pH-Responsive Low-Power Upconversion Based on Sandwichlike

May 22, 2017 - With the aid of nF900 software, the prompt fluorescence lifetime (τf) of RhB was obtained under the detection of nF lamp, while the de...
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pH-Responsive Low-Power Upconversion Based on Sandwichlike Palladiumphthalocyanine and Rhodamine B Rongkang Hao,† Changqing Ye,*,† Xiaomei Wang,*,†,‡ Lin Zhu,† Shuoran Chen,† Jiawei Yang,† and Xutang Tao‡ †

Jiangsu Key Laboratory for Environmental Functional Materials, Institute of Chemistry, Biology and Materials Engineering, Suzhou University of Science and Technology, Suzhou 215009, P. R. China ‡ State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, P. R. China S Supporting Information *

ABSTRACT: Sandwhichlike palladiumphthalocyanine was found to possess highly efficient NIR triplet sensibilization with an increasing of ligand number, due to the triplet energy level (ET) enhancing and the triplet lifetime (3τ0) prolonging and resulting in efficient triplet−triplet energy transfer (TTT). Also, rhodamine B (RhB) was found to act as reactive triplet acceptor that emits red-to-yellow upconversion when doped with sandwhichlike complex. By tuning pH value, the three-layered phthalocyanine/RhB system presents so far the highest red-to-yellow upconversion efficiency (ΦUC) as large as 10.5% (λex = 655 nm, 1W·cm−2), which has provided a simple approach to develop not only the new-type highly efficient NIR sensitizers but also the TTA-UC supported pH response applied in biomedicine.

when exposed to visible light,16,17 the red-to-yellow upconversion with rubrene as acceptor is not durable.18,19 So, exploration of highly efficient NIR sensitizer and stable acceptor for red-to-yellow upconversion is of important significance. In this work, we reported newly synthesized sandwichlike palladiumphthalocyanines (PdPc2 and Pd2Pc3) that exhibit highly efficient NIR triplet sensibilization with an increasing of ligand number. Also, it was found that both complexes show improving solubility in common organic solvents owing to the sandwiched structure reducing intermolecular stacking, relative to single-layered PdPc (see Figure 1). Three-layered phthalocyanine (Pd2Pc3) doped with rhodamine B (RhB) as triplet acceptor presents so far the highest red-to-yellow upconversion efficiency (ΦUC) as large as 10.5% (λex = 655 nm, 1W·cm−2) by tuning pH values, which has provided a simple approach to exploring the new-type NIR sensitizers and promoting low-powered upconversion efficiency without the difficulties. Importantly, the strong linear correlation between upconversion and pH value in the scope of 3−7 implies the TTA-UC-based pH response applied in biomedicine becoming possible.

1. INTRODUCTION Triplet−triplet annihilation (TTA) upconversion systems capable of generating short-wavelength radiation from longwavelength light sources are very desirable because of their wide application potentials.1−5 In general terms, TTA upconversion (TTA-UC) is facilitated by selective excitation of strongly absorbing sensitizer in the long wavelength region that internally converts to the long-lived lowest energy triplet excited state. The Dexter-type triplet−triplet energy transfer then occurs from the sensitizer to the acceptor and finally between two excited acceptors, the latter producing the desired upconversion fluorescence.6 For the purpose of solar energy or biomedicine application, the upconversion systems of red-toshort wavelength light are more interesting since they can make the best of harvesting the sunlight in the near-infrared (NIR) region and exploiting the NIR window of the human body. Among red-to-short upconversion, the red-to-yellow has been investigated significantly, of which the typical pair of acceptor/ sensitizer is almost entirely fixed on 5,6,11,12-tetraphenylnaphthacene (rubrene)7−11 doped with large planar peripherally substituted metalloporphyrins, such as metalated tetrabenzoporphyrins,11−13 metalated tetranaphthaloporphyrins,8,14 and metalated tetraquinoxalineporphyrins7,9,15 as well as metalated tetraanthraporphyrins.10 These complexes usually need heavy and complicated synthesis, and, moreover, their triplet sensibilization is also less than satisfactory. As a result, the present red-to-yellow upconversion efficiency is low. On the other hand, since rubrene is readily photo-oxidized and always accompanied by the decomposition of corresponding peroxide © 2017 American Chemical Society

Received: March 24, 2017 Revised: May 15, 2017 Published: May 22, 2017 13524

DOI: 10.1021/acs.jpcc.7b02785 J. Phys. Chem. C 2017, 121, 13524−13531

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The Journal of Physical Chemistry C

