lead(II) phthalocyanine 4 and to investigate chemical stability of the complex, in situ spectroelectrochemical characterization of the complex was performed. Fig. 9 represents in situ spectroelectrochemical and in situ electrocolorimetric analysis results of lead(II) phthalocyanine 4 recorded in DCM/TBAP. Under open circuit potential, lead(II) phthalocyanine 4 give the Q band at 723 nm, n-p* transition band at 646 nm, and the B band at 396 nm (red spectrum in Fig. 9a). When 0.75 V was applied to the working electrode, due to the reduction of neutral [PbIIPc2] species to the monoanionic lead(II) phthalocyanine 4 species, spectrum of the complex immediately changes and the final spectrum (blue spectrum in Fig. 9a) characterizing [PbIIPc3]1 species. Decreasing the Q band at 723 nm and observation of a new band at 610 nm characterizing a Pc based reduction reactions of MPcs bearing redox inactive metal center. Well resolved isosbestic points 657 and 775 nm illustrates chemical stability of the reduced [PbIIPc3]1 species. Distinct color changes from green (x = 0.2934 and y = 0.3764) to deep blue (x = 0.2443 and y = 0.2901) is observed as given in Fig. 9d. During the second reduction reaction, while the Q band at 723 nm, the np* transition band at 646 nm and the band at 610 nm decrease in intensity, a new band is enhanced at 547 nm (Fig. 9b). These spectral changes are in harmony with the spectral changes characterizing formation of dianionic MPc species. Thus the second reduction reaction (IVa/IVc) of PbPc could be easily assigned to [PbIIPc3]1/[PbIIPc4]2 process. After the seconds, reduction reaction, 0.00 V was applied to the working electrode and spectral changes were followed. Returning the spectrum of [PbIIPc4]2 to the original spectrum of [PbIIPc2] supported chemical stability and chemical reversibility of the reduction reactions of PbPc. Clear

isosbestic points recorded during the second reduction reactions shows the stability of dianionic [PbIIPc4]2. Deep blue color of [PbIIPc3]1 species changes to purple (x = 0.2934 and y = 0.3764) after the second reduction reaction (Fig. 9d). Spectral changes given in Fig. 9c, indicate decomposition of the complex during the oxidation reaction. This decomposition reaction could not be resulted from the demetallzation reaction of the complex, since all complexes discussed above decomposed during the oxidation reactions. Similar spectral changes were observed during the reduction of lead(II) phthalocyanine 4 in DMSO/TBAP with those recorded in DCM/TBAP (Fig. 10). As shown in Fig. 10a, the Q band at 713 nm decreases in intensity and a new band increases at 612 nm during the first reduction reaction. Clear isosbestic points at 675 and 759 nm and color change from light green (x = 0.3355 and y = 0.362) to cyan (x = 0.3043 and y = 0.343) are observed during this reduction reaction. During the second reduction reaction, spectral changes characterizing the formation of dianionic [PbIIPc4]2 species are observes as shown in Fig. 10b. Observation of a new band at 550 nm and decreasing of the Q band without a shift are characteristic spectral changes recorded during the second reduction reaction of MPcs. Fig. 10b inset shows the spectral changes recorded during the third reduction reaction and formation of [PbIIPc5]3 species from [PbIIPc4]2 species. When 0.0 V potential was applied after the third reduction reaction, the spectrum of [PbIIPc5]3 turned to the original spectrum of [PbIIPc2], which indicates chemical stability and chemical reversibility of the reduction reactions of lead(II) phthalocyanine 4 in DMSO like recorded in DCM. Like recorded in DCM, spectral changes characterizing decomposition of oxidized lead(II) phthalocyanine 4 species are also observed in DMSO/TBAP electrolyte

Fig. 10. In-situ UV–vis spectral changes of PbPc in DMSO/TBAP. (a) Eapp = 0.75 V. (b) Eapp = 1.20 V. Eapp = 1.80 V. (c) Eapp = 1.00 V. (d) Chromaticity diagram (each symbol represents the color of electro-generated species; &: [PbIIPc2], :[PbIIPc3]1; 4: [PbIIPc4]2; $: [PbIIPc1]+1).

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(Fig. 10c). As shown in the chromaticity diagram of lead(II) phthalocyanine 4 given in Fig. 10d, less distinctive color changes are observed in DMSO than those observed in DCM. 4. Conclusion According to spectral data (UV–vis, IR, 1H NMR, 13C NMR, elemental analysis data), metal free, nickel(II), lead(II) and zinc(II) phthalocyanines 2–5 were successfully synthesized. Electrochemical and spectroelectrochemical results of the complexes support proposed structure of the complexes 2–5. Changing the ions in the Pc core of the complexes only affected the easy of the electron transfer reactions. Although PbPc type complexes were reported as instable during redox reactions, PbPc studied here gives chemically stabile anionic species. PbPc shows distinct color differences between the electrogenerated anionic species in DCM/TBAP electrolyte system, which indicate its possible application in the display technologies, e.g. electrochromic and data storage application. Acknowledgement This study was supported by the Research Fund of Karadeniz Technical University, (Project no: 5369), Trabzon-Turkey. References [1] X. Jia, F.F. Yang, J. Li, J.Y. Liu, J.P. Xue, J. Med. Chem. 56 (2013) 5797–5805. [2] E. Ranyuk, N. Cauchon, K. Klarskov, B. Guerin, J.E. van Lier, J. Med. Chem. 56 (2013) 1520–1534. [3] S. Yano, S. Hirohara, M. Obata, Y. Hagiya, S. Ogura, A. Ikeda, H. Kataoka, M. Tanaka, T. Joh, J. Photochem. Photobiol. C 12 (2011) 46–67. [4] F. Cicoira, N. Coppede, S. Iannotta, R. Martel, Appl. Phys. Lett. 98 (2011) 183– 303. [5] M. Ince, M.V. Martínez-Díaz, J. Barbera, T. Torres, J. Mater. Chem. 21 (2011) 1531–1536. [6] S. Nagel, M. Lener, C. Keil, R. Gerdes, L. Łukasz, S.M. Gorun, J. Phys. Chem. C 115 (2011) 8759–8767. [7] F.I. Bohrer, A. Sharoni, C. Colesniuc, J. Park, I.K. Schuller, A.C. Kummel, J. Am. Chem. Soc. 129 (2007) 5640–5646. [8] S. Schumann, R.A. Hatton, T.S. Jones, J. Phys. Chem. C 115 (2011) 4916–4921. [9] S.S. James, G. Richard, S. Pong, R.F. Steven, H. Heckmann, M. Hanack, J. Phys. Chem. A 104 (2000) 1438–1449. [10] D. Atilla, N. Saydan, M. Durmus, A.G. Gürek, T. Khan, A. Rück, J. Photochem. Photobiol. A Chem. 186 (2007) 298–307.

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