Tetra and octa(2,6-di-iso-propylphenoxy)-substituted phthalocyanines:

Article abstract of DOI:10.1142/S1088424612004495

This work reports on the synthesis of novel metal free, zinc, aluminum, gallium and indium tetra and octa (2,6-di-iso-propylphenoxy)-substituted phthalocyanine derivatives. UV-visible and ¹H NMR analyses confirm that a non-planar conformation,

Full text of DOI:10.1142/S1088424612004495

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2012; 16: 163–174
DOI: 10.1142/S1088424612004495
Published at http://www.worldscinet.com/jpp/
Tetra and octa(2,6-di-iso-propylphenoxy)-substituted
phthalocyanines: a comparative study among their
photophysicochemical properties
Ahmad Tuhla◊, Wadzanai Chidawanayikab, Hamada Mohamed Ibrahima, Nouria Al-
Awadia, Christian Litwinskib, Tebello Nyokongb◊, Haider Behbehania, Hacene Manaac
and Saad Makhseed*a
a Department of Chemistry, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait
b Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa
c Department of Physics, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait
Received 10 September 2011
Accepted 23 October 2011
ABSTRACT: This work reports on the synthesis of novel metal free, zinc, aluminum, gallium and
indium tetra and octa (2,6-di-iso-propylphenoxy)-substituted phthalocyanine derivatives. UV-visible
and 1H NMR analyses confirm that a non-planar conformation, adapted by the phenoxy substituents due
to steric interaction in both derivative series, perfectly discourage cofacial aggregation. Fluorescence
quantumyieldsvaryasafunctionofthenumberofsubstituentsontheringperiphery,whilethefluorescence
lifetimes display no distinct trend. Triplet quantum yields are significantly larger for the tetra 2,6-di-iso-
propylphenoxy-substituted derivatives relative to their corresponding octa-substituted species. However
there was no overall trend in the triplet lifetime values. For almost all of the phthalocyanine derivatives,
singlet oxygen was produced with relatively good quantum yields. This study explores the possibility of
fine-tuning their physicochemical properties by simple structural modification.
KEYWORDS: di-iso-propylphenoxy, phthalocyanine, photochemistry, photophysics.
MPcderivativescontainingdiamagneticnon-transition
INTRODUCTION
centrals, are photoactive, and are often employed in
Phthalocyanines (Pcs) and derivatives thereof show
photosensitization [11–13]. Interest in improving
potential as photosensitizers for many applications
the photophysical properties of the Pc ring system
including in non-linear optics and photodynamic therapy
continues to grow rapidly. This has stirred research into
(PDT) [1–8]. For these applications, large triplet state
examining new Pc compounds whose structures promote
yield and long triplet state lifetimes are required.
good photophysical behavior [14–16]. Therefore
With their p-conjugated planar organic structure,
this work is focused on the synthesis and subsequent
Pcs interact strongly with light over most of the visible
photophysicochemical characterization of novel tetra
spectrum. Diversification of the phthalocyanine by
and octa di-iso-propylphenoxy substituted metal free,
substitution at the ring periphery, axial position and
aluminum, zinc, gallium and indium phthalocyanines. For
central metal atom could alter the solubility, electronic
the di-iso-propylphenoxy tetra substituted complexes,
absorption characteristics and photosensitizing behavior
the chlorine atom is also present on the periphery of the
of Pcs [9, 10].
ring. Halogenated phthalocyanines have been reported
to show improved photosensitizer activity for PDT
compared to non-halogenated derivatives [17–19]. With
the exception of octa-substituted zinc derivative [20, 21]
SPP full member in good standing
to date there have been no reports on the synthesis
of tetra-substituted derivatives. Ameliorating the
*Correspondence to: Saad Makhseed, email: saad.makhseed@
ku.edu.kw, tel: +965 24985538, fax: +965 24816482
Copyright © 2012 World Scientific Publishing Company

164
A. TUHL ET AL.
photosensitizing efficiency of phthalocyanines may also
be achieved by the presence of four chlorine atoms on the
ring periphery (for complexes 4a–8a), due to the heavy
atom effect [22, 23]. Moreover, electron-withdrawing
groups (i.e. chlorine atoms) are known to increase the
photo-oxidative stability, which could impart additional
features for such material in PDT and non-linear optical
(NLO) applications [24, 25].
The aim is thus to establish the influence of the
different metal centers and substitution patterns, with
respect to the photophysicochemical properties of the
complexes.
gallium phthalocyanines respectively. Phthalonitriles
3a and 3b were prepared by the well-known base
catalyzed nucleophilic aromatic substitution reaction
between 4,5-dichlorophthalonitrile and the anion of the
sterically hindered phenol (2,6-di-iso-propylphenol)
in good yield [26]. Following the metal-ion-mediated
reaction procedure, the precursor 3a undergoes cyclo-
tetramerisation in quinoline using the appropriate metal
salt (AlCl3, GaCl3, InCl3 and Zn(OAc)2) with a catalytic
amount of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) to
afford metal-containing derivatives 5a–8a in acceptable
yields as a mixture of inseparable structural isomers.
Cyclization of 3a in pentanol with lithium metal (i.e.
the most routinely followed procedure) to prepare the
metal-free phthalocyanine derivative (4a) was avoided to
protect the peripheral chlorine atoms from the possibility
of being displaced by the alkyloxide anion. Therefore,
the synthesis of 4a was best achieved in reasonable yield
simply by urea-promoted cyclization of 3a in quinoline.
