Investigation of Excited-State Photophysical Properties of Water

May 12, 2017 - The photophysical properties of the excited states of the water-soluble gallium corrole (Ga(tpfc) (SO3Na)2) and fluorescence lifetime i...
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Investigation of Excited-State Photophysical Properties of Water-Soluble Gallium Corrole Li-Li Wang, Hui Wang, Fan Cheng, Zhen-Hua Liang, Chufeng Liu, Yuan Li, WeiQian Wang, Su-Hong Peng, Xue Wang, Xiao Ying, Liang-Nian Ji, and Haiyang Liu J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 12 May 2017 Downloaded from http://pubs.acs.org on May 18, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Investigation of Excited-State Photophysical Properties of Water-Soluble Gallium Corrole Li-Li Wang,1 Hui Wang,*,1 Fan Cheng,2 Zhen-Hua Liang,2 Chu-Feng Liu,1 Yuan Li,3 Wei-Qian Wang,1 Su-Hong Peng,2 Xue Wang,1 Xiao Ying,3 Liang-Nian Ji1,4 and Hai-Yang Liu*,2 1

State Key Laboratory of Optoelectronics Materials and Technologies, Sun Yat-Sen University, Guangzhou 510275, China

2

Department of Chemistry, South China University of Technology, Guangzhou 510641, China

3

Department of Applied Physics, South China University of Technology, Guangzhou 510641, China 4

School of Chemistry and Chemical Engineering/MOE Laboratory of Bioinorganic and Synthetic Chemistry, Sun Yat-Sen University, Guangzhou 510275, China

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ABSTRACT

The photophysical properties of the excited states of the water-soluble gallium corrole (Ga(tpfc)(SO3Na)2) and fluorescence lifetime imaging in three kinds of living cell lines have been investigated using the steady-state and time-resolved spectroscopic techniques. In comparison with gallium corrole (Ga(tpfc)(py)) in six selected solvents: Ga(tpfc)(SO3Na)2 shows the much weaker emission and B-band absorption, and the Q-band absorption exhibits red-shift of about 15 nm, the fluorescence lifetime decreases, while the triplet lifetime increases. The internal transition time from the B-band to the Q-band of Ga(tpfc)(SO3Na)2 is similar to that of Ga(tpfc)(py) in toluene. The high polarity of the water has an important impact on the dynamics of the singlet and triplet states, which may result in efficient electron transfer to the oxygen. This observation has been confirmed by the EPR experiment. Without the aid of carrier proteins, Ga(tpfc)(SO3Na)2 can penetrated into three kinds of living cell lines: mouse fibroblast (L-929), human breast cancer (MCF-7) and human prostate carcinoma (PC-3M). Efficient energy transfer and intracellular high accumulation has been observed in PC-3M cell line, which results in the fluorescence quench and shorter lifetime.

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INTRODUCTION Corrole is a tetrapyrrole macrocycle with one of meso-carbon atoms substituted by a direct pyrrole-pyrrole link.1-2 Unique photophysical and photochemical properties of the corroles have resulted in their use in some current and emerging technologies, including oxygen sensors,3 organic photovoltaic solar cells,4-5 and photodynamic therapy (PDT).6-8 Specifically, the sulfonated gallium corroles are water soluble, and function both for tumor detection and targeting therapeutic agents because of their intense fluorescence and cytotoxicity.9-11 It has been confirmed that the combination of a breast cancer-targeted cell penetration protein (HerPBK10) and a sulfonated gallium corrole (S2Ga) can elicit tumor cell death while sparing healthy tissue such as the heart.9-10 The water-soluble corroles have been found to exhibit good DNA photocleavage activity.6, 11-14 These studies call for a photophysical investigation on the watersoluble corroles under physiological condition, which has special relevance to the understanding of the mechanism of the effective corrole delivery and tumor-targeted toxicity. Physiological fluid is an aqueous solution. Water molecule possesses a high solvent polarity.15 Previous studies revealed that both the spectral and dynamical features of tetrapyrrole complexes were significantly affected by the polarity of the solvent.16-18 In addition, the water is a protic solvent, which can lead to the formation of the hydrogen bonding between the fluorine atoms of corroles and the hydrogen atoms of water,19-20 and the oxygen atom of the water molecule can act as axial ligand with corrole complexes.21 These may further modify the photophysical properties of the corroles.21-23 Up to now, there is no report about the effects of aqueous media on the photophysical properties of the water-soluble corroles. It is known that the delivery of the toxicity of corroles in PDT is usually initiated through interacting with the oxygen molecules. Previous work has confirmed that singlet oxygen (1O2)

