Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 55–62
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
The influence of the various central metals on photophysical and photochemical properties of benzothiazole-substituted phthalocyanines Asiye Nas a, Gülsev Dilber a, Mahmut Durmusß b, Halit Kantekin a,⇑ a b
Department of Chemistry, Karadeniz Technical University, 61080 Trabzon, Turkey Gebze Institute of Technology, Department of Chemistry, PO Box 141, Gebze 41400, Kocaeli, Turkey
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The fluorescence quantum yields and
The fluorescence quantum yields and lifetimes, fluorescence quenching studies and singlet oxygen quantum yields and photodegradation studies of tetra-benzothiazole substituted metal-free (H2Pc, 1), lead(II) (PbPc, 2) and zinc(II) (ZnPc, 3) phthalocyanine compounds were investigated in tetrahydrofuran (THF) solution. The influence of the various central metal ions (zinc, lead or without metal) on the photophysical and photochemical parameters was also investigated and compared.
lifetimes of phthalocyanines. The fluorescence quenching studies of phthalocyanines. The singlet oxygen quantum yields and photodegradation studies of phthalocyanines. The influence of the metal ions on the photophysical and photochemical parameters.
a r t i c l e
i n f o
Article history: Received 1 April 2014 Received in revised form 21 June 2014 Accepted 25 June 2014 Available online 3 July 2014 Keywords: Phthalocyanine Photophysical Photochemical Fluorescence Singlet oxygen Photodegradation
a b s t r a c t The photophysical (fluorescence quantum yields and lifetimes, fluorescence quenching studies by 1,4benzoquinone (BQ)) and photochemical (singlet oxygen quantum yields and photodegradation studies under light irradiation) properties of tetra-benzothiazole substituted metal-free (H2Pc, 1), lead (II) (PbPc, 2) and zinc(II) (ZnPc, 3) phthalocyanine compounds were investigated in tetrahydrofuran (THF) solution. All of these compounds did not show any aggregation and they produced good singlet oxygen (especially ZnPc). The influence of the various central metal ions (zinc, lead or without metal) on the photophysical and photochemical parameters was also investigated and compared. Ó 2014 Elsevier B.V. All rights reserved.
Introduction ⇑ Corresponding author. Tel.: +90 462 377 25 89; fax: +90 462 325 31 96. E-mail address: [email protected] (H. Kantekin). http://dx.doi.org/10.1016/j.saa.2014.06.135 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
Phthalocyanines (Pcs) are synthetic macrocyclic compounds containing four p-fused isoindole units which were synthesized
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accidentally in 1928 during the preparation of phthalimide from phthalic anhydride and ammonia [1]. Because these compounds have a large 18 p-electron conjugated system, they absorb strongly in the near-IR region, giving a characteristic blue–green color [2]. They are generally stable against to acids, bases, moisture, heat, light and solvents [1,3]. Phthalocyanine compounds, the synthetic analogs of the naturally occurring porphyrins, are functional materials in many different fields. Metallophthalocyanines (MPcs) have been shown to behave as semiconductors [4,5], catalysts [6,7], liquid crystals [8,9], chemical sensors [10,11] and especially in photodynamic therapy [12–14]. Photodynamic therapy (PDT) is a medical treatment of cancer tissues which employs the combination of light and photosensitizing agent like porphyrins, phthalocyanines, etc. [15,16]. A major objective for this cancer treatment is the selective destruction of tumor cells without damage to normal healthy tissues [17,18]. Phthalocyanines are porphyrin-like second generation sensitizers for photodynamic therapy of cancer [19]. In order to be used for PDT, a phthalocyanine derivative (photosensitizer) must have a long wavelength absorption in the red region [20]. The intense absorptions of the phthalocyanine derivatives in the red region of the visible spectrum make them suitable for PDT. Another important circumstance is that the phthalocyanines have low solubility in aqueous media or common organic solvents, whereas water-soluble phthalocyanines are considered as the new generation of photosensitizers [20]. Adding some substituents to peripheral positions [21] leads to phthalocyanine derivatives soluble in several organic solvents. Also adding sulfo or quaternized ammonium groups increase the solubility of phthalocyanines in aqueous media [17]. With the increasing solubility of phthalocyanines, their usage as the new generation of photosensitizers also increases in photodynamic therapy. The heterocycles, which are important not only due to their abundance but also because of their chemical, biological and technical significance, are the largest group of organic compounds. Thiazoles, which are heterocyclic compounds that contain both sulfur and nitrogen, are members of the heterocycles that include imidazoles and oxazoles [22]. Thiazoles, which are consisting of a 5-membered 1,3-thiazole ring fused to a benzene ring, are so-called benzothiazoles. Many benzothiazole derivatives are found to possess a number of biological activities such as antifungal, antidiabetic, antiviral, antitubercular and anti-inflammatory activities [23]. In recent years, there are number of paper dealing with thiazoles, which have both sulfur and nitrogen atom [24,25], but there are very few articles about photophysical and photochemical properties of the thiazole substituted phthalocyanines [26,27]. Whereas as a substituent the sulfur atom behaves as electron donating atom, which can influence the electronic spectra of complexes [28]. As a substituent group, benzothiazoles were used because they enhance the photosensitizer activities of complexes because of synergistic effect [26]. The PDT properties of the phthalocyanine molecules are vigorously influenced by the presence and nature of central metal ion and substitute group [28]. It is known that diamagnetic ions with a closed shell such as Zn2+, Al3+, Si4+ result in both high triplet quantum yields and relatively long triplet lifetimes [29]. While ZnPc complexes are especially well known for their high photosensitizing abilities, unmetallated phthalocyanines show very little PDT effect [30]. In this study, the photophysical (fluorescence lifetime and quantum yields, fluorescence quenching studies by BQ and photochemical (singlet oxygen and photodegradation quantum yields) properties of metal-free (H2Pc, 1), lead(II) (PbPc, 2) and zinc(II) (ZnPc, 3) phthalocyanines (Fig. 1) were investigated in THF for the first time. In addition, the influence of the various central metal
