Germanium triazatetrabenzcorrole and germanium phthalocyanine: Synthesis, fluorescence and singlet oxygen generation

Article abstract of DOI:10.1142/S1088424618500670

The synthesis, fluorescence properties and singlet oxygen generation capability of germanium tetrabenzotriazacorrole (LGeTBC), germanium phthalocyanine (Cl2GePc) and their derivatives are described. Measurements include UV-vis absorption spectra, fluoresc

Full text of DOI:10.1142/S1088424618500670

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2018; 22: 1–6
DOI: 10.1142/S1088424618500670
Published at http://www.worldscinet.com/jpp/
Germanium triazatetrabenzcorrole and germanium
phthalocyanine: Synthesis, fluorescence and singlet
oxygen generation
a,b
a
Xian-Fu Zhang* and Xiaojie Sun
a
Institute of Applied Photochemistry, Hebei Normal University of Science and Technology, Qinhuangdao,
Hebei Province, 066004, China
MPC Technology, Hamilton, ON L8S 3H4, Canada
b
Dedicated to Professor Naisheng Chen on the occasion of his 80th birthday
Received 16 April 2018
Accepted 30 April 2018
ABSTRACT: The synthesis, fluorescence properties and singlet oxygen generation capability of
germanium tetrabenzotriazacorrole (LGeTBC), germanium phthalocyanine (Cl GePc) and their
2
derivatives are described. Measurements include UV-vis absorption spectra, fluorescence emission
spectra, fluorescence quantum yields, fluorescence lifetimes, and singlet delta oxygen quantum
yields. LGeTBC and its derivatives exhibit quite different spectral and fluorescence properties from
their phthalocyanine precursor. Both LGeTBC and Cl GePc show high singlet delta oxygen quantum
2
yields and suitable fluorescence quantum yields, indicating that they can act as good singlet oxygen
photosensitizers for photodynamic therapy.
KEYWORDS: triazatetrabenzocorrole, phthalocyanine, corrole, singlet oxygen, photodynamic therapy.
low oxidation potentials, the ability to stabilize high-
valent oxidation states, and high reduction potentials
INTRODUCTION
Triazatetrabenzcorroles (TBCs) are large p-
[4, 9, 10, 24]. We also studied the photophysical and
conjugated macrocyclic compounds derived from phtha-
locyanines (Pcs) as shown in Scheme 1. The synthesis,
characterization and properties of TBCs have been
reviewed recently [1]. A TBC molecule is obtained by
eliminating one bridging nitrogen atom in a Pc molecule
photochemical properties of phosphorous and silicon
TBCs [1–3, 11]. We now extend the study to germanium
TBC, including the synthesis, fluorescence properties
and singlet oxygen generation capability of germanium
tetrabenzotriazacorrole (LGeTBC) and its derivative
[
2–18]. Pc materials have been the focus of study in
L’GeTBCR (Scheme 1).
4
diverse areas for over a century and have been applied
as dyes, pigments, chemical sensors, non-linear optics,
liquid crystals and photosensitizers in photodynamic
therapy [19–22]. TBCs, however, are relatively new.
Only P, Ge, and Si have been inserted into the central
cavity of TBC and isolated as pure stable complexes
EXPERIMENTAL
Materials and instruments
[
2–10]. Studies on physical and chemical properties of
All experiments were carried out in redistilled DMF.
Zinc phthalocyanine (ZnPc) was the product of Tokyo
Kasei. All other reagents were analytical grade and
TBCs are even rarer, with only phosphorous and silicon
TBCs having been explored [2, 3, 9–11, 23]. Based on
the known data, TBCs show unique properties such as
1
used as received. H NMR spectra were recorded at
room temperature on a Bruker DMX 300 MHz NMR
spectrometer. MSspectrawererecordedeitheronaBruker
APEX II or Autoflex III MALDI-TOF spectrometer.
*
Correspondence to: Xian-Fu Zhang, email: zhangxianfu@
tsinghua.org.cn.
