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A new unsymmetrical phthalocyanine with a single o-carborane substituent
Journal of Organometallic Chemistry 781 (2015) 53e58
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Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem
A new unsymmetrical phthalocyanine with a single o-carborane substituent € Nilgün Ozgür, Ilgın Nar, Ahmet Gül, Esin Hamuryudan* Department of Chemistry, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 November 2014 Received in revised form 30 December 2014 Accepted 14 January 2015 Available online 23 January 2015
Unsymmetrical zinc phthalocyanine carrying a single o-carborane substituent on the peripheral position was synthesized in a multistep reaction sequence. At the first step, metal-free phthalocyanine 3 was prepared by cyclotetramerization of 4,5-di(hexylthio)phthalonitrile and 4-(2-hydroxyethylthio)phthalonitrile in pentanol in the presence of lithium; and zinc phthalocyanine 4 was prepared by insertion of Zn(II) salt. After esterification of phthalocyanine 4 with 4-pentynoic acid, terminal alkynyl bearing precursor phthalocyanine 5 was obtained. At the last step, reaction of decaborane with phthalocyanine 5 yielded phthalocyanine 6 with the desired o-carborane moiety. The new phthalocyanine compounds were characterized by elemental analysis, NMR, FT-IR, UVevis and mass spectral data. Aggregation behaviour and electrochemical properties of these newly synthesized unsymmetrical phthalocyanine derivatives were also reported. © 2015 Elsevier B.V. All rights reserved.
Keywords: Phthalocyanine Carborane BNCT Unsymmetric Zinc
Introduction Multifaceted electrochemical and spectroscopic properties of phthalocyanines have attracted a great deal of attention from researchers in diverse fields [1] such as catalysis [2], nonlinear optics [3], photovoltaic cells, solar energy conversion [4], chemical sensors [5,6], photodynamic cancer therapy [7,8], etc. The peripheral or nonperipheral substituents of phthalocyanines such as alkyl chains or bulky groups increase the solubility properties of corresponding phthalocyanines and make them beneficial in different application areas [9,10]. However, there are some limitations of using symmetrical phthalocyanines. Hence, recently, researchers have been interested in low symmetrical phthalocyanines which enrich the specifications and make them usable in a more controlled manner [11e15]. In addition, the boron containing cluster systems have recently become an extensively studied class of compounds due to their three-dimensional structure: aromaticity, high chemical and thermal stability and incorporation into various organic molecules as substituents [16]. Boron containing cluster systems such as carborane, cobalt bis dicarballide and dodecaborate anion are used in substitution of
* Corresponding author. Tel.: þ90 212 285 68 26. E-mail address: [email protected] (E. Hamuryudan). http://dx.doi.org/10.1016/j.jorganchem.2015.01.011 0022-328X/© 2015 Elsevier B.V. All rights reserved.
phthalocyanines and these phthalocyanine derivatives are potential sentisizers for photodynamic therapy (PDT) and boron neutron capture therapy (BNCT) [17e24]. Furthermore, substitution of phthalocyanines with o-carborane group enhances the properties of the compound as it displays a rich chemistry. The substituted ocarboranes are electrochemically more powerful. The redox activity of phthalocyanine may improve when it is substituted with o-carborane group due to the high stability, electrochemistry and ease of functionalization related to o-carborane [25e29]. In this study, we have synthesized an unsymmetrical phthalocyanine carrying a single o-carborane unit on the periphery through a multistep reaction sequence. We also report spectroscopic characterization, aggregation behaviour and electrochemical properties of these newly synthesized unsymmetrical phthalocyanine derivatives. Experimental section Materials and equipment Precursor phthalonitriles 4,5-di(hexylthio)phthalonitrile (1) and 4-(2-hydroxyethylthio) phthalonitrile (2) were prepared according to the literature [30,31]. All necessary reagents and solvents were of reagent grade quality obtained from commercial sources. Silica gel (Kieselgel 60, 200e400 mesh) was chosen as stationary phase for column chromatography. FT-IR spectra were
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obtained on a Perkin Elmer Spectrum One FT-IR spectrophotometer. 1H and 11B NMR spectra were taken in CDCl3 and pyridine d5 solutions, recorded on Agilent VNMRS 500 MHz spectrometer. Electronic spectra were measured on Scinco LabProPlus UV/vis spectrophotometer and mass spectra were performed on Bruker microflex LT MALDI-TOF mass spectrometer. Electrochemical measurements were carried out with a Gamry Reference 600 potentiostat/galvanostat. Cyclic voltammetry (CV) and square-wave voltammetry (SWV) were performed in a threeelectrode cell with a platinum disk (0.071 cm2 surface area) working, a platinum wire counter and a saturated calomel reference electrodes. Saturated calomel electrode (SCE) was connected to the solution through a bridge. Dichloromethane (DCM) containing 0.1 mol dm3 tetrabutylammonium perchlorate (TBAP) was used as supporting electrolyte. In situ UVevis absorption spectra were measured by an Ocean Optics HR2000 þ diode array spectrophotometer. For in situ spectroelectrochemical measurements, an optically transparent thin layer quartz cell of path length 1 mm was employed, in which a platinum gauze electrode, a platinum wire and a SCE were used for the working, counter and reference electrodes, respectively.
1352.52 [M þ H]þ; anal. calcd. for C70H92ZnN8OS7; C, 62.21; H, 6.86; N, 8.29; found: C, 62.25; H, 6.77; N, 8.35%. [2,3,9,10,16,17-Hexakis(hexylthio)-23-1pentynyloxyethylthiophthalocyaninato] zinc(II) (5) 10 mg of 4-pentynoic acid (0.12 mmol) and 20 mL of anhydrous dichloromethane were added to a three-necked 50 mL flask. After the 4-pentynoic acid was solved, 150 mg (0.11 mmol) of 4 and 24 mg (0.12 mmol) of dicyclohexylcarbodiimide (DCC) were added. Then 7 mg (0.055 mmol) of N,N-dimethylaminopyridine (DMAP), dissolved in 1.5 mL of anhydrous dichloromethane, was added dropwise. The mixture was stirred for 48 h at room temperature under N2 atmosphere. The solution was filtered and the filtrate was placed in the freezer for 30 min and then filtered again. Following the evaporation of the solvent, purification of the solid crude product 5 was accomplished by column chromatography with dichloromethane: methanol (50:1) as eluents to give a dark green solid. Yield: 95 mg (59.8%); m.p. > 200 C. IR y (cm1): 3310 (alkynyl eCH), 2950e2850 (alkyl eCH), 2202 (-C^CH), 1720 (eC]O); UVevis lmax (nm) in THF: 360, 630, 698; MALDI-TOF MS (matrix DHB) m/z: 1432.47 [M þ H]þ; anal. calcd. for C75H96ZnN8O2S7; C, 62.93; H, 6.76; N, 7.83; found: C, 63.01; H, 6.71; N, 7.88%.
