Synthesis, photophysical and photochemical properties of substituted zinc phthalocyanines

Article abstract of DOI:10.1039/b704345g

The synthesis, photophysical and photochemical properties of the 4-({3,4,5-tris-[2-(2-ethoxyethoxy)ethyloxy]benzyl}oxy) and 4-({3,4,5-tris-[2-(2- ethoxyethoxy)ethyloxy]benzyl}thio) zinc(ii) phthalocyanines are reported for the first time. The new compound

Full text of DOI:10.1039/b704345g

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PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis, photophysical and photochemical properties of substituted zinc
phthalocyanines†
Ilke Gu¨rol,a Mahmut Durmus¸,b,c Vefa Ahsena,b and Tebello Nyokong*c
Received 21st March 2007, Accepted 24th May 2007
First published as an Advance Article on the web 2nd July 2007
DOI: 10.1039/b704345g
The synthesis, photophysical and photochemical properties of the 4-({3,4,5-tris-[2-(2-ethoxyethoxy)-
ethyloxy]benzyl}oxy) and 4-({3,4,5-tris-[2-(2-ethoxyethoxy)ethyloxy]benzyl}thio) zinc(II)
phthalocyanines are reported for the first time. The new compounds have been characterized by
elemental analysis, IR, 1H and 13C NMR spectroscopy, electronic spectroscopy and mass spectra.
General trends are described for photodegradation, singlet oxygen, fluorescence and triplet excited state
quantum yields, and triplet state and fluorescence lifetimes of these compounds in dimethylsulfoxide
(DMSO). The fluorescence of the complexes was quenched by benzoquinone (BQ). The effects of the
substitution on the photophysical and photochemical parameters of the zinc(II) phthalocyanines (6, 7
and 8) are also reported. Photophysical and photochemical properties of phthalocyanine complexes are
very useful for PDT applications. The substituted Zn(II) phthalocyanines showed high triplet and
singlet oxygen quantum yields. High singlet oxygen quantum yields are very important for Type II
mechanism. Thus, these complexes show potential as Type II photosensitizers.
Addition of groups to the peripheral positions of MPc com-
plexes is known to influence the properties of the MPc to a
Introduction
large degree.18–22 For instance, the peripheral substituents increase
the distance between the planar macrocycle rings carrying the
p-electrons thereby making solvation easier. Solvents affect aggre-
gation in phthalocyanine complexes. Organic solvents are known
to reduce aggregation whereas aqueous media result in highly
aggregated complexes. However, many phthalocyanine complexes
remain aggregated even in non-aqueous solutions.23–25 Aromatic
solvents such as benzene or toluene are known to give narrow
Q bands for phthalocyanines whereas broadening is observed in
non-aromatic solvents.26
In this work, we report on the effects of substitutions on the
photophysical and photochemical parameters of zinc phthalocya-
nine derivatives in DMSO. Aggregation behaviour, photophysical
(triplet state lifetimes and quantum yields, and fluorescence life-
time and quantum yields) and photochemical (singlet oxygen and
photodegradation quantum yields) properties were investigated.
This work also explores the effects of ring substitutions on the
fluorescence quenching of zinc phthalocyanines by benzoquinone
(BQ) using the Stern–Volmer relationship. Since PDT activity is
mainly based on singlet oxygen, its production was determined by
the dye-sensitised photooxidation of 1,3-diphenylisobenzofuran
(DPBF), a specific scavenger of this toxic species.27 Studies of
the photostability of zinc phthalocyanines during photosensitized
reactions is also of immense importance.
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.1,2 There has been growing interest
in the use of phthalocyanines in a variety of new high technology
fields including semiconductor devices,3,4 Langmuir–Blodgett
films,5 electrochromic display devices,6 gas sensors,7 liquid
crystals,8 non-linear optics9 and various catalytic processes.10 The
attractive characteristics of phthalocyanines in these applications
arise from their great diversity, thermal and chemical stability,
redox versatility and intense colour.
Recently, phthalocyanine complexes have found applications
as photosensitizers in photodynamic therapy (PDT) since dia-
magnetic central metals, such as Zn or Mg enhance phototoxicity
of phthalocyanines.11–14 Zinc phthalocyanine complexes have
attracted much interest because of their appreciably long triplet
lifetimes.15–17 Such long lifetimes constitute a great advantage
since the number of diffusional encounters between the triplet
excited state and ground state molecular oxygen increases with
the lifetime of the excited state. The introduction of peripheral
and non-peripheral substituents onto the MPc ring is expected
to affect both the triplet quantum yield and lifetime, hence it is
desirable to carry out studies on the effects of substituents on these
parameters.
aTUBITAK-Marmara Research Center, Materials Institute, P.O. Box 21,
Gebze, 41470, Turkey
Experimental
bGebze Institute of Technology, Department of Chemistry, P.O. Box 141,
Gebze, 41400, Kocaeli, Turkey
Materials
cDepartment of Chemistry, Rhodes University, Grahamstown, 6140, South
Africa. E-mail: t.nyokong@ru.ac.za; Fax: +27 46 622 5109; Tel: +27 46
603 8260
† Electronic supplementary information (ESI) available: Experimental
details on equipment used. See DOI: 10.1039/b704345g
Quinoline, dimethylsulfoxide (DMSO), n-hexane, chloroform
(CHCl3), dichloromethane (DCM) were dried as described in
Perrin and Armarego28 before use. Zinc acetate, K2CO3, Na2CO3,
3782 | Dalton Trans., 2007, 3782–3791
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anhydrous Na2SO4, deuterated DMSO-d6, diethyleneglycolmono-
ethylether were purchased from commercial suppliers. Column
chromatography was performed on silica gel 60 (0.04–0.063 mm)
and Bio-Beads S-X1. {3,4,5-Tris(2-(2-ethoxyethoxy)ethyloxy]-
phenyl}methanol,29 4-nitrophthalonitrile (1),30 and 4,5-dichloro-
phthalonitrile (2)31 were synthesised and purified according to
literature procedures.
