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Synthesis, photophysical, photochemical and SO2 sensing properties of ball-type phthalocyanines substituted with carboxyl groups
Inorganic Chemistry Communications 103 (2019) 75–81
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Synthesis, photophysical, photochemical and SO2 sensing properties of ball-type phthalocyanines substituted with carboxyl groups
T
GRAPHICAL ABSTRACT
ABSTRACT
Ball-type phthalocyanines substituted with carboxyl group at non-peripheral positions were synthesized and characterized by elemental analysis, Fourier-transform infrared, 1H nuclear magnetic resonance, and electronic spectroscopies and mass spectrometry. The fluorescence quantum yields and corresponding lifetimes of singlet and triplet states were determined. It was found that the obtained lifetime of triplet states ranges from 80 to 820 μs for the complexes. In order to get more detailed information about the charge transport mechanism and SO2 sensing potential of these compounds impedance spectroscopy measurements were carried out. The estimated model parameters for ac conduction mechanism indicated that the correlated barrier hopping model is the best to represent the dependence of the frequency exponent on temperature for all compounds investigated. From the SO2 sensing test of the compounds it was found that compound 4b has great potential for use of SO2 sensing element.
Phthalocyanines (Pcs), which have 18 π aromatic macrocycle, were first discovered by chance in 1907 [1–4]. Since then, the synthesis of Pcs with different substitutions and their application have extensively developed in broad range of fields such as new materials in optical, electronic, and in medicinally photodynamic therapy (PDT) [5–7]. More recently, a new type of phthalocyanine compound, which is called ball-type phthalocyanine, was synthesized by Tamilova et al. [8,9]. Ball-type compounds structurally contain four bridged substituents on peripheral position of each benzene ring of the two face-to-face Pc molecules. In this regard, there are structural differences between the ball-type and their parent mono phthalocyanines. Ball-type Pcs offer ample opportunities in the design of novel classes for medicinal applications. Due to the wide range of interactions between the face-to-face Pc rings and/or the two metal centres, these classes of novel compounds exhibit interesting spectroscopic, electrochemical, electrical, and fluorescence properties [10–22]. However, the studies concerning the non-peripherally substituted ball-type Pc derivatives are still succinct in
literature. Furthermore, to the researchers' knowledge, photochemical characteristics of these compounds have not been focused in detail. On the other hand, the presence of functional groups in the molecule may improve the photo physicochemical and PDT activities of photosensitizers. The many molecules containing carboxyl group may be attached to biological molecules which can interact with the amino groups through an amide bond [23]. The photo physicochemical properties of Pcs are strongly influenced by the presence and nature of the central diamagnetic metals, such as zinc, magnesium, aluminium. In some cases MPcs containing central metals such as Zn has much better photo physicochemical properties [24]. In this regard, this is one of the case study be conducted pertaining to the singlet oxygen and photodegradation quantum yields of ball-type phthalocyanines as photosensitizers in PDT. In this study, we synthesized carboxyl groups substituted novel ball-type H4Pc2 4a, Mg2Pc2 4b, Zn2Pc2 4c and Co2Pc2 4d bridged by oxygen at the non-peripheral positions (Scheme 1). Electrical properties and SO2 gas sensing performance of these novel
https://doi.org/10.1016/j.inoche.2019.02.040 Received 14 October 2018; Received in revised form 21 February 2019; Accepted 26 February 2019 Available online 04 March 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
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NC
CN
HO COOH
O2 N
+
HO
(1)
(2) i
NC
CN
O
COOH O
NC
CN
(3)
ii
O
O N
HOOC
N N N
N M
COOH N
N
N
O
O O
O N N N
HOOC
N
N M
N
COOH
N
N O
(4) 4: a b c d M: H2 Mg Zn Co
O
Scheme 1. i: DMSO, rt., 12 days, K2CO3 ii: n-pentanol, DBU, reflux, overnight, metal salt.
compounds were also investigated. Details regarding the materials and equipment and the informations for electrical and gas sensing measurements are provided as Supplementary information (ESI) [25]. The equations and the details for the photophysicochemical studies (The fluorescence (ΦF), triplet (ΦT) singlet oxygen (ΦΔ), photodegredation (Φd) quantum yields and triplet lifetime (τT)) of complexes were assessed using comparative methods reported in the literature [26]. All of the measurements were not performed on the complex 4d due to containing a paramagnetic metal center. The target precursors were prepared by a nucleophilic aromatic substitution reaction between 1 and 2 in DMSO. Ball-type derivatives 4 were accomplished by using in n-pentanol in the presence of DBU at the reflux temperature. The synthesis, physical measurements (Yield, FT-IR, 1H NMR, etc.) of 3,4-bis(2,3-dicyanophenoxy)benzoic acid (3) and general procedure for synthesis of ball-type Pcs (4) are given in detail in the Supporting information. Column chromatography on silica gel using CHCl3, THF, MeOH and glacial CH3COOH (only for 4b) as mobile phase was used to purify complexes 4a-d. Complex 4a was separated as a first fraction by column chromatography on silica gel with gradient of chloroform (CHCl3), tetrahydrofuran (THF) and methanol (CH3OH) whereas complex 4b was eluted as a second fraction by column chromatography on silica gel with gradient of glacial acetic acid. H4Pc2, 4a may be obtained during synthesis of Mg2Pc2, 4b derivatives due to size of Mg. Thereafter both of the complexes were achieved by separate reactions based on conditions described above.
