Synthesis, electrochemical and spectroelectrochemical properties of peripherally tetra-imidazole substituted metal free and metallophthalocyanines

Dyes and Pigments 96 (2013) 483e494

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Synthesis, electrochemical and spectroelectrochemical properties of peripherally tetra-imidazole substituted metal free and metallophthalocyanines Hakkı Türker Akçay a, Rıza Bayrak a, Ümit Demirbas¸ a, Atıf Koca b, Halit Kantekin a, *, irmenciog lu a Ismail Deg a b

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

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 July 2012 Received in revised form 27 August 2012 Accepted 12 September 2012 Available online 27 September 2012

The synthesis and characterization of novel organosoluble peripherally tetra imidazole substituted metal-free (4), zinc(II) (5), nickel(II) (6), cobalt(II) (7), lead(II) (8) and oxo-titanium(IV) (9) phthalocyanines are described for the first time in this study. Electrochemical and spectroelectrochemical measurements exhibit that while copper phthalocyanine gives only ring-based electron transfer reactions, incorporating redox active metal centers, Co(II), and Ti(IV)OPc, into the phthalocyanine core extends the redox richness of the phthalocyanine ring with the metal-based reduction and oxidation couples of the metal centers in addition to the common phthalocyanine ring-based electron transfer processes. In-situ electrocolorimetric measurements of the complexes allow quantification of color coordinates of the each electrogenerated anionic and cationic redox species. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Synthesis Phthalocyanine Imidazole Electrochemistry Spectroelectrochemistry Soluble

1. Introduction The synthesis and characterization of the new compounds that are used in production of the materials supporting requirements of the manufacturer is one of the most important research topics of the chemists. Phthalocyanines and their metal complexes, thanks to special properties such as strong delocalized 18 p-electronic structure, good thermal stability and visible area optical properties, play an important role in design of the new high-tech materials such as chemical sensors [1e3], electrochromic displaying systems [4], non-linear optics [5], solar cells [6], photo-voltaic optics, molecular electronics [7], semiconductors [8], liquid crystals [9], optical storage devices [10], laser dyes [11], catalyst [12] and photo dynamic therapeutic agents (PDT) [13]. Additionally, imidazoles, five membered ring system containing two nitrogen atoms, are important heterocyclic compounds. The uses of imidazoles can be listed as P38 MAP kinase [14], antivascular disrupting, antitumor activity [15], ionic liquids [16], anion sensors [17], electrical and optical materials [18e20]. Porphyrins containing imidazole moieties have been prepared and optical, electronic and catalytic properties investigated [21e27]. To the best of our knowledge, imidazole substituted phthalocyanines are present in a few lu and co-workers have prepared reports [28e32]. Recently Bekarog * Corresponding author. Tel.: þ90 462 377 25 25; fax: þ90 462 325 31 96. E-mail address: [email protected] (H. Kantekin). 0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dyepig.2012.09.005

imidazole substituted phthalocyanines and investigated electrical and electrochemical properties of these compounds [33,34]. In our study, triphenylimidazole moieties were substituted peripherally on phthalocyanine macrocycle and its Zn(II), Ni(II), Co(II), Pb(II) and Ti(IV) complexes have been prepared. It is well documented that MPcs (metallo-phthalocyanine) have proven to be functional species for electrosensors [35,36] electrochromic devices [37,38] and unique macromolecular complexes that catalyze many target species such as CO2 [39,40], CO [41], and Hþ [42e45] because of their rich redox behavior. The electrochemical properties of MPcs can be altered by changing the metal and/or the substituents. For this purpose, the electrochemical and spectroelectrochemical properties of these newly synthesized MPcs bearing redox active CoII, and TiIVO and redox inactive ZnII, NiII and PbII metal centers and an electron donor imidazole sunstituents have been investigated with voltammetric and insitu spectroelectrochemical measurements. 2. Experimental 2.1. Materials and equipment All reactions were carried out under dry and oxygen free nitrogen atmosphere using schlenk system. DMF (dimethylformamide) was dried and purified as described by Perrin and Armarego [46], 4-(4,5diphenyl-1H-imidazol-2-yl) 3-methoxyphenol (1) [47] and

