Synthesis and photophysicochemical properties of novel zinc phthalocyanines mono substituted with carboxyl containing functional groups

Article abstract of DOI:10.1016/j.jphotochem.2012.09.007

This work reports on the synthesis and the physicochemical properties of novel unsymmetrically substituted zinc phthalocyanines (complexes 8, 10 and 11) and their symmetrically substituted counterparts (complexes 9, 12 and 13). The new complexes and their

Full text of DOI:10.1016/j.jphotochem.2012.09.007

Journal of Photochemistry and Photobiology A: Chemistry 250 (2012) 18–24
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Synthesis and photophysicochemical properties of novel zinc phthalocyanines
mono substituted with carboxyl containing functional groups
Nomasonto Rapulenyane, Edith Antunes∗, Nkosiphile Masilela, Tebello Nyokong
Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 July 2012
Received in revised form
10 September 2012
Accepted 20 September 2012
Available online 29 September 2012
This work reports on the synthesis and the physicochemical properties of novel unsymmetrically substi-
tuted zinc phthalocyanines (complexes 8, 10 and 11) and their symmetrically substituted counterparts
(complexes 9, 12 and 13). The new complexes and their counterparts were successfully structurally char-
acterized by IR, NMR, mass spectral and elemental analyses. Low fluorescence quantum yields (0.032) and
lifetimes (0.91 ns) were obtained for the symmetrical ZnOTPc (complex 9) compared to the higher fluo-
rescence quantum yields (0.15, 0.13, 0.10, 0.09 and 0.29) and lifetimes (4.4, 1.69, 1.69, 2.16 and 3.23 ns)
obtained for ZnMPCPc (8), ZnTDTPc (12), ZnMCapPc (10), ZnMCafPc (11) and ZnTCPPc (13), respectively.
All the complexes showed the ability to produce singlet oxygen with the highest triplet quantum yields
obtained for 8 and 10 (0.80 and 0.65 respectively). High triplet lifetimes (109–286 ␮s) were obtained for
all complexes. Complex 8 showed the longest triplet and fluorescence lifetimes as well the largest triplet
state and singlet oxygen quantum yields.
Keywords:
Zinc phthalocyanine
Fluorescence
Triplet yield
Singlet oxygen
Photodegradation
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
This work reports on the synthesis of zinc phthalocyanines
which are asymmetrically substituted at a peripheral position
Phthalocyanines (Pcs) have drawn a great deal of attention
due to their unique chemical and physical properties and conse-
quently they have been extensively explored for their potential use
in a number of applications ranging from medicinal, industrial to
environmental [1–3]. Pcs have in particular attracted attention in
photodynamic therapy (PDT), a cancer treatment using a photo-
sensitizer (e.g. Pcs), laser light and oxygen to kill cancerous cells.
Phthalocyanines have been chosen for this particular application
because they meet the requirements of the photosensitizer; they
are non-toxic in the absence of light, locate mainly in the cancerous
cells and clear from the body quickly after therapy [4,5]. Phthalo-
cyanines also demonstrate a strong absorption of red light allowing
more effective light penetration into tissues [6–8]. The application
for PDT calls for specific phthalocyanine structures with distinct
physical and chemical properties hence immense research has gone
into the preparation and incorporation of substituents that improve
such properties. One of which includes the synthesis of phthalocya-
nine derivatives with different substituents on the ring. The use of
Pc substituents containing one carboxyl group allows for the possi-
bility of forming a covalent linkage to biological molecules, which
are specific to cancer, through an amide bond.
