Synthesis, fluorescence, excited triplet state properties and singlet oxygen generation of para-(tert-butylphenoxy) substituted phthalocyanines containing group IV A central elements

Article abstract of DOI:10.1016/j.dyepig.2013.06.010

Phthalocyanines containing group IV A central elements and non-peripheral para-(tert-butyl)-phenoxy substituents, i.e. M(OH)₂PC(α-t- butyl-phenoxy)₄ (M is Si, Ge, Sn and Pb respectively), were synthesized. The effects of the central elements on the photosensitizing and photophysical properties (quantum yield of singlet oxygen formation, quantum yield and lifetime of lowest lying excited triplet- and singlet state) were investigated by laser flash photolysis, time correlated single photon counting, steady state fluorescence and absorption spectra. The incorporation of large atoms significantly enhances the efficiency of excited triplet state and singlet oxygen formation. The triplet quantum yield is increased 2.5 times to 0.75 for the lead tetrasubstituted phthalocyanine relative to that of 0.30 determined for the silyl analog, while the triplet lifetime is longer than 120 μs. Correspondingly, the quantum yield of singlet oxygen formation is 0.64 for the lead tetrasubstituted phthalocyanine and is 0.26 for the silyl compound. All of the PC complexes maintain reasonably good fluorescence characteristics with the shortest fluorescence lifetime measured being 3.14 ns, and the lowest fluorescence quantum yield of 0.18. These properties indicate that some of the PC complexes may be good candidates for singlet oxygen photosensitization.

Full text of DOI:10.1016/j.dyepig.2013.06.010

Dyes and Pigments 99 (2013) 480e488
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis, fluorescence, excited triplet state properties and singlet
oxygen generation of para-(tert-butylphenoxy) substituted
phthalocyanines containing group IV A central elements
,
b
,
a
a
a
a
Xian-Fu Zhang a , Xiaona Shao , Hongyan Tian , Xiaojie Sun , Kaixue Han
*
a Chemistry Department, Hebei Normal University of Science and Technology, Qinhuangdao, Hebei Province 066004, China
b MPC Technology, Hamilton, ON L8S 3H4, Canada
a r t i c l e i n f o
a b s t r a c t
Article history:
Phthalocyanines containing group IV A central elements and non-peripheral para-(tert-butyl)-phenoxy
substituents, i.e. M(OH)2PC( -t-butyl-phenoxy)4 (M is Si, Ge, Sn and Pb respectively), were synthesized.
Received 23 February 2013
Received in revised form
10 June 2013
Accepted 11 June 2013
Available online 18 June 2013
a
The effects of the central elements on the photosensitizing and photophysical properties (quantum yield
of singlet oxygen formation, quantum yield and lifetime of lowest lying excited triplet- and singlet state)
were investigated by laser flash photolysis, time correlated single photon counting, steady state fluo-
rescence and absorption spectra. The incorporation of large atoms significantly enhances the efficiency of
excited triplet state and singlet oxygen formation. The triplet quantum yield is increased 2.5 times to 0.75
for the lead tetrasubstituted phthalocyanine relative to that of 0.30 determined for the silyl analog, while
Keywords:
Phthalocyanine
Singlet oxygen
Triplet state
Fluorescence
SieGeeSnePb
Photodynamic therapy (PDT)
the triplet lifetime is longer than 120 ms. Correspondingly, the quantum yield of singlet oxygen formation
is 0.64 for the lead tetrasubstituted phthalocyanine and is 0.26 for the silyl compound. All of the PC
complexes maintain reasonably good fluorescence characteristics with the shortest fluorescence lifetime
measured being 3.14 ns, and the lowest fluorescence quantum yield of 0.18. These properties indicate
that some of the PC complexes may be good candidates for singlet oxygen photosensitization.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
PCs, especially Al, Zn and SiPC complexes, have been shown to be
effective in PDT [1], the photophysical properties for these PCs have
Photodynamic therapy (PDT) of tumors involving certain types
of photosensitizers has been approved in clinic use for the treat-
ment of several diseases [1e4]. A photosensitizer (PS) is a light-
absorbing substance that generates reactive oxygen species
(mainly singlet oxygen) and is not consumed in the reaction.
Developing new PSs is currently an active field to address several
needs in PDT [1e4], including (i) high light absorption within the
optimal window (650e850 nm), (ii) a high quantum yield and long
lifetime for the excited triplet state, (iii) a low tendency to form
aggregates, (iv) amphiphilicity and (v) high selectivity for the target
tissue.
also been extensively studied [8e42]. Nonetheless, the photody-
namic activity of many metal PC complexes (e.g. germanium PCs,
tin PCs) has not been explored in depth, especially when compared
to Al, Zn and Si PC complexes [1]. The incorporation of central
metals with a high atomic number such as germanium, tin and lead,
which enhance the intersystem crossing (ISC) from S1 (the lowest
lying excited singlet state) to T1 (the lowest lying excited triplet
state), is of current research interest, since this will increase the
quantum yield of triplet state formation (FT) and improve the
quantum efficiency of singlet oxygen generation (FD). This method
to improve photosensitizing capability is actually one way to
harness heavy atom effect (HAE) [43e45].