ions (zinc, lead or without metal) on the photophysical and photochemical parameters was also investigated and compared. Experimental Materials 4-(2-(Benzo[d]thiazol-2-yl)phenoxy) substituted phthalocyanine derivatives (H2Pc (1), PbPc (2) and ZnPc (3) were prepared according to procedure in the relevant literature [31]. Unsubstituted zinc(II) phthalocyanine (ZnPc) were purchased from Sigma Aldrich. All other solvents and reagents were of reagent grade and used as received. Results and discussion Synthesis and characterization The general synthetic route for the synthesis of the phthalocyanine compounds ((H2Pc (1), PbPc (2) and ZnPc (3)) is given in related literature [31]. Briefly, 4-(2-(benzo[d]thiazol-2-yl)phenoxy-substituted H2Pc (1) was prepared by cyclotetramerization of 4-(2-(benzo[d]thiazol-2-yl)phenoxy)phthalonitrile in the presence of 1,8-diazabicyclo[5.4.0] undec-7-ene in dry n-pentanol at 160oC for 24 h. Tetra-substituted PbPc (2) and ZnPc (3) compounds were also obtained by cyclotetramerization of 4-(2-(benzo[d]thiazol-2yl)phenoxy)phthalonitrile in the presence of related metal salts (PbO) or (Zn(ac)2 and two drops of 1,8-diazabicyclo[5.4.0] undec7-ene in dry n-pentanol 160 °C for 24 h, respectively. All phthalocyanines were purified by column chromatography on silica gel using chloroform: methanol solvent system (93:7, 91:9, 93:7, respectively for compounds 1–3) as eluent. UV–Vis absorption spectra As mentioned in our previous paper, the absorption spectrum of H2Pc (1) includes a splitting intense Q-band at kmax : 700 and 665 nm with shoulders at kmax : 638 and 605 nm and the B-band at kmax : 383 nm [31]. For compounds 2 and 3 the Q-bands were observed at kmax : 707 and 673 nm, respectively. In the absorption spectra, PbPc (2) was red shifted compared to H2Pc (1) and ZnPc (3). For compound 2, the large red shift was attributed to highly deformed phthalocyanine skeleton because of the central metal ion (Pb2+) which was not fitting into the cavity of phthalocyanine molecule due to large atomic size of lead atom. Namely, phthalocyanine ligand deformation causes in the red shifting of the Q-band [32]. In this study, the aggregation behavior of the studied phthalocyanine compounds (1–3) was investigated at different concentrations in THF, as the concentration was increased, the intensity of absorption of the Q band also increased and there were no new bands due to the aggregated species for the studied phthalocyanine compounds (1–3). Beer–Lambert law was obeyed in these compounds in the concentrations ranging from 2 10 6 to 12 10 6 M in THF. Fluorescence spectra The fluorescence emission and excitation wavelength maxima values were given in Table 1. The obtained stokes shifts were 4 nm for H2Pc (1) and PbPc (2) compounds and 6 nm for ZnPc (3) compound. For the H2Pc (1) and ZnPc (3) compounds, the excitation spectra were similar in the absorption spectra in THF. This attributes that the nuclear configuration of the ground state and the excited state are similar and not affect by the excitation. Fig. 2a–c show absorption, fluorescence emission and excitation spectra of compounds 1–3 in THF.
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(a)
(b)
(c) Fig. 1. Chemical structure of (a) H2Pc (1), (b) PbPc (2) and (c) ZnPc (3) compounds.
Table 1 Absorption, excitation and emission spectral data for tetra-substituted H2Pc (1), PbPc (2) and ZnPc (3) compounds in THF.
a
Compound
Solvent
Q band kmax (nm)
log e
Excitation kEx (nm)
Emission kEm (nm)
Stokes shift DStokes (nm)
H2Pc (1) PbPc (2) ZnPc (3) Std-ZnPca
THF THF THF THF
700–665 707 673 666
5.11–5.09 5.20 5.29 5.19
699–664 699–665 676 666
703 703 682 673
4 4 6 7
Data from Ref. [37].
On the other hand, there were differences between the excitation and absorption spectra for the PbPc (2) in that the Q band of the excitation showed two peaks unlike that of the absorption spectrum. This observed difference for the PbPc (2) on the excitation could be due to the larger lead metal being out from the core of the Pc ring. Photophysical studies Fluorescence quantum yields (UF) and lifetimes (sF) The fluorescence quantum yield (UF) values of the studied Pcs (1–3) were calculated and obtained results were given in Table 2. The UF values of the H2Pc (1) and ZnPc (3) compounds were higher in comparison with standard ZnPc in THF. However, the PbPc (2) compound exhibited lower UF values according to the standard ZnPc
in THF, which citing that there are more changes in PbPc (2) compound following excitation as mentioned in fluorescence section. The fluorescence lifetime (sF) is an average time is a molecule spends its excited state before fluorescing, and its founded value is directly related to that of fluorescence quantum yield. From the details in Table 2, this connection between the fluorescence lifetimes (sF) and the fluorescence quantum yield (UF) values were clearly observed. Fluorescence lifetime (sF) values depend on the environment and the nature of a fluorophere. As reflected by the data in Table 2, compound 3 has the highest sF value when compared to other studied Pc compounds. For all studied compounds, the sF values were lower than standard ZnPc in THF. At the same time, the sF values of these compounds were different from each other, suggesting that the variety of the metal ions in the framework of the Pc ring effects sF values of compounds.