250 instrument (Mono AlKα, hν = 1486.6 eV, 15 KV, 150 W, 500 μm beam spot; the fixed transmittance of the energy analyzer is 30 eV). Absorption spectra were recorded on a Hitachi U-3500 spectrophotometer, while the steady-state fluorescence spectra and time-resolved decay curves were measured on an Edinburgh FLS 920 fluorophotometer equipped with a timecorrelated single-photon counting (TCSPC) card. The used solvent in the absorption and fluorescence measurements is the mixed alcohol solvents (n-propanol/ethylene glycol (v/v: 1/2)) with the concentration at 10 μM. The phosphorescence spectra were measured by an Edinburgh FLS-980 fluorophotometer under excitation of 660 nm at room temperature (N 2 atmosphere, CHCl3 solvent, 10 μM). For decay fluorescence lifetime measurements, the excitation wavelength and detection wavelength are the respective maximum absorption wavelength and maximum fluorescence emission, respectively. With the aid of nF900 software, the prompt fluorescence lifetime (τf) of RhB was obtained under the detection of nF lamp, while the delayed fluorescence lifetime (τDF) of sensitizer (PdPc2 and Pd2Pc3) was obtained under the detection of microsecond xenon flash lamp (Edinburgh Analytical Instruments, μF900). For all radiation decays, monoexponential or diexponential fits give acceptable statistics parameters of χ2 < 1.1 (χ2 is the “reduced chi-square”). Nanosecond transient absorption (TA) difference spectra correlated with triplet decay dynamics for sensitizers without and with acceptor were measured with nanosecond timeresolved transient absorption spectroscopy that were measured on a computer-controlled ultrafast Nd:YAG laser/OPO system at 10 Hz. A 600 nm long-pass filter was placed between the OPO and the sample to filter out residual second and the third YAG harmonics. 2.2. TTA Upconversion Measurements. Diode solid state laser (655 nm) was used as the excitation source for TTAupconversion. The laser power was measured with photodiode detector. For TTA-upconversion experiments, the mixed solutions containing sensitizer and acceptor were degassed for about 15 min with N2. Then the solution was excited with the laser. The upconverted fluorescence was observed with PR655 Spectra Scan colorimeter.

Figure 1. Molecular structures of palladiumphthalocyanines (PdPc, PdPc2, and Pd2Pc3) and their photographs dispersed in CHCl3 (10 μM).

2. EXPERIMENTAL SECTION 2.1. Chemicals, Characterization, and Measurements. All chemicals were purchased from Aldrich or Acros Chemical Co. and used without any further purification. The synthesis and characterization of three palladium(II)phthalocyanines (PdPc, PdPc2, and Pd2Pc3) can be found in the Supporting Information section (Figures S1−S6). IR spectra were measured on a Nicolet FT-IR 5DX instrument using solid samples dispersed in KBr disks. Mass spectra were taken on Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR-MS) produced by Bruker Company (Bruker Solarix X) with 2-[(2E)-3-(4-tertbutylphenyl)-2-methylprop-2-enylidene]malonitrile (DCTB) as the matrix. 1H NMR was performed on an INOVA-400 spectrometer using THF-d8 with the addition of ∼1 vol % N2H4·H2O as solvent. The hydrazine hydrate was used to reduce the paramagnetic neutral species to the diamagnetic monoanion species. X-ray photoelectron spectra (XPS) measurements were carried out on the Thermo ESCALAB

Figure 2. Absorption spectra (a) and fluorescence spectra (b) of three palladium-phthalocyanines in n-propanol/glycol (v/v: 1/2) at 10 μM (inset: the phosphorescence of PdPc2 and Pd2Pc3 in CHCl3 at 10 μM under rt and N2 atmosphere). 13525

DOI: 10.1021/acs.jpcc.7b02785 J. Phys. Chem. C 2017, 121, 13524−13531

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Table 1. Absorption, Delayed Fluorescence, Phosphorescence, and Corresponding Lifetime of PdPc2 and Pd2Pc3 in nPropanol/Glycol (v/v: 1/2) at 10 μM

a

sensitizer

λabs max (nm)

λfluo max (nm)

λphors(nm)a

τDF1

τDF2

ET1 (eV)

PdPc2 Pd2Pc3

336(B) 655(Q) 347(B) 657(Q)

695 697

899 870

2.44 μs (29.97%) 2.21 μs (30.23%)

15.82 μs (69.03%) 17.69 μs (69.7%)

1.38 1.43

3

τ0 (μs) 7.85 13.46

In CHCl3 at 10 μM under rt and N2 atmosphere.

