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Polyhedron 26 (2007) 3323–3335 www.elsevier.com/locate/poly Synthesis, photophysical and photochemical properties of tetraand octa-substituted gallium and indium phthalocyanines Mahmut Durmusß b a,b , Tebello Nyokong a,* a Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa Gebze Institute of Technology, Department of Chemistry, P.O. Box 141, Gebze 41400, Turkey Received 13 February 2007; accepted 6 March 2007 Available online 12 March 2007 Abstract The synthesis, photophysical and photochemical properties of the tetra- and octa-[4-(benzyloxyphenoxy)] substituted gallium(III) and indium(III) phthalocyanines are reported for the first time. The new compounds have been characterized by elemental analysis, IR, 1H NMR spectroscopy and electronic spectroscopy. General trends are described for quantum yields of photodegredation, fluorescence quantum yields and lifetimes, triplet lifetimes and triplet quantum yields as well as singlet oxygen quantum yields of these compounds in dimethylsulfoxide (DMSO). Substituted indium phthalocyanine complexes (7b–9b) showed much higher quantum yields of triplet state and shorter triplet lifetimes, compared to the substituted GaPc derivatives due to enhanced intersystem crossing (ISC) in the former. The gallium and indium phthalocyanine complexes showed phototransformation during laser irradiation due to ring reduction. The singlet oxygen quantum yields (UD), which give an indication of the potential of the complexes as photosensitizers in applications where singlet oxygen is required (Type II mechanism) ranged from 0.51 to 0.94. Thus, these complexes show potential as photodynamic therapy of cancer. 2007 Elsevier Ltd. All rights reserved. Keywords: Phthalocyanine; Photosensitizer; Gallium; Indium; Quantum yields; Singlet oxygen; Photodegredation 1. Introduction Phthalocyanines, a family of aromatic macrocycles based on an extensive delocalized 18-p electron system, are known not only as classical dyes in practical use but also as modern functional materials in scientific research [1]. There has been growing interest in the use of phthalocyanines in a variety of new high technology fields including semiconductor devices [2], Langmuir–Blodgett films [3], electrochromic display devices [4], gas sensors [5], liquid crystals [6], non-linear optics [7] and various catalytic processes [8]. The attractive characteristics of phthalocyanines in these applications arise from their great diversity, thermal and chemical stability, redox versatility and intense colour. * Corresponding author. Tel.: +27 46 6038260; fax: +27 46 6225109. E-mail address: [email protected] (T. Nyokong). 0277-5387/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2007.03.007 One of their most important applications is functioning as photosensitizers for photodynamic therapy (PDT) [9– 12]. Due to the intense absorption in the visible region, high efficiency to generate reactive oxygen species (such as singlet oxygen), and low dark toxicity, phthalocyanines have been used in this avenue for the treatment of various cancers and photoinactivation of viruses [13–15]. However, their insolubility in common organic solvents causes difficulties for many applications, rendering the syntheses of soluble derivatives an important task. Phthalocyanine derivatives of increased solubility have been obtained using substituents such as alkyl, alkoxy, alkylthio chains and bulky groups. Peripheral substitution with bulky groups or long alkyl, alkoxy or alkylthio chains leads to phthalocyanine products which are soluble in apolar solvents. Sulfo or quaternary ammonium groups enhance solubility in aqueous media over a wide pH range of aqueous solutions. The size and the nature of the substituents are not 3324 M. Durmusß, T. Nyokong / Polyhedron 26 (2007) 3323–3335 the only criteria for the solubility of the substituted phthalocyanines; the change in symmetry caused by the substituents is also important. Generally, tetra substituted phthalocyanines are more soluble than symmetrically octasubstituted ones due to the formation of four positional isomers in the case of tetra substituted analogues [16]. According to their substituent positions two types of tetrasubstituted macrocycles which show significant differences in their chemical and physical behaviour can be distinguished. Substitution at the more sterically crowded a (non-peripheral) position show reduced aggregation tendencies more than substitution at b (peripheral) position [17,18]. It has been established that non-aggregated phthalocyanines are extremely important and potentially useful for PDT applications [19]. Molecular aggregation of phthalocyanines, which is an intrinsic property of these Cl RO CN CN ROH, DMF CN K2CO3, RT CN NO2 OR 1 4 OR N N N MCl3, quinoline N 180 ˚ C, 7 hrs M N N RO N N OR M = Ga (7a) In (7b) OR Cl CN O2N CN RO ROH, DMF N RO CN K2CO3, RT CN 2 N N MCl3, quinoline N 180 ˚ C, 7 hrs M OR N N N N 5 OR M = Ga (8a) In (8b) RO OR Cl Cl CN MCl3, quinoline N RO N CN K2CO3, RT Cl N N CN RO ROH, DMF RO CN 180 ˚ C, 7 hrs 6 3 RO M OR N OR N N RO N OR M = Ga (9a) In (9b) R = O Scheme 1. Synthesis of 4-benzyloxyphenoxy tetra- and octa-substituted gallium and indium phthalocyanine