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0.12
(a)
700 0.1
Absorption
Emission
600
Excitation Emission
400
Absorption
0.06
Absorption
Intensity (a.u.)
0.08
Excitation
500
300 0.04 200 0.02 100
0 500
550
600
650
700
750
0 800
Wavelength (nm) 0.2
350
(b)
0.18
300 0.16 Excitation
250
Intensity (a.u.)
Absorption
Absorption
0.12
200 0.1
Excitation
150
Emission
0.08
Absorption
0.14 Emission
0.06
100
0.04 50 0.02 0 500
550
600
650
700
750
0 800
Wavelength (nm) 900
0.25
(c)
800
Excitation 0.2
700 Excitation Emission
Emission
Absorption
0.15
500 400
Absorption
Intensity (a.u.)
600
0.1 300
Absorption 200
0.05
100 0 500
550
600
650
700
750
0 800
Wavelength (nm) Fig. 2. Absorption, excitation and emission spectra of compounds (a) for compound 1, (b) for compound 2 and (c) for compound 3. Excitation wavelength: 650 nm.
The natural radiative lifetime (s0) and the rate constants for fluorescence (kF) values of studied compounds were also listed in Table 2. For the all compounds, the s0 values were lower than the standard ZnPc in THF. On the contrary, the rate constant for fluorescence (kF) values of these compounds were higher than standard ZnPc in THF.
Fluorescence quenching studies by 1,4-benzoquinone (BQ) The fluorescence quenching of the studied phthalocyanine compounds (1–3) by the addition of increasing concentrations of BQ is to obey Stern–Volmer kinetics. Quinones have highly electron affinities, and their involvement in electron transfer is also well documented [33]. Moreover, MPc
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A. Nas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 55–62 Table 2 Photophysical and photochemical parameters of unsubstituted and tetra-substituted H2Pc (1), PbPc (2) and ZnPc (3) compounds in THF.
a b c
Compound
Solvent
UF
sF (ns)
s0 (ns)
kFa (s
H2Pc (1) PbPc (2) ZnPc (3) Std-ZnPc
THF THF THF THF
0.297 0.138 0.320 0.25b
2.082 1.119 2.436 2.72b
7.012 8.110 7.612 10.90b
14.26 12.33 13.14 9.17b
1
) (107)
Ud (10 4)
UD
8.90 34.30 0.92 20.00b
0.27 0.51 0.67 0.53c
The rate constant for fluorescence. Values calculated using kF = UF/sF. Data from Ref. [37]. Data from Ref. [38].
are to be easily reduced. Fluorescence quenching of MPc compounds by BQ is formed via excited state electron transfer from the MPc to BQ. The energy of the lowest excited state for quinones is greater than the energy of the excited singlet state of MPc compounds [34,35]. MPc interacts with increasing concentrations of BQ and their fluorescence spectra were monitored. There were no shifts in wavelength or additional peak, suggesting that there could be any ground state interaction between Pc compounds and BQ. Fig. 3 shows the quenching of the compound 3 by BQ as an example. Stern–Volmer plots for all studied compounds exhibit linear lines, depicting diffusion-controlled quenching mechanism (Fig. 4). The KSV and biomolecular quenching constant (Kq) values of the studied phthalocyanine compounds (1–3) were given in Table 3, and these values are lower than unsubstituted ZnPc in THF. The substitution of the Pc ring seemed to decrease KSV value of these compounds. As seen from Table 3, the compound 2 has the highest KSV value; while the compound 1 has the lowest KSV value in THF.
2.4 2.2
1 2
2
3
Io/I
1.8 1.6 1.4 1.2 1 0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
[BQ] Fig. 4. Stern–Volmer plots for BQ quenching of tetra-substituted Pcs (1–3) (C = 1.00 10 5 M) in THF. [BQ] = 0, 0.008, 0.016, 0.024, 0.032 and 0.040 M.
Photochemical studies Table 3 Fluorescence quenching data of tetra-substituted H2Pc (1), PbPc (2) and ZnPc (3) compounds in THF.
Singlet oxygen quantum yields Singlet oxygen plays a significant role for the using of the phthalocyanine compounds in PDT application as photosensitizers. It is formed via energy transfer between the triplet state of a phthalocyanine compound and ground state molecular oxygen, and this energy transfer must be highly efficient to generate large amounts of singlet oxygen. Singlet oxygen quantum yields (UD) were determined by chemical method using DPBF (1,3-diphenylisobenzofuran) as a singlet
a
Compound
Solvent
KSV (M
H2Pc (1) PbPc (2) ZnPc (3) Std-ZnPca
THF THF THF THF
22.93 32.13 28.14 48.48
1
)
Kq (dm3 mol
1
s
1
) (1010)
1.10 2.87 1.15 1.78
Data from Ref. [37].