4b was synthesized using the lithium method. Due to
the asymmetrical arrangement of the substituents in
the periphery of the macrocycle, all 4a–8a products are
obtained as a mixture of structural isomers
RESULTS AND DISCUSSION
Synthesis and characterization
The phthalonitrile precursors (3a and 3b) mono and
di-substituted with di-iso-propylphenoxy were employed
to synthesize the tetra (Scheme 1) and octa (Scheme 2)
substituted metal free, aluminum, zinc, indium and
which are highly soluble in most organic
solvents e.g. dichloromethane (DCM),
CHCl3, tetrahydrofuran (THF), toluene and
OH
hexane thus facilitating their purification
by column chromatography. Each of 4a–8a
complexes occurs as a mixture of structural
isomers which are difficult to separate these
Cl
Cl
CN
O
CN
(a)
+
CN
Cl
CN
3a
1
2
isomers by column chromatography or
high performance liquid chromatography
(HPLC) technique. The octa-substituted
derivatives (5b–8b) were prepared by
following the same procedure used to
prepare the tetra 2,6-di-iso-propylphenoxy-
substituted complexes (5a–8a). The identity
and purity of the prepared complexes
(b)
Cl
O
1
were confirmed by H NMR, IR, UV-vis
spectroscopies, fast atom bombardment
(FAB) mass spectroscopy and elemental
analysis. All the techniques employed
gave further confirmation of the predicted
structural configuration of the molecules as
detailed in the experimental section.
N
N
N
N
N
O
Cl
N
M
Cl
O
N
N
Structural and self-association characte-
rization using proton NMR
O
Cl
The efficient p–p stacking of disc-like
molecules such as phthalocyanines results
in aggregation, even in dilute solution,
into dimer, trimer and higher oligomers.
Such intrinsic behavior can considerably
affect their properties (e.g. opto-electronic
4a-8a
M = H2 (4a); Zn (5a); AlCl (6a); GaCl (7a); InCl (8a)
Scheme 1. Synthetic route to tetra-substituted 2,6-di-iso-propylphenoxy
phthalocyanines 4a–8a. Reagents and conditions: (a) anhydrous K2CO3, DMF,
45 °C, 24 h; (b) appropriate metal salt, quinoline, 180 °C, 12 h, inert atmosphere
properties) and of course limit their uses
in the intended applications such as in the
Copyright © 2012 World Scientific Publishing Company
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TETRA AND OCTA(2,6-DI-ISO-PROPYLPHENOXY)-SUBSTITUTED PHTHALOCYANINES
165
findings impart that the chlorine atom next
to the bulky phenoxy substituent in 3a is
big enough to introduce steric hindrance
which prevents rotation around the aryl ether
OH
Cl
Cl
CN
CN
CN
CN
O
O
(a)
linkage and consequently forces the phenoxy
substituent out of the plane of the target
molecule, where the iso-propyl groups adopt a
configuration, leading to two different methyl
hydrogen environments. Thus, the resulting
conformational arrangement adopted by the
substituents in 3a would interestingly ensure
that cofacial self-association of the derived
Pc cores is prohibited. Variable-temperature
studies using NMR techniques showed that
the coalescence of the methyl hydrogen peaks
for 3b and 3a occurs at 308 K and 348 K,
respectively, corresponding to a free energy
of activation (DG*) of 65.8 and 75.8 KJ.mol-1,
respectively (Figs 1 and 2 for 348 K, Table 1).
This energy barrier difference between
the phthalonitriles under investigation can be
attributed to the greater steric effect imposed
by the chlorine atom as a result of interaction
with the mono phenoxy substituent in 3a,
in comparison with the effect of the second
phenoxy substituent containing 3b which
provides more freedom of movement for the
conformational processes required for methyl
interchange.
+
1
2
3b
(b)
O
O
N
N
N
N
N
O
O
O
O
N
M
N
N
O
O
The intrinsic tendency of phthalocyanine
moleculestowardaggregationcanusuallyresult
in a poor quality NMR spectrum characterized
by the broadening and up field shift of proton
peaks [29, 35]. Therefore, we use the NMR
technique along with UV-vis spectroscopy
to evaluate the self-association behavior of
4b-8b
M = H2 (4b); Zn (5b); AlCl (6b); GaCl (7b); InCl (8b)
Scheme 2. Synthetic route to octa-substituted 2,6-di-iso-propylphenoxy
phthalocyanine 4b–8b. Reagents and conditions: (a) anhydrous K2CO3,
DMF, 80 °C, 24 h; (b) appropriate metal salt, quinoline, 180 °C, 12 h, inert
atmosphere
NLO and PDT fields [9, 15, 27–30]. UV-vis [31–33]
and NMR [34] spectroscopic techniques were, therefore,
used to evaluate the aggregation behavior of the target
complexes.
1
It was noted previously that the H NMR analyses of
the disubstituted phthalonitrile 3b demonstrated that the
phenyl rings bearing two iso-propyl groups are forced out
of the plane of the benzene-containing dicarbonitriles due
to the bulkiness exerted by the presence of two isopropyl
groups [21]. This informative conformation was clearly
1
detected by the H NMR spectrum which displays a
doublet of doublets for the methyl groups at 298 K due
to the frozen rotation about the aryl-oxygen bonds on the
NMR time-scale as shown in Fig. 1. Surprisingly, the
1H NMR spectra of the mono-substituted phthalonitrile
3a at 298 K shows the same conformation adapted by
the phenoxy groups in the derived phthalonitrile 3b
and correspondingly displays two environments for
the methyl hydrogens of the iso-propyl groups. Such
1
Fig. 1. The H NMR spectra of the aliphatic region (methyl
groups of the di-iso-propyl units) of 4-chloro-5-(2,6-di-iso-
propylphenoxy) phthalonitrile (3a) at different temperatures in
DMSO-d6 (a) 298 K and (b) 348 K
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166
A. TUHL ET AL.
signal patterns, including the absence of both broadening
and chemical shifts of the prepared complexes, were
essentially independent of the concentration and the
appearance of the spectra remained unchanged over the
concentration range between 1 × 10-4 and 1 × 10-2 M at
room temperature [35]. Such behavior can be attributed
to the position adopted by the bulky phenoxy substituents
relative to the Pc core that successfully prevents efficient
p–p stacking of the macrocycle units.