with one phenyl carboxy (8), captopril (10) and caffeic acid (11)
substituent and their symmetrically substituted derivatives: zinc
octaoctylthiophthalocyaninane (ZnOTPc, complex 9) zinc tetra
diethylaminoethanethio phthalocyanine (ZnTDTPc, 12) and zinc
tetra carboxy phenoxy phthalocyanine (13), Schemes 1–3. Periph-
eral substitution of phthalocyanines with caffeic acid or captopril
is reported here for the first time. The syntheses of complexes 9 [9],
12 [10] and 13 [11] have been reported before. Caffeic acid is a phe-
nolic antioxidant which inhibits lipoprotein oxidation induced by
peroxyl radicals. It is known to protect against phthalocyanine sen-
sitized photohemolysis of human erythrocytes [12]. On the other
hand captopril is an angiotensin converting enzyme (ACE) inhibitor
used for the treatment of hypertension and some types of conges-
tive heart failure [13]. Captopril is a photodamage mitigating agent
hence it may protect phthalocyanine photosensitizers during PDT.
The sulfur groups aid in the red shifting of the Q band which is
desired for application in PDT. The photophysical and photochemi-
cal properties of these complexes are also investigated in this work
as potential photosensitizers for PDT.
2. Experimental
2.1. Materials
∗
Captopril, caffeic acid, 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU), diphenylisobenzofuran (DPBF), 1-pentanol and zinc
Corresponding author. Tel.: +27 46 6038801; fax: +27 46 6225109.
E-mail address: e.antunes@ru.ac.za (E. Antunes).
1010-6030/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotochem.2012.09.007

N. Rapulenyane et al. / Journal of Photochemistry and Photobiology A: Chemistry 250 (2012) 18–24
19
Scheme 1. Synthesis of low symmetry monocarboxy phenoxy zinc phthalocyanine (complex 8) and the molecular structures of octaoctylthiophthalocyaninato zinc (complex
9) and tetracarboxyphenoxy ZnPc (complex 13).
acetate dehydrate were purchased from Sigma–Aldrich. Potas-
sium carbonate, quinoline, dimethyl formamide (DMF), dimethyl
sulfoxide (DMSO) and tetrahydrofuran (THF) were purchased
from SAARCHEM. 3,6-Di(octylthio)-4,5-dicyanobenzene (1)
[14], 4-(3,4-dicyanophenoxy)benzoic acid (2) [15] and 1,2-
bis(diethylaminoethanelthiol) (5) [10] were synthesized as
reported in literature.
absorption spectra were performed on a Shimadzu UV-2550 spec-
trophotometer. 1H nuclear magnetic resonance signals (1H NMR)
were recorded on a Bruker AMX 400 MHz NMR spectrometer. Ele-
mental analysis was done using a Vario-Elementar Microcube ELIII.
Steady state fluorescence excitation and emission spectra were
recorded on a Varian Eclipse spectrofluorimeter. Mass spectral data
were collected with a Bruker AutoFLEX III Smartbeam TOF/TOF
Mass spectrometer. The instrument was operated in positive or
negative ion mode using an m/z range of 400–3000 amu. The volt-
age of the ion sources was set at 19 and 16.7 kV for ion sources 1
and 2 respectively, while the lens was set at 8.50 kV. The reflec-
tor 1 and 2 voltages were set at 21 and 9.7 kV respectively for
2.2. Equipment
Infrared spectra were recorded on using a Perkin Elmer Spec-
trum 100 FT-IR (ATR) spectrometer. Ground state electronic
Scheme 2. Synthesis of low symmetry monocaffeic zinc phthalocyanine (complex 10).

20
N. Rapulenyane et al. / Journal of Photochemistry and Photobiology A: Chemistry 250 (2012) 18–24
Scheme 3. Synthesis of low symmetry monocaptopril zinc phthalocyanine (complex 11) and the molecular structure of tetrakis-diethylaminoethylthiol zinc phthalocyaninato
(complex 12).
positive ion mode. The spectra were acquired using ␣-cyano-4-
hydroxycinnamic acid as the MALDI matrix, using a 355 nm Nd:YAG
laser. Low resolution ESI-MS data was acquired in the negative ion
mode on a FinniganMAT LCQ spectrometer equipped with an ion
trap.