500 450
[BQ]=0 400
Intensity (a.u.)
350 300 250 200
[BQ]=saturated
150 100 50 0
654
674
694
714
734
754
774
794
Wavelength (nm) Fig. 3. Fluorescence emission spectral changes of 3 (1 10 5 M) during fluorescence quenching studies in THF. [BQ] = 0, 0.008, 0.016, 0.025, 0.032, 0.040 M and saturated with BQ (inset: Stern–Volmer plots for BQ quenching of 3 in THF).
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(a) 1,2
Absorbance
1
0,8
0,6
0,4
0,2
0 300
350
400
450
500
550
600
650
700
750
800
750
800
Wavelength (nm)
(b)
1,6
(c) 1,4
Absorbance
1,2
1
0,8
0,6
0,4
0,2
0 300
350
400
450
500
550
600
650
700
Wavelength (nm) Fig. 5. Absorption changes during the determination of singlet oxygen quantum yield (a) for compound 1, (b) for compound 2 and (c) for compound 3 in THF using DPBF as a singlet oxygen quencher.
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2 1,8 1,6
Absorbance
1,4 1,2 1 0,8 0,6 0,4 0,2 0 300
350
400
450
500
550
600
650
700
750
800
Wavelength (nm) Fig. 6. Absorption spectral changes during the investigation of the photodegradation quantum yield of compound 3 in THF (inset: plot of absorbance versus time).
oxygen scavenger in THF. The obtained results were given in Table 2. A time-dependent decrease in the DPBF concentration was observed at 417 nm by following a decrease in its absorbance (Fig. 5, compounds 1, 2 and 3 were given as examples). The absence of any significant changes in the Q band intensities of the studied Pc compounds supports that there was no defect on its structure during singlet oxygen measurement. It is clear from Table 2 that ZnPc derivative (compound 3) show higher UD value than their H2Pc (1) and PbPc (2) counterparts and also standard ZnPc in THF. Generally, zinc(II) phthalocyanines generate highly singlet oxygen since the d10 configuration of the central Zn2+ ion, which make them appropriate photosensitizers for PDT application. Considering data obtained in THF, the lowest UD value observed for H2Pc (1), suggesting inefficient energy transfer from the triplet state to molecular oxygen. Photodegradation quantum yields Photodegradation process can be used to determinate stability of Pcs which include decreasing of the Pc’s Q band intensity under the light irradiation. The stability of these molecules is particularly important for using as photocatalysts. Photodegradation is characterized by decreasing of absorption spectra of Pcs without accompanying shift in position or appearing of new peaks in the visible region. The stabilities of all studied compounds against to light irradiation were determined in THF by monitoring the absorption spectra of these compounds with the increasing irradiation time (Fig. 6, compound 3 was given as an example). The observed spectral changes confirm that photodegradation is not associated with photo transformation into different forms of phthalocyanine absorbing in the visible region. While typical values for stable phthalocyanines are of the order of 10 6, these values for unstable phthalocyanines are of the order of 10 3 [36]. The Ud values were listed in Table 2 for all compounds. According to the data obtained from the table, the ZnPc (3) is more stable than the other Pc compounds (1 and 2) because of having lower Ud value. Conclusions In this paper, the photophysical and photochemical properties of tetra-benzothiazole substituted metal-free (H2Pc, 1), lead (II) (PbPc, 2) and zinc(II) (ZnPc, 3) phthalocyanine compounds were
examined. These properties of studied phthalocyanine compounds (1–3) were calculated and compared with each other. The UF of 3 (UF = 0.320) was relatively higher compared to the corresponding H2Pc (1) (UF = 0.297) and the standard ZnPc (UF = 0.25) in THF. In similar, when the fluorescence quantum lifetimes (sF) of related phthalocyanine compounds were compared, the sF of 3 is the higher than the others. However, sF of standard ZnPc is higher than studied phthalocyanines because of the metal ion effect in the Pc ring. The fluorescence quenching of the 1–3 by BQ in different concentration obeys Stern–Volmer kinetics. For all studied compounds, Stern–Volmer plots exhibit linear lines, depicting diffusion-controlled quenching mechanism. The studied compounds (1–3) produced singlet oxygen in THF and these compounds, especially compound 3 which has the highest singlet oxygen quantum yield (UD = 0.67 for compound 3), are good candidates for photodynamic therapy. According to the photodegradation quantum yield (Ud) values of the studied phthalocyanines, the Ud of compound 3 is more stable than the other compounds because of lower Ud value. One can be derived from conducted study; the clear inference is that benzothiazole-substituted ZnPc (3) would be used as a potential photosensitizer agent due to its highest UD values in photodynamic therapy which is a fairly spectacular alternative treatment method of cancer diseases today. Acknowledgement This study was supported by the Research Fund of Karadeniz Technical University, Project No.: 2010.111.002.5 (TrabzonTurkey). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.06.135. References [1] A.L. Thomas, Phthalocyanine Research and Applications, CRC Press, Boca Raton, Florida, 1990. [2] J.Y. Liu, P.C. Lo, D.K.P. Ng, Struct. Bond. 135 (2010) 169–210. [3] P. Erk, H. Hengelsberg, in: K.M. Kadish, K.M. Smith, R. Guilard (Eds.), The Porphyrin Handbook, vol. 19: Applications of Phthalocyanines, Academic Press, San Diego, California, 2003.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
The influence of the various central metals on photophysical and photochemical properties of benzothiazole-substituted phthalocyanines Asiye Nas a, Gülsev Dilber a, Mahmut Durmusß b, Halit Kantekin a,⇑ a b
Department of Chemistry, Karadeniz Technical University, 61080 Trabzon, Turkey Gebze Institute of Technology, Department of Chemistry, PO Box 141, Gebze 41400, Kocaeli, Turkey
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The fluorescence quantum yields and
The fluorescence quantum yields and lifetimes, fluorescence quenching studies and singlet oxygen quantum yields and photodegradation studies of tetra-benzothiazole substituted metal-free (H2Pc, 1), lead(II) (PbPc, 2) and zinc(II) (ZnPc, 3) phthalocyanine compounds were investigated in tetrahydrofuran (THF) solution. The influence of the various central metal ions (zinc, lead or without metal) on the photophysical and photochemical parameters was also investigated and compared.