Ground state electronic absorption and fluorescence
spectra
Figures 3 and 4 show the ground state electronic
absorption spectra pertaining to the metal free, aluminum,
zinc, indium and gallium tetra-di-iso-propylphenoxy
(4a–8a) and octa-di-iso-propylphenoxy (4b–8b)
phthalocyanines, respectively in DMF. All spectra
remain monomeric up to 2 × 10-5 M (Beer–Lambert law
was obeyed below these concentrations) and exhibit
sharp bands typical of non-aggregated species. The
presence of axial ligands associated with the aluminum,
indium and gallium central metal atoms, is particularly
useful in preventing aggregation. Axial ligands in this
position restrict the stacking interaction of multiple MPc
molecules, often reflected by broad and blue shifted
absorption bands in the UV-vis spectrum [36–38].
1
Fig. 2. The H NMR spectra of the aliphatic region (methyl
groups of the di-iso-propyl units) of 4,5-bis-(2,6-di-iso-
propylphenoxy) phthalonitrile (3b) at different temperatures in
CDCl3 (a) 273 K and (b) 308 K
Table 1. Activation energy calculations of 4-chloro-5-(2,6-
di-iso-propylphenoxy)phthalonitrile (3a) in comparison with
the 4,5-bis-(2,6-di-iso-propylphenoxy)phthalonitrile (3b)
Materials
Tc, K
Du, Hz
DG*, kJ.mole
3a
3b
348
308
31.8
45.6
75.8
65.8
the prepared complexes. The NMR spectra for 4a–8a
the prepared complexes recorded in CDCl3 were almost
identical and show complicated and overlapped resonant
peaks which were difficult to assign due to the mixture of
isomers characteristic of the prepared Pcs. Nonetheless,
integration of the peaks correctly corresponded to the
expected total number of protons for each complex, thus
confirming the relative purity of the complexes which
is supported by the elemental and mass spectrometry
analysis. For example, the NMR spectrum of 5a consists
of two distinct aliphatic and one aromatic region: several
overlapped doublet peaks at e: 1.10–1.60 ppm belonging
to the methyl protons, four septet peaks in the range of e:
3.21–4.21 ppm attributed to the methylene protons and
complicated multiplet peaks at e: 7.39–9.05 ppm related
to the aromatic protons. The qualities of 1H NMR spectra
of 4a–8a complexes remain unchanged even after the
Fig. 3. Normalized ground state electronic absorption spectra
of (i) 4a, (ii) 5a, (iii) 6a, (iv) 7a and (v) 8a in DMF
1
addition of trace amount of pyridine-D5 into the NMR
sample solution. Although, the overlapped aromatic
peaks are separated from each other due to the chemical
shift induced by pyridine-D5. In addition, all octa-
substituted derivatives (4b–8b) show very simple and
well resolved spectra with sharp peaks in both aromatic
and aliphatic regions due to their high symmetry and
the total absence of aggregation. Indeed, the defined
Fig. 4. Normalized ground state electronic absorption spectra
of (i) 4b, (ii) 5b, (iii) 6b, (iv) 7b and (v) 8b in DMF
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TETRA AND OCTA(2,6-DI-ISO-PROPYLPHENOXY)-SUBSTITUTED PHTHALOCYANINES
167
Complexes 5a, 6a and 8a show a slight red shift in
a decrease in electronegativity on going from Al to In,
which leads to higher p-electron density of the system
and therefore the lower energy gap between HOMO and
LUMO is consequently observed.
the Q-band maxima relative to the corresponding octa-
substituted complexes 5b, 6b and 8b proving that the
highest occupied molecular orbital (HOMO) to lowest
unoccupied molecular orbital (LUMO) energy gap is
reduced upon the introduction of chlorine atoms [24,
25, 39]. As anticipated, the electronic spectra of the
tetra and octa (2,6-di-iso-propylphenoxy)-substituted
phthalocyanine complexes showed monomeric behavior
in solution using different organic solvents (CH2Cl2,
CHCl3, THF and DMF) as confirmed by the position and
the appearance of intense Q-band peaks. The spectrum
in less coordinating solvents such as chloroform was
similar to that in DMF (Fig. 3) which is typical of
non-aggregated species belonging to D4h symmetry.
However, the monomeric Q-band position of 5b is blue
shifted relative to that of 5a proving that the HOMO-
LUMO energy gap is reduced upon the introduction
of chlorine atoms. This marginal red shift, caused by
chlorine atoms, is consistent with the concept that the
chlorine atoms are participating in the p-electron system
of the Pc complexes and act as electron donor substituents
by mesomeric effects which may provoke noticeable
changes in photoelectric behavior [22, 23, 40].
Similarly, a shift to longer wavelengths with an
increase in central metal size is due to an increase in
electron density, where the order of Q-band positions
is Zn2+ < Al3+ < Ga3+ < In3+ (Table 2) for the 4a–8a
complexes. For the octa-substituted complexes the trend
is the same as for the 4a–8a complexes, except 7b and
8b have the same wavelength. The notable red shift for
InPc derivatives may be attributed either to distortion
of the macrocycle in which the indium (i.e. the largest
size among the transition metals used in this study) is
out of the p-planar system creating a non-planar system
or weakness of the coordination M–N bond [41], due to
With the exception of InPc and GaPc derivatives,
all the molecules showed fluorescence emission and
excitation spectra typical of phthalocyanines, where the
excitation is similar to absorption and both are mirror
images of the fluorescence emission, Fig. 5 for 6a in
DMF. The behavior of InPc and GaPc derivatives could
be related to their size which may result in changes in
symmetry upon excitation. A flexible s-bond connecting
the phenoxy ring to the local MPc ring may allow for
photoinduced twisting which distorts the MPc molecule.
Stokes shifts were typical of MPc derivatives [11].
Photophysicochemical parameters
Fluorescence quantum yields and lifetimes.
Fluorescence quantum yields reflect the fraction of
absorbing molecules, excited to the singlet state and
subsequently deactivated via radiative means. Thus
the quantum yield is a ratio of the number of photons
emitted relative to the number of photons absorbed [42].
Variations in fluorescence quantum yield values have
been explained by a number of prevailing conditions
including temperature, molecular structure and solvent
parameters (polarity, viscosity, and refractive index). In
this case, considering molecules with the same number
of substituents, there is a trend towards lower quantum
yields with increase in size of central metal for the 4a–8a
species i.e. Zn(5a) > Ga(7a) > In(8a) (Table 3) and
similarly for the octa-di-iso-propyl-substituted species.