2.3. Synthesis
2.3.1. Phthalonitriles
2.3.1.1. Captopril phthalonitrile (4, Scheme 2). Captopril phthaloni-
trile (4) was synthesized as follows: to dry DMSO (10 ml)
under nitrogen atmosphere, captopril (0.8 g, 3.68 mmol) and 4-
nitrophthalonitrile (3a, 1 g, 5.78 mmol) were added and the
mixture left to stir for 15 min. K2CO3 (2.5 g, 18.09 mmol) was
added to the mixture over 2 h. The mixture was stirred for
Fluorescence lifetimes were measured using a time correlated
single photon counting setup (TCSPC) (FluoTime 200, Picoquant
GmbH) with a diode laser (LDH-P-670 with PDL 800-B, Picoquant
GmbH, 670 nm, 20 MHz repetition rate, 44 ps pulse width). Flu-
orescence 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 8 nm was used to
select the required emission wavelength band. The response func-
tion of the system, which was measured with a scattering Ludox
solution (DuPont), had a full width at half-maximum (FWHM)
of 300 ps. All luminescence decay curves were measured at the
maximum of the emission peak and lifetimes were obtained by
deconvolution of the decay curves using the FluorFit software pro-
gram (PicoQuant GmbH, Germany). The support plane approach
[16] was used to estimate the errors of the decay times.
Photo-irradiations were done using a general electric quartz line
lamp (300 W). A 600 nm glass cut off filter (Schott) and a water filter
were used to filter off ultraviolet and infrared radiations respec-
tively. An interference filter (Intor, 700 nm with a band width of
40 nm) was additionally placed in the light path before the sam-
ple. Light intensities were measured with a POWER MAX 5100
(Molelectron detector incorporated) power meter.
a
total of 48 h at room temperature. After 48 h, the prod-
uct formed was precipitated out by acetone, filtered and air
dried.
Yield: 76.3%. IR [(KBr) ꢀmax/cm−1]: 2235 (C N), 700 (C
S C),
2921 (S C), 3440 (O H). 1H NMR (D2O): ı, ppm 8.32 (1H, s,
Ar H), 8.25 (1H, d, Ar H), 7.60 (1H, m, Ar H), 2.87–4.10 (3H,
m, N CH2, CH), 1.50–2.92 (7H, m, CH2, S CH2), 1.08 (3H, m,
CH3). 13C NMR (D2O with a drop of MeOD): ı, ppm 175.1
(C O), 169.8 (C O), 142.2 (Ar S), 135.3 (Ar C), 124.0 (Ar C),
123.9 (Ar C), 121.2 (Ar CN), 119.8 (Ar CN), 118.6 (CN), 118.1
(CN) 57.1 (N CH), 43.3 (N CH2), 35.8 (CH), 32.6 (S CH2), 26.8
(CH2), 24.9 (CH2), 19.8 (CH3). ESI-MS m/z: Calcd: 343. Found:
[M]− 343.
2.3.1.2. Caffeic acid phthalonitrile (7, Scheme 3). Synthesis and
purification of this phthalonitrile was as described for 4 except caf-
feic acid and 4.5-dichloro-1.2-dicyanobenzene (3b) were employed
instead of captopril and 4-nitrophthalonitrile (3a).
Yield: 92%. IR [(KBr) ꢀmax/cm−1]: 2249 (C N), 1363 (R
O R),
3300 (O H). 1H NMR (DMSO-d6): ı, ppm 9.87 (1H, br s, OH), 8.52
(1H, d, Ar H), 8.32 (2H, m, Ar H), 8.17 (1H, dd, HC CH), 8.01 (1H, d,
Ar H), 7.58 (1H, m, Ar H), 7.52 (1H, d, HC CH). 13C NMR (DMSO-
d6): ı, ppm 171.0 (C O), 150.3 (Ar O), 149.8 (Ar O), 145.8 (Ar),
144.7 (Ar), 141.1 (C C), 129.9 (Ar), 127.0 (Ar), 125.1 (Ar), 124.8
(Ar), 123.8 (Ar), 121.4 (C C), 120.8* (Ar, CN, overlapped*), 114.4*
(Ar, overlapped*). ESI-MS m/z: Calcd: 304. Found: [M−H]− 303.