Synthesis 2,3,9,10,16,17-Hexakis(hexylthio)-23hydroxyethylthiophthalocyanine (3) A mixture containing 0.432 g (1.2 mmol) of 4,5-di(hexylthio) phthalonitrile 1 and 0.08 g (0.4 mmol) of 4- (2-hydroxyethylthio) phthalonitrile 2 in 3 ml pentanol was heated and stirred at 140 C under N2 in a sealed tube. After 15 min, 0.015 g (2 mmol) lithium metal was added to the mixture and refluxed for 4 h. The reaction mixture was cooled to ambient temperature and then poured into 25 ml methanol and acidified with acetic acid until the crude product precipitated. In this mixture, lithium phthalocyanine derivatives were converted into metal free phthalocyanines. The precipitate was centrifuged and washed several times with hot methanol to remove unreacted materials. Finally, the green residue was chromatographed on silica gel using dichloromethane: methanol as the eluent, changing from 150:1 to 75:1 (v/v), to result in 3 as a green solid on the second fraction. Yield: 0.13 g (26%); m.p. > 200 C. IR y (cm1): 3300 (eOH), 3290 (eNH), 2950e2850 (alkyl eCH); 1H NMR (CDCl3) d ppm: 8.42e7.57 (m, 9H, AreH), 4.71 (s, H, OH), 4.19 (m, 2H, OCH2), 3.68 (m, 2H, SCH2), 3.56e3.24 (m, 12H, SCH2), 2.03e1.26 (m, 48H, eCH2), 1.05 (m, 18H, CH3), 4.3 (br, 2H, NeH); UVevis lmax (nm) in THF: 342, 694, 725; MALDI-TOF MS (matrix DHB) m/z: 1287.85 [M]þ; anal. calcd. for C70H94N8OS7; C, 65.28; H, 7.36; N, 8.70; found: C, 65.17; H, 7.45; N, 8.78%.
[2,3,9,10,16,17-Hexakis(hexylthio)-23-1-(o-carboranyl) propanoxy ethylthio phthalocyaninato]zinc(II) (6) 0.019 g (0.154 mmol) of decaborane (B10H14) was dissolved in a mixture of 10 mL dry toluene and 6 mL dry acetonitrile. The reaction mixture was heated to 90 C. After 2 h, 0.20 g (0.140 mmol) phthalocyanine 5 was added. This mixture was heated to reflux for 48 h. After the reaction mixture was cooled to room temperature, it was filtered, and all solvents were evaporated. Purification of the solid crude product 6 was accomplished by column chromatography with THF as eluent to give a green solid. Yield: 0.086 g (40.0%); m.p. > 200 C. IR y (cm1): 2950e2850 (alkyl eCH), 2571 (-BH); 1H NMR (pyridine d5): 9.73e9.45 (m, 6H, AreH), 8.97e8.38 (m, 3H, AreH), 4.74 (m, 2H, CH2), 4.41 (s, H, CH), 3.92 (m, 2H, OCH2), 3.80 (m, 2H, SCH2), 3.56 (m, 12H, SCH2), 3.22e1.10 (b, 10H, BH), 2.84 (m, 2H, CH2OC]O), 2.13 (m, 2H, CCH2), 2.03 (m, 12H, SCCCH2), 1.70e1.30 (m, 24H, CCH2), 0.93 (m, 18H, CH3); 11B NMR (pyridine d5): d, ppm 2.34 (b, 1B); 5.34 (b, 1B), 9.18 (b, 3B), 11.21 (b, 5B).; UVevis lmax (nm) in THF: 361, 632, 700; MALDI-TOF MS (matrix DHB) m/z: 1551.72 [M]þ, 1352.090 [M-C5H17B10O]þ; anal. calcd. for C75H108B10ZnN8O2S7; C, 58.05; H, 7.02; N, 7.22; found: C, 57.96; H, 6.93; N, 7.28%. Results and discussion Synthesis and characterization
[2,3,9,10,16,17-Hexakis(hexylthio)-23hydroxyethylthiophthalocyaninato]zinc(II) (4) A solution of 0.1 g (0.08 mmol) 3 and 0.044 g (0.24 mmol) anhydrous Zn(CH3COO)2 was refluxed in 2 ml of 1-pentanol with stirring for 4 h under N2 atmosphere. The resulting suspension was cooled to ambient temperature and then poured into 20 ml methanol. The precipitate was filtered off, washed successively with water, hot methanol, hot aceton and hot hexane. The purification was carried out by column chromatography on silica gel using dichloromethane: methanol (50:1) as the eluent to result in 4 as a green solid. Yield: 0.10 g (94.4%); m.p. > 200 C. IR y (cm1): 3360 (eOH), 2950e2850 (alkyl eCH); 1H NMR (CDCl3 þ 1 drop pyridine d5) d ppm: 9.10e8.82 (br, 5H, AreH), 8.55 (s, 1H, AreH), 8.03 (m, 2H, AreH), 7.62 (s, 1H, AreH), 4.64 (s, H, OH), 4.15 (m, 2H, OCH2), 3.84 (m, 2H, SCH2), 3.44e2.86 (m, 12H, SCH2), 2.02e1.72 (m, 24H, CH2), 1.44e1.22 (m, 24H, CH2), 0.96 (m, 18H, CH3); UVevis lmax (nm) in THF: 361, 629, 699; MALDI-TOF MS (matrix DHB) m/z:
4,5-Di(hexylthio)phthalonitrile (1) and 4- (2-hydroxyethylthio) phthalonitrile (2) were chosen as precursors and they were synthesized as reported in the literature [30,31]. Scheme 1 shows the synthetic pathway to 2,3,9,10,16,17-hexakis(hexylthio)-23-hydroxy ethylthiophthalocyanine (3) and [2,3,9,10,16,17-hexakis(hexylthio)23-hydroxyethylthiophthalocyaninato]zinc(II) (4). A3B type metal free phthalocyanine derivative 3 was synthesized in two steps. Preparation of the metal-free phthalocyanine begins with the lithium templated cyclotetramerization of hexylthio and hydroxyethylthio substituted phthalonitriles in a high boiling point solvent (1-pentanol) to obtain dilithium phthalocyanine [14,32]. In the second step, dilithium phthalocyanine was converted to metal-free phthalocyanine 3 by acidification with acetic acid. Column chromatography was used for the isolation of the A3B type product with 26% yield. Metallation was achieved by refluxing 3 in 1-pentanol in the presence of zinc (II) acetate under N2 atmosphere. The product
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Scheme 1. Synthesis of phthalocyanine complexes 3, 4 and 5.
was compatible with those reported earlier [15]. Steglich esterification was achieved with dicyclohexylcarbodiimide (DCC)/N,Ndimethylaminopyridine (DMAP) by reacting the 4-pentynoic acid with 4 in the presence of CH2Cl2 [33]. The reaction was carried out at ambient temperature by stirring the mixture for 24 h under N2 atmosphere. During the reaction, urea derivatives was formed in
only slight amounts which were generally removed easily by routine techniques including precipitation and column chromatography. For the synthesis of the phthalocyanine 6; phthalocyanine 5 and decaborane (B10H14) were reacted in a mixture of acetonitrile and toluene to give the o-carboran substituted phthalocyanine 6 in 40% yields as shown in Scheme 2 [24].
