Synthesis, photophysical and photochemical studies on long chain zinc phthalocyanine derivatives

Synthetic Metals 158 (2008) 839–847

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Synthesis, photophysical and photochemical studies on long chain zinc phthalocyanine derivatives Abimbola Ogunsipe a,b , Mahmut Durmus¸ a,c , Devrim Atilla c , Ays¸e Gül Gürek c , Vefa Ahsen c,d , Tebello Nyokong a,∗ a

Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa Department of Chemistry, University of Lagos, Lagos, Nigeria Gebze Institute of Technology, Department of Chemistry, P.O. Box 141, 41400, Gebze, Kocaeli, Turkey d TUBITAK-Marmara Research Center, Materials Institute, P.O. Box 21, 41470, Gebze, Kocaeli, Turkey b c

a r t i c l e

i n f o

Article history: Received 26 September 2007 Received in revised form 12 April 2008 Accepted 5 June 2008 Available online 31 July 2008 Keywords: Phthalocyanine Quantum yields Singlet oxygen Photodynamic therapy Benzoquinone Quenching

a b s t r a c t The synthesis and characterization of 2,9,16,23-chloro-3,10,17,24-triethyleneoxysulphanylphthalocyaninato zinc(II) (CTESZnPc) is described. The photophysics and photochemistry of CTESZnPc and those of tetrakis(triethyleneoxysulphanyl)zinc phthalocyanine (TTESZnPc) and octakis(triethyleneoxysulphanyl)zinc phthalocyanine (OTESZnPc), are presented and compared with those of unsubstituted zinc phthalocyanine (ZnPc). The presence of triethyleneoxysulphanyl substituents on the ZnPc ring gave rise to higher values of singlet oxygen (˚ ) and photodegradation (˚Pd ) quantum yields in DMF. However, TTESZnPc, OTESZnPc and CTESZnPc are less fluorescent than ZnPc, judging from their fluorescence quantum yield (˚F ) values. Fluorescence was lower in toluene than in DMF due to aggregation in the former solvent. Triplet quantum yield (˚T ) values were found to increase with the presence of substituents on the ZnPc ring, while triplet lifetimes ( T ) were found to vary linearly with the logarithms of solvent viscosities. The fluorescences of the substituted ZnPc complexes were effectively quenched by benzoquinone (BQ), and the quenching data analyzed by the Stern–Volmer equation. The Stern–Volmer constants and the diffusion-controlled bimolecular rate constants were calculated. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Metallophthalocyanines (MPcs) constitute a class of 18 ␲electron aromatic macrocycles that have been a focus of attention because of their exclusive properties. MPcs exhibit two major absorption bands: the Q band in the visible (red) region and the Soret band in the ultraviolet (UV) region of the electromagnetic spectrum. The substituted Pc derivatives are functional as active components in various processes driven by visible light [1,2]. In this respect, they have found applications in photochemical and photovoltaic cells [3]. MPcs exhibit prolific photosensitizing tendencies due to their strong UV–vis light absorption. They have been used successfully in photodynamic therapy (PDT) of cancer [4–7]. In order to fully harness the inherent photosensitizing and energy-transducing potentials of MPcs, photophysicochemical studies on the complexes are essential. Thiol-derivatized MPc complexes show rich spectroscopic and photochemical properties. For example, they are known to absorb at longer wavelengths [8–13]

∗ Corresponding author. Tel.: +27 46 603 82 60; fax: +27 46 622 5109. E-mail address: [email protected] (T. Nyokong). 0379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2008.06.007

than other substituted metallophthalocyanine complexes, which is a very useful feature for application in PDT, optoelectronics and near-IR devices. MPc complexes readily aggregate in solution [14] and aggregation is not desired for PDT and many other applications. The introduction of either long chains or bulky substituents to the periphery of the macrocycle should prevent the aggregation [15]. In addition to these features, the function of the poly(oxyethylene) groups is to enhance the solubility of the phthalocyanines in water, rendering them easier to administer in medical applications. This work reports on the photophysicochemical behavior of tetrakis(triethyleneoxysulphanyl) zinc phthalocyanine (TTESZnPc, Fig. 1), octakis(triethyleneoxysulphanyl) zinc phthalocyanine (OTESZnPc, Fig. 1) and octakis-[(2,9,16,23-chloro-3,10,17,24-triethyleneoxysulphanyl)]zinc phthalocyanine (CTESZnPc, Scheme 1). The effects of substituents and solvents on the photophysical and photochemical parameters of zinc phthalocyanine (ZnPc), zinc tetra(tert-butylphenoxy)phthalocyanine[ZnPc(TBPh)4 ], zinc octa(methylphenoxy phthalocyanine [ZnPc(MPh)8 ], zinc tetranitrophthalocyanine [ZnPc(NO2 )4 ], zinc octachlorophthalocyanine (ZnPcCl8 ), zinc tetrasulfophthalocyanine [ZnPc(SO3 − )4 ], a mixture of zinc mono-, di-, tri- and tetrasulfophthalocyanine [ZnPc(SO3 − )mix ] and zinc naphthalocyanine (ZnNPc) have been

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A. Ogunsipe et al. / Synthetic Metals 158 (2008) 839–847

Scheme 1. Synthetic pathway to CTESZnPc; (i) DMSO, K2 CO3 , 50 ◦ C, 48 h. (ii) n-Hexanol, DBU, zinc(II)acetate, 18 h reflux.

phenoxy ring were found to be photochemically unstable contrary to electron-withdrawing groups. Diebold and co-workers observed a decrease in the ˚F value on going from ZnPc to ZnPcF16 followed by an increase for the ZnPc(C(CF3 )2 F)8 F8 complex [19]. The ability to accept electrons giving a dianion species is the dominant feature of quinone chemistry. When a chlorophyll-like substance (e.g., MPc) absorbs energy, a good percentage of this energy may be lost as fluorescence. If, however, this fluorescence could be prevented (quenched), then the excitation energy could be re-directed and made available for chemical use. In light of this, a mixture of MPc and quinone should be a good mimicker of the natural photosynthetic system; with the MPc being the light harvester, and the quinone, the energy transducer. An investigation into the light-harvesting and energy-transducing tendencies of some MPc-benzoquinone (MPc-BQ) systems is presented in this work. Quinones are known to quench the excited singlet states of MPc under red light excitation [20,21]. This work also explores the effects of substituents and solvent on the fluorescence properties of the MPcs and on the quenching of the MPcs by benzoquinone (BQ) using the Stern–Volmer relationship.