A laser flash photolysis system was used for the determination
of triplet decay kinetics. The excitation pulses were produced by a
Quanta-Ray Nd:YAG laser operating at 532 nm (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 Kratos Lis Projekte 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 256 laser pulses. Triplet lifetimes
were determined by exponential fitting of the kinetic curves using
OriginPro 8 software.
2.3.2. Phthalocyanines
Complexes 9 and 12 were synthesised as reported before in
[9,10] respectively.

N. Rapulenyane et al. / Journal of Photochemistry and Photobiology A: Chemistry 250 (2012) 18–24
21
2.3.2.1. Hexakis {8,11,16,19,42,27-(octylthiol)-1-(4-phenoxy car-
boxy) phthalocyanine} Zn (Scheme 1, ZnMPCPc (8)). Phthalonitriles
(1) (0.168 g, 1.20 mmol) and (2) (0.035 g, 0.4 mmol) were sus-
pended in 10 mL of 1-pentanol in the presence of zinc acetate
(0.048 g, 0.22 mmol) and heated at 130 ◦C, under nitrogen atmo-
sphere. The solution was left to stir for 18 h. On cooling to room
temperature, methanol was added to the product and the resulting
precipitate centrifuged and washed several times with methanol.
The product was dissolved in a minimum amount of CHCl3
and chromatographed on a neutral alumina column and eluted
first with CHCl3 followed by a THF:MeOH (6:4) to remove the
intermediate fractions.
where F and FStd are the areas under the fluorescence curves of the
MPc derivatives and the standard, respectively. A and AStd are the
absorbances of the sample and standard at the excitation wave-
length, and n and nStd are the refractive indices of the solvents used
for the sample and standard, respectively. ZnPc in DMF was used
as a standard, ˚F = 0.3 [18].
Triplet quantum yields were determined using a comparative
method based on triplet decay, using Eq. (2):
ꢂASTampleεSTtd
ꢂASTtdεSTample
Sample
T
˚
= ˚STtd
(2)
where ATSample and ASTtd are the changes in the triplet state
Yield: 23% IR (KBr, cm−1): 3422 (O H), 2917–2850 (C
stretch), 1730 (C O), 1464–1367 (C H bending), 1236–1035
(C C), 962, 929, 803 (C
C). 1H NMR (DMSO-d6): ı, ppm 8.03
H
absorbance of the sample and the standard, respectively. εTSample
and εSTtd are the triplet state extinction coefficients for the sam-
ple and standard, respectively. ˚STtd is the triplet state quantum
O
S
(2H, d, Phenoxy H), 7.88 (1H, d, Ar H), 7.57 (1H, d, Ar H), 7.39 (1H,
s, Ar H), 7.19 (2H, d, Phenoxy H), 7.00 (6H, m, Pc H), 2.93–2.70
(12H, m, S CH2), 2.34–1.23 (72H, q, CH2), 0.86 (18H, m-CH3).
UV/vis (DMSO), ꢁmax nm (log ε): 698 (4.36), 634 (4.35), 337
yield for the standard. Unsubstituted ZnPc was used as a standard,
Std
˚
= 0.58 for ZnPc in DMF [19].
T
εSTample and εTStd were determined from the molar extinction
(4.54). Calculated for C87H116N8S6ZnO3: C 66.24, H 7.64, N 6.96,
coefficients of their respective ground singlet states (εS and εSStd),
the changes in absorbance of the ground singlet states (ꢂAS and
S 11.95; Found: C 66.78, H 8.01, N 7.00, S 12.23 MALDI TOF MS m/z:
Calcd: 1578. Found: [M]+ 1578.