Photochemical parameters
Singlet oxygen quantum yields. Singlet oxygen (UD) quantum
yield determinations were carried out using the experimental
set-up described in ESI†.38,39 Typically, a 2 mL portion of the
respective substituted ZnPc (6, 7 and 8) solutions (absorbance
∼1 at the irradiation wavelength) containing the singlet oxygen
quencher was irradiated in the Q band region with the photo-
irradiation set-up described above. UD values were determined
in air using the relative method with 1,3-diphenylisobenzofuran
(DPBF) as a singlet oxygen chemical quencher in DMSO (eqn (5)):
Photophysical parameters
Fluorescence quantum yields and lifetimes. Fluorescence quan-
tum yields (UF) were determined by the comparative method
(eqn (1)),32,33
Std
RI
UD = UStd
(5)
abs
D
Std
R
Iabs
FAStdg2
FStdAg2Std
Std
UF = UStd
(1)
where UD is the singlet oxygen quantum yield for the standard
F
ZnPc (UD = 0.67 in DMSO,40 R and RStd are the DPBF
Std
photobleaching rates in the presence of the respective samples (6, 7
and 8) and standard, respectively, Iabs and IabsStd are the rates of light
absorption by the samples (6, 7 and 8) and standard, respectively.
The concentrations of DPBF in the solutions were calculated using
the determined value of log e = 4.36 at 417 nm (DPBF in dimethyl-
sulfoxide, DMSO). The light intensity used for UD determinations
was found to be 9.27 × 1015 photons s−1 cm−2. The error in the de-
termination of UD was ∼10% (determined from several UD values).
where F and FStd are the areas under the fluorescence curves of the
samples (6, 7 and 8) and the standard respectively. A and AStd are
the respective absorbances (which was ∼0.05 in all solvents used)
of the sample and standard at the excitation wavelengths and g
and gStd are the refractive indices of solvents used for the sample
and standard, respectively. Unsubstituted ZnPc in DMSO (UF =
0.18)34 was employed as the standard.
Natural radiative lifetimes were (s0) were determined using Pho-
tochemCAD Program35 which uses the Strickler–Berg equation,
and fluorescence lifetimes (sF) are given by eqn (2)
Photodegradation quantum yields. Photodegradation quan-
tum yields were determined using eqn (6),
sF
UF =
(2)
(C0 − Ct)VNA
s0
Ud =
(6)
IabsSt
Using the sF values, rate constants for fluorescence (kf), inter-
system crossing (kISC), internal conversion (kIC) and photodegra-
dation (kd) were estimated.
where C0 and Ct are the samples’ (6, 7 and 8) 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 (6, 7 and 8). A
light intensity of 3.09 × 1016 photons s−1 cm−2 was employed for
Ud determinations.
Triplet quantum yields (UT) and lifetimes (sT). The de-aerated
solutions of the respective substituted ZnPc complexes (6, 7 and
8) were introduced into a 1 cm pathlength spectrophotometric
cell and irradiated at the Q band maxima using the laser system
described in the electronic supplementary information (ESI).†
Triplet quantum yields (UT) were determined by a comparative
method using triplet decay,36 eqn (3)
Fluorescence quenching by benzoquinone (BQ). Fluorescence
quenching experiments on the substituted ZnPc (6, 7 and 8) were
carried out by the addition of different concentrations of BQ to
a fixed concentration of the complexes, and the concentrations
of BQ in the resulting mixtures were 0, 0.008, 0.016, 0.024, 0.032
and 0.040 mol dm−3. The fluorescence spectra of substituted ZnPc
(6, 7 and 8) at each BQ concentration were recorded, and the
changes in fluorescence intensity related to BQ concentration by
the Stern–Volmer (SV) equation41 (eqn (7)):
DASTampleeSTtd
DASTtdeSTample
USTample = USTtd
(3)
Sample
Std
where DAT
and DAT are the changes in the triplet state
absorbances of the samples (6, 7 and 8) and standard, respectively,
eT and eTStd, the triplet state extinction coefficients for the samples
Sample
Std
(6, 7 and 8) and standard, respectively. UT
and UT are the
I0
I
triplet quantum yields of the sample and standard respectively. The
= 1 + KSV[BQ]
(7)
Std
standard employed was zinc phthalocyanine (ZnPc) with UT
0.65 for ZnPc37 in DMSO.
=
where I0 and I are the fluorescence intensities of fluorophore
in the absence and presence of quencher, respectively, [BQ] is
the concentration of the quencher, and KSV is the Stern–Volmer
constant, and is the product of the bimolecular quenching constant
(kq) and the fluorescence lifetime sF, i.e.
Quantum yields of internal conversion (UIC) were obtained from
eqn (4), which assumes that only three processes (fluorescence,
intersystem crossing and internal conversion), jointly deactivate
the excited singlet state of substituted ZnPc complexes.
UIC = 1 − (UF + UT)
(4)
KSV = kqsF
(8)
Triplet lifetimes were determined by exponential fitting of the
kinetic curves using OriginPro 7.5 software.
The ratios I0/I were calculated and plotted against [BQ]
according to eqn (7), and KSV determined from the slope.
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Synthesis
{3,4,5-Tris[2-(2-ethoxyethoxy) ethyloxy]phenyl}methanethiol.
phenyl}methanol. The amounts of the reagents employed were:
3,4,5-tris-[2-(2-ethoxyethoxy)ethyloxy]phenyl}methanethiol (4 g,
7.68 mmol), 4-nitrophthalonitrile (1.33 g, 7.68 mmol) and anhy-
drous potassium carbonate (1.69 g, 12.26 mmol) in 10 mL dried
DMSO. Yield: 2.60 g (52%). Mp = 85 ◦C. IR [(KBr) mmax/cm−1]:
{3,4,5-Tris[2-(2-ethoxyethoxy)ethyloxy]phenyl}methanol (10 g,
20 mmol) was dissolved in concentrated HCl (37%, 5.82 mL),
and thiourea (1.54 g, 20 mmol) was added and then, the mixture
was refluxed for 24 h under stirring. To the resulting solution,
a solution of potassium hydroxide (3.6 g, 64 mmol) in 22 mL
H2O (degassed with argon) was added under argon atmosphere.