complexes. M. Durmusß, T. Nyokong / Polyhedron 26 (2007) 3323–3335 large pconjugated systems, provides an efficient non-radiative energy relaxation pathway, thereby shortening the excited state lifetimes and greatly reducing the photosensitizing efficiency [20,21]. The nature of peripheral substituents influences the degree of aggregation, with bulky groups reducing this phenomenon [22]. There is a continuous effort to extend the chemistry of metallophthalocyanines. Nonetheless, gallium(III) and indium(III) phthalocyanine chemistry appears not to have been fully explored especially when compared to the development of aluminium (which is the other group IIIA metal) phthalocyanine complexes. Aluminium phthalocyanine complexes have been used as PDT agents. However there are only a few studies about the photochemical and photophysical properties of gallium and indium phthalocyanine compounds [23–25]. Closed shell, diamagnetic ions, such as Zn2+, Al3+ and Ga3+, give phthalocyanine complexes with both high triplet quantum yields and long lifetimes [26]. To our knowledge, octasubstituted gallium or indium phthalocyanines have not been reported before. This is first report about synthesis of octasubstituted gallium and indium phthalocyanines. 4-Benzyloxyphenoxy substitution on gallium and indium-based phthalocyanines have not been reported before also. Thus we report on the synthesis of 4-benzyloxyphenoxy-substituted gallium (ClGaPcs) and indium (ClInPcs) phthalocyanines, tetrasubstituted at the non-peripheral (7a,b), peripheral (8a,b) positions and peripherally octasubstituted (9a,b) with 4-benzyloxyphenoxy group (Scheme 1). The development and elucidation of photophysical and photochemical properties of new phthalocyanine complexes are of fundamental importance, hence this work presents the study of these parameters. Specific phthalocyanines can thus be tailored such that they consist of certain properties which are required for various applications since the possibility of combining an unlimited number and type of substituents with a great number of central metals is infinite. 2. Experimental 2.1. Materials Quinoline, dimethylsulfoxide (DMSO), methanol, hexane, chloroform (CHCl3), dichloromethane (DCM), tetrahydrofuran (THF), acetone, ethanol and dimethylformamide (DMF) were dried as described in Perrin and Armarego [27] before use. Gallium(III) chloride, indium(III) chloride, deuterated CDCl3, 1,3-diphenylisobenzofuran (DPBF) and 4-benzyloxyphenol were purchased from Aldrich. Column chromatography was performed on silica gel 60 (0.04– 0.063 mm) and preparative thin layer chromatography was performed on silica gel 60 P F254. 3-Nitrophthalonitrile (1) [28], 4-nitrophthalonitrile (2) [29], 4,5-dichlorophthalonitrile (3) [30], 3-(benzyloxyphenoxy)phthalonitrile (4) [31], 4-(benzyloxyphenoxy)phthalonitrile (5) [31], 4,5-bis-(benzyloxyphenoxy)-phthalonitrile (6) [31], unsubstituted gallium(III) 3325 phthalocyanine [32] and unsubstituted indium(III) phthalocyanine [32] were synthesized and purified according to literature procedures. 2.2. Equipment UV–Vis spectra were recorded on a Cary 500 UV–Vis/ NIR spectrophotometer. Fluorescence excitation and emission spectra, were recorded on a Varian Eclipse spectrofluoremeter using 1 cm pathlength cuvettes at room temperature. IR spectra (KBr pellets) were recorded on a Perkin–Elmer spectrum 2000 FTIR spectrometer. 1H NMR spectra were recorded using a Bruker EMX 400 MHz NMR spectrometer. Elemental Analyses were performed at the University of Cape Town. 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, 700 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. Triplet absorption and decay kinetics were recorded on a laser flash photolysis system, the excitation pulses were produced by a Quanta-Ray Nd: YAG laser providing 400 mJ, 90 ns pulses of laser light at 10 Hz, pumping a Lambda-Physik FL3002 dye (Pyridin 1 dye in methanol). Single pulse energy was 7 mJ. The analyzing beam source was from a Thermo Oriel xenon arc lamp, and a photomultiplier tube was used as a detector. Signals were recorded with a two-channel digital real-time oscilloscope (Tektronix TDS 360); the kinetic curves were averaged over 256 laser pulses. 2.3. Photophysical parameters 2.3.1. Fluorescence quantum yields and lifetimes Fluorescence quantum yields (UF) were determined by the comparative method (Eq. (1)) [33,34], F AStd g2 UF ¼ UF ðStdÞ ð1Þ F Std A g2Std where F and FStd are the areas under the fluorescence emission curves of the samples (7a,b, 8a,b and 9a,b) and the standard, respectively. A and AStd are the respective absorbances of the samples and standard at the excitation wavelengths, respectively. g and gStd are the refractive indexes of the solvents used for the sample and standard, respectively. Unsubstituted ZnPc (in DMSO) (UF = 0.18) [35] was employed as the standard. Both the samples and standard were excited at the same wavelength. The absorbance of the solutions at the excitation wavelength ranged between 0.04 and 0.05. Fluorescence (sF) and natural radiative (s0) lifetimes were determined using PhotochemCAD program which uses the Strickler–Berg equation [36]. 