Titanyl phthalocyanine and its soluble derivatives: Highly efficient photosensitizers for singlet oxygen production

Article abstract of DOI:10.1016/j.jphotochem.2009.12.002

The photophysical properties of titanyl phthalocyanine (TiOPc), tetra(β-phenoxy) titanyl phthalocyanine (TiOPc(β-OPh)₄) and tetra(α-phenoxy) titanyl phthalocyanine (TiOPc(α-OPh)₄), were investigated in homogeneous solution. Absorptio

Full text of DOI:10.1016/j.jphotochem.2009.12.002

Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 232–237
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Titanyl phthalocyanine and its soluble derivatives: Highly efficient
photosensitizers for singlet oxygen production
Xian-Fu Zhanga,b,∗, Yun Wanga, Lihong Niub
a Chemistry Department, Hebei Normal University of Science and Technology, Qinhuangdao, Hebei Province 066004, China
b Chemistry Department, Tsinghua University, Beijing 100084, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 August 2009
Received in revised form 6 November 2009
Accepted 2 December 2009
Available online 11 December 2009
The photophysical properties of titanyl phthalocyanine (TiOPc), tetra(␤-phenoxy) titanyl phthalocya-
nine (TiOPc(␤-OPh)4) and tetra(␣-phenoxy) titanyl phthalocyanine (TiOPc(␣-OPh)4), were investigated
in homogeneous solution. Absorption, fluorescence, triplet state and singlet oxygen [O2(1ꢀg)] sensiti-
zation studies are reported. The phthalocyanines present high triplet quantum yields (˚T) of 0.81, 0.85
for TiOPc(␤-OPh)4 and TiOPc(␣-OPh)4, respectively, while they still maintain long triplet lifetime (ꢁT)
of 80 ␮s and 69 ␮s. This makes them produce O2(1ꢀg) very efficiently with quantum yields (˚ꢀ) of
0.79 (TiOPc), 0.76 (TiOPc(␤-OPh)4) and 0.86 (TiOPc(␣-OPh)4). The ground state of all the phthalocya-
nines shows a strong absorption band in the red region (690 nm for TiOPc, 698 nm for TiOPc(␤-OPh)4,
and 718 nm for TiOPc(␣-OPh)4 with ε > 150,000 M−1 cm−1 in DMSO), which is located in the optimal
optical window of photodynamic therapy of tumor (PDT). For the singlet excited state, the following
data were determined in DMSO: energy levels, Es = 1.75 0.04 eV, lifetime, ꢁf = 4.0 0.1 ns and fluo-
rescence quantum yield, ˚f = 0.13–0.06 in air-saturated solution. The high ˚ꢀ, reasonable long ꢁf and
sufficiently good ˚f make the phthalocyanines not only excellent for O2(1ꢀg) production as photosensi-
tizers, but also good for fluorescent imaging of tumors and the biodistribution detection of the sensitizers
themselves.