ꢂASStd) and changes in the triplet state absorptions (ꢂAT and ꢂASTtd
)
2.3.2.2. Tris
{11,19,27-(1,2-diethylaminoethylthiol)-2-(captopril)
according to Eqs. (3a) and (3b):
phthalocyanine} Zn (Scheme 2, ZnMCapPc (10)). Diethylaminoethyl-
thiol phthalonitrile (5) (0.50 g, 1.20 mmol), (4) (0.275 g, 0.4 mmol)
and zinc acetate (0.045 g, 0.21 mmol) were dissolved in quinoline
(10 ml) and the mixture refluxed for 12 h. The mixture was cooled
and dissolved in CHCl3:MeOH 1:1 ratio and chromatographed
using aluminum oxide with CHCl3:MeOH as an eluent.
ꢂAT
εSTample = εS
(3a)
ꢂAS
ꢂASTtd
ꢂASStd
εSTtd = εSStd
(3b)
Yield: 33%. IR (KBr, cm−1): 3420 (OH), 2956 (C H stretch),
Both the sample and the standard were excited at the same
relevant wavelength.
1646 (N
C O), 1315, 1119 (C N stretch), 806 (C S
C). 1H NMR
(DMSO-d6): ı, ppm 10.22 (1H, br s, OH), 9.25 (1H, d, Ar H), 8.72
(1H, d, Ar H), 8.46 (1H, d, Ar H), 8.18 (3H, m, Ar H), 7.73 (3H,
d, Ar H), 7.51 (3H, m, Ar H), 4.24–4.06 (2H, m, CH), 3.89–3.76
(10H, m, S CH2, N CH2), 2.40–2.06 (42H, m, CH2, CH3). UV/vis
(DMF), ꢁmax nm (log ε): 687 (4.07), 621 (3.35), 335 (3.6). Calcu-
lated C59H68N12O3S4Zn: C 59.64, H 5.77, N 14.15, S 10.80; Found:
C 60.96, H 6.00, N 13.00, S 10.60. MALDI TOF MS m/z: Calcd: 1187.
Found: [M+2H]+ 1189.
2.5. Singlet oxygen quantum yield
Singlet oxygen quantum yield (˚ꢂ) values were determined
under ambient conditions using the relative method with DPBF act-
ing as a singlet oxygen chemical quencher in DMF for complexes
8–12. Eq. (4) was employed for calculating singlet oxygen quantum
yields:
Std
RI
2.3.2.3. Tris {11,19,27-(1,2-diethylaminoethylthiol)-1,2(caffeic acid)
phthalocyanine} Zn (Scheme 3, ZnMCafPc (11)). The synthesis was
similar to that of complex 10, except the caffeic acid phthaloni-
trile (7) was used instead of captopril phthalonitrile (4). Amounts
of reagents were the same as for 10. The product was chro-
matographed over alumina, using 1:1 ratio of CHCl3:THF and
THF:MEOH respectively for purification.
abs
˚
= ˚Std
.
(4)
ꢂ
ꢂ
RStdIabs
where ˚Sꢂtd is the singlet oxygen quantum yield for the standard
(ZnPc, ˚Sꢂtd = 0.56 in DMF [18] and 0.67 in DMSO) [20]. R and RStd
are the DPBF photobleaching rates in the presence of the respec-
tive MPcs (complexes 8–12), under investigation and the standard
respectively. Iabs and I
MPcs and the standard, respectively. Solutions of the MPcs with an
absorbance of ∼0.5 at the irradiation wavelength were prepared in
the dark, irradiated at the Q band region, keeping the DPBF absorp-
tion at 417 nm at ∼1.
