The synthesis and photophysical properties of peripherally and non-peripherally substituted ball-type Mg(ii) and Zn(ii) phthalocyanines

Article abstract of DOI:10.1039/c0dt00920b

Newly synthesized ball-type Zn(ii) and Mg(ii) phthalocyanines containing four 1,1′-binaphthol substituents at peripheral and non-peripheral positions are presented. The structures of the synthesized compounds were characterized by using elemental analysis

Full text of DOI:10.1039/c0dt00920b

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PAPER
The synthesis and photophysical properties of peripherally and
non-peripherally substituted ball-type Mg(II) and Zn(II) phthalocyanines†
Mevlude Canlicaa,b and Tebello Nyokong*b
Received 28th July 2010, Accepted 29th November 2010
DOI: 10.1039/c0dt00920b
Newly synthesized ball-type Zn(II) and Mg(II) phthalocyanines containing four 1,1¢-binaphthol
substituents at peripheral and non-peripheral positions are presented. The structures of the synthesized
compounds were characterized by using elemental analysis, UV-vis, FT-IR, 1H-NMR and mass
spectroscopies. The UF values were 0.33, 0.08, 0.20 and 0.08 for 6–9, respectively. The UT values were
0.56, 0.85, 0.64 and 0.88 for 6–9, respectively. All the complexes showed reasonably large triplet
lifetimes with tT values of 710 (6), 170 (7), 1490 (8) and 380 ms (9) in DMSO. These complexes offer
potential as photosensitizers in photodynamic therapy.
compounds or the central metal. The distance between the two Pc
1. Introduction
units affects the degree of interaction between the rings.
Metallophthalocyanines (MPcs) have been studied as materials for
Due to these interactions, the Q-bands in the electronic spectra
many applications, including molecular electronics,1 non-linear
of ball-type MPc complexes and their voltammetric signals are
optics,2 liquid crystals,3 photosensitizers4,5 and electrocatalysis.6
usually much broader (or split) compared to those for their parent
There is considerable interest in MPc complexes containing non-
monomers. In this study, the photophysical properties of ball-type
transition metals for use as photosensitizers in the relatively
complexes are presented by us for the first time.
new method of cancer treatment called photodynamic therapy
Thus, in this paper, we describe the synthesis and photophysical
(PDT).4,5,7–10 The mode of operation in PDT is based on visi-
properties of new symmetrically-substituted ball-type MgPc and
ble light excitation of a tumor-localized photosensitizer. After
ZnPc complexes containing substituents at the non-peripheral and
excitation, energy is transferred from the photosensitizer (in
peripheral positions (Scheme 1). We compare the effects of a vs. b
its triplet excited state) to ground state oxygen (3O2), forming
substitution on their photophysical properties.
singlet oxygen (1O2). High triplet state quantum yields and long
triplet lifetimes are required for efficient photosensitization. The
synthesis, spectroelectrochemical and electrocatalytic behaviour
2. Experimental
of ball-type phthalocyanine derivatives have been extensively
studied since these molecules were published in the literature for
2.1 Materials
the first time in 2002.11,12 Ball-type phthalocyanine derivatives
show spectroscopic and electrochemical properties that differ
significantly from their parent monomer.13 However, studies on
non-peripherally substituted ball-type MPc derivatives and those
containing bulky substituents (hence less aggregated) are still
limited. The photophysical behaviour of ball-type molecules have
also not received much attention.
Our interest in ball-type complexes is due to the possibility
of intramolecular interactions between the Pc rings and/or the
metal centres of these compounds. The electronic properties of
ball-type Pcs can change dramatically, depending on the bridging
3-Nitrophthalonitrile (1) and 4-nitrophthalonitrile (2) were syn-
thesized as reported in the literature.14 1,1¢-Binaphthyl-8,8¢-diol
(3) was obtained from commercial suppliers. Anhydrous mag-
nesium(II) chloride and zinc(II) acetate were purchased from
Sigma-Aldrich. Dimethylsulfoxide (DMSO) and chloroform were
obtained from Saarchem. Silica gel for column chromatography
was purchased from Merck. All solvents were dried and purified
as described by Perrin and Armarego.15 All other reagents were
used as received.
2.2 Equipment
UV-vis absorption spectra were obtained using a Varian Cary 500
UV-Vis/NIR instrument. Fluorescence excitation and emission
spectra were recorded using a Varian Eclipse spectrophotometer.