Copyright © 2018 World Scientific Publishing Company

2
X.-F. ZHANG AND X. SUN
Scheme 1. The chemical structures of Cl GePc, L’ GePcR , LGeTBC, and L’GeTBCR
4
2
2
4
IR spectra were recorded at room temperature on a
Shimadzu FTIR-8900 spectrometer. UV-vis spectra were
recorded on a Shimadzu 4500 spectrophotometer using
Synthesis of L’ GePcR . This preparation and purifi-
2 4
cationwasperformedaccordingtotheproceduredescribed
in the literature [25]. 4-(para-tert-butyl phenoxy) metal-
free phthalocyanine (0.31g) was placed in a 25 mL flask,
and freshly distilled quinoline (10 mL) was added. After
10 min of argon bubbling, germanium tetrachloride
(0.5 mL) was added, and the mixture was stirred at
240°C for 2 h. After cooling down, stirring and argon
bubbling were stopped. Adding n-hexane (50 mL) to the
mixture, precipitation was formed and filtered. The dried
deep blue solid was purified by column chromatography
using CH Cl /ethanol (95/5 v/v) as eluent. Yield: 31%.
1
cm matched quartz cuvettes.
Synthesis of Cl GePc (dichlorogermanium (IV)
2
phthalocyanine). This preparation and purification was
performed according to the procedure in the literature
[
25]. A mixture of GeCl (20 g, 0.094 mol) and metal-
4
free phthalocyanine (50 g, 0.39 mol) was placed in
quinoline (100 ml) and brought very slowly to reflux with
constant stirring. Heating continued at 240°C for 4 h with
stirring. The hot mixture was filtered and the solid was
extracted in a Soxhlet extractor with successive portions
of DMF, xylene and acetone. Purification was carried
out by heating a 300 mg sample in a vacuum sublimator
at 450–460°C for 2 h. Anal. Calcd for C H N GeC1 :
2
2
-
1
IR (KBr)/cm : 743, 955, 1271, 1464, 1506, 1717 (n Pc
ring); 1053 (n Ge–O); 1364, 1404, 2926, 2963 (n CH );
3
1088, 1238 (n Ar–O–C); 3306 (n OH); UV-vis (DMF)
lmax/nm: 359, 689; MALDI-TOF-MS (m/z) cal.: 1196.4
3
2
16
8
2
+
C, 58.69; H, 2.46; N, 17.07; Cl, 10.81. Found: C, 58.32;
found: 1178.3 [M–H O] .
2
-1
H, 2.25; N, 16.840; Cl, 10.34. IR (KBr) nmax/cm : 1523,
334, 1288, 1167, 1121, 905, 756, 727 (Pc ring stretching
mode), 1084, 1423, 1364, 669. UV-vis (DMF) lmax/nm:
55 (0.45), 607 (0.21), 644 (0.21), 673 (1.13), MS m/z
Synthesis of L’GeTBCR4. This synthesis and
purification was carried out by following the reported
procedure [5]. As shown in Scheme 1, a mixture of
L GePcR (0.21 g, 0.3 mmol) and NaBH4 (0.30 g,
1
3
2
4
+
(
MALDI-TOF) [M + H] calculated 657.1, found 657.3.
0.80 mmol) was stirred at 145°C in diethylene glycol
dimethyl ether (5 mL) for 2 h in an argon gas atmosphere.
Water (20 mL) was added and filtered. The dried solid
was purified by column chromatography using CH Cl /
Synthesis of LGeTBC. This synthesis and purification
was carried out by following the reported procedure [5]
Scheme 1). Cl GePc (2.0 g, 3.0 mmol) was mixed with
(
2
2
2
NaBH (0.37 g, 0.98 mmol) in a mixture of benzyl alcohol
ethanol (95/5 v/v) as eluent. Yield: 21%. UV-vis (DMF)
4
(
50 mL) and anisole (25 mL). The mixture was stirred
lmax/nm: 454, 672; MALDI-TOF-MS (m/z) cal.: 1165.3
+
at 160°C for 2 h in an Ar atmosphere. The hot mixture
was filtered under reduced pressure, and then anisole
was evaporated under reduced pressure at 100°C. The
residual was then washed with ether, and the obtained
solid was repeatedly extracted with hot xylene. The green
extract was evaporated under reduced pressure at 70°C.