Yield: 33%. IR (KBr, cm−1): 3464 (O H), 2970–2958 (C
stretch), 1753 (C O), 1445–1373 (C N stretching), 1231–1098
(C C), 895, 739 (C
C). 1H NMR (DMSO-d6): ı, ppm 8.60
H
Std
abs
are the rates of light absorption by the
O
S
(3H, m, Ar H), 8.35 (3H, m, Ar H), 7.97 (3H, m, Ar H), 7.78
(2H, m, Ar H), 7.70 (1H, s, CH), 7.28 (1H, d, Ar H), 6.88 (1H, s,
Ar H), 6.69 (1H, d, Ar H), 6.59 (1H, s, CH), 4.03–2.76 (6H, m, CH2),
2.69–1.96 (18H, m, CH2), 1.64–0.94 (18 H, m, CH3). UV/vis (DMF),
ꢁmax nm (log ε): 701 (4.92), 634 (4.26), 338 (4.45). Calculated for
C59H59N11O4S3Zn: C 61.72, H 5.18, N 13.43, S 8.38; Found: C 61.60,
H 5.68, N 13.19, S 8.53. MALDI TOF MS m/z: Calcd: 1147. Found:
[M−7H] 1140.
3. Results and discussion
3.1. Synthesis and characterization
The synthesis of asymmetrically substituted phthalocyanines is
a relatively complicated procedure compared to the symmetrically
substituted counterparts. Low yields are obtained due to extensive
purification procedures. Relatively good yields were obtained for
complexes 10 and 11 after continuous washing in methanol and
purification by column chromatography, whereas for complex 8,
the yield was very low. It is expected that a mixture structural iso-
mers is obtained. With respect to unsymmetrical Pcs bearing bulky
2.4. Photophysical parameters
Fluorescence quantum yields (˚F) were determined by the com-
parative method [17] using (Eq. (1)),
F · AStd · n2
˚
F
= ˚F
·
(1)
Std
FStd · A · n2
Std

22
N. Rapulenyane et al. / Journal of Photochemistry and Photobiology A: Chemistry 250 (2012) 18–24
substituents, particularly at the non-peripheral position. Céspedes-
Guirao et al. have previously reported on the preferential formation
and purification of one pure regioisomer simply by column chro-
matography [21]. It is therefore possible that with complexes 8, 10
and 11, the one isomer is present in greater quantities. Complexes
10 and 11 and their symmetrical counterpart (12) showed solu-
bility in most organic solvents including DMSO, dichloromethane
(DCM), methanol, acetone, chloroform and tetrahydrofuran (THF).
Complex 8, however, was found to be only soluble in DMF and
DMSO.
Characterization of the complexes was achieved using
infrared-red,
ultraviolet–visible,
matrix-assisted
laser
desorption/ionization-time of flight (MALDI-TOF) mass, pro-
ton nuclear magnetic resonance (1H NMR) spectroscopies and
elemental analyses. The 1H NMR spectra of all the complexes
showed characteristic signals for the aromatic Pc ring protons
between 7.0 and 11.0 ppm corresponding to the required number
of Pc protons for each of the complexes. Moreover the 1H NMR
spectra showed the signals at 10.22 and 4.24 ppm specific to
complex 10 for the carboxyl and the pyrolidine proton moiety
(N CH), respectively. Complex 11 showed more peaks due to the
aromatic ring found in the caffeic acid moiety. The syntheses and
characterization of complexes 9 [9] and 12 [10] have already been
reported.
Phthalonitrile cyclization was proved by the absence of the
sharp C N vibration peak (∼2230 cm−1) in the IR spectra of all
MPc complexes. Infra-red spectra further showed the presence of
C
H stretching and bending alkyl groups bands around 2900 cm−1
and 1400 cm−1 respectively, and a C
S C stretch between 800 and
Fig. 1. Ground state absorption spectra of (a) complexes 8 and 9 and (b) complexes
(10, 11 and 12) in DMF.
990 cm−1 for all complexes. MALDI-TOF MS further confirmed the
formation of the complexes (complexes 8, 10 and 11) with the
molecular ion peaks observed at 1578, 1189 and 1140 amu respec-
tively as expected.
of 12–25 nm (Table 1) were observed upon comparison of the exci-
tation and emission spectra of the Pc complexes (8–12) in DMF.