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and hydroxyl radical (•OH) can cleavage DNA and leads to tumor cell death.24-25 Therefore, the understanding of the quenching mechanism of the singlet and triplet excited-states of watersoluble corroles is of particular importance for improving the effect of PDT. In this article, the photophysical properties of the excited states of a water-soluble corrole: Ga(tpfc)(SO3Na)2 and its interaction with the oxygen have been investigated using the steadystate and time-resolved spectroscopic techniques. The fluorescence lifetime imaging of three kinds of living cell lines after uptake of Ga(tpfc)(SO3Na)2 have been examined, and the results show a high accumulation in the human prostate carcinoma (PC-3M) cell while it is spare in a mouse fibroblast (L-929) cells. The possible mechanism has been discussed. EXPERIMENTAL SECTION Sample preparation. Ga(tpfc)(SO3Na)2 was synthesized according to a previously reported procedure.26 The synthesis of Ga(tpfc)(py) has been reported elsewhere.27-28 Figure S1 (Supporting Information) shows the molecular structures of both gallium corroles. Cell lines and culture conditions. L-929 (mouse fibroblast), MCF-7 (human breast cancer), and PC-3M (human prostate carcinoma) cells were obtained from the Experimental Animal Center of Sun Yat-Sen University. The Ga(tpfc)(SO3Na)2 solution (50 µM) were prepared with phosphate buffered saline (PBS, pH = 7.4). The cells were incubated with Ga(tpfc)(SO3Na)2 at 37˚C for 1 h after seeded in a laser confocal microscopy 35 mm2 Petri dish (MatTek, America) for 24 h, and then washed 3 times with PBS to remove the remaining corrole molecules and dead cells. Optical measurements. Samples. Ga(tpfc)(SO3Na)2 was dissolved in Tris–HCl buffer (5 mM Tris, 50 mM NaCl, pH = 7.2). Solutions of Ga(tpfc)(py) were prepared with six selected solvents, namely,

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dimethylformamide (DMF), acetonitrile (CH3CN), methanol, ethanol, tetrahydrofuran (THF), and toluene. Steady-State Spectroscopy. Steady-state absorption spectra were recorded by a PerkinElmer Lambda 850 UV-vis spectrometer (PE Company). The steady-state emission spectra were measured using a PerkinElmer LS55 luminescence spectrometer (PE Company), with the excitation wavelength of 560 nm. The fluorescence quantum yields were estimated using 5,10,15,20-tetraphenylporphyrin H2TPP in toluene as a reference (ΦF(std) = 0.11).28 Both spectroscopic measurements were carried out using a solution concentration of ~5 µM. In addition, a FLS920 spectrofluorimeter (Edinburgh Instruments) was used to measure the singlet oxygen luminescence of Ga(tpfc)(py) in toluene and Ga(tpfc)(SO3Na)2 in Tris-HCl buffer against H2TPP in toluene as a reference (Φ∆(std) = 0.70).28 Time-resolved Spectroscopy. Femtosecond transient absorption spectra were recorded with 400-nm excitation, using a pump-probe apparatus described previously.29 Nanosecond transient absorption measurements were performed on another pump-probe setup previously reported, after excitation at 532 nm.28 The triplet absorption decay curves were well fitted with a singleexponential function convoluted with a Gaussian response function. The triplet quantum yield of Ga(tpfc)(SO3Na)2 was calculated using TPPS in water as a reference (ΦT(std) = 0.76, εT(std) ≈ 1.30 × 105 M-1cm-1).30-31 Emission lifetimes were measured on a streak camera system (Hammatsu C1587) with excitation at 420 nm, which has been described in detail elsewhere.28 The fluorescence decay profiles of both corroles at the corresponding emission peak wavelengths have been well fitted with a single-exponential function convoluted with a Gaussian response function. In addition, the fluorescence spectra and lifetimes of Ga(tpfc)(SO3Na)2 in PBS buffer at