The IR spectrum of 3 clearly indicates the present of CN and the CN vibrational peak observed at 2232 cm−1. A diagnostic feature of the formation of 4a-d from the phthalodinitrile derivatives 3, is the disappearance of the sharp CN vibration. The remaining IR spectra were very similar for compounds 3 and 4a-d showed Ar-O-Ar peaks at 1266, 1169, 1170, 1170, 1170 cm−1, respectively. 1 H NMR spectra of 4a-c, which were recorded in DMSO were similar to each other. Complex 4a-c showed complex patterns due to the mixed isomer character of these complexes and were complicated, however, expected protons were obtained in their respective regions. The COOH protons of 3 were observed as a broad signal at around 3.55 ppm and the signal disappeared by deuterium exchange. The aromatic protons appeared at 7.37, 7.43, 7.64, 7.85, 8.02 and 8.05 ppm integrating for a total of 9 for protons for 4. The 1H NMR spectra of 4a-c show the aromatic protons between 8.00 and 7.00 ppm, integrating for a total of 44, 40 and 40 for protons, respectively. 1H NMR measurements could not be obtained for 4d due to the paramagnetic nature of the cobalt metal cation. In addition, in the 1H NMR spectrum of 4a-c, the OH protons are observed as signals around at 3.3, 2.8 and 2.9 ppm, respectively. On the other hand, the inner protons of the H4Pc2 ring were not observed clearly below 0 ppm as indicated in the literature. These results confirm that the complexes have been synthesized successfully. In the mass spectrum of compound 3, the presence of molecular ion peak [M]+ at m/z 406 confirmed the proposed structures. The purified phthalocyanines were further characterized by mass spectra. For the 76
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compound 4a and its Mg, Zn and Co complexes (4b-d), protonated, mainly water, sodium and potassium adducted ion peaks were observed. In the case of compound 4a and 4b, protonated molecular ion signals were observed beside the other adducts and fragment signals. For 4c and 4d sodium, potassium and water adduct signals were observed except protonated molecular ion signals. MALDI-TOF-MS spectra for all compounds were yielded protonated ion signals (for 4a and 4b) and different number of sodium, potassium and water adducts and also some fragment ions from these adducts higher and lower masses from the mass of protonated molecular ion signal. Poor protonated ion signal intensities were mainly resulted from the low proton affinity to the compounds, beside the high affinity of sodium and potassium to the compounds in the gas-phase. On the other hand, these high number of adducts and fragment signals were because of the dimeric or olygomeric forms of the complex compounds having high number of carboxylic acid functional groups. Because of these oligomeric species in the gas-phase, different and high number of fragmentations could be occurred under the laser firing and mass spectrometric conditions. As a result, more signals were not because of the impurities (all signals were defined checking the mono isotopic masses of each signals and matched with adducts and fragments species isotopic mass distributions), those were because of the fragmentation of the different oligomeric forms under the mass spectrometric conditions in the gas-phase. All these adduct and fragment signals were indicated and defined on the MALDITOF-MS spectra in Fig.S7–S15. (Please see the Supplementary data for the spectra in Fig. S7–S15). The expected mass values corresponded with the found values for all complexes. These results confirm that the complexes have been synthesized successfully. The complexes have good purity but the yield after purification was low. The low yield can be partly due to the loss of the substance during purification and side products. The UV–Vis spectra in DMSO are shown in Fig. 1 for 4a-d. The Q band maxima are listed in Table 1 in DMSO and THF. The absorption spectra of monomeric MPcs are characterized by intense electronic absorptions between 600 and 750 nm, to give the characteristic Q band, with a molar absorptivity often exceeding 105 L mol−1 cm−1 [28]. On the other hand, the UV–Vis spectra of the ball-type Pcs are slightly different from their monomer counterparts such as a broad Q band. A broad shoulder was observed around 620 nm for 4a-d, suggesting some degree of aggregation [8–19,27]. The UV–Vis spectra of 4a-d showed absorptions at 687/715, 695, 692, 680 nm in Q band region, respectively, in THF. On the other hand, the Q-band absorptions in DMSO were observed at 694/718, 698, 696, 686 nm for 4a-d, respectively. It is well documented in literature that the position of the Q band of Pc compounds strongly depends on the refractive index of the solvent, the coordination of donor solvent molecules to the metal ion in the cavity of the macrcyclic ligand, substitued groups or metal ions The observed red
shifted Q band in DMSO can be attributed to high refractive index of DMSO. On the other hand substituted Pcs such as carboxyl group are known to undergo strong bonding involving the carboxylic acid group, resulting in broad spectra, the split Q bands or new band appearance. This is most likely the situation may be appear in UV–Vis spectra or 1H NMR spectra [28,29]. Complexes showed B band at around 318 and 360 (except 4d) nm in both THF and DMSO which can be attributed to transitions from the deeper π levels to the LUMO. It was also observed that the intensity of the B bands is higher than the Q band this may be due to intramolecular interactions between the Pc rings for 4a-d. The spectrum of 4d displayed charge transfer band at 440 nm. The electron-withdrawing groups and the distance between the two Pc units of the ball-type molecule considerably affect the degree of the interaction between the rings. It can be seen from the electronic absorption spectrum of 4a-d that the Q band was broad. The broadening of the Q band can be attributed to intramolecular interactions as well as face-to-face aggregation of Pcs. Fluorescence in MPcs is usually short lived, of the order 10−8 s. Fluorescence properties, such as fluorescence intensity and fluorescence quantum yield (ΦF), of the MPc is influenced by several factors which include but are not limited to, aggregation, solvent properties, concentration (quenching), nature of the central metal atom, substituent type (particularly halogenation) and photo-induced energy transfer [30]. The absorption and fluorescence excitation spectra of complex 4ac in DMSO are shown in Fig. 2. The excitation, absorption and emission spectral data are listed in Table 1. The fluorescence emission spectrum of complex 4a-c showed a maximal emission peak at 726, 708, 707 nm with excitation at 721, 699, 699 nm, respectively. It was observed the proximity of the wavelength of each component of the Q band absorption to the Q band maxima of the excitation spectra for all complexes. This finding suggests that the nuclear configurations of the ground and excited states are similar and not affected by excitation. For complex 4a, the emission spectrum is not split, since metal free Pcs are known to fluoresce with only main peak which has been assigned as the 0–0 transition of the fluorescence [27]. The emission peak is narrower since aggregates are not known to fluoresce. Stoke's shifts range from 9 to 11 nm for 4a-c, and are typical of MPc complexes [31]. The fluorescence quantum yield ΦF values are typical of MPc complexes. The ΦF values are 0.065, 0.12 and 0.056 in DMSO for 4a-c, respectively. The low ΦF values were observed for ball-type derivatives. This is attributed to the ball-type structure which may encourage intersystem crossing to the triplet state due to its larger size, as mentioned above [11–14,16]. Face-to-face interaction of the two monomers in dimers is expected to decrease the energy gap between the singlet state and the triplet state and enhance the formation of triplet state (i.e. intersystem crossing increases) decreasing fluorescence [14]. However, in this work the ΦF values are similar to those of monomeric
Fig. 1. Absorption spectra for 4a-d in DMSO, (concentration = 10−5 M). 77
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Table 1 UV–Vis absorption (Q-band), emission and excitation spectral data, and photophysical data for the phthalocyanines 4 in THF and DMSO. λmax/nm THF
4a 4b 4c 4d
DMSO
λAbs
λAbs
λEms
λExc
ΔλStokes
ΦF
ΦT
ΦΔ
Φd(×10−6)
ΦISC
SΔ
τT/μs
687/715 695 692 680
694/717 698 696 686
726 708 707 –
695/721 699 699 –
9 10 11 –
0.065 0.120 0.056 –
0.11 0.57 0.73 –
0.087 0.083 0.173 –
1.32 1.72 1.88 0.00
0.825 0.310 0.210 –
0.79 0.14 0.24 –
80 820 340 –
Fig. 2. Absorbance (a), excitation (b), emission (c) spectra of complex 4a-c in DMSO, Excitation λmax: 615, 619, 618 nm, respectively.