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4-nitrophthalonitrile (2) [48] were prepared as described in the literature. 1H NMR, 13C NMR spectra were recorded on a Varian XL-200 NMR spectrometer DMSO d6 (deuterated dimethylsulfoxide), and chemical shifts were reported (d) relative to Me4Si (tetramethylsilane) as internal standard. IR spectra were recorded on a PerkineElmer Spectrum One FT-IR spectrometer with ATR technique. The MS spectra were measured with a Thermo Quantum Access Mass spectrometer with H-ESI probe. Methanol, chloroform were used as solvents in mass analysis and all mass analysis were conducted in positive ion mode. Elemental analysis was performed on a Costech ECS 4010 instrument, UVeVis spectra were recorded by Perkin Elmer Lambda 25 spectrometer, using 1 cm path length cuvettes at room temperature. Melting points were measured by an electrothermal apparatus. 2.2. Electrochemical measurements The cyclic voltammetry (CV) and square wave voltammetry (SWV) measurements were carried out with Gamry Reference 600 potentiostat/galvanostat controlled by an external PC and utilizing a threeelectrode configuration at 25  C. The working electrode was a Pt disc with a surface 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 (tetrabutylammonium perchlorate) in extra pure DCM (dichloromethane) and DMSO (dimethylsulfoxide) was employed as the supporting electrolyte at a concentration of 0.10 mol dm3. For each measurement, the reference electrode tip was moved as close as possible to the working electrode so that uncompensated resistance of the solution was a smaller fraction of the total resistance, and therefore the potential control error was low. Moreover, IR compensation was also applied to the voltammetric measurements to minimize the potential control error. 2.3. In-situ spectroelectrochemical and in-situ electrocolorimetric measurements UVeVis absorption spectra and chromaticity diagrams were measured by an Ocean Optics QE65000 diode array spectrophotometer. In-situ spectroelectrochemical measurements were carried out by utilizing a three-electrode configuration of thin-layer quartz thin-layer spectroelectrochemical cell at 25  C. The working electrode was a semipermeable Pt sheet. 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 Ocean Optics QE65000 diode array spectrophotometer at color measurement mode by utilizing a threeelectrode 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.

tablet) ymax/cm1: 3067 (-NH), 3049 y(AreCH), 2230 (C^N), 1595 y(C]N), 1561e1480 y(C]C), 1287e1250 y(CeOeC)/(CeN), 1133 d(CeN), 1087e1071 d(CeOeC), 953 d(CH). 1H NMR (DMSO-d6), (d: ppm): 12.70 (s, 1H/NH), 8.08-8.03 (d, 1H/ArH), 7.91 (s, 1H/ArH), 7.80e7.73 (d, 2H/ArH), 7.54e7.51 (m, 4H/ArH), 7.39e7.35 (m, 8H/ ArH), 3.82 (s, 3H/OCH3). 13C NMR (DMSO-d6), (d: ppm): 164.10, 154.04, 147.63, 143.77, 139.05, 132.31, 131.40, 130.67, 130.24, 125.66, 124.09, 123.33, 121.24, 119.41, 118.88, 118.35, 113.13, 110.54, 105.00, 58.85. MS (ESI), (m/z): Calculated: 468.16; Found: 469.43 [M þ H]þ. 2.4.2. Synthesis of phthalocyanines (4-9) The phthalocyanine compounds were synthesized according to literature [32]. Metallo-phthalocyanines (5e9) were synthesized with addition of stoichiometric amounts of related anhydrous metal salts (Zn(CH3COO)2; NiCl2; CoCl2; PbO; Ti(OBu)4). 2.4.2.1. Metal-free phthalocyanines. The green solid product was purified by column chromotography with chloroform:methanol (100:2) as eluent. Yield: 45 mg (15%), mp > 300  C. Anal.calc. for C120H82N16O8: C, 76.83; H, 4.41; N, 11.95; Found: C, 76.71; H, 4.45; N, 11.83. IR (KBr tablet) ymax/cm1: 3234, 3057, 1646e1603, 1490, 1474, 1450, 1233, 1169, 1117, 1092, 1014, 963, 842. 1H NMR (DMSO-d6), (d: ppm): 12.66 (s, 4H/NH), 8.18e7.90 (m, 8H/ArH), 7.80e7.73 (m, 8H/ ArH), 7.50e7.33 (bm, 48H/ArH), 3.80 (s, 12H/OCH3). UVevis (DMSO) lmax/nm: [(105 3, dm3 mol1 cm1)]. 705 (4.86), 667 (4.85), 633 (4.17) 599 (4.17), 326 (56.04). MS (ESI), (m/z): Calculated: 1875.65, Found: 1876.93 [M þ H]þ. 2.4.2.2. Synthesis of zinc (II) phthalocyanine (5). Eluent for column chromatography: chloroform:methanol (100:1.5). Yield: 105 mg (40.6%), mp > 300  C. Anal. Calc. for C120H80N16O8Zn: C, 74.31; H, 4.16; N, 11.56; Found: C, 74.42; H, 4.12; N, 11.50. IR (KBr tablet) ymax/ cm1: 3055, 1649e1601, 1491, 1470, 1452, 1232, 1160, 1116, 1095, 1012, 968, 840. 1H NMR (DMSO-d6), (d: ppm): 12.75 (s, 4H/NH), 8.10e7.92 (m, 8H/ArH), 7.87e7.69 (m, 8H/ArH), 7.51e7.30 (bm, 48H/ ArH), 3.83 (s, 12H/OCH3). UVevis (DMSO) lmax/nm: [(105 3, dm3 mol1 cm1)]: 682 (4.86), 616 (4.09), 348 (4.63). MS (ESI), (m/ z): Calculated: 1937.57, Found: 1938.86 [M þ H]þ. 2.4.2.3. Synthesis of nickel (II) phthalocyanine (6). Eluent for column chromatography: chloroform:methanol (100:3). Yield: 80 mg (31%), mp > 300  C. Anal.calc. for C120H80N16O8: C, 74.57; H, 4.17; N, 11.60. Found: C, 74.50; H, 4.22; N, 11.68. IR (KBr tablet) ymax/ cm1: 3058, 1645e1606, 1490, 1472, 1450, 1231, 1155, 1122, 1093, 1010, 965, 849. 1H NMR (DMSO-d6), (d: ppm): 12.73 (bs, 4H/NH), 8.06e7.70 (bm, 16H/ArH), 7.50e7.22 (bm, 48H/ArH), 3.80 (bs, 12H/ OCH3). UVevis (DMSO) lmax/nm: [(105 3, dm3 mol1 cm1)]: 678 (4.96), 647 (4.59), 613 (4.47), 323 (5.06). MS (ESI), (m/z): Calculated: 1931.57, Found: 1954.57 [M þ Na]þ.