This mixture was refluxed and stirred for an additional 3 h,
under argon. The two-phase system which resulted was cooled
and separated, and the upper aqueous phase was acidified with
dilute HCl and extracted with dichloromethane (3 × 25 mL).
The dichloromethane layer was combined with the oily organic
layer from the reaction mixture, and this solution was dried over
Na2SO4, filtered and evaporated to give a thick liquid. This was
purified by column chromatography on aluminium oxide 90 active
neutral using n-hexane : CH2Cl2 = 5 : 1 as eluent. Yield: 1.5 g
(% 19). IR [(KBr) mmax/cm−1]: 3036 (ArCH), 2974–2870 (CH2),
=
3040 (ArCH), 2960–2860 (CH2), 2220 (C≡N), 1580 (C C), 1470,
1380, 1100 (C–O–C). 1H NMR (CDCl3): d, ppm 7.56–7.54 (1 H,
d, ArCH), 7.50 (1 H, s, ArCH), 7.40 (1 H, d, ArCH), 6.55 (2 H, s,
ArCH), 4.10 (8 H, m, ArOCH2), 3.79–3.44 (24 H, m, OCH2), 1.12
(9 H, m, CH3). 13C NMR (500 MHz, CDCl3): d, ppm 153.21 (Car),
146.80 (Car), 138.55 (ArCH), 133.52 (Car), 130.78 (Car), 130.50
(ArCH), 129.84 (ArCH), 116.43 (Car), 115.68 (C≡N), 115.30
(C≡N), 111.48 (Car), 108.57 (ArCH), 72.64–66.86 (OCH2), 37.33
(SCH2), 15.42 (CH3). Calc. for C33H46N2O9S (646.79): C 61.28; H
7.17; N 4.34; Found: C 61.09; H 7.11; N 4.20. MS (ES-MS), m/z:
Calc. 646.7; Found: 647.2 [M + H]+.
4-Chloro-5-({3,4,5-tris[2-(2-ethoxyethoxy)ethyloxy]benzyl}thio)
phthalonitrile (5). Synthesis and purification was as outlined
for 3 except 3,4,5-tris-[2-(2-ethoxyethoxy)ethyloxy]phenyl}metha-
nethiol was employed instead of 3,4,5-tris-[2-(2-ethoxyethoxy)-
ethyloxy]phenyl}methanol and 4,5-dichlorophthalonitrile (2) was
employed instead of 4-nitrophthalonitrile (1). The amounts
of the reagents employed were: 3,4,5-tris-[2-(2-ethoxyethoxy)-
ethyloxy]phenyl}methanethiol (5 g, 9.60 mmol), 4,5-dichloro-
phthalonitrile (0.94 g, 4.80 mmol) and anhydrous potassium
carbonate (4.8 g, 30.00 mmol) in 10 mL dried DMSO. Yield:
1,1 g (31%). Mp = 90 ◦C. IR [(KBr) mmax/cm−1]: 3036 (ArCH),
2558 (SH), 1589 (C C), 1113(C–O–C). 1H NMR (500 MHz,
=
CDCl3): d, ppm 6.56 (2 H, s, ArCH), 4.18(6 H, t, ArOCH2), 3.87
(6 H, t, ArOCH2CH2), 3.73 (2 H, d, ArCH2), 3.66–3.51 (18 H, m,
OCH2), 1.81(1 H, t, SH), 1.23 (9 H, t, CH3). 13C NMR (500 MHz,
CDCl3): d, ppm 152.63 (Car), 137.40 (Car), 136.49 (Car), 108.72
(ArCH), 72.58–61.21 (OCH2), 29.23 (ArCH2), 15.17 (CH3). Calc.
for C25H44O9S (520.68): C 57.67; H 8.52; Found: C 56.82; H 8.35.
MS (ES-MS), m/z: Calc. 520.6; Found: 521.8 [M + H]+.
4-({3,4,5-Tris-[2-(2-ethoxyethoxy)ethyloxy]benzyl}oxy) phthalo-
nitrile (3). The 3,4,5-tris-[2-(2-ethoxyethoxy)ethyloxy]phenyl}-
methanol (3 g, 5.94 mmol) was dissolved in dried dimethylsulfox-
ide (7.5 cm3) under argon atmosphere and 4-nitrophthalonitrile
(1.03 g, 5.95 mmol) was added. After stirring for 10 min, finely
ground anhydrous potassium carbonate (1.24 g, 8.94 mmol) was
added in portions during 2 h with efficient stirring. The reaction
mixture was stirred under argon atmosphere at room temperature
for 24 h, followed by evaporation of the solvent under reduced
pressure. Water (5 cm3) was added and the aqueous phase was ex-
tracted with dichloromethane (3 × 10 cm3). The combined extracts
were treated first with sodium carbonate solution (5%), then with
water and dried over anhydrous sodium sulfate. Dichloromethane
was removed under reduced procedure. Purification of this product
was accomplished by column chromatography with silica gel
(eluent : n-hexane–dichloromethane = 5 : 2). Yield: 1 g (27%).
Mp = 47 ◦C. IR [(KBr) mmax/cm−1]: 3036 (ArCH), 2980–2850
2970–2866 (CH2), 1596 (C C), 1120 (C–O–C). 1H NMR
=
(CDCl3): d, ppm 7.65 (1 H, s, ArCH), 7.45 (1 H, s, ArCH), 6.58
(2 H, s, ArCH), 4.10 (8 H, m, OCH2), 3.80–3.44 (24 H, m, OCH2),
1.15 (9 H, m, CH3). 13C NMR (CDCl3) 153.32 (ArCH), 146.02
(Car), 139.23 (Car), 136.18 (ArCH), 133.44 (Car), 129.44 (ArCH),
114.70 (Car), 112.11 (C≡N), 108.70 (ArCH), 72.65–66.88 (OCH2),
37.26 (ArCH2), 15.41 (CH3). Calc. for C33H45ClN2O9S (681.24):
C 58.18; H 6.66; N 4.11; Found: C 57.85; H 6.23; N 3.94. MS
(ES-MS), m/z: Calc. 681.2; Found: 682.2 [M + H]+.