lifetimes of phthalocyanines. The fluorescence quenching studies of phthalocyanines. The singlet oxygen quantum yields and photodegradation studies of phthalocyanines. The influence of the metal ions on the photophysical and photochemical parameters.
a r t i c l e
i n f o
Article history: Received 1 April 2014 Received in revised form 21 June 2014 Accepted 25 June 2014 Available online 3 July 2014 Keywords: Phthalocyanine Photophysical Photochemical Fluorescence Singlet oxygen Photodegradation
a b s t r a c t The photophysical (fluorescence quantum yields and lifetimes, fluorescence quenching studies by 1,4benzoquinone (BQ)) and photochemical (singlet oxygen quantum yields and photodegradation studies under light irradiation) properties of tetra-benzothiazole substituted metal-free (H2Pc, 1), lead (II) (PbPc, 2) and zinc(II) (ZnPc, 3) phthalocyanine compounds were investigated in tetrahydrofuran (THF) solution. All of these compounds did not show any aggregation and they produced good singlet oxygen (especially ZnPc). The influence of the various central metal ions (zinc, lead or without metal) on the photophysical and photochemical parameters was also investigated and compared. Ó 2014 Elsevier B.V. All rights reserved.
Introduction ⇑ Corresponding author. Tel.: +90 462 377 25 89; fax: +90 462 325 31 96. E-mail address: [email protected] (H. Kantekin). http://dx.doi.org/10.1016/j.saa.2014.06.135 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
Phthalocyanines (Pcs) are synthetic macrocyclic compounds containing four p-fused isoindole units which were synthesized
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accidentally in 1928 during the preparation of phthalimide from phthalic anhydride and ammonia [1]. Because these compounds have a large 18 p-electron conjugated system, they absorb strongly in the near-IR region, giving a characteristic blue–green color [2]. They are generally stable against to acids, bases, moisture, heat, light and solvents [1,3]. Phthalocyanine compounds, the synthetic analogs of the naturally occurring porphyrins, are functional materials in many different fields. Metallophthalocyanines (MPcs) have been shown to behave as semiconductors [4,5], catalysts [6,7], liquid crystals [8,9], chemical sensors [10,11] and especially in photodynamic therapy [12–14]. Photodynamic therapy (PDT) is a medical treatment of cancer tissues which employs the combination of light and photosensitizing agent like porphyrins, phthalocyanines, etc. [15,16]. A major objective for this cancer treatment is the selective destruction of tumor cells without damage to normal healthy tissues [17,18]. Phthalocyanines are porphyrin-like second generation sensitizers for photodynamic therapy of cancer [19]. In order to be used for PDT, a phthalocyanine derivative (photosensitizer) must have a long wavelength absorption in the red region [20]. The intense absorptions of the phthalocyanine derivatives in the red region of the visible spectrum make them suitable for PDT. Another important circumstance is that the phthalocyanines have low solubility in aqueous media or common organic solvents, whereas water-soluble phthalocyanines are considered as the new generation of photosensitizers [20]. Adding some substituents to peripheral positions [21] leads to phthalocyanine derivatives soluble in several organic solvents. Also adding sulfo or quaternized ammonium groups increase the solubility of phthalocyanines in aqueous media [17]. With the increasing solubility of phthalocyanines, their usage as the new generation of photosensitizers also increases in photodynamic therapy. The heterocycles, which are important not only due to their abundance but also because of their chemical, biological and technical significance, are the largest group of organic compounds. Thiazoles, which are heterocyclic compounds that contain both sulfur and nitrogen, are members of the heterocycles that include imidazoles and oxazoles [22]. Thiazoles, which are consisting of a 5-membered 1,3-thiazole ring fused to a benzene ring, are so-called benzothiazoles. Many benzothiazole derivatives are found to possess a number of biological activities such as antifungal, antidiabetic, antiviral, antitubercular and anti-inflammatory activities [23]. In recent years, there are number of paper dealing with thiazoles, which have both sulfur and nitrogen atom [24,25], but there are very few articles about photophysical and photochemical properties of the thiazole substituted phthalocyanines [26,27]. Whereas as a substituent the sulfur atom behaves as electron donating atom, which can influence the electronic spectra of complexes [28]. As a substituent group, benzothiazoles were used because they enhance the photosensitizer activities of complexes because of synergistic effect [26]. The PDT properties of the phthalocyanine molecules are vigorously influenced by the presence and nature of central metal ion and substitute group [28]. It is known that diamagnetic ions with a closed shell such as Zn2+, Al3+, Si4+ result in both high triplet quantum yields and relatively long triplet lifetimes [29]. While ZnPc complexes are especially well known for their high photosensitizing abilities, unmetallated phthalocyanines show very little PDT effect [30]. In this study, the photophysical (fluorescence lifetime and quantum yields, fluorescence quenching studies by BQ and photochemical (singlet oxygen and photodegradation quantum yields) properties of metal-free (H2Pc, 1), lead(II) (PbPc, 2) and zinc(II) (ZnPc, 3) phthalocyanines (Fig. 1) were investigated in THF for the first time. In addition, the influence of the various central metal