Fig. 1. Molecular structures of TTESZnPc, OTESZnPc and CTESZnPc.

2. Experimental 2.1. Materials and equipment

reported in the literature [16,17]. It was found that the presence of peripheral substituents on the macrocycle enhances the yield of the triplet state. Among the different substituents, the sulfonated derivative, ZnPc(SO3 − )mix , has the longest triplet lifetime ( T ) and the highest singlet oxygen quantum yield (˚ ). Furthermore, octasubstituted phthalocyaninato zinc complexes were synthesized and their photochemistry studied [18]. The substituents included cholesterol, estrone, naphthol and phenoxy groups substituted with CH3 , C(CH3 )3 , NO2 , NH2 , COH, COOH, and H. In general, complexes containing electron-donating groups attached to the

ZnPc, chlorophyll a and 1,3-diphenylisobenzofuran (DPBF) were purchased from Aldrich and used as received. 1-Mercapto4,7,10-trioxaundecane (1) [22], 4,5-dichlorophthalonitrile (2) [23], TTESZnPc and OTESZnPc, Fig. 1 [24] were synthesized and purified according to literature procedures. N,N -dimethylformamide (DMF, SAARCHEM) and dimethylsulfoxide (DMSO SAARCHEM) were freshly distilled. All reagents and solvents (of reagent-grade quality) were obtained from commercial suppliers, and were dried before use, as described by Perrin and Armarego [25].

A. Ogunsipe et al. / Synthetic Metals 158 (2008) 839–847

Elemental analyses were obtained from Thermo Finnigan Flash 1112. Infrared spectra were recorded on a Bio-Rad FTS 175C FT-IR spectrophotometer. UV–visible absorption spectra were recorded with a Shimadzu 2001 UV Pc spectrophotometer and a Varian 500 UV–vis/NIR spectrophotometer. Mass spectra were recorded on a Thermo LCQ DECA XP-Max spectrometer. 1 H and 13 C NMR spectra were recorded in CDCl3 solutions on a Varian 500 MHz spectrometer. Fluorescence excitation and emission spectra were recorded on a Varian Eclipse spectrofluorometer. Triplet absorption and decay kinetics were recorded on a laser flash photolysis system, the excitation pulses were produced by an Nd:YAG laser (Quanta-Ray, 1.5 J/90 ns) pumping a dye laser (Lambda Physic FL 3002, Pyridin 1 in methanol). The analyzing beam source was from a Thermo Oriel xenon arc lamp, and a photomultiplier tube was used as detector. Signals were recorded with a two-channel digital real-time oscilloscope (Tektronix TDS 360); the kinetic curves were averaged over 256 laser pulses. Photo-irradiations were done using a General Electric Quartz line lamp (300 W). A 600 nm glass cut off filter (Schott) and a water filter were used to filter off ultraviolet and infrared radiations, respectively. An interference filter (Intor, 670 nm with a band width of 40 nm) was additionally placed in the light path before the sample. Light intensities were measured with a POWER MAX5100 (Molelectron detector incorporated) power meter. 2.2. Photophysical parameters 2.2.1. Fluorescence quantum yields Fluorescence quantum yields (˚F ) were determined by the comparative method [26–28] (Eq. (1)), using chlorophyll a in ether (˚F = 0.32) [29], as the reference. ˚F = ˚Std F

AreaF AbsStd n2 AreaStd Absn2Std F

(1)

where AreaF and AreaStd are the areas under the fluorescence emisF sion curves of the sample and standard, respectively. Abs and AbsStd , the absorbances of the sample and standard, respectively; and n and nStd , the refractive indices of the solvents used for sample and standard, respectively. Both the sample and reference were excited at the same wavelength. 2.2.2. Triplet quantum yields (˚T ) and lifetimes ( T ) The solutions for triplet state quantum yields and lifetimes were introduced into 1 cm path length spectrophotometric cell, deaerated using nitrogen and irradiated at the Q band maxima. Triplet state quantum yields (˚T ) were determined using a comparative method [30] using ZnPc (in different solvents) as a standard, Eq. (2). ˚T = ˚Std T

AT εStd T

(2)

AStd εT T AStd T

are the changes in the triplet state where AT and absorbances of the ZnPc derivatives and the standard, respectively; , the triplet state molar extinction coefficients for the εT and εStd T ZnPc derivative and the standard, respectively; ˚Std , the triplet T quantum yield for the ZnPc standard (˚Std = 0.65 in DMSO [31], T 0.58 in DMF [32], 0.65 in toluene [33] and 0.65 in pyridine [17,34]). Triplet lifetimes were determined by exponential fitting of the decay kinetics using OriginPro 7.5 software. Quantum yields of internal conversion (˚IC ) were obtained from Eq. (3), which assumes that only the three intrinsic processes (fluorescence, intersystem crossing and internal conversion), jointly

841

deactivate the excited singlet state of an MPc molecule. ˚IC = 1 − (˚F + ˚T )

(3)