Keywords:
Photosensitizer
Singlet oxygen
Titanyl phthalocyanine
Photodynamic therapy
Photophysics
Fluorescence
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
[6–9]. The related studies for the molecular state of TiOPc and
its derivatives in solution are still rare [10,11], and the correla-
Phthalocyanine (Pc) complexes (Scheme 1) belong to a wide
class of ␲-conjugated metallomacrocycles, which are of great inter-
est due to their various important applications in modern science
and technology [1,2]. Although all transition metals can coordi-
nate to Pc ligands, only a few of them (Zn, Ti, Pd) can form highly
photoactive complexes, such as titanyl phthalocyanine (TiOPc)
and zinc phthalocyanine (ZnPc), owing to the closed shell nature
of the electronic configuration of Ti4+ or Zn2+, similar to that of
main group metal ions. Other transition metal Pc complexes usu-
ally show very short lifetime for their S1, T1 state and very low
fluorescence emission efficiency [3]. Although TiOPc have found
many more important applications in modern technologies than
ZnPc and its derivatives, such as high sensitive photoconductors
in laser printing and photo coping [4], the studies on synthesis
and physical properties, however, are mainly placed on the latter
[5]. Our understanding on the photophysical properties of TiOPc
mainly stays on the various crystalline forms of its solid state
tion of photophysical properties with their chemical structures is
much less explored compared to the case of ZnPc. On the basis
of our previous reports on the photosensitizing and other prop-
erties of molecular ZnPc and its derivatives [12–21], we now
extend the studies to TiOPc and its derivatives. It was found in
this study that its quantum yield of singlet oxygen (O2(1ꢀg)) for-
mation is among the highest for various PCs. O2(1ꢀg) is generally
accepted to be a key intermediate in photodynamic therapy (PDT),
and produced by energy transfer from an electronically excited
triplet state of sensit−izer molecule S to the ground state of molec-
ular oxygen, O2(3˙g ) [22,23]. An active research area in PDT
involves the search for and characterization of photosensitizers
that improve upon the presently used hematoporphyrin derivative
(HpD, Photofrin) [24]. Basic requirements for a PDT photosensitizer
include optimal light absorbing properties in the therapeutic win-
dow, where HpD is very weakly absorbing [25]. Many researchers,
include us, have synthesized quite some Pcs meeting the con-
ditions, but only a few of them have such a high efficiency of
O2(1ꢀg) generation as TiOPc family. This shows that TiOPc and its
derivatives have good potential for use in PDT, which leads us to
synthesize well soluble TiOPc derivatives as shown in Scheme 1
and measure their photophysical and photosensitizing properties.
∗
Corresponding author at: Chemistry Department, Tsinghua University, 202W
Gongwuguan, Beijing 100084, China. Tel.: +86 10 627 82456; fax: +86 10 627 70304.
E-mail address: zhangxianfu@tsinghua.org.cn (X.-F. Zhang).
1010-6030/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2009.12.002

X.-F. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 232–237
233
Scheme 1. Synthesis of phenoxy substituted TiOPc.
The results should provide us with structure-property relation-
2.2. Synthesis
ships.
2.2.1. 4-Phenoxyphthalonitrile
Phenol (1.87 g, 20 mmol) and 4-nitrophthalonitrile (3.46 g,
20 mmol) were added and stirred to dissolve in dry DMSO (30 ml)
under argon. To this solution was added four equal portions of
anhydrous K2CO3 (total 4.14 g, 30 mmol) every other 15 min during
1.0 h and the mixture was kept stirring at room temperature. After
24 h total reaction time, the mixture was poured into ice-water
(50 ml), thus forming a precipitate that was filtered, washed by
cold water to neutral and dried in air. Recrystallization twice from
ethanol yielded a white product. Yield: 3.3 g (73%). m.p. 100–101 ◦C.
IR [(KBr) ꢂmax/cm−1]: 3047 (Ar-H), 2233 (CN), 1600–1450 (C C in
Ph), 1235 (C–O–C). 1H NMR (d6-DMSO, ppm): ı 7.75 (d, 1H, Ar-H),
7.44 (t, 2H, Ar-H), 7.21–7.33 (m, 3H, Arꢀ-H), 7.06 (d, 2H, Arꢀ-H). MS
(m/z): 221 (M + 1)+.
2. Experimental
2.1. Materials and instruments
Dimethylformamide (DMF), dimethylsulfoxide (DMSO),
tetrahydrofuran (THF), chloroform and dichloromethane quinoline
were dried and redistilled before use. Titanyl Phthalocyanine
was a product of Aldrich. Tetra(phenoxy) Titanyl Phthalocyanines
and their precursors were synthesized as described below by
modifying procedures in literature [26]. All other reagents were
analytical grade and used as received. 1H NMR spectra were
recorded at room temperature on a Bruker dmx 300 MHz NMR
spectrometer. MS spectra were recorded either on a Bruker APEX
II or Autoflex III Maldi-TOF spectrometer. IR spectra were recorded
at room temperature on a Shimadzu FTIR-8900 spectrometer.