FT-IR data (KBr pellets) were recorded using a Perkin-Elmer
spectrum 2000 FTIR spectrometer. H NMR spectra were ob-
tained using a Bruker EMX 400 MHz spectrometer. Elemental
analyses were performed on a Vario-Elementar Microcube EL III
aYildiz Technical University, Chemistry Department, Inorganic Chem-
istry Division, Davutpasa Campus, 34220, Esenler, Istanbul, Turkey.
E-mail: mcanlica@yahoo.com
bRhodes University, Chemistry Department, P.B. Box 6140, Grahamstown,
South Africa. E-mail: t.nyokong@ru.ac.za; Fax: + 27 46 6225109; Tel: +
27 46 6038260
1
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c0dt00920b
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Scheme 1 A synthetic scheme for non-peripherally and peripherally substituted ball-type metallophthalocyanines.
analyser. Mass spectra data were collected by a Bruker AutoFLEX
III Smartbeam MALDI/TOF mass spectrometer. The instrument
was operated in positive ion mode using an m/z range of 400–3000.
The voltage of the ion sources were set at 19 and 16.7 kV for ion
sources 1 and 2, respectively, while the lens was set at 8.50 kV.
Reflector 1 and 2 voltages were set at 21 and 9.7 kV, respectively.
Spectra were acquired using dithranol as the MALDI matrix using
a 354 nm Nd:YAG laser.
Fluorescence lifetimes were measured using a time-correlated
single photon counting (TCSPC) setup (Fluo Time 200, Picoquant
GMbH) with a diode laser as the excitation source (LDH-P-670
driven by PDL 800-B, 670 nm, 20 MHz repetition rate, Picoquant
GmbH). Fluorescence was detected under the magic angle with
a peltier cooled photomultiplier tube (PMT) (PMA-C 192-N-M,
Picoquant GmbH) and integrated electronics (PicoHarp 300E,
Picoquant GmbH). A monochromator with a spectral width of
about 4 nm was used to select the required emission wavelength.
The response function of the system, which was measured with
a scattering Ludox solution (DuPont), had a full-width at half-
maximum (FWHM) of about 300 ns. The ratio of stop to start
pulses was kept low (below 0.05) to ensure good statistics. All
luminescence decay curves were measured at the maximum of the
emission peak. The data were analyzed with the program FluoFit
(Picoquant GmbH). The support plane approach16 was used to
estimate the errors of the decay times.
300 MHz digital real-time oscilloscope (Tektronix TDS 3032C);
kinetic curves were averaged over 256 laser pulses. Triplet lifetimes
were determined by exponential fitting of the kinetic curves using
the program OriginPro 7.5.
2.3 Syntheses
The target precursors were prepared by a nucleophilic aromatic
substitution reaction between 3-nitrophthalonitrile (1) or 4-
nitrophthalonitrile (2) and 1,1¢-binaphthyl-8,8¢-diol (3) in the solid
phase in the absence solvent or catalysts (Scheme 1).
2.3.1. 3,4¢-(1,1¢-Binaphthyl-8,8¢-diylbis(oxy))diphthalonitrile
(4). Compound 3 (3.31 g, 11.6 mmol) was dissolved in dry
DMSO (15 mL) and compound 1 (4.0 g, 23.1 mmol) was added
under an inert atmosphere. To this reaction mixture was added
finely ground anhydrous potassium carbonate (3.19 g, 23.1 mmol).
After 4 h of stirring at room temperature, further potassium
carbonate (0.79 g, 5.8 mmol) was added, and this same amount
was added again after 24 h of stirring. After a total of 96 h of
stirring, the reaction mixture was poured into water (50 mL),
resulting in the formation of a light yellow precipitate. The crude
product was centrifuged and further purified by chromatography
over a silica gel column using CHCl3 as the eluent. The product
was recrystallized from ethanol, and then the pure product was
dried using P2O5. Yield: 2.5 g. IR(KBr) nmax/cm-1: 3080 (Ar-CH),
2230 (C N), 1570 (C C), 1259 (C–O–C). 1H NMR (DMSO-d6):
d/ppm 8.21 (2H, d, Ar-H), 8.08 (2H, d, Ar-H), 7.63 (2H, t, Ar-H),
7.59 (2H, t, Ar¢-H), 7.51–7.57 (4H, m, Ar-H), 7.37 (2H, d, Ar¢-H),
7.34 (2H, t, Ar¢-H), 7.26 (2H, d. Ar¢-H). Calc. for C36H18N4O2: C,
80.29; H, 3.37; N, 10.40%. Found: C, 79.37; H, 3.68; N, 9.96%.