3326 M. Durmusß, T. Nyokong / Polyhedron 26 (2007) 3323–3335 2.3.2. Triplet quantum yields and lifetimes The de-aerated solutions of the respective gallium(III) and indium(III) phthalocyanine complexes were introduced into a 1 cm pathlength spectrophotometric cell and irradiated at the Q-band maxima with the laser system described above. Triplet quantum yields (UT) were determined by a comparative method using triplet decay [23], Eq. (2): USample T Sample eStd Std DAT T ¼ UT Sample DAStd T eT DASample T Q-band region using the set-up described above. DPBF degradation at 417 nm was monitored. The light intensity used for UD determinations was found to be 9.21 · 1015 photons s1 cm2. The error in the determination of UD was 10% (determined from several UD values). Photodegradation quantum yields (Ud) were determined using Eq. (5) Ud ¼ ð2Þ DAStd T and are the changes in the triplet state where absorbances of the samples (7a,b, 8a,b and 9a,b) and standard, respectively; eSample and eStd T T , the triplet state extinction coefficients for the samples (7a,b, 8a,b and 9a,b) and standard, respectively. The standard employed was ZnPc in DMSO, UStd T ¼ 0:65 [37]. Quantum yields of internal conversion (UIC) were obtained from Eq. (3), which assumes that only three processes (fluorescence, intersystem crossing and internal conversion), jointly deactivate the excited singlet state of peripherally and non-peripherally tetra- and octa-substituted gallium(III) and indium(III) phthalocyanine complexes (7a,b, 8a,b and 9a,b): ðC0 Ct Þ V N A I abs S t ð5Þ where C0 and Ct are the samples (7a,b, 8a,b and 9a,b) 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 samples (7a,b, 8a,b and 9a,b). A light intensity of 3.07 · 1016 photons s1 cm2 was employed for Ud determinations. 2.4. Synthesis ð4Þ 2.4.1. 1,(4)-Tetrakis(4benzyloxyphenoxyphthalocyaninato) gallium(III) (7a) A mixture of anhydrous gallium(III) chloride (0.40 g, 2.3 mmol), 3-(benzyloxyphenoxy)phthalonitrile (4) (1.50 g, 4.6 mmol) and quinoline (5 mL, doubly distilled over CaH2) was stirred at 180 C for 7 h under nitrogen atmosphere. After cooling, the solution was dropped into the ethanol. The green solid product was precipitated and collected by filtration and washed with ethanol. The crude product was dissolved in CH2Cl2 and then filtered. After filtering and concentrating, the dark green waxy product was purified by passing through a silica gel column, using THF as the eluting solvent. Furthermore this product was purified with preparative thin layer chromatography (silica gel) using CHCl3 solvent system. Yield: 0.42 g (26%). UV–Vis (DMSO): kmax nm (log e) 353 (4.62), 643 (4.48), 716 (5.20). IR [(KBr) mmax/cm1]: 3055 (Ar–CH), 3027 (Ar–CH), 2945–2852 (CH), 1586 (C@C), 1194 (C–O–C). 1 H NMR (CDCl3): d, ppm 8.40–8.91 (4H, m, Pc–H), 7.63–7.93 (4H, m, Pc–H), 7.31–7.57 (24H, m, Phenyl–H, Pc–H), 7.04–7.28 (12H, m, Phenyl–H), 6.81–6.98 (4H, m, Phenyl–H), 5.11 (8H, m, CH2). Anal. Calc. for C84H56ClGaN8O8: C, 71.52; H, 4.00; N, 7.94. Found: C, 71.06; H, 3.91; N, 7.81%. where UStd D is the singlet oxygen quantum yield for the standard (ZnPc in DMSO, UZnPc ¼ 0:67) [40], R and RStd are D the DPBF photobleaching rates in the presence of the respective samples (7a,b, 8a,b and 9a,b) and standard, respectively; Iabs and I Std abs are the rates of light absorption by the samples (7a,b, 8a,b and 9a,b) and standard, respectively. To avoid chain reactions induced by DPBF in the presence of singlet oxygen [41], the concentration of DPBF was lowered to 3 · 105 mol dm3. Solution of sensitizer (absorbance 1.5 at the irradiation wavelength) containing DPBF were prepared in the dark and irradiated in the 2.4.2. 1,(4)-Tetrakis(4benzyloxyphenoxyphthalocyaninato) indium(III) (7b) Synthesis and purification was as outlined for 7a except anhydrous InCl3 was employed instead of anhydrous GaCl3. The amounts of the reagents employed were: 4 (1.50 g, 4.6 mmol), indium(III) chloride (0.51 g, 2.3 mmol) and quinoline (5 ml). Yield: 0.37 g (22%). UV–Vis (DMSO): kmax nm (log e) 355 (4.58), 641 (4.44), 716 (5.18). IR [(KBr) mmax/cm1]: 3057 (Ar–CH), 3027 (Ar– CH), 2949–2856 (CH), 1582 (C@C), 1192 (C–O–C). 1H NMR (CDCl3): d, ppm 8.79–9.18 (4H, m, Pc–H), 7.86– UIC ¼ 1 ðUF þ UT Þ ð3Þ Triplet lifetimes were determined by exponential fitting of the kinetic curves using OriginPro 7.5 software. 2.3.3. Singlet oxygen and photodegradation quantum yields Singlet oxygen (UD) and photodegradation (Ud) quantum yield determinations were carried out using the experimental set-up described above [38,39]. Typically, a 2 mL portion of the respective peripheral and non-peripheral tetra- and octa-substituted gallium(III) and indium(III) phthalocyanine (7a,b, 8a,b and 9a,b) solutions containing the singlet oxygen quencher was irradiated in the Q-band region with the photo-irradiation set-up described above [38,39]. UD values were determined in air using the relative method with DPBF as singlet oxygen chemical quencher in DMSO (Eq. (4)): UD ¼ UStd D R I Std abs RStd I abs M. Durmusß, T. Nyokong / Polyhedron 26 (2007) 3323–3335 8.12 (4H, m, Pc–H), 7.31–7.58 (28H, m, Phenyl–H, Pc–H), 7.23–7.28 (4H, m, Phenyl–H), 7.10–7.18 (4H, m, Phenyl– H), 6.88–6.97 (4H, m, Phenyl–H), 5.12 (8H, m, CH2). Anal. Calc. for C84H56ClInN8O8: C, 69.31; H, 3.88; N, 7.70. Found: C, 69.19; H, 3.80; N, 7.60%. 2.4.3. 