UV–vis absorption measurements were made with either a HP
8451A or Shimadzu 4500 spectrophotometer in 10 mm quartz
cuvettes.
2.2.2. 3-Phenoxyphthalonitrile
The synthesis and purification procedures were the same as
that described for 4-phenoxyphthalonitrile, using phenol (1.87 g,
20 mmol) and 3-nitrophthalonitrile (3.46 g, 20 mmol). Yield: 3.07 g
(68%). m.p. 117–118 ◦C. IR [(KBr) ꢂmax/cm−1]: 3082 (Ar-H), 2231

234
X.-F. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 232–237
(CN), 1600–1450 (C C in Ph), 1245 (C–O–C). 1H NMR (d6-DMSO,
ppm): ı 7.78–7.85 (m, 2H, Ar-H), 7.49–7.55 (t, 2H, Ar-H, Arꢀ-H),
7.25–7.34 (m, 4H, Arꢀ-H). MS (m/z): 221 (M + 1)+.
incident at the sample was attenuated to a few mJ per pulse. Time
profiles at a series of wavelengths from which point-by-point spec-
tra were assembled were recorded with the aid of a PC controlled
kinetic absorption spectrometer. The concentrations of the target
compounds were typically 10 ␮M providing A355 = 0.3 in a 10 mm
cuvette.
The triplet–triplet absorption coefficients (εT) of the samples
were obtained using the singlet depletion method [28], and the
following equation was used to calculate the εT:
2.2.3. 2,9(10),16(17),23(24)-Tetra(ˇ-phenoxy) titanyl
phthalocyanine
4-Phenoxy-phthalonitrile (1.32 g, 6.0 mmol), two drops of DBU
and urea (0.1 g) were dissolved in quinoline (10 ml) under dry
argon. After the temperature was raised to 190 ◦C, Ti(OC4H9)4
(0.75 ml, 2.2 mmol) was then added with a syringe to the solution
and the reaction mixture stirred at 190 ◦C for 6 h under argon. Most
solvent was then removed by the vacuum distillation. After cooling,
30 ml hexane was added to the mixture and stirred. The resulting
mixture was then filtered and washed with hexane and methanol.
The dried crude product was chromatographed with CH2Cl2-THF
(v/v, 9:1) as eluent. Yield: 0.76 g (55%). UV/vis (DMSO): ꢃmax nm
(log ε): 354 (4.73), 366 (4.72), 630 (4.50), 698 (5.20). IR [(KBr)
ꢃmax/cm−1]: 1234 (C–O–C), 970 (Ti O). 1H NMR (CDCl3, ppm): ı
8.62–8.90 (t, q, t; 4H; Pc–H), 8.34–8.51 (s, s, d; 4H; Pc–H), 7.64–7.78
(m, 4H, Pc–H), 7.54–7.61 (m, 8H, Phenyl-2,6-H), 7.42–7.50 (m,
8H, Phenyl-3,5-H), 7.30–7.41 (m, 4H, Phenyl-4-H). MALDI-TOF-MS
(m/z): 945.4 (M + 1)+.
ꢀAT
εT = εS
(1)
ꢀAS
where ꢀAS and ꢀAT are the absorbance change of the triplet
transient difference absorption spectrum at the minimum of the
bleaching band and the maximum of the positive band, respec-
tively, and εS is the ground-state molar absorption coefficient at
the UV–vis absorption band maximum. Both ꢀAS and ꢀAT were
obtained from the triplet transient difference absorption spectra.
The triplet quantum yield ˚T was obtained by comparing the
ꢀAT of the optically matched sample solution at 355 nm in a
1 cm cuvette to that of the reference, ZnPc (˚T = 0.65 0.02, εT
(480) = 30,000 1000 M−1 cm−1) [29], using the equation:
εZTnPc
ꢀAT
ꢀAZTnPc
˚
T
= ˚ZTnPc
,
(2)
2.2.4. 1,5(8),9(12),13(16)-Tetra(˛-phenoxy) titanyl
phthalocyanine
εT
where the superscripts represent the reference, ꢀAT is the
absorbance of the triplet transient difference absorption spectrum
at the selected wavelength, and εT is the triplet state molar absorp-
tion coefficient.