Such behavior is expected on the grounds of spin-orbit
coupling induced by the respective central metal ions.
In3+ being the largest central metal ion induces a greater
Table 2. Absorption, fluorescence emission and fluorescence excitation spectral data for MPc complexes 4a–8a and 4b–8b and
ZnPc in DMF
MPc
Q-band lmax, nm
Log e
Emission lmax, nm
Excitationa lmax, nm
Stoke’s Shift, nm
ZnPc
4a
669
671, 700
680
5.37
5.21
5.35
685
707
697
669
671, 700
677
16
7
5a
20
6a
7a
8a
683
688
693
5.29
5.27
5.21
696
702
712
683
692
704
13
10
8
4b
5b
667, 701
675
5.30
5.57
709
693
667, 703
676
6
17
6b
679
5.48
697
679
8
7b
8b
690
690
5.43
5.38
710
713
703
705
7
8
a Excitation at 580 nm for MPc complexes.
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168
A. TUHL ET AL.
decay with mono-exponential behavior with fluorescence
lifetimes that range between 1–10 ns [43–45]. All phtha-
locyanines, in this study, exhibit mono-exponential decay
kinetic behavior typical of monomeric phthalocyanine
species as shown by a typical fluorescence decay curve for
7b in Fig. 6. The data in Table 3 shows minimal variation
with respect to the number of substituents; however a
change in central metal is more significant. A similar effect
to the fluorescence quantum yields is observed for the most
part for fluorescence lifetimes, where the presence of large
central metals such as Ga and In experience shortened
lifetimes as a consequence of the heavy atom mediated
effect. This has also been reported previously for Ga and In
derivatives [15, 16]. As with fluorescence quantum yields,
the aluminum derivatives 6a and 6b deviate from typical
behavior to give fluorescence lifetimes higher than those
of a corresponding Pc substituted with larger central metal
atoms i.e. Zn2+, Ga3+ and In3+. Aluminum Pcs generally
tend to exhibit elevated fluorescence yields [41, 44, 46–48]
due to the small size of Al.
Triplet quantum yields and lifetimes. The spin
forbidden process of intersystem crossing to populate
the triplet-excited state is a result of spin-orbit coupling.
Phthalocyanine species that encounter the strongest spin-
orbit coupling effects are often accompanied by high
triplet quantum yields (FT) which give a measure of the
fraction of absorbing molecules that undergo intersystem
crossing to the triplet excited state. The FT and triplet
lifetimes (tT) pertaining to the molecules explored in this
work are shown in Table 3. The order of variance, with
respect to the octa-substituted derivatives, is consistent
with the strengths of induced spin-orbit coupling in
the complexes, i.e. species metalated with In3+ show the
highest FT values with the lowest data occurring for the
Zn species, although aluminum was expected to give
lower values due to its size. For the 4a–8a complexes
Fig. 5. Normalized ground state electronic absorption (i),
fluorescence emission (ii) and fluorescence excitation (iii) of
6a in DMF
effect, in terms of spin-orbit coupling, thereby increasing
the likelihood of the spin-forbidden intersystem crossing
(ISC) process to populate the triplet state and an attendant
decrease in the spin-allowed fluorescence. Aluminum
serves as an exception, due to its small size, with
fluorescence quantum yields higher than the Zn metalated
species (Table 3). The effect of introducing eight di-
iso-propylphenoxy substituents to the Pc ring periphery is
insignificant in terms of quantum yield values, relative to
the tetra 2,6-di-iso-propylphenoxy-substituted derivatives
(Table 3). However, 6b gives higher yields relative to the
tetra 2,6-di-iso-propylphenoxy-substituted derivative,
6a. This may be in relation to the presence of the chloro-
substituent groups in 6a which have a greater effect, in
this case, in terms of deactivation of molecules in the
excited singlet via non-radiative means such as ISC rather
than by fluorescence, as a result of the heavy atom effect.
The fluorescence lifetime gives an indication of how
much time the molecule spends in the excited state prior to
relaxation back to the ground state. Metallophthalocyanines
Table 3. Photophysicochemical parameters of MPc complexes 4a–8a and 4b–8b and ZnPc in DMF
MPc
FF
FT
0.5854
FIC
FD
SD
tF, ns ( 0.04)
tT, ms
ZnPc
0.1754
0.12
0.25
0.05
0.56 [57]
0.52
0.97
0.63
3.14
3.69
330 [58]
17
4a
0.83 (700 nm)
0.81
5a
6a
7a
0.12
0.14
0.07
0.07
0.01
0.12
0.58
0.25
0.35
0.72
0.30
0.43
2.87
5.52
3.86
69
12
13
0.85
0.81
8a
4b
5b
0.02
0.12
0.12
0.77
0.21
0.01
0.56
0.56
0.53
0.26
0.73
0.56
0.81
2.17
5.52
3.00
19
10
15
0.87 (702 nm)
0.32
6b
0.24
0.40
0.64
0.36
0.90
5.88
17
7b
8b
0.09
0.01
0.51
0.67
0.40
0.32
0.45
0.42
0.88
0.63
3.53
2.76
6
13
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TETRA AND OCTA(2,6-DI-ISO-PROPYLPHENOXY)-SUBSTITUTED PHTHALOCYANINES
169
Fig. 7. Transient absorption spectrum of 7a in DMF
(concentration = 1.34 × 10-5 M)
Fig. 6. Fluorescence decay curve of 7b (red) and instrument
response function (IRF, black) in DMF
to ground state molecular oxygen (3Sg). The efficiency
with which singlet oxygen is generated is quantified by
the singlet oxygen quantum yield (FD) which varies as a
functionoftripletstatequantumyield, tripletstatelifetime,
efficiency of energy transfer (SD), triplet state quenching
effectofsubstituentsandthetripletstateenergy.Inessence,
large triplet state quantum yields should be accompanied
in turn by large singlet oxygen quantum yields as shown
in Table 3, where 4a–8a derivatives generally give larger
quantum yield values compared to their octa-substituted
counterparts. Compounds 6a and 7a are exceptions,
expressing much lower yields than expected based on
their triplet quantum yields. In these complexes, the
energy transfer process proves to be slightly less efficient
as expressed by SD which is a measure of excitation
energy transfer efficiency. However, values near unity,
as shown in Table 3 for the octa-substituted derivatives,
there seems to be minimal effect with increase in size
of the central metal. Interestingly the presence of eight
2,6-di-iso-propylphenoxy ring substituents gave reduced
FT values suggesting that an increase in substituent groups
brings about a quenching effect. Such an effect may be
in relation to the “loose bolt effect” [49]. A similar effect
has been reported for InPc complexes substituted with
large phenoxy substituents [16]. The effect is associated
with the loss of energy, through internal conversion, as a
result of C–H vibrations [49]. An increase in the number
of 2,6-di-iso-propylphenoxy groups, from four to eight
(tetra to octa), gives rise to an increase in C–H bonds
and concomitant increased rate of internal conversion
and thus a more pronounced “loose bolt” effect. Worthy
of note is the transient absorption spectra of 7a, as an
example, shown in Fig. 7. The figure indicates the
appearance of a broadened peak centered near 700 nm
whereas the absorption spectrum corresponding to this
complex shows a maximum at 688 nm (Table 2). Such
changes in optical spectra are often a result of symmetry
loss, often depicted by splitting or broadening of bands,
following photoexcitation [50].As stated above, a flexible
s-bond connecting the phenoxy ring to the local MPc ring
allows for photoinduced twisting which distorts the MPc
molecule and thus gives rise to the changes observed.