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different pH values were recorded by using the streak camera system (Hammatsu C1587) with excitation at 420 nm. Fluorescence lifetime imaging (FLIM). FLIM was carried out on a Renishaw confocal imaging spectrometer system (inVia Reflex), with excitation at 400 nm and detection at 615 nm. The method has been described elsewhere.32 Confocal microscopy imaging. Steady-state emission spectra of Ga(tpfc)(SO3Na)2 in living cell lines were recorded with 400-nm excitation and 615-nm detection, using a Leica TCS SP5 confocal laser scanning microscope (Leica Inc., America). Electron paramagnetic resonance (EPR) measurement. EPR spectra were obtained at Xband on a Bruker A300-10-12 spectrometer (9.87 GHz) before and after irradiation. The scanning parameters were: scan number, 3; modulation amplitude, 1 G; modulation frequency, 100 kHz; microwave power, 19.86 mW; sweep width, 100 G; sweep time, 40.96 s; time constant, 40.96 ms; center filed, 3505 G; receiver gain, 1 × 103. The samples were irradiated with a 150 W CW xenon lamp (LHX150), using a glass filter to cut the wavelengths below 520 nm. Two commonly used spin-trapping agents, 2,2,6,6-tetramethylpiperidine (TEMP) and 5,5′dimethylpyrroline N-oxide (DMPO), were employed to trap the

1

O2 and •OH species,

respectively. It is well-known that the paramagnetic 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) may be generated from the reaction of 1O2 with TEMP, and •OH reacts with DMPO may yield the paramagnetic DMPO-OH adduct.33-34 Ga(tpfc)(SO3Na)2 was dissolved in deionized water for the EPR measurement. All measurements were carried out at room temperature (296K). The samples were exchanged after each experiment, and no obvious photo-degradation was observed in the spectrometers applied in this study.

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Quantum-chemical calculations. Theoretical calculations on Ga(tpfc)(SO3Na)2 in water and Ga(tpfc)(py) in toluene were done with Gaussian 09 software. To evaluate the effect of different solvents, a polarizable continuum model (PCM) has been applied.35 Full geometry optimizations of the corroles were performed with the density functional theory (DFT) employing B3LYP exchange-correlation function,36-37 for which LANL2DZ and 6-311G** basis sets were used for gallium atom and the other atoms, respectively. The excited-state energies were calculated with time-dependent density functional theory (TD-DFT)38 at the same level of theory. The energy gap between the S0 and the T1 state, E(T1), were determined to be 1.62 and 1.54 eV for Ga(tpfc)(SO3Na)2 and Ga(tpfc)(py), respectively. RESULTS AND DISCUSSION Steady-state absorption spectra. Figure 1 shows the steady-state absorption spectra of Ga(tpfc)(py) in six select solvents and Ga(tpfc)(SO3Na)2 in Tris-HCl buffer, which display an intense B band near 422 nm and several weak Q bands in the region of 500 ~ 650 nm, respectively. Ga(tpfc)(SO3Na)2 exhibits the weakest B-band absorption and a largest Q-band redshift of about 15 nm compared to Ga(tpfc)(py) in six selected solvents. Compared to the nonpolar solvents (toluene and THF), Ga(tpfc)(py) in the four kinds of polar solvents exhibits the decrease of the B-band absorption and Q-band red shift. The formation of the hydrogen bonding in Tris-HCl buffer also can further decrease the B- and Q-band absorption of Ga(tpfc)(SO3Na)2 and Ga(tpfc)(py) in both ethanol and methanol, which are significantly lower than other polar solvents. The axial ligation of gallium atom with the oxygen atom of the water molecule may have contribution to the red-shift of the Q-band absorption of Ga(tpfc)(SO3Na)2.21, 39 This is different from that of Ga(tpfc)(py), and the effect of the axial ligation results in blue-shifted Q

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bands in DMF and THF vs. CH3CN and toluene, respectively.40 All the photophysical parameters of both gallium corroles are listed in Table 1.