2.3. Photochemical parameters 2.3.1. Singlet oxygen and photodegradation quantum yields Singlet oxygen (˚ ) and photodegradation (˚Pd ) quantum yield determinations were carried out using the experimental set-up described above [17,18,35]. Typically, a 2 ml portion of the respective MPc solution (absorbance ∼ 0.2 at the irradiation wavelength) containing the singlet oxygen quencher was irradiated in the Q band region with the photo-irradiation set-up described above. ˚ Values were determined in air using the relative method with 1,3dipenylisobenzofuran (DPBF) as singlet oxygen chemical quencher in the various solvents, using Eq. (4): ˚ = ˚Std 

Std RIabs Std R Iabs

(4)

where ˚Std is the singlet oxygen quantum yield for the ZnPc stan

= 0.67 in DMSO [36], 0.56 in DMF [37], 0.58 in toluene dard (˚Std 

[38], 0.61 in pyridine [38] and 0.53 in THF [39]); R and RStd are the DPBF photobleaching rates in the presence of the respective MPc Std are the rates of light absorpand standard, respectively; Iabs and Iabs tion using the MPc and standard, respectively. The light intensity used for ˚ determinations was found to be 5.35 × 1015 photons s−1 cm−2 . The error in the determination of ˚ was ∼ 10% (determined from several ˚ values). Photodegradation quantum yields (˚Pd ) were determined using Eq. (5), ˚Pd =

(C0 − Ct )V NA Iabs St

(5)

where C0 and Ct are the MPc concentrations before and after irradiation, respectively, V is the reaction volume, NA the Avogadro’s constant, S the irradiated cell area and t the irradiation time. Iabs is the overlap integral of the radiation source light intensity and the absorption of the MPc. A light intensity of 4.82 × 1016 photons s−1 cm−2 was employed for ˚Pd determinations. 2.3.2. Stern–Volmer relationship for fluorescence quenching of MPcs by benzoquinone (BQ) Fluorescence quenching experiments on the various MPc complexes were carried out by the addition of different concentrations (gradually increasing) of BQ to a fixed concentration of the complex, and the concentrations of BQ in the resulting mixtures were 0, 0.0028, 0.0056, 0.0084, 0.0112 and 0.0140 mol dm−3 . The fluorescence spectra of MPc complexes at each BQ concentration were recorded, and the changes in fluorescence intensity are related to BQ concentration by the Stern–Volmer (SV) equation (Eq. (6)) [40]: I0 = 1 + KSV [BQ] I

(6)

where I0 and I are the fluorescence intensities of fluorophore in the absence and presence of quencher, respectively, and KSV is the Stern–Volmer constant. The ratios I0 /I were calculated and plotted against [BQ] according to Eq. (6), and KSV determined from the slope. The bimolecular quenching constant (kq ) values in respective solvents were calculated using the Smoluchowski equation (Eq. (7)) [41]: kq =

4NA (Df + Dq )(rf + rq ) 1000

(7)

where NA is the Avogadro’s constant, Df and Dq , the diffusion coefficients of the fluorophore and quencher, respectively; and rf and rq , the molecular radii of the fluorophore and quencher, respectively.

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The diffusion coefficient D (of the fluorophore or quencher) is given by the Stokes–Einstein equation (Eq. (8)) [41]:

substituted groups afford the isomer mixture of phthalocyanine derivatives [43]. In all cases, a mixture of four possible structural isomers is obtained. The four probable isomers can be designed kT D= (8) by their molecular symmetry as C4h , C2v , Cs and D2h . The 2(3)6r substituted compounds always occur in the expected statistical where k is the Boltzman constant, T, the absolute temperature, , mixture of 12.5% C4h -, 25% C2v -, 50% Cs - and 12.5% D2h -isomer. the solvent’s viscosity and r, the molecule’s radius. r Values were But for the 1(4)-substituted ones the composition depends on the determined using ACD/ChemSketch software. central metal ion and the structure of the peripheral substituent [44]. We expect that phthalocyanine compound (CTESZnPc) was 2.4. Synthesis prepared as a statistical mixture of four regioisomers due to the various possible positions of triethyleneoxy and chloro side chains 4-chloro-5-(triethyleneoxysulphanyl) phthalonitrile (3): 4,5relative to one another. No attempt was made to separate the Dichlorophthalonitrile (2) (1.97 g, 10 mmol) was dissolved in isomers of the complex (CTESZnPc). anhydrous dimethylsulfoxide (15 ml) under argon and 1-mercaptoThe same route as reported in the literature [45] was applied 4,7,10-trioxaundecane (1) (1.62 g, 12 mmol) was added. After to prepare disubstituted dinitrile derivative (3) from 1-mercaptostirring for 15 min at room temperature, dry and finely powdered 4,7,10-trioxaundecane (1) and 4,5-dichloro phthalonitrile (2). The potassium carbonate (2.50 g, 18 mmol) was added portion-wise phthalocyanine derivative (CTESZnPc) was obtained from the reacover 0.5 h with efficient stirring. The reaction mixture was stirred tion of the dicyano compound 3 in the presence of corresponding under argon at 50 ◦ C for 48 h. After cooling, the brown mixture was metal salt and DBU in 1-hexanol at reflux temperature. The synfiltered off and the solvent was evaporated under reduced pressure. thetic pathway is shown in Scheme 1. The oily product was purified by column chromatography on silica Elemental analysis results and the spectral investigations (1 H gel using dichloromethane:MeOH (100:1) as the eluent. Yield: NMR, 13 C NMR, FT-IR, UV–vis and MS) on the newly synthesized 1.02 g (40%). IR max (cm−1 ) (KBr pellet): 3040 (CHar ), 2925–2820, intermediate (3) and phthalocyanine derivative (CTESZnPc) are in 2241(C N), 1580, 1540, 1480, 1420, 1390, 1300, 1200, 1150–1060. accordance with the proposed structures. MS (ES) m/z (%): 341 (80) [M + 1]+ . 1 H NMR (CDCl3 ): ı 3.26 (t, 2H, Spectral comparison of the IR spectra of each step gave some SCH2 ), 3.37 (s, 3H, OCH3 ), 3.56 (t, 2H, CH2 ), 3.64 (t, 2H, CH2 ), 3.69 hints as to the nature of the product. The IR spectrum of 3 was easily (t, 4H, CH2 ), 3.84 (t, 2H, CH2 ), 7.70 (s, 1H, CHar ), 7.74 (s, 1H, CHar ). verified with the sharp C N vibrations at 2241 cm−1 which disap13 C NMR (CDCl , Decoupled): ı 32.23 (SCH ), 59.30 (OCH ), 69.60 peared after conversion into zinc phthalocyanine. The IR spectrum 3 2 3 (CH2 ), 70.87–71.10 (CH2 ), 72.13 (CH2 ), 111.49 (C N), 114.50 (Car ), of CTESZnPc showed CH2 and CH3 groups at 2926 and 2854 cm−1 , 114.79 (Car ), 115.09 (C N), 129.84 (Car H), 133.32 (ClCar ), 136.36 and the C = N group stretch around 1600 cm−1 . (Car H), 147.04 (SCar ). Calc. for C15 H17 N2 O3 SCl: C 52.86%, H 5.03%, N The 1 H and 13 C NMR spectra of compounds 3 and CTESZnPc 8.22%; Found C 52.90%, H 5.10%, N 8.16%. in CDCl3 gave the characteristic chemical shifts for the structures 2,9,16,23-chloro-3,10,17,24-triethyleneoxysulphanylphthalocyaninato as expected. The 1 H and 13 C NMR spectra (Figs. S1 to S6) and the zinc(II) (CTESZnPc): A mixture of 3 (0.554 g, 1.63 mmol), anhyTables showing NMR spectral data (Table S1 and S2) are provided drous Zn(AcO)2 (30.00 mg, 0.50 mmol), 0.07 ml (0.45 mmol) as supplementary data for both complexes. 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU) and dried n-hexanol A close investigation of the mass spectra of the dinitrile (3) (5 ml) were heated to reflux for 18 h under argon in a roundderivative and phthalocyanine derivative (CTESZnPc) confirmed the bottomed flask. The resulting green suspension was cooled and proposed structures. The mass spectra of 3 and CTESZnPc were the crude product was precipitated by addition of hexane. The obtained by Electron Spray (ES) and the molecular ion peaks at m/z: crude green product was purified by column chromatography (Silica gel, CH2 Cl2 :MeOH 15:1). Yield: 210 mg (36%). IR max (cm−1 ) KBr pellet: 3055 (CHar ), 2926–2854 (CH), 1600 (C N), 1350 (C–N), 1281 (C–O–C), 1200, 1160–1090. MS (ES), m/z (%): 1428 (100) [M + 1]+ , 1282 (55) [M-{(CH2 CH2 O)3 CH3 }]+ , 1250 (50) [M-{S(CH2 CH2 O)3 CH3 }]+ , 1135 (45) [M-2{(CH2 CH2 O)3 CH3 }]+ , 989 (10) [M-3{(CH2 CH2 O)3 CH3 }]+ , 843 (5) [M-4{(CH2 CH2 O)3 CH3 }]+ . 1 H NMR (DMSO-d ): ı 3.23 (s, 12H, OCH ), 3.44 (t, 8H, SCH ), 3.61 6 3 2 (t, 8H, CH2 ), 3.73 (t, 8H, CH2 ), 3.76–3.87 (m, 16H, CH2 ), 4.17 (t, 8H, CH2 ), 8.40–8.88 (m, 8H, CHar ). 13 C NMR (DMSO-d6 , Decoupled): ı 33.10 (S–CH2 ), 58.77 (CH3 ), 69.60 (CH2 ), 70.10–70.50 (CH2 ), 72.00 (CH2 ), 119.5 (Car ), 120.55 (CHar ), 122.94 (CHar ), 133.05 (Car ), 136.75 (Car ), 139.11(Car ), 151.03 (C N). Calc. for C60 H68 Cl4 N8 O12 S4 Zn: C, 50.44%; H, 4.80%; N, 7.84%; Found C, 50.52%; H, 4.85%; N, 7.60%. 3. Results and discussion 3.1. Synthesis and characterization Generally, substituted phthalocyanines are prepared by cyclotetramerization of substituted phthalonitriles or 1,3-diimino1H-isoindoles [42]. 2(3),9(10),16(17),23(24)-Tetra-substituted phthalocyanines can be synthesized from 4-substituted phthalonitriles while 1(4),8(11),15(18),22(25)-tetra-substituted phthalocyanines are obtained from 3-substituted analogues [42]. Also, phthalonitrile derivatives containing two different

Fig. 2. Mass spectrum (ES–MS) of CTESZnPc.

A. Ogunsipe et al. / Synthetic Metals 158 (2008) 839–847

Fig. 3. Ground state electronic absorption spectra of ZnPc (a), TTESZnPc (b) and OTESZnPc (c) in DMF. Concentration ∼ 5 × 10−6 mol dm−3 .