High values of FT for 4a–8a derivatives could also be
due to the presence of a halogen (chlorine atom).
1
reflect more efficient energy transfer to produce O2 as
opposed to MPc complexes modified by tetra-substitution
with 2,6-di-iso-propylphenoxy.
EXPERIMENTAL
Materials
All solvents including dimethylformamide (DMF),
dichloromethane (DCM), and tetrahydrofuran (THF)
were of reagent grade and purchased from Aldrich. Any
purification required was carried out according to Perrin
and Amarego [52].
Thin layer chromatography (TLC) was performed
using Polygram sil G/UV 254 TLC plates and
visualization was carried out by ultraviolet light at 254
nm and 350 nm. Column chromatography was performed
using Merck silica gel 60 of mesh size 0.040–0.063 mm.
2,6-di-iso-propylphenol (2), 4,5-dichlorophthalonitrile
(1), diphenylisobenzofuran (DPBF) and 1,8-diazabicyclo
[5.4.0]undec-7-ene (DBU) were purchased from esta-
blished suppliers (Sigma-Aldrich, Merck, TCI Europe).
All other reagents were purchased from suppliers and
used without further purification.
The triplet lifetimes determined for the complexes
conform to no particular trend ranging from 10–69 ms
(Table 3). Such low lifetimes in DMF are common for
phthalocyanines containing a large central metal such
as In, Ga and Cd, corresponding to high triplet state
quantum yields [15, 16, 50, and 51]. The reason for the
low lifetime for the AlPc derivatives is not clear, but
could be related to the substituents. Unsubstituted ZnPc
has a long lifetime compared to complexes 5a and 5b,
showing the effects of the substituents.
Singlet oxygen quantum yields. The generation of
singlet oxygen (1O2) is a result of the transfer of energy
from a phthalocyanine molecule in the triplet excited state
Copyright © 2012 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2012; 16: 169–174

170
A. TUHL ET AL.
Instrumentation
DMF (with ΦF = 0.17 [54]). Similarly triplet quantum
yield (ΦT) values were determined according to literature
[55] using ZnPc in DMF as a standard (ΦT = 0.56 [56].
Fluorescencelifetimeswereobtainedbydeconvolution
of the decay curves, obtained by time correlated single
photon counting (TCSPC), using the FluoFit Software
program (PicoQuant GmbH, Germany). Triplet lifetimes
(τT) were determined by exponential fitting of the kinetic
curves using OriginPro 7.5 software.
Quantum yields of internal conversion (ΦIC) were
obtained from Equation 1, which assumes that only
the three intrinsic processes fluorescence, intersystem
crossing and internal conversion; jointly deactivate the
excited singlet state of an MPc molecule.
IR spectra were recorded on a Perkin Elmer system
2000 FTIR. All absorption spectra were undertaken on
a Varian Cary 5 UV-vis spectrometer and fluorescence
emission and excitation spectra using a Varian Eclipse
spectrofluorimeter. 1H NMR spectra were recorded using
a Bruker DPX 400 or BrukerAvance II 600. Elemental
Analyses were carried out using a LECO Elemental
Analyzer CHNS 932. Mass analyses were obtained using
a VG Autospec-Q. DSC analyses were carried out on a
Shimadzu DSC-50. FAB mass spectra were obtained
using a Micro-Mass Tofspec 2E spectrometer.
Fluorescence lifetimes were measured using a time
correlatedsinglephotoncounting(TCSPC)setup(FluoTime
200, Picoquant GmbH). The excitation source was a diode
laser (LDH-P-670 with PDL 800-B, Picoquant GmbH, 670
nm,20MHzrepetitionrate,44pspulsewidth).Fluorescence
was detected under the magic angle with a peltier
cooled photomultiplier tube (PMT) (PMA-C 192-N-M,
Picoquant) 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 band. 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 ps. 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 was analyzed with the program
FluoFit (Picoquant GmbH). The support plane approach
was used to estimate the errors of the decay times [42].
A laser flash photolysis system was used for the
determination of triplet absorption and decay kinetics.
The excitation pulses were produced by a Quanta-Ray
Nd: YAG laser (1.5 J/9 ns), pumping a Lambda Physik
FL 3002 dye laser (Pyridin 1 in methanol). The analyzing
beam source was from a Thermo Oriel 66902 xenon arc
lamp, and a KratosLisProjekte MLIS-X3 photomultiplier
tube was used as the detector. Signals were recorded with
a two-channel 300 MHz digital real-time oscilloscope
(Tektronix TDS 3032C); the kinetic curves were averaged
over 128 laser pulses.