Scheme 2. Synthesis of phtalocyanine complex 6.
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The phthalocyanines 3, 4, 5 and 6 are soluble in solvents such as dichloromethane, tetrahydrofuran, chloroform, dimethylformamide and ethyl acetate. Characterization of the newly synthesized phthalocyanine derivatives was achieved by a combination of methods including elemental analysis, FT-IR, UVevis, 1H NMR, 11B NMR, and MALDI-TOF spectral data. The data are consistent with the proposed structures. The IR spectra of metal-free 3 and metallo phthalocyanine 4 were very similar, apart from the eNH stretching vibrations of the inner phthalocyanine core in the metal-free molecule, which were assigned to a band at 3290 cm1. Hexylthio and ethylthio side chains were verified by the observation of an intense aliphatic CeH stretching band. The FT-IR spectrum of 5 showed the ≡CeH absorption band at 3312 cm1 and eC^CH group absorption band at 2202 cm1. Besides, absorption band of eC]O was observed at 1720 cm1. The fact that the band of eBH stretching vibration was detected at 2571 cm1 with disappearance of the bands at 3312 cm1 and 2202 cm1 in the FT-IR spectrum was significant evidence of the formation of carborane cluster on phthalocyanine 6. The 1H NMR spectra of metallo phthalocyanine 4 was identical to that of metal-free phthalocyanine 3 in deuterated chloroform and a trace amount of deuterated pyridine. The ordinary shielding of the inner core protons leading to a chemical shift in negative ppm region pertains to metal-free phthalocyanine, which was observed at the 3.87 ppm. 1H NMR spectrum of 4 indicated the aromatic protons at around d 9.10e7.62 ppm, the SCH2 protons as multiplets at d 3.84 and 3.44e2.86 ppm. The aliphatic protons eCH3 and eCH2 protons adjacent to them appeared at around d 0.96 and 2.02e1.22 ppm as multiplets. The OH protons were observed at d 4.71 ppm for metal-free 3, d 4.64 ppm for zinc(II) 4 phthalocyanines and the signal disappeared by deuterium exchange with D2O [30]. The 1H NMR spectrum of 6 showed a broad signal of BH protons at 3.22e1.10 ppm and the signal of the CCH proton on ocarborane at 4.41 ppm. The 11B-NMR spectra of 6 showed four broad signals. This pattern is typical of the expected o-carborane system. The elemental analyses of the compounds verified the proposed structures. MALDI-TOF MS spectrum showed molecular ion peaks at m/z ¼ 1287.85 [M]þ for metal-free 3, 1352.52 [M þ H]þ for metallo phthalocyanine 4, 1432.47 [M þ H]þ for metallo phthalocyanine 5 and 1551.72 as [M]þ for o-carborane containing phthalocyanine 6 (Fig. 1).
Ground state absorption spectra and aggregation properties Substituted metal free and metallo phthalocyanines show characteristic electronic absorption spectra with two strong bands, one in the visible region at 600e700 nm (Q band) attributed to the pep* transition from the HOMO to the LUMO of the phthalocyanine ring and the other in the UV region at approximately 300e400 nm (B-Soret) band [34]. Q band absorptions were observed at 694 and 725 nm for 3 as a splitted band in THF. In the UVevis spectrum of phthalocyanine 4, an intense sharp band was recorded at 699 nm with weaker absorption peak at 629 nm in THF. Zinc phthalocyanines 5 and 6 showed the characteristic Q bands at 698 and 699 nm with no splits and weaker absorptions at 630 and 631 nm, respectively, in THF. The soret bands of phthalocyanine derivatives 3e6 were observed at 342, 361, 360 and 361 nm, respectively. Aggregation reduces solubility and the excited state lifetime and complicates the characterization and purification of phthalocyanine molecules because of composed dimers, trimers and higher oligomers [35,36]. Decreasing aggregation tendency of phthalocyanines can be achieved by combining bulky groups in the a or b positions at the benzo units and axial substitution of the central metal ion. From this point of view, new unsymmetrical phthalocyanine derivatives were synthesized and their aggregation properties were investigated by recording UVevis absorptions of phthalocyanine 6 at different concentrations in THF. As shown in Fig. 2, intensity of absorption of the Q-band for 6 was increased in parallel with increasing concentration from 2 106 M to 12 106 M without any new absorption band adjacent to Q band. Considering these results, phthalocyanine 6 had monomeric structure and obeyed BeereLambert law in this concentration range. The Q-band positions and intensities were changed by the refractive index of the solvents as shown in Fig. 3. The largest blue shift of the Q-band was recorded in EtOAc at 695 nm, with the greatest red shift in DCM at 708 nm [37]. Electrochemical and spectroelectrochemical studies To examine the reduction and oxidation potentials of unsymmetrical phthalocyanine complexes 4, 5 and 6, we undertook electrochemical studies by means of cyclic voltammetry (CV) and square wave voltammetry (SWV) in dichloromethane using
Fig. 1. MALDI-TOF mass spectrum of 6.