2.4. Synthesis

2.4.2.4. Synthesis of cobalt (II) phthalocyanine (7). Eluent for column chromatography: chloroform:methanol (100:1). Yield: 85 mg (33%), mp > 300  C. Anal.calc. for C120H80N16O8Co: C, 74.56; H, 4.17; N, 11.59. Found: C, 74.49; H, 4.10; N, 11.68. IR (KBr tablet) ymax/cm1: 3062, 1648e1612, 1495, 1470, 1456, 1233, 1159, 1127, 1095, 1013, 960, 852. UVeVis (DMSO) lmax/nm: [(105 3, dm3 mol1 cm1)]: 669 (4.85), 608 (4.27), 328 (5,07). MS (ESI), (m/z): Calculated: 1932.57, Found: 1933.95 [M þ H]þ.

2.4.1. 4-[4-(4,5-diphenyl-1H-imidazol-2-yl)3-methoxyphenoxy] phthalonitrile (3) The phthalonitrile derivative was synthesized according to literature [32]. The compound (3) was crystallized from acetone/ ethanol solvent system. Yield 0.8 g (59%). Anal.calc. for C30H20N4O2: C, 76.91; H, 4.30; N, 11.96; Found: C, 76.86; H, 4.34; N, 11.91. IR (KBr

2.4.2.5. Synthesis of lead (II) phthalocyanine (8). Eluent for column chromatography with chloroform:methanol (100:2). Yield: 43 mg (15.4%), mp > 300  C. Anal. Calc. for C120H80N16O8Pb: C, 69.25; H, 3.87; N, 10.77; Found: C, 69.41; H, 3.75; N, 10.92. IR (KBr tablet) ymax/cm1: 3057, 1648e1601, 1490, 1472, 1451, 1230, 1160, 1115, 1092, 1012, 963, 845. 1H NMR (DMSO-d6), (d: ppm): 12.66 (s, 4H/

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NH), 8.12e7.93 (m, 8H/ArH), 7.85e7.72 (m, 8H/ArH), 7.46e7.24 (m, 48H/ArH), 3.85 (s, 12H/OCH3). UVeVis (DMSO) lmax/nm: [(105 3, dm3 mol1 cm1)]: 708 (4.83), 647 (4.14), 322 (5,04). MS (ESI), (m/ z): Calculated: 2080.61, Found: 2081.44 [M þ H]þ. 2.4.2.6. Synthesis of oxo-titanium (IV) phthalocyanine (9). Eluent for column chromatography with chloroform:methanol (100:0.5). Yield: 45 mg (17.4%), mp > 300  C. Anal. Calc. for C120H80N16O9Ti: C, 74.37; H, 4.16; N, 11.56; Found: C, 74.45; H, 4.24; N, 11.42. IR (KBr tablet) ymax/cm1: 3050, 1645e1601, 1493, 1470, 1457, 1231, 1163, 1114, 1092, 1013, 965, 842. 1H NMR (DMSO-d6), (d: ppm): 12.78 (s, 4H/NH), 8.12e7.7.65 (bm, 16H/ArH), 7.52e7.18 (bm, 48H/ ArH), 3.91 (s, 12H/OCH3). UVevis (DMSO) lmax/nm: [(105 3, dm3 mol1 cm1)]: 705 (4.91), 671 (4.61), 639 (4.36), 610 (4.14), 325 (4.96). MS (ESI), (m/z): Calculated: 1937.58, Found: 1938.93 [M þ H]þ. 3. Results and discussion 3.1. Synthesis and characterization The synthetic route of novel compounds is depicted in Fig. 1. The key compound substituted phthalonitrile 3 were obtained by nucleophilic aromatic nitro displacement reaction of 4-nitrophthalonitrile with compound 1 by the catalysis of K2CO3 as base. These type of aromatic nitro substitution reactions are generally carried out by strong nucleophiles under dipolar aprotic conditions [49]. All spectral