FIC = 1 - (FF + FT)
(1)
The fraction of the excited triplet state quenched by
ground state molecular oxygen S∆ was calculated using
Equation 2:
Φ∆
ΦT
S∆ =
(2)
Singlet oxygen quantum yields
Singlet oxygen quantum yield values (ΦD) values were
determined in air using the relative method with DPBF
acting as a singlet oxygen chemical quencher in DMF,
using Equation 3:
Std
abs
R.I
Φ∆ = ΦSTtd
(3)
RStd .Iabs
Std
where Φ is the singlet oxygen quantum yield for the
∆
Std
standard ZnPc (Φ = 0.56 in DMF [57]); R and RStd are
∆
the DPBF photobleaching rates in the presence of the
respective MPc and standard respectively; Iabs and I
are the rates of light absorption by the MPc and standard
respectively. The concentration of DPBF was lowered
to ~ 3 × 10-5 M for all solutions, to avoid chain reactions
[57]. DPBF degradation was monitored at ~417 nm.
Std
abs
Synthesis
Photo-irradiations for singlet oxygen determinations
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. The light intensity
was measured with a POWER MAX 5100 (Molelectron
detector incorporated) power meter and found to be
2.97 × 1016 photons.s-1.
4-chloro-5-(2,6-di-iso-propylphenoxy)phthalo-
nitrile (3a). To a stirred solution of 4,5-dichloro-
phthalonitrile (1, 1.97 g, 10 mmol) and 2,6-di-iso-
propylphenol (2, 1.8 g, 10 mmol) in dry DMF (150 mL)
finely ground anhydrous potassium carbonate (4.8 g, 35
mmol) was added. The reaction mixture was heated at
45 °C under nitrogen for 24 h. On cooling, the reaction
mixture was poured into acidified water. The resulting
precipitate was collected by filtration and washed with
distilled water, then air-dried. The crude product was
then recrystallized from methanol to give 3a. Yield 2.85
g (84%) as a white crystalline solid, mp 181 °C. Anal.
calcd. for C20H19ClN2O: C, 70.90; H, 5.65; N, 8.27%.
Found: C, 70.78; H, 5.58; N, 8.59. IR (KBr): n, cm-1
Photophysicochemical parameters
Fluorescence quantum yields (ΦF) were determined as
reported in literature [53] using ZnPc as a standard in
Copyright © 2012 World Scientific Publishing Company
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TETRA AND OCTA(2,6-DI-ISO-PROPYLPHENOXY)-SUBSTITUTED PHTHALOCYANINES
171
2233 (CN). 1H NMR (400 MHz, CDCl3, 25 °C): d, ppm
7.46–9.63 (m, 20H, Pc Ar-H and Ar′-H). MS (FAB): m/z
1417 (calcd. for [M]+ 1416.6608).
1.14 (d, 6H, 2-CH3), 1.22 (d, 6H, 2-CH3), 2.75 (sept,
2H, 2-CH3CHCH3 ), 6.76 (s, 1H, Pc Ar-H), 7.30 (d, 2H,
Ar-H), 7.37 (t, 1H, Ar-H), 7.91 (s, 1H, Pc Ar-H). MS
(EI): m/z 338 (calcd. for [M]+ 338.5998].
2,9,16,23-tetrachloro-3,10,17,24-tetra(2,6-di-iso-
propylphenoxy)phthalocyaninatogallium
chloride
(7a). Gallium chloride used as metal salt. Eluent used:
DCM-ethyl acetate (4:1) to give 7a. Yield 0.19 g (17.5%)
as a green powder, mp > 300 °C. Anal. calcd. for
C80H76Cl5N8O4Ga: C, 65.79; H, 5.25; N, 7.67%. Found:
C, 65.41; H, 5.36; N, 7.39. IR (KBr): n, cm-1 3963 (Ar-H),
1606 (C=N). UV-vis (DMF): λmax, nm (log e) 688 (5.27).
1H NMR (600 MHz, CDCl3, 25 °C): d, ppm 1.04–1.80
(m, 48H, 16-CH3), 3.21–4.25 (m, 8H, 4-CH3CHCH3),
7.43–9.71 (m, 20H, Pc Ar-H and Ar′-H). MS (FAB): m/z
1460 (calcd. for [M]+ 1459.5859).
2,9,16,23-tetrachloro-3,10,17,24-tetra(2,6-di-iso-
propylphenoxy)phthalocyanine (4a). A solution of
4-chloro-5-(2,6-di-iso-propylphenoxy)phthalonitrile (3a,
1.00 g, 2.95 mmol), and urea (0.35 g, 6 mmol), in dry
quinoline (10 mL) was heated at 180°C under nitrogen
for 12 h. On cooling the reaction mixture was poured
into 200 mL of stirred distilled water and neutralized
with hydrochloric acid. The resulting solid product
was collected by filtration, washed with water and
methanol. The crude product was purified by column
chromatography on silica (eluent DCM) to give 4a. Yield
0.28 g (28%) as a greenish-blue powder, mp > 300 °C.
Anal. calcd. for C80H78Cl4N8O4: C, 70.79 H, 5.79; N,
8.26%. Found: C, 71.18; H, 5.53; N, 7.95. IR (KBr):
2,9,16,23-tetrachloro-3,10,17,24-tetra(2,6-di-iso-
propylphenoxy)phthalocyaninatoindium
chloride
(8a). Indium chloride used as metal salt. Eluent used:
hexane-DCM (1:5) to give 8a. Yield 0.25 g (22.5%)
as a green powder, mp > 300 °C. Anal. calcd. for
C80H76Cl5N8O4In: C, 63.82; H, 5.09; N, 7.44%. Found:
C, 64.22; H, 5.27; N, 7.54. IR (KBr): n, cm-1 3964 (Ar-H),
1605 (C=N). UV-vis (DMF): λmax, nm (log e) 693 (5.21).