The residue was again washed with ether and dried in a
vacuum for 1 day at 100°C. Yield: 37%. IR: n (KBr)/
cm 2924, 2855 (CH ), 1100 (Ge–O–C), 1636, 1580,
1
(
found: 1166.4 [M + H] .
Photophysics
The absorption and fluorescence spectra, fluorescence
quantum yields and excited singlet-state lifetimes, as
well as triplet properties were investigated at room
temperature in DMF. Steady-state fluorescence spectra
were acquired on a FLS 920 with 1 nm slit width for
both excitation and emission monochromators. All
spectra were corrected for the sensitivity of the photo-
max
-
1
2
526, 1439, 1364, 766, 638. UV-vis (DMF)：lmax/nm
A) = 415 (0.80), 444 (2.04), 599 (0.23), 659 (1.02). MS
+
m/z (MALDI-TOF) [M + H] calculated 680.1, found
80.4.
multiplier tube. The fluorescence quantum yield (F )
f
0
6
was calculated by F = F A F /(F A ), in which F is the
f
s
0
f
0
s
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J. Porphyrins Phthalocyanines 2018; 22: 2–6

GERMANIUM TRIAZATETRABENZCORROLE AND GERMANIUM PHTHALOCYANINE
3
integrated fluorescence intensity, A is the absorbance
ether as the solvent for L’GeTBCR makes the treatment
4
at the excitation wavelength, the subscript 0 stands for
much easier, since this solvent is miscible with water.
Therefore, the product can be precipitated easily by
adding water and filtering. Benzyl alcohol for LGeTBC
synthesis is very difficult to be removed to get a dry
a reference compound and s represents samples. Zinc
0
phthalocyanine was used as the reference (Ff = 0.30)
[
26]. Excitation wavelengths of 610 nm corresponding
4
+
to S0 to S1 transitions were employed. The sample
and reference solutions were prepared with the same
product. The ligand linked to Ge is different when the
solvent in the reaction is different (Scheme 1). The UV-vis
and MS data are consistent with the product structure.
Due to the coexistence of many aromatic H atoms, the
absorbance (A ) at the excitation wavelength (near 0.09
i
per cm). All solutions were air saturated.
1
Fluorescence lifetime of S was measured by the
H NMR signal is not split and not informative. LGeTBC
1
Time-Correlated Single Photon Counting Method
and L’GeTBCR are stable both as a solid and in solutions
4
(
(
Edinburgh FLS920 spectrophotometer) with diode laser
50 ps FWHM) excitation at 672 nm, and emission was
of ethanol, DMF, and DMSO for over two years.
monitored at 690 nm.
Absorption and fluorescence properties
UV-vis absorption spectra of the four compounds are
shown in Fig. 1. The UV data are collected in Table 1.
Cl GePc and L’ GePcR show the typical absorption
spectra of phthalocyanines, in which two distinct bands
occur, i.e. a weak B band at about 350 nm and a sharp
Q band in the near IR (about 680 nm). The absorption
spectra of LGeTBC and its derivative, however, are very
different from that of their corresponding Pc precursor.
The relative peak position and height between the B band
and the Q band of the TBCs changed very significantly
Singlet oxygen identification
For measurement of singlet oxygen, typically, a 2 ml
portion of the respective sample solutions that contained
diphenylisobenzofuran (DPBF) was irradiated at 660 nm
in air-saturated DMF. To avoid chain reactions induced by
DPBF in the presence of singlet oxygen, the concentration
of DPBF was lowered to ~3 × 10 mol dm . 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
2
2
4
-5
-3
.