Phthalocyanines (PCs) belong to a class of synthetic aromatic
dyes which are structurally similar to the porphyrins [5e7]. Some
In this report a systematic study was made on group IV A (Si, Ge,
Sn Pb) PC complexes (Fig. 1), which are all substituted at one a-
position of each benzene moiety by a para-(tert-butyl)phenoxy.
Various synthesis strategies were explored. All aspects of photo-
physics related to photosensitizing properties of the complexes
were covered. For the b-substituted Ge and Sn PC complexes, Maree
and Nyokong reported the synthesis and photosensitizing proper-
ties [46].
* Corresponding author. Chemistry Department, Hebei Normal University of
Science and Technology, Qinhuangdao, Hebei Province 066004, China. Tel./fax: þ86
0335 8387040.
E-mail addresses: zhangxianfu@tsinghua.org.cn, zhangxf111@hotmail.com
(X.-F. Zhang).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.06.010

X.-F. Zhang et al. / Dyes and Pigments 99 (2013) 480e488
481
added and the green precipitate was filtered and washed with
water. The dried crude product was dissolved in dichloromethane
and purified by column chromatography (silica gel) using
dichloromethane as the mobile phase. Yield: 39%. UVevis (EtOH):
lmax nm 334, 691, 717. IR [(KBr) nmax/cmꢁ1]: 750, 941, 1256, 1412,
1589, 1697 (PC ring); 1335, 1412, 2953, 2961 (CH3); 1026, 1175 (Are
OeC). 1H NMR (CDCl3):
d, ppm 7.49e7.89 (12H, m, PceH), 7.01e7.47
(16H, m, phenyleH), 1.21e1.65 (36H, m, CH3). MALDI-TOF-MS m/z:
Calculated 1107.4; found 1107.6 [M þ 1]þ. Calcd. for C72H66N8O4 C,
78.09: H, 6.01; N, 10.12. Found: C 77.58, H, 6.21; N, 10.27.
2.2.3. Synthesis of tetra(
phthalocyanine, Si(OH)2PcR4 (6)
a
ꢁ4-tert-butylphenoxy) silicon
Compound 3-(4-tert-butylphenoxy)phthalonitrile was con-
verted to 3-(4-tert-butylphenoxy)1,3-diiminoisoindoline in the first
step. To methanol (40 mL) containing CH3ONa (0.070 g, 1.4 mmol),
3-(4-tert-butylphenoxy)phthalonitrile (2.76 g, 10 mmol) was
added. Ammonia was then bubbled through the solution with
stirring for 1 h at room temperature. The temperature was then
raised to 70 ꢀC and the reaction was maintained for 9 more hrs with
stirring and N2 bubbling. After cooling down, methanol was
removed by distillation. The solid was then washed with water,
filtered, and dried under vacuum.
Fig. 1. Chemical structure of ML2PCR4 complexes 5 to 9, R is located at 1, 5(8), 9(12),
and 13(16), respectively.
3-(4-tert-Butylphenoxy)1,3-diiminoisoindoline (0.55 g, 1.86 m
mol) and SiCl4 (0.30 mL, 2.63 mmol) were added into freshly distilled
quinoline (3 mL) and stirred at 200 ꢀC under argon for 1 h. Acetone
(10 mL) was added, the resulted mixture was filtered and washed
with acetone. The filtrate and the solvent were evaporated under
vacuum. The product was purified by column chromatography (silica
gel) by using CH2Cl2 as eluent. Yield: 27%. UVevis (EtOH): lmax nm
346, 699. IR [(KBr) nmax/cmꢁ1]: 617, 748, 982, 1252, 1479, 1589 (PC
ring); 1015 (SieO); 1335, 2868, 2903, 2961 (CH3); 1070,1245 (AreOe
2. Experimental
2.1. Reagents and apparatus
All reagents for synthesis were analytical grade and used as
received. Ethanol and dimethylformide (DMF) were dried and
redistilled before use. UVevisible spectra were recorded on a
StellarNet BLACK Comet C-SR diode array miniature spectropho-
tometer connected to deuterium and halogen lamp by Optical fiber
using 1 cm matched quartz cuvettes. A Shimadzu FTIR-8900 spec-
trometer was used to record IR spectra at room temperature. MS
spectra were recorded either on a Bruker APEX II or Autoflex III
Maldi-TOF spectrometer. The measurement of 1H NMR spectra was
carried out at room temperature on a Bruker DMX 400 MHz NMR
spectrometer.
C).1H NMR (CDCl3):
d, ppm 7.57e7.95 (12H, m, PceH), 7.03e7.62 (16H,
m, phenyleH), 1.21e1.69 (36H, m, CH3). MALDI-TOF-MS m/z: Calcu-
lated 1166.5; found 1149.5 [M ꢁ OH]ꢁ. Calcd. for C72H66N8O6Si C,
74.07; H, 5.70; N, 9.60. Found: C, 73.66; H, 5.51; N, 9.39.
