Light-Controlled Generation of Singlet Oxygen within a Discrete Dual

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Light-Controlled Generation of Singlet Oxygen within a Discrete Dual-Stage Metallacycle for Cancer Therapy Yi Qin, Li-Jun Chen, Fangyuan Dong, Shu-Ting Jiang, GuangQiang Yin, Xiaopeng Li, Yang Tian, and Hai-Bo Yang J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 14 May 2019 Downloaded from http://pubs.acs.org on May 14, 2019

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Light-Controlled Generation of Singlet Oxygen within a Discrete DualStage Metallacycle for Cancer Therapy Yi Qin,† Li-Jun Chen,†,* Fangyuan Dong,† Shu-Ting Jiang,† Guang-Qiang Yin,† Xiaopeng Li,‡ Yang Tian,†,* and Hai-Bo Yang†,* Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062, P. R. China. ‡ Department of Chemistry, University of South Florida, Tampa, Florida 33620, United States †

ABSTRACT: Noninvasive control over the reversible generation of singlet oxygen (1O2) has found the practical significance in benefiting photodynamic therapy. In this study, we developed a new dual-stage metallacycle (M) by using photosensitizer and photochromic-switch as the functional building blocks, which enables the noninvasive “off-on” switching of 1O2 generation through the efficient intramolecular energy transfer. Due to the proximal placement of the functional entities within the well-defined metallacyclic scaffold, 1O2 generation in the ring-closed form state of photochromic-switch (C-M) is quenched by photo-induced energy transfer, whereas the generation of 1O2 in the ring-open form state (O-M) is activated upon light irradiation. More interestingly, the metallacycle-loaded nanoparticles with relatively high stability and water solubility were prepared, which allow for the delivery of metallacycles to cancer cells via endocytosis. Their theranostic potential has been systematically investigated both in vitro and in vivo. Under the light irradiation, the designed ring-open form nanoparticles (O-NPs) show remarkable higher cytotoxicity against cancer cells compared to the ring-closed form nanoparticles (C-NPs). In vivo experiments also revealed that tumors can be very efficiently eliminated by the designed nanoparticles under light irradiation with the ability to regulate in vivo generation of singlet oxygen. All these results demonstrated that the supramolecular coordination complexes with a dual-stage state provide a highly efficient nanoplatform for noninvasive control over the reversible generation of 1O2, thus allowing for their promising application in tumor treatment and beyond that.

INTRODUCTION Singlet oxygen (1O2) has been extensively studied over the past decades because of its wide applications in catalysis, photochemistry, and photodynamic therapy (PDT).1 Usually, 1O can be generated through energy transfer from an excited 2 photosensitizer to the ground-state molecular oxygen.2 Recently, the development of activatable photosensitizer system, in which the generation of 1O2 can be controlled in response to the target stimulus, is of great interest.3,4 For example, the controlled generation of 1O2 can reduce the side effects and unselective killing of healthy cells by minimizing nonspecific photodamage from the undesirably generated 1O2 in PDT.5 Thus a great number of activatable photosensitizers have been prepared, which can be activated upon the environmental changes such as pH6 and polarity change7 etc. However, such systems sometimes suffer from imprecise control over the generation of 1O2 because of the involved irreversible or passive interactions.8,9 Therefore, the development of highly efficient activatable photosensitizers that can realize the noninvasive control over the reversible generation of 1O2 is still in great need. It should be noted that light can offer precise noninvasive spatiotemporal control on the property of photochromic-switch.10-12 In this sense, a

system that inherently integrates both photosensitizer and photochromic-switch with the precise number and position could provide an opportunity to reversibly switch off-on the singlet oxygen generation13-15, thus allowing for the promising selective photodynamic therapy. Well-defined supramolecular coordination complexes (SCCs)16-20 are an emerging class of architectures that have captured widespread research interest because of their design flexibility and viability with the extensive applications in molecular recognition, sensing, catalysis, and drug delivery and so on.21-30 We recently demonstrated that discrete metallacycle provides an efficient platform for energy transfer31 since it is very convenient to introduce multiple functional moieties into the well-defined skeleton with the stoichiometry and position of individual functional group being precisely controlled.32 Inspired by our previous study on multiple functionalized metallacycles through coordinationdriven self-assembly33-37, herein, we present a new dual-stage metallacycle M (Figure 1) containing both porphyrin photosensitizer and diarylethene photochromic-switch moieties with the precise number and position. It should be noted that such well-defined metallacyclic scaffold provides a proximal placement of both functional entities for efficient