Fluorescence lifetimes were obtained for all complexes using
TCSPC. The decay curve for complex 8 as an example is shown
in Fig. 4, as a representative for the rest of the Pcs. The complex
showed a mono-exponential fluorescence decay curve, suggesting
that the solution had only one species which fluoresces. Mono-
exponential behavior was observed for all complexes as evidenced
by a single lifetime in Table 1. Higher fluorescence quantum yields
(˚F = 0.15, 0.10, 0.13 and 0.29) were obtained for complexes 8, 10,
12 and 13, respectively, compared to (˚F = 0.03 and 0.09 for com-
plexes 9 and 11, respectively (Table 1). For complex 13, the reported
values are in DMSO, while the rest of the complexes are in DMF. ˚F
values in DMF and DMSO have been found to be comparable [15].
Complex 8 has a larger ˚F value compared to the non-symmetrical
3.2. Ground state UV–visible spectral characterisation
Fig. 1A shows the ground state electronic absorption spectra of
complexes 8 and 9 in DMF. Q band peak maxima are listed in Table 1.
A broadening of the Q-band at 698 nm was observed for complex
8; this is commonly associated with a loss of symmetry in asym-
metrically substituted phthalocyanines [22–24]. The symmetrical
counterpart, complex 9 possessing additional sulfur groups (and
the absence of a COOH electron withdrawing group) had a red-
shifted narrow Q-band compared to 8. Fig. 1B shows the spectra of
complex 12 and its asymmetrical counterpart’s: complexes 10 and
11 in DMF. Complex 12, the symmetrical Pc, has a Q-band at 706 nm
whereas the asymmetrical counterparts (complexes 10 and 11) had
blue shifted Q-band absorption maxima at 683 and 699 nm respec-
tively, Table 1. The shift was expected as electron deficient groups
were introduced. The spectral shift observed (in complexes 8, 10
and 11) also confirmed the formation of unsymmetrical phthalo-
cyanines.
As shown in Fig.
2 (using complex 11 as an exam-
ple), Beer Lambert law was obeyed at concentrations below
3.23 × 10−5 mol dm−3. Complexes 8 and 10 also show the same
trend.
3.3. Photophysical studies
Fig. 3 shows the absorption, emission and excitation spectra
of complex 11 in DMF. The excitation spectra were similar to the
absorption spectra and both were mirror images of the emission
spectra, confirming lack of aggregation and no change in symme-
try of the molecule upon excitation. Similar fluorescence behavior
was observed for the rest of the complexes. Typical Pc Stokes shifts
Fig. 2. Ground state electronic absorption spectra of complex 11 at various con-
centrations: (i) 3.23 × 10−5, (ii) 2.77 × 10−5, (iii) 2.43 × 10−5, (iv) 2.16 × 10−5, (v)
1.94 × 10−5, (vi) 1.76 × 10−5 and (vii) 1.62 × 10−5 mol dm−3 in DMF.

N. Rapulenyane et al. / Journal of Photochemistry and Photobiology A: Chemistry 250 (2012) 18–24
23
Table 1
Photophysical and photochemical parameters (in DMF) for the mono substituted and symmetrically substituted phthalocyanines.
Sample
Absꢁ
(nm)
Emsꢁ
(nm)
ꢂStokes (nm)
˚
T
ꢃT (␮s)
˚
F
ꢃF (ns) 0.003
˚
ꢂ
Sꢂ
max
max
ZnMPCPc (8)
ZnOTPc (9)
ZnMCapPc (10)
ZnMCafPc (11)
ZnTDTPc (12)
ZnTCPPca (13)
698
782
683
699
706
678
710
810
693
711
729
690
12
25
10
12
23
12
0.80
0.62
0.65
0.57
0.58
0.47
286
177
148
109
144
181
0.15
0.03
0.10
0.09
0.13
0.29
4.4
0.77
0.61
0.26
0.25
0.56
0.50
0.96
0.98
0.40
0.44
0.96
0.96
1.69
2.16
1.69
3.23
a
Values from Ref. [11] and in DMSO.