ions (zinc, lead or without metal) on the photophysical and photochemical parameters was also investigated and compared. Experimental Materials 4-(2-(Benzo[d]thiazol-2-yl)phenoxy) substituted phthalocyanine derivatives (H2Pc (1), PbPc (2) and ZnPc (3) were prepared according to procedure in the relevant literature [31]. Unsubstituted zinc(II) phthalocyanine (ZnPc) were purchased from Sigma Aldrich. All other solvents and reagents were of reagent grade and used as received. Results and discussion Synthesis and characterization The general synthetic route for the synthesis of the phthalocyanine compounds ((H2Pc (1), PbPc (2) and ZnPc (3)) is given in related literature [31]. Briefly, 4-(2-(benzo[d]thiazol-2-yl)phenoxy-substituted H2Pc (1) was prepared by cyclotetramerization of 4-(2-(benzo[d]thiazol-2-yl)phenoxy)phthalonitrile in the presence of 1,8-diazabicyclo[5.4.0] undec-7-ene in dry n-pentanol at 160oC for 24 h. Tetra-substituted PbPc (2) and ZnPc (3) compounds were also obtained by cyclotetramerization of 4-(2-(benzo[d]thiazol-2yl)phenoxy)phthalonitrile in the presence of related metal salts (PbO) or (Zn(ac)2 and two drops of 1,8-diazabicyclo[5.4.0] undec7-ene in dry n-pentanol 160 °C for 24 h, respectively. All phthalocyanines were purified by column chromatography on silica gel using chloroform: methanol solvent system (93:7, 91:9, 93:7, respectively for compounds 1–3) as eluent. UV–Vis absorption spectra As mentioned in our previous paper, the absorption spectrum of H2Pc (1) includes a splitting intense Q-band at kmax : 700 and 665 nm with shoulders at kmax : 638 and 605 nm and the B-band at kmax : 383 nm [31]. For compounds 2 and 3 the Q-bands were observed at kmax : 707 and 673 nm, respectively. In the absorption spectra, PbPc (2) was red shifted compared to H2Pc (1) and ZnPc (3). For compound 2, the large red shift was attributed to highly deformed phthalocyanine skeleton because of the central metal ion (Pb2+) which was not fitting into the cavity of phthalocyanine molecule due to large atomic size of lead atom. Namely, phthalocyanine ligand deformation causes in the red shifting of the Q-band [32]. In this study, the aggregation behavior of the studied phthalocyanine compounds (1–3) was investigated at different concentrations in THF, as the concentration was increased, the intensity of absorption of the Q band also increased and there were no new bands due to the aggregated species for the studied phthalocyanine compounds (1–3). Beer–Lambert law was obeyed in these compounds in the concentrations ranging from 2 10 6 to 12 10 6 M in THF. Fluorescence spectra The fluorescence emission and excitation wavelength maxima values were given in Table 1. The obtained stokes shifts were 4 nm for H2Pc (1) and PbPc (2) compounds and 6 nm for ZnPc (3) compound. For the H2Pc (1) and ZnPc (3) compounds, the excitation spectra were similar in the absorption spectra in THF. This attributes that the nuclear configuration of the ground state and the excited state are similar and not affect by the excitation. Fig. 2a–c show absorption, fluorescence emission and excitation spectra of compounds 1–3 in THF.
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(a)
(b)
(c) Fig. 1. Chemical structure of (a) H2Pc (1), (b) PbPc (2) and (c) ZnPc (3) compounds.
Table 1 Absorption, excitation and emission spectral data for tetra-substituted H2Pc (1), PbPc (2) and ZnPc (3) compounds in THF.
a
Compound
Solvent
Q band kmax (nm)
log e
Excitation kEx (nm)
Emission kEm (nm)
Stokes shift DStokes (nm)
H2Pc (1) PbPc (2) ZnPc (3) Std-ZnPca
THF THF THF THF
700–665 707 673 666
5.11–5.09 5.20 5.29 5.19
699–664 699–665 676 666
703 703 682 673
4 4 6 7
Data from Ref. [37].
On the other hand, there were differences between the excitation and absorption spectra for the PbPc (2) in that the Q band of the excitation showed two peaks unlike that of the absorption spectrum. This observed difference for the PbPc (2) on the excitation could be due to the larger lead metal being out from the core of the Pc ring. Photophysical studies Fluorescence quantum yields (UF) and lifetimes (sF) The fluorescence quantum yield (UF) values of the studied Pcs (1–3) were calculated and obtained results were given in Table 2. The UF values of the H2Pc (1) and ZnPc (3) compounds were higher in comparison with standard ZnPc in THF. However, the PbPc (2) compound exhibited lower UF values according to the standard ZnPc
in THF, which citing that there are more changes in PbPc (2) compound following excitation as mentioned in fluorescence section. The fluorescence lifetime (sF) is an average time is a molecule spends its excited state before fluorescing, and its founded value is directly related to that of fluorescence quantum yield. From the details in Table 2, this connection between the fluorescence lifetimes (sF) and the fluorescence quantum yield (UF) values were clearly observed. Fluorescence lifetime (sF) values depend on the environment and the nature of a fluorophere. As reflected by the data in Table 2, compound 3 has the highest sF value when compared to other studied Pc compounds. For all studied compounds, the sF values were lower than standard ZnPc in THF. At the same time, the sF values of these compounds were different from each other, suggesting that the variety of the metal ions in the framework of the Pc ring effects sF values of compounds.
