Two-Photon Excited FRET Dyads for Lysosome-Targeted Imaging and Photodynamic Therapy

Article abstract of DOI:10.1021/acs.inorgchem.8b01581

Two-photon excitable fluorescent dyes with integrated functions of targeted imaging and photodynamic therapy (PDT) are highly desired for the development of cancer theranostic agents. Herein, fluorescence resonance energy transfer (FRET) dyads, AceDAN-H

Full text of DOI:10.1021/acs.inorgchem.8b01581

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Two-Photon Excited FRET Dyads for Lysosome-Targeted Imaging
and Photodynamic Therapy
Mengliang Zhu,† Jinghui Zhang,† Yabin Zhou,‡ Peipei Xing,†,§ Lei Gong,† Chaorui Su,† Dongdong Qi,†
Hongwu Du,‡ Yongzhong Bian,*,† and Jianzhuang Jiang*,†
†Beijing Key Laboratory for Science and Application of Functional Molecular and Crystalline Materials, Department of Chemistry,
University of Science and Technology Beijing, Beijing 100083, China
‡Department of Biology, University of Science and Technology Beijing, Beijing 100083, China
§Beijing Vocational College of Agriculture (Beiyuan), Beijing 100012, China
S
* Supporting Information
ABSTRACT: Two-photon excitable fluorescent dyes with integrated functions
of targeted imaging and photodynamic therapy (PDT) are highly desired for the
development of cancer theranostic agents. Herein, fluorescence resonance energy
transfer (FRET) dyads, AceDAN-H2Por-Lyso (1a) and AceDAN-ZnPor-Lyso
(1b), were developed for two-photon excited (TPE) lysosome-targeted
fluorescence imaging and PDT of cancer cells. Under one-photon or two-
photon excitation, the AceDAN donor can effectively transfer the excited state
energy to the porphyrin acceptor via high efficient FRET, leading to the
generation of deep-red fluorescence and singlet oxygen for cell imaging and
PDT, respectively. 1a and 1b exhibit high photocytotoxicity and low dark
cytotoxicity, in addition to strong lysosomal targeting capability in living cells. By
taking the advantages of the two-photon absorption properties of the AceDAN
donor and the properly distributed S1 and T1 states of the porphyrin acceptor,
the AceDAN-porphyrin dyads 1a and 1b have been successfully applied to TPE-fluorescence imaging for tracking the significant
morphology changes of cancer cells under two-photon laser irradiation.
approach for regulating the emission wavelength is to
incorporate a TPE dye into a fluorescence resonance energy
transfer (FRET) system as the energy donor.12 Then, by
choosing a proper FRET acceptor, the intrinsic emission of a
TPE dye could be translated to the emission of the acceptor
with a tunable manner.13
INTRODUCTION
■
Fluorescence imaging is a prominent detection technique for
biomedical research due to the high-resolution, real-time, and
nondestructive features.1 The precise diagnosis and fluores-
cence-guided treatment of disease are pursued by applying
analyte-specific and stimuli-responsive fluorescent probes to
bioimaging.2 In this respect, one encouraging strategy is to
integrate the fluorescence imaging and the photodynamic
therapy (PDT) modalities to formulate theranostic agents for
the direct visualization and synchronized treatment of
tumors.3,4 However, most of the current fluorophores for
theranostic agents are based on one-photon excitation (OPE)
by ultraviolet−visible (UV−vis) light, which may suffer from
low penetration depth and high background noise.5 Alter-
natively, two-photon excitation (TPE)-based fluorophores
allow for excitation with near-infrared (NIR) light,6,7 which
are feasible for achieving the advantages of deep tissue
penetration, high spatial resolution, low background signal,
and minimized side effects for TPE-fluorescence imaging8 and
TPE-PDT.9 Nevertheless, conventional TPE dyes mostly emit
in a relatively short and fixed spectral range of blue to green
color (400−550 nm),8 which may also restrict the clinical
translation of TPE-fluorescence imaging. One of the goals of
the current research is to develop TPE dyes with near-infrared
(NIR) light emission characteristics.10,11 An attractive
On the other hand, porphyrins have shown great potential in
fluorescence imaging and PDT treatment of tumors14,15 due to
the superior photophysical properties and well established