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, 9 ns pulses of
laser light at 10 Hz, pumping a Lambda-Physic FL 3002 dye
(Pyridin 1 in methanol). The analyzing beam source was from
a Thermo Oriel xenon arc lamp, and a photomultiplier tube was
used as the detector. Signals were recorded with a two-channel
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2.3.2 4,4¢-(1,1¢-Binaphthyl-8,8¢-diylbis(oxy))diphthalonitrile
(5). The synthesis of 5 was the same as that for 4, except that
compound 2 was employed instead of 1; the amounts of the
reagents were the same. Yield: 2.1 g. IR(KBr) (nmax/cm-1: 3072
2.4. Photophysical Studies
2.4.1 Fluorescence quantum yields. Fluorescence quantum
yields (UF) were determined by the comparative method17,18
(eqn (1)):
1
(Ar-CH), 2232 (C N), 1586 (C C), 1278 (C–O–C). H-NMR
(DMSO-d6): d/ppm 8.20 (2H, d, Ar-H), 8.08 (2H, d, Ar-H), 7.85
(2H, d, Ar-H), 7.55 (2H, t, Ar¢-H), 7.49 (2H, s, Ar-H), 7.39 (2H, t,
Ar-H), 7.30 (2H, d, Ar¢-H), 7.16 (2H, d, Ar¢-H), 6.93 (2H, d, Ar¢-
H). Calc. for C36H18N4O2: C, 80.29; H, 3.37; N, 10.40%. Found:
C, 79.43; H, 4.05; N, 9.83%.
FAStd n2
(1)
FF = F
F(Std) F AnS2td
Std
where F and FStd are the areas under the fluorescence curves of
6–9 and the standard, respectively, A and AStd are the respective
absorbances of the sample and standard at the excitation wave-
length (which was ~0.05 in all solvents used), and n and nstd are the
refractive indices of the solvents used for the sample and standard,
respectively. ZnPc (UF = 0.20 in DMSO) was employed as the
standard.19
General synthetic procedure for complexes 6–9
A mixture of complex 4 or 5 (100 mg) and magnesium(II) chloride
(100 mg) for 6 and 8, respectively, and zinc acetate (100 mg) for
7 and 9, respectively, was ground in a quartz crucible and heated
in sealed glass tube for 15 min (6 and 8) or for 10 min (7 and 9),
under an argon atmosphere at 550 ◦C (6 and 8) or at 500 ◦C (7 and
9). After cooling to room temperature, the dark-green reaction
products were washed with hot methanol and hot water. The
products were separated by column chromatography on silica gel
using chloroform and a gradient of chloroform–methanol up to
50%. Finally, pure dark-green products were obtained by column
chromatography over silica gel and eluting with chloroform.
2.4.2 Triplet quantum yields and lifetime
The solutions for triplet quantum yields and lifetimes were
introduced into a 1.0 mm path length UV-visible spectropho-
tometric cell de-aerated using nitrogen and irradiated at the
Q-band maximum. Triplet state quantum yields (UT) of 6–9
were determined by the triplet absorption method20 using zinc
phthalocyanine (ZnPc) as a standard; eqn (2):
DATeTStd
FT = FSTtd
2.3.3 1¢,11¢,15¢,25¢-[Tetrakis(4,4¢-(1,1¢-binaphthyl-8,8¢-diyl-
(oxy))diphenyl)]bis-phthalocyaninatodimagnesium(II) (6). Yield:
0.038 g. UV-vis (CHCl3) lmax/nm (log e/dm-3 mol-1 cm-1): 698
(4.98). IR[(KBr) nmax/cm-1: 3054 (Ar-CH), 1581 (C C), 1243 (C–
O–C). 1H NMR (DMSO-d6): d/ppm 8.10–7.22 (64H, Ar-H), 7.05
(8H, d, Ar¢-H). Anal. calc. for C144H72N16O8Mg2: C, 78.51; H, 3.29;
N, 10.17%. Found: C, 78.09; H, 3.48; N, 9.91%. MALDI-TOF-MS
m/z calc. 2202.82. Found [M + H]+: 2203.54.