4-({3,4,5-Tris-[2-(2-ethoxyethoxy)ethyloxy]benzyl}oxy) phthalo-
cyaninato zinc (6). A mixture of 3 (250 mg, 0.40 mmol),
anhydrous zinc(II) acetate (18.35 mg, 0.10 mmol), 2 mL quinoline
(freshly distilled over CaH2) were heated to reflux for 16 h under
argon atmosphere in a round-bottomed flask. The resulting green
suspension was cooled and the crude product was precipitated
by addition of n-hexane. The crude green product was purified by
column chromatography (Bio-Beads S-X1, CH2Cl2). Yield : 30 mg
=
(CH2), 2220 (C≡N), 1580 (C C), 1470, 1380, 1120 (C–O–C).
◦
1H NMR (CDCl3): d, ppm 7.78–7.76 (1 H, d, ArCH), 7.60 (1 H,
s, ArCH), 7.52 (1 H, d, ArCH), 6.50 (2 H, s, ArCH), 4.16 (8 H,
m, ArOCH2), 3.70–3.41 (24 H, m, OCH2), 1.05 (9 H, m, CH3).
13C NMR (500 MHz, CDCl3): d, ppm 164.69 (Car), 148.65 (Car),
135.49 (ArCH), 134.04 (Car), 132.50 (Car), 119.39 (ArCH), 117.77
(ArCH), 115.60 (Car), 114.85 (C≡N), 111.87 (Car), 107.08 (ArCH),
71.52–63.59 (OCH2), 15.19 (CH3). Calc. for C33H46N2O10 (630.73):
C 62.84; H 7.35; N 4.44; Found: C 62.09; H 7.15; N 4.31. MS (ES-
MS), m/z: Calc. 630.7; Found: 631.3 [M + H]+.
(12%). Mp = 175 C. IR [(KBr) mmax/cm−1] 3040 (ArCH), 2920–
1
=
2850 (CH2), 1590 (C C), 1480, 1340, 1140–1070 (C–O–C). H
NMR (500 MHz, DMSO-d6): d, ppm 7.85–7.60 (8 H, b, ArCH),
7.00–6.78 (4 H, b, ArCH), 6.50–6.48 (8 H, b, ArCH), 5.21 (8 H, s,
ArCH2O) 4.20–3.90 (120 H, m, OCH2), 1.05 (36 H, m, CH3). 13
C
NMR (500 MHz, DMSO-d6): d, ppm 152.01 (Car), 108.05 (ArCH),
73.66–72.18 (OCH2), 70.53–64.91 (OCH2), 15.12 (CH3). Calc. for
C132H184N8O40Zn (2588.3): C 61.25; H 7.17; N 4.33; Found C 61.05;
H 7.01; N 4.03. MS (ES-MS), m/z: Calc. 2588.3; Found: 2589.2
[M + H]+.
4-({3,4,5-Tris-[2-(2-ethoxyethoxy)ethyloxy]benzyl}thio) phthalo-
nitrile (4). Synthesis and purification was as outlined for 3 except
3,4,5-tris-[2-(2-ethoxyethoxy)ethyloxy]phenyl}methanethiol was
employed instead of 3,4,5-tris-[2-(2-ethoxyethoxy)ethyloxy]-
4-({3,4,5-Tris-[2-(2-ethoxyethoxy)ethyloxy]benzyl}thio) phthalo-
cyaninato zinc (7). Complex 7 was prepared and purified using
the same procedure described for preparation of 6 using complex
3784 | Dalton Trans., 2007, 3782–3791
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4 instead of 3. Amounts of reagents were: 4 (250 mg, 0.39 mmol),
anhydrous zinc(II) acetate (18.35 mg, 0.10 mmol), 2 mL quinoline.
Yield : 25 mg (10%). Mp = 200 ◦C. IR [(KBr) mmax/cm−1] 3040
elemental analysis, mass spectra of the new organic intermediate
compounds and phthalocyanines were obtained by the ES-MS
technique.
=
(ArCH), 2920–2850 (CH2), 1590 (C C), 1480, 1340, 1140–1070
The 1H and 13C NMR spectra of dinitrile (3, 4 and 5)
compounds were recorded in CDCl3, but DMSO-d6 was used for
Pcs compounds (6, 7 and 8). We were able to obtain good aliphatic
NMR signals for both 13C and 1H NMR spectroscopy. The 1H and
13C NMR spectra of 3, 4 and 5 compounds in CDCl3 confirmed
1
(C–O–C). H NMR (500 MHz, DMSO-d6): d, ppm 7.40–7.20
(8 H, b, ArCH), 6.85–6.78 (4 H, b, ArCH), 6.45–6.40 (8 H, b,
ArCH), 4.20–3.90 (120 H, m, OCH2), 1.04 (36 H, m, CH3). 13C
NMR (500 MHz, DMSO-d6): d, ppm 152.04 (Car), 106.35 (ArCH),
73.45–72.08 (OCH2), 70.53–64.91 (OCH2), 39.60 (SCH2), 15.52
(CH3). Calc. for C132H184N8O36S4Zn (2652.6): C, 59.77; H, 6.99; N,
4.22; Found C 58.95; H 6.36; N, 4.13. MS (ES-MS), m/z: Calc.
2652.6; Found: 2653.2 [M + H]+.
1
the proposed structure. The H NMR analysis of compounds 3,
4 and 5 in deuterated chloroform, showed methyl protons as a
triplet at 1.05 ppm. The aliphatic OCH2 protons occurred as
1
multiplets between 4.20 and 3.40 ppm. The H NMR spectrum
of the aromatic protons in phthalonitriles 3 and 4 appeared as a
doublet between 7.78 and 7.76 ppm (for 3) and between 7.56 and
7.54 ppm (for 4), another doublet was observed at 7.52 ppm (for
3) and 7.40 ppm (for 4), and a singlet at 7.60 ppm (for 3) and
7.50 ppm (for 4). In addition, the aromatic protons of {3,4,5-tri[2-
(2-ethoxyethoxy)ethyloxy]benzyl appeared as a singlet at 6.50 ppm
and 6.55 ppm for 3 and 4, respectively. On the other hand, the 1H
NMR spectrum of the other nitrile compound 5 shows as three
different singlets at 7.65 ppm, 7.45 ppm and 6.58 ppm as expected
for aromatic protons.