synthetic chemistry.16 In particular, metal-free-17 and some
metallo-porphyrins18 possess bright deep-red emissions from
singlet excited states (S1) due to the high extinction
coefficients in the vis−NIR region and the substantial
fluorescence quantum yields, which are desired for bioimaging
applications. Furthermore, these porphyrins also have long-
lived triplet excited states (T1),19 which are essential for the
type-II PDT photosensitizers to generate singlet oxygen
(1O2).20,21 Together, porphyrin derivatives could simultane-
ously perform the dual functions of imaging agents and PDT
photosensitizers by properly manipulating and distributing the
S1 and T1 states.22
Received: June 7, 2018
© XXXX American Chemical Society
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Nevertheless, the cellular uptake efficacy and subcellular
localization should be considered as well for the development
of effective imaging and PDT agents.23 Due to the short
lifetime of singlet oxygen (1O2), the PDT outcome strongly
depends on the subcellular localization of photosensitizers,24
and organelle-targeted photosensitizers can specifically accu-
mulate and efficiently induce apoptosis of cancer cells.25,26 The
lysosome, the recycling center of cells, is a kind of vesicle
structure containing numerous hydrolases.27 Singlet oxygen
generated by activating lysosomal targeting photosensitizers
can induce lysosomal damage and release hydrolytic enzymes
into the cytoplasm, thereby leading to cell necrosis.15,28
On the basis of the above wisdom, herein we present the
design of new functional porphyrins for two-photon excited
lysosome-targeted fluorescence imaging and PDT. A two-
photon absorption (TPA) donor and a lysosome-targeted
moiety were attached to a porphyrin acceptor forming the new
theranostic agents AceDAN-H2Por-Lyso (1a) and AceDAN-
ZnPor-Lyso (1b; Scheme 1). Upon two-photon excitation
The lysosome-targeted dyads AceDAN-H2Por-Lyso (1a)
and AceDAN-ZnPor-Lyso (1b) were constructed from 5,15-
diaryl-porphyrin precursors (4a and 4b) via Suzuki−Miyaura
coupling reactions32 with phenylboronic acid ester derivatives
of MPL and AceDAN successively (Scheme S1). The reference
compounds H2Por-Lyso (3a) and ZnPor-Lyso (3b) were
synthesized by a similar procedure. The reference compound 2
was obtained by the methylation of 7 with methyl iodide
(Scheme S2). All of the new compounds were sufficiently
characterized by H NMR, H−1H COSY, 13C NMR, and
13C−1H COSY spectroscopies and mass spectrometry and
elemental analysis (Figures S1−S9, Supporting Information).
Photophysical Properties. The photophysical data of
AceDAN-H2Por-Lyso (1a), AceDAN-ZnPor-Lyso (1b), and
the reference compounds 2, H2Por-Lyso (3a) and ZnPor-Lyso
(3b), are summarized in Table S1, Supporting Information.
The ground state absorption spectra of 1a and 1b show the
characteristic Soret bands (ca. 420 nm) and Q bands (500−
650 nm) of porphyrins in addition to a less intense and broad
absorption band of the AceDAN moiety centered at 370 nm
(Figure 1a), which resembles the linear superimposition of
those of compounds 3a or 3b with 2, respectively, indicating
1
1
Scheme 1. Structures of AceDAN-H2Por-Lyso (1a);
AceDAN-ZnPor-Lyso (1b); and the Reference Compounds
AceDAN (2), H2Por-Lyso (3a), and ZnPor-Lyso (3b)
(TPE) with NIR light (740 nm), the donor could effectively
transfer the excited state energy to the porphyrin acceptor via
FRET. Then deep-red fluorescence (600−750 nm) and singlet
oxygen (1O2) were generated from the porphyrin acceptor,
which were utilized successfully for lysosome-targeted imaging
and PDT of cancer cells.
RESULTS AND DISCUSSION
■
Design and Synthesis. 2-Acetyl-6-dimethylaminonaph-
thalene (AceDAN),29 as a typical TPE chromophore, was
selected to pair up with a metal-free or zinc porphyrin (H2Por
or ZnPor) to formulate the FRET dyads. A morpholine (MPL)
moiety was also incorporated into the TPE-FRET system for
lysosome-targeted cell imaging30 and to improve the PDT
efficiency by lysosomal photodamage induced cell death.31 The
introduction of 3,5-di-tert-butylphenyl groups at the remaining
meso-sites was expected to suppress the tendency of
aggregation; thus it could increase the solubility and be
beneficial for the fluorescent emission and singlet oxygen
generation.