(2)
DATStdeT
Std
where DAT and DAT
are the changes in the triplet state
Std
absorbances of 6–9 and the standard, respectively, eT and eT
are the triplet state molar extinction coefficients for 6–9 and the
Std
standard, respectively and UT is the triplet quantum yield for
the standard, ZnPc (UT = 0.65 in DMSO21). eT and eT were
Std
determined from the molar extinction coefficients of the respective
ground singlet state (eS and eSStd) and the change in absorbance of
the ground singlet state (DAS and DASStd) according to eqn (3):
2.3.4 1¢,11¢,15¢,25¢-[Tetrakis(4,4¢-(1,1¢-binaphthyl-8,8¢-diyl-
(oxy))diphenyl)]bis-phthalocyaninatodizinc(II) (7). Yield: 0.037 g.
UV-vis (CHCl3) lmax/nm (log e/dm-3 mol-1 cm-1): 696 (4.60).
IR[(KBr) nmax/cm-1: 3055 (Ar-CH), 1579 (C C), 1245 (C–O–C).
1H-NMR (DMSO-d6): d/ppm 8.10–7.20 (64H, Ar-H), 7.02 (8H,
d. Ar¢-H). Anal. calc. for C144H72N16O8Zn2: C, 75.69; H, 3.18; N,
9.81%. Found: C, 75.10; H, 3.24; N, 9.73%. MALDI-TOF-MS
m/z calc. 2285.50. Found [M]+: 2285.50.
DAT
DAS
eT = eS
(3)
Quantum yields of internal conversion (UISC) were obtained from
eqn (4), which assumes that only three processes (fluorescence,
intersystem crossing and internal conversion) jointly deactivate
the excited singlet state of complexes 6–9.
2.3.5 2¢,10¢,16¢,24¢-[Tetrakis(4,4¢-(1,1¢-binaphthyl-8,8¢-diyl-
(oxy))diphenyl)]bis-phthalocyaninatodimagnesium(II) (8). Yield:
0.035 g. UV-vis (CHCl3) lmax/nm (log e/dm-3 mol-1 cm-1): 690
(4.42). IR[(KBr) nmax/cm-1: 3054 (Ar-CH), 1581 (C C), 1231 (C–
O–C). 1H-NMR (DMSO-d6): d/ppm 8.23–7.16 (64H, d, Ar¢-H),
6.95 (8H, d, Ar¢-H). Anal. calc. for C144H72N16O8Mg2: C, 78.51; H,
3.29; N, 10.17%. Found: C, 77.88; H, 3.36; N, 10.09%. MALDI-
TOF-MS m/z calc. 2202.82. Found [M]+: 2202.69.
UIC = 1 - (UF + UT)
(4)
3. Results and discussion
3.1. Synthesis and characterization
New ball-type binuclear Mg(II)Pc and Zn(II)Pc complexes
6–9 were prepared by the reaction of compounds 4 and 5 with
MgCl2 and zinc acetate. The cyclotetramerization of phthalonitrile
derivatives 4 and 5 to form MPc derivatives 6–9 was accomplished
without a solvent in the presence of metal salts at 500 and 550 ◦C.
Column chromatography on silica gel using CHCI3 as the mobile
phase (or a gradient of chloroform and methanol) was used
to purify complexes 4–9. The structure and purity of the MPc
derivatives was confirmed by UV-vis, IR and mass spectral data,
and elemental analyses. The results of elemental analysis differed
2.3.6 2¢,10¢,16¢,24¢-[Tetrakis(4,4¢-(1,1¢-binaphthyl-8,8¢-diyl-
(oxy))diphenyl)]bis-phthalocyaninatodizinc(II) (9). Yield: 0.038 g.
UV-vis (CHCl3) lmax/nm (log e/dm-3mol-1cm-1): 684 (4.85).
IR[(KBr) nmax/cm-1: 3054 (Ar-CH), 1579 (C C), 1232 (C–O–
1
C). H NMR (DMSO-d6): d/ppm 8.20–7.15 (64H, Ar¢-H), 6.93
(8H, d, Ar¢-H). Anal. calc. for C144H72N16O8Zn2: C, 75.69; H, 3.18;
N, 9.81%. Found: C, 75.97; H, 3.94; N, 9.44%. MALDI-TOF-MS
m/z calc. 2285.50. Found [M]+: 2285.37.
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by less than 1% from the calculated values, showing the purity of
the complexes.