ΦUC

2 ⎛ A r ⎞⎛ Fs ⎞⎛ ηs ⎞ = 2Φr ⎜ ⎟⎜ ⎟⎜⎜ ⎟⎟ ⎝ A s ⎠⎝ Fr ⎠⎝ ηr ⎠

PdPc2 (ET1, 1.38 eV) and would be in favor of triplet−triplet energy transfer between sensitizer to acceptor (TTT). Timeresolved fluorescence spectra (Figure 3) reveal that both

(1)

TTA-upconversion efficiency (ΦUC) is calculated by eq 1, where the subscript “s” represents sample that is either sensitizer or acceptor, while “r” stands for the reference (here, ZnPc was used as reference). Φr is the fluorescence quantum yield of the reference (ZnPc, 32%).20 A is the absorbance, F is the integrated fluorescence intensity, and η is the refractive index of solutions. That is, ηr = 1.4310 (ZnPc in DMF solvent) and ηs = 1.4276 (binary sensitizer/acceptor in npropanol/glycol (v/v: 1/2)). The equation is multiplied by a factor of 2, accounting for the fact that two absorbed photons are required to produce one upconverted photon.6 2.3. The pH-Responsive Titration Procedures. To 5.0 mL of upconversion solutions, such as RhB/PdPc2 (3 mM/1 μM) and RhB/Pd2Pc3 (3 mM/1 μM) in n-propanol/glycol (v/ v: 1/2), was added 1 μL of HCl or KOH aqueous solution with different concentrations. Upon each addition, the pH values were measured by pH meter and the upconversion spectra were monitored on real-time under excitation of 655 nm (1 W· cm−2). A blank experiment (without H+ or OH− ion) was carried out via the same procedure to check the effects of added water on the upconversion spectra. Meanwhile, it was fount that the refractive index of solution was hardly changed during the pH-responsive titration processes.

Figure 3. Delayed fluorescence decay curves of PdPc2 and Pd2Pc3 in npropanol/glycol (v/v: 1/2) at 10 μM under room temperature and nitrogen atmosphere.

sandwichlike complexes possess the delayed fluorescence (DF) feature that comes from the reverse intersystem crossing (RISC: T1 → S1).22,23 Also, three-layered Pd2Pc3 possesses longer DF lifetime (τDF2 = 17.69 μs) than two-layered PdPc2 (τDF2 = 15.82 μs). Since the DF kinetics is strongly influenced by the triplet energy migration dynamics,15 three-layered Pd2Pc3 with longer DF lifetime would have longer triplet lifetime and promote the triplet−triplet energy transfer (TTT). Transient absorption (TA) difference spectrum of PdPc2 in the region of 380−800 nm was obtained upon 655 nm excitation. As shown in Figure 4a, the negative absorption signals observed at ∼680 nm correspond to the ground state bleach of the Q-bands, while the broad positive excited tripletstate absorptions between 475 and 575 nm show the typical feature for the transient absorption of triplet excited state.12 Fitting the TA difference spectra at 500 nm with the aid of analysis software gives the corresponding transient decay, resulting in single exponential decay with triplet lifetime (3τ0) at 7.85 μs (Figure 4b and Table 1). Under the identical fitting conditions (Figure 4c), the transient decay of TA difference spectra of PdPc2 is remarkably decreased to 3.33 μs when doped with RhB. The fact that the triplet-state decay time of PdPc2 shortens from 7.85 to 3.33 μs was interpreted as the triplet−triplet energy transfer (TTT) indeed occurs with TTT efficiency (ΦTTT) at 57.6%.24 Similarly, the TA difference spectra (Figure 5a) correlated with the transient decay curves of Pd2Pc3 exhibiting its triplet lifetime (3τ0) at 13.46 μs (Figure 5b and Table 1), while its transient decay of TA difference spectra is decreased to 4.68 μs when doped with RhB (Figure 5c) with TTT efficiency (ΦTTT) at 65.2%. As can be seen, three-layered