78
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Fig. 3. Triplet lifetime for 4c in DMSO.
phthalocyanines, showing the interaction between the two rings is minimal. In addition, low ΦF values can be observed often for aggregated MPcs [32]. Triplet state properties including the triplet lifetimes (τT) and quantum yields (ΦT) of the Pcs are normally determined using laser flash photolysis. Table 1 shows this ΦT and τT values of 4a-c in DMSO. The triplet state quantum yield values are 0.11, 0.57, 0.73 for 4a-c, respectively. As known in the literature, these high values of ΦT values suggest more efficient intersystem crossing (ISC) in the presence of the carboxyl substituents for substituted complexes, corresponding to low ΦF values. The metastable triplet state properties intrinsic to phthalocyanines are particularly sensitive to changes that deactivate fluorescence. Yet, in here, complex 4a has both low ΦT and ΦF due to face-toface interaction and electron withdrawing resonance effect at nonperipherally position. A triplet decay curve of change in absorbance (ΔA) versus time in seconds is obtained from the experiment and from this the triplet lifetime can be determined. A typical triplet state decay curve for 4c is shown in Fig. 3 as an example (please see the Supplementary information for further information on the other complexes). Obtained lifetimes are 80, 820, 340 μs for 4a-c, respectively. The order of the lifetime values among the substituted complexes was 4b > 4c > 4a. The complex 4b, Mg2Pc2 have a long time triplet lifetime. Phthalocyanines with the same substituents at the same position bearing a different metal in the central cavity also show different triplet lifetimes and quantum yields. Fig. 4 shows spectral changes observed for 4c as an example (see
Supplementary data for the other complexes) during photolysis of complexes with increasing time from 0 to 60 min in the presence of DPBF and all experimental results are summarized in Table 1. DPBF degradation of the complexes 4a, b, and c at 415 nm was monitored with UV–Vis spectroscopy. Singlet oxygen can be obtained through an energy transfer process between excited triplet state of Pcs and ground state molecular oxygen. The ΦΔ value for a viable photosensitizer is related to the population of the triplet state, the lifetime and the effectiveness of energy transfer process. The ΦΔ values are 0.087, 0.083 and 0,173 for 4a, 4b and 4c, respectively. The order of the ΦΔ values among the substituted complexes was 4b < 4a < 4d in DMSO. The phthalocyanine complexes have high singlet oxygen quantum yields due to a high value of the product of triplet quantum yield. The observed low value of ΦΔ is not consistent with the previous findings, which may be an important challenge of ball-type structure [29]. The energy transfer efficiency (SΔ) between excited triplet state photosensitizer and the ground state molecular oxygen of the unmetaled Pc were found to be significantly higher than that those obtained for the metal complexes, the order of the SΔ values among the substituted complexes was 4b < 4c < 4a in DMSO. These values are compatible with the triplet state lifetimes. The low SΔ for 4b and 4c may also be due to partial oxidation, the face-to-face interaction between the two rings. Photodegradation is the decomposition of phthalocyanine upon illumination. Complexes 4a-d were found to be stable upon exposure to illumination for 60 min with the photodegradation quantum yield value
Fig. 4. Time-dependent photobleaching of DPBF absorption in the presence of 4c in DMSO. 79
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Fig. 5. The photobleaching spectra for 4b in DMSO.
in the order of 10−6, which is a good indicator of the high photostability of complexes [33] The data are presented in the Table 1. The obtained spectrum for 4b as an example (Please see the Supplementary data for the other complexes) is depicted in Fig. 5. Dc. conductivity measurements on compounds of 4a, 4b, 4c and 4d were carried out in the temperature range 300–450 K in a vacuum (< 10−3 mbar). In order to avoid from sorption-desorption process, rapid cooling method was used for temperature dependent conductivity measurements. From the comparison of the measured conductivity of the films, we found that the conductivity of the film of 4b is the highest among the investigated other phthalocyanine films. The order of conductivities observed for these compounds are 4b > 4d > 4c > 4a at all temperatures investigated. Because of its high conductivity, we focused on the compound of 4b. The results of the dc conductivity measurements also showed that the measured dc conductivity of the compounds depend exponentially on temperature which indicates the applicability of Eq. (1). dc
=
0
exp
EA kT
coordination in Pc unit has a significant effect on the SO2 sensing properties as suggested by Paoletti et al. It was observed that the compound of ball-type Mg2Pc2 (4b) exhibits better sensing performance towards to various concentrations of SO2 gas at room temperature. A typical plot in the complex impedance plane for one of the best sensor is shown in Fig. S18. The sensor was manufactured of a 4b film with a thickness of 100 nm. 4b coated IDT was exposed to various concentrations of SO2 and complex impedance spectra were taken after reaching the adsorption equilibrium. With the examination of the impedance spectra clear differences could be seen in the dependence of ω on both real and imaginary part of impedance between 300 and 1500 ppm SO2. It is well known from the present literature that the sensing mechanism of SO2 gas by phthalocyanines, unlike other well known simple gas molecule, is not well known. It is well documented in the literature that the gas sensing performance of the phthalocyanines can be affected by the conjugated π-electron system, by the peripheral attachment of additional atoms or groups, and the presence of hetero atoms and the central metal atom [38]. It was reported by Bohrer et al. [39] and Park et al. [40] that the detection of analyte molecules by phthalocyanines to be governed primarily by coordination to the metal center. The interaction between the analyte and the metallophthalocyanine surface produces a modulation in the electronic levels of phthalocyanine available for π–π* transitions. As results, a doping effect due to the adsorbed analytes onto the phthalocyanine film is basically the cause of the changes in the impedance of the sensing layer. Although phthalocyanines compounds have similar molecular structure, due to the nature of the central metal ion, they have quite different physical and chemical properties. In order to be sure that the observed SO2 response of the 4 based sensor is stable and repeatable, the response-recovery characteristics of the 4 based sensor to repeated exposure and cutting of 1500 ppm SO2 gas was also investigated. It was found that the response-recovery behavior of the sensors is highly reproducible. It suggests that the compounds are stable in contact with various concentrations of SO2 gas. In this work, we presented the synthesis of ball-type phthalocyanines. The complexes were characterized using elemental analysis and 1 H NMR, FTIR, UV–Vis spectroscopies and mass spectrometry. We observed that Mg2Pc2 had longer triplet lifetime than those of Zn2Pc2. Electrical conduction and SO2 sensing capability of the compounds were also investigated by impedance spectroscopy technique. Measurements revealed that dominant conduction mechanism in these compounds is correlated barrier hopping. The photophysical and photochemical behaviours of the compounds indicated that are valuable as potential photosensitizers for PDT as well as SO2 sensing test results indicated that compound 4b has great potential for use of SO2 sensor.