1H NMR (600 MHz, CDCl3, 25 °C): d, ppm 0.94–1.46
(m, 48H, 16-CH3), 3.02–3.39 (m, 8H, 4-CH3CHCH3),
7.42–9.65 (m, 20H, Pc Ar-H and Ar′-H). MS (FAB): m/z
1505 (calcd. for [M]+ 1504.6809).
n, cm-1 3964 (Ar-H), 1607 (C=N). UV-vis (DMF): λmax
,
1
nm (log e) 668 (5.13), 703 (5.21). H NMR (600 MHz,
CDCl3, 25 °C): d, ppm -0.53 (s, 2H, -NH), 1.15–1.37 (m,
48H, 16-CH3), 3.20–4.24 (m, 8H, 4-CH3CHCH3), 7.45–
9.51 (m, 20H, Pc Ar-H and Ar′-H). MS (FAB): m/z 1357
(calcd. for [M]+ 1356.4259).
A general procedure for synthesis of complexes 5a–8a
is as follows. A mixture of 3a (1.00 g, 2.95 mmol) and
excess of anhydrous metal salt (40 mg) in dry quinoline
(5 mL) and few drops of DBU was stirred at 180 °C under
nitrogen for 12 h. The reaction mixture was poured into
stirred distilled water (200 mL) on cooling, and the solid
product was collected by filtration, washed with water
and methanol. The crude product was purified by column
chromatography on silica using the appropriate eluent as
stated for each complex below.
4,5-bis-(2,6-di-iso-propylphenoxy)phthalonitrile
(3b) [20]. Dry potassium carbonate (30.00g, 217 mmol)
was added to a solution of 4,5-dichlorophthalonitrile
(1, 10.0 g, 50.7 mmol) and 2,6-di-iso-propylphenol (2,
22.25 g, 125.0 mmol) in dry dimethylformamide and were
reacted according to the procedure used for 3a to give the
crude compound which was purified by recrystallization
from methanol to give 3b. Yield 18.3 g (75%) as a white
solid, mp 161–163 °C. Anal. calcd. for C32H36N2O2: C,
79.95; H, 7.50; N, 5.83%. Found: C, 79.91; H, 7.60; N,
5.80. IR (KBr): n, cm-1 3082, 2228, 1590, 1500, 1342,
2,9,16,23-tetrachloro-3,10,17,24-tetra(2,6-di-iso-
propylphenoxy)phthalocyaninatozinc (5a). Zinc
acetate used as metal salt. Eluent used: hexane-DCM
(1:1) to give 5a. Yield 0.20 g (19%) as a greenish-blue
powder, mp > 300 °C. Anal. calcd. for C80H76Cl4N8O4 Zn:
C, 67.63; H, 5.39; N, 7.88%. Found: C, 67.37; H, 5.49;
N, 7.54. IR (KBr): n, cm-1 3962 (Ar-H), 1605 (C=N).
1
1096, 880, 795. H NMR (400 MHz; CDCl3): d, ppm
7.25 (m, 6H, Ar-H), 6.70 (s, 2H, Pc Ar-H), 2.88 (sept,
4H, J = 6.7 Hz, CH3CHCH3), 1.20 (d, 12H, J = 6.7 Hz,
CH3CHCH3), 1.10 (d, 12H, J = 6.7 Hz, CH3CHCH3). MS
(EI): m/z 480 (calcd. for [M]+ 480.2778).
1
UV-vis (DMF): λmax, nm (log e) 680 (5.35). H NMR
2,3,9,10,16,17,23,24-octa(2,6-di-iso-propylphe-
noxy)phthalocyaninato (4b) [20]. To a solution of
(600 MHz, CDCl3, 25 °C): d, ppm 1.10–1.60 (m, 48H,
16-CH3), 3.21–4.21 (m, 8H, 4-CH3CHCH3), 7.39–9.05
(m, 20H, Pc Ar-H and Ar′-H). MS (FAB): m/z 1417
(calcd. for [M]+ 1419.7903).
4,5-bis(2,6-di-iso-propylphenoxy)phthalonitrile
(3b,
0.20 g, 0.42 mmol), in refluxing anhydrous 1-pentanol
(3 mL) and under nitrogen, an excess of lithium was
added. The reaction mixture was refluxed for 5 h and
then cooled and acetic acid added (1 mL, 0.1 M). The
solvent was removed under reduced pressure to give the
crude product. Purification by column chromatography
on SiO2, eluting with DCM to give 4b. Yield 0.08 g
(40%) as a green solid, mp > 300 °C. Anal. calcd. for
C128H146N8O8: C, 79.88; H, 7.65; N, 5.82%. Found: C,
80.04; H, 7.70; N, 5.70. IR (film): n, cm-1 3576, 2962,
1439, 1328, 1265, 1184, 1093, 1017, 877, 754. UV-vis
(DMF): λmax, nm (log e) 701 (5.30). 1H NMR (400 MHz,
2,9,16,23-tetrachloro-3,10,17,24-tetra(2,6-di-iso-
propylphenoxy)phthalocyaninatoaluminum chloride
(6a). Aluminum chloride used as metal salt. Eluent
used: ethyl acetate-THF (4:1) to give 6a. Yield 0.29 g
(28%) as a green powder, mp > 300 °C. Anal. calcd. for
C80H76Cl5N8O4Al: C, 67.77; H, 5.40; N, 7.90%. Found: C,
67.45; H, 5.87; N, 7.65. IR (KBr): n, cm-1 3963 (Ar-H),
1607 (C=N). UV-vis (DMF): λmax, nm (log e) 683 (5.29).
1H NMR (600 MHz, CDCl3, 25 °C): d, ppm 1.07–1.45
(m, 48H, 16-CH3), 3.14–3.45 (m, 8H, 4-CH3CHCH3),
Copyright © 2012 World Scientific Publishing Company
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172
A. TUHL ET AL.
CDCl3): d, ppm 8.13 (br, 8H, Pc Ar-H), 7.50 (m, 24H,
Ar-H), 3.37 (sept, 16H, J = 6.5 Hz, CH3CHCH3), 1.28
(br m, 96H, CH3CHCH3), -0.84 (s, 2H, -NH). MS (FAB):
m/z 1925.71 (calcd. for [M]+ 1923.1272).