Singlet oxygen quantum yield (˚ꢀ) determinations were car-
ried out using the chemical trapping method [30]. Typically, a
2 ml portion of the respective TiOPc solutions that contained
diphenylisobenzofuran (DPBF) was irradiated at 670 nm in air-
saturated dimethylsufoxide. ˚ꢀ values were obtained by the
relative method using ZnPc as the reference (Eq. (3)):
The procedure for synthesis and purification was the same
as that described for tetra(␤-phenoxy) titanyl phthalocyanine,
using 3-phenoxyphthalonitrile (1.32 g, 6.0 mmol) and Ti(OC4H9)4
(0.75 ml, 2.2 mmol). Yield: 0.41 g (30%). ꢃmax nm (log ε): 324 (5.12),
360 (4.99), 644 (4.72), 718 (5.43). IR [(KBr) ꢃmax/cm−1]: 1247
(C–O–C), 971 (Ti O). 1H NMR (CDCl3, ppm): ı 7.62–9.10 (m, 12H;
Pc–H), 7.14–7.19 (m, 20H; Ar–H). MALDI-TOF-MS (m/z): 945.3
(M + 1)+.
2.3. Photophysical measurements
k
kref
Iaref
˚
= ˚ref
,
(3)
ꢄ
DMSO and other solvents were dried and freshly distilled before
use. Measurements were carried out at room temperature of 22 ◦C.
UV–vis absorption measurements were made in DMSO with a
HP 8451A or Shimadzu 4500 spectrophotometer in 10 mm quartz
cuvettes, sample concentrations were adjusted so that the Q-band
absorption maxima were between 0.20–2.0. Fluorescence spectra
up to 900 nm were monitored using a PerkinElmer LS 55, with
2.5 nm slits. All spectra were corrected for the sensitivity of the
photo-multiplier tube. The fluorescence quantum yield (˚f) was
ꢄ
Ia
where ˚rꢀef is the singlet oxygen quantum yield for the standard
(0.65 for ZnPc in DMSO) [31], k and kref are the DPBF photo bleach-
ing rate constants in the presence of the respective samples and
standard, respectively; Ia and Iref are the rates of light absorption at
a
the irradiation wavelength of 670 nm by the samples and standard,
respectively. Their ratio can be obtained by Eq. (4):
ref
670
Iaref
1 − 10−A
calculated by ˚f = FsA0˚0/(F0As), in which F is the integrated fluo-
f
=
,
(4)
Ia
1 − 10−A
670
rescence intensity, A is the absorbance at excitation wavelength, the
subscript 0 stands for a reference compound and s represents sam-
ples. Zinc phthalocyanine was used as the reference (˚0f = 0.20)
To avoid chain reactions induced by DPBF in the presence
of singlet oxygen, the concentration of DPBF was lowered to
∼3 × 10−5 mol dm−3. A solution of sensitizer (absorbance ∼0.65 at
the irradiation wavelength) that contained DPBF was prepared in
the dark and irradiated in the Q-band region. DPBF degradation at
415 nm was monitored. The error in the determination of ˚ꢀwas
∼10% (determined from several ˚ꢀ values).
[27]. Excitation wavelengths of 610 nm corresponding to S0 to S1
transitions were employed. The sample and reference solutions
were prepared with the same absorbance (Ai) at the excitation
wavelength (near 0.09 per cm). All solutions were air-saturated.
Fluorescence lifetime of S1 was measured by time-correlated single
photon counting method (Edinburgh FL-900 spectrophotometer)
with excitation at 660 nm by a CdS diode laser (50 ps FWHM)
and emission was monitored at 700 nm. Transient spectra were
recorded in degassed DMSO (prepared by bubbling with Argon for
20 min) with an Edinburgh LP-920 laser flash photolysis system.