485

data support the proposed structure 3. According to IR spectral data, new vibration appeared at 2230 cm1 clearly indicated that structure 3 have nitrile group. In addition the vibration of NH group of imidazole moiety observed at 3067 cm1 due to intermolecular hydrogen bonding [32]. In 1H NMR spectrum of compound 3, the disappearance of the OH signal of 1 and presence of additional aromatic protons indicated that subtitution was accomplished. In 13C NMR of compound 3, the new peaks at 118,88 ppm and 118,35 ppm belonging to nitrile carbons indicated that the substitution occurred. Stable molecular ion [M þ H]þ peak at 469.43 in the mass spectra of compound 3 was showed that target compound successfully prepared. Also elemental analysis data of compound (3) was satisfactory. In spite of the presence of many synthetic methods for preparing substituted phthalocyanines in literature [50e52], most of the authors prefer cyclotetramerization of substituted phthalonitrile or 1,3-diimino-1H-isoindoles [53]. Tetra substituted phthalocyanines prepared from 4-substituted phthalonitriles are obtained as a mixture of four different possible isomers (C4h, C2v, Cs and D2h). In this work, we used 4-substituted phthalonitrile (3) as starting compound for synthesis of the metal-free- and metallo-phthalocyanines (Zn, Ni, Co, Pb and TiO) and did not make an effort to separate the isomeric mixture. All new phthalocyanines were characterized by IR, 1H NMR, mass spectrometry, elemental analysis and UV/Vis. All spectral data support the proposed structures (4e9). In the IR spectra of all novel phthalocyanines, the eChN vibration of compound 3 disappeared, the absence of eChN vibration in the

Fig. 1. Synthetic route of novel phthalocyanine compounds.

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phthalocyanine IR spectral data supports the proposed target structures. The rest of the spectra are similar to that of compund 3. The main difference between the IR spectral data of metal-free and of metallophthalocyanines is the inner core eNH vibration for metal free phthalocyanine (4) that was observed at 3234 cm1. The 1H NMR spectra of compound 7 could not be determined because of the presence of paramagnetic cobalt ion [54]. The 1H NMR spectra of phthalocyanines 4, 5, 6, 8 and 9 were taken DMSOd6 at room temperature. The spectra of the compounds were very similar. For metal free phthalocyanine 4, the typical shielding of inner core protons could not be observed due to the strong aggregation between phthalocyanine molecules [55]. In addition, mass spectra and elemental analysis results of the compounds 4e9 were good correlation with proposed structure. 3.2. UVeVis absorption spectra All phthalocyanines were soluble in most of organic solvents such as, THF (tetrahydrofuran), dichloromethane, chloroform, DMF, DMSO and pyridine. For metallo phthalocyanines 5e9 in DMSO, the Q bands caused by p / p* transitions were observed at 682, 678, 669, 705 and 708 nm respectively (Figs. 2 and 3). The shoulders of these metallophthalocyanines were observed at: 616 for 5, 613 for 6, 608 for 7, 647 for 8 and 639 for 9. For Q band regions of metallophthalocyanines, the longer wavelength absorptions are due to the monomeric species and shorter wavelength absorptions (shoulders) are due to the aggregated species [56]. So, in DMSO at 1  105 mol dm3 concentration, monomeric behaviour of metallophthalocyanines were proved by the dominance of the longer wavelength absorptions. The B bands arising from deeper p levels to LUMO were observed at: 347, 323, 328, 322 and 325 nm for 5, 6, 7, 8 and 9 respectively (Figs. 2 and 3). Differently from metallophthalocyanines, the Q band absorptions of metal-free phthalocyanines split to Qx and Qy and there are two strong absorption bands in visible region. In DMSO, the splitting Q band of compound 4 was observed at 705 and 667 nm with shoulders at 633 and 599 nm, evidence of non-degenerate D2h symmetry. Additionally, B-band of compound 4 was observed at 326 nm (Fig. 2). 3.3. Voltammetric measurements Electrochemical analyses of the complexes were investigated in solution to propose possible applications of the complexes in different fields of electrochemical technologies. Thus, CV and SWV analysis of

Fig. 2. UVevis absorption spectra changes of compounds 4, 5 and 6 in DMSO at 1  105 mol dm3.

Fig. 3. UVevis absorption spectra changes of compounds 7, 8 and 9 in DMSO at 1  105 mol dm3.

the complexes were performed in deaerated DCM and/or DMSO/TBAP electrolyte system on a Pt working electrode. Table 1 lists 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), and difference between the first oxidation and reduction processes (DE1/2). As shown in Table 1 E1/2 and DE1/2 values of the complexes are in agreements with the similar complexes in the literature [57e67]. Changing the redox inactive metal centers with the redox active ones increases the redox richness of the complex with extra electron transfer processes of the metal centers. Changing of the solvent especially affect the oxidation processes of CoPc due to the coordinating of DMSO the CoIIIPc form of the complex, while just shift the redox potential of the others due to the electron donating ability differences of DCM and DMSO. When compared with the similar complexes in the literature, Redox couples of the complexes generally shift to the negative potentials due to the electron donating ability of the imidazole groups on the substituents. Figs. 4 and 5 illustrate the CV and SWV responses of ZnPc and NiPc respectively recorded in DMSO/TBAP electrolyte system. ZnPc displays three reductions (R1 at 0.82 V, R2 at 1.25 Vand R3 at 1.56 V) and an oxidation couple (O1) at 0.78 V. While the first reduction and first oxidation couples are electrochemically reversible, the others are in quasi-reversible range with respect to DEp of the couples. However, only the first reduction and first oxidation couples are both electrochemically and chemically reversible. The first reduction and first oxidation processes are purely diffusion controlled processes with respect to unit Ip,a/Ip,c ratio at all scan rates and linear changes of Ip with y1/2. The effect of coupled chemical reactions to the electron transfer reactions is illustrated with Ip,a/Ip,c ratio change as a function of the scan rate. Unity of the Ip,a/Ip,c ratios at all scan rates indicate purely diffusion controlled and chemically reversible behavior of the R1 and O1 processes. However, R2 and R3 are not chemically reversible with respect to the Ip,a/Ip,c ratio [68]. These behaviors indicate existence of a chemical reaction succeeding the R2 process. Moreover, reversibility of these couples can be concluded with respect to symmetry and peak current ratios of the waves recorded during the forward and reverse SWV scans (Fig. 4b). NiPc gives similar redox responses with ZnPc as shown in Fig. 5. However all redox processes of NiPc are both electrochemically and chemically reversible at all scan rates. When compared with ZnPc, redox processes of NiPc shift slightly to the positive potentials due to the smaller effective nuclear charge of NiPc than ZnPc.