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Fig. 2. Absorption spectra of compound 6 in THF at different concentrations.
tetrabutylammonium perchlorate (TBAP) as supporting electrolyte system on a platinum working electrode. Cyclic and square wave voltammograms of 4 are shown in Fig. 4. It displayed two one electron reductions and an oxidation versus SCE. The E1/2 (DE/mV) values were determined as 0.97 V and 1.32 V for the first and the second reduction waves and 0.70 V for oxidation at 0.1 V s1 scan rate. In Fig. 5 o-carborane containing phthalocyanine complex 6 showed very similar responses with phthalocyanine complexes 4 and 5. It displayed two one electron reduction process at 0.89 V and 1.21 V and one oxidation process at 0.68 V versus SCE at 0.1 V s1. The results we obtained are tabulated on Table 1 and the electrochemical behaviour of phthalocyanines 4, 5 and 6 were compared with zinc phthalocyanine with similar substituents other than o-carborane. Spectroelectrochemical measurements were executed to enlighten the nature of the redox processes. It is known that the oxidation state of the central zinc ion with completely full d-orbitals does not change [1,13,19,38e44]. Thus, all reduction and oxidation processes of the complexes should be referred to phthalocyanine ring or connected group. Compound 6 indicated only phthalocyanine ring-based reduction or oxidation during spectroelectrochemical measurements. The spectral changes of 6 are shown in Fig. 6. Fig. 6a shows the in situ UVevis spectral changes during controlled potential reduction of 6 at 0.90 V. The
Fig. 4. Cyclic and square wave voltamograms of 4 in DCM/TBAP.
Fig. 5. Cyclic and square wave voltamograms of 6 in DCM/TBAP.
intensity of the Q band at 705 nm increases with a shift to 699 nm while the intensity of B band at 359 nm increases with a shift to 375 nm during the reduction of 6 at 0.9 V potential application. In the spectrum, there are clear isosbestic points at 360, 407, 668, and 712 nm. All of these data support that the reduction is a phthalocyanine ring reduction. Decrease in the Q band intensity at 698 nm Table 1 The electrochemical potentials of phthalocyanine derivatives.
Fig. 3. Absorption spectra of compound 6 in different solvents. Concentration: 105 M.
Compound
Solvent
4 5 6 ZnPc ZnPc ZnPc
DCM DCM DCM DCM THF DCM
Ox2
Oxı
RR1
RR2
Ref.
0.805 0.36 0.814
0.70 0.71 0.68 0.60 0.14 0.633
0.97 0.87 0.89 0.79 1.37 0.78
1.32 1.22 1.21 1.10 1.76 1.027
tw. tw. tw. [40] [41] [42]
tw: This work. RR: Ring Reduction. Ox: Oxidation.
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References
Fig. 6. Absorption spectral changes in the electrolysis at an applied potential of (a) 0.90 V and (b) 1.3 V vs. SCE for the first and second reduction of 6 in deaerated DCM containing TBAF. The arrows show the direction of the changes.
without shift and decrease in the B band intensity with a shift to 382 nm indicate the second ring reduction process during the potential application at 1.3 V (Fig. 6b). During the oxidation process of 6, the decrease in the absorption of the Q band and B band in intensity without shift shows ring based oxidation. Conclusions A new soluble unsymmetrical zinc phthalocyanine with an ocarborane moiety on the periphery has been reported in this work for the first time. The electrochemical and spectroelectrochemical studies show that all synthesized zinc phthalocyanine complexes give ring-based redox processes. Further studies on the applicability of the phthalocyanine with carborane group in both PDT and BNCT are in progress. Acknowledgements This work was supported by TUBITAK (Project number: 211T052). AG thanks Turkish Academy of Sciences (TUBA) for partial support.
[1] C.C. Leznoff, A.B.P. Lever, Phthalocyanines: Properties and Applications, VCH Publishers, New York, 1990-1996. [2] A.B. Sorokin, Chem. Rev. 113 (2013) 8152e8191. [3] H.S. Nalwa, T. Saito, A. Kakuta, T. Iwayanagi, J. Phys. Chem. 97 (1993) 10515e10517. [4] C.C. Wamser, M.G. Walter, A.B. Rudine, J. Porphyr. Phthalocyanines 14 (2010) 759e792. € € Bekarog €pel, Z.Z. Oztürk, lu, Appl. Organomet. Chem. [5] R. Zhou, F. Josse, W. Go O. 10 (1996) 557e577. € [6] V. Ahsen, D. Atilla, A.G. Gürek, N. Kılınç, Z.Z. Oztürk, J. Porphyr. Phthalocyanines 13 (2009) 1179e1187. [7] T. Stuchinskaya, M. Moreno, M.J. Cook, D.R. Edwards, D.A. Russell, Photochem. Photobiol. Sci. 10 (2011) 822e831. [8] C.M. Whitacre, D.K. Feyes, T. Satoh, J. Grossmann, J.W. Mulvihill, H. Mukhtar, N.L. Oleinick, Clin. Cancer Res. 6 (2000) 2021e2027. [9] M. Selcukoglu, E. Hamuryudan, Dyes Pigm. 74 (2007) 17e20. lu, E. Hamuryudan, Dyes Pigm. 100 (2014) 32e40. [10] S. Sürgün, Y. Arslanog €. Bekaroǧlu, Chem. Ber. 127 (1994) 2009e2012. [11] S. Dabak, A. Gül, o [12] M. Garcia-Iglesias, J.-H. Yum, R. Humphry-Baker, S.M. Zakeeruddin, P. Pechy, P. Vazquez, E. Palomares, M. Gratzel, M.K. Nazeeruddin, T. Torres, Chem. Sci. 2 (2011) 1145e1150. [13] B.N. S¸en, H. Mert, H. Dinçer, A. Koca, Dyes Pigm. 100 (2014) 1e10. € _ Ozçes [14] I. ¸ meci, A.K. Burat, Z.A. Bayır, J. Organomet. Chem. 750 (2014) 125e131. [15] D. Topkaya, F. Dumoulin, V. Ahsen, U. Isci, Dalton Trans. 43 (2014) 2032e2037. [16] B.P. Dash, R. Satapathy, J.A. Maguire, N.S. Hosmane, New J. Chem. 35 (2011) 1955e1972. € hrle, I.B. Sivaev, D. Gabel, V.I. Bregadze, J. Porphyr. Phthalocyanines 05 [17] D. Wo (2001) 767e781. [18] V.I. Bregadze, I.B. Sivaev, A. Gül, A.F. Mironov, Macroheterocycles 5 (2012) 292e301. €z, I. Nar, A. Gül, J. Organomet. Chem. 755 (2014) 64e71. [19] B. Birso mus¸, Spectrochim. Acta Part A Mol. [20] G.Y. Atmaca, C. Dizman, T. Eren, A. Erdog Biomol. Spectrosc. 