A Nd:YAG laser (Continuum surelite II, 355 nm and 7 ns FWHM)
was used as excitation source. The analyzing light was from a
xenon lamp. The laser and analyzing light beams perpendicularly
passed through a quartz cell with an optical path length of 1 cm.
The signal was displayed and recorded on a Tektronix TDS 3012B
oscilloscope and an Edinburgh LP900 detector. The laser energy
3. Results and discussion
3.1. Synthesis
The reactions used to prepare the phenoxy substituted TiOPcs,
are summarized in Scheme 1. Phenoxy substituted TiOPcs were
selected for the study because the aromatic electrophilic reactions
of phenol such as that used for 4-OPhPN and 3-OPhPN (Scheme 1)
gave products in much better yield over that of alcohol under the
condition at room temperature. It was found that the product yield

X.-F. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 232–237
235
Fig. 1. Normalized UV–vis absorption spectra of TiOPC and its derivatives in DMSO.
Fig. 2. The fluorescence spectra of three Pcs measured under same conditions, the
excitation wavelength is 610 nm, absorbances at 610 nm were adjusted to be 0.090.
was quite low when n-pentanol, a common solvent for Pc synthe-
sis, was employed, and products are mixed with the metal free Pcs.
This suggests that titanium is harder to form complex with Pc than
zinc, copper, nickel etc. Raising reaction temperature by using high
b.p. solvent quinoline solved the problem. The reaction time, how-
ever, needs to be controlled since the Pcs, especially ␣-phenoxy
substituted TiOPC, could decompose slowly at the temperature. To
remove quinoline by distillation after the reaction, vacuum is highly
suggested to lower the b.p. and hence reduce the decomposition.
is not a desired feature for PDT since the H dimer or aggregate is
non-photoactive. ZnPc(␤-X)4 (X = I, Br, Cl), for example, forms H
dimer in DMSO in significant amount [12]. TiOPcs are non-planar
in which the Ti is out of the ␲ plane structures, which is likely the
reason of less aggregation than ZnPcs.
3.3. Fluorescence
The three Pcs showed different fluorescence behavior, as plot-
ted in Fig. 2 in which three spectra were measured with the
excitation at 610 nm. The fluorescence emission peak for TiOPc
was observed at 701 nm, but the substitutions caused the red
shift of the emission maximum and the significant reduction
of their emission intensity. The bathochromic shift for TiOPc(␣-
OPh)4 relative to TiOPc is 38 nm, which is larger than 14 nm
for TiOPc(␤-OPh)4. The fluorescence quantum yields were mea-
sured by reference to ZnPc and summarized in Table 1. ˚f of
TiOPc is 0.13 and the same magnitude as but smaller than that
of ZnPc. The peripherally ␤-tetrasubstituted complex show larger
Both substituted TiOPcs show excellent solubility (≥0.55 mmol L−1
)
in most organic solvents, which makes purification by column
chromatography convenient. Maldi-TOF mass spectra, IR and 1H
NMR showed satisfied results that are agreed with their structures.
UV–vis spectra were obtained as expected, as shown below.
3.2. Ground-state absorption spectra
The normalized UV–vis absorption spectra of the Pc compounds
measured in DMSO solutions are displayed in Fig. 1, along with that
of unsubstituted TiOPc. All showed similar spectral behavior, i.e., a
strong but narrow Q-band centered around 700 nm and a B-band
located at ca. 350 nm, which are typical for metallophthalocyanines
in molecular state. As expected, the ␣ substitution caused the larger
red shift of Q-band than that of the ␤ substitution. The detailed
spectral data are collected in Table 1. The larger bathochromic shift
of the ␣-substitution is due to linear combinations of the atomic
orbital (LCAO) coefficients at the ␣ positions of the HOMO being
greater than those at the ␤ positions. As a result, the HOMO level
is destabilized more at the ␣ position than it is at the ␤ position.
Essentially, the energy gap (ꢀE) between the HOMO and LUMO
becomes smaller, resulting in a bathochromic shift [32,33].