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487

Table 1 Voltammetric data of the complexes with the related MPcs. All voltammetric data were given versus SCE. Complex ZnPc (in DMSO)

Ring oxidations a

E1/2 DEp (mV) c Ip,a/Ip,c a E1/2 b DEp (mV) c Ip,a/Ip,c a E1/2 b DEp (mV) c Ip,a/Ip,c a E1/2 b DEp (mV) c Ip,a/Ip,c a E1/2 b DEp (mV) c Ip,a/Ip,c a E1/2 b DEp (mV) c Ip,a/Ip,c a E1/2 b DEp (mV) c Ip,a/Ip,c b

NiPc (in DMSO)

CoPc (in DMSO)

PbPc (in DMSO)

PbPc (in DCM)

TiOPc (in DMSO)

TiOPc (in DCM)

a b c d e f g

e e e e e e e e e e e e 1.07 84 0.45 e e e e e e

0.78 65 0.92 0.80 160 0.65 0.95 130 0.57 1.05e e e 0.70 70 0.95 0.80 e e 0.96 e e

MIII/MII

MII/MI

e e e e e e 0.43 80 0.91 e e e e e e e e e e e e

e e e e e e 0.40 90 0.97 e e e e e e 0.48f (0.83g) 60 (63g) 0.95 (0.86g) 0.47f (0.81g) 62 (64g) 1.20 (1.76g)

0.82 58 0.95 0.79 100 0.90 1.38 59 0.86 0.78 70 0.96 0.75 60 0.95 0.63 62 0.92 0.62 61 0.90

d

Ref

1.60

tw

1.59

tw

0.83

tw

1.83

tw

1.45

tw

1.28

tw

1.28

tw

DE1/2

Ring reductions 1.25 80 0.55 1.16 60 0.98 e e e 1.04 51 0.84 1.02 70 0.92 0.99 65 0.88 0.96 60 0.93

1.56 85 0.45 e e e e e e 1.50 55 0.68 1.41 120 0.72 1.70 e e 1.35 e e

E1/2 values ((Epa þ Epc)/2) were given versus SCE and Fc/Fcþ(in parenthesis)at 0.100 Vs1 scan rate. DEp ¼ Epa-Epc. Ip,a/Ip,c for reduction,Ip,c/Ip,afor oxidation processes. DE1/2 ¼ E1/2 (first oxidation)- E1/2 (first reduction). Recorded by SWV. The process is assigned to TiIVOPc/TiIIIOPc. The process is assigned to TiIVOPc/TiIIOPc. tw: This work.

Fig. 6 illustrates the CV and DPV of CoPc in DMSO/TBAP. Within the electrochemical window of DMSO/TBAP, CoPc undergoes two one-electron oxidation, and two reversible one-electron reduction processes. For the reduction couples, anodic to cathodic peak separations (DEp) changed from 58 to 130 mV with the scan rates from 10 to 500 mV s1 support electrochemical reversibility of the electron transfer. Unity of the Ipa/Ipc ratio of the first reduction couple with the scan rate and linear variation of the Ipc with square root of the scan rates indicated the purely diffusion controlled

electron transfer mechanism of the process. Chemical and electrochemical reversibility is illustrated by the similarity of the peak currents and symmetry of the peaks in the forward and reverse SWVs (Fig. 6b). Oxidation processes are affected from the vertex potential (Fig. 6a inset and 6b). When the vertex potential is returned before the irreversible O2 process (0.95 V), O1 process is electrochemically and chemically reversible. However O1 process gets chemically irreversible, when the vertex potential passes the O2 process. This data indicate that the second oxidation couple is

Fig. 4. (a) CVs of ZnPc at various scan rates on a Pt working electrode in DMSO/TBAP. (b) SWV of ZnPc recorded with SWV parameters: step size ¼ 5 mV; pulse size ¼ 100 mV; Frequency ¼ 25 Hz.

Fig. 5. (a) CVs of NiPc at various scan rates on a Pt working electrode in DMSO/TBAP. (b) SWV of NiPc recorded with SWV parameters: step size ¼ 5 mV; pulse size ¼ 100 mV; Frequency ¼ 25 Hz.

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Fig. 6. (a) CVs of CoPc at various scan rates on a Pt working electrode in DMSO/TBAP. (b) SWV of CoPc recorded with SWV parameters: step size ¼ 5 mV; pulse size ¼ 100 mV; Frequency ¼ 25 Hz.

complicated with a chemical reaction. It is well documented that CoIII oxidation state of the central metal of CoPc easily coordinate with the coordinating solvent DMSO and form an equilibrium as in Equation (1) [69].