Figure 1. Absorption (a) and emission spectra (b) of AceDAN-
H2Por-Lyso (1a), AceDAN-ZnPor-Lyso (1b), and 2 in CHCl3 (2
μM) with excitation at 370 nm.
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the absence of strong electronic interactions between the
porphyrin and AceDAN units in the ground state (Figure S10,
Supporting Information).
Upon excitation at 370 nm, where the AceDAN moiety
absorbs most of the light, 1a and 1b give the characteristic
emissions of H2Por (λmax = 654 nm, τ = 7.78 ns, Φem = 0.14)
and ZnPor (λmax = 606 nm, τ = 1.62 ns, Φem = 0.036),
respectively, and a very weak emission band of AceDAN (λmax
= 436−438 nm, Φem = 0.011−0.013; Figures 1b and S11,
Supporting Information). As expected, the AceDAN emissions
of 1a and 1b are significantly quenched in comparison with
that of the reference 2 (λmax = 440 nm, Φem = 0.49), while the
emissions of H2Por and ZnPor are obviously enhanced relative
to those of 3a and 3b under the same experimental conditions
(Figure S12, Supporting Information). This observation
indicates the occurrence of intramolecular FRET from the
AceDAN donor to the porphyrin acceptor, which is further
supported by comparing the excitation spectra of 1a and 1b
with those of 3a and 3b in the range of 300−400 nm (where
the absorption of the AceDAN moiety dominates, Figure S12
in the Supporting Information). The efficiencies of energy
transfer (ηEET) were estimated to be up to 97% for 1a and 98%
for 1b,18 which are believed to be due to the effective spectral
overlaps between the AceDAN emission and the porphyrin
absorption as well as the short distance between the two
fluorophores (Figures S13 and S14, Supporting Informa-
tion).33
The two-photon absorption cross section (δTPA) values of
1a, 1b, and 2 in the range of 720−880 nm were determined by
using the two-photon excited fluorescence measurement
technique.6 The peak δTPA values of 1a (112 GM) and 1b
(95 GM) are obtained at 740 nm (Figure 2a), which are
obviously inherited from the AceDAN moiety, as the reference
compound 2 (120 GM) exhibits a larger δTPA value than 3a
(25 GM) and 3b (12 GM) at this wavelength.8 Under the
optimum two-photon excitation at 740 nm with femtosecond
pulses, compound 2 exhibits a strong fluorescence emission
peak at 440 nm, which is translated to the characteristic
emissions of porphyrin in the deep-red region (Figure 2b). In
addition, 1a and 1b show enhanced two-photon fluorescence
emission responses in comparison with 3a and 3b under the
same excitation conditions (Figure S15, Supporting Informa-
tion). These results confirm that the two-photon absorption of
the AceDAN donor could be utilized to efficiently generate the
excited states of the porphyrin acceptor via FRET, thus
indicating the high potential in TPE-fluorescence imaging and
TPE-PDT.
Figure 2. Two-photon absorption cross section (a) and two-photon
excited emission spectra (b) of AceDAN-H2Por-Lyso (1a), AceDAN-
ZnPor-Lyso (1b), and 2 in CHCl3 (5 μM) with two-photon
excitation at 740 nm with 100 fs pulses.
Lysosome-Targeted Cell Imaging under One-Photon
Excitation. The cell imaging performance was first evaluated
under one-photon excitation by confocal laser scanning
microscopy (CLSM). To verify the lysosome-targeting ability,
a commercial lysosome-targeting dye, LysoTracker Green
DND-26, was applied to costain A549 cells with 1a and 1b,
respectively. The A549 cells were incubated with 1a or 1b (8
μM in DMEM, with 0.4% DMSO and 0.1% Cremophor EL,34
v/v) at 37 °C for 15 min and then incubated with LysoTracker
Green DND-26 (100 nM) for another 5 min. As shown in
Figure 3, the red fluorescence from 1a and 1b overlapped well
with the green fluorescence from DND-26. The Pearson’s
coefficients35 of the red channel and green channel were
calculated to be 0.81 and 0.86 for 1a and 1b, respectively. And
the overlap coefficients were 0.85 and 0.89 for 1a and 1b,
respectively (Figure S16, Supporting Information). Moreover,
Figure 3. (a) Colocalization images of A549 cells stained with 1a (8
μM, red channel, λex = 405 nm, λem = 550−750 nm) and DND-26
(100 nM, green channel, λex = 488 nm, λem = 500−550 nm). (b)
Colocalization images of A549 cells stained with 1b (8 μM, red
channel, λex = 405 nm, λem = 550−750 nm) and DND-26 (100 nM,
green channel, λex = 488 nm, λem = 500−550 nm). The orange lines in
the red and green channels represent the region of interest (ROI)
across the A549 cells.