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intramolecular energy transfer, thus allowing for the noninvasive control over the reversible generation of 1O2. In this dual-stage metallacycle, the efficient generation of 1O from the porphyrin photosensitizer is observed when the 2 diarylethene unit is in the ring-open form (O-M). On the contrary, 1O2 generation is severely suppressed when the diarylethene is converted into the ring-closed form (C-M), which can be then reactivated and fully recovered through the selective irradiation at the distinct wavelengths. Furthermore, metallacycle-loaded nanoparticles (O-NPs and C-NPs, respectively) containing the dual-stage metallacycle M within the hydrophobic interior were prepared to investigate the possibility for off−on switching on 1O2 generation in vitro and in vivo. To our delight, as shown in Figure 1, the resultant nanoparticles are found to generate 1O2 efficiently when they are activated in the “on state” (O-NPs). The photoactivity of the ring-closed form nanoparticles (C-NPs) is effectively quenched in the “off state” with the inadvertent producing of 1O irradiation. More interestingly, PDT effect 2 upon investigation revealed that, after these nanoparticles were indwelled in tumor

Figure 1. Schematic diagrams of light-controlled generation of 1O within a discrete dual-stage metallacycle for cancer therapy: 2 (a) Schematic representation of the controllable generation of 1O2 in metallacycle M and NPs. mPEG-DSPE agents are used for nanoparticles formation. Nanoparticles in ring-open form state (O-NPs) are in the on state of photosensitization and generate 1O2. Nanoparticles in ring-closed form state (C-NPs) are in the off state of photosensitization and do not generate 1O2. (b) Schematic

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illustration of O-NPs accumulation in tumor tissue followed by EPR effect and PDT effect.

microenvironment through the enhanced permeability and retention (EPR) effect, O-NPs exhibited the superior anticancer outcomes than C-NPs in combating against tumor models after treatments. In other words, these switchable dualstage nanoplatforms can realize the effective cancer therapy through the controllable irradiation, thus exhibiting the potentials for clinical treatment of cancer in the future.

RESULTS AND DISCUSSION Synthesis and Characterization. The 120° diarylethenecontaining acceptor A was easily synthesized in a few steps as indicated in the Supporting Information (SI). A 120° donor ligand D substituted with porphyrin moiety, which is a wellknown 1O2 photosensitizer38,39, was adopted to interact with ligand A to afford the discrete metallacycle M. By stirring a mixture of diplatinum-(II) acceptor A and donor D in a 1:1 ratio in a solution of acetone and water (v:v = 5:1), a hexagonal metallacycle M containing both porphyrin and diarylethene units was obtained (Fig. 2) as monitored by multinuclear NMR spectroscopy. The sharp NMR signals in both the 1H and 31P NMR spectra (Fig. S1-S2) along with the presence of cross peaks in two-dimensional (2-D) COSY and 1H-1H NOESY NMR spectra (Fig. S3-S4) indicated the existence of a highly symmetric complex and ruled out the formation of oligomers (detail discussion can be found in the SI). Moreover, in ESI-TOF-MS spectrum, two peaks at m/z = 1257 and 1537, corresponding to different charge states resulted from the loss of hexafluorophosphate counterions [M6PF6−]6+ and [M-5PF6−]5+ species, respectively, were observed. These peaks were isotopically resolved and in good agreement with their theoretical distributions (Fig. S5), thus providing further evidence for the formation of discrete metallacycle M.

Figure 2. Self-assembly of porphyrin-containing 120° donor D or model 120° donor D’ and the diplatinum acceptor A into discrete hexagonal metallacycle M or M’.