Fig. 7. (a) CVs of PbPc recorded with different switching potentials at 0.100 Vs1 scan rate on a Pt working electrode in DMSO/TBAP. (b) SWV of PbPc recorded with SWV parameters: step size ¼ 5 mV; pulse size ¼ 100 mV; Frequency ¼ 25 Hz.

Fig. 8. (a) CVs of PbPc recorded with different switching potentials at 0.100 Vs1 scan rate on a Pt working electrode in DCM/TBAP. (b) SWV of PbPc recorded with SWV parameters: step size ¼ 5 mV; pulse size ¼ 100 mV; Frequency ¼ 25 Hz.

h i h i CoIII Pc2 þ DMSO4 DMSO  CoIII Pc2

(1)

PbPc gives a one-electron oxidation and three one-electron reduction processes in DMSO/TBAP (Fig. 7). While the first and

Fig. 9. (a) CVs of TiOPc recorded with different switching potentials at 0.100 Vs1 scan rate on a Pt working electrode in DMSO/TBAP. (b) SWV of TiOPc recorded with SWV parameters: step size ¼ 5 mV; pulse size ¼ 100 mV; Frequency ¼ 25 Hz.

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Fig. 10. (a) CVs of TiOPc recorded with different switching potentials at 0.100 Vs1 scan rate on a Pt working electrode in DCM/TBAP. (b) SWV of TiOPc recorded with SWV parameters: step size ¼ 5 mV; pulse size ¼ 100 mV; Frequency ¼ 25 Hz.

489

second reduction couples are electrochemically and chemically reversible with respect to DEp and the Ipa/Ipc ratio of the couples, the third reduction process is complicated with a chemical reaction. As shown in Fig. 7, when the vertex potential passes the R3 process, the chemical reversibility of the previous couples are disturbed and a wave assigned to the chemical reaction products (CP) at 0.55 V is recorded during the reverse potential scan. The oxidation couple (O1) at 1.05 V is completely irreversible. To investigate effect of the solvent to the electrochemical behavior of the complex, CV and SWVs responses of PbPc were also recorded in DCM, a non-coordinating and nonpolar solvent (Fig. 8). PbPc gives three well-resolved reversible reduction (R1 at 0.75 V, R2 at 1.02 V and R3 at 1.41 V). These redox couples shift to the positive potentials in DCM due to the less electron donor ability of DCM, when compared with those in DMSO. The reduction processes are more reversible and diffusion controlled in DCM, while they are complicated with chemical reactions. PbPc also illustrates two oxidation reactions, O1 at 0.70 V and O2 at 1.09 V in DCM and the first oxidation process is a reversible process. Figs. 9 and 10 show CV and SWVs of TiOPc in DMSO/TBAP and DCM/TBAP respectively. TiOPc illustrates very similar redox responses in both electrolyte systems. Five reduction and a completely irreversible oxidation processes are recorded. While the first four reduction processes are reversible or quasi-reversible, the fifth one (R5) is an irreversible reduction process. R5 process is followed with a chemical reaction, which decreases the chemical reversibility of the previous electron transfer reactions. The product of the chemical reaction gives a wave at around 0.20 V during the reverse potential scan. This chemical reaction is dominant in DMSO with respect to DCM solvent.

Fig. 11. In-situ UVeVis spectral changes of PbPc in DMSO. a) Eapp ¼ 0.90 V b) Eapp ¼ 1.20 V(Inset: Eapp ¼ 1.60 V). c) Eapp ¼ 1.10 V d) Chromaticity diagram (each symbol II 4 : PbIIPc5, : PbIIPc1. represents the color of electro-generated species; n: PbIIPc2, : PbIIPc3, : Pb Pc ,

O

P

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Fig. 12. In-situ UVeVis spectral changes of CoPc in DMSO. a) Eapp ¼ 0.50 V b) Eapp ¼ 1.50 V c) Eapp ¼ ¼ 0.60 V d) Chromaticity diagram (each symbol represents the color of : [CoIPc3]2, : [CoIIIPc2]þ1. electro-generated species; n: [CoIIPc2], :[CoIPc2]1,

O

3.4. Spectroelectrochemical measurements To perform the assignments for the redox processes recorded with the CVs and SWVs of the complexes spectroelectrochemical studies were employed. It is well documented in the literature that ZnPc, NiPc and PbPc gives only ring based electron transfer processes [69e74]. Spectroelectrochemical result of ZnPc and NiPc are in harmony with those reported in the literature. In our previous papers we reported that PbPc complexes demetalized and then decomposed during the spectroelectrochemical measurements under the applied potential [74e76]. However, PbPc studied here give reversible anionic species without demetalization under the applied potential. The Fig. 11 shows in-situ UVeVis spectral changes observed upon controlled potential reductions and oxidation of PbPc in DMSO/TBAP electrolyte system. During the first reduction at 0.90 V (Fig. 11a), the Q band at 715 nm decreases in intensity without shifting, while two new bands are recorded at 614 and 652 nm. These spectral changes clearly indicated reduction of PbIIPc2 to PbIIPc3 [69e76]. To check the stability of the reduced species and the chemical reversibility of the process, 0.0 V was applied after the first reduction process. The spectra of the PbIIPc3 species completely tuned to the original spectrum of PbIIPc2, which indicate the chemical reversibility of the process. We have observed well-defined isosbestic points at 400, 675, and 753 nm, which demonstrate that the reduction proceeds cleanly in deoxygenated DMSO to give a single type reduced species, monomeric PbIIPc3 species. Without any potential application, the solution of PbIIPc2 is light green (x ¼ 0. 0.338 and y ¼ 0.373) (point n in Fig. 11d). As the potential is stepped from 0 to 0.90 V, the color of the neutral PbIIPc2