Figure 1. Steady-state absorption (solid lines) and emission (dotted lines) spectra of Ga(tpfc)(py) and Ga(tpfc)(SO3Na)2. Fluorescence spectra and lifetimes. The fluorescence spectra of Ga(tpfc)(SO3Na)2 and Ga(tpfc)(py) are displayed in Figure 1, too. All the sample exhibit the strong emissions between 580 to 640 nm and the shoulders between 650 to 680 nm. Ga(tpfc)(SO3Na)2 exhibits the weakest emission with a peak at 623 nm and the largest red-shift (~ 20 nm compared to Ga(tpfc)(py) in toluene). Ga(tpfc)(py) in the polar solvents (ethanol, methanol, CH3CN and DMF) present the weaker emissions and red shifts compared to the nonpolar solvents (toluene and THF). It is interesting that the fluorescence intensities of Ga(tpfc)(py) in ethanol and methanol are significantly higher than those in CH3CN and DMF although the lower absorbances of the former. Although ΦF of Ga(tpfc)(SO3Na)2 is lower than that of Ga(tpfc)(py) in the six selected solvents, it still much larger than that of water-soluble porphyrin TPPS.30 Furthermore, the polarity of the solvent may increase the macrocycle distortion of the corrole and lead to the decrease of the energy gap between the S1 and S0 state, therefore, the red-shift of the

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fluorescence spectra can be observed. In this case, the nonradiative decay rate (knr) of the S1 state may be enhanced and result in the shorter fluorescence lifetime as shown in Table 1. On other hand, Ga(tpfc)(py) exhibits the slightly longer lifetime of the fluorescence in DMF and THF vs. CH3CN and toluene, respectively, which should be due to the influence of the axial ligation.41 Femtosecond transient absorption (TA) spectra. Figure S2 exhibits the femtosecond TA spectra of Ga(tpfc)(py) and Ga(tpfc)(SO3Na)2 within 420 ~ 700 nm, of which the positive signal corresponds to the absorption (ESA) of the population in excited states and the negative one is the ground-state bleaching (GB) or the stimulated emission (SE) signal.42 The positions of GB bands are coincident with those of absorption bands of the ground state, and the position of SE signal is the same with that of the emission peak. In addition, a sharp minus peak around 460 nm appears at 0.05 ps and disappears as soon as the pump and probes no longer overlap, which is a stimulated Raman amplification (SRA) signal associated with the O-H or C-H stretching mode of the solvent.43 Compared to the 0.05-ps spectrum of Ga(tpfc)(py) in toluene, that of Ga(tpfc)(SO3Na)2 reveals a broader ESA band extending over the range of 446 ~ 574 nm (Figure 2b), which may be due to the red-shift of the Q bands, resulting in less ground-state absorption intensity between 562 and 574 nm (Figure 1). Herein, the ESA band of Ga(tpfc)(py) is weaker than that of Ga(tpfc)(SO3Na)2, which may be attributed to the effect of the aggregation of Ga(tpfc)(py) in toluene at the concentration of 100 µM (Figures 2 and S3).

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Figure 2. Femtosecond TA spectra of Ga(tpfc)(py) (a) and Ga(tpfc)(SO3Na)2 (b) monitored from 0.05 to 10 ps. The sample concentration was ~100 µM. The corresponding steady-state absorption (blue dotted lines) and emission (red dotted lines) spectra are shown in each subfigure. The analysis method of singular-value decomposition and global lifetime fitting (SVD-GLF)42, 44

is employed to resolve the excited-state absorption spectra, and the corresponding kinetics are

based on a proposed consecutive kinetic model as follows: hv τ21 S0 →M1   →M2 ,

where τ21 is the time constant of relaxation from the transient species M1 to M2. Recomposed spectra and kinetics of M1 and M2 obtained from SVD-GLF are shown in Figures S4 and S5. In the case of Ga(tpfc)(py) in toluene, the M1 and M2 species may be assigned to be the S2 and S1 state, respectively. Hence, τ21 estimated as 256 fs is the time constant of internal conversion from the S2 to S1 state, which is close to the reported lifetime (280 fs) of S2-state fluorescence of Ga(tpfc)(py) in benzene.45 It has been known that the fluorescence quantum yield of S2 state of