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In order to avoid the possible interference of the dark-purple porphyrin units during the observation of the color change from the photochromic groups and give more evidence on the photochromism of the diarylethene-containing metallacycle, a model metallacycle M’ from the self-assembly of diarylethene-containing acceptor A and a typical 120° dipyridine donor ligand D’ without porphyrin moiety was also designed and synthesized (Fig. 2). Under similar conditions, a bulk colorless sample was obtained (Fig. S6-S10, the detailed synthetic and characterization can be found in the SI). Photochromism of Metallacycles M and M’. With these metallacycles in hand, we first examined their photochromic properties. In the photochromic study, a UV lamp (8.0 W) was used as ultraviolet light to perform the photocyclization and the absorbance of metallacycles M and M’ was recorded from 0 s to 60 s. A 500 W xenon lamp with the long pass filter (> 470 nm) was employed to perform the cycloreversion reaction and the absorbance of metallacycles M and M’ was recorded from 0 min to 40 min. When the ring-open form of metallacycle M’ in CH2Cl2 solution was irradiated under UV light (365 nm), an obvious color change from colorless to purple was clearly observed accompanied with an increase in a new absorption peak at 560 nm (Fig. S11a), suggesting the photoisomerization from the ring-open form (O-M’) to the ring-closed form (C-M’). Moreover, the initial color and spectrum fully recovered upon irradiation with the visible light (> 470 nm), indicating a full cycloreversion of the diarylethene moiety (Fig. S11b). Additionally, such photochromic switching process showed good reversibility, and no apparent degradation was observed after several cycles (Fig. S11c).

Figure 3. (a) The absorption spectrum of the ligand and metallacycle M. Inset: photographs of the color change of metallacycle M solutions. (b) Absorption spectral changes of metallacycle M (10 µM in CH2Cl2) under irradiation at 365 nm. The insets show the amplifying absorption spectral changes from 450 nm to 700 nm and the absorbance at 560 nm. (c) Fatigue resistance of metallacycle M (10 µM in CH2Cl2) upon alternating UV (365 nm) and visible light (> 470 nm) irradiation. (d) Changes in emission intensity of metallacycle M at 652 nm over a few cycles of photoisomerization. (10 µM in CH2Cl2, λex = 420 nm).

In the case of porphyrin-containing metallacycle M, the ring-open form exhibited an extremely intense peak at 419 nm and four weak peaks (514, 549, 590, and 646 nm) in the visible region (Fig. 3a), which agreed to the characteristic absorption of porphyrin units.40-42 Irradiation at 365 nm led to an increase in absorption between 500 to 680 nm (Fig. 3b), which was attributed to the ring-closed diarylethene moieties. This observation indicated that the presence of porphyrin units does not affect the photoswitching of metallacycle M. The structural transformation during the photoisomerization process of metallacycles M and M’ was also monitored by NMR spectroscopy (Fig. S12-S15). For example, in the 1H NMR spectrum of metallacycle M, two sets of signals at 2.91 and 7.28 ppm, corresponding to protons on the methyl groups and thiophene rings, respectively, were gradually disappeared upon irradiation at 365 nm, and two new peaks appeared at 2.60 and 6.70 ppm (Fig. S14), which were attributed to the resulting ring-closed form (C-M). Meanwhile, we could assess the photoconversion yield based on the integration of the corresponding ring-closed photostationary state (PSS) in 19F NMR, which was determined to be 94% for metallacycle M (Fig. S15) and > 99% for metallacycle M’ (Fig. S13), respectively. Therefore, with this high photoconversion yield, the ring-closed form metallacycle C-M can be obtained by UV irradiation of the ring-open form metallacycle O-M. Moreover, photoswitching between the ring-open O-M and the ringclosed C-M can be repeated several times by alternating irradiation with UV and visible light (Fig. 3c), indicating the good fatigue resistance of dual-state metallacycle M. Control over the Generation of Singlet Oxygen in Metallacycle. After the successful realization of photochromism of both metallacycles M and M’, we then investigated the controlled generation of singlet oxygen in porphyrin containing dual-stage metallacycle M in DMF. The ring-closed form (C-M) and ring-open form (O-M) of metallacycle M were adopted to compare the quenching efficiency of 1O2 generation, which was monitored by the turnon fluorescence of singlet-oxygen sensor green (SOSG).43,44 We selected a Soret band (λ = 420 nm) to excite the samples to circumvent the potential interference of the switching operation. It was found that, after irradiation at 420 nm for several minutes, the sample O-M exhibited an obvious increase in fluorescence intensity (Fig. S16a), suggesting an excellent photosensitizing ability of metallacycle M, whereas the ring-closed form sample C-M showed almost a complete quenching of 1O2 under the same condition (Fig. S16c). O-M was about 24 times as efficient as C-M in 1O2 generation (Fig. 4a and Table S1, detailed calculation and discussion can be found in SI), thus demonstrating the excellent control of the generation of singlet oxygen. Since the distance between the porphyrin photosensitizer and diarylethene photochromic-switch moieties in metallacycle M is short enough to perform an efficient energy transfer (Fig. S20), a proposed mechanism for the control of 1O generation in M was shown in Fig. 4c, which was centered 2 on the competitive energy transfer pathways of porphyrin emission between the ring-closed diarylethene units and 3O2. DFT calculations on the lowest singlet and triplet state