340 for 3 and m/z: 1428 for CTESZnPc was observed. The regions of the molecular ion and of the other bigger fragment ions are shown in Fig. 2, together with corresponding leaving groups for compound CTESZnPc. In addition to the [M + 1]+ peak at 1428, fragment ions corresponding to the loss of [(CH2 CH2 O)3 CH3 ] ([M-146]+ ), [S(CH2 CH2 O)3 CH3 ] ([M-178]+ ), 2x (CH2 CH2 O)3 CH3 ] ([M-293]+ ), 3x [(CH2 CH2 O)3 CH3 ] ([M-439]+ ) and 4x [(CH2 CH2 O)3 CH3 ] ([M-585]+ ) were easily identified. 3.2. Ground state electronic absorption and fluorescence spectra The ground state electronic absorption spectra of ZnPc, TTESZnPc and OTESZnPc (Fig. 3) and CTESZnPc (not shown) show that these species are monomeric in DMF (up to ∼2 × 10−5 M). The Q band positions of TTESZnPc, OTESZnPc and CTESZnPc are red-shifted relative to that of ZnPc, Table 1, implying that the HOMO-LUMO energy gap of the Pc ring is reduced on introducing the triethyleneoxysulphanyl (and/or chloro) substituents. In toluene, however, the substituted ZnPc derivatives are aggregated (Figs. 4 and 5). For example, OTESZnPc has two bands that are not vibronic in origin at 676 and 708 nm (Fig. 5) and CTESZnPc at 651 and 693 nm; the higher energy bands being assigned to the aggregates. This is typical of aggregation behavior in MPc complexes [46]. Toluene is a non-coordinating solvent, so would not be axially ligated to the zinc central atom, thereby giving room for ␲–␲ stacking between adjacent rings. The lack of such aggregation in DMF suggests that DMF, a coordinating solvent, is axially ligated to the central zinc atom thus preventing columnar aggregation. Addition of Triton X-100 (a surfactant) to the toluene solutions of

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Fig. 4. Ground state electronic absorption spectra of CTESZnPc in toluene (a), acetone (b), THF (c), pyridine (d) and 1-chloronaphthalene (e). Concentration ∼ 5 × 10−6 mol dm−3 .

TTESZnPc, OTESZnPc and CTESZnPc resulted in disaggregation, as evident from the sharp increase in the intensity of the monomer peak and an attendant collapse of the aggregate peak (Fig. 5). ZnPc is monomeric in toluene. The ground state electronic absorption spectra of CTESZnPc in different solvents are shown in Fig. 4. CTESZnPc is monomeric in coordinating solvents like DMSO, DMF and pyridine, but aggregated in toluene, benzene and chloroform, which are non-coordinating. It is obvious that the Q band position varies as the solvent is changed. It has been well established that spectra of organic compounds in solution are subject to a general polarization red shift when compared to the gas phase spectrum [47]; and differential red shifting in solution has been explained in terms of the solvents’ refractive indices [38]. Fig. 6 shows the plot of Q absorption band positions of CTESZnPc versus solvents’ refractive indices, where it could be inferred that the Q band wavelength varies directly as the refractive index of the solvent. Fluorescence emission spectra of the MPcs in DMF (with 630 nm excitation) are mirror images of the absorption spectra (Fig. 7) as expected, with Stokes’ shifts typical of MPc complexes in solution. The emission wavelengths are listed in Table 1 in DMF and toluene. The fluorescence excitation spectra are similar to the absorption spectra, showing that the absorbing species (monomeric) are also the fluorescing species in DMF. In toluene, however, only ZnPc displayed regular fluorescence behavior, i.e., fluorescence excitation spectrum being the same as absorption spectrum and a mirror image of the emission spectrum. The fluorescence excitation spectra of TTESZnPc, OTESZnPc and CTESZnPc in toluene do not coincide with their absorption spectra, due to aggregation. The emission spectra were the mirror image of the absorption spectra of the com-

Table 1 Spectral, photopysical and photochemical parameters for ZnPc, TTESZnPc, OTESZnPc, CTESZnPc in DMF and toluene Solvent

Compound

Q Abs max (nm) (Log ε)

Q Ems max (nm)

˚F

˚T

˚IC

 T (␮s)

˚

˚Pd (×10−5 )

DMF

ZnPc TTESZnPc OTESZnPc CTESZnPc

670 (5.37) 689 (5.30) 705 (5.28) 693 (5.11)

676 699 714 710

0.17 0.15 0.13 0.09

0.58 0.64 0.73 0.72

0.25 0.21 0.14 0.19

330 210 240 230

0.56 0.58 0.66 0.67

2.35 3.28 4.56 4.01

Toluene

ZnPc TTESZnPc OTESZnPc CTESZnPc

672 (5.14) 660 (4.71), 691 (4.98, 5.25a ) 676 (4.07), 708 (4.11, 4.66a ) 650 (4.40), 693 (4.32, 5.18a )

677 701 716 701

0.07 0.04 0.03 0.03

0.65 0.67 0.70 0.71

0.28 0.29 0.27 0.26

340 180 210 190

0.58 0.52 0.41 0.44

0.93 –b –b –b

Q Abs: Q band absorption maxima of phthalocyanine complexes. Q Ems: Q band emission maxima of phthalocyanine complexes. : Wavelength; ε: extinction coefficient; ˚F : fluorescence quantum yield; ˚T : triplet quantum yield; ˚IC : internal conversion quantum yield;  T : triplet lifetime; ˚ : singlet oxygen quantum yield; ˚Pd : photodegradation quantum yield. a Values in the presence of Triton X-100. b Not determined since photodegradation was accompanied by phototransformation.

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Fig. 5. UV–vis absorption spectra of OTESZnPc in toluene (in the presence and absence of Triton X-100). (Concentration of OTESZnPc = 3.40 × 10−5 mol dm−3 .)

