Ratiometric Fluorescent Detection of Pb2+ by FRET-Based

Nov 20, 2017 - Sensitive and selective detection of Pb2+ is a very worthwhile endeavor in terms of both human health and environmental protection, as ...
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Ratiometric Fluorescent Detection of Pb2+ by FRET-Based Phthalocyanine-Porphyrin Dyads Dongli Zhang,† Mengliang Zhu,† Luyang Zhao,† Jinghui Zhang,† Kang Wang,† Dongdong Qi,† Yang Zhou,‡ Yongzhong Bian,*,† and Jianzhuang Jiang*,† †

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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 ‡ College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan 250014, China S Supporting Information *

ABSTRACT: Sensitive and selective detection of Pb2+ is a very worthwhile endeavor in terms of both human health and environmental protection, as the heavy metal is fairly ubiquitous and highly toxic. In this study, we designed phthalocyanine−porphyrin (Pc-Por) heterodyads, namely, H2Pc-α-ZnPor (1) and H2Pc-β-ZnPor (2), by connecting a zinc(II) porphyrin moiety to the nonperipheral (α) or peripheral (β) position of a metal-free phthalocyanine moiety. Upon excitation at the porphyrin Soret region (420 nm), both of the dyads exhibited not only a porphyrin emission (605 nm) but also a phthalocyanine emission (ca. 700 nm), indicating the occurrence of intramolecular fluorescence resonance energy transfer (FRET) processes from the porphyrin donor to the phthalocyanine acceptor. The dyads can selectively bind Pb2+ in the phthalocyanine core leading to a red shift of the phthalocyanine absorption and thus a decrease of spectral overlap between the porphyrin emission and phthalocyanine absorption, which in turn suppresses the intramolecular FRET. In addition, the binding of Pb2+ can highly quench the emission of phthalocyanine by heavy-metal ion effects. The synergistic coupled functions endow the dyads with remarkable ratiometric fluorescent responses at two distinct wavelengths (F605/F703 for 1 and F605/F700 for 2). The emission intensity ratio increased as a linear function to the concentration of Pb2+ in the range of 0−4.0 μM, whereas the detection limits were determined to be 3.4 × 10−9 and 2.2 × 10−8 M for 1 and 2, respectively. Furthermore, by comparative study of 1 and 2, the effects of distance and relative orientation between Pc and ZnPor fluorophores on the FRET efficiency and sensing performance were highlighted, which is helpful for further optimizing such FRET systems.

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Pb2+ sensors were designed by use of small molecules,10−12 DNAzymes,13,14 proteins,15 polymers,16 and nanomaterials.17,18 However, most of these Pb2+ sensors are based on the detection of single-emission intensities, which can be unfavorably affected by various environmental and instrumental factors. In comparison, ratiometric fluorescent sensors employ the ratio of two emission intensities at different wavelengths, which are suited to self-calibration via dual-channel detection and thus allow precise analyses of Pb2+ and other analytes.19−24 One of the most practicable strategies for design of ratiometric fluorescent sensors is the analyte-induced modulating of intramolecular fluorescence resonance energy transfer (FRET),25,26 by which the nonradiative energy transfer efficiency can been effectively tuned within a donor−acceptor system, leading to opposite change in the intensities of the donor fluorescence (Fd) and the acceptor fluorescence (Fa). As a result, sensitive emission ratio (Fd/Fa) changes can be

INTRODUCTION Lead is one of the most toxic heavy-metal pollutants and has been widely distributed in the environment.1 The accumulation of lead in the human body has substantial deleterious effects on the nervous, cardiovascular, digestive, and reproductive systems, causing serious illness and, in particular, developmental delay of children.2 Therefore, the World Health Organization (WHO) recommends that Pb(II) level in drinking water be limited to 10 ppb.3 So far, several conventional analytical methods, such as atomic absorption spectrometry (AAS),4 inductively coupled plasma atomic emission spectrometry (ICP-AES),5 inductively coupled plasma mass spectrometry (ICP-MS),6 and anodic stripping voltammetry (ASV),7 have been employed for measuring lead ion with detection limits typically lower than the critical concentration in drinking water. Nevertheless, fluorescent detection has become one of the most convenient and inexpensive methods for Pb2+ analysis with high sensitivity and selectivity,8 particularly in biological systems.9 Despite Pb2+ is known as a strong fluorescent quencher by spin−orbital coupling or electron transfer, a number of elegant fluorescent © 2017 American Chemical Society