energies of these two building blocks A and D showed that there was a large energy gap between the ring-open form A (O-A) and ring-closed form A (C-A) at the singlet and triplet energy, which supported the possibility of reversibly adjusting energy transfer from porphyrin to diarylethene. The lowest triplet energy of D (1.33 eV) is higher than the lowest triplet energy of C-A (1.03 eV) but lower than the lowest triplet energy of O-A (2.32 eV) (Table S2). So the energy transfer can occur from D to the ring-closed form C-A rather than to the ring-open form O-A. Meanwhile, the lowest triplet energy of C-A is a little higher than the lowest excited state of 3O2 (0.97 eV)45, which affords an efficient way to suppress energy transfer from D to 3O2.

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nanoparticles since nanoparticles (NPs) have been considered as a promising platform to transport medicine into live cells.4647 The hydrophobic segment of the polymer can package metallacycle M in the core of NPs, while the peripheral PEG groups can increase the solubility in water, thus allowing for the formation of the micellar superstructures. It should be noted that such nanoparticles containing metallacycles could prolong the circulation time and prevent the premature release of metallacycles, thus minimizing the toxicity to the normal tissue. Two kinds of nanoparticles, O-NPs and C-NPs, which contained the ring-open form metallacycles O-M and the ringclosed form metallacycles C-M, respectively, were obtained to directly compare their efficacy of 1O2 generation. As shown in Fig. 5, the spherical micellar structures with a diameter around 60 nm were observed for both O-NPs and C-NPs in transmission electron microscopy (TEM) images. Dynamic light scattering (DLS) experiments of O-NPs and C-NPs showed a unimodal peak distribution with an averaged hydrodynamic diameter around 61 nm and 58 nm, respectively (Fig. 5b and 5d), which were comparable to the results from TEM images. The existence of metallacycle M in the nanoparticles was confirmed by energy dispersive spectrometry (EDS), in which platinum element was observed (Fig. S21). Moreover, the UV-vis absorption and fluorescence spectra of NPs were almost identical to the spectra of metallacycle M (Fig. S22), suggesting that the encapsulation of the metallacycles into nanoparticles had a negligible effect on their photophysical properties. In addition, stability of the obtained nanoparticles was examined by using DLS, which revealed no obvious destruction in 48 h (Fig. S23), thus confirming the good stability of both O-NPs and C-NPs in water.

Figure 4. (a) Fluorescence response of SOSG upon treatment with O-M and C-M for 30 min. Irradiation light for singlet oxygen generation was at 420 nm. λex = 504 nm and λem = 560 nm. (b) Fluorescence response of SOSG upon treatment with O-NPs and C-NPs for 50 min. Irradiation light for singlet oxygen generation was at 420 nm. λex = 504 nm and λem = 529 nm. (c) The proposed mechanism of energy transfer (ET) in the metallacycle M.

The feasibility of the reversible energy transfer between porphyrin and diarylethene units in metallacycle M was then evaluated by recording the emission spectra. As shown in Fig. S17, metallacycle M gave two emission peaks at 652 nm and 717 nm. Upon irradiating with 365 nm light, the emission was gradually quenched by the energy transfer from porphyrin to the ring-closed form diarylethene. A recovery of emission was achieved upon the subsequent irradiation by visible light (λ > 470 nm) due to the reverse photoisomerization. Good reversibility was observed by recording the changes in emission intensity at 652 nm upon alternating cycles of switch on/off, indicating an acceptable fatigue resistance. Light-Controlled Generation of Singlet Oxygen in NPs. In order to evaluate the applicability of the dual-stage metallacycle system in live cells, an amphiphilic polymer, mPEG-DSPE, was used to incorporate metallacycle M into the

Figure 5. (a) TEM image and (b) DLS result of O-NPs. (c) TEM image and (d) DLS result of C-NPs.