2.2.4. Synthesis of tetra(
phthalocyanine, Ge(OH)2PcR4 (7)
Tetra(
ꢁ4-tert-butylphenoxy) phthalocyanine (0.31 g, 0.28 m
aꢁ4-tert-butylphenoxy) germanium
a
mol) and freshly distilled quinoline (5 mL) were added into 25 mL
three-necked flask. After bubbling argon for 10 min, GeCl4 (0.5 mL,
4.4 mmol) was added quickly. The solution was stirred for 2 h at
240 ꢀC. After cooling down, acetone (10 mL) was added, the
resulting mixture was filtered and washed with acetone. The
filtrate and the solvent were evaporated under vacuum. The
product was purified by column chromatography (silica gel) by
using CH2Cl2 as eluent. Yield: 33%. UVevis (EtOH): lmax nm 342,
714. IR [(KBr) nmax/cmꢁ1]: 640, 743, 972, 1259, 1483, 1589 (PC ring);
1015 (GeeO); 1340, 2866, 2963 (CH3); 1080, 1213 (AreOeC). 1H
2.2. Synthesis
2.2.1. Synthesis of 3-(4-tert-butylphenoxy)phthalonitrile (3)
3-Nitrophthalonitrile (1.73 g, 0.01 mol) and 4-tert-butylphenol
(1.50 g, 0.01 mol) were dissolved in DMSO (30 mL), LiOH (0.42 g,
0.01 mol) was then added. The resulted mixture was then stirred at
50 ꢀC for 24 h under N2 atmosphere. The product solution was
cooled and then poured into NaCl solution (10% aqueous, 100 mL),
the precipitated solid was filtered, washed with water and dried
under vacuum. White needles were obtained after recrystallization
NMR (CDCl3): d, ppm 7.60e8.01 (12H, m, PceH), 7.06e7.59 (16H, m,
in toluene. Yield: 51%. m.p. 116e118 ꢀC. IR (KBr),
n
(cmꢁ1): 3083,
phenyleH), 1.17e1.75 (36H, m, CH3). MALDI-TOF-MS m/z: Calcu-
lated 1212.4; found 1195.5 [M_OH]ꢁ. Calcd. for C72H66GeN8O6 C,
71.35; H, 5.49; N, 9.25. Found: C, 70.79; H, 5.09; N, 9.47.
3014, 2232 (C^N), 1695, 1632, 1563, 1506, 1485, 1465, 1429, 1386,
1351, 1275 (CeOeC), 1258, 1222, 1165, 1142, 1015, 938, 917. 1H NMR
(400 MHZ, CDCl3, ppm):
d 1.353 (s, 9H, t-butyl), 6.992 (d, 2H,
J ¼ 7 Hz, AreH), 7.245 (d, 1H, J ¼ 6 Hz, AreH), 7.510 (d, 2H, J ¼ 7 Hz,
AreH), 7.799 (m, 2H, AreH), 7.835 (m, 1H, AreH). MS, m/z: 298.95
[M þ Na]þ.
2.2.5. Synthesis of tetra(
phthalocyanine, Sn(OH)2PcR4 (8)
a
ꢁ4-tert-butylphenoxy) tin
3-(4-tert-Butylphenoxy)phthalonitrile (0.276 g, 1 mmol) and
dried N,N-dimethylaminoethanol (5 mL) were mixed and stirred at
150 ꢀC for 4 h under argon atmosphere in the presence of two drops
of DBU as catalyst. SnCl2 (0.19 g, 1 mmol) was then added at 170 ꢀC,
the reaction was maintained for 1 h. After cooling, water (50 mL)
was added and the green precipitate was collected by filtration and
washed with water. The dried crude product was dissolved in
dichloromethane and purified by column chromatography (silica
2.2.2. Synthesis of tetra(
H2PcR4 (5)
aꢁ4-tert-butylphenoxy) phthalocyanine,
3-(4-tert-Butylphenoxy)phthalonitrile (0.276 g, 1 mmol) and
dried N,N-dimethylamino-ethanol (5 mL) were mixed and stirred
at 150 ꢀC for 4 h under argon atmosphere in the presence of two
drops of DBU as catalyst. After cooling down, water (50 mL) was

482
X.-F. Zhang et al. / Dyes and Pigments 99 (2013) 480e488
gel) using dichloromethane as the mobile phase. Yield: 43%. UVevis
(EtOH): lmax nm 331, 624, 691. IR [(KBr) nmax/cmꢁ1]: 608, 827, 972,
solvent, the subscript 0 stands for a reference compound and s
represents samples. ZnPc was used as the reference ðF0f ¼ 0:18Þ in
DMSO [47]. The excitation wavelength is 610 nm, which is corre-
sponding to the vibronic band of S0 to S1 transitions. The absor-
bance of a sample and the reference solutions was adjusted so that
they both have roughly the same absorbance (Ai is about 0.050) at
the excitation wavelength.
1259, 1477, 1593, 1651 (PC ring); 1047 (SneO); 1339, 1364, 1396,
1
2868, 2963 (CH3); 1090, 1211 (AreOeC). H NMR (CDCl3):
d
, ppm
9.20e9.42 (4H, m, Pc-H), 8.91e9.12 (4H, m, Pc-H), 8.00e8.23 (4H,
m, Pc-H), 7.55e7.80 (16H, m, phenyl-H), 1.30e1.62 (36H, m, t-butyl).