3. RESULTS AND DISCUSSION 3.1. Absorption, Fluorescence, and Phosphorescence Spectra. Absorption spectra of three palladium(II)phthalocyanines in mixed alcohol solvents are presented in Figure 2a, where the B-band and Q-band are bathochromic from PdPc (330, 587, 650 nm) to PdPc2 (336, 592, 655 nm) to Pd2Pc3 (347, 597, 657 nm). The molar extinction coefficients (ε) of the maximum absorptive Q-band are increased in order of Pd2Pc3 (1.80 × 105 M−1·cm−1) > PdPc2 (1.61 × 105 M−1· cm−1) > PdPc (2.9 × 104 M−1·cm−1), demonstrating that sandwichlike complexes (PdPc2 and Pd2Pc3) exhibit stronger absorptive Q-band than single-layered PdPc. This would be beneficial to triplet−triplet energy transfer (TTT) between sensitizer and acceptor and result in efficient upconversion emission.21 Under the excitation of Q-band, three complexes emit fluorescence at 690 nm (PdPc), 695 nm (PdPc2), and 697 nm (Pd2Pc3), respectively, accompanied by the fluorescence intensity in the order of Pd2Pc3> PdPc2 ≫ PdPc (Figure 2b). Absorption, fluorescence, and phosphorescence properties of sandwichlike complexes are shown in Table 1. Noted that both sandwichlike complexes present almost the same fluorescence peaks locating at ∼696 nm that correspond to 1.78 eV (ES1); however, their phosphorescence positions are much different (Figure 2b, inset). The phosphorescence peak of three-layered Pd2Pc3 (870 nm) is hypsochromic shifted 29 nm relative to PdPc2 (899 nm), indicating that Pd2Pc3 possesses higher triplet energy level (ET1, 1.43 eV) than 13526

DOI: 10.1021/acs.jpcc.7b02785 J. Phys. Chem. C 2017, 121, 13524−13531

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Figure 4. Nanosecond TA difference spectra (a) of PdPc2 (1 μM) measured at several delay times in propanol/glycol (v/v: 1/2) at excitation at 655 nm (2.0 mJ/pulse) and the corresponding decay curves without (b) and with (c) RhB (3 mM); the continuous lines are the fits of the experimental data with single exponential functions.

with slope of 2, confirming that the TTA-UC is a two-photon process.25 Beyond 400 mW·cm−2, the linear power dependence implies that TTA becomes the main deactivation channel for the acceptor triplets and the resulting maximum TTA-UC efficiency (ΦUC) has been reached.26 Both RhB/PdPc2 and RhB/Pd2Pc3 show the upconversion peak at 604 nm that is redshifted 24 nm relative to the prompt fluorescence of RhB (580 nm) (Figure S11). We measured that the maximum fluorescence peak of RhB is red-shifted from 580 to 604 nm (Figure S12a), accompanied by the fluorescence lifetime increasing from 2.92 to 6.65 ns in the range of 10 μM−1 mM, suggesting that RhB produces the aggregation at higher concentration (Figure S12b). Thus, anti-Stocks (Eex − EUC) at 0.16 eV between the excitation light at 655 nm (1.89 eV) and the upconversion at 604 nm (2.05 eV) was obtained. Relative to RhB/PdPc2, RhB/Pd2Pc3 presents about 1.3-fold increase in upconversion intensity. The stronger upconversion for RhB/Pd2Pc3 can be explained by the Jablonski diagram (Figure 8). Since three-layered Pd2Pc3 possesses higher triplet level (1.43 eV) and longer triplet lifetime (13.46 μs) than PdPc2 (1.38 eV and 7.85 μs) (see Table 1), three-layered Pd2Pc3 can effectively promote Dextertype triplet−triplet energy transfer (TTT), resulting in larger TTT efficiency (65.2%) than double-layered PdPc2 (57.5%)

phthalocyanine (Pd2Pc3) presents longer triplet lifetime (13.46 μs) and higher triplet energy level (1.43 eV), resulting in efficient triplet−triplet energy transfer (65.2%), relative to double-layered phthalocyanine (PdPc2) with shorter triplet lifetime (7.85 μs) and lower triplet energy level (1.38 eV) and resulting in smaller TTT efficiency (57.6%). 3.2. Triplet−Triplet Annihilation Upconversion. Selecting rhodamine B (RhB) acts as triplet acceptor due to its photostability being better than rubrene. The binary systems of RhB/PdPc2 and RhB/Pd2Pc3 present yellow upconversion under excitation of red diode laser (655 nm, ∼1 W·cm−2). As expected, the RhB/Pd2Pc3 system exhibits much more bright yellow upconversion than RhB/PdPc2 (Figure 6), confirming that sandwichlike palladiumphthalocyanine shows efficient NIR triplet sensibilization with an increasing of ligand number. The optimized red-to-yellow upconversion is obtained by the concentration-dependent upconversion measurements (Figures S7−S10). Meanwhile, the power-dependent upconversion spectra show that the yellow upconversion was rapidly enhanced with increasing incident excitation powers (Figure 7a−b), accompanied by a DF increase from the sensitizer. There are two regions with the slope of 2 and around 1, respectively. The logarithmic plot of upconversion integral versus the excited power (at 50−400 mW·cm−2) is obtained 13527