The IR spectra of 4 and 5 clearly indicate the absence of
OH groups in 3. The C N vibrational peaks were observed
at 2230 and 2232 cm-1 for compounds 4 and 5, respectively. A
diagnostic feature of the formation of 6–9 from the phthalodinitrile
derivatives is the disappearance of the sharp –CN vibration at
2230 and 2232 cm-1 for 4 and 5, respectively. The IR spectra of
4–9 showed Ar–O–Ar peaks at 1235–1255 cm-1.
1
The H NMR spectra of 4 and 5 show the aromatic protons
appearing between d 8.21–7.26 and 8.08–6.93, respectively, inte-
grating to a total of 18 for 4 and 5. In the H NMR spectrum
Fig. 1 Absorption spectra of complexes 6–9 in DMSO, concentration =
1 ¥ 10-5 mol dm-3.
1
of 5, a sharp singlet signal was observed at d 7.49 belonging
to Ar-H. The spectrum of 4 has three triplets at d 7.63, 7.59
and 7.34, whereas the spectrum of 5 shows triplets at d 7.55 and
absorptions at 698 and 696 nm, whereas complexes 8 and 9 have Q-
band absorptions at 690 and 684 nm in CHCl3 (Table 1). It is well
known that a-substitution results in a red-shifting of the spectra of
MPcs.23,24 This is due to the electron density enhancement caused
by substitution at the a-position.
1
7.39. The H NMR spectra of Pcs 6–9 were recorded in DMSO
and were similar to each other. The aromatic protons appeared
at d 8.10–7.22, 8.10–7.20, 8.23–7.16 and 8.20–7.15, and d 7.05,
7.02, 6.95 and 6.93, respectively, and integrating to a total of
72 for 6–9. The elemental analysis results were also consistent
with the proposed structures of 4–9. The purified monofunctional
phthalocyanines were further characterized by mass spectroscopy.
The expected mass values corresponded to the found values for
all the complexes using dithranol as the matrix. The protonated
molecular ion peak of complexes 6 and the molecular ion peak
complex of 7–9 were observed at 2203.54, 2285.50, 2202.69 and
2285.37 amu, respectively. These values are for the most abundant
isotope (see the ESI, Fig. S1†).
The UV-vis spectra in CHCl3, shown in Fig. 1, for 6–9 are also in
agreement with the expected structures. The phthalocyanines show
typical electronic spectra with two absorption bands, one in the
UV region in the range 300–400 nm (B band) and the other in the
visible region in the range 600–700 nm (Q band). The characteristic
Q band transition of metallophthalocyanines with D4h symmetry is
observed as a single band of high intensity in the visible region. The
aggregation behaviour of Pc is depicted as a coplanar association
of rings progressing from monomer to dimer and higher order
complexes, and is dependent on the concentration, nature of the
solvent, substituents, metal ions and temperature.22 Aggregation
in MPcs is typified by a broadened or split Q-band, with the high
energy band being due to the aggregate and the low energy band
being due to the monomer.
The well-defined band close to 620 nm in ball-type phthalo-
cyanines is due to exciton coupling between the two Pc rings.13
The spectra of complexes 8 and 9 in DMSO show two bands
at ~640 and 613 nm. The latter is attributed to intermolecular
interactions between the rings, in agreement with the literature.
This band is also observed for complexes 6 and 7. The band at
~640 nm is attributed to aggregation, as is typical for phthalocya-
nines. For complexes 6 and 7, this band is not observed; however,
for complex 7, aggregation is evidenced by broadening of the Q-
band. Aggregation is not evident for complex 6. Thus, it is clear
that there is more aggregation for complexes 8 and 9, which are
peripherally substituted compared to non-peripherally substituted
complexes 6 and 7, as is typical for phthalocyanines.25 However,
the Lambert–Beer law was obeyed for the complexes in CHCl3 for
concentrations of less than 1 ¥ 10-5 M.