2,(3)-Tetrakis(4benzyloxyphenoxyphthalocyaninato) gallium(III) (8a) Synthesis and purification was as outlined for 7a except 5 was employed instead of 4. The amounts of the reagents employed were: 5 (1.50 g, 4.6 mmol), gallium(III) chloride (0.40 g, 2.3 mmol) and quinoline (5 ml). Yield: 0.62 g (38%). UV–Vis (DMSO): kmax nm (log e) 354 (4.82), 622 (4.55), 693 (5.24). IR [(KBr) mmax/cm1]: 3057 (Ar–CH), 3027 (Ar–CH), 2947–2854 (CH), 1617 (C@C), 1193 (C–O–C). 1H NMR (CDCl3): d, ppm 8.21–8.82 (8H, m, Pc–H), 7.61–7.77 (4H, m, Pc–H), 7.36–7.58 (28H, m, Phenyl–H), 7.16–7.23 (8H, m, Phenyl–H), 5.16 (8H, m, CH2). Anal. Calc. for C84H56ClGaN8O8: C, 71.52; H, 4.00; N, 7.94. Found: C, 71.24; H, 3.79; N, 7.96%. 2.4.4. 2,(3)-Tetrakis(4benzyloxyphenoxyphthalocyaninato) indium(III) (8b) Synthesis and purification was as outlined for 7a except 5 was employed instead of 4 and anhydrous InCl3 was employed instead of anhydrous GaCl3. The amounts of the reagents employed were: 5 (1.50 g, 4.6 mmol), indium(III) chloride (0.51 g, 2.3 mmol) and quinoline (5 ml). Yield: 0.59 g (35%). UV–Vis (DMSO): kmax nm (log e) 361 (4.78), 625 (4.46), 697 (5.16). IR [(KBr) mmax/cm1]: 3056 (Ar–CH), 3027 (Ar–CH), 2950–2857 (CH), 1617 (C@C), 1192 (C–O–C). 1H NMR (CDCl3): d, ppm 8.29– 8.91 (8H, m, Pc–H), 7.58–7.77 (4H, m, Pc–H), 7.41–7.56 (28H, m, Phenyl–H), 7.15–7.22 (8H, m, Phenyl–H), 5.16 (8H, m, CH2). Anal. Calc. for C84H56ClInN8O8: C, 69.31; H, 3.88; N, 7.70. Found: C, 69.38; H, 3.74; N, 7.49%. 2.4.5. 2,3-Octakis(4-benzyloxyphenoxyphthalocyaninato) gallium(III) (9a) Synthesis and purification was as outlined for 7a except 6 was employed instead of 4. The amounts of the reagents employed were: 6 (2.00 g, 3.8 mmol), gallium(III) chloride (0.33 g, 1.9 mmol) and quinoline (5 ml). Yield: 0.84 g (40%). UV–Vis (DMSO): kmax nm (log e) 358 (4.98), 620 (4.68), 692 (5.43). IR [(KBr) mmax/cm1]: 3058 (Ar–CH), 3027 (Ar–CH), 2949–2857 (CH), 1602 (C@C), 1190 (C– O–C). 1H NMR (CDCl3): d, ppm 8.62–9.16 (8H, m, Pc–H), 8.09–8.41 (8H, m, Phenyl–H), 7.36–7.61 (48H, m, Phenyl–H), 7.15–7.24 (16H, m, Phenyl–H), 5.12 (16H, d, CH2). Anal. Calc. for C136H96ClGaN8O16: C, 74.13; H, 4.39; N, 5.09. Found: C, 74.01; H, 4.79; N, 5.24%. 2.4.6. 2,3-Octakis(4-benzyloxyphenoxyphthalocyaninato) indium(III) (9b) Synthesis and purification was as outlined for 7a except 6 was employed instead of 4 and anhydrous InCl3 was employed instead of anhydrous GaCl3. The amounts of 3327 the reagents employed were: 6 (2.00 g, 3.8 mmol), gallium(III) chloride (0.42 g, 1.9 mmol) in quinoline (5 ml). Yield: 0.92 g (43%). UV–Vis (DMSO): kmax nm (log e) 362 (5.02), 623 (4.69), 696 (5.43). IR [(KBr) mmax/cm1]: 3058 (Ar–CH), 3027 (Ar–CH), 2951–2859 (CH), 1600 (C@C), 1187 (C–O–C). 1H NMR (CDCl3): d, ppm 8.60– 9.21 (8H, m, Pc–H), 8.18–8.59 (8H, m, Phenyl–H), 7.31– 7.57 (48H, m, Phenyl–H), 7.03–7.22 (16H, m, Phenyl–H), 5.15 (16H, ss, CH2). Anal. Calc. for C136H96ClInN8O16: C, 72.65; H, 4.30; N, 4.98. Found: C, 72.83; H, 4.76; N, 5.05%. 3. Results and discussion 3.1. Synthesis and characterization Phthalocyanines are prepared by cyclotetramerization of phthalonitriles or 1,3-diimino-1H-isoindoles. Octasubstituted phthalocyanines can be synthesized from 4,5-dichlorophthalonitriles [30]. 2(3),9(10),16(17),23(24)Tetrasubstituted phthalocyanines can be synthesized from 4-substituted phthalonitriles while 1(4),8(11),15(18),22 (25)-tetrasubstituted phthalocyanines are obtained from 3-substituted analogues [42]. In both cases a mixture of four possible structural isomers are obtained for tetrasubstituted phthalocyanines. The four probable isomers can be designed by their molecular symmetry as C4h, C2v, Cs and D2h [43]. In this study, synthesized tetrasubstituted phthalocyanine compounds are obtained as isomer mixtures as expected. No attempt was made to separate the isomers of 7a,b and 8a,b. The preparation of phthalocyanine derivatives from the aromatic nitriles occur under different reaction conditions. The syntheses of metallophthalocyanine complexes (7a,b, 8a,b and 9a,b) were achieved by treatment of phthalonitriles 4, 5 and 6 with InCl3 or GaCl3 in freshly distilled quinoline (Scheme 1). Because gallium and indium are large atoms, high energy is required to insert the metal ion into the phthalocyanine ring, thus a high-boiling solvent (such as quinoline) is used to achieve this purpose. Complexes 7a and 7b are non-peripherally (a), and 8a and 8b are peripherally (b) tetrasubstituted, whereas complexes 9a and 9b are peripherally octasubstituted. Column and preparative thin layer chromatography with silica gel was employed to obtain the pure products from the reaction mixtures. Generally, phthalocyanine complexes are insoluble in most organic solvents; however introduction of substituents on the ring increases the solubility. All complexes (7a,b, 8a,b and 9a,b) exhibited excellent solubility in organic solvents such as dichloromethane, chloroform, THF, toluene, DMSO. The new compounds were characterized by UV–Vis, IR and NMR spectroscopies and elemental analysis. The analyses are consistent with the predicted structures as shown in the experimental section. After conversion into gallium or indium phthalocyanines, the characteristic C„N stretch at 2230 cm1 of 3328 M. Durmusß, T. Nyokong / Polyhedron 26 (2007) 3323–3335 phthalonitriles 4, 5 and 6 disappeared, indicative of metallophthalocyanine formation. The complexes showed characteristic vibrations due to ether groups (C–O–C) 1187–1194 cm1, aromatic CH stretching at ca. 3058– 3027 cm1 because the compounds contain different aromatic rings and aliphatic CH stretching at ca. 2949– 2857 cm1. The 1H NMR spectra (Fig. 1) of tetrasubstituted phthalocyanine derivatives (7a,b and 8a,b) show complex patterns due to the mixed isomer character of these compounds and they have different aromatic protons. Octasubstituted derivative (9a,b) also show complex patterns due to the different aromatic protons. The complexes were found to be pure by 1H NMR with all the substituents and ring protons observed in their respective regions. The resonances belonging the phthalocyanine ring protons of non-peripherally tetrasubstituted complexes were observed between 8.40–8.91, 7.63–7.93 and 7.31–7.57 ppm (overlap with phenyl protons) for 7a; 8.79–9.18, 7.86–8.12 and 7.31–7.58 ppm (overlap with phenyl protons) for 7b (Fig. 1), integrating to a total of 12 protons expected for both non-peripheral and peripheral protons. The phenyl protons were observed between 7.31–7.57, 7.04–7.28 and 6.81–6.98 ppm integrating for 24 (overlap with 4 phthalocyanine ring protons), 12 and 4, respectively and totalling 36 protons as expected for 7a and between 7.31–7.58, 7.23–7.28, 7.10–7.18 and 6.88–6.97 ppm integrating for 28 (overlap with 4 phthalocyanine ring protons), 4, 4 and 4, respectively and totally 36 protons as expected for 7b (Fig. 1). For the peripherally tetrasubstituted complexes the resonance belonging the phthalocyanine ring protons were observed between 8.21–8.82 and 7.61–7.77 ppm for 8a and between 8.29–8.91 and 7.58–7.77 ppm for 8b integrating for 8 and 4 protons respectively giving a total of 7.0 8.00 4.07 8.0 3.1.1. Ground state electronic absorption and fluorescence spectra The ground state electronic absorption spectra showed monomeric behaviour evidenced by a single (narrow) Qband, typical of metallated phthalocyanine complexes (Fig. 2) [44]. In DMSO, the Q-bands were observed at: 716 (7a), 716 (7b), 693 (8a), 697 (8b), 692 (9a) and 696 (9b), Table 1. The Q-bands of the a-tetrasubstituted complexes (7a and 7b) are red-shifted by 23–19 nm, when compared to the corresponding b-tetrasubstituted (8a and 8b), and by 24 and 20 nm when compared with octasubstituted 4.01 4.06 4.00 9.0 28.03 4.05 ppm (t1) 12 protons as expected for both peripherally tetrasubstituted complexes. The phenyl protons were observed between 7.36–7.58 and 7.16–7.23 ppm for 8a and between 7.41–7.56 and 7.15–7.22 ppm for 8b, integrating for 28 and 8 protons respectively and giving a total of 36 protons for each complex as expected. The resonances belonging to phthalocyanine ring protons of octasubstituted complexes were observed between 8.62–9.16 ppm for 9a and 8.60– 9.21 ppm for 9b, integrating 8 protons as expected. The phenyl protons for octasubstituted complexes were observed between 8.09–8.41, 7.36–7.61, 7.15–7.24 ppm for 9a and between 8.18–8.59, 7.31–7.57, 7.03–7.22 ppm for 9b, integrating for 8, 48 and 16, respectively, giving a total of 72 protons as expected. The methyl protons were observed at ca. 5.10 ppm, integrating 8 protons for tetrasubstituted (7a,b, 8a,b) and 16 protons for octasubstituted (9a,b) complexes. The methyl protons were observed as multiplets for tetrasubstituted complexes (7a,b, 8a,b) due to the mixed isomer character of these complexes. They were observed as two singlets for octasubstituted complexes (9a,b), due to the two type of CH2 protons, with one of them near the Cl atom and the far from Cl atom. 6.0 Fig. 1. 1H NMR spectrum of complex 7b in CDCl3. 5.0 Edited by Foxit Reader Copyright(C) by Foxit Software Company,2005-2007 For Evaluation Only. M. Durmusß, T. Nyokong / Polyhedron 26 (2007) 3323–3335 a 3329 2 9a Absorbance 1.6 7a 8a 1.2 0.8 8a 9a 7a 0.4 0 300 400 500 600 700 800 Wavelength (nm) b 1.6 Absorbance 9b 1.2 0.8 8b 7b 9b 8b 0.4 0 300 7b 400 500 600 700 800 Wavelength (nm) Fig. 2. Absorption spectra of: (a) 7a, 8a and 9a; (b) 7b, 8b and 9b in DMSO. Concentration = 4 · 106 mol dm3. (9a and 9b) complexes in DMSO (Table 1). The observed red spectral shift is typical of phthalocyanines with substituents at the non-peripheral positions and has been explained [45,46] to be due to linear combinations of the atomic orbitals (LCAO) coefficients at the non-peripheral positions of the HOMO being greater than those at the peripheral positions. As a result, the HOMO level is destabilized more at the non-peripheral position than it is at the peripheral position. Essentially, the energy gap (DE) between the HOMO and LUMO becomes smaller, resulting in a bathochromic shift. The shoulder between 400 and 450 nm may be due to charge transfer from the electron-rich ring to the electron-poor metal. The B-bands are broad due to the superimposition of the B1 and B2 bands in the 330–360 nm region [45]. Aggregation is usually depicted as a coplanar association of and it is dependent on the concentration, nature of the solvent, nature of the substituents, complexed metal ions and temperature [47,48]. The aggregation behaviour of the phthalocyanine complexes (7a,b, 8a,b and 9a,b) was also investigated at different concentrations in DMSO. In DMSO, as the concentration was increased, the intensity of absorption of the Q-band also increased and there were no new bands (normally blue shifted) due to the aggregated species for all complexes (7a,b, 8a,b and 9a,b) (Fig. 3 for complex 7a). Beer–Lambert law was obeyed for all of the Table 1 Absorption, excitation and emission spectral data for unsubstituted, tetra- and octa-substituted gallium and indium phthalocyanine compounds in DMSO Compound Q-band, kmax (nm) (log e) Excitation, kEx (nm) Emission, kEm (nm) Stokes shift, DStokes (nm) ClGaPc 7a 8a 9a 680 716 693 692 5.15 5.20 5.24 5.43 680 717 696 695 691 728 704 702 11 12 11 10 ClInPc 7b 8b 9b 686 716 697 696 4.46 5.18 5.16 5.43 689 728 709 707 700 739 716 713 14 23 19 17 M. Durmusß, T. Nyokong / Polyhedron 26 (2007) 3323–3335 Absorbance 3 2 Absorbance 3330 3 2 (A) 1 0 0 0.000005 0.00001 0.000015 Concentration (F) 1 0 300 400 500 600 700 800 Wavelength (nm) Fig. 3. Absorption spectra of 7a in DMSO at different concentrations: 14 · 106 (A), 12x106 (B), 10 · 106 (C), 8 · 106 (D), 6 · 106 (E), 4 · 106 (F) mol dm3. a Absorption Excitation Intensity a.u. Emission Excitation Emission Absorption 500 550 600 650 700 750 800 Wavelength (nm) b Excitation Intensity a.u. Absorption Excitation Emission Absorption Emission 500 550 600 650 700 750 800 Wavelength (nm) Fig. 4. Absorption, fluorescence emission and excitation spectra of (a) 8a and (b) 8b in DMSO. Excitation wavelength = 660 nm for 8a and 665 nm for 8b. Edited by Foxit Reader Copyright(C) by Foxit Software Company,2005-2007 For Evaluation Only. M. Durmusß, T. Nyokong / Polyhedron 26 (2007) 3323–3335 compounds in the concentrations ranging from 1.4 · 105 to 4 · 106 mol dm3. The ClGaPc derivatives (7a, 8a, 9a) showed the same fluorescence behaviour, which was different from the ClInPc derivatives. The latter complexes (7b, 8b, 9b) also showed fluorescence behaviour which was similar to each other. Fig. 4 shows fluorescence emission, absorption and excitation spectra of complexes 8a and 8b as examples of the ClGaPc and ClInPc derivatives, respectively. Fluorescence emission peaks were observed at: 728 nm for 7a, 704 nm for 8a, 702 nm for 9a, 739 nm for 7b, 716 nm for 8b and 713 nm for 9b in DMSO (Table 1). The observed Stokes shifts are typical of MPc complexes (Table 1). The shape of the excitation spectra were similar to absorption spectra and both were mirror images of the fluorescent spectra for the ClGaPc complexes (Fig. 4a). However in terms of wavelength, the excitation spectra was slightly red-shifted when compared to the absorption spectra, suggesting that the nuclear configurations change following excitation. For the tetra- and octa-substituted indium phthalocyanine complexes 7b, 8b and 9b, the shape of excitation spectra was different from the absorption spectra in that the Q-band of the former showed splitting, Fig. 4b, unlike the narrow Q-band of the latter. This suggests that there are changes in the molecule following excitation most likely Table 2 Photophysical and photochemical parameters of unsubstituted, tetra- and octa-substituted gallium and indium phthalocyanine compounds in DMSO Compound sT (ls) UF UT UIC Ud (·105) UD SD ClGaPc 7a 8a 9a 200 280 210 200 0.30 0.15 0.23 0.13 0.69 0.77 0.75 0.74 0.01 0.08 0.02 0.13 0.93 0.27 1.92 0.50 0.41 0.69 0.62 0.51 0.59 0.98 0.82 0.69 ClInPc 7b 8b 9b 50 40 50 70 0.018 0.013 0.017 0.017 0.91 0.97 0.91 0.89 0.07 0.02 0.07 0.09 3.43 0.30 0.97 0.27 0.61 0.94 0.87 0.78 0.67 0.97 0.95 0.88 3331 due to loss of symmetry. The fluorescence spectra of complexes 7b, 8b and 9b were also broad. Thus the Q-band maxima of the excitation and absorption spectra were different (Table 1) due to the difference in the ground and excited state species. This was however not observed for the ClGaPc complexes. The difference in the behaviours of ClGaPc and ClInPc on excitation could be due to the larger indium metal being more displaced from the core of the phthalocyanine ring, and the displacement being more pronounced on excitation hence a loss of symmetry. 3.2. Photophysical properties 3.2.1. Fluorescence lifetimes and quantum yields The fluorescence quantum yields (UF) of ClGaPc complexes 7a, 8a and 9a are typical of MPc complexes, but are lower than for the unsubstituted ClGaPc (Table 2). This suggests that the substituents quench the excited singlet state. The peripherally tetrasubstituted complex 8a show marginally larger UF values, suggesting less quenching of the excited singlet state by peripheral tetrasubstitution compared to non-peripheral tetra- and octasubstitution. For the ClInPc complexes (7b, 8b and 9b), the UF values were very low due to enhancement of intersystem crossing (ISC) by the presence of a heavier indium atom in these complexes. The enhanced ISC will also result in increased triplet quantum yields and decreased triplet lifetimes as will be discussed below. Lifetimes of fluorescence (sF, Table 3) were calculated using the Strickler–Berg equation. Using this equation, a good correlation has been [34] found between experimentally and the theoretically determined lifetimes for the unaggregated molecules as is the case in this