After confirming the successful incorporation of metallacycles into NPs, their photosensitization as well as 1O2 controllability was then evaluated. As expected, for O-NPs, an obvious turn-on fluorescence (at 529 nm) of SOSG was observed upon irradiation at 420 nm, indicating its excellent


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photosensitizing ability, while for C-NPs, no obvious change of emission intensity at 529 nm of SOSG was observed (Fig. 4b and S24). The singlet oxygen generation efficiency of ONPs was about 155 times faster than that of C-NPs (Table S3), thus verifying the controllability on 1O2 generation. In Vitro PDT Studies. Porphyrin-based photosensitizer can display the dual functions of fluorescence imaging and photodynamic therapy in many cases.48-49 To probe the cellular uptake behavior and imaging capability of the resultant nanoparticles, confocal laser scanning microscopic (CLSM) studies were performed on HeLa cells. Herein, metallacycleloaded nanoparticles in two different states, O-NPs and CNPs, respectively, were employed to illustrate their switching ability. It should be noted that the diameters of the nanoparticles are both about 60 nm, which allows the nanoparticles going into the cancer cells through the enhanced permeability and retention (EPR) effect.50 Due to the same method for the formation of nanoparticles, O-NPs and C-NPs have a similar uptake ability. As expected, the HeLa cells exhibited red fluorescence after incubation with O-NPs for 2.0 h, and became brighter when the incubation time was extended to 4.0 h, while the ring-closed form C-NPs displayed the relatively weak fluorescence signal under the same experimental conditions (Fig. 6a). This result was consistent with the hypothesis that energy transfer happened from porphyrin to diarylethene in the C-NPs. The red fluorescence signal was observed in the same regions as the blue fluorescence signal from the DAPI stain, indicating the uptake of nanoparticles by the cells and eventual accumulation in the nucleus.

Figure 6. (a) CLSM images of HeLa cells treated with 10 µM ONPs and C-NPs, respectively. (b) PDT treatment with O-NPs and C-NPs in HeLa cells. Incubation time = 24 h. Irradiation time = 30 min.

To evaluate the feasibility of nanoparticles in cancer therapy, PDT studies in vitro were further evaluated by using a 3(4’,5’-dimethylthiazol-2’-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay with concentrations ranging from 0.125 to 16 μM of nanoparticles O-NPs and C-NPs. Notably, both O-NPs and C-NPs presented the relatively low dark cytotoxicity at low concentrations (Fig. S27). As shown in Fig. 6b, the scrammed C-NPs obviously showed relatively lower photocytotoxicity than that of O-NPs because of the suppressed generation of toxic 1O2 of C-NPs in Hela cell. Apparently, although C-NPs could be transported into cell, their low 1O2 generation efficiency remarkably decreased the damage to tumor cells. Moreover, in the case of O-NPs, light irradiation triggered in situ generation of toxic 1O2 in nuclei and maximized the photodynamic therapeutical efficacy. The IC50 of O-NPs under the irradiation has been determined to be 1.926 ± 0.027 µM towards Hela cell, which is moderate but comparable to some of the reported systems.51-53 This result supported the energy-transfer-based control of 1O2 generation for PDT through the photochromic switch in this study. Preliminary In Vivo PDT Response. Motivated by the above results, in vivo antitumor efficiency of NPs was then evaluated in immunocompetent mouse models by using subcutaneous tumor model of human cervical cancer HeLa in the right axilla region of BALB/c mice. After the primary tumor volume reached ~50 mm3, the mice were randomly divided into five treatment groups. In the experimental groups, the O-NPs or C-NPs dispersed in PBS buffer were directly injected into the tumor site of each mouse to investigate the PDT efficiency. In addition, mice treated with PBS, O-NPs and C-NPs without light irradiation, respectively, served as the dark controls. After treatment, the therapeutic effects were assessed by monitoring the change in tumor volume and tumor weight as well as by hematoxylin and eosin (H&E) staining of the tumor tissues. As depicted in Fig. 7a, once the mice were injected with O-NPs and then received 30-min light irradiation, tumor growth was remarkably retarded. Tumor sizes even became smaller after the 7th day, which substantially demonstrated the successful tumor suppression of O-NPs. For comparison, the scrambled C-NPs with light irradiation only inhibited tumor growth to some extent owing to the 1O2 quenching as mentioned above. It should be noted that, due to the unsatisfactory penetration depth of the light at 420 nm, we adopted the injection of nanoparticles into the tumor with the light irradiation for several times. It was found that the tumor was almost completely eradicated at the end upon being treated with O-NPs. In the cases of the controlled experiments, the unobvious inhibition of tumor growth was observed when mice were injected with nanoparticles yet without light irradiation (Fig. S28). When the mice were sacrificed at the 15th day and all the tumor tissues were peeled, a considerable difference in the therapeutical efficacy was further intuitively reflected by the average tumor weight (Fig. 7c) and images of tumor tissues (Fig. 7d). Additionally, the negligible changes of