4-Chloro-5-({3,4,5-tris[2-(2-ethoxyethoxy)ethyloxy]benzyl}thio)
phthalocyaninato zinc (8). Complex 8 was prepared and purified
using the same procedure as described for preparation of 6 using
complex 5 instead of 3. Amounts of reagents were: 5 (500 mg,
0.73 mmol), anhydrous zinc(II) acetate (33.67 mg◦, 0.18 mmol),
2 mL quinoline. Yield : 130 mg (25%). Mp = 210 C. IR [(KBr)
−1
=
mmax/cm ] 3050 (ArCH), 2930–2850 (CH2), 1580 (C C), 1530,
1
1335, 1140–1070 (C–O–C). H NMR (500 MHz, DMSO-d6): d,
ppm 7.87 (4 H, b, ArCH), 7.31 (4 H, b, ArCH), 5.81 (8 H, s,
ArCH), 4.25 (32 H, m, OCH2), 3.96–3.47 (m, 96 H, OCH2), 1.04
(36 H, m, CH3). 13C NMR (500 MHz, DMSO-d6): d, ppm 152.2
(Car), 106.1 (ArCH), 71.60–66.21 (OCH2), 40.6–39.6 (S–CH2),
15.2 (CH3). Calc. for C132H180Cl4N8O36S4Zn (2790.4): C 56.82; H
6.50; N 4.02; Found C 56.80; H 6.39; N 3.98. MS(ES-MS), m/z:
Calc. 2790.4; Found: 2791.3 [M + H]+.
The 13C NMR spectrum of dinitrile (3), methyl carbon atoms
showed a signal at 15.2 ppm for CH3, the aliphatic OCH2 carbon
atoms showed 11 different signals between 63.5 ppm and 71.5 ppm.
The nitrile carbon atoms were observed at 114.8 ppm. For dinitrile
compound 4, methyl carbon atoms for CH3 are observed at
15.4 ppm, SCH2 group is observed at 37.3 ppm, while methyl
carbon atoms for CH2 showed 10 different signals between at
72.6 ppm and 66.8 ppm. The nitrile carbon atoms were observed
at 115.7 ppm and 115.3 ppm. The 13C NMR spectra of other
dinitrile 5, showed methyl carbon atoms at 15.4 ppm for CH3
carbon atoms. The aliphatic OCH2 carbon atoms appeared as
10 different signals between 72.6 and 66.8 ppm and the aliphatic
SCH2 carbon atom was observed at 37.2 ppm. The nitrile carbon
atoms were observed at 112.1 ppm. The mass spectra of 3, 4 and
5 exhibit [M + H]+ peaks at 631.3, 647.2 and 682.2 by the ES-MS
technique.
Results and discussion
Synthesis and characterization
The compound 3,4,5-tris[2-(2-ethoxyethoxy)ethyloxy]phenyl}-
methanol was converted into thiol compound using thiourea.
The synthesis of the phthalocyanines and their intermediate
phthalonitriles were performed by using procedures similar to
those described in the literature with some modification.31,42
Briefly, the mono-aryloxy/arylthia phthalonitriles (3 and 4) were
prepared by reaction of 3,4,5-tris-[2-(2-ethoxyethoxy)ethyloxy]-
phenyl}methanol and 3,4,5-tris-[2-(2-ethoxyethoxy)ethyloxy]-
phenyl}methanethiol with 4-nitrophthalonitrile in dry dimethyl-
sulfoxide (DMSO) as a solvent at room temperature in the presence
of solid K2CO3. The substituted phthalonitrile 5 was prepared
by reaction of 3,4,5-tris-[2-(2-ethoxyethoxy)ethyloxy]phenyl}-
methanethiol with 4,5-dichlorophthalonitrile under the same
conditions as for 3 and 4.
1
The H NMR spectra of 6, 7 and 8 have broad absorptions
when compared with that of phthalonitrile derivatives. It is
likely that broadness is due to both chemical exchange caused
by aggregation–disaggregation equilibrium and the fact that the
product obtained in this reaction is a mixture of four positional
isomers which are expected to show chemical shifts which
slightly differ from each other. The poly(oxyethylene) substituted
phthalocyanines were found to be pure by 1H NMR with all
the substituents and ring protons observed in their respective
regions. The phthalocyanine complexes (6, 7 and 8) showed the
phthalocyanine ring protons as unresolved multiplets (most likely
due to the presence of isomers). The phthalocyanine ring protons
were observed in the range 7.85 to 6.48 ppm for 6, 7.40 to 6.40 ppm
for 7 and 7.87 to 5.81 ppm for 8. The Ar-CH2 protons were
observed at 5.21 ppm for 6, 5.52 ppm for 7 and 5.81 ppm for 8 as
singlet. The other CH2 protons were observed between 4.20 and
3.90 ppm for 6, 4.20 and 3.90 ppm for 7 and 4.25 and 3.47 ppm for
8 as multiplets. The CH3 protons were observed at ∼1.04 ppm
for all phthalocyanine complexes as multiplets. Although the
presence of isomers as well as phthalocyanine aggregation at the
concentrations used for the NMR measurements may lead to
The functionalized phthalonitriles were converted into the
substituted Zn(II)Pc derivatives 6, 7 and 8, following a one pot
synthetic cyclotetramerization and metal insertion procedure. The
reactions were performed in dried quinoline in the presence of zinc
acetate at 180 ◦C for 16 h under an argon atmosphere (Scheme 1).
Column chromatography with Bio-Beads was employed to obtain
the pure products from reaction mixtures. These phthalocyanines
were very soluble in a number of solvents such as chloroform,
acetone, tetrahydrofuran (THF), DMSO, methanol and N,Nꢀ-
dimethylformamide (DMF).
The complexes could be thoroughly investigated by 1H and 13
NMR, IR and UV-vis spectroscopy in solution. In addition to
C
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Scheme 1 Synthesis route of substituted zinc(II) phthalocyanine complexes.
broadening of the aromatic signals, the observed spectra of all
the complexes were relatively well-resolved.
sharp peaks disappeared and the colour changed after conversion,
indicative of phthalocyanines formation.