(1)
where, EA is an activation energy, T is temperature, k is Boltzmann's constant and σ0 is a constant of proportionality. The value of the activation energy was derived from the slope of the log σdc versus 1/T graphs, with the aid of Eq. (1), and was found to be 0.92 eV for compound 4a, 0.84 eV for compound 4b, 0.80 eV for compound 4c and 0.68 eV for compound 4d. These findings reveal that the central metal atom play an important role in dc conduction. The observed differences between Pc molecules investigated can be correlated with the size of the central metal atom. It was reported previously that the oxidation potential and morphological modifications can be achieved by the change of central metal atom [34–36]. The variation of the real part of the complex conductivity, σa.c(ω), with frequency in the range 20–2 × 106 Hz for the studied compounds was investigated at different temperature values between 300 and 450 K. The frequency dependence of real conductivity in the 4b thin film on a double logarithmic scale at different selected temperatures is presented in Fig. S16. The variation of the frequency exponent s, with temperature for the investigated samples is depicted in Fig. S17. According to Paoletti et al. [37] that the interaction between the target gas molecule and the MPc unit is determined by the metal ion coordination and the peripheral attachment of additional atoms or groups that enhance or diminish the ionisation energy. Therefore in the first step of our investigation, we focused on better understand the function of the metal ion coordination in SO2 sensing performance. For this purpose, the sensing performance of the compounds for the same concentration of SO2 at room temperature was first investigated. The response-recovery measurements verified that the metal ion 80
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Acknowledgements
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This work was supported by the Research Fund of the Yildiz Technical University (Project No: 2011-01-02-GEP07), partially supported by Ministry of Development-Republic of Turkey with the project Number: 2016K121230, as well as Yildiz Technical University in Istanbul, Turkey and Rhodes University in Grahamstone, South Africa. The authors are also thankful to Prof. Tebello Nyokong for providing us a grant and laboratory facilities during the photophysical and photochemical measurements. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.inoche.2019.02.040. References [1] R. Polley, H. Heckmann, M. Hanack, Methods of Organic Chemistry (HoubenWeyl), 4 Edition, Vol. E9 Georg Thieme Verlag, Stuttgart, 1997, pp. 717–842. [2] N.B. McKeown, Phthalocyanine Materials: Structure, Synthesis and Function (1998). [3] H. de Diesbach, E. von der Weid, Helv. Chim. Acta 10 (1927) 886–888. [4] A. Braun, J. Tcherniac, Ber. Dtsch. Chem. Ges. 40 (1907) 2709–2714. [5] J. Simon, C. Sirlin, J. Appl. Chem. 61 (1989) 1625–1629. [6] S.B. Brown, E.A. Brown, I. Walker, Lancet Oncol. 5 (2004) 497–508. [7] D. Dini, M. Hanack, K.M. Kadish, K.M. Smith, R. Guilard (Eds.), The Porphyrin Handbook, Academic Press, 2003, pp. 1–36. [8] A.Y. Tolbin, A.V. Ivanov, L.G. Tomilova, N.S. Zefirov, Mendeleev Commun. 12 (2002) 96–97. [9] A.Y. Tolbin, A.V. Ivanov, L.G. Tomilova, N.S. Zefirov, J. Porphyrins Phthalocyanines 7 (2003) 162–166. [10] G. de la Torre, G. Bottari, M. Sekita, A. Hausmann, M.G. Dirk, T. Torres, Chem. Soc. Rev. 42 (2013) 8049–8105. [11] M. Canlıca, I.N. Booysen, T. Nyokong, Polyhedron 30 (2011) 508–514. [12] M. Canlıca, T. Nyokong, Dalton Trans. 40 (2011) 1497–1502. [13] M. Canlıca, T. Nyokong, Dalton Trans. 40 (2011) 5285–5290. [14] M. Canlıca, T. Nyokong, Polyhedron 31 (2012) 704–709. [15] M. Canlica, M. Coskun, A. Altindal, T. Nyokong, J. Porphyrins Phthalocyanines 16 (2012) 855–860. [16] M. Canlica, A. Altindal, T. Nyokong, J. Porphyrins Phthalocyanines 16 (2012) 826–832. [17] N. Nwaji, D.O. Oluwole, J. Mack, M. Louzada, S. Khene, J. Britton, T. Nyokong, Dyes Pigments 140 (2017) 417–430. [18] E. Yabas, E. Bagda, E. Bagda, Dyes Pigments 120 (2015) 220–227.