137 (2015) 244e249. [21] D. Pietrangeli, A. Rosa, A. Pepe, G. Ricciardi, Inorg. Chem. 50 (2011) 4680e4682. [22] D. Pietrangeli, A. Rosa, S. Ristori, A. Salvati, S. Altieri, G. Ricciardi, Coord. Chem. Rev. 257 (2013) 2213e2231. [23] E. Friso, G. Roncucci, D. Dei, M. Soncin, C. Fabris, G. Chiti, P. Colautti, J. Esposito, L. De Nardo, C. Riccardo Rossi, D. Nitti, F. Giuntini, L. Borsetto, G. Jori, Photochem. Photobiological Sci. 5 (2006) 39e50. € hrle, O. Tsaryova, A. Semioshkin, D. Gabel, O. Suvorova, J. Organomet. [24] D. Wo Chem. 747 (2013) 98e105. [25] J.H. Morris, H.J. Gysling, D. Reed, Chem. Rev. 85 (1985) 51e76. [26] L. Weber, J. Kahlert, L. Bohling, A. Brockhinke, H.-G. Stammler, B. Neumann, R.A. Harder, P.J. Low, M.A. Fox, Dalton Trans. 42 (2013) 2266e2281. e pni [27] A. Korotvi cka, I. Snajdr, P. St cka, I. Císarov a, Z. Janousek, M. Kotora, Eur. J. Inorg. Chem. 2013 (2013) 2789e2798. [28] H. Tricas, M. Colon, D. Ellis, S.A. Macgregor, D. McKay, G.M. Rosair, A.J. Welch, I.V. Glukhov, F. Rossi, F. Laschi, P. Zanello, Dalton Trans. 40 (2011) 4200e4211. [29] M. Fox, C. Nervi, A. Crivello, A. Batsanov, J.K. Howard, K. Wade, P. Low, J. Solid State Electrochem. 13 (2009) 1483e1495. [30] I. Ozcesmeci, A.I. Okur, A. Gul, Dyes Pigm. 75 (2007) 761e765. [31] A.G. Gurek, O. Bekaroglu, J. Chem. Dalton Trans. (1994) 1419e1423. [32] M.J. Lin, J.D. Wang, J. Mol. Struct. 837 (2007) 284e289. [33] B. Akkurt, E. Hamuryudan, Dyes Pigm. 79 (2008) 153e158. [34] M. Ozcesmeci, I. Ozcesmeci, E. Hamuryudan, Polyhedron 29 (2010) 2710e2715. [35] M. Kostka, P. Zimcik, M. Miletin, P. Klemera, K. Kopecky, Z. Musil, J. Photochem. Photobiol. A Chem. 178 (2006) 16e25. [36] E.M. Maya, A.W. Snow, J.S. Shirk, R.G.S. Pong, S.R. Flom, G.L. Roberts, J. Mater. Chem. 13 (2003) 1603e1613. [37] M. Kryjewski, M. Nowak, P. Kasprzycki, P. Fita, C. Radzewicz, T. Goslinski, J. Mielcarek, Inorg. Chem. Commun. 27 (2013) 56e59. lu, A. Koca, M.B. Koçak, Synth. Met. 182 (2013) 1e8. [38] H.R.P. Karaog € _ Ozçes _ [39] I. ¸ meci, A.K. Burat, Y. Ipek, A. Koca, Z.A. Bayır, Electrochim. Acta 89 (2013) 270e277. € € Bekaroǧlu, Polyhedron 16 (1997) [40] A.R. Ozkaya, A.G. Gürek, A. Gül, O. 1877e1883. [41] S.Z. Topal, U. Isci, U. Kumru, D. Atilla, A.G. Gurek, C. Hirel, M. Durmus, J.B. Tommasino, D. Luneau, S. Berber, F. Dumoulin, V. Ahsen, Dalton Trans. 43 (2014) 6897e6908. [42] A. Kalkan, A. Koca, Z.A. Bayır, Polyhedron 23 (2004) 3155e3162. € [43] S¸. Ozçelik, A. Koca, A. Gül, Polyhedron 42 (2012) 227e235. lu, A. Koca, E. Hamuryudan, Dyes Pigm. 88 (2011) 135e142. [44] Y. Arslanog
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Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem
A new unsymmetrical phthalocyanine with a single o-carborane substituent € Nilgün Ozgür, Ilgın Nar, Ahmet Gül, Esin Hamuryudan* Department of Chemistry, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 November 2014 Received in revised form 30 December 2014 Accepted 14 January 2015 Available online 23 January 2015
Unsymmetrical zinc phthalocyanine carrying a single o-carborane substituent on the peripheral position was synthesized in a multistep reaction sequence. At the first step, metal-free phthalocyanine 3 was prepared by cyclotetramerization of 4,5-di(hexylthio)phthalonitrile and 4-(2-hydroxyethylthio)phthalonitrile in pentanol in the presence of lithium; and zinc phthalocyanine 4 was prepared by insertion of Zn(II) salt. After esterification of phthalocyanine 4 with 4-pentynoic acid, terminal alkynyl bearing precursor phthalocyanine 5 was obtained. At the last step, reaction of decaborane with phthalocyanine 5 yielded phthalocyanine 6 with the desired o-carborane moiety. The new phthalocyanine compounds were characterized by elemental analysis, NMR, FT-IR, UVevis and mass spectral data. Aggregation behaviour and electrochemical properties of these newly synthesized unsymmetrical phthalocyanine derivatives were also reported. © 2015 Elsevier B.V. All rights reserved.
Keywords: Phthalocyanine Carborane BNCT Unsymmetric Zinc
Introduction Multifaceted electrochemical and spectroscopic properties of phthalocyanines have attracted a great deal of attention from researchers in diverse fields [1] such as catalysis [2], nonlinear optics [3], photovoltaic cells, solar energy conversion [4], chemical sensors [5,6], photodynamic cancer therapy [7,8], etc. The peripheral or nonperipheral substituents of phthalocyanines such as alkyl chains or bulky groups increase the solubility properties of corresponding phthalocyanines and make them beneficial in different application areas [9,10]. However, there are some limitations of using symmetrical phthalocyanines. Hence, recently, researchers have been interested in low symmetrical phthalocyanines which enrich the specifications and make them usable in a more controlled manner [11e15]. In addition, the boron containing cluster systems have recently become an extensively studied class of compounds due to their three-dimensional structure: aromaticity, high chemical and thermal stability and incorporation into various organic molecules as substituents [16]. Boron containing cluster systems such as carborane, cobalt bis dicarballide and dodecaborate anion are used in substitution of
* Corresponding author. Tel.: þ90 212 285 68 26. E-mail address: [email protected] (E. Hamuryudan). http://dx.doi.org/10.1016/j.jorganchem.2015.01.011 0022-328X/© 2015 Elsevier B.V. All rights reserved.