(Fig. 1). Contrary to that of Cl GePc, the B band of
2
4
15 nm was monitored.
LGeTBC absorbs much more strongly than its Q band.
Compared to that of the corresponding Pc, the position
of the B band in a TBC is red shifted 90 nm, while the
Q band of the TBC is blue shifted about 15 nm. The
shape of the B band and the Q band of a TBC is also quite
different from that of the corresponding Pc. The structural
difference between a TBC and its corresponding Pc is the
absence of a bridge nitrogen, as displayed in Scheme 1.
The missing of the nitrogen atom reduces both the size
and the planarity of the p-conjugated system in a TBC
molecule, which leads to the property changes (Table 1).
RESULTS AND DISCUSSION
Synthesis
For the synthesis of both unsubstituted LGeTBC and
substituted L’GeTBCR (Scheme 1), the reaction of the
Pc precursors with NaBH proceeds smoothly to yield
the product. However, the use of diethyl glycol dimethyl
4
4
Fig. 1. UV-vis absorption (Left) and emission spectra (Right) of Cl GePc, LGeTBC, L’ GePc(R) , and LGeTBC(R) in DMF.
2
2
4
4
Inset, left, is the plot of absorbance at 645 nm against concentration of L’ GePc(R) , which deviates from linear relation due to
2
4
H-aggregation
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J. Porphyrins Phthalocyanines 2018; 22: 3–6

4
X.-F. ZHANG AND X. SUN
Table 1. The photophysical parameters of LGeTBC, L’GeTBCR and Cl GePc in DMF***
4
2
Cl GePc
LGeTBC
444, 659
L’ GePcR
L’GeTBCR4
454, 672
2
2
4
lmax, abs [nm]
355, 673
5.45
681
359, 689
5.34
703
loge
5.22
5.41
lmax, em [nm]
673
690
Stokes shift [nm]
8
14
14
18
Ff
0.13
5.65
0.83
0.10
0.14
5.12
0.12
0.11
tf [ns]
FD
1.26, 5.37 (18%)
0.59
1.07, 4.41 (52%)
0.55
-1
-1
*
**: lmax,abs: wavelength at absorption maximum; e: molar absorption coefficient, [M cm ]; lmax, em:
wavelength at emission maximum; F : fluorescence quantum yield; t : fluorescence lifetime; F : singlet
f
f
D
oxygen quantum yield.
Fig. 2. Normalized fluorescence emission spectra with excitation at 610 nm (absorbance 0.090)
Comparing to the unsubstituted cases, the presence of
The fluorescence emission data are summarized in
the p-tert-butyl phenoxy on the Pc and TBC ring causes
the red shift of the absorption peak, which is due to
the electron-donating nature of this group. L’ GePcR
showed an additional peak at 645 nm, which indicates the
presence of H-aggregation for this compound. Figure 1
Table 1. Fluorescence quantum yield (F ) was determined
f
using the comparative method with ZnPc in DMF as a
standard (F = 0.30). All compounds have a F of 0.12 ±
2
4
f
f
0.02, for example, the value of F obtained is 0.13 for
f
Cl GePc and 0.10 for LGeTBC. This F value is good
2
f
(
inset) shows that the absorbance–concentration plot
for fluorescence imaging and diagnosing during PDT
treatment.
of L’ GePc(R) deviates from linear relation, also an
2
4
indication of its H-aggregation.
The emission decay at 690 nm was determined using
time-correlated single photon counting with excitation at
672 nm using a 50 ps diode laser (Fig. 3), from which
The fluorescence emission spectra of the four
compounds are also shown in Fig. 1. The emission spectra
of the TBCs are very similar to those of phosphorous and
silicon TBCs reported in the literature [1, 4, 13]. The
emission maximum of LGeTBC occurs at 673 nm, which
is blue shifted 8 nm from that of Cl GePc. Similar to the
case of absorption, the presence of O–Ph–C(CH ) on
the Pc and TBC ring causes the red shift of the emission
peak. The excitation spectrum is mirror symmetrical to
its fluorescence emission (Fig. 2).
the fluorescence lifetime (t ) was calculated. The single
f
exponential fitting gives satisfactory results for Cl GePc
2
and L’ GePcR ; the t are 5.65 and 5.12 ns, respectively.