Aromatic 13C NMR signals were observed at 152.0, 108.0, 73.6–
72.1, 70.5–67.2, 66.2, 64.9 and 15.1 ppm for 6, at 152.0, 106.3,
73.4–72.0, 70.5–68.1, 66.4, 64.9, 39.6 and 15.5 ppm for 7 and at
152.2, 106.1, 71.6–69.9, 66.2, 40.6–39.6 and 15.2 ppm for 8.
In the mass spectra of phthalocyanines obtained by the relatively
soft ES-MS technique, the molecular ion peaks were observed at
m/z 2589.2 for 6, 2653.2 for 7 and 2791.3 for 8. Another common
point in all phthalocyanines mass spectra is the clear appearance
of the small dimerization peaks.
Ground state electronic absorption and fluorescence spectra
The electronic spectra of the complexes showed characteristic
absorption in the Q band region at 696 nm for 6, 686 nm for
7 and 699 nm for 8 in DMSO, Table 1. The B band region was
observed around 355–365 nm in DMSO (Fig. 1). The spectra
showed monomeric behaviour evidenced by a single (narrow) Q
band, typical of metallated phthalocyanine complexes for 6, 7 and
8 in DMSO.43 The complexes were slightly aggregated in DMSO
(Fig. 1). The Q band positions of the substituted complexes (6,
7 and 8) are red-shifted relative to that of unsubstituted ZnPc,
Comparison of the IR spectra at each step gave some insights
on the nature of the products. The IR spectra of all the nitrile
compounds (C≡N) showed an absorption at 2230 cm−1. These
Table 1 Absorption, excitation and emission spectral data for unsubstituted (ZnPc), and substituted (6, 7 and 8) zinc phthalocyanine complexes in
DMSO
Compound
Q band kmax/nm
log e
Excitation kEx/nm
Emission kEm/nm
Stokes shift DStokes/nm
6
696
686
699
672
5.08
5.05
5.06
5.14
697
687
699
672
709
697
709
682
13
11
10
10
7
8
ZnPc
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Fig. 1 Absorption spectra of substituted zinc phthalocyanine complexes
(6, 7 and 8) in DMSO. Concentration = 4 × 10−6 mol dm−3
.
implying that the HOMO–LUMO energy gap of the phthalocya-
nine ring is reduced on introducing the substituents. The Q bands
of the tetrathiasubstituted complex (7) were blue-shifted by 10 nm
when compared to the corresponding tetraoxysubstituted complex
(6) and 13 nm when compared to the chlorotetrasubstituted
complex (8) in DMSO (Table 1). The blue shifting is surprising
since sulfur is more electron-donating than oxygen, the former
generally results in red shifting. The B-bands are broad due to
the superimposition of the B1 and B2 bands in the 355 to 365 nm
region.
Aggregation is usually depicted as a coplanar association of
rings progressing from monomer to dimer and higher order
complexes. It is dependent on the concentration, nature of the
solvent, nature of the substituents, complexed metal ions and
temperature.44 In the aggregated state the electronic structure of
the complexed phthalocyanine rings are perturbed resulting in
alternation of the ground and excited state electronic structures.45
Generally in MPc complexes, the peak due to aggregation is
blue shifted with respect to the monomer. In this study, the
aggregation behaviour of the phthalocyanine complexes (6, 7
and 8) are investigated at different solvents (DMSO, toluene,
dichloromethane (DCM), chloroform, THF, DMF and methanol
(MeOH)) (Fig. 2 for complex 8). While complex 8 did not show
aggregation in DMF, chloroform and THF, it showed only a small
broadness of the Q band in DCM, DMSO and toluene, suggesting
minimal aggregation. Toluene is a non-coordinating solvent, so
would not be axially ligated by the zinc central atom, thereby giving
room for p–p stacking between adjacent rings. In MeOH however,
all substituted complexes are aggregated. For instance, complex
8 has two non-vibrational bands at 643 and 676 nm (Fig. 2); the
higher energy band being assigned to the aggregate in MeOH. This
is typical of aggregation behaviour in MPc complexes.43 Addition
of Triton X-100, (a surfactant) to the MeOH solutions of the
substituted complexes (6, 7 and 8) resulted in disaggregation, as
evident from the sharp increase in the intensity of the monomer
peak and an attendant collapse of the aggregate peak (Fig. 3 for
complex 7) in MeOH. We suggest that the high aggregation in
MeOH is due to the H-bonds between-OH groups of the solvent
and polyoxy chain groups of the phthalocyanine complexes.
The aggregation behaviour of the phthalocyanine complexes
(6, 7 and 8) was also investigated at different concentrations in
DMSO (Fig. 4 for complex 7). In DMSO, as the concentration was
increased, the intensity of absorption of the Q band also increased
for all complexes (6, 7 and 8) (Fig. 4 for complex 7). Beer–Lambert
law was obeyed for all of these compounds in the concentrations
Fig. 2 Absorption spectra of the compound 8 in different solvents:
Concentration = 8 × 10−6 mol dm−3
.
Fig. 3 Absorption spectral changes for complex 7 observed on addition
of Triton X-100 in methanol.
Fig. 4 Aggregation behaviour of 7 in DMSO at different concentrations:
14 × 10−6 (A), 12 × 10−6 (B), 10 × 10−6 (C), 8 × 10−6 (D), 6 × 10−6 (E),
4 × 10−6 (F) mol dm−3
.
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ranging from 1.4 × 10−5 to 4 × 10−6 mol dm−3, showing that
the aggregation observed in Fig. 2 for DMSO solutions was
insignificant.