phthalocyanines and these phthalocyanine derivatives are potential sentisizers for photodynamic therapy (PDT) and boron neutron capture therapy (BNCT) [17e24]. Furthermore, substitution of phthalocyanines with o-carborane group enhances the properties of the compound as it displays a rich chemistry. The substituted ocarboranes are electrochemically more powerful. The redox activity of phthalocyanine may improve when it is substituted with o-carborane group due to the high stability, electrochemistry and ease of functionalization related to o-carborane [25e29]. In this study, we have synthesized an unsymmetrical phthalocyanine carrying a single o-carborane unit on the periphery through a multistep reaction sequence. We also report spectroscopic characterization, aggregation behaviour and electrochemical properties of these newly synthesized unsymmetrical phthalocyanine derivatives. Experimental section Materials and equipment Precursor phthalonitriles 4,5-di(hexylthio)phthalonitrile (1) and 4-(2-hydroxyethylthio) phthalonitrile (2) were prepared according to the literature [30,31]. All necessary reagents and solvents were of reagent grade quality obtained from commercial sources. Silica gel (Kieselgel 60, 200e400 mesh) was chosen as stationary phase for column chromatography. FT-IR spectra were
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obtained on a Perkin Elmer Spectrum One FT-IR spectrophotometer. 1H and 11B NMR spectra were taken in CDCl3 and pyridine d5 solutions, recorded on Agilent VNMRS 500 MHz spectrometer. Electronic spectra were measured on Scinco LabProPlus UV/vis spectrophotometer and mass spectra were performed on Bruker microflex LT MALDI-TOF mass spectrometer. Electrochemical measurements were carried out with a Gamry Reference 600 potentiostat/galvanostat. Cyclic voltammetry (CV) and square-wave voltammetry (SWV) were performed in a threeelectrode cell with a platinum disk (0.071 cm2 surface area) working, a platinum wire counter and a saturated calomel reference electrodes. Saturated calomel electrode (SCE) was connected to the solution through a bridge. Dichloromethane (DCM) containing 0.1 mol dm3 tetrabutylammonium perchlorate (TBAP) was used as supporting electrolyte. In situ UVevis absorption spectra were measured by an Ocean Optics HR2000 þ diode array spectrophotometer. For in situ spectroelectrochemical measurements, an optically transparent thin layer quartz cell of path length 1 mm was employed, in which a platinum gauze electrode, a platinum wire and a SCE were used for the working, counter and reference electrodes, respectively.
1352.52 [M þ H]þ; anal. calcd. for C70H92ZnN8OS7; C, 62.21; H, 6.86; N, 8.29; found: C, 62.25; H, 6.77; N, 8.35%. [2,3,9,10,16,17-Hexakis(hexylthio)-23-1pentynyloxyethylthiophthalocyaninato] zinc(II) (5) 10 mg of 4-pentynoic acid (0.12 mmol) and 20 mL of anhydrous dichloromethane were added to a three-necked 50 mL flask. After the 4-pentynoic acid was solved, 150 mg (0.11 mmol) of 4 and 24 mg (0.12 mmol) of dicyclohexylcarbodiimide (DCC) were added. Then 7 mg (0.055 mmol) of N,N-dimethylaminopyridine (DMAP), dissolved in 1.5 mL of anhydrous dichloromethane, was added dropwise. The mixture was stirred for 48 h at room temperature under N2 atmosphere. The solution was filtered and the filtrate was placed in the freezer for 30 min and then filtered again. Following the evaporation of the solvent, purification of the solid crude product 5 was accomplished by column chromatography with dichloromethane: methanol (50:1) as eluents to give a dark green solid. Yield: 95 mg (59.8%); m.p. > 200 C. IR y (cm1): 3310 (alkynyl eCH), 2950e2850 (alkyl eCH), 2202 (-C^CH), 1720 (eC]O); UVevis lmax (nm) in THF: 360, 630, 698; MALDI-TOF MS (matrix DHB) m/z: 1432.47 [M þ H]þ; anal. calcd. for C75H96ZnN8O2S7; C, 62.93; H, 6.76; N, 7.83; found: C, 63.01; H, 6.71; N, 7.88%.
Synthesis 2,3,9,10,16,17-Hexakis(hexylthio)-23hydroxyethylthiophthalocyanine (3) A mixture containing 0.432 g (1.2 mmol) of 4,5-di(hexylthio) phthalonitrile 1 and 0.08 g (0.4 mmol) of 4- (2-hydroxyethylthio) phthalonitrile 2 in 3 ml pentanol was heated and stirred at 140 C under N2 in a sealed tube. After 15 min, 0.015 g (2 mmol) lithium metal was added to the mixture and refluxed for 4 h. The reaction mixture was cooled to ambient temperature and then poured into 25 ml methanol and acidified with acetic acid until the crude product precipitated. In this mixture, lithium phthalocyanine derivatives were converted into metal free phthalocyanines. The precipitate was centrifuged and washed several times with hot methanol to remove unreacted materials. Finally, the green residue was chromatographed on silica gel using dichloromethane: methanol as the eluent, changing from 150:1 to 75:1 (v/v), to result in 3 as a green solid on the second fraction. Yield: 0.13 g (26%); m.p. > 200 C. IR y (cm1): 3300 (eOH), 3290 (eNH), 2950e2850 (alkyl eCH); 1H NMR (CDCl3) d ppm: 8.42e7.57 (m, 9H, AreH), 4.71 (s, H, OH), 4.19 (m, 2H, OCH2), 3.68 (m, 2H, SCH2), 3.56e3.24 (m, 12H, SCH2), 2.03e1.26 (m, 48H, eCH2), 1.05 (m, 18H, CH3), 4.3 (br, 2H, NeH); UVevis lmax (nm) in THF: 342, 694, 725; MALDI-TOF MS (matrix DHB) m/z: 1287.85 [M]þ; anal. calcd. for C70H94N8OS7; C, 65.28; H, 7.36; N, 8.70; found: C, 65.17; H, 7.45; N, 8.78%.