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Ga(tpfc)(py) is as small as about 1.1 × 10-4.45 The M1 spectrum may be just a combination of GB and ESA signals; and then, the pure ESA spectrum as shown in Figure 3a of the S2 state can be achieved by adding ground state absorption signal of Ga(tpfc)(py), which exhibits a broad ESA band located in the region of 443 ~ 700 nm with a maximum at 475 nm. However, the pure ESA spectrum as shown in Figure 3c of the S1 state is obtained by adding both ground state absorption and S1-state emission signals of Ga(tpfc)(py), which reveals a broad ESA band located in the region of 443 ~ 700 nm with a maximum at 460 nm. As shown in Figure 3e, the decay time of S2 is consistent with the rise time of S1, indicating a nearly unity efficiency of S2 → S1 internal conversion of Ga(tpfc)(py). It is in accordance with the earlier study of Ga(tpfc)(py).45 Likewise, the species associated with Figures 3b and 3d are assigned to be the S2 and S1 states of Ga(tpfc)(SO3Na)2, respectively, and the time constant of S2 → S1 internal conversion is determined as 259 fs in Tris-HCl buffer (Figure 3f). It is interesting to find that the τ21 values in toluene and Tris-HCl buffer are almost identical.

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Figure 3. Absorption spectra (a ~ d) and kinetics (e ~ f) of the S1 and S2 states obtained by SVDGLF analysis of the TA spectra shown in Figure 2. The τ21 values of Ga(tpfc)(py) in six selected solvents shown in Table 1, which increase with the increase of solvent polarity. For all the solvents, the largest τ21 value is revealed in DMF. On the other hand, axial ligation with solvent molecule may affect the lifetime of S2 state. It is known that Ga(tpfc)(py) is five-coordinate when dissolved in non-coordinating solvents, and it may bind with a coordinating solvent to form a six-coordinated complex,41 which may slow down the dynamical evolution of Ga(tpfc)(py) in DMF compared to CH3CN although the difference of the ε values of both solvents is very small. However, as mentioned above, Ga(tpfc)(SO3Na)2 is four-coordinate and it may become five-coordinate after axial ligation with water, which can decrease the value of τ21.21 Thus, the τ21 value of Ga(tpfc)(SO3Na)2 is affected by both solvent polarity and axial ligation with solvent molecules. Triplet optical properties. Figure 4 shows the nanosecond TA spectra of Ga(tpfc)(SO3Na)2 in aerated and deaerated Tris-HCl buffer, which are dominated by a broad positive absorption band ranging from 440 to 530 nm with the maximum around 455 nm due to the T1 → Tn absorption and one negative absorption band at 590 nm, corresponding to the ground-state bleaching. Using the singlet depletion method, the T1 → Tn molar absorption coefficient εT may be calculated to be 2.12 × 104 M-1cm-1 for Ga(tpfc)(SO3Na)2 in Tris-HCl buffer, which is about half of that of Ga(tpfc)(py) in toluene (3.93 × 104 M-1cm-1).28, 46 The triplet-state quantum yield (ΦT) is 0.68, which is similar to that of Ga(tpfc)(py) in toluene (0.69).28 The rate constant (kISC) of S1 → T1 intersystem crossing (35.2 × 107 s-1) for Ga(tpfc)(SO3Na)2 in Tris-HCl buffer is significantly larger than that of Ga(tpfc)(py) in toluene (23.4 × 107 s-1),28 which may be due to the smaller energy gap between the S1 and T1 state according to the theoretical calculation.

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Figure 4. Nanosecond TA spectra of Ga(tpfc)(SO3Na)2 in aerated (a) and deaerated (b) Tris-HCl buffer monitored after 532-nm excitation (1.0 mJ/pulse). The insets show the decay profiles monitored at 455 nm. The sample concentration was ~100 µM. The T1-state lifetime (τT) of Ga(tpfc)(SO3Na)2 in aerated buffer is fitted to be 2.95 µs (Figure 4a), which is much longer than those of Ga(tpfc)(py) in six selected solvents (0.33 ~ 0.60 µs, Table 1 and Figure S6). In deaerated buffer (Figure 4b), the T1-state lifetime of Ga(tpfc)(SO3Na)2 is about 297 µs and also much longer than that of Ga(tpfc)(py) in toluene. This may be attributed to the larger energy gap between the T1 and S0 states of Ga(tpfc)(SO3Na)2 in the environment of the aqueous solution. On other hand, the lower concentration of molecular oxygen (O2) in aerated buffer ([O2] = 0.28 mM) compared to toluene ([O2] = 2.1 mM) may further increase the τT value of Ga(tpfc)(SO3Na)2.28, 47-48 Using the values of τT and τT0, the