starts to changes and a light blue color (x ¼ 0.297 and y ¼ 0.339) (point  in Fig. 11d) of monoanionic PbIIPc3 was obtained at the end of the first reduction. Fig. 11b illustrates the spectral changes during the second reduction process. Formation of a band at 553 nm and decreasing of the bands at 614, 652, and 715 nm can characterize the further ring-based reduction of PbIIPc3 to PbIIPc4 species [69e76]. Applying 0.90 V return the spectrum of PbIIPc4 to the spectrum of PbIIPc3, which indicated the chemical reversibility of the second reduction process. Clear isosbestic points at 455 and 601 nm in the spectra are recorded. Color of the dianionic species, PbIIPc4, is recorded as bluish purple (x ¼ 0.304 and y ¼ 0.28) with in-situ colorimetric measurements (point O in Fig. 11d). Inset in Fig. 11b represents the spectral changes at 1.60 V potential application. During the third reduction process, while the band at 614 nm remains as unchanged, a new band is recorded at 774 nm. At the same time, the bands at 652, and 715 nm disappear completely. Decreasing of the Q band without shift and observation of a new band at MLCT (metal to ligand charge transfer) region (the band at 774 nm) are characteristic changes for a ring-based reduction process. Color of the trianionic species is recorded as purple (x ¼ 0.326 and y ¼ 0.274) with in-situ colorimetric measurements (point P in Fig. 11d). Applying the previous process potentials did not returned the spectrum back, which indicate the chemical irreversibility of the third reduction process. To confirm the origin of the oxidation process (O1), in-situ UVeVis changes are recorded under the controlled potential application at 1.10 V (Fig. 11c). During this process, the Q band decreases in intensity, while a new band increases at 610 nm. These spectral changes are characteristics of a ring-based oxidation process in MPc complexes, thus the couple O1 is

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491

Fig. 13. In-situ UVeVis spectral changes of TiOPc in DMSO. a) Eapp ¼ 0.55 V b) Eapp ¼ 0.70 V c) Eapp ¼ ¼ 0.90 V d) Eapp ¼ ¼ 1.30 V.

easily assigned to PbIIPc2/PbIIPc1 redox process. Color of the monocationic species, PbIIPc1, is recorded as light blue (x ¼ 0.306 and y ¼ 0.35) with in-situ colorimetric measurements (point in Fig. 11d). Fig. 12 represents in-situ UVeVis spectral changes and in-situ recorded chromaticity diagram of CoPc in DMSO/TBAP during the electron transfer processes. Shifting of the Q band from 655 nm to 708 nm and observation of a new band at 470 nm characterize formation of [CoIPc2]1 species under the applied potential at

0.50 V (Fig. 12a) [74e77]. This process resulted in clear isosbestic points at 565 and 690 nm in the spectra and a color changes from bluish green (x ¼ 0.267 and y ¼ 0.357) to green (x ¼ 0.345 and y ¼ 0.397) as shown in the chromaticity diagram (Fig. 12d). These spectroscopic changes assigns the process R1 to [CoIIPc2]/ [CoIPc2]1- process. The spectral changes recorded during the reduction of [CoIPc2]1- at 1.50 V indicates a ring-based redox process. Because, the Q band at 690 nm decreases without shift

Fig. 14. In-situ UVeVis spectral changes of TiOPc in DMSO. a) ¼ 1.00 V b) Chromaticity diagram (each symbol represents the color of electro-generated species; n: [TiIVOPc2], : : [TiIIIOPc3]2, [TiIIIOPc3]2; : [TiIIOPc3]3; : [TiIIOPc4]4; : [TiIVOPc2]þ1. [TiIIIOPc2]1,

O

O

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while a new broad band is recorded at 550 nm (Fig. 12b). Fig. 12c represents the spectral changes during the first oxidation process of the complex. Under 0.60 V potential application, the Q band shifts from 665 nm to 680 nm with increasing, which indicates a metalbased oxidation process, [CoIIPc2]/[CoIIIPc2]1þ [74e77]. Distinct color changes during the electron transfer reactions represented in the chromaticity diagram in Fig. 12d, indicates possible application of the complex as an electrochromatic material.

1.00 V during the oxidation reaction, since all bands decrease in intensity as shown in Fig. 14a. Color changes of the TiOPc in solution during the redox processes were recorded with in-situ electrocolorimetric measurements and given in Fig. 14b. When we collect the responses of CV, SWV and spectroelectrochemical measurements and the data from the literature, we can propose the mechanism the following mechanism for the electrochemical responses of TiOPc.