[2,3,9,10,16,17-Hexakis(hexylthio)-23-1-(o-carboranyl) propanoxy ethylthio phthalocyaninato]zinc(II) (6) 0.019 g (0.154 mmol) of decaborane (B10H14) was dissolved in a mixture of 10 mL dry toluene and 6 mL dry acetonitrile. The reaction mixture was heated to 90 C. After 2 h, 0.20 g (0.140 mmol) phthalocyanine 5 was added. This mixture was heated to reflux for 48 h. After the reaction mixture was cooled to room temperature, it was filtered, and all solvents were evaporated. Purification of the solid crude product 6 was accomplished by column chromatography with THF as eluent to give a green solid. Yield: 0.086 g (40.0%); m.p. > 200 C. IR y (cm1): 2950e2850 (alkyl eCH), 2571 (-BH); 1H NMR (pyridine d5): 9.73e9.45 (m, 6H, AreH), 8.97e8.38 (m, 3H, AreH), 4.74 (m, 2H, CH2), 4.41 (s, H, CH), 3.92 (m, 2H, OCH2), 3.80 (m, 2H, SCH2), 3.56 (m, 12H, SCH2), 3.22e1.10 (b, 10H, BH), 2.84 (m, 2H, CH2OC]O), 2.13 (m, 2H, CCH2), 2.03 (m, 12H, SCCCH2), 1.70e1.30 (m, 24H, CCH2), 0.93 (m, 18H, CH3); 11B NMR (pyridine d5): d, ppm 2.34 (b, 1B); 5.34 (b, 1B), 9.18 (b, 3B), 11.21 (b, 5B).; UVevis lmax (nm) in THF: 361, 632, 700; MALDI-TOF MS (matrix DHB) m/z: 1551.72 [M]þ, 1352.090 [M-C5H17B10O]þ; anal. calcd. for C75H108B10ZnN8O2S7; C, 58.05; H, 7.02; N, 7.22; found: C, 57.96; H, 6.93; N, 7.28%. Results and discussion Synthesis and characterization
[2,3,9,10,16,17-Hexakis(hexylthio)-23hydroxyethylthiophthalocyaninato]zinc(II) (4) A solution of 0.1 g (0.08 mmol) 3 and 0.044 g (0.24 mmol) anhydrous Zn(CH3COO)2 was refluxed in 2 ml of 1-pentanol with stirring for 4 h under N2 atmosphere. The resulting suspension was cooled to ambient temperature and then poured into 20 ml methanol. The precipitate was filtered off, washed successively with water, hot methanol, hot aceton and hot hexane. The purification was carried out by column chromatography on silica gel using dichloromethane: methanol (50:1) as the eluent to result in 4 as a green solid. Yield: 0.10 g (94.4%); m.p. > 200 C. IR y (cm1): 3360 (eOH), 2950e2850 (alkyl eCH); 1H NMR (CDCl3 þ 1 drop pyridine d5) d ppm: 9.10e8.82 (br, 5H, AreH), 8.55 (s, 1H, AreH), 8.03 (m, 2H, AreH), 7.62 (s, 1H, AreH), 4.64 (s, H, OH), 4.15 (m, 2H, OCH2), 3.84 (m, 2H, SCH2), 3.44e2.86 (m, 12H, SCH2), 2.02e1.72 (m, 24H, CH2), 1.44e1.22 (m, 24H, CH2), 0.96 (m, 18H, CH3); UVevis lmax (nm) in THF: 361, 629, 699; MALDI-TOF MS (matrix DHB) m/z:
4,5-Di(hexylthio)phthalonitrile (1) and 4- (2-hydroxyethylthio) phthalonitrile (2) were chosen as precursors and they were synthesized as reported in the literature [30,31]. Scheme 1 shows the synthetic pathway to 2,3,9,10,16,17-hexakis(hexylthio)-23-hydroxy ethylthiophthalocyanine (3) and [2,3,9,10,16,17-hexakis(hexylthio)23-hydroxyethylthiophthalocyaninato]zinc(II) (4). A3B type metal free phthalocyanine derivative 3 was synthesized in two steps. Preparation of the metal-free phthalocyanine begins with the lithium templated cyclotetramerization of hexylthio and hydroxyethylthio substituted phthalonitriles in a high boiling point solvent (1-pentanol) to obtain dilithium phthalocyanine [14,32]. In the second step, dilithium phthalocyanine was converted to metal-free phthalocyanine 3 by acidification with acetic acid. Column chromatography was used for the isolation of the A3B type product with 26% yield. Metallation was achieved by refluxing 3 in 1-pentanol in the presence of zinc (II) acetate under N2 atmosphere. The product
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55
Scheme 1. Synthesis of phthalocyanine complexes 3, 4 and 5.
was compatible with those reported earlier [15]. Steglich esterification was achieved with dicyclohexylcarbodiimide (DCC)/N,Ndimethylaminopyridine (DMAP) by reacting the 4-pentynoic acid with 4 in the presence of CH2Cl2 [33]. The reaction was carried out at ambient temperature by stirring the mixture for 24 h under N2 atmosphere. During the reaction, urea derivatives was formed in
only slight amounts which were generally removed easily by routine techniques including precipitation and column chromatography. For the synthesis of the phthalocyanine 6; phthalocyanine 5 and decaborane (B10H14) were reacted in a mixture of acetonitrile and toluene to give the o-carboran substituted phthalocyanine 6 in 40% yields as shown in Scheme 2 [24].
Scheme 2. Synthesis of phtalocyanine complex 6.
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The phthalocyanines 3, 4, 5 and 6 are soluble in solvents such as dichloromethane, tetrahydrofuran, chloroform, dimethylformamide and ethyl acetate. Characterization of the newly synthesized phthalocyanine derivatives was achieved by a combination of methods including elemental analysis, FT-IR, UVevis, 1H NMR, 11B NMR, and MALDI-TOF spectral data. The data are consistent with the proposed structures. The IR spectra of metal-free 3 and metallo phthalocyanine 4 were very similar, apart from the eNH stretching vibrations of the inner phthalocyanine core in the metal-free molecule, which were assigned to a band at 3290 cm1. Hexylthio and ethylthio side chains were verified by the observation of an intense aliphatic CeH stretching band. The FT-IR spectrum of 5 showed the ≡CeH absorption band at 3312 cm1 and eC^CH group absorption band at 2202 cm1. Besides, absorption band of eC]O was observed at 1720 cm1. The fact that the band of eBH stretching vibration was detected at 2571 cm1 with disappearance of the bands at 3312 cm1 and 2202 cm1 in the FT-IR spectrum was significant evidence of the formation of carborane cluster on phthalocyanine 6. The 1H NMR spectra of metallo phthalocyanine 4 was identical to that of metal-free phthalocyanine 3 in deuterated chloroform and a trace amount of deuterated pyridine. The ordinary shielding of the inner core protons leading to a chemical shift in negative ppm region pertains to metal-free phthalocyanine, which was observed at the 3.87 ppm. 1H NMR spectrum of 4 indicated the aromatic protons at around d 9.10e7.62 ppm, the SCH2 protons as multiplets at d 3.84 and 3.44e2.86 ppm. The aliphatic protons eCH3 and eCH2 protons adjacent to them appeared at around d 0.96 and 2.02e1.22 ppm as multiplets. The OH protons were observed at d 4.71 ppm for metal-free 3, d 4.64 ppm for zinc(II) 4 phthalocyanines and the signal disappeared by deuterium exchange with D2O [30]. The 1H NMR spectrum of 6 showed a broad signal of BH protons at 3.22e1.10 ppm and the signal of the CCH proton on ocarborane at 4.41 ppm. The 11B-NMR spectra of 6 showed four broad signals. This pattern is typical of the expected o-carborane system. The elemental analyses of the compounds verified the proposed structures. MALDI-TOF MS spectrum showed molecular ion peaks at m/z ¼ 1287.85 [M]þ for metal-free 3, 1352.52 [M þ H]þ for metallo phthalocyanine 4, 1432.47 [M þ H]þ for metallo phthalocyanine 5 and 1551.72 as [M]þ for o-carborane containing phthalocyanine 6 (Fig. 1).