body weight were also observed of all mice by continuous monitor throughout the entire treatment (Fig. 7b), suggesting the negligible systemic cytotoxicity of metallacycle-loaded nanoparticles during PDT. The satisfactory therapeutic efficacy with the minimal systemic toxicity was further confirmed via standard H&E staining on tissue sections from different treatment groups. As shown in Fig. S29, no obvious physiological morphology changes were found in major organs (heart, liver, spleen, lung, and kidney) of those treated groups compared with the PBS groups, indicating the low systemic toxicity during the PDT process. In contrast, tumor treated with O-NPs with light irradiation exhibited massive damage (Fig. 7e), suggesting the death of many tumor cells. Notably, few tumor cells showed apoptosis or necrosis in the tumor tissues for the controlled groups without irradiation, and part of tumor cells were damaged in the C-NPs treated tumor tissues. These results were consistent with the antitumor data observed in vivo. It should be pointed out that although C-NPs still showed some phototoxicity, the toxicity was reduced compared to O-NPs, which is helpful for silencing systemic toxicity. Ideally, switching operation can turn on 1O2 generation in O-NPs, enabling the desired control of PDT results. Such results confirmed our hypothesis that energy-transfer-based control of 1O generation can be realized in the cell and tumor model. 2

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defined scaffold. By incorporating porphyrin and diarylethene moieties into the building blocks for coordination-driven selfassembly of a dual-stage metallacycle, the excellent photoisomerization property of diarylethene enables the resultant metallacycle to act as a supramolecular switch for fine-tuning of the efficiency of 1O2 generation. To the best of our knowledge, it provides the first successful example of the spatiotemporal controllable 1O2 release in the discrete supramolecular coordination complexes (SCCs) including metallacycles or metallacages. In addition, combining the resultant metallacycles with a PEG-modified long-circulating liposome led to the formation of nanoparticles containing metallacycles that can realize the controlled 1O2 release in both aqueous media and in cancer cells. In vivo investigation demonstrated that the ring-open form O-NPs upon light activation clearly caused the phototoxicity to the tumor, while only slight inhibition of the tumor growth was observed for the ring-closed form C-NPs. These investigations successfully established the discrete supramolecular coordination complexes imbedded within nanoparticles as a promising platform for selective photodynamic therapy. In summary, this work demonstrated the great potential of developing multifunctional discrete metallacycles for selective therapy of cancers with the controllable 1O2 release, thus providing a blueprint for the development of SCCs with applications in the field of biotechnology and biomaterials.

ASSOCIATED CONTENT Supporting Information. Synthesis, characterization, and other experimental details. This material is available free of charge via the internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected] * [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT

Figure 7. In vivo PDT efficacy on Hela tumor-bearing mice. (a) Tumor growth inhibition curves after PDT treatment. (b) Mice body weight curves with relevant treatments. (c) Tumor weight curves with relevant treatments. (d) Photo of tumors of each group after PDT. One tumor in the O-NPs + light group was completely eradicated at the end point. (e) Histology of tumor slices of mice in various groups after PDT treatment.

H.-B. Y. thanks NSFC/China (No. 21625202), Innovation Program of Shanghai Municipal Education Commission (No. 2019-01-07-00-05-E00012), and Program for Changjiang Scholars and Innovative Research Team in University for financial support. Y. T. thanks National Natural Science Foundation of China (Nos. 21635003 and 21827814) and Innovation Program of Shanghai Municipal Education Commission (No. 201701070005E00020). We greatly appreciate Prof. Wenbo Bu at ECNU for his generous help with biological experiments.

CONCLUSIONS

REFERENCES

In summary, we have successfully constructed a supramolecular dual-stage system with the capability to reversibly control 1O2 generation by unifying both photosensitizer and photochromic-switch within a well-

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Table of contents (TOC)

Light-Controlled Generation of Singlet Oxygen within a Discrete DualStage Metallacycle for Cancer Therapy Yi Qin,† Li-Jun Chen,†,* Fangyuan Dong,† Shu-Ting Jiang,† Guang-Qiang Yin,† Xiaopeng Li,‡ Yang Tian,†,* and Hai-Bo Yang†,*


Light-Controlled Generation of Singlet Oxygen within a Discrete Dual

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