3.2. Fluorescence spectra, quantum yields and lifetimes
The absorption and fluorescence excitation spectra of complexes
6–9 in DMSO are shown in Fig. 2. The excitation, absorption and
emission spectral data are listed in Table 1. All complexes showed
a similar fluorescence behaviour. The absorption and excitation
spectra show the same Q-band maxima. However, the absorption
spectra are broadened compared to the emission and excitation
spectra due to aggregation in the former. The peaks due to exciton
coupling are, however, not clear upon excitation, suggesting
relaxation of the coupling. The proximity of the wavelength of each
Complexes 6 and 7 show Q band absorptions at 690 and
693 nm, whereas complexes 8 and 9 show Q band absorptions
at 685 and 684 nm in DMSO. The band maxima are listed in Table
1 in both CHCl3 and DMSO. Complexes 6 and 7 show Q-band
Table 1 UV-vis absorption (Q band) emission and excitation spectral data, and photophysical data for phthalocyanines 6–9
max/nm (log e/dm-3 mol-1 cm-1)
CHCl3 DMSO
abs (log e) abs /nm
l
Solvent→
Complex↓
l
l
l
ems/nm
l
exc/nm
Stokes shift/nm
UF
tT/ms
UT
UIC
tF/ns
6
7
8
9
698 (4.98)
696 (4.60)
690 (4.42)
684 (4.85)
690
693
685
684
697
699
695
695
689
693
686
685
8
6
9
10
0.33
0.08
0.20
0.08
710
170
1490
380
0.56
0.85
0.64
0.88
0.11
0.07
0.16
0.04
5.38
2.35
5.09
2.67
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similar and not affected by excitation. The emission spectra are
mirror images of the excitation spectra. Stokes shifts ranged from
6 to 10 nm, typical of MPc complexes. The effect of the point
of substitution is not clear from the nature of the excitation and
emission spectra.
The UF values ranged from 0.08 to 0.33 in DMSO. The
fluorescence quantum yield (UF) values were typical26 of MPc
complexes for complex 8. The UF values were low for the ZnPc
derivatives (7 and 9). This is attributed to the central Zn metal,
which is heavier than Mg and hence encourages intersystem
crossing to the triplet state, as will be confirmed below.
The fluorescence quantum yield of the octaphenoxy-substituted
zinc phthalocyanine monomer has been reported to be 0.17 in
DMF.26 This value is much larger than that reported here for the
ZnPc ball-type dimers (complexes 7 and 9), which may suggest
a self-quenching process of the two Pc ring complexes in ball-
type complexes. Face-to-face interaction of the two monomers in
dimers will decrease the energy gap between the singlet state and
the triplet state, and enhance the formation of the triplet state (i.e.
intersystem crossing increases), so decreasing fluorescence.27 The
UF values for complexes 7–9 will, however, also be underestimated
due to aggregation. We believe that aggregation and the heavy
atom effect are the predominant factors affecting the UF values of
the complexes, since for complex 6, which is not aggregated, a UF
value higher than that typical for monomeric phthalocyanines is
observed.
Non-peripherally substituted complex 6 showed a larger UF
compared to the corresponding peripherally substituted 8, proba-
bly due to there being less aggregation in the former, as observed in
Fig. 1. Aggregation reduces the likelihood of radiative deactivation
through the dissipation of energy by aggregates.
The fluorescence decay profiles for the complexes were mono-
exponential. Due to the heavy atom effect, fluorescence lifetime
(tF) values are higher for complexes 6 and 8 when compared to
complexes 7 and 9. The tF values range from 2.35 to 5.38 (Table 1).
Triplet quantum yields (UT) and lifetimes (sT)
The triplet quantum yields (UT) and lifetimes of the complexes are
listed Table 1; Fig. 3 shows a representative triplet decay curve for
the complexes (using complex 7 as an example). UT represents
the fraction of absorbing molecules that undergo intersystem
crossing to the metastable triplet excited state. Therefore, factors
that induce spin–orbit coupling will certainly populate the triplet
excited state. High UT values, and correspondingly low UF values
(for ZnPc derivatives 7 and 9), suggest a more efficient intersystem
crossing (ISC). The UT values for 6–9 are 0.56, 0.85, 0.64 and
0.88, respectively. The low values of UT for the MgPc derivatives,
when compared to the ZnPc derivatives, are accounted for by
the heavy atom effect of the Zn central metal, which encourages
intersystem crossing, resulting in larger UT and correspondingly
lower UF values, as discussed above. Complexes 6–9 have low
UIC values, which may be explained by strong intramolecular
interactions between the Pc rings, probably due to the cofacial
structure.
Fig. 2 (a) Absorbance, (b) excitation and (c) emission spectra of
complexes 6–9 in DMSO. Excitation wavelength = 620, 616, 614 and
616 nm for 6–9, respectively.