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the variations of intensity profiles in the linear region of
interest (ROI; given by orange lines in Figure 3) are almost
synchronous in the two channels (Figure S17, Supporting
Information). These observations indicate that 1a and 1b are
colocalized with the commercial LysoTracker, thus confirming
the specific lysosomal targeting in living cells.
Singlet Oxygen Generation and Photocytotoxicity
under One-Photon Excitation. To evaluate the potential as
effective photosensitizers for PDT, the singlet oxygen
generation of 1a and 1b under one-photon excitation was
first performed using a chemical trapping method36 with 1,3-
diphenylisobenzofuran (DPBF) as an efficient 1O2 scavenger.37
From the time-dependent absorption changes of DPBF
(Figure S18, Supporting Information), it is clear that the
presence of 1a and 1b leads to the fast decay of DPBF upon
yellow light (λ > 490 nm) irradiation, while 1a induces a
higher decay rate of DPBF than that of 1b, indicating a higher
1O2 generation efficiency of the former. The quantum yields of
1O2 production (ΦΔ) were then determined by measuring the
NIR phosphorescence of 1O2 (ca. 1275 nm)38 upon excitation
at 370 nm. Distinct emissions around 1275 nm were observed
in the presence of 1a and 1b, Figure 4a. The integral intensities
1
of O2 emissions are linearly proportion to the optical density
(OD = 0.01−0.05) values of the utilized photosensitizers,
Figure 4b. Thus, by reference to tetraphenylporphyrin (TPP,
ΦΔ = 0.55),39 the O2 quantum yields were obtained for 1a
1
(0.57) and 1b (0.66), respectively. The substantial 1O2
generation of 1a and 1b is believed to be mainly based on
the intramolecular FRET from the excited AceDAN donor to
the porphyrin acceptor;40 thereafter, the intersystem crossing
(ISC) of the excited porphyrin moiety and the intermolecular
3
triplet−triplet energy transfer to the ground state of O2 are
involved.14 In addition, it is noted that 1b showed a higher ΦΔ
value than 1a, which can be attributed to the different
distribution of S1 and T1 states between H2Por and ZnPor.22
The cytotoxicities of 1a and 1b (1−8 μM) were examined
on A549 cells in the dark and under light irradiation (λex > 490
nm, 18 J·cm−2), respectively. As shown in Figure 5, 1a and 1b
were essentially noncytotoxic to A549 cells (viability > 90% at
8 μM) in the absence of light, but displayed significant
photocytotoxicity with increasing concentration from 1 to 8
μM. According to the survival curves in the presence of light,
the impressive photocytotoxicity with IC50 values of 3.1 and
4.6 μM were obtained for 1a and 1b, respectively.
Figure 4. (a) The NIR phosphorescence emission spectra of singlet
oxygen obtained from O2-saturated solutions of 1a, 1b, and TPP (OD
= 0.05) with excitation at 370 nm in CHCl3. (b) The relative intensity
1
of O2 emission versus absorption at 370 nm.
Two-Photon Excited Cell Imaging and PDT. TPE-
fluorescence imaging of 1a and 1b toward A549 cells was
performed under the irradiation of a 740 nm femtosecond laser
(115 mW, 80 MHz, 140 fs) by CLSM. The morphology
changes of cells were tracked within 0−30 min to access the
TPE-PDT feasibility (Figure 6). Both 1a and 1b could get into
A549 cells after 15 min of incubation and show strong red
fluorescence under two-photon excitation. The images of two-
photon (TP) mode are coincident with those of one-photon
(OP) mode and bright field, indicating the promising TPE-
fluorescence imaging performance of 1a and 1b.