Ground state absorption spectra and aggregation properties Substituted metal free and metallo phthalocyanines show characteristic electronic absorption spectra with two strong bands, one in the visible region at 600e700 nm (Q band) attributed to the pep* transition from the HOMO to the LUMO of the phthalocyanine ring and the other in the UV region at approximately 300e400 nm (B-Soret) band [34]. Q band absorptions were observed at 694 and 725 nm for 3 as a splitted band in THF. In the UVevis spectrum of phthalocyanine 4, an intense sharp band was recorded at 699 nm with weaker absorption peak at 629 nm in THF. Zinc phthalocyanines 5 and 6 showed the characteristic Q bands at 698 and 699 nm with no splits and weaker absorptions at 630 and 631 nm, respectively, in THF. The soret bands of phthalocyanine derivatives 3e6 were observed at 342, 361, 360 and 361 nm, respectively. Aggregation reduces solubility and the excited state lifetime and complicates the characterization and purification of phthalocyanine molecules because of composed dimers, trimers and higher oligomers [35,36]. Decreasing aggregation tendency of phthalocyanines can be achieved by combining bulky groups in the a or b positions at the benzo units and axial substitution of the central metal ion. From this point of view, new unsymmetrical phthalocyanine derivatives were synthesized and their aggregation properties were investigated by recording UVevis absorptions of phthalocyanine 6 at different concentrations in THF. As shown in Fig. 2, intensity of absorption of the Q-band for 6 was increased in parallel with increasing concentration from 2 106 M to 12 106 M without any new absorption band adjacent to Q band. Considering these results, phthalocyanine 6 had monomeric structure and obeyed BeereLambert law in this concentration range. The Q-band positions and intensities were changed by the refractive index of the solvents as shown in Fig. 3. The largest blue shift of the Q-band was recorded in EtOAc at 695 nm, with the greatest red shift in DCM at 708 nm [37]. Electrochemical and spectroelectrochemical studies To examine the reduction and oxidation potentials of unsymmetrical phthalocyanine complexes 4, 5 and 6, we undertook electrochemical studies by means of cyclic voltammetry (CV) and square wave voltammetry (SWV) in dichloromethane using
Fig. 1. MALDI-TOF mass spectrum of 6.
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57
Fig. 2. Absorption spectra of compound 6 in THF at different concentrations.
tetrabutylammonium perchlorate (TBAP) as supporting electrolyte system on a platinum working electrode. Cyclic and square wave voltammograms of 4 are shown in Fig. 4. It displayed two one electron reductions and an oxidation versus SCE. The E1/2 (DE/mV) values were determined as 0.97 V and 1.32 V for the first and the second reduction waves and 0.70 V for oxidation at 0.1 V s1 scan rate. In Fig. 5 o-carborane containing phthalocyanine complex 6 showed very similar responses with phthalocyanine complexes 4 and 5. It displayed two one electron reduction process at 0.89 V and 1.21 V and one oxidation process at 0.68 V versus SCE at 0.1 V s1. The results we obtained are tabulated on Table 1 and the electrochemical behaviour of phthalocyanines 4, 5 and 6 were compared with zinc phthalocyanine with similar substituents other than o-carborane. Spectroelectrochemical measurements were executed to enlighten the nature of the redox processes. It is known that the oxidation state of the central zinc ion with completely full d-orbitals does not change [1,13,19,38e44]. Thus, all reduction and oxidation processes of the complexes should be referred to phthalocyanine ring or connected group. Compound 6 indicated only phthalocyanine ring-based reduction or oxidation during spectroelectrochemical measurements. The spectral changes of 6 are shown in Fig. 6. Fig. 6a shows the in situ UVevis spectral changes during controlled potential reduction of 6 at 0.90 V. The
Fig. 4. Cyclic and square wave voltamograms of 4 in DCM/TBAP.
Fig. 5. Cyclic and square wave voltamograms of 6 in DCM/TBAP.
intensity of the Q band at 705 nm increases with a shift to 699 nm while the intensity of B band at 359 nm increases with a shift to 375 nm during the reduction of 6 at 0.9 V potential application. In the spectrum, there are clear isosbestic points at 360, 407, 668, and 712 nm. All of these data support that the reduction is a phthalocyanine ring reduction. Decrease in the Q band intensity at 698 nm Table 1 The electrochemical potentials of phthalocyanine derivatives.
Fig. 3. Absorption spectra of compound 6 in different solvents. Concentration: 105 M.
Compound
Solvent
4 5 6 ZnPc ZnPc ZnPc
DCM DCM DCM DCM THF DCM
Ox2
Oxı
RR1
RR2
Ref.
0.805 0.36 0.814
0.70 0.71 0.68 0.60 0.14 0.633
0.97 0.87 0.89 0.79 1.37 0.78
1.32 1.22 1.21 1.10 1.76 1.027
tw. tw. tw. [40] [41] [42]
tw: This work. RR: Ring Reduction. Ox: Oxidation.
58
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References
Fig. 6. Absorption spectral changes in the electrolysis at an applied potential of (a) 0.90 V and (b) 1.3 V vs. SCE for the first and second reduction of 6 in deaerated DCM containing TBAF. The arrows show the direction of the changes.
without shift and decrease in the B band intensity with a shift to 382 nm indicate the second ring reduction process during the potential application at 1.3 V (Fig. 6b). During the oxidation process of 6, the decrease in the absorption of the Q band and B band in intensity without shift shows ring based oxidation. Conclusions A new soluble unsymmetrical zinc phthalocyanine with an ocarborane moiety on the periphery has been reported in this work for the first time. The electrochemical and spectroelectrochemical studies show that all synthesized zinc phthalocyanine complexes give ring-based redox processes. Further studies on the applicability of the phthalocyanine with carborane group in both PDT and BNCT are in progress. Acknowledgements This work was supported by TUBITAK (Project number: 211T052). AG thanks Turkish Academy of Sciences (TUBA) for partial support.
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