Fig. 6. Q band position of CTESZnPc as a function of solvent’s refractive index.

plexes only after addition of Triton X-100. In the absence of Triton X-100, the observation of only one peak in the fluorescence emission spectra of TTESZnPc, OTESZnPc and CTESZnPc in toluene shows that only the monomeric species fluoresce in these complexes. 3.3. Photophysical properties 3.3.1. Fluorescence quantum yields Fluorescence quantum yield (˚F ) depends on many factors which include molecular structure [48,49], solvent parameters

Fig. 7. Normalized fluorescence excitation and emission spectra of TTESZnPc in DMF.

[17,38] and temperature, to mention a few. Here, we consider the effects of molecular structure and solvent on ˚F . The presence of triethyleneoxysulphanyl (and/or chloro) substituents on ZnPc tends to make the ring less fluorescent, judging from the lower ˚F values for the substituted derivatives compared to ZnPc (Table 1). It appears that the substituents create an path through which excitation energy can ‘leak out’ (via ISC or IC); and logically the effect is slightly less pronounced in TTESZnPc, which has four substituent units, as opposed to OTESZnPc and CTESZnPc, which have eight substituent units. Comparison of the ˚F values for ZnPc (which is unaggregated in toluene unlike the rest of the complexes) in DMF and toluene suggests that the MPcs are less fluorescent in toluene than in DMF. ˚F values for CTESZnPc in different solvents are shown in Table 2; the highest value is obtained in 1-chloronaphthalene (0.18), while the lowest, in toluene (0.03). The low value in the latter solvent is due to its aggregated nature. In general the values are in the range reported for substituted MPc complexes containing oxo bridges [16] or thio bridged OTiPc complexes containing polyoxyethylene substituents [50]. 3.3.2. Triplet quantum yields and lifetimes Triplet quantum yields (˚T ) and lifetimes ( T ) of all four complexes are listed in Table 1. ˚T values of substituted ZnPc derivatives (TTESZnPc, OTESZnPc and CTESZnPc) in a particular solvent (DMF or toluene) are larger than that of unsubstituted ZnPc, which implies that the presence of triethyleneoxysulphanyl (and/or chloro) substituents probably bring about stronger spin-orbit coupling, thereby enhancing intersystem crossing in these complexes. It can then be said that ISC predominates over IC in deactivating

Table 2 Spectral, photophysical and photochemical parameters for CTESZnPc in different solvents Solvent

n

 (cP)

Q Abs max (nm)

Q Ems max (nm)

˚F

˚T

˚IC

 T (␮s)

˚

˚Pd (×10−5 )

THF Toluene DMF Pyridine DMSO CLNc

1.406 1.497 1.430 1.509 1.479 1.633

0.48 0.59 0.78 0.94 1.99 3.02

691 650, 693 693 698 696 702

700 701 710 720 708 718

0.16 0.03 0.09 0.12 0.13 0.18

–a 0.71 0.72 0.68 0.77 –a

–a 0.26 0.19 0.20 0.10 –a

170 190 230 250 310 340

0.52 0.44 0.67 0.63 0.71 –a

0.87 –b 4.01 3.88 3.21 1.62

n: Solvent’s refractive index; : solvent viscosity; Q Abs: Q band absorption maxima of phthalocyanine complexes. Q Ems: Q band emission maxima of phthalocyanine complexes. : wavelength; ˚F : fluorescence quantum yield; ˚T : triplet quantum yield; ˚IC : internal conversion quantum yield;  T : triplet lifetime; ˚ : singlet oxygen quantum yield; ˚Pd : photodegradation quantum yield. a Not determined due to lack of standards. b Not determined since photodegradation was accompanied by phototransformation. c CLN = chloronaphthalene.

A. Ogunsipe et al. / Synthetic Metals 158 (2008) 839–847

Fig. 8. Dependence of CTESZnPc’s triplet lifetime on logarithm of solvent viscosities.

the singlet excited states of the substituted ZnPc derivatives. This is manifested in the lower values of ˚IC for CTESZnPc, TTESZnPc and OTESZnPc, compared with ZnPc. The increase in ˚ISC induced by Cl substituents in CTESZnPc is not unexpected: chlorine, being a heavy atom provokes spin-orbit coupling, resulting in populating the triplet excited state. However, the enhancement effect of triethyleneoxysulphanyl substituents on ˚T has not been reported in the literature. In toluene the observation of lower values of ˚T for ZnPc compared to the substituted derivatives is surprising due to the aggregation of the latter complexes in this solvent. However, the values are marginally lower in TTESZnPc (tetra-substituted) compared to its octasubstituted counterparts. We believe that the extra four substituent units (triethyleneoxysulphanyl and chloro, respectively) in OTESZnPc and CTESZnPc would further enhance intersystem crossing in these complexes. In Table 2, solvent effects on ˚T values of CTESZnPc are small. The ˚T values in DMSO Table 2 are similar to those reported thio bridged OTiPc complexes containing polyoxyethylene substituents [50]. Triplet lifetimes ( T ) ranged from 170 to 340 ␮s in the various solvents in Tables 1 and 2. The  T values were lower for the substituted derivatives in DMF compared to unsubstituted ZnPc, corresponding to increase in ˚T on ring substitution. In toluene the decrease in the  T values for the substituted complexes compared to ZnPc is also due to the aggregated nature of these complexes in this solvent. A relationship could be observed between  T values of CTESZnPc and solvent viscosity (Fig. 8 and Table 2). Fig. 8 shows that there exists a direct relationship between the logarithms of solvent viscosities and CTESZnPc’s triplet lifetimes. It is generally expected that polarity plays a role in photophysical properties of molecules, but this is not the case in this work. In THF and toluene (with low-viscosities), there is a greater possibility for non-radiative deactivation of the excited state through internal conversion. Aggregation in toluene will play a major role.