The Q-band absorption maxima of the TiOPcs are about 20 nm
longer than other common metal Pcs, such as Mg, AlCl, Zn, and Ga,
which are more suitable for PDT.
˚
F
values, suggesting less quenching of the excited singlet state
by peripheral ␤-tetrasubstitution compared to non-peripheral ␣-
tetrasubstitution, which is generally true for all metallo Pcs [5].
The absorption and fluorescence emission spectra are compared
in Fig. 3. The fluorescence excitation spectra are quite similar to the
absorption spectra and not shown. Each absorption spectrum is
basically the mirror image of the corresponding fluorescence spec-
trum for all the complexes. The S1 excitation energy (Es) is then
able to be calculated from each cross point and included in Table 1.
3.4. Fluorescence lifetime
The fluorescence decays in Fig. 4, were satisfactorily described
by monoexponential functions. In all cases, the weighted residuals
(such as that for TiOPc(␤-OPh)4 on bottom of Fig. 4), their autocor-
relation function and reduced chi-squared (ꢅ2 < 1.20) were used to
judge the quality of the fit. The decay lifetimes (ꢁf) are given in
Table 1. ꢁf for ␣- and ␤-substituted TiOPcs is 4.0 and 4.1 ns, respec-
tively, exhibited little difference from 4.0 ns of TiOPc itself. Ishii and
No indications for dimerization or aggregation in their ground
states were found for the three Pcs in DMSO, which is evidenced
by the absence of the typical broad dimeric band in their UV–vis
spectra. Dimerization or aggregation occurs for many Pcs [12], but
Table 1
Photophysical parameters for the Pcs in DMSO.
ꢃabs (nm)
Log ε
ꢃem (nm)
Stoke shift (nm)
Es (eV)
ꢁf (ns)
ꢃT–T (nm)
ꢁT (␮s)
ꢀεT
˚
T
˚
f
˚
ꢀ
TiOPc(␣-OPh)4
TiOPc(␤-OPh)4
TiOPc
718
698
690
5.4
5.2
–
738
715
701
20
17
11
1.71
1.76
1.79
4.0
4.1
4.0
570
530
–
79
69
150,710
128,220
–
0.85
0.81
–
0.06
0.10
0.13
0.86
0.76
0.79

236
X.-F. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 232–237
Fig. 3. Comparison of normalized absorption and fluorescence spectra of in DMSO. Excitation wavelength = 610 nm.
Kobayashi have given the most complete compilation for the ꢁf val-
ues of various Pcs [3], but no data on TiOPc or its derivatives were
included, probably because of lacking of such reports. The ꢁf values
measured in this study for TiOPcs are comparable to that of ZnPcs
and other main group metal Pcs, typically in the range from 2 ns
to 9 ns [3], while CoPc, NiPc, FePc, etc., transition metal Pcs show
much shorter values which are usually only several ps [3]. This is
apparently because of the closed shell nature of Ti4+ which reduces
the spin–orbital interactions between the Ti4+ and the ␲ system of
the Pcs, so that the TiOPcs are highly photoactive.
OPh)4 and TiOPc(␤-OPh)4 showed similar spectral features, i.e.,
negative absorption in the Q- and B-band regions, a broad positive
absorption maximum near 570 nm and 530 nm, respectively. The
positive bands are separated from the ground-state bleaching with
well defined isosbestic points, and the bleaching recovery kinetics
are synchronous to the absorption decay kinetics, indicating con-
comitant behavior, i.e., as the positive absorbing transient decays,
the ground state is repopulated. It is very likely that the transient
3.5. Triplet state transient spectra and kinetics
Fig. 5 shows the transient spectrum changes for TiOPc(␣-OPh)4
after laser excitation with 7 ns pulses at 355 nm. Both TiOPc(␣-
Fig. 5. The transient absorption spectra of TiOPc(␣-OPh)4 in degassed DMSO with
laser excitation at 355 nm (top). The bottom contains decays at 580 nm and rise at
710 nm. [TiOPc(␣-OPh)4] = 13 ␮M.