After a 740 nm laser scan for 10 min, the fluorescence of 1a
and 1b in A549 cells was diffused and enhanced, suggesting the
disruption of lysosomes and other membrane structures in
cytoplasm. As the irradiation time was prolonged, blebs were
formed on the cell boundaries accompanied by obvious cell
shrinkage, which may indicate the rupture of the cell
membrane. After 30 min of laser scanning, A549 cells loaded
with 1a or 1b had lost their morphological integrity or even
Figure 5. Dark and photocytotoxicity (λex > 490 nm, light dose 18 J·
cm−2) of 1a and 1b (1−8 μM) on A549 cells obtained by a MTT
assay.
obviously deformed, leaving only residues, which represents
cell death. For comparison, the cells without photosensitizer
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fraction was collected and evaporated. Repeated chromatography
(CHCl3) followed by recrystallization from CHCl3 and ethanol gave
1
pure target compound as a purple powder (20 mg, 34%). H NMR
(CDCl3, 400 MHz, 293 K): δ 8.87 (d, J = 4.0 Hz, 6H), 8.84 (d, J =
4.0 Hz, 2H), 8.38 (s, 1H), 8.19 (d, J = 8.0 Hz, 2H), 8.13 (d, J = 8.0
Hz, 2H), 8.09 (s, 4H), 7.98 (d, J = 8.0 Hz, 1H), 7.92 (d, J = 12.0 Hz,
1H), 7.80 (s, 2H), 7.73 (d, J = 8.0 Hz, 1H), 7.62 (d, J = 8.0 Hz, 2H),
7.43 (dd, J = 8.0 Hz, J = 4.0 Hz, 1H), 7.28 (d, J = 8.0 Hz, 2H), 7.15
(s, 1H), 5.05 (s, 2H), 4.41 (t, J = 4.0 Hz, 2H), 3.84 (t, J = 4.0 Hz,
4H), 3.43 (s, 3H), 3.01 (t, J = 4.0 Hz, 2H), 2.73 (t, J = 4.0 Hz, 4H),
2.70 (s, 3H), 1.53 (s, 36H), −2.71 (s, 2H). 13C NMR (CDCl3, 100
MHz, 293 K): δ 197.9, 158.7, 149.9, 148.9, 141.4, 138.0, 137.7, 135.7,
135.1, 131.2, 130.5, 130.1, 126.5, 125.5, 125.1, 124.9, 121.6, 121.2,
116.6, 113.0, 105.8, 67.2, 66.4, 58.1, 56.7, 54.4, 39.2, 35.2, 31.9, 26.6.
MALDI-TOF-MS, m/z calcd for C80H86N6O3 (M+): 1178.7. Found:
1178.8. Anal. calcd for C80H86N6O3: C, 81.46; H, 7.35; N, 7.12.
Found: C, 81.23; H, 7.39; N, 7.15.
5,15-Bis[3,5-di(tert-butyl)phenyl]-10-[4-(2-morpholinoethoxy)-
phenyl]-20-[4-[[(6-acetyl-2-naphthalenyl)methylamino]methyl]-
phenyl]porphyrinatozinc (1b). By using the procedure described
above with 6b (52 mg, 0.05 mmol) instead of 6a as the starting
material, 1b was obtained as a reddish purple powder (31 mg, 50%).
1H NMR (CDCl3, 400 MHz, 293 K): δ 8.89−8.95 (m, 8H), 8.35 (s,
1H), 8.18 (d, J = 8.0 Hz, 2H), 8.10 (d, J = 8.0 Hz, 2H), 8.07 (s, 4H),
7.95 (d, J = 8.0 Hz, 1H), 7.89 (d, J = 8.0 Hz, 1H), 7.78 (s, 2H), 7.71
(d, J = 8.0 Hz, 1H), 7.59 (d, J = 8.0 Hz, 2H), 7.41 (d, J = 8.0 Hz, 1H),
7.22 (d, J = 8.0 Hz, 2H), 7.13 (s, 1H), 5.04 (s, 2H), 4.30 (s, 2H), 3.47
(s, 4H), 3.41 (s, 3H), 2.87 (s, 2H), 2.67 (s, 3H), 2.46 (s, 4H), 1.53 (s,
36H). 13C NMR (CDCl3, 100 MHz, 293 K): δ 197.9, 158.4, 150.0,
148.5, 142.6, 142.3, 138.0, 137.2, 135.5, 135.0, 131.2, 131.1, 130.5,
130.0, 126.4, 125.5, 124.9, 124.8, 122.5, 122.4, 120.7, 116.6, 112.7,
105.7, 66.8, 66.1, 57.9, 56.7, 54.0, 39.2, 35.2, 31.9, 26.6. MALDI-
TOF-MS, m/z calcd for C80H84N6O3Zn (M+): 1240.6. Found:
1241.1. Anal. calcd for C80H84N6O3Zn: C, 77.31; H, 6.81; N, 6.76.
Found: C, 77.83; H, 6.76; N, 6.71.
Figure 6. Two-photon (TP, red channel, λex = 740 nm) and one-
photon (OP, red channel, λex = 405 nm, merged with bright field)
confocal images of A549 cells treated with 8 μM of 1a (a) and 1b (b).
Images were obtained after different times (0−30 min) of laser
irradiation (740 nm, 115 mW, 80 MHz, 140 fs).
showed no significant change under the same irradiation
(Figure S19, Supporting Information). These observations
clearly demonstrate that 1a and 1b possess strong photo-
cytotoxicity under two-photon excitation, thus they can
hopefully be photosensitizers for TPE-PDT.
ASSOCIATED CONTENT
* Supporting Information
■
S
CONCLUSION
■
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b01581.
In summary, we have developed AceDAN-porphyrin dyads 1a
and 1b for lysosome-targeted two-photon excited fluorescence
imaging and PDT of cancer cells. Upon one-photon or two-
photon excitation of the AceDAN donor, intramolecular FRET
processes occurred with high energy transfer efficiencies (ηEET
= 97−98%), leading to the formation of the excited porphyrin
acceptor, from which deep-red fluorescence (600−750 nm)
and singlet oxygen were generated and utilized for cell imaging
and PDT simultaneously. 1a and 1b exhibited strong
lysosome-targeting abilities and low cytotoxicity in the dark,
while displaying high photocytotoxicity with IC50 values as low
as 3.1 and 4.6 μM under irradiation (λex > 490 nm, 18 J·cm−2).
The AceDAN-porphyrin dyads were also successfully used in
TPE-fluorescence imaging for tracking the significant morphol-
ogy changes of A549 cells under the irradiation of a 740 nm
femtosecond laser, thus demonstrating a promising TPE-FRET
strategy for the development of cancer theranosis.
Chemicals and instruments, measurement of the photo-
physical properties, dark and photocytotoxicity assay,
cell culture and confocal microscopy studies, synthesis
and characterization details, DFT calculation of dyad 1b,
Schemes S1 and S2, Figures S1−S19, and Table S1
(PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: yzbian@ustb.edu.cn.
*E-mail: jianzhuang@ustb.edu.cn.
■
ORCID
Yongzhong Bian: 0000-0003-0621-3683
Jianzhuang Jiang: 0000-0002-4263-9211
Notes
EXPERIMENTAL SECTION
■
Synthesis and Characterization of 1a and 1b. 5,15-Bis[3,5-
di(tert-butyl)phenyl]-10-[4-(2-morpholinoethoxy)phenyl]-20-[4-
[[(6-acetyl-2-naphthalenyl)methylamino]methyl]phenyl]porphyrin
(1a). To a solution containing compound 6a (48.5 mg, 0.05 mmol) in
toluene (10 mL) were added Cs2CO3 (82 mg, 0.25 mmol),
Pd(PPh3)4 (12 mg, 0.01 mmol), and compound 8 (22.8 mg, 0.055
mmol, 1.1 equiv). The resulting reaction mixture was refluxed for 15 h
under N2 and then cooled to room temperature. The solvent was
removed under reduced pressure. The residue was chromatographed
on a silica gel column (CH2Cl2/C2H5OH = 100:2), where the first
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the National Natural Science
Foundation of China (21471015, 21631003 and 21671017),
Beijing Natural Science Foundation, and Fundamental
Research Funds for the Central Universities is gratefully
acknowledged.
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Oxygen Generation via Two-Photon Excited FRET. J. Am. Chem. Soc.
2004, 126, 5380−5381.
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DOI: 10.1021/acs.inorgchem.8b01581
Inorg. Chem. XXXX, XXX, XXX−XXX