MALDI-TOF-MS m/z: Calculated 1258.4; found 1241.5 [M ꢁ OH]ꢁ.
Calcd. for C72H66N8O6Sn C, 68.74; H, 5.29; N, 8.91. Found: C, 68.29;
H, 5.58; N, 8.46.
Time-correlated single photon counting method was used to
measure fluorescence lifetime (
diode laser (70 ps FWHM), the emission was monitored at the
wavelength of a band maximum. f was obtained by a convolution
f) þ B to the
sf) with excitation at 672 nm by a
2.2.6. Synthesis of tetra(
phthalocyanine, PbPcR4 (9)
To a DMF solution of tetra(
a
ꢁ4-tert-butylphenoxy) lead
s
procedure that fits the summation of IRF and I(0)e(ꢁt/
s
a
ꢁ4-tert-butylphenoxy) phthalocy-
measure I(t), in which IRF is the measured instrument response, I(t)
is the fluorescence intensity at time t after laser excitation, B is a
constant (fluorescence background). The software is a product of
the instrument supplier.
anine (59 mg, 0.05 mmol), lead diacetate (11.9 mg, 0.05 mmol) was
added. The resulted solution was stirred under argon atmosphere at
200 ꢀC for 1.5 h. After cooling, water (50 mL) was added and the
green precipitate was collected by filtration and washed with wa-
ter. The dried crude product was dissolved in dichloromethane and
purified by column chromatography (silica gel) using dichloro-
methane as the mobile phase. Yield: 57%. UVevis (EtOH): lmax/nm
331, 624, 691. IR [(KBr) nmax/cmꢁ1]: 644, 748, 974, 1250, 1477, 1508,
1597 (PC ring); 1015 (PbeO); 1331, 1364, 2866, 2959 (CH3); 1065,
2.3.2. Laser flash photolysis
An Edinburgh LP920 laser flash photolysis system was used to
obtain transient absorption spectra in DMF. The concentrations of
the target compounds were typically 15
m
M providing A355 ¼ 0.30
in 10 mm cuvettes. The solutions were either air-saturated or
deoxygenated by bubbling with argon for 20 min in a capped
quartz cuvettes. The optical path length was 1 cm. The excitation
source was A Nd:YAG laser (Brio, 355 nm and 4 ns FWHM). The
analyzing light was from a pulsed xenon lamp, perpendicular to
the excitation laser. The signal was recorded on a R928B detector
and displayed by a Tektronix TDS 3012B oscilloscope. The laser
energy incident at the sample was attenuated to ca. 10 mJ per
pulse. Time profiles at a series of wavelengths were recorded
with the aid of a computer controlled kinetic absorption spec-
trometer, from which point by-point spectra were sliced and
assembled by the L900 software provided by the instrument
supplier.
1213 (AreOeC). 1H NMR (DMSO):
d, ppm 8.39e9.04 (12H, m, Pce
H), 7.71e8.01 (16H, m, phenyleH), 1.29e1.34 (36H, m, CH3). MALDI-
TOF-MS m/z: Calculated 1312.5; found 1313.3 [M þ H]þ. Calcd. for
C72H64N8O4Pb C, 65.89; H, 4.91; N, 8.54. Found: C, 66.49; H, 4.52; N,
8.37.
2.3. Photophysical measurements
2.3.1. Fluorescence
Fluorescence spectra and lifetimes were acquired on a FLS 920 of
Edinburgh Instruments using 1 cm quartz cuvettes. All fluorescence
spectra were corrected for the sensitivity of the photo-multiplier
tube. The absorption, fluorescence and excited triplet state prop-
erties were investigated at room temperature (22 ꢀC). All ethanol
solutions were air saturated for absorption, fluorescence spectra
and fluorescence quantum yield (Ff) measurements.
Ff was computed by Eq. (1)
The triplet quantum yield (FT) was computed by Eq. (2) by using
ZnPc (FT ¼ 0.65 in 1-propanol) as a reference [48],
ZnPc
D
3
D
D
AT
3
T
FZTnPc
$
$
;
(2)
T
FT
¼
D
AZTnPc
2
Fs A0 ns
in which
spectrum at the selected wavelength, and
state molar absorption coefficient. of the samples were ob-
D
AT is the absorbance of the tripletetriplet absorption
Ff
¼
Ff0
$
$
;
(1)
2
D
3
T
is the triplet
F0
A
s n0
D
3
T
in which A is the absorbance at excitation wavelength, F is the in-
tegrated fluorescence intensity, n is the refractive index of the
tained by using the singlet depletion method according to Eq.
(3) [48].
Fig. 2. The synthetic route for SiPcR4.