work. Thus we believe that the values obtained using this equation are a good measure of fluorescence lifetimes. The sF values were within the range reported for MPc complexes [34]. sF values were lower for substituted complexes 7a,b, 8a,b and 9a,b when compared to unsubstituted derivatives, Table 3, suggesting quenching of fluorescence by the ring substituents. For the substituted complexes, longer sF values were Table 3 Rate constants for various excited state deactivation processes of unsubstituted, tetra- and octa-substituted gallium and indium phthalocyanine compounds in DMSO Compound Fluorescence lifetime, sF (ns) Natural radiative lifetime, s0 (ns) kaF (s1) (·108) kbISC (s1) (·109) ClGaPc 7a 8a 9a 3.71 1.39 1.90 0.70 11.96 9.3 8.26 5.49 0.83 1.07 1.21 1.82 0.18 0.55 0.39 1.05 0.03 0.57 0.10 1.85 4.60 0.96 9.10 2.50 ClInPc 7b 8b 9b 0.90 0.12 0.16 0.09 50.20 9.03 9.35 5.34 0.19 1.07 1.09 1.87 1.01 8.08 5.68 9.88 0.77 1.14 4.56 10.33 68.60 7.50 19.40 3.80 a b c d kF is the rate constant for fluorescence. Values calculated using kF = UF/sF. kISC is the rate constant for intersystem crossing. Values calculated using kISC = UT/sF. kIC is the rate constant for internal conversion. Values calculated using kIC = UIC/sF. kd is the rate constant for photodegredation. Values calculated using kd = Ud/sT. kcIC (s1) (·108) kdd (s1) 3332 M. Durmusß, T. Nyokong / Polyhedron 26 (2007) 3323–3335 obtained for the peripherally tetrasubstituted complexes 8a and 8b compared to non-peripherally tetrasubstituted (7a and 7b) and octasubstituted complexes (9a and 9b), suggesting more quenching by peripheral tetrasubstitution when compared to non-peripheral ones and octasubstitution. The sF values ClInPc complexes were lower than for the ClGaPc complexes due to the fact that indium atom is larger than gallium atom. The natural radiative lifetime (s0) values of substituted complexes (7a,b, 8a,b and 9a,b) were lower than unsubstituted complexes (ClGaPc and ClInPc). The rate constants for fluorescence (kF) however increased on going from unsubstituted ClGaPc (or ClInPc) to the corresponding substituted complexes 7a,b, 8a,b and 9a,b (Table 3). There was an increase in kF values on going from tetrasubstituted to octasubstituted ClGaPc and ClInPc complexes. Substituted complexes 7a–9a and 8b– 9b also showed larger rate constants for internal conversion (kIC, Table 3), compared to unsubstituted derivatives. There was no clear trend found for kIC (Table 3) and quantum yields for internal conversion (UIC, Table 2). In addition larger rate constants for intersystem crossing (kISC) for complexes 7a–9a and 7b–9b compared to the respective unsubstituted ClGaPc and ClInPc complexes confirmed improved intersystem crossing to the triplet state for the former. 3.2.2. Triplet lifetimes and quantum yields Transient differential spectrum for complex 8a in DMSO is shown in Fig. 5, and shows a maximum at 520 nm, hence the triplet lifetimes and yields were determined at this wavelength. Fig. 6 shows the triplet decay curves of the complexes (using complex 8a in DMSO as an example). Table 2 shows that the triplet lifetimes for the ClGaPc complexes ranged from 200 to 280 ls, with unsubstituted ClGaPc showing the lowest triplet quantum 0.1 0 400 450 500 550 600 650 700 750 800 0.007 0.008 Wavelength (nm) A -0.1 A 0.04 -0.2 0.02 0 400 -0.3 440 480 520 560 600 Wavelength (nm) -0.4 Fig. 5. Transient differential spectrum of complex 8a in DMSO. 0.003 A 0.002 0.001 0 0 0.001 0.002 0.003 0.004 0.005 0.006 Time, s Fig. 6. Triplet decay curve of 8a in DMSO at 520 nm. Excitation wavelength = 693 nm. M. Durmusß, T. Nyokong / Polyhedron 26 (2007) 3323–3335 1.6 Before Laser 1.2 Absorbance 3333 After Laser 0.8 0.4 After Laser Before Laser 0 500 550 600 650 700 750 800 Wavelength (nm) Fig. 7. Absorption spectral changes of compound 8b before and after laser irradiation. DPBF Absorbance yield (UT) when compared to substituted complexes 7a–9a. Due to enhanced ISC, indium phthalocyanine complexes (ClInPc and 7b–9b) showed much higher quantum yields of triplet state and shorter triplet lifetimes when compared to corresponding gallium phthalocyanine (ClGaPc and 7a– 9a) complexes. There is not a huge difference between UT of octa- and tetra-substituted derivatives of each of ClGaPc or ClInPc. Fig. 7 shows that for substituted complexes (7a–9a and 7b–9b) and unsubstituted derivatives, there was a change is spectra following laser irradiation. The spectral changes involved the decrease in the Q-band and an 1 1.2 0.8 0.4 0 0 10 0 sec 0.8 Absorbance increase in the absorption near 590 nm. However, on exposure of the solution to air, the Q-band increased in intensity and the band around 590 nm decreased suggesting that this band is due to a reduction products of the complexes. The first ring reduction in MPc complexes is characterized by a decrease in the Q-band and the formation of weak bands between 500 and 600 nm [49]. Thus during laser irradiation, the ClGaPc and ClInPc derivatives were partly transformed to an anion (Pc3) species. The suggested mechanism for the formation of Pc3 in the presence of H donors is shown by Eqs. (6)–(8): 20 30 40 Time (sec) 0.6 0.4 40 sec 0.2 0 300 400 500 600 700 800 Wavelength (nm) Fig. 8. A typical spectrum for the determination of singlet oxygen quantum yield. This determination was for compound 7a in DMSO at a concentration of 3 · 105 mol dm3. 