DOI: 10.1021/acs.jpcc.7b02785 J. Phys. Chem. C 2017, 121, 13524−13531

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The Journal of Physical Chemistry C

Figure 5. Nanosecond TA difference spectra (a) of Pd2Pc3 (1 μM) measured at several delay times in propanol/glycol (v/v: 1/2) at excitation at 655 nm (2.0 mJ/pulse) and the corresponding decay curves without (b) and with (c) RhB (3 mM); the continuous lines are the fits of the experimental data with single exponential functions.

pH response between pH = 3 and 7 in alcohol media. For the pristine solutions of RhB/PdPc2 and RhB/Pd2Pc3 in the mixed alcohol media, the pH values were measured at ∼5.2. As shown in Figure 9a, addition of HCl aqueous into the binary system of RhB/PdPc2 makes the pH value decrease from 5.2 to 4 and to 3; correspondingly, the upconversion intensity (IUC) was quenched from 5041 to 4285 and to 3643. However, when pH values were increased from 5.2 to 6 and to 7 by addition of KOH aqueous, the upconversion intensity (IUC) was enhanced from 5041 to 5620 and to 6233. Continuing titration of OH− into solution until pH = 8, the upconversion intensity does not enhance but for dramatic quenching. Such sensitivity in the range of pH = 3−7 can be fitted as the following linear equation: IUC = 659pH + 1629 (Figure 9a, inset). Similar condition was observed in binary RhB/Pd2Pc3 (Figure 9b). When the pH value was decreased from 5.3 to 4 and to 3, the upconversion intensity (IUC) was quenched from 6843 to 5775 and to 4760; while pH values are increased from 5.3 to 6 and to 7, the upconversion intensity enhances from 6843 to 7742 and to 8645. At pH = 8, the upconversion intensity suddenly quenches. Thus, the IUC ∼ pH linear equation is IUC = 968pH + 1847 in the range of pH = 3− 7 (Figure 9b, inset). Accordingly, the TTA-UC efficiencies in the scope of pH = 3−7 were calculated and are presented in

Figure 6. Realistic photographs of upconversion of RhB/PdPc2 (3 mM/1 μM) and RhB/Pd2Pc3 (3 mM/1 μM) under excitation of 655 nm (1 W/cm2) in n-propanol/glycol (v/v: 1/2) at N2 atmosphere.

when doped with RhB. From the point view of rhodamine B (RhB), the reduced energy level (Eagg) of aggregation state enables its TTA to take place rather efficiently (Figure 8). A combination of efficient TTT and TTA processes promotes red-to-yellow upconversion effectively, which results in the upconversion efficiency (ΦUC) of RhB/PdPc2 at 6.8% and RhB/Pd2Pc3 at 8.4%, upon excitation 655 nm (∼1 W·cm−2). 3.3. The pH-Responsive Low-Power Upconversion. Interestingly, RhB/PdPc2 and RhB/Pd2Pc3 systems present the 13528

DOI: 10.1021/acs.jpcc.7b02785 J. Phys. Chem. C 2017, 121, 13524−13531

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Figure 7. Power-dependent upconversion of RhB/PdPc2 (a) and RhB/Pd2Pc3 (b) in n-propanol/glycol (v/v: 1/2) at optimized concentration (3 mM/1 μM) at N2 under 655 nm excitation (insets: logarithmic plots of upconversion integral versus power density).

Figure 9. pH-responsive upconversion intensity of RhB/PdPc2 (a) and RhB/Pd2Pc3 (b) in n-propanol/glycol (v/v: 1/2) at N2 atmosphere, excited with 655 nm/1 W/cm2 laser (insets: pH value versus the max upconversion intensity).