X.-F. Zhang et al. / Dyes and Pigments 99 (2013) 480e488
483
Fig. 3. The synthetic route for GePCR4, SnPcR4 and PbPcR4.
causes DPBF bleaching [49]. Typically, a 3 mL portion of the
respective 5 M PS solution that contained 30 M DPBF was irra-
diated by a LED lamp (660 nm) from StellarNet corp. in air saturated
DMF. FD value was calculated by Eq. (4) using ZnPc as the reference
(Eq. (4)):
D
AT
D
3
¼
(3)
3 SDAS
m
m
T
3
S is the molar absorption coefficient at the UVevis absorption band
maximum for the ground-state. AS and AT in Eq. (3) are the
absorbance change at the minimum of the bleaching band and the
maximum of the positive band, respectively. Both AS and AT were
D
D
k Iaref
FrDef
;
(4)
D
D
FD
¼
kref
Ia
obtained from the triplet transient difference absorption spectra.
where FrDef is the singlet oxygen quantum yield for the standard (0.56
for ZnPc in DMF) [50], k and kref is the DPBF photo-bleaching rate
constant for the sample and ZnPc, respectively; Ia and Iaref is the rate of
light absorption at the irradiationwavelength of 660 nm by the sample
and standard, respectively. Their ratio can be obtained by Eq. (5).
2.3.3. Singlet oxygen generation
Singlet oxygen quantum yields (FD) were determined by the
chemical trapping method, since one molecule of singlet oxygen can
react with one molecule of diphenylisobenzofuran (DPBF) which
ref
Iaref
Ia
1 ꢁ 10ꢁA
670
¼
;
(5)
1 ꢁ 10ꢁA
670
DPBF degradation was monitored by UVevis absorption spec-
trum. The error in the determination of FD was w10% (determined
from several FD values).
3. Results and discussion
3.1. Synthesis
The synthetic routes to the desired products are shown in Figs. 2
and 3. K2CO3, a base that is often effective in the reaction of a
phenol with nitro substituted phthalonitrile at room temperature,
did not work well in reaction (i) that generates compound 3.
Instead, a strong base LiOH and a higher temperature were needed
to effect reaction (i). Initial attempts to prepare the PCs 6, 7, 8, 9 by
the very common procedure (Fig. 4) by using the condensation of
compound 3 with PbCl4 (or Pb(CH3COO)2) or SnCl4 were not suc-
cessful, although the common method (Fig. 4 top) worked well for
MPCR4 (M ¼ Zn, Cu, InCl, GaCl, TiO) [51e54].
Fig. 4. The synthetic routes that did not lead to the desired product.

484
X.-F. Zhang et al. / Dyes and Pigments 99 (2013) 480e488
Table 1
The photophysical properties of compounds ML2PCR4 in ethanol.a
labs, nm
lex, nm
lem, nm
Ff
s
f, ns
FT
sT
,
ms
lT-T,nm
FD
kf, 109 sꢁ1
Si(OH)2PcR4
Ge(OH)2PcR4
Sn(OH)2PcR4
PbPcR4
699
711
691
691
698
714
690
691
718
725
706
707
0.66
0.52
0.45
0.18
5.29
4.23
3.14
3.21
0.30
0.34
0.43
0.75
267
129
125
121
593
565
553
578
0.26
0.30
0.36
0.64
0.13
0.12
0.14
0.055
a
labs: absorption maximum. lex: excitation maximum. lem: emission maximum. Ff: fluorescence quantum yield.
T: triplet lifetime. FT: quantum yield for triplet state. lTeT: T1eTn absorption maximum. FD: quantum yield for singlet oxygen. kf: the rate constant of
emission process, calculated by kf ¼ Ff sf
sf: fluorescence lifetime. c
2: chi square values for sf fitting
is within 1.00e1.03.
s
/
.
are affected significantly by the change of the central element. The
increase of atom size remarkably enhances the efficiency of triplet
state and singlet oxygen formation whilst good fluorescence char-
acteristics were retained. The details of these properties are
described in the following section.
3.2.1. Ground state UVevis absorption spectra
The UVevis absorption spectra of the phthalocyanines in
ethanol or DMF all showed similar spectral behavior, and the
spectra shown in Fig. 5 are typical. It can be seen that the Q band
of Si(OH)2PcR4 shows a vibronic shoulder with an intense narrow
peak at 699 nm, while the B band is located at 331 nm.
Ge(OH)2PcR4 Q band showed a red-shifted maximum (711 nm)
relative to that of Si(OH)2PcR4. Sn(OH)2PcR4, PbPcR4, however,
exhibited blue-shifted Q band maxima (both are 691 nm) relative
to that of SiPcR4. The absorption maxima (labs) are included in
Table 1.
Fig. 5. Normalized UVevis absorption spectra of MPcR4 in ethanol.
The blue shift is a reflection of the planarity of the
p-system.
With the increase of the atomic size from Si to Pb, Pb and Sn
cannot be inserted into the cavity of PC but are located
Two different pathways are required to obtain the four phtha-
locyanines in this study. For Si(OH)2PcR4, reaction (iii) in Fig. 2 was
employed in which compound 4 was the starting material. This
procedure, however, did not work for Sn(OH)2PcR4 and PbPcR4.
Reaction (v) in Fig. 3 proved to be successful in which Ge was
inserted into the isolated metal-free phthalocyanine 5 in quinoline.