(DMF): λmax, nm (log e) 690 (5.38). 1H NMR (400 MHz,
CDCl3): d, ppm 8.17 (s, 8H, Pc Ar-H), 7.61 (t, 8H, J =
7.6 Hz, Ar-H), 7.52 (br s, 16H, Ar-H), 3.35 (br s, 16H,
CH3CHCH3), 1.28 (br m, 96H, CH3CHCH3). MS (FAB):
m/z 2072 (calcd. for [M]+ 2071.3822).
2,3,9,10,16,17,23,24-octa(2,6-di-iso-propylphe-
noxy)phthalocyaninatozinc (5b) [20]. A stirred solution
of 4,5-bis (2,6-di-iso-propylphenoxy)phthalonitrile (3b,
0.30 g, 0.6 mmol) and zinc(II) acetate (0.1 g, 0.6 mmol)
in quinoline (1 mL) was heated at 180 °C for 12 h under
nitrogen. The reaction mixture was cooled to room
temperature and poured into methanol (15 mL). The
crude product was purified by column chromatography
on SiO2, eluting with hexane-DCM (4:6) to give 5b.Yield
0.2 g (67%) as a green solid, mp > 300 °C. Anal. calcd.
for C128H144N8O8Zn: C, 77.59; H, 7.32; N, 5.66%. Found:
C, 77.68; H, 7.52; N, 5.52. IR (film): n, cm-1 2961, 1612,
1456, 1462, 1413, 1353, 1269, 1186, 1095, 1050, 904,
864, 799, 777, 755, 729. UV-vis (DMF): λmax, nm (log e)
CONCLUSION
Thesynthesisofnovelmetalfree,zinc,aluminum,gallium
and indium phthalocyanines terminated by four or eight
di-iso-propylphenoxy groups has been reported. Supporting
characterization techniques confirm the predicted structural
configuration of these molecules. It was found that the
steric factor produced by the peripheral substituents play a
major role in perfectly preventing, the common aggregation
1
behavior of Pc complexes as confirmed by H NMR and
UV-vis spectroscopic techniques. Consequently the effects
of substituents (i.e. phenoxy or chlorine atoms) or the
central metals on the physicochemical properties have
been investigated. Low fluorescence quantum yields were
accompanied by large triplet quantum yields, particularly
for the tetra-substituted species. Considering the central
metal atom, an increase in the size of the central metal
atom yielded a similar effect in response to the heavy atom
effect. Fluorescence lifetimes fall within the range typical
of phthalocyanines, with the aluminum species (6a and
6b) showing extended lifetimes in the S1 state relative to
the other Pc derivatives. All phthalocyanine derivatives in
this study generally produced singlet oxygen with relatively
good quantum yields. The photophysicochemical properties
determined thus suggest that these molecules may be
considered for either PDT or optical limiting applications.
1
675 (5.57). H NMR (400 MHz, CDCl3): d, ppm 8.17
(s, 8H, Pc Ar-H), 7.62 (t, 8H, J = 7.8 Hz, Ar-H), 7.5 (br
s, 16H, Ar-H), 3.46 (br s, 16H, CH3CHCH3), 1.30 (br m,
96H, CH3CHCH3). MS (FAB): m/z 1988 (calcd. for [M]+
1986.4915).
Complexes 6b–8b were synthesized and purified as
outlined for (5b).
2,3,9,10,16,17,23,24-octa(2,6-di-iso-propylphe-
noxy)phthalocyaninatoaluminum chloride (6b).
Aluminum(III) chloride used as metal salt. Eluent used:
hexane-DCM-ethyl acetate (4:4:1) to give 6b. Yield 0.12
g (20%) as a green solid, mp > 300 °C. Anal. calcd. for
C128H144ClN8O8Al: C, 77.48; H, 7.26; N, 5.65%]. Found:
C, 77.52; H, 7.45; N, 5.80. IR (film): n, cm-1 2962, 1621,
1463, 1414, 1354, 1330, 1272, 1185, 1093. UV-vis
(DMF): λmax, nm (log e) 691 (5.48). 1H NMR (400 MHz,
CDCl3): d, ppm 8.22 (s, 8H, Pc Ar-H), 7.61 (t, 8H, J =
7.8 Hz, Ar-H), 7.50 (br s, 16H, Ar-H), 3.43 (br s, 16H,
CH3CHCH3), 1.27 (br m, 96H, CH3CHCH3). MS (FAB):
m/z 1982 (calcd. for [M]+ 1983.5457).
2,3,9,10,16,17,23,24-octa(2,6-di-iso-propylphenoxy)
phthalocyaninatogallium chloride (7b). Gallium(III)
chloride used as metal salt. Eluent used: DCM to give
7b. Yield 0.04 g (40%) as a green solid, mp > 300 °C.
Anal. calcd. for C128H144ClGaN8O8: C, 75.85; H, 7.11; N,
5.53%. Found: C, 75.60; H, 7.98; N, 5.78. IR (film): n,
cm-1 2963, 1610, 1452, 1400, 1265, 1185, 1090. UV-vis
(DMF): λmax, nm (log e) 690 (5.43). 1H NMR (400 MHz,
CDCl3): d, ppm 8.19 (s, 8H, Pc Ar-H), 7.60 (t, 8H, J =
7.6 Hz, Ar-H), 7.48 (r s, 16H, Ar-H), 3.41 (br s, 16H,
CH3CHCH3), 1.33 (br m, 96H, CH3CHCH3). MS (FAB):
m/z 2025 (calcd. for [M]+ 2026.2872).
Acknowledgements
AT wishes to thank the College of Graduate Studies,
Kuwait University for a Ph.D. candidacy. The financial
support of ANALAB and SAF grants (Nos. GS 01/01,
GS 01/05 and GS 03/01), Kuwait University, and the
support by the Department of Science and Technology
(DST) and National Research Foundation (NRF), South
Africa through DST/NRF Chairs Initiative for Professor
of Medicinal Chemistry and Nanotechnology and Rhodes
University are gratefully acknowledged. WC thanks
Rhodes University for funding.
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