Table 2. PH-Responsive TTA-UC Efficiency (ΦUC) in the Scope of pH = 3−7 pH sensitizer

3

4

5.2/5.3

6

7

PdPc2/RhB Pd2Pc3/RhB

5.1 5.9

5.9 7.2

6.8 8.4

7.8 9.5

8.7 10.5

described as the following linear equation: ΦUC = 0.91pH + 2.29 (χ2 = 0.999) in the range of pH = 3−7 (Figure 10i). This can be explained that the upconversion efficiency (ΦUC) is relevant to the fluorescence quantum yield (ΦF) of acceptor (RhB), according to the equation of ΦUC = ΦF × ΦTTT × ΦTTA; at the same time, pH value is related to the activity of RhB molecules. A combination of both factors results in the upconversion efficiency scaling linearly with the pH values (see Figure S13). Similar condition for the RhB/Pd2Pc3 system is also observed in Table 2. That is, the upconversion efficiency (ΦUC) is increased from 5.9% to 10.5% when the pH value changes from 3 to 7, which gives the pH-responsive upconversion efficiency (ΦUC) linear equation as ΦUC = 1.15pH + 2.51 (χ2 = 0.999) in the range of pH = 3−7 (Figure 10ii). It is worth mentioning that the RhB/Pd2Pc3 system exhibits more sensitive pH-dependent upconversion (Figure 10), which is undoubtedly attributed to Pd2Pc3 being a more

Figure 8. Energy-level diagram of the upconversion (UC) processes. ISC, intersystem crossing; TTT, triplet−triplet energy transfer; TTA, triplet−triplet annihilation. S0, ground state; S1, excited state; T1, triplet state of complexes; TRhB, triplet state of RhB; Eagg, aggregation energy level of RhB.

Table 2, where the pristine RhB/PdPc2 solution (pH = 5.2) possesses the upconversion efficiency (ΦUC) at 6.8%. When the pH value is increased from 5.2 to 7, the ΦUC value is increased from 6.8% to 8.7%. However, when the pH value is decreased from 5.2 to 3, the ΦUC value is decreased from 6.8% to 5.1%. Thus, the pH-responsive upconversion efficiency (ΦUC) can be 13529

DOI: 10.1021/acs.jpcc.7b02785 J. Phys. Chem. C 2017, 121, 13524−13531

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The Journal of Physical Chemistry C Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to National Natural Science (51673143, 51303122, 51603141), Excellent Innovation Team in Science and Technology of Education Department of Jiangsu Province, the Priority Academic Program Development of Education Department of Jiangsu Province (PAPD), the Innovation project of postgraduate scientific research (CXLX11-0970), and Collaborative Innovation Center for the financial support.

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Figure 10. pH-dependent upconversion efficiency of RhB/PdPc2 (3 mM/1 μM) and RhB/Pd2Pc3 (3 mM/1 μM) in n-propanol/glycol (v/ v: 1/2) at N2 under 655 nm excitation.

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efficient triplet sensitizer. This has provided a simple approach to increase low-powered upconversion efficiency without the difficulties and also is particularly suitable for biological upconversion pH detection.27

4. CONCLUSION We have demonstrated for the first time that more sandwichpalladiumphthalocyanine can act as highly efficient NIR triplet senzitizers, due to enhancing triplet energy level (ET) and triplet lifetime (τP), which results in efficient TTT process. With easy synthesis and highly efficient triplet sensibilization, the molecular design of sandwich architecture might be a new concept for exploring the new-type NIR upconversion sensitizers. Meanwhile, it was first reported that rhodamine B (RhB) as reactive triplet acceptor presents so far the highest red-toyellow upconversion efficiency (ΦUC) as large as 10.5% (λex = 655 nm, 1W·cm−2), when doped with three-layered phthalocyanine complex. By tuning pH values (3−7), the pHresponsive upconversion efficiency (ΦUC) exhibits the strong linear relation (ΦUC = 1.15pH + 2.51), showing potential application of biological pH detection by near-infrared (NIR) excitation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b02785. Synthesis and characterization of palladium(II)phthalocyanines; Figures S1−S13 (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xiaomei Wang: 0000-0002-5336-9608 Xutang Tao: 0000-0001-5957-2271 13530

DOI: 10.1021/acs.jpcc.7b02785 J. Phys. Chem. C 2017, 121, 13524−13531

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DOI: 10.1021/acs.jpcc.7b02785 J. Phys. Chem. C 2017, 121, 13524−13531