DMSO is minimal (as judged by the obeying of Beer’s law) hence
the Strickler–Berg equation still applies. Thus, we believe that
the values obtained using this equation are a good measure of
fluorescence lifetimes. sF values are lower, Table 3, for complexes
6, 7 and 8 when compared to unsubstituted ZnPc in DMSO,
suggesting that the substituents have a quenching effect. The
sF value for complex 8 is higher than complexes 6 and 7 in
DMSO. The natural radiative lifetime (s0) values of substituted
complexes (6, 7 and 8) are larger than for unsubstituted complex
(ZnPc) in DMSO. The s0 values among substituted complexes
decrease as follows: 8 > 7 > 6 > ZnPc, Table 3. The rate constants
for fluorescence (kF) values of the substituted complexes (6, 7 and
8) are lower compared to unsubstituted ZnPc complex. Lower
kF values were obtained for the chlorotetrasubstituted complex 8
compared to monotetrasubstituted (6 and 7) complexes. The kF
values among substituted complexes decreases as follows: ZnPc >
6 > 7 > 8, Table 3. The substituted complexes (6 and 8) showed
lower rate constant (kIC) and quantum yield (UIC) for internal
conversion in DMSO when compared to unsubstituted ZnPc,
Table 3 and Table 2, respectively, suggesting less quenching of the
excited singlet state by the substituents for the former complexes.
Complex 7 showed larger kIC when compared to unsubstituted
ZnPc. In addition complexes 6, 7 and 8 showed larger rate
constants for intersystem crossing (kISC) values when compared
to unsubstituted ZnPc in DMSO. The kISC values order among
substituted complexes is: 6 > 7 > 8 > ZnPc, Table 3.
All the complexes (6, 7 and 8) showed similar fluorescence
behaviour in DMSO. Fig. 5 shows the absorption, fluorescence
emission and excitation spectra for complexes (6, 7 and 8) in
DMSO (using complex 6 as an example in DMSO). Fluorescence
emission peaks were observed at: 709 nm for 6, 697 nm for 7 and
709 nm for 8 in DMSO (Table 1). The excitation spectra were
similar to absorption spectra and both were mirror images of
the fluorescent spectra in DMSO (Fig. 5). The proximity of the
wavelength of each component of the Q-band absorption to the Q
band maxima of the excitation spectra for all complexes suggests
that the nuclear configurations of the ground and excited states
are similar and not affected by excitation in DMSO. The observed
Stokes shifts were 13, 11 and 10 nm for complexes 6, 7 and 8,
respectively (Table 1) are typical of MPc complexes in DMSO.
Triplet lifetimes and quantum yields. The transient spectrum
for complex 6 in DMSO is shown in Fig. 6, and shows a maximum
at 530 nm, hence the triplet lifetimes and yields were determined
at this wavelength for complexes 6, 7 and 8. Fig. 7 shows the triplet
decay curves of the complexes (using complex 6 in DMSO as an
example).
Fig. 5 Absorption, excitation and emission spectra of the for compound
6 in DMSO. Excitation wavelength = 650 nm.
Photophysical and photochemical properties
Fluorescence lifetimes and quantum yields. The fluorescence
quantum yields (UF) of all complexes were similar, Table 2.
The UF values of the substituted complexes (6, 7 and 8) are
lower compared to unsubstituted ZnPc in DMSO. The low UF
value for substituted complexes (6, 7 and 8) in DMSO can be
ascribed to small aggregation tendencies of the molecules in this
solvent (Fig. 1). Aggregation reduces the likelihood of radiative
deactivation (fluorescence) through dissipation of energy by the
aggregates.
Lifetimes of fluorescence (sF) were calculated using the
Strickler–Berg equation.35 Using this equation, a good correla-
tion has been found33 for the experimentally and theoretically
determined fluorescence lifetimes for the unaggregated molecules
as is the case in this work. As stated above the aggregation in
Fig. 6 Transient differential spectrum of complex 6 in DMSO.
The triplet lifetime (sT) values were lower for complexes 6 and 8
when compared to unsubstituted ZnPc while longer for complex
7, Table 2. Lower sT value for complex 8 could be explained by the
heavy atom effect of the additional chlorine substituents which
would increase the quantum yield (see discussion below) of the
triplet state and inevitably lower the triplet life time. On this bases
we would expect the sT values for 7 to be shorter than for 6, this
is not the case in Table 2, suggesting that the heavy atom effect
between oxygen and sulfur is not significant in these complexes, as
shown also by blue shifting of the spectra (discussed above) of the
thiol derivative 7 compared to 6 and 8.
Table 2 Photophysical and photochemical parameters of unsubstituted
(ZnPc) and substituted (6, 7 and 8) zinc phthalocyanine complexes in
DMSO. Triplet absorption wavelength used = 510 nm for ZnPc and 530 nm
for 6–8
Compound
UF
sT/ls
UT
UIC
Ud × 10−5
UD
SD
6
0.10 280
0.08 370
0.10 240
0.18 350
0.87 0.03 0.22
0.67 0.25 1.63
0.89 0.01 0.26
0.65 0.17 2.61
0.73 0.84
0.65 0.97
0.80 0.90
0.67 1.03
7
8
ZnPc
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Table 3 Rate constants for various excited state deactivation processes of unsubstituted (ZnPc) and substituted (6, 7 and 8) zinc phthalocyanine
complexes in DMSO
Compound
Fluorescence lifetime sF/ns
Natural radiative lifetime s0/ns
akF × 108/s−1
bkISC × 108/s−1
ckIC × 107/s−1
dkd/s−1
6
0.81
0.78
1.16
1.22
8.13
9.82
11.56
6.80
1.23
1.02
0.86
1.47
10.74
8.59
7.67
5.32
3.70
32.02
0.86
0.007
0.044
0.010
0.074
7
8
ZnPc
13.9
a kF is the rate constant for fluorescence. Values calculated using kF = UF/sF. b kISC is the rate constant for intersystem crossing. Values calculated using
kISC = UT/sF. c
k
IC is the rate constant for internal conversion. Values calculated using kIC = UIC/sF. d kd is the rate constant for photodegradation. Values
calculated using kd = Ud/sT.
Fig. 7 Triplet decay curve of 6 in DMSO. Monitored at 530 nm.
Fig. 8 A typical spectrum for the determination of singlet oxygen
quantum yield. This determination was for compound 6 in DMSO at
The triplet quantum yields (UT) for substituted complexes (6,
7 and 8) in DMSO are higher when compared to unsubstituted
ZnPc in DMSO, though marginally so for 7. The higher values of
UT suggest efficient intersystem crossing (ISC) in the presence of
the substituents for substituted complexes (6, 7 and 8), resulting in
high UT values. Complex 8 showed larger UT value when compared
to 7 and marginally higher than 6 in DMSO (Table 2), due to the
heavy atom effect discussed above.