2
4
f
The fluorescence decay of two TBCs, however, shows
bi-exponential behavior, which is very likely due to the
isomerization of TBCs (Fig. 4). The isomer TBC-II in
2
3
3
4
+
Fig. 4 is more stable than TBC-I, since Ge in TBC-II
4
+
is chelated by two six-membered rings, while Ge in
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GERMANIUM TRIAZATETRABENZCORROLE AND GERMANIUM PHTHALOCYANINE
5
Fig. 3. Time profile of fluorescence decay with excitation at 672 nm diode laser (50 ps). The emission was monitored at 690 nm, the
2
concentration of dyes is ca. 2.0 mM. Bottom: fitting residues (c < 1.15 for all fittings)
shorter than its corresponding Pc due to
the isomerization.
Singlet oxygen production
Singlet oxygen quantum yield (FD)
values were determined by the Chemical
Trapping Method [27] with irradiation
at 660 nm in air-saturated DMF. F was
D
computed by the relative method using
ZnPc as reference (Equation 1). FD
values are also included in Table 1.
Fig. 4. The isomerization of LGeTBCs. TBC-II is more stable than TBC-I
ref
a
k I
ref
Φ∆ = Φ∆
,
(1)
ref
k
Ia
TBC-I is ligated by one six-member ring and one five-
member ring (Ge–N –C –C –N ), therefore the long-
lived emitting component is likely from TBC-II. The
fluorescence lifetime and quantum yield of a TBC is
3
6
5
4
ref
D
where F is the singlet oxygen quantum yield for the
ref
standard (0.65 for ZnPc in DMF) [28], k and k are the
Fig. 5. The change of absorption spectrum upon irradiation time in air saturated DMF containg 5 mDBF and 5 mM photosensitizer
with irradiation at 660 nm. Inset is the plot of concentration at 415 nm against irradiation time
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6
X.-F. ZHANG AND X. SUN
DPBF photobleaching rate constants in the presence of
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ref
the samples and standard, respectively; I and I are the
a
a
rates of light absorption at the irradiation wavelength by
the samples and standard, respectively. DPBF degradation
at 415 nm was monitored. Figure 5 displays the decrease
of [DPBF] upon irradiation time, for which the first order
kinetics was observed.
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BothTBCs show good yield of singlet oxygen formation
(
≥ 0.55). The F of Cl GePc is as high as 0.83, indicating it
D
2
is an excellent photosensitizer. The F of L’ GePcR is low
D
2
4
due to its aggregation as indicated by UV-vis absorption
spectra (Fig. 1, left). An additional peak at 645 nm of
H-aggregates for L’ GePcR , together with the inset of
2
4
Fig. 1, the absorbance–concentration plot of L’ GePc(R)
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CONCLUSION
We synthesized two germanium TBC compounds
and their Pc precursors. We measured their UV-vis
absorption spectra, fluorescence properties and singlet
oxygen quantum yields. The results tell that the
absence of a bridging nitrogen in Cl GePc leads to a
remarkable change in various properties from its Pc
4
81–491.
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precursor. Nevertheless, both LGeTBC and Cl GePc
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show high singlet delta oxygen quantum yields and
suitable fluorescence quantum yields, indicating that
they can act as good singlet oxygen photosensitizers for
photodynamic therapy.
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Acknowledgments
We thank the financial support from the Hebei
ProvincialHundredTalentsPlan(ContractE2013100005)
and the Hebei Provincial Science Foundation (Contract
B2014407080).
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Supporting information
1
Figures S1–S3 are given in the supplementary material.
This material is available free of charge via the Internet at
http://www.worldscinet.com/jpp/jpp.shtml
2
2
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