845

Fig. 9. Photodisaggregation and photodegradation of TTESZnPc in toluene. Concentration = 1.29 × 10−5 mol dm−3 .

uisites for appreciable production of singlet oxygen. In DMF, ZnPc has a lower ˚ value than its substituted counterparts, Table 1. This is expected, based on the fact that the former has a lower ˚T value in DMF; hence the number of diffusional encounters between their triplet states and ground state molecular oxygen is higher than that in ZnPc. In toluene (in the absence of Triton X100), however, TTESZnPc, OTESZnPc and CTESZnPc exhibit lower ˚ values than ZnPc, an observation easily relatable to the aggregation of substituted ZnPc complexes in this solvent. Aggregation has been identified to constitute a major hindrance to the effective production of singlet oxygen. Aggregates take up electronic energy and convert it to vibrational motion [51], i.e., shortens the triplet lifetimes of photosensitizers. As a result, molecular interaction with ground state molecular (triplet) oxygen is reduced resulting in ˚ values which are lower than expected. Table 2 shows that ˚ values of CTESZnPc are marginally lower in THF and toluene compared to pyridine, DMSO and DMF. The lower ˚ values in toluene could also be due to aggregation of the substituted derivatives in this solvent. This observation could easily be traced to the triplet lifetimes of the CTESZnPc in the various solvents.  T is shorter in THF (and toluene) than in the other solvents, hence there is not as much time (in THF and toluene) as in DMSO (for example) for the excited triplet CTESZnPc molecules to interact with ground state molecular oxygen. The aftermath of this is lower energy transfer (from triplet photosensitizer to

3.4. Photochemical properties Singlet oxygen is formed from a bimolecular interaction between the triplet state of a sensitizer and ground state (triplet) molecular oxygen. As a result, the extent of singlet oxygen generation should depend on, and be limited by the population of molecules in the triplet state, the lifetime and the energy of this state. High triplet quantum yield, long triplet lifetime and proximity in energy between the excited triplet state of the MPc complexes and triplet molecular oxygen are indispensable prereq-

Fig. 10. Fluorescence quenching of OTESZnPc by benzoquinone (BQ) in toluene; inset: Stern–Volmer plot for the quenching course. Concentration = 2 × 10−6 mol dm−3 .

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A. Ogunsipe et al. / Synthetic Metals 158 (2008) 839–847

Table 3 Rate constants for various excited state deactivation processes of ZnPc, TTESZnPc, OTESZnPc and CTESZnPc complexes in DMF and toluene Solvent

Compound

KSV (M−1 )

kq (×10−10 M−1 s−1 )

DMF

TTESZnPc OTESZnPc CTESZnPc

77.83 69.84 75.23

1.99 1.99 1.99

Toluene

TTESZnPc OTESZnPc CTESZnPc

83.93 77.75 80.56

2.63 2.63 2.63

KSV : Stern–Volmer constant; kq : Bimolecular quenching constant.

Table 4 Fluorescence quenching (by benzoquinone) data for CTESZnPc in different solvents Solvent

 (cP)

KSV (M−1 )

kq (×10−10 M−1 s−1 )

THF Toluene DMF Pyridine Benzonitrile DMSO Chloronaphthalene

0.48 0.59 0.78 0.94 1.16 1.99 3.02

83.02 80.56 75.23 64.79 53.45 29.10 15.09

3.24 2.63 1.99 1.65 1.34 0.78 0.51

: Solvent viscosity; KSV : Stern–Volmer constant; kq : bimolecular quenching constant.

3.5. Fluorescence quenching studies by benzoquinone [BQ] oxygen) efficiency in THF and toluene, than in DMSO, DMF and pyridine. Photodegradation quantum yield (˚Pd ) results (Table 1) show that the substituted derivatives of ZnPc are more prone to photodegradation than ZnPc itself (in DMF where aggregation did not occur). This implies that the substituents actually reduce the photostability of the ZnPc skeleton. As photodegradation is known to be a singlet oxygen-mediated process [52,53], the trend in the variation of ˚ (in DMF) is the same as that of ˚Pd , with the octasubstituted derivatives (OTESZnPc and CTESZnPc) exhibiting higher values than the tetra-substituted TTESZnPc. In toluene (where TTESZnPc, OTESZnPc and CTESZnPc are aggregated), the molecules generate less singlet oxygen, and so are expected to degrade more slowly. However, the situation is more complex; initially, a slight photodisaggregation was observed on photolysis (Fig. 9: inset), as evidenced by the increase in the Q band intensity with isosbestic points ‘a’ and ‘b’ in the spectra. Continued irradiation resulted in synchronous phototransformation and photodegradation (Fig. 9). We propose here that the involvement of aggregates in photodegradation is indirect, through dissociation into monomers. Hence the rate of photobleaching should depend on, and be restricted by the rate of disaggregation. ˚Pd values of CTESZnPc in different solvents are shown in Table 2, with the values being comparable in DMSO, DMF and pyridine. The extremely low value in THF suggests that other factors play roles in the photodegradation of CTESZnPc in this solvent. The relatively low value in 1-chloronapnthalene could be due to the higher viscosity (3.02 cP) of this solvent; which tend to attenuate incident light to a greater extent than in the other solvents. Consequently, the efficiency of a photoreaction is expected to decrease in such solvent.

Fig. 11. Dependence of TTESZnPc’s KSV with solvent viscosity.