Fig. 4. Fluorescence decay curves of the Pcs at 700 nm in DMSO with excitation at
660 nm. The concentration of Pcs is controlled at 10 ␮M.

X.-F. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 232–237
237
observable on the nanosecond and longer time scale has a broad
continuous absorption between ca. 400 and ca. 800 nm, superim-
posed upon which is an intense ground-state bleaching signal.
In air or oxygen saturated solutions the transient absorptions
were effectively quenched. The shape of the broad positive absorp-
tion is similar to those found previously with other Pcs except the
red shift of absorption maxima. By analogy the positive absorp-
tion band can be assigned to the triplet–triplet (T1–Tn) absorption.
The red shift of absorption maxima from ca. 490 nm of other
Pcs is reasonable, since the ground-state absorption spectrum of
TiOPc(␣-OPh)4 and TiOPc(␤-OPh)4 is also red shifted about 50 nm
and 30 nm from 670 nm of ZnPc, respectively. The appearance of
the isosbestic point between positive absorption and ground-state
bleaching indicated that the species near 570 nm or 530 nm is
responsible for the ground-state repopulation. Therefore we can
conclude that the positive peaks arise from a T1 to Tn transition.
The values of the T1–Tn absorption coefficient and triplet state
quantum yields for the Pcs were calculated and collected in Table 1.
The time profiles (bottom of Fig. 5) at the maximum wavelengths
were recorded from which kinetic parameters were extracted. The
triplet state lifetime (ꢁT) is 69 ␮s and 79 ␮s for them, and the triplet
quantum yield (˚T) is 0.81 and 0.85, respectively. ˚T and ꢁT are the
two major factors that govern the efficiency of singlet oxygen for-
mation [12]. The ˚T values for the TiOPcs are larger than that of
other closed shell Pcs [3], which is an advantage for them to be
singlet oxygen photosensitizers. The ꢁT values, however, are some-
what smaller than, but still in the same order to that of other closed
shell Pcs [3]. In cases studied in this report, triplet state decays were
first order, indicating no triplet-triplet annihilation.
stituted titanyl phthalocyanines. These complexes are monomeric
in DMSO solution. The phthalocyanines showed high singlet oxy-
gen quantum yields ranging from 0.76 to 0.85 because of their
large triplet quantum yields and sufficient long triplet lifetime. The
Pcs still maintain long fluorescence lifetime and good fluorescence
quantum yields. Together with their strong absorption in the red
region around 700 nm, all these properties give an indication of
the potential of the complexes as photosensitizers in applications
where singlet oxygen is required (Type II mechanism), in particular
as Type II photosensitizers for PDT.
Acknowledgement
We thank HUBUST for financial support.
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Singlet oxygen quantum yields (˚ꢀ) were determined in DMSO
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of DPBF absorption maximum was recorded and plotted against
irradiation time for quantitative kinetic analysis (Section 2). The
˚
values for three Pcs are 0.79, 0.76 and 0.86, all are very high
ꢀ
compared to other Pcs.
The high ˚ꢀ values can be attributed to their high ˚T values,
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ket is in the order of 109 M−1 s−1 and [O2] is ∼2.0 × 10−3 mol dm−3
.
˚
is mainly determined by ˚T, ꢁT, and ket. Because the ꢁ is suf-
ficient long such that (ꢁT)−1 ꢁ ket[O ] < ket[O ]/Á, hence ˚ T ≈ ˚T:
ꢀ
2
2
ꢀ
ket[O2]
ꢁT−1 + ket[O2]/Á
˚
= ˚T
.
(5)
ꢀ
Note that the ˚T of CuPc is also as high as 0.90 [3], but its
is much lower as of the short ꢁT value of 0.035 ␮s [3], so that
ꢀ ≈ 0.065˚T. Generally ˚ ≈ 0.5˚T, when ꢁT = 0.5 ␮s.
˚
˚
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ꢀ
ꢀ
4. Conclusion
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In conclusion, we have measured and compared the photophysi-
cal and photosensitizing properties of ␤- and ␣-tetra-phenoxy sub-