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oxygen quenching rate constant kqT of Ga(tpfc)(SO3Na)2 in aerated buffer is calculated as 1.19 × 109 M-1s-1, which is slightly larger than that of Ga(tpfc)(py) in toluene (0.94 × 109 M-1s-1).28 Table 1 Spectral and dynamical parameters of two studied gallium corroles in different solvents. Solvent

εa

λA (B-band) b (nm)

λA (Q-band) c (nm)

λF d (nm)

ΦF e

τ21 f (fs)

τF g (ns)

knr h (107 s-1)

τT i (µs)

Ga(tpfc)(py) toluene

2.3

424

571, 596

603

0.31

256

2.88

24.16

0.46

THF

7.5

420

569, 596

604

0.30

423

3.39

20.56

0.33

ethanol

24.6

423

575, 602

612

0.24

553

2.99

25.46

0.47

methanol

33.0

422

573, 599

609

0.23

575

2.71

28.50

0.46

CH3CN

37.5

421

573, 601

610

0.20

717

2.27

35.38

0.41

DMF

37.6

423

573, 597

607

0.21

989

2.49

31.59

0.60

259

1.93

42.30

2.95

Ga(tpfc)(SO3Na)2 Tris-HCl

81.0

424

588, 611

623

0.18

The dielectric constants of the selected solvents. See the reference.49 b,c Peak wavelengths of the absorption B and Q bands, respectively. d Peak wavelengths of the fluorescence spectra. e Fluorescence quantum yields in aerated solutions. f Time constants of internal conversion from the S2 to S1 state. g Fluorescence lifetimes in aerated solutions. h Nonradiative decay rate constants.28 i Triplet lifetimes in aerated solutions. a

Electron transfer from Ga(tpfc)(SO3Na)2 to oxygen. Our previous study has demonstrated that the efficient energy transfer can occur from Ga(tpfc)(py) to oxygen (O2).28 For Ga(tpfc)(SO3Na)2, the interaction with O2 should be strongly affected by the solvent of water. Figure S7 shows the emission spectra of 1O2 of Ga(tpfc)(py) and H2TPP in toluene and Ga(tpfc)(SO3Na)2 in Tris-HCl buffer. The quantum yields (Φ∆) of 1O2 generation of Ga(tpfc)(py) estimated as 0.43 is similar to the earlier reported value.28 The emission from Ga(tpfc)(SO3Na)2 is too weak to be detected. Figure 5 displays the EPR spectra of Ga(tpfc)(SO3Na)2 in water and

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Ga(tpfc)(py) in toluene. Both samples have not exhibit any signal before irradiation as shown in Figure 5a. Upon the addition of spin-trapping agent TEMP, the typical TEMPO signals have been observed,50 which indicates that 1O2 is photo-generated as shown in Figure 5b. Ga(tpfc)(py) exhibit much stronger signal than that of Ga(tpfc)(SO3Na)2. This is consistent with the result of measurement of 1O2 emission in Figure S7. Figure 5c shows the superimposed four-line signal with hyperfine splitting structure of Ga(tpfc)(SO3Na)2 with DMPO, which is the characteristic of DMPO−OH spin adduct34 and suggests that •OH has been generated by electron transfer (ET) from Ga(tpfc)(SO3Na)2 to O2. No any signal has been observed in Ga(tpfc)(py). The thermodynamic calculation further confirms the possibility of ET from Ga(tpfc)(SO3Na)2 to O2. The driving force (∆G) can be estimated based on the well-known Rhem-Weller equation,51

}

∆G = e { E ( ox, D ) − E ( red , A ) − E (T1 )

(1)

where the oxidation potential E(ox, D) is measured as 1.21 V (vs. NHE) for Ga(tpfc)(SO3Na)2 in water (pH = 7.2),29 and the reduction potential E(red, A) is reported as -0.16 V (vs. NHE) for O2 in neutral aqueous solution.52 The value of ∆G is -0.25 eV, indicating that ET from Ga(tpfc)(SO3Na)2 to O2 is feasible. The value of E(red, A) of O2 in toluene and the value of E(ox, D) of Ga(tpfc)(py) are reported as -0.56 and 0.99 V (vs. NHE), respectively, which results in ∆G = 0.01 eV.27, 53-54 Therefore it is unlikely that ET will occur from Ga(tpfc)(py) to O2.