All complexes showed reasonably large triplet lifetimes, with
tT ranging from 170 to 1490 ms. The MgPc derivatives (6
and 8) showed longer tT values when compared to the ZnPc
derivatives (7 and 9) due to the heavy atom effect of Zn.
component of the Q-band absorption to the Q-band maximum
of the excitation spectrum for all the complexes suggests that
the nuclear configurations of the ground and excited states are
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Acknowledgements
This work was supported by the Department of Science and
Technology (DST) and the National Research Foundation (NRF),
South Africa through DST/NRF South African Research Chairs
Initiative for Professor of Medicinal Chemistry and Nanotechnol-
ogy, as well as Rhodes University.
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6 N4-Macrocyclic Metal Complexes, ed. J. Zagal, F. Bedioui and J. P.
Dodelet, Springer, New York, 2006.
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Phthalocyanines, 2001, 5, 161.
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249.
Fig. 3 Triplet decay curve for complex 7.
Peripheral substitution resulted in longer tT values compared to
non-peripheral substitution when considering the same central
metal; compare non-peripherally substituted complex 6 (tT
=
710 ms) and peripherally substituted complex 8 (tT = 1490
ms) or non-peripherally substituted complex 7 (tT = 170 ms)
and peripherally substituted 9 (tT = 390 ms). The value of tT
~1.5 ms is longer compared to phthalocyanine complexes in
general.26 Also, complex 6 has a higher UT and a longer tT when
compared to its monomer.28 This can be attributed to strong
intramolecular interactions between the Pc rings. This is due to
the fact that the face-to-face interaction of the two monomers in
dimers will decrease the energy gap between the singlet state and
the triplet state, and enhance the formation of the triplet state
(as discussed above), consequently decreasing the fluorescence
intensity but increasing the triplet state yield.27
9 E. Ben-Hur and W. S. Chan, Phthalocyanines: Properties and Materials,
in The Porphyrin Handbook, ed. K. M. Kadish, K. M. Smith and R.
Guilard, Academic Press, New York, 2003, ch. 117, pp. 19.
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Porphyrins Phthalocyanines, 2003, 7, 162.
13 O. Bekaroglu, in Functional Phthalocyanine Molecular Materials, ed. J.
Jiang, 2010, vol. 135, 105.
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1987, 6, 1463.
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Chemicals, Pergamon Press, Oxford, 2nd edn, 1980.
16 J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Kluwer
Academic/Plenum Publishers, New York, 2nd edn, 1999.
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18 J. Fu, X. Y. Li, D. K. P. Ng and C. Wu, Langmuir, 2002, 18, 3843.
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20 P. Kuba´t and J. Mosinger, J. Photochem. Photobiol., A, 1996, 96, 93.
21 T. H. Tran-Thi, C. Desforge and C. C. Thiec, J. Phys. Chem., 1989, 93,
1226.
Conclusions
We have reported on the synthesis of two new phthalodinitrile
derivatives for the preparation of novel binuclear Pcs of the
ball-type, substituted at non-peripheral and peripheral positions
with aromatic groups. The synthesis and photophysical properties
of the new ball-type Mg(II)Pc and Zn(II)Pc complexes were
presented. The tT values are longer and UT values are lower for
the non-peripherally substituted derivatives. The MgPc derivatives
(6 and 8) showed longer tT values when compared to the ZnPc
derivatives (7 and 9). All the complexes showed reasonably high
triplet state quantum yields and long triplet lifetimes, which are
required for efficient photosensitization. Thus, these complexes
show potential for the PDT of cancer.
22 H. Engelkamp and R. J. M. Nolte, J. Porphyrins Phthalocyanines, 2000,
4, 454.
23 M. J. Stillman and T. Nyokong, in Phthalocyanines: Properties and
Applications, ed. C. C. Leznoff and A. B. P. Lever, VCH, New York,
1989, pp. 1.
24 T. Nyokong and H. Isago, J. Porphyrins Phthalocyanines, 2004, 8, 1083.
25 T. Nyokong and H. Isago, J. Porphyrins Phthalocyanines, 2004, 8, 1083.
26 T. Nyokong, Coord. Chem. Rev., 2007, 251, 1707.
27 J. Rena, X.-L. Zhao, Q.-C. Wang, C. F. Ku, D.-H. Qu, C.-P. Chang and
H. Tian, Dyes Pigm., 2005, 64, 179.
28 M. Canlica and T. Nyokong, Inorg. Chim. Acta, 2010, 363, 3384.
1502 | Dalton Trans., 2011, 40, 1497–1502
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The Royal Society of Chemistry 2011
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