Stern–Volmer (SV) plots (Eq. (6)), Fig. 10, for the substituted ZnPc derivatives (in DMF and toluene) were linear with R2 values very close to unity (∼0.999), on excitation of MPc at 630 nm, with KSV values ranging between 70 and 84 M−1 , Table 3. As indicated above, fluorescence occurs from the monomer in toluene. The linearity of these plots indicates that fluorescence quenching is reasonably described by a collisional quenching mechanism. The magnitudes of KSV values (Table 3) for the complexes reflect their degrees of interaction with BQ. KSV is higher for TTESZnPc than for its octasubstituted counterparts, with OTESZnPc showing the least value. In different solvents, the KSV values for BQ quenching of CTESZnPc varies directly with the solvents’ viscosities, Fig. 11. It is reasonable to imagine that the frequency of collisions between a fluorophore and quencher would be higher in low-viscosity solvents, Fig. 11. A viscous solvent would retard molecular movements, thereby lowering rate of collision. This is attested to by the relative values of the diffusion-controlled bimolecular rate constant (kq ) in the various solvents (Eq. (7), Table 4). Likewise in Table 3, it could be noted that KSV values of the complexes are larger in toluene than in DMF. In calculating kq values (Eq. (7)), a basic assumption was made, that the molecular radii of the complexes are roughly the same (21.69 Å), as determined using ChemSketch software. 4. Conclusion This work has demonstrated the effects of substituents and solvents on the absorption and fluorescence properties of zinc phthalocyanine and its triethyleneoxysulphanyl-substituted derivatives. All zinc phthalocyanine complexes are monomeric in DMF while substituted complexes are aggregated in toluene. Addition of Triton X-100, to the toluene solutions of substituted complexes resulted in disaggregation, as evident from the sharp increase in the intensity of the monomer peak and an attendant collapse of the aggregate peak. We have shown that the presence of substituents on an MPc ring could create paths for non-radiative deactivation of the excited state and that the solvent effects on fluorescence cannot be ignored. The presence of triethyleneoxysulphanyl (and/or chloro) substituents on ZnPc tends to make the ring less fluorescent, judging from the lower ˚F values for the substituted derivatives compared to ZnPc. Comparison of the ˚F values in DMF and toluene suggests that the MPcs are less fluorescent in toluene than in DMF due to the aggregation in toluene. Triplet quantum yields (˚T ) values of substituted ZnPc derivatives are larger than that of unsubstituted ZnPc, which implies that the presence of triethyleneoxysulphanyl (and/or chloro) substituents probably bring about stronger spin-orbit coupling, thereby enhancing intersystem crossing in these complexes. Triplet lifetimes ( T ) ranged from 170 to 340 ␮s in the various solvents. Photodegradation quantum yield (˚Pd ) results show that the substituted derivatives of

A. Ogunsipe et al. / Synthetic Metals 158 (2008) 839–847

ZnPc are more prone to photodegradation than ZnPc itself. This work has also showed the quenching of MPc fluorescence by benzoquinone. The possible use of ZnPc derivatives in PDT cannot be overemphasized; considering the fact that the effectiveness of photodynamic action is chiefly based on the generation of singlet oxygen, suffice is it to conclude that ZnPc derivatives studied in this work are potential candidates as regards consideration in PDT. Singlet oxygen quantum yields (˚ ) ranged from 0.41 to 0.71 in the various solvents especially CTESZnPc complex has largest value in DMSO. Photosens® , which has been in use for PDT has a singlet oxygen quantum yield of 0.42, which is about the least value obtained for the ZnPc derivatives studied. Acknowledgements This work was supported by the National Research Foundation of South Africa (NRF GUN # 2053657) and TUBITAK (Kariyer-104T217) project as well as Rhodes University. AO thanks ANSTI/UNESCO for Staff exchange visit fellowship. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.synthmet.2008.06.007. References [1] Y.M. Lim, S.H. Park, S.H. Song, C.J. Park, H. Ryu, J.G. Jee, H.S. Yang, Bull. Korean Chem. Soc. 20 (1999) 701–704. [2] M. Casstevens, M. Samok, J. Pfleger, P.N. Prasad, J. Chem. Phys. 92 (1996) 2019–2024. [3] D. Wöhrle, D. Meissner, Adv. Mater. 3 (1991) 129–138. [4] D. Wöhrle, J. Gitzel, G. Krawczyk, E. Tsuchida, H. Ohno, T. Nishisaka, J. Macromol. Sci. Chem.: A 25 (1988) 1227–1254. [5] N.A. Kuznetsova, N.S. Gretsova, V.M. Derkacheva, O.L. Kaliya, E.A. Luk’yanets, J. Porphyrins Phthalocyanines 7 (2003) 147–154. [6] R. Edrei, V. Gottfried, J.E. Van Lier, S. Kimel, J. Porphyrins Phthalocyanines 2 (1998) 191–199. [7] J.D. Spikes, Photochem. Photobiol. 43 (1986) 691–699. [8] D. Atilla, G. Aslibay, A.G. Gürek, H. Can, V. Ahsen, Polyhedron 26 (2006) 1061–1069. [9] A.G. Gürek, Ö. Bekaro˘glu, J. Chem. Soc., Dalton Trans. (1994) 1419–1423. [10] K. Ozoemena, T. Nyokong, J. Chem. Soc., Dalton Trans. (2002) 1806–1811. [11] I. Yılmaz, A.G. Gürek, V. Ahsen, Polyhedron 24 (2005) 791–798. [12] K. Ozoemena, T. Nyokong, Inorg. Chem. Commun. 6 (2003) 1192–1195. [13] Y. Arslano˘glu, A.M. Sevim, E. Hamuryudan, A. Gül, Dyes Pigments 68 (2006) 129–132. [14] K. Tabata, K. Fukushima, K. Oda, I.J. Okura, J. Porphyrins Phthalocyanines 4 (2000) 278–284. [15] M.T.M. Choi, P.P.S. Li, D.K.P. Ng, Tetrahedron 56 (2000) 3881–3887. [16] T. Nyokong, Coord. Chem. Rev. 251 (2007) 1707–1722. [17] A. Ogunsipe, J.Y. Chen, T. Nyokong, New J. Chem. 7 (2004) 822–827.

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