The reduction of TiIVOPc2 species has been a subject of some controversy, with some reports proposing ring reduction and others suggesting metal reductions followed with the ring reduction processes. Silver et al. [78] studied the thin film voltammetric behaviour of TiOPc complex. They showed that the complex coated on ITO electrode displayed two ring reduction processes and decompose during the oxidation process. Nyokong and co workers [79] studied the solution electrochemistry of titanyl phthalocyanine derivatives and reported that these complexes gave three reduction processes assigned to two electron metal center reduction from TiIVOPc2 to TiIIOPc2 for first reduction couple, a combination of one-electron ligand reduction from TiIIOPc2 to TiIIOPc3 and a metal center one-electron reduction from TiIIOPc3 to TiIOPc3 for second reduction and finally one electron ligand reduction from TiIOPc3 to TiIOPc4. In our previous study oneelectron metal center reductions from TiIVOPc2 to TiIIIOPc2 and from TiIIIOPc2 to TiIIOPc2 for the first two reduction couples and finally one electron ligand reduction for both tetrakis(2dimethylaminoethylsulfanyl) phthalocyaninato oxotitanium(IV) and octakis(2-dimethylaminoethylsulfanyl) phthalocyaninato oxotitanium(IV) complexes were reported [80]. We reported the same assignments for the (3)-Tetra[4-(thiophen-3-yl)-phenol]phthalocyaninato oxotitanium(IV) complex [61] To perform the assigments of the redox couples of TiOPc in this study, performed spectroelectrochemical measurements were given in Fig. 13. During the controlled potential reduction of TiOPc at 0.55 V (Fig. 13a), while the absorption of the Q-band corresponding to the neutral [TiIVOPc2] species shifts from 704 to 635 nm, a new band is recorded at 590 nm. As shown in Fig. 13a, the process occurred with clear isosbestic points at 640 and 756 nm in the spectra which demonstrate that there is no chemical reaction complicating the electron transfer reaction. These spectral changes are typical for a metal-based reduction process of titanyl phthalocyanine complexes, [TiIVOPc2]/[TiIIIOPc2]1 [61,77e80]. During the second reduction of TiOPc at 0.70 V, the absorption for the Q band at 635 nm and the band at 590 nm decrease slightly, while a new band is recorded at 530 nm. These spectral changes are easily assigned to a ring reduction process, [TiIIIOPc2]1/[TiIIIOPc3]2 redox process [61,77e80]. The spectral changes given in Fig. 13c indicate the spectral changes recorded during the third reduction at 0.90 V. During this process, while the bands at 590 and 635 nm decrease in intensity, a new sharp band is recorded at 676 nm. At the same time, the band at 544 nm increases in intensity. These spectral changes characterizes a metal-based reduction process and assigned to the [TiIIIOPc3]2/[TiIIOPc3]3. The spectral changes during the fourth reduction process indicate a ring-based reduction process (Fig. 13d). TiOPc decomposes under the applied potential at

4. Conclusions In the presented work, the syntheses of new imidazole substituted phthalonitrile and its corresponding metal free, zinc(II), nickel(II), cobalt(II), lead(II) and oxo-titanium(IV) phthalocyanines were described and these novel compounds were characterized by elemental analysis, FT-IR, 1H NMR, 13CN MR, electronic spectroscopy and mass spectra. Electrochemical and spectroelectrochemical measurement revealed that incorporation redox active metal centers, CoII, TiIVO, and MnIIIOAc into the phthalocyanine core extend the redox richness of the Pc ring with the reversible metal-based reduction and oxidation couples in addition to the common Pc ring-based electron transfer processes. This expanded redox behaviour of the complexes are the desired properties of the electrochemical applications, especially, electrocatalytic, electrochromic and electrosensing applications. Solvent of the electrolyte affects the redox properties of the complexes which have redox active metal center due to the coordination ability differences of the solvents. In-situ electrocolorimetric measurements of the new complexes allow quantification of color coordinates of the each electrogenerated anionic and cationic redox species. Different color of the electrogenerated species indicates their possible application in the display technologies, e.g. electrochromic and data storage application. Presence of O2 in the electrolyte system influences the redox couples of the complexes due to the interaction between O2 and MPcs having redox active metal center, which is the desired properties for the fuel cell applications of the complexes as cathode active material. Multiple, reversible, and metal and ring based electron transfer reactions and distinct color changes of the TiOPc complex indicate its possible application in various electrochemical technologies such as electrocatalytic, electrosensor and electrochromatic applications. Acknowledgements This study was supported by Yıldız Technical University (Project No: 2012-01-02-KAP03), Marmara University (Project No: FEN-CYLP-110411-0098), TUBITAK (Project No: 111T179) and Research Fund of Karadeniz Technical University (Project No: 2010.111.002.1). References [1] Leznoff CC, Lever ABP. Phthalocyanines properties and applications, vol. 1. New York: VCH Publisher; 1989. [2] Parra V, Bouvet M, Brunet J, Rodríguez-Méndez ML, Saja JA. On the effect of ammonia and wet atmospheres on the conducting properties of different lutetium bisphthalocyanine thin films. Thin Solid Films 2008;516:9012e9.

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