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0.12
(a)
700 0.1
Absorption
Emission
600
Excitation Emission
400
Absorption
0.06
Absorption
Intensity (a.u.)
0.08
Excitation
500
300 0.04 200 0.02 100
0 500
550
600
650
700
750
0 800
Wavelength (nm) 0.2
350
(b)
0.18
300 0.16 Excitation
250
Intensity (a.u.)
Absorption
Absorption
0.12
200 0.1
Excitation
150
Emission
0.08
Absorption
0.14 Emission
0.06
100
0.04 50 0.02 0 500
550
600
650
700
750
0 800
Wavelength (nm) 900
0.25
(c)
800
Excitation 0.2
700 Excitation Emission
Emission
Absorption
0.15
500 400
Absorption
Intensity (a.u.)
600
0.1 300
Absorption 200
0.05
100 0 500
550
600
650
700
750
0 800
Wavelength (nm) Fig. 2. Absorption, excitation and emission spectra of compounds (a) for compound 1, (b) for compound 2 and (c) for compound 3. Excitation wavelength: 650 nm.
The natural radiative lifetime (s0) and the rate constants for fluorescence (kF) values of studied compounds were also listed in Table 2. For the all compounds, the s0 values were lower than the standard ZnPc in THF. On the contrary, the rate constant for fluorescence (kF) values of these compounds were higher than standard ZnPc in THF.
Fluorescence quenching studies by 1,4-benzoquinone (BQ) The fluorescence quenching of the studied phthalocyanine compounds (1–3) by the addition of increasing concentrations of BQ is to obey Stern–Volmer kinetics. Quinones have highly electron affinities, and their involvement in electron transfer is also well documented [33]. Moreover, MPc
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A. Nas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 55–62 Table 2 Photophysical and photochemical parameters of unsubstituted and tetra-substituted H2Pc (1), PbPc (2) and ZnPc (3) compounds in THF.
a b c
Compound
Solvent
UF
sF (ns)
s0 (ns)
kFa (s
H2Pc (1) PbPc (2) ZnPc (3) Std-ZnPc
THF THF THF THF
0.297 0.138 0.320 0.25b
2.082 1.119 2.436 2.72b
7.012 8.110 7.612 10.90b
14.26 12.33 13.14 9.17b
1
) (107)
Ud (10 4)
UD
8.90 34.30 0.92 20.00b
0.27 0.51 0.67 0.53c
The rate constant for fluorescence. Values calculated using kF = UF/sF. Data from Ref. [37]. Data from Ref. [38].
are to be easily reduced. Fluorescence quenching of MPc compounds by BQ is formed via excited state electron transfer from the MPc to BQ. The energy of the lowest excited state for quinones is greater than the energy of the excited singlet state of MPc compounds [34,35]. MPc interacts with increasing concentrations of BQ and their fluorescence spectra were monitored. There were no shifts in wavelength or additional peak, suggesting that there could be any ground state interaction between Pc compounds and BQ. Fig. 3 shows the quenching of the compound 3 by BQ as an example. Stern–Volmer plots for all studied compounds exhibit linear lines, depicting diffusion-controlled quenching mechanism (Fig. 4). The KSV and biomolecular quenching constant (Kq) values of the studied phthalocyanine compounds (1–3) were given in Table 3, and these values are lower than unsubstituted ZnPc in THF. The substitution of the Pc ring seemed to decrease KSV value of these compounds. As seen from Table 3, the compound 2 has the highest KSV value; while the compound 1 has the lowest KSV value in THF.
2.4 2.2
1 2
2
3
Io/I
1.8 1.6 1.4 1.2 1 0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
[BQ] Fig. 4. Stern–Volmer plots for BQ quenching of tetra-substituted Pcs (1–3) (C = 1.00 10 5 M) in THF. [BQ] = 0, 0.008, 0.016, 0.024, 0.032 and 0.040 M.
Photochemical studies Table 3 Fluorescence quenching data of tetra-substituted H2Pc (1), PbPc (2) and ZnPc (3) compounds in THF.
Singlet oxygen quantum yields Singlet oxygen plays a significant role for the using of the phthalocyanine compounds in PDT application as photosensitizers. It is formed via energy transfer between the triplet state of a phthalocyanine compound and ground state molecular oxygen, and this energy transfer must be highly efficient to generate large amounts of singlet oxygen. Singlet oxygen quantum yields (UD) were determined by chemical method using DPBF (1,3-diphenylisobenzofuran) as a singlet
a
Compound
Solvent
KSV (M
H2Pc (1) PbPc (2) ZnPc (3) Std-ZnPca
THF THF THF THF
22.93 32.13 28.14 48.48
1
)
Kq (dm3 mol
1
s
1
) (1010)
1.10 2.87 1.15 1.78
Data from Ref. [37].