complexes with lifetimes within the range generally observed for
Pc’s. It is expected that when ˚F is high, ˚T will be low. But this
is not the case (Table 1) where for complex 8 both these parame-
ters are the highest. The high values of ˚T = 0.80 coupled with ˚F
of 0.15 (total = 0.95), leaves very little room for loss of energy via
non-radiative processes. This makes complex 8 a very good pho-
tosensitizer. The symmetrical counterpart, 9, has a low ˚F and a
relatively low ˚T, showing less efficiency for triplet state popu-
lation compared to 8. The symmetrical tetracarboxyphenoxy ZnPc
complex (13) on the other hand has a higher fluorescence quantum
yield (˚F = 0.29) [11], but a lower ˚T (0.47) when compared to com-
plex 8 [11]. It may be prudent to note that the data for complex 13
whereacquiredin DMSO[11]. Forcomplexes10and11, unsymmet-
rical substitution with captopril has an advantage over symmetrical
substitution in 12, whereas 11 shows no advantage over 12. Triplet
lifetimes should be short where triplet yields are large. This is
not observed for complex 8, which has both long lifetimes and
large triplet yields in Table 1. Again this shows that complex 8 is
a promising photosensitizer. Symmetrically substituted counter-
part of 8 (complex 9) has lower ˚T and triplet lifetimes compared
to 8, showing the advantage of asymmetrical substitution with
the carboxyphenoxy substituent. For complexes 10 and 11 there
is not an advantage of asymmetrical substitution with capto-
pril or caffeic acid since the lifetimes are nearly the same or
complex 12.
Fig. 3. Ground state absorption (black, i), fluorescence excitation (red, ii) and emis-
sion (yellow, iii) of complex 11 in DMF. (For interpretation of the references to color
in this figure legend, the reader is referred to the web version of the article.)
counterpart complex 9 which is symmetrically substituted. How-
ever, the literature ˚F value for a tetra subsituted carboxyphenoxy
ZnPc (13) is the highest at ˚F = 0.29 [11]. The fact that 8 and 13
with carboxyphenoxy substituents show high ˚F suggests that this
substituent enhances fluorescence. Comparing 12 (symmetrically
substituted) with asymmetrically substituted counterparts 10 and
11, shows a decrease in ˚F for the latter which could suggest it
is not the asymmetric substitution but the quenching effect of the
substituents that affect the ˚F values. ˚F corresponds to ꢃF in that
complexes 8 and 13 with the large ˚F also have the long ꢃF val-
ues. Conversely complex 9 with the lowest ˚F values also gave the
shortest ꢃF. The trend is not clear for complexes 10–12.
3.4. Photochemical properties
A good photosensitizer is measured by its ability to produce
toxic singlet oxygen. Fig. 6 shows the chemical photodegrada-
tion of DPBF (from 0 to 60 s) which was used as a quencher
during singlet oxygen quantum yield determinations for com-
plex 8 (as with all Pcs studied). The complex’s Q-band remained
unchanged over the irradiation period of singlet oxygen production,
The triplet decay curve for complex 8 (Fig. 5) was found to obey
first order kinetics. Similar curves were observed for the rest of
Fig. 4. Photoluminescence decay of complex 8 in DMF.
Fig. 5. Triplet decay curves for complex 11 in DMF.

24
N. Rapulenyane et al. / Journal of Photochemistry and Photobiology A: Chemistry 250 (2012) 18–24
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Acknowledgements
This work was supported by the Department of Science and
Technology (DST) and National Research Foundation (NRF), South
Africa through DST/NRF South African Research Chairs Initiative
for Professor of Medicinal Chemistry and Nanotechnology, an NRF
CSUR/KFD grant as well as Rhodes University.