3334 M. Durmusß, T. Nyokong / Polyhedron 26 (2007) 3323–3335 Absorbance 1.2 Absorbance 1 0.8 1.5 0 sec 1 0.5 3000 sec 0 0 0.6 500 1000 1500 2000 2500 3000 Time (sec) 0.4 0.2 0 300 400 500 600 700 800 Wavelength (nm) Fig. 9. The photodegredation of compound 8b in DMSO showing the disappearance of the Q-band at 10 min intervals. MPc þ hm ! 3 MPc 3 ð6Þ MPc þ S–H ! MPc þ S–H MPc þ O2 ! MPc þ O 2 þ ð7Þ ð8Þ S ¼ solvent 3.3. Photochemical properties 3.3.1. Singlet oxygen quantum yields Singlet oxygen quantum yields (UD) were determined in DMSO using DPBF as a chemical quencher. The disappearance of DBPF was monitored using UV–Vis spectrometer. Many factors are responsible for the magnitude of the determined quantum yield of singlet oxygen including; triplet excited state energy, ability of substituents and solvents to quench the singlet oxygen, the triplet excited state lifetime and the efficiency of the energy transfer between the triplet excited state and the ground state of oxygen. Because of the presence of oxygen during the determination of singlet oxygen quantum yields (UD) the photoreduction of complexes was not observed during singlet oxygen studies as shown by Fig. 8 for complex 7a (as an example) in DMSO. There was no decrease in the Q-band of formation of new bands. The values of UD were higher for complexes (7a,b, 8a,b and 9a,b) when compared to respective unsubstituted ClGaPc or ClInPc complexes. The trend of singlet oxygen quantum yields among the corresponding substituted complexes was non-peripherally tetrasubstituted (7a,b) > peripherally tetrasubstituted (8a,b) > octasubstituted (9a,b) for both of substituted gallium and indium phthalocyanine complexes in DMSO. The magnitude of the SD (=UD/UT) represents the efficiency of quenching of the triplet excited state by singlet oxygen. Most of the substituted complexes showed SD of near unity (Table 2), suggesting efficient quenching of the triplet state by singlet oxygen. The SD values were less than unity for unsubstituted ClGaPc and ClInPc complexes, suggesting inefficient quenching of the triplet state by singlet oxygen. 3.3.2. Photodegradation studies Degradation of the molecules under irradiation can be used to study their stability and this is especially important for those molecules intended for use as photo catalysts. The collapse of the absorption spectra without any distortion of the shape confirms clean photodegradation not associated with phototransformation. The spectral changes observed for all the complexes (7a,b, 8a,b and 9a,b) during irradiation are as shown in Fig. 9 (using complex 8b as an example in DMSO) and hence confirm photodegradation occurred without phototransformation. All the complexes showed about the same stability with Ud of the order of 105. Again there was no photoreduction of the complexes during photodegradation studies in the presence of oxygen, hence the magnitudes of the observed Ud are not due to phototransformation. The rate constant for photodegradation (kd) values were lower for substituted indium phthalocyanine complexes 7b, 8b and 9b, compared to unsubstituted ClInPc, while there was no clear trend for the ClGaPc derivatives (Table 3). 4. Conclusions In conclusion, we have synthesized new peripherally and non-peripherally tetra-, and octa-(4-benzyloxyphenoxy) substituted gallium and indium phthalocyanines. These phthalocyanine complexes are monomeric in solution. The difference in the behaviours of ClGaPc and ClInPc complexes on excitation could be due to the larger indium metal being more displaced from the core of the phthalocyanine ring, and the displacement being more pronounced on excitation hence a loss of symmetry. Although the fluorescence quantum yields (UF) of ClGaPc complexes are typical of MPc complexes, the UF values for the ClInPc complexes were very low due to presence of a heavier indium atom in these complexes. 4-Benzyloxyphenoxy substituted gallium phthalocyanine complexes (7a–9a) showed long triplet lifetimes and high triplet quantum M. Durmusß, T. Nyokong / Polyhedron 26 (2007) 3323–3335 yields. Due to enhanced ISC, 4-benzyloxyphenoxy indium phthalocyanine complexes (7b–9b) showed much higher quantum yields of triplet state and shorter triplet life times. The gallium and indium phthalocyanine complexes showed phototransformation when using laser irradiation due to the ring reduction. The substituted complexes (7a,b, 8a,b and 9a,b) gave good singlet oxygen quantum yields ranging from 0.51 (for 9a) and 0.94 (for 7b). The singlet oxygen quantum yields (UD), which give an indication of the potential of the complexes as photosensitizers in applications where singlet oxygen is required (Type II mechanism). Thus, these complexes show potential as Type II photosensitizers for photodynamic therapy of cancer. Acknowledgement This work was supported by the National Research Foundation of South Africa (NRF GUN # 2053657) as well as Rhodes University. References [1] C.C. Leznoff, A.B.P. Lever, Phthalocyanines, Properties and Applications, vols. 1–4, VCH, New York, 1989, 1993, 1996. [2] M. 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