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Figure 5. EPR spectra of Ga(tpfc)(SO3Na)2 (0.3 mM) in water and Ga(tpfc)(py) (0.3 mM) in toluene monitored before (a) and after irradiation (b, c) in the presence of spin trap TEMP or DMPO (360 mM). (b)

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O2: Ga(tpfc)(py) (aN = 1.56 mT and g-value = 2.012) and

Ga(tpfc)(SO3Na)2 (aN = 1.72 mT and g-value = 2.012). (c) •OH: Ga(tpfc)(SO3Na)2 (aN = aH = 1.48 mT). FLIM images of Ga(tpfc)(SO3Na)2 in three types of cell lines. Figure 6 shows the absorption and fluorescence spectra of free Ga(tpfc)(SO3Na)2 in PBS buffer. The absorption and emission peaks are located at 424 and 610 nm, respectively, and the fluorescence lifetime at the emission peak is about 2.15 ns.

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Figure 6. Absorption (solid line) and fluorescence (dotted lines) spectra of Ga(tpfc)(SO3Na)2 in PBS buffer. The autofluorescence spectra of three kinds of cell lines are displayed in Figures 7a ~ 7c (black curves) with a broad emission from 500 to 700 nm. After incubation with Ga(tpfc)(SO3Na)2, the new emission peaks (red curves) appear around 615 nm, indicating that Ga(tpfc)(SO3Na)2 has penetrated into the cells. The emission peaks of Ga(tpfc)(SO3Na)2 in L-929 and PC-3M cells exhibit about 10 nm red-shift compared to free Ga(tpfc)(SO3Na)2 in PBS. For PC-3M cell line, the autofluorescence around 530 nm has been strongly quenched after uptake of Ga(tpfc)(SO3Na)2, which may be attributed to the efficient fluorescence resonance energy transfer (FRET) from the endogenous chromophores to Ga(tpfc)(SO3Na).55 Therefore, the emission around 615 nm is ascribed to Ga(tpfc)(SO3Na)2, while the emission from L-929 and PC-3M cells should be the contribution of the autofluorescence and the fluorescence of Ga(tpfc)(SO3Na)2. The fluorescence relaxation dynamics of the three kinds of cells after uptake of Ga(tpfc)(SO3Na)2 at 615 nm are well-fitted with a two-term exponential function. The initial

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fast relaxation τ1 and the slow decay τ2. Figures 7d ~ 7f display the τ1 FLIM images of the three kinds of cells after uptake of Ga(tpfc)(SO3Na)2. For PC-3M cell, the distribution of fluorescence lifetime is 350 ~ 380 ps, which is much shorter than those in MCF-7 ( 400 ~ 460 ps ) and L-929 ( 460 ~ 680 ps ). The τ2 distribution ( 1.89 ~ 2.61 ns, see Figure S8 ) of PC-3M should be due to the emission from the Ga(tpfc)(SO3Na)2 monomers. The τ2 distributions of L-929 and MCF-7 cells are similar, and the mean lifetime τ2 is 2.11 ns ( see Figure S8 ), which are the contribution of the autofluorescence and Ga(tpfc)(SO3Na)2 monomers. Figures 7g ~ 7i show the images of the steady-state fluorescence intensity of the three kinds of cell lines at 615 nm. Ga(tpfc)(SO3Na)2 penetrates into the cells after 1 h incubation and display significant fluorescence throughout the cytoplasm region, and Ga(tpfc)(SO3Na)2 in PC-3M exhibits the weakest emission.

