Download and octa-substituted gallium and indium phthalocyanines

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].
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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
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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
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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.
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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.
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