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Mevlüde Canlıcaa, , Birsel Can Ömürb, Bekir Salihc Yildiz Technical University, Chemistry Department, Inorganic Chemistry Division, Davutpasa Campus, 34220 Esenler- Istanbul, Turkey b Yildiz Technical University, Physics Department, Davutpasa Campus, 34220 Esenler- Istanbul, Turkey c Hacettepe University, Chemistry Department, 06800 Cankaya-Ankara, Turkey E-mail address: [email protected] (M. Canlıca). ⁎
a
Corresponding author. 81
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Synthesis, photophysical, photochemical and SO2 sensing properties of ball-type phthalocyanines substituted with carboxyl groups
T
GRAPHICAL ABSTRACT
ABSTRACT
Ball-type phthalocyanines substituted with carboxyl group at non-peripheral positions were synthesized and characterized by elemental analysis, Fourier-transform infrared, 1H nuclear magnetic resonance, and electronic spectroscopies and mass spectrometry. The fluorescence quantum yields and corresponding lifetimes of singlet and triplet states were determined. It was found that the obtained lifetime of triplet states ranges from 80 to 820 μs for the complexes. In order to get more detailed information about the charge transport mechanism and SO2 sensing potential of these compounds impedance spectroscopy measurements were carried out. The estimated model parameters for ac conduction mechanism indicated that the correlated barrier hopping model is the best to represent the dependence of the frequency exponent on temperature for all compounds investigated. From the SO2 sensing test of the compounds it was found that compound 4b has great potential for use of SO2 sensing element.
Phthalocyanines (Pcs), which have 18 π aromatic macrocycle, were first discovered by chance in 1907 [1–4]. Since then, the synthesis of Pcs with different substitutions and their application have extensively developed in broad range of fields such as new materials in optical, electronic, and in medicinally photodynamic therapy (PDT) [5–7]. More recently, a new type of phthalocyanine compound, which is called ball-type phthalocyanine, was synthesized by Tamilova et al. [8,9]. Ball-type compounds structurally contain four bridged substituents on peripheral position of each benzene ring of the two face-to-face Pc molecules. In this regard, there are structural differences between the ball-type and their parent mono phthalocyanines. Ball-type Pcs offer ample opportunities in the design of novel classes for medicinal applications. Due to the wide range of interactions between the face-to-face Pc rings and/or the two metal centres, these classes of novel compounds exhibit interesting spectroscopic, electrochemical, electrical, and fluorescence properties [10–22]. However, the studies concerning the non-peripherally substituted ball-type Pc derivatives are still succinct in
literature. Furthermore, to the researchers' knowledge, photochemical characteristics of these compounds have not been focused in detail. On the other hand, the presence of functional groups in the molecule may improve the photo physicochemical and PDT activities of photosensitizers. The many molecules containing carboxyl group may be attached to biological molecules which can interact with the amino groups through an amide bond [23]. The photo physicochemical properties of Pcs are strongly influenced by the presence and nature of the central diamagnetic metals, such as zinc, magnesium, aluminium. In some cases MPcs containing central metals such as Zn has much better photo physicochemical properties [24]. In this regard, this is one of the case study be conducted pertaining to the singlet oxygen and photodegradation quantum yields of ball-type phthalocyanines as photosensitizers in PDT. In this study, we synthesized carboxyl groups substituted novel ball-type H4Pc2 4a, Mg2Pc2 4b, Zn2Pc2 4c and Co2Pc2 4d bridged by oxygen at the non-peripheral positions (Scheme 1). Electrical properties and SO2 gas sensing performance of these novel
https://doi.org/10.1016/j.inoche.2019.02.040 Received 14 October 2018; Received in revised form 21 February 2019; Accepted 26 February 2019 Available online 04 March 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
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NC
CN
HO COOH
O2 N
+
HO
(1)
(2) i
NC
CN
O
COOH O
NC
CN
(3)
ii
O
O N
HOOC
N N N
N M
COOH N
N
N
O
O O
O N N N
HOOC
N
N M
N
COOH
N
N O
(4) 4: a b c d M: H2 Mg Zn Co
O
Scheme 1. i: DMSO, rt., 12 days, K2CO3 ii: n-pentanol, DBU, reflux, overnight, metal salt.
compounds were also investigated. Details regarding the materials and equipment and the informations for electrical and gas sensing measurements are provided as Supplementary information (ESI) [25]. The equations and the details for the photophysicochemical studies (The fluorescence (ΦF), triplet (ΦT) singlet oxygen (ΦΔ), photodegredation (Φd) quantum yields and triplet lifetime (τT)) of complexes were assessed using comparative methods reported in the literature [26]. All of the measurements were not performed on the complex 4d due to containing a paramagnetic metal center. The target precursors were prepared by a nucleophilic aromatic substitution reaction between 1 and 2 in DMSO. Ball-type derivatives 4 were accomplished by using in n-pentanol in the presence of DBU at the reflux temperature. The synthesis, physical measurements (Yield, FT-IR, 1H NMR, etc.) of 3,4-bis(2,3-dicyanophenoxy)benzoic acid (3) and general procedure for synthesis of ball-type Pcs (4) are given in detail in the Supporting information. Column chromatography on silica gel using CHCl3, THF, MeOH and glacial CH3COOH (only for 4b) as mobile phase was used to purify complexes 4a-d. Complex 4a was separated as a first fraction by column chromatography on silica gel with gradient of chloroform (CHCl3), tetrahydrofuran (THF) and methanol (CH3OH) whereas complex 4b was eluted as a second fraction by column chromatography on silica gel with gradient of glacial acetic acid. H4Pc2, 4a may be obtained during synthesis of Mg2Pc2, 4b derivatives due to size of Mg. Thereafter both of the complexes were achieved by separate reactions based on conditions described above.