Reaction (vi) and (vii) are necessary to prepare Sn(OH)2PcR4 and
PbPcR4, but 5 was not isolated during the one-pot two-step reac-
tion, in which the first step was to allow 5 to form. The reaction did
not lead to 8 (or 9) when 3 and SnCl2 (or PbAc2) were heated under
reflux in n-pentanol in the presence of DBU.
outside PC
p-ring. The mutual interaction leads to the deviation
of PbPC ring from planarity to a shuttlecock shaped structure
[55e57].
The BeereLambert law plots for all of the compounds in ethanol
showed good linearity in the range of concentrations investigated
(0.50e20 mM), indicating no aggregation in the ground state.
3.2.2. Fluorescence properties
The fluorescence emission spectra of ML2PcR4 are compared in
Fig. 6. Their spectral shape are all similar and typical of that for PCs,
although the maximal wavelengths of the emission peaks
are influenced by the central elements. Table 1 includes the exci-
tation and emission maxima (lex and lem). The effect of the central
elements on lex or lem is the same as their influence on labs
(Table 1).
3.2. Photophysics
Table 1 summarizes the photophysical properties of compounds
6, 7, 8, 9. Both the fluorescence and triplet excited state properties
Fig. 6. (A) The normalized fluorescence emission spectra of MPcR4 (M ¼ Si, Ge, Sn) in EtOH with excitation at 610 nm (absorbance 0.050). (B) The normalized excitation (emission at
730 nm) and emission spectra of PbPcR4 in EtOH (with excitation at 610 nm).

X.-F. Zhang et al. / Dyes and Pigments 99 (2013) 480e488
485
the atomic nucleus can impart an electromagnetic force on the
electron causing it to change its spin. This can only occur if the
electron is alone in an orbital and thus free from the influences of
another electron (the case in S1 state). The Hamiltonian for spin-
orbit interaction (Hseo) is strongly proportional to the nuclear
charge as shown in Eq. (6), which is intractably proportional to the
size of the atom [58].
z4e2
3
Hseo
¼
L$S;
(6)
8
p
0me2c2
In which e is the elementary charge of an electron,
3
0
is the
permittivity of the vacuum, me is the mass of an electron, c is the
speed of light, and z is the atomic number of the acting nucleus. The
vector portion is the dot product of the electron’s angular mo-
mentum, L, and spin, S. The magnitude of spin-orbit coupling is
thus greatly affected by the size of the nucleus.
Fig. 7. Fluorescence decay of MPCR4 (M ¼ Si, Ge, Sn) in EtOH with excitation at 672 nm
diode laser (70 ps), the emission was monitored at emission maximum, the concen-
The stronger spin-orbit coupling results in the faster ISC from S1
tration of dyes is ca. 2.0 mM. Bottom: fitting residues for SiPCR4.
to T1, i.e. a larger kisc arises, which leads to a smaller Ff and sf, since
sf ¼ (kf þ kic þ kisc
by kf ¼ Ff
three ML2PCR4 (M
)
ꢁ1, and Ff ¼ kf/(kf þ kic þ kisc). kf can be obtained
The symmetry between the excitation and emission spectra
holds for all ML2PCR4 compounds. The stokes shift were about
17(ꢂ3) nm, the small values indicate only a slight change of the
molecular geometry upon photoexcitation.
/sf, the values are also included in Table 1. kf values for the
¼
Si, Ge, Sn) are about the same, ca.
0.13 ꢃ 109 sꢁ1, but reduced remarkably to 0.55 ꢃ 108 sꢁ1 for PbPCR4.
The decrease of kf in PbPCR4 is due to the significant bending of the
The fluorescence decays in Fig. 7 are satisfactorily described
by monoexponential functions. In all cases, the weighted re-
siduals (Fig. 7 bottom) are small, and the reduced chi-squared
values (c2 in Table 1) are 1.02 ꢂ 0.02, indicating a good agree-
PbPC p-system, so that a strain force exists and favors the internal
conversion by heat release.
ment of the monoexponential fits. The decay lifetimes (
s
f) are
3.3. Triplet excited state properties
given in Table 1.
Ff is monotonically lowered with the increase of atomic size
(Table 1). So is the case for f. This lowering of Ff and f is generally
due to the enhancement of the spin-orbit coupling between d-or-
bitals of the metal ion and PC -ring electrons. The angular
movement of the electron through the magnetic field created by
The transient absorption spectra for the PC complexes are dis-
played in Fig. 8. Laser flash photolysis (LFP) was carried out with
laser excitation (4 ns) pulse at 355 nm. The samples for the spectra
were dissolved in DMF and purged with argon for 20 min before the
measurement. The transient spectra show common features: 1) the
s
s
p
Fig. 8. The transient absorption spectra in argon saturated DMF with laser excitation (4 ns) at 355 nm (absorbance was adjusted to w0.3).