The kF, kIC and kISCvalues are lowest for complex 8 containing
chloro substituents. The kIC value was the highest for complex 7
containing SR substituents, suggesting higher loss of energy via
vibrational relaxation caused by the sulfur ligands when compared
to oxygen in complex 6.
a concentration of 3 × 10−5 mol dm−3
.
(Table 2), due to the higher UT value. The magnitude of the SD
(= UD/UT) represents the efficiency of quenching of the triplet
excited state by singlet oxygen. Complexes 6, 7 and 8 showed
SD of near unity (Table 2), suggesting efficient quenching of the
triplet state by triplet oxygen. Complex 6 showed lower SD when
compared to the other substituted complexes (7 and 8) in DMSO.
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 photocatalysts.
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 6, 7 and 8 during irradiation are as shown in Fig. 9
(using complex 7 as an example in DMSO) and hence confirm
photodegradation occurred without phototransformation.
Singlet oxygen quantum yields. Singlet oxygen quantum yields
(UD) were determined in DMSO using a chemical method and 1,3-
diphenylisobenzofuran (DPBF) as a quencher. The disappearance
of DPBF was monitored using a 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.
There was no change in the Q band intensity during the UD
determinations, Fig. 8 (using complex 6 as an example in DMSO),
confirming that complexes are not degraded during singlet oxygen
studies. The UD values of tetraoxysubstituted complex 6 and
chlorotetrasubstituted complex 8 were higher, and tetrathiasub-
stituted complex 7 was lower when compared to unsubstituted
ZnPc in DMSO. The UD values follow the same trend as the UT
values as expected. The UD value of the chlorotetrasubstituted
(8) was higher than other tetrasubstituted complexes (6 and 7)
Fig. 9 The photodegradation of compound 7 in DMSO showing the
disappearance of the Q-band at ten minutes intervals.
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Table 4 Fluorescence quenching data for unsubstituted (ZnPc) and
substituted (6, 7 and 8) zinc phthalocyanine complexes in DMSO
Table 2 shows that all substituted complexes (6, 7 and 8) were
more stable to degradation compared to unsubstituted ZnPc
in DMSO. Thus the substitution of ZnPc seems to increase
the stability of the complexes in DMSO. Complexes 6 and 8
showed the same stability whereas complex 7 was less stable when
compared to other substituted complexes. The rate constants for
photodegradation (kd) of substituted complexes (6, 7 and 8) were
lower than unsubstituted ZnPc in DMSO (Table 3).
Compound
K
SV/M−1
kq/1010 dm3 mol−1 s−1
6
16.09
13.87
12.37
31.90
1.98
1.77
1.06
2.61
7
8
ZnPc
Fluorescence quenching studies by benzoquinone [BQ]. In the
presence of BQ quencher, a reaction takes place between the
excited zinc phthalocyanine complexes and the BQ molecules.
The fluorescence quenching of zinc phthalocyanine complexes
by benzoquinone (BQ) in DMSO was found to obey Stern–
Volmer kinetics, which is consistent with diffusion-controlled
bimolecular reactions. Fig. 10 shows the quenching of complex
6 by BQ in DMSO as an example. The slope of the plots
shown at Fig. 11 gave KSV values. The KSV and bimolecular
quenching constant (kq) values for the BQ quenching of zinc
phthalocyanine complexes in DMSO are listed in Table 4. The
KSV values of the substituted zinc phthalocyanine complexes (6, 7
and 8) are lower than unsubstituted ZnPc in DMSO. The KSV
values among the substituted complexes decreased as follows:
6 > 7 > 8 in DMSO. The bimolecular quenching constant (kq)
values of the substituted zinc phthalocyanine complexes (6, 7
and 8) also lower than for unsubstituted ZnPc in DMSO. The
bimolecular quenching rate constants were found to be close to the
diffusion-controlled limits, ∼1010 M−1 s−1. The kq values among the
substituted complexes decreased as follows: 6 > 7 > 8 in DMSO.
Conclusions
In conclusion, we have prepared and characterized new 3,4,5-tris-
[2-(2-ethoxyethoxy)ethyloxy]phenyl}methanol and 3,4,5-tris-[2-
(2-ethoxyethoxy)ethyloxy]phenyl}methanethiol substituted zinc
phthalocyanines for the first time. In solution, the spectra showed
generally monomeric behaviour evidenced by a single (narrow) Q
band, typical of metallated phthalocyanine complexes for 6, 7 and
8. The complexes showed small broad band around 650 nm due
to a small aggregation in DMSO, which did not affect Beer’s law
behaviour. The substituted complexes showed similar and typical
fluorescence behaviour in DMSO. The fluorescence quantum
yields of substituted complexes were also similar. Although triplet
quantum yield values of substituted complexes were higher than
unsubstituted ZnPc, the triplet lifetime values were lower when
compared to unsubstituted ZnPc except for complex 7. The sub-
stituted complexes (6, 7 and 8) have good singlet oxygen quantum
yields especially chlorotetrasubstituted complex (8) which has the
highest value. 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 0.65 to 0.80. Thus, these complexes show
potential as Type II photosensitizers. The substitution of ZnPc
with 3,4,5-tris-[2-(2-ethoxyethoxy)ethyloxy]phenyl}methanol
and 3,4,5-tris-[2-(2-ethoxyethoxy)ethyloxy]phenyl}methanethiol
groups seem to increase the photostability of the complexes in
DMSO. The stability order among the substituted complexes
was 6 > 8 > 7 in DMSO. The substituted complexes (6, 7
and 8) showed lower Ksv and kq values when compared to the
unsubstituted ZnPc in DMSO.
Fig. 10 Fluorescence emission spectral changes of
6
(1.50
×
10−5 mol dm−3) on addition of different concentrations of BQ in DMSO.
[BQ] = 0, 0.008, 0.016, 0.024, 0.032, 0.040 mol dm−3
.
Acknowledgements
This work was supported by the National Research Foundation
of South Africa (NRF GUN # 2053657) as well as Rhodes
University.
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3790 | Dalton Trans., 2007, 3782–3791
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The Royal Society of Chemistry 2007
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