Figure 7. (a ~ c) Fluorescence spectra of the three kinds of cell lines before (black curves) and after uptake of Ga(tpfc)(SO3Na)2 (red curves). (d ~ f) The τ1 FLIM images of the three kinds of

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cell lines after uptake of Ga(tpfc)(SO3Na)2, the detection wavelength is 615 nm. (g ~ i) Fluorescence intensity images of the three kinds of cell lines after uptake of Ga(tpfc)(SO3Na)2. The FLIM image as shown in Figure 7f exhibits the very high accumulation of Ga(tpfc)(SO3Na)2, indicating the significant specificity of Ga(tpfc)(SO3Na)2 to PC-3M compared to other two kinds of cell lines. Similar aggregate of Ga(tpfc)(SO3Na)2 has been observed in L929 cell although the seldom penetration of Ga(tpfc)(SO3Na)2 into the cell. The previous studies have shown that the aggregate of porphyrin molecules can result in the red-shift of the emission.56 In our case, the red-shift of the emission peaks of Ga(tpfc)(SO3Na)2 in both PC-3M and L-929 should be also due to the aggregation of Ga(tpfc)(SO3Na)2 because the pH change has not induce the significant shift of the fluorescence peaks and the variation of the fluorescence lifetime of Ga(tpfc)(SO3Na)2 ( see Figures 6 and S9 ).57 Furthermore, the aggregates of Ga(tpfc)(SO3Na)2 in PC-3M cell has been confirmed by the quenching of the emission,58 while the stronger emission in L-929 cell as shown in Figure 7g may be due to the presence of the autofluorescence around 615 nm as shown in Figure 7a. Figure 7b shows that much less Ga(tpfc)(SO3Na)2 has entered into the MCF-7 cell compared to the PC-3M cell, which is consistent with the earlier report.8-9, 59-60 The absence of the red-shift of the fluorescence peak and the FLIM image suggest that no significant aggregate has formed in MCF-7 cell. The results indicate that intracellular environment may have impact on the aggregation of Ga(tpfc)(SO3Na) and its interaction with the endogenous chromophores, which leads to the significant change of the luminescence properties including spectrum, lifetime and intensity of Ga(tpfc)(SO3Na)2 in the three kinds of cell lines. CONCLUSION

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The photophysical properties of the water-soluble corrole: Ga(tpfc)(SO3Na)2 are reported for the first time and analysed on the basis of the solvent effect. The high polarity of water lead to the red shift of Q-band of Ga(tpfc)(SO3Na)2 toward to the phototherapeutic window and much longer lifetime of the triplet state, which is of particular importance for photodynamic therapy. Furthermore, the electron transfer from the triplet state of Ga(tpfc)(SO3Na)2 to O2, leading to the formation of the •OH species, while it is absent for Ga(tpfc)(py) in toluene. The result is consistent with our early observation of the cleavage of DNA by the •OH species.50 The FLIM results exhibit the significant accumulation in the human prostate carcinoma PC-3M cell line even without the aid of carrier proteins. Further investigation on the mechanism of the photophysics and luminescence in various cancer cells is underway. ASSOCIATED CONTENT Supporting Information. The molecular structures of Ga(tpfc)(SO3Na)2 and Ga(tpfc)(py), the femtosecond TA spectra of Ga(tpfc)(SO3Na)2 in Tris-HCl buffer and Ga(tpfc)(py) in six selected solvents, the spectra and kinetics of the transient species obtained by SVD-GLF analysis of the TA spectra, the triplet spectra and kinetics of Ga(tpfc)(SO3Na)2 and Ga(tpfc)(py) in aerated solutions, the singlet oxygen luminescence from optically matched toluene solutions of Ga(tpfc)(py) and standard H2TPP, and Tris-HCl buffer solution of Ga(tpfc)(SO3Na)2, the FLIM images of the living cells incubated with Ga(tpfc)(SO3Na)2, and the fluorescence lifetimes of Ga(tpfc)(SO3Na)2 in PBS buffer at different pH values. The Supporting Information is available free of charge. AUTHOR INFORMATION Corresponding Author

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* E-mail [email protected] (H.W.). * E-mail [email protected] (H.Y.L.). ACKNOWLEDGMENT This work was financially supported by National Natural Science Foundation of China (Nos. 61178037 and 21371059), National Basic Research Program (973 Program) of China under Grant 2013CB922403, and the Open Fund of the State Key Laboratory of optoelectronic Materials and Technologies (Sun Yat-Sen University, No. OEMT-2015-KF-05). REFERENCES (1)

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