500 450
[BQ]=0 400
Intensity (a.u.)
350 300 250 200
[BQ]=saturated
150 100 50 0
654
674
694
714
734
754
774
794
Wavelength (nm) Fig. 3. Fluorescence emission spectral changes of 3 (1 10 5 M) during fluorescence quenching studies in THF. [BQ] = 0, 0.008, 0.016, 0.025, 0.032, 0.040 M and saturated with BQ (inset: Stern–Volmer plots for BQ quenching of 3 in THF).
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(a) 1,2
Absorbance
1
0,8
0,6
0,4
0,2
0 300
350
400
450
500
550
600
650
700
750
800
750
800
Wavelength (nm)
(b)
1,6
(c) 1,4
Absorbance
1,2
1
0,8
0,6
0,4
0,2
0 300
350
400
450
500
550
600
650
700
Wavelength (nm) Fig. 5. Absorption changes during the determination of singlet oxygen quantum yield (a) for compound 1, (b) for compound 2 and (c) for compound 3 in THF using DPBF as a singlet oxygen quencher.
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2 1,8 1,6
Absorbance
1,4 1,2 1 0,8 0,6 0,4 0,2 0 300
350
400
450
500
550
600
650
700
750
800
Wavelength (nm) Fig. 6. Absorption spectral changes during the investigation of the photodegradation quantum yield of compound 3 in THF (inset: plot of absorbance versus time).
oxygen scavenger in THF. The obtained results were given in Table 2. A time-dependent decrease in the DPBF concentration was observed at 417 nm by following a decrease in its absorbance (Fig. 5, compounds 1, 2 and 3 were given as examples). The absence of any significant changes in the Q band intensities of the studied Pc compounds supports that there was no defect on its structure during singlet oxygen measurement. It is clear from Table 2 that ZnPc derivative (compound 3) show higher UD value than their H2Pc (1) and PbPc (2) counterparts and also standard ZnPc in THF. Generally, zinc(II) phthalocyanines generate highly singlet oxygen since the d10 configuration of the central Zn2+ ion, which make them appropriate photosensitizers for PDT application. Considering data obtained in THF, the lowest UD value observed for H2Pc (1), suggesting inefficient energy transfer from the triplet state to molecular oxygen. Photodegradation quantum yields Photodegradation process can be used to determinate stability of Pcs which include decreasing of the Pc’s Q band intensity under the light irradiation. The stability of these molecules is particularly important for using as photocatalysts. Photodegradation is characterized by decreasing of absorption spectra of Pcs without accompanying shift in position or appearing of new peaks in the visible region. The stabilities of all studied compounds against to light irradiation were determined in THF by monitoring the absorption spectra of these compounds with the increasing irradiation time (Fig. 6, compound 3 was given as an example). The observed spectral changes confirm that photodegradation is not associated with photo transformation into different forms of phthalocyanine absorbing in the visible region. While typical values for stable phthalocyanines are of the order of 10 6, these values for unstable phthalocyanines are of the order of 10 3 [36]. The Ud values were listed in Table 2 for all compounds. According to the data obtained from the table, the ZnPc (3) is more stable than the other Pc compounds (1 and 2) because of having lower Ud value. Conclusions In this paper, the photophysical and photochemical properties of tetra-benzothiazole substituted metal-free (H2Pc, 1), lead (II) (PbPc, 2) and zinc(II) (ZnPc, 3) phthalocyanine compounds were
examined. These properties of studied phthalocyanine compounds (1–3) were calculated and compared with each other. The UF of 3 (UF = 0.320) was relatively higher compared to the corresponding H2Pc (1) (UF = 0.297) and the standard ZnPc (UF = 0.25) in THF. In similar, when the fluorescence quantum lifetimes (sF) of related phthalocyanine compounds were compared, the sF of 3 is the higher than the others. However, sF of standard ZnPc is higher than studied phthalocyanines because of the metal ion effect in the Pc ring. The fluorescence quenching of the 1–3 by BQ in different concentration obeys Stern–Volmer kinetics. For all studied compounds, Stern–Volmer plots exhibit linear lines, depicting diffusion-controlled quenching mechanism. The studied compounds (1–3) produced singlet oxygen in THF and these compounds, especially compound 3 which has the highest singlet oxygen quantum yield (UD = 0.67 for compound 3), are good candidates for photodynamic therapy. According to the photodegradation quantum yield (Ud) values of the studied phthalocyanines, the Ud of compound 3 is more stable than the other compounds because of lower Ud value. One can be derived from conducted study; the clear inference is that benzothiazole-substituted ZnPc (3) would be used as a potential photosensitizer agent due to its highest UD values in photodynamic therapy which is a fairly spectacular alternative treatment method of cancer diseases today. Acknowledgement This study was supported by the Research Fund of Karadeniz Technical University, Project No.: 2010.111.002.5 (TrabzonTurkey). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.06.135. References [1] A.L. Thomas, Phthalocyanine Research and Applications, CRC Press, Boca Raton, Florida, 1990. [2] J.Y. Liu, P.C. Lo, D.K.P. Ng, Struct. Bond. 135 (2010) 169–210. [3] P. Erk, H. Hengelsberg, in: K.M. Kadish, K.M. Smith, R. Guilard (Eds.), The Porphyrin Handbook, vol. 19: Applications of Phthalocyanines, Academic Press, San Diego, California, 2003.
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