The IR spectrum of 3 clearly indicates the present of CN and the CN vibrational peak observed at 2232 cm−1. A diagnostic feature of the formation of 4a-d from the phthalodinitrile derivatives 3, is the disappearance of the sharp CN vibration. The remaining IR spectra were very similar for compounds 3 and 4a-d showed Ar-O-Ar peaks at 1266, 1169, 1170, 1170, 1170 cm−1, respectively. 1 H NMR spectra of 4a-c, which were recorded in DMSO were similar to each other. Complex 4a-c showed complex patterns due to the mixed isomer character of these complexes and were complicated, however, expected protons were obtained in their respective regions. The COOH protons of 3 were observed as a broad signal at around 3.55 ppm and the signal disappeared by deuterium exchange. The aromatic protons appeared at 7.37, 7.43, 7.64, 7.85, 8.02 and 8.05 ppm integrating for a total of 9 for protons for 4. The 1H NMR spectra of 4a-c show the aromatic protons between 8.00 and 7.00 ppm, integrating for a total of 44, 40 and 40 for protons, respectively. 1H NMR measurements could not be obtained for 4d due to the paramagnetic nature of the cobalt metal cation. In addition, in the 1H NMR spectrum of 4a-c, the OH protons are observed as signals around at 3.3, 2.8 and 2.9 ppm, respectively. On the other hand, the inner protons of the H4Pc2 ring were not observed clearly below 0 ppm as indicated in the literature. These results confirm that the complexes have been synthesized successfully. In the mass spectrum of compound 3, the presence of molecular ion peak [M]+ at m/z 406 confirmed the proposed structures. The purified phthalocyanines were further characterized by mass spectra. For the 76
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compound 4a and its Mg, Zn and Co complexes (4b-d), protonated, mainly water, sodium and potassium adducted ion peaks were observed. In the case of compound 4a and 4b, protonated molecular ion signals were observed beside the other adducts and fragment signals. For 4c and 4d sodium, potassium and water adduct signals were observed except protonated molecular ion signals. MALDI-TOF-MS spectra for all compounds were yielded protonated ion signals (for 4a and 4b) and different number of sodium, potassium and water adducts and also some fragment ions from these adducts higher and lower masses from the mass of protonated molecular ion signal. Poor protonated ion signal intensities were mainly resulted from the low proton affinity to the compounds, beside the high affinity of sodium and potassium to the compounds in the gas-phase. On the other hand, these high number of adducts and fragment signals were because of the dimeric or olygomeric forms of the complex compounds having high number of carboxylic acid functional groups. Because of these oligomeric species in the gas-phase, different and high number of fragmentations could be occurred under the laser firing and mass spectrometric conditions. As a result, more signals were not because of the impurities (all signals were defined checking the mono isotopic masses of each signals and matched with adducts and fragments species isotopic mass distributions), those were because of the fragmentation of the different oligomeric forms under the mass spectrometric conditions in the gas-phase. All these adduct and fragment signals were indicated and defined on the MALDITOF-MS spectra in Fig.S7–S15. (Please see the Supplementary data for the spectra in Fig. S7–S15). The expected mass values corresponded with the found values for all complexes. These results confirm that the complexes have been synthesized successfully. The complexes have good purity but the yield after purification was low. The low yield can be partly due to the loss of the substance during purification and side products. The UV–Vis spectra in DMSO are shown in Fig. 1 for 4a-d. The Q band maxima are listed in Table 1 in DMSO and THF. The absorption spectra of monomeric MPcs are characterized by intense electronic absorptions between 600 and 750 nm, to give the characteristic Q band, with a molar absorptivity often exceeding 105 L mol−1 cm−1 [28]. On the other hand, the UV–Vis spectra of the ball-type Pcs are slightly different from their monomer counterparts such as a broad Q band. A broad shoulder was observed around 620 nm for 4a-d, suggesting some degree of aggregation [8–19,27]. The UV–Vis spectra of 4a-d showed absorptions at 687/715, 695, 692, 680 nm in Q band region, respectively, in THF. On the other hand, the Q-band absorptions in DMSO were observed at 694/718, 698, 696, 686 nm for 4a-d, respectively. It is well documented in literature that the position of the Q band of Pc compounds strongly depends on the refractive index of the solvent, the coordination of donor solvent molecules to the metal ion in the cavity of the macrcyclic ligand, substitued groups or metal ions The observed red
shifted Q band in DMSO can be attributed to high refractive index of DMSO. On the other hand substituted Pcs such as carboxyl group are known to undergo strong bonding involving the carboxylic acid group, resulting in broad spectra, the split Q bands or new band appearance. This is most likely the situation may be appear in UV–Vis spectra or 1H NMR spectra [28,29]. Complexes showed B band at around 318 and 360 (except 4d) nm in both THF and DMSO which can be attributed to transitions from the deeper π levels to the LUMO. It was also observed that the intensity of the B bands is higher than the Q band this may be due to intramolecular interactions between the Pc rings for 4a-d. The spectrum of 4d displayed charge transfer band at 440 nm. The electron-withdrawing groups and the distance between the two Pc units of the ball-type molecule considerably affect the degree of the interaction between the rings. It can be seen from the electronic absorption spectrum of 4a-d that the Q band was broad. The broadening of the Q band can be attributed to intramolecular interactions as well as face-to-face aggregation of Pcs. Fluorescence in MPcs is usually short lived, of the order 10−8 s. Fluorescence properties, such as fluorescence intensity and fluorescence quantum yield (ΦF), of the MPc is influenced by several factors which include but are not limited to, aggregation, solvent properties, concentration (quenching), nature of the central metal atom, substituent type (particularly halogenation) and photo-induced energy transfer [30]. The absorption and fluorescence excitation spectra of complex 4ac in DMSO are shown in Fig. 2. The excitation, absorption and emission spectral data are listed in Table 1. The fluorescence emission spectrum of complex 4a-c showed a maximal emission peak at 726, 708, 707 nm with excitation at 721, 699, 699 nm, respectively. It was observed the proximity of the wavelength of each component of the Q band absorption to the Q band maxima of the excitation spectra for all complexes. This finding suggests that the nuclear configurations of the ground and excited states are similar and not affected by excitation. For complex 4a, the emission spectrum is not split, since metal free Pcs are known to fluoresce with only main peak which has been assigned as the 0–0 transition of the fluorescence [27]. The emission peak is narrower since aggregates are not known to fluoresce. Stoke's shifts range from 9 to 11 nm for 4a-c, and are typical of MPc complexes [31]. The fluorescence quantum yield ΦF values are typical of MPc complexes. The ΦF values are 0.065, 0.12 and 0.056 in DMSO for 4a-c, respectively. The low ΦF values were observed for ball-type derivatives. This is attributed to the ball-type structure which may encourage intersystem crossing to the triplet state due to its larger size, as mentioned above [11–14,16]. Face-to-face interaction of the two monomers in dimers is expected to decrease the energy gap between the singlet state and the triplet state and enhance the formation of triplet state (i.e. intersystem crossing increases) decreasing fluorescence [14]. However, in this work the ΦF values are similar to those of monomeric
Fig. 1. Absorption spectra for 4a-d in DMSO, (concentration = 10−5 M). 77
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Table 1 UV–Vis absorption (Q-band), emission and excitation spectral data, and photophysical data for the phthalocyanines 4 in THF and DMSO. λmax/nm THF
4a 4b 4c 4d
DMSO
λAbs
λAbs
λEms
λExc
ΔλStokes
ΦF
ΦT
ΦΔ
Φd(×10−6)
ΦISC
SΔ
τT/μs
687/715 695 692 680
694/717 698 696 686
726 708 707 –
695/721 699 699 –
9 10 11 –
0.065 0.120 0.056 –
0.11 0.57 0.73 –
0.087 0.083 0.173 –
1.32 1.72 1.88 0.00
0.825 0.310 0.210 –
0.79 0.14 0.24 –
80 820 340 –
Fig. 2. Absorbance (a), excitation (b), emission (c) spectra of complex 4a-c in DMSO, Excitation λmax: 615, 619, 618 nm, respectively.