486
X.-F. Zhang et al. / Dyes and Pigments 99 (2013) 480e488
Fig. 9. Triplet decays and ground state recovery of MPCR4 in argon saturated DMF (left and middle). Triplet decays in air saturated DMF with laser excitation (4 ns) at 355 nm (right).
negative absorption in the Q-band region showed peaks matching
the corresponding ground state absorption; 2) the broad shape of
the positive absorption is similar to the T1eTn transient spectra
found previously for other similar PCs [33,59e61]; 3) the positive
bands are separated from the ground state bleaching with well
defined isosbestic points; 4) the decrease of the positive band is
accompanied by the increase of the negative band; 5) the bleaching
recovery kinetics is synchronous with the absorption decay kinetics
(shown in Fig. 9). The concurrent behavior indicates that as the
positive absorbing transient decays, the ground state is repopu-
lated. In summary, the positive absorptions are due to T1eTn
absorption decay while the negative bands are attributed to the
recovery of S0eS1 absorptions; the spectral changes with time
reflect a two state transformation: T1 / S0.
The T1eTn absorption maximum (lTeT in Table 1) is a function of
the central elements. lTeT decreases with the atomic size, but
PbPCR4 showed slightly different behavior (Table 1).
Typical transient decay curves are given in Fig. 5. The synchro-
nous ground state recovery is also shown. These curves can all be
fitted by the monoexponential function. The triplet lifetimes are
collected in Table 1. The
s
T values are all larger than 120
ms, the PC
complex with a heavy central metal exhibits shorter
s
T values than
that of Si(OH)2PCR4. The lifetimes are comparable to those of other
PCs [48], and are sufficiently long for photosensitizing the pro-
duction of singlet oxygen.
In air saturated DMF the triplet species decayed at a much faster
rate (Fig. 9 right), the sT values are 0.45, 0.54, 0.38 and 0.39 ms for
ML2PCR4 (M ¼ Si, Ge, Sn, Pb), respectively. This is due to a very
efficient molecular oxygen quenching. The rate constants by oxy-
gen quenching (ket) range from 0.92
ꢃ
109 Mꢁ1 sꢁ1 to
1.31 ꢃ109 Mꢁ1 sꢁ1. The values are close to one-ninth of the diffusion
rate of O2 in the solvent.
This effective oxygen quenching also indicates that the positive
absorptions are due to T1eTn triplet absorptions. The ket values
obtained above are comparable to the rate constants obtained for
O2 quenching of the triplet excited states of a variety of porphyrins
and phthalocyanines [62].
FT for each of the four compounds in DMF is given in Table 1. In
contrast to Ff change, FT becomes larger with increasing the atomic
size, as shown in Table 1. The incorporation of large atoms signifi-
cantly enhances the efficiency of triplet state formation. FT is
increased 2.5 times to 0.75 for PbPCR4 from 0.30 for Si(OH)2PCR4,
indicating an effective heavy atom effect. The summation of FT and Ff
in all cases is less than 1, suggesting the presence of internal
conversion.
3.4. Singlet oxygen formation by DPBF chemical trapping
The quantum yields for singlet oxygen formation (FD) were
measured in air saturated DMF by using the chemical trapping
method with irradiation at 660 nm. Photosensitized DPBF degra-
dation was monitored by UVevis spectroscopy. Fig. 10 displays the
variation of DPBF absorbance upon irradiation, for which the
pseudo zero-order kinetics could be applied (Fig. 10).
FD is also collected in Table 1. The coordination of large atoms
remarkably enhances the efficiency of singlet oxygen formation.
The value of FD for each PC is close to its FT, suggesting a good
efficiency of energy transfer, considering the two competing
processes:
Fig. 10. Top: The variation of the absorption spectra in Sn(OH)2PcR4 (6
sensitized system containing 32 M DPBF in DMF, with irradiation at 660 nm. Bottom:
The plot of DPBF absorbance at 410 nm against time, and the linear fitting.
mM) photo-
m

X.-F. Zhang et al. / Dyes and Pigments 99 (2013) 480e488
487
[12] Rapulenyane N, Antunes E, Masilela N, Nyokong T. Synthesis and photo-
physicochemical properties of novel zinc phthalocyanines mono substituted
with carboxyl containing functional groups. J Photochem Photobiol A Chem
2012;250:18e24.
ket
3
1
T1ðPSÞ þ O2 S ðPSÞ þ O ; T energy transfer to form
!
0
2
1
singlet oxygen
[13] Makhseed S, Tuhl A, Samuel J, Zimcik P, Al-Awadi N, Novakova V. New highly
soluble phenoxy-substituted phthalocyanine and azaphthalocyanine de-
rivatives: synthesis, photochemical and photophysical studies and atypical
aggregation behavior. Dyes Pigment 2012;95:351e7.
T ðPSÞk S ðPSÞ; T1 natural decay
d
!
1
0
since ket[O2] values are 1.84e2.56 ꢃ 106
s
ꢁ1, which is 300 times
T),
[14] Saka ET, Gol C, Durmus M, Kantekin H, Biyikhoglu Z. Photophysical, photo-
chemical and aggregation behavior of novel peripherally tetra-substituted
phthalocyanine derivatives. J Photochem Photobiol A Chem 2012;241:67e78.