78
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Fig. 3. Triplet lifetime for 4c in DMSO.
phthalocyanines, showing the interaction between the two rings is minimal. In addition, low ΦF values can be observed often for aggregated MPcs [32]. Triplet state properties including the triplet lifetimes (τT) and quantum yields (ΦT) of the Pcs are normally determined using laser flash photolysis. Table 1 shows this ΦT and τT values of 4a-c in DMSO. The triplet state quantum yield values are 0.11, 0.57, 0.73 for 4a-c, respectively. As known in the literature, these high values of ΦT values suggest more efficient intersystem crossing (ISC) in the presence of the carboxyl substituents for substituted complexes, corresponding to low ΦF values. The metastable triplet state properties intrinsic to phthalocyanines are particularly sensitive to changes that deactivate fluorescence. Yet, in here, complex 4a has both low ΦT and ΦF due to face-toface interaction and electron withdrawing resonance effect at nonperipherally position. A triplet decay curve of change in absorbance (ΔA) versus time in seconds is obtained from the experiment and from this the triplet lifetime can be determined. A typical triplet state decay curve for 4c is shown in Fig. 3 as an example (please see the Supplementary information for further information on the other complexes). Obtained lifetimes are 80, 820, 340 μs for 4a-c, respectively. The order of the lifetime values among the substituted complexes was 4b > 4c > 4a. The complex 4b, Mg2Pc2 have a long time triplet lifetime. Phthalocyanines with the same substituents at the same position bearing a different metal in the central cavity also show different triplet lifetimes and quantum yields. Fig. 4 shows spectral changes observed for 4c as an example (see
Supplementary data for the other complexes) during photolysis of complexes with increasing time from 0 to 60 min in the presence of DPBF and all experimental results are summarized in Table 1. DPBF degradation of the complexes 4a, b, and c at 415 nm was monitored with UV–Vis spectroscopy. Singlet oxygen can be obtained through an energy transfer process between excited triplet state of Pcs and ground state molecular oxygen. The ΦΔ value for a viable photosensitizer is related to the population of the triplet state, the lifetime and the effectiveness of energy transfer process. The ΦΔ values are 0.087, 0.083 and 0,173 for 4a, 4b and 4c, respectively. The order of the ΦΔ values among the substituted complexes was 4b < 4a < 4d in DMSO. The phthalocyanine complexes have high singlet oxygen quantum yields due to a high value of the product of triplet quantum yield. The observed low value of ΦΔ is not consistent with the previous findings, which may be an important challenge of ball-type structure [29]. The energy transfer efficiency (SΔ) between excited triplet state photosensitizer and the ground state molecular oxygen of the unmetaled Pc were found to be significantly higher than that those obtained for the metal complexes, the order of the SΔ values among the substituted complexes was 4b < 4c < 4a in DMSO. These values are compatible with the triplet state lifetimes. The low SΔ for 4b and 4c may also be due to partial oxidation, the face-to-face interaction between the two rings. Photodegradation is the decomposition of phthalocyanine upon illumination. Complexes 4a-d were found to be stable upon exposure to illumination for 60 min with the photodegradation quantum yield value
Fig. 4. Time-dependent photobleaching of DPBF absorption in the presence of 4c in DMSO. 79
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Fig. 5. The photobleaching spectra for 4b in DMSO.
in the order of 10−6, which is a good indicator of the high photostability of complexes [33] The data are presented in the Table 1. The obtained spectrum for 4b as an example (Please see the Supplementary data for the other complexes) is depicted in Fig. 5. Dc. conductivity measurements on compounds of 4a, 4b, 4c and 4d were carried out in the temperature range 300–450 K in a vacuum (< 10−3 mbar). In order to avoid from sorption-desorption process, rapid cooling method was used for temperature dependent conductivity measurements. From the comparison of the measured conductivity of the films, we found that the conductivity of the film of 4b is the highest among the investigated other phthalocyanine films. The order of conductivities observed for these compounds are 4b > 4d > 4c > 4a at all temperatures investigated. Because of its high conductivity, we focused on the compound of 4b. The results of the dc conductivity measurements also showed that the measured dc conductivity of the compounds depend exponentially on temperature which indicates the applicability of Eq. (1). dc
=
0
exp
EA kT
coordination in Pc unit has a significant effect on the SO2 sensing properties as suggested by Paoletti et al. It was observed that the compound of ball-type Mg2Pc2 (4b) exhibits better sensing performance towards to various concentrations of SO2 gas at room temperature. A typical plot in the complex impedance plane for one of the best sensor is shown in Fig. S18. The sensor was manufactured of a 4b film with a thickness of 100 nm. 4b coated IDT was exposed to various concentrations of SO2 and complex impedance spectra were taken after reaching the adsorption equilibrium. With the examination of the impedance spectra clear differences could be seen in the dependence of ω on both real and imaginary part of impedance between 300 and 1500 ppm SO2. It is well known from the present literature that the sensing mechanism of SO2 gas by phthalocyanines, unlike other well known simple gas molecule, is not well known. It is well documented in the literature that the gas sensing performance of the phthalocyanines can be affected by the conjugated π-electron system, by the peripheral attachment of additional atoms or groups, and the presence of hetero atoms and the central metal atom [38]. It was reported by Bohrer et al. [39] and Park et al. [40] that the detection of analyte molecules by phthalocyanines to be governed primarily by coordination to the metal center. The interaction between the analyte and the metallophthalocyanine surface produces a modulation in the electronic levels of phthalocyanine available for π–π* transitions. As results, a doping effect due to the adsorbed analytes onto the phthalocyanine film is basically the cause of the changes in the impedance of the sensing layer. Although phthalocyanines compounds have similar molecular structure, due to the nature of the central metal ion, they have quite different physical and chemical properties. In order to be sure that the observed SO2 response of the 4 based sensor is stable and repeatable, the response-recovery characteristics of the 4 based sensor to repeated exposure and cutting of 1500 ppm SO2 gas was also investigated. It was found that the response-recovery behavior of the sensors is highly reproducible. It suggests that the compounds are stable in contact with various concentrations of SO2 gas. In this work, we presented the synthesis of ball-type phthalocyanines. The complexes were characterized using elemental analysis and 1 H NMR, FTIR, UV–Vis spectroscopies and mass spectrometry. We observed that Mg2Pc2 had longer triplet lifetime than those of Zn2Pc2. Electrical conduction and SO2 sensing capability of the compounds were also investigated by impedance spectroscopy technique. Measurements revealed that dominant conduction mechanism in these compounds is correlated barrier hopping. The photophysical and photochemical behaviours of the compounds indicated that are valuable as potential photosensitizers for PDT as well as SO2 sensing test results indicated that compound 4b has great potential for use of SO2 sensor.