[15] Zhang XF, Guo WF. Imidazole functionalized magnesium phthalocyanine
photosensitizer: modified photophysics, singlet oxygen generation and
photooxidation mechanism. J Phys Chem A 2012;116:7651e7.
[16] Tombe S, Chidawanyika W, Antunes E, Priniotakis G, Westbroek P, Nyokong T.
Physicochemical behavior of zinc tetrakis (benzylmercapto) phthalocyanine
when used to functionalize gold nanoparticles and in electronspun fibers.
J Photochem Photobiol A Chem 2012;240:50e8.
larger than that of the competing decay rate of T1 itself (kd ¼ 1/
s
kd values are 0.38e0.82 ꢃ 104
s
ꢁ1).
4. Conclusions
We have synthesized ML2PCR4 compounds in which M is Si,
Ge, Sn, and Pb respectively, which includes all possible group IV
elements in the periodic table. The preparation is not trivial since
the common procedures reported in literature by using the
condensation of compound 3 failed. The ML2PCR4 compounds
harness the heavy atom effect to enhance triplet and singlet
oxygen formation.
[17] Uslan C, Sesalan BS. Synthesis of novel DNA-interacting phthalocyanines. Dyes
Pigment 2012;94:127e35.
[18] Zeballos NCL, Vior MCG, Awruch J, Dicelio LE. An exhaustive study of a novel
sulfur-linked adamantane tetrasubstituted zinc(II) phthalocyanine incorpo-
rated into liposomes. J Photochem Photobiol A Chem 2012;235:7e13.
[19] Uslan C, Sesalan BS, Durmus M. Synthesis of new water soluble phthalocya-
nines and investigation of their photochemical, photophysical and biological
properties. J Photochem Photobiol A Chem 2012;235:56e64.
The photophysical and photochemical processes related to PDT
were revealed by comparing the steady state and transient spectra
of S0, S1, and T1 of these compounds. The insertion of large atoms
into PC complexes significantly enhances the efficiency of triplet
state and singlet oxygen formation. The triplet quantum yield is
increased 2.5 times to 0.75 for PbPCR4 from 0.30 for Si(OH)2PCR4,
[20] Pereira JB, Carvalho EFA, Faustino MAF, Fernandes R, Neves M, Cavaleiro JAS,
et al. Phthalocyanine thio-pyridinium derivatives as antibacterial photosen-
sitizers. Photochem Photobiol 2012;88:537e47.
[21] Nombona N, Antunes E, Chidawanyika W, Kleyi P, Tshentu Z, Nyokong T.
Synthesis, photophysics and photochemistry of phthalocyanine-epsilon-
polylysine conjugates in the presence of metal nanoparticles against Staphy-
lococcus aureus. J Photochem Photobiol A Chem 2012;233:24e33.
[22] Ke MR, Yeung SL, Fong WP, Ng DKP, Lo PC. A phthalocyanine-peptide con-
jugate with high in vitro photodynamic activity and enhanced in vivo tumor-
retention property. Chem Eur J 2012;18:4225e33.
[23] Nombona N, Maduray K, Antunes E, Karsten A, Nyokong T. Synthesis of
phthalocyanine conjugates with gold nanoparticles and liposomes for
photodynamic therapy. J Photochem Photobiol B Biol 2012;107:35e44.
[24] Ke MR, Ng DKP, Lo PC. A pH-responsive fluorescent probe and photosensitiser
based on a self-quenched phthalocyanine dimer. Chem Commun 2012;48:
9065e7.
[25] Iqbal Z, Masilela N, Nyokong T, Lyubimtsev A, Hanack M, Ziegler T. Spectral,
photophysical and photochemical properties of tetra- and octaglycosylated
zinc phthalocyanines. Photochem Photobiol Sci 2012;11:679e86.
[26] Zhang XF, Guo WF. Indole substituted zinc phthalocyanine: improved pho-
tosensitizing ability and modified photooxidation mechanism. J Photochem
Photobiol A Chem 2011;225:117e24.
while the triplet lifetime is longer than 120 ms. Correspondingly, the
quantum yield of singlet oxygen formation is promoted to 0.64 for
PbPCR4 from 0.26 for Si(OH)2PCR4. In the mean time, these PC
complexes still maintain reasonably good fluorescence character-
istics. These properties indicate that some of the PC complexes are
good candidates for singlet oxygen photosensitizers but the toxicity
of lead compounds to the human body must be noted.
Acknowledgments
This work has been supported partially by Hebei Provincial
Science Foundation (Contract B2010001518) and HBUST (Contract
CXTD2012-05).
[27] Durmus M, Yaman H, Gol C, Ahsen V, Nyokong T. Water-soluble quaternized
mercaptopyridine-substituted zinc-phthalocyanines: synthesis, photo-
physical, photochemical and bovine serum albumin binding properties. Dyes
Pigment 2011;91:153e63.
[28] Vior MCG, Monteagudo E, Dicelio LE, Awruch J. A comparative study of a novel
lipophilic phthalocyanine incorporated into nanoemulsion formulations:
photophysics, size, solubility and thermodynamic stability. Dyes Pigment
2011;91:208e14.
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