(1)
where, EA is an activation energy, T is temperature, k is Boltzmann's constant and σ0 is a constant of proportionality. The value of the activation energy was derived from the slope of the log σdc versus 1/T graphs, with the aid of Eq. (1), and was found to be 0.92 eV for compound 4a, 0.84 eV for compound 4b, 0.80 eV for compound 4c and 0.68 eV for compound 4d. These findings reveal that the central metal atom play an important role in dc conduction. The observed differences between Pc molecules investigated can be correlated with the size of the central metal atom. It was reported previously that the oxidation potential and morphological modifications can be achieved by the change of central metal atom [34–36]. The variation of the real part of the complex conductivity, σa.c(ω), with frequency in the range 20–2 × 106 Hz for the studied compounds was investigated at different temperature values between 300 and 450 K. The frequency dependence of real conductivity in the 4b thin film on a double logarithmic scale at different selected temperatures is presented in Fig. S16. The variation of the frequency exponent s, with temperature for the investigated samples is depicted in Fig. S17. According to Paoletti et al. [37] that the interaction between the target gas molecule and the MPc unit is determined by the metal ion coordination and the peripheral attachment of additional atoms or groups that enhance or diminish the ionisation energy. Therefore in the first step of our investigation, we focused on better understand the function of the metal ion coordination in SO2 sensing performance. For this purpose, the sensing performance of the compounds for the same concentration of SO2 at room temperature was first investigated. The response-recovery measurements verified that the metal ion 80
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Acknowledgements
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This work was supported by the Research Fund of the Yildiz Technical University (Project No: 2011-01-02-GEP07), partially supported by Ministry of Development-Republic of Turkey with the project Number: 2016K121230, as well as Yildiz Technical University in Istanbul, Turkey and Rhodes University in Grahamstone, South Africa. The authors are also thankful to Prof. Tebello Nyokong for providing us a grant and laboratory facilities during the photophysical and photochemical measurements. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.inoche.2019.02.040. References [1] R. Polley, H. Heckmann, M. Hanack, Methods of Organic Chemistry (HoubenWeyl), 4 Edition, Vol. E9 Georg Thieme Verlag, Stuttgart, 1997, pp. 717–842. [2] N.B. McKeown, Phthalocyanine Materials: Structure, Synthesis and Function (1998). [3] H. de Diesbach, E. von der Weid, Helv. Chim. Acta 10 (1927) 886–888. [4] A. Braun, J. Tcherniac, Ber. Dtsch. Chem. Ges. 40 (1907) 2709–2714. [5] J. Simon, C. Sirlin, J. Appl. Chem. 61 (1989) 1625–1629. [6] S.B. Brown, E.A. Brown, I. Walker, Lancet Oncol. 5 (2004) 497–508. [7] D. Dini, M. Hanack, K.M. Kadish, K.M. Smith, R. Guilard (Eds.), The Porphyrin Handbook, Academic Press, 2003, pp. 1–36. [8] A.Y. Tolbin, A.V. Ivanov, L.G. Tomilova, N.S. Zefirov, Mendeleev Commun. 12 (2002) 96–97. [9] A.Y. Tolbin, A.V. Ivanov, L.G. Tomilova, N.S. Zefirov, J. Porphyrins Phthalocyanines 7 (2003) 162–166. [10] G. de la Torre, G. Bottari, M. Sekita, A. Hausmann, M.G. Dirk, T. Torres, Chem. Soc. Rev. 42 (2013) 8049–8105. [11] M. Canlıca, I.N. Booysen, T. Nyokong, Polyhedron 30 (2011) 508–514. [12] M. Canlıca, T. Nyokong, Dalton Trans. 40 (2011) 1497–1502. [13] M. Canlıca, T. Nyokong, Dalton Trans. 40 (2011) 5285–5290. [14] M. Canlıca, T. Nyokong, Polyhedron 31 (2012) 704–709. [15] M. Canlica, M. Coskun, A. Altindal, T. Nyokong, J. Porphyrins Phthalocyanines 16 (2012) 855–860. [16] M. Canlica, A. Altindal, T. Nyokong, J. Porphyrins Phthalocyanines 16 (2012) 826–832. [17] N. Nwaji, D.O. Oluwole, J. Mack, M. Louzada, S. Khene, J. Britton, T. Nyokong, Dyes Pigments 140 (2017) 417–430. [18] E. Yabas, E. Bagda, E. Bagda, Dyes Pigments 120 (2015) 220–227.
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Mevlüde Canlıcaa, , Birsel Can Ömürb, Bekir Salihc Yildiz Technical University, Chemistry Department, Inorganic Chemistry Division, Davutpasa Campus, 34220 Esenler- Istanbul, Turkey b Yildiz Technical University, Physics Department, Davutpasa Campus, 34220 Esenler- Istanbul, Turkey c Hacettepe University, Chemistry Department, 06800 Cankaya-Ankara, Turkey E-mail address: [email protected] (M. Canlıca). ⁎
a
Corresponding author. 81