Received: September 3, 2017 Published: November 20, 2017 14533

DOI: 10.1021/acs.inorgchem.7b02261 Inorg. Chem. 2017, 56, 14533−14539

Article

Inorganic Chemistry

performance were highlighted, which is helpful for further optimizing such FRET systems.

acquired at two distinct wavelengths in response to a target analyte. To develop new ratiometric fluorescent sensors with improved performance for heavy-metal ions, we recently proposed a synergistically coupled mechanism of a spectral overlap modulated FRET with a metal-chelating quenching of fluorescence, as exemplified by a boron-dipyrromethene− porphyrin (BODIPY-Por) dyad that could provide exceptionally large changes in the intensity ratio of two distinct emissions for the selective detection of Ag+ with high sensitivity in solution and living cells.27 Herein, we present new FRET systems of covalently linked phthalocyanine−porphyrin(Zn) heterodyads (Scheme 1) for the ratiometric fluorescent

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RESULTS AND DISCUSSION Molecular Design and Synthesis. Conjugated tetrapyrrole derivatives are superior candidates for the construction of FRET-based fluorescent sensors owing to their readily tunable photophysical and ion-binding properties.22,23,27,28 Specifically, a Pc ring contains four isoindole nitrogen atoms forming the inner coordination cavity, which is able to bind various metal ions. However, because of the large ionic size, Pb2+ is unable to completely enter the central cavity of Pc ring but sits atop the ligand.29 This coordination geometry tends to activate the stereochemical lone pair of Pb2+ thus increasing the reactivity to Pc ligands, which is suggested to be responsible for the selectivity of Pc ring toward Pb2+.30,31 On the other hand, the intense fluorescent emission of a zinc(II) porphyrin (ZnPor) fluorophore well overlaps the strong Q-absorption of a metalfree phthalocyanine (H2Pc) fluorophore; therefore, we choose a ZnPor donor and a H2Pc acceptor to formulate the FRET pair. The distance and relative orientation between the donor and acceptor fluorophores can be readily adjusted by connecting a ZnPor moiety to the nonperipheral (α) or peripheral (β) position of a H2Pc moiety. The dyads H2Pc-α-ZnPor (1) and H2Pc-β-ZnPor (2) were synthesized by a mixed cyclic tetramerization of 4,5-bis(2,6dimethylphenoxy)phthalonitrile with the porphyrin-containing phthalonitriles32 3 or 4 in refluxing n-pentanol. The desired products were isolated by column chromatography with acceptable yields of ca. 20% and characterized by mass and NMR spectroscopies and elemental analysis (see the Experimental Section and Supporting Information Scheme S1 and Figures S1−S4 for details). Spectroscopic Properties and FRET Efficiency. The electronic absorption and emission spectra of dyads 1 and 2 were measured in a mixed solvent of THF/CH3OH (4:1 v/v) (Figure 1 and Table 1). The most strong absorption at 425 nm is the ZnPor Soret band, and the weak absorption at 558 nm is attributed to the ZnPor Q-band. The broad band at ca. 350 nm is assigned to the H2Pc Soret band. The two intense absorptions at 663−700 nm and the vibronic band at 640 nm are due to the H2Pc Q-band, while the other vibronic band around 600 nm is an overlap of the H2Pc and ZnPor Q bands. The absorption spectra of dyads 1 and 2 are very similar to the sum of those of individual components, indicating the absence of strong electronic interaction between the H2Pc and ZnPor units in the ground state. However, in comparison with those of 2, the H2Pc Q absorptions of 1 appear at longer wavelength,

Scheme 1. Schematic Molecular Structures of Dyads 1 and 2

detection of Pb2+. Furthermore, by comparative study of H2Pc-α-ZnPor (1) (Pc = phthalocyanine) and H2Pc-β-ZnPor (2), the effects of distance and relative orientation between Pc and Por fluorophores on the FRET efficiency and sensing

Figure 1. Electronic absorption (a) and emission (b) spectra of dyads 1 and 2 with excitation at 420 nm (2 μM in THF/CH3OH, 4:1 v/v). 14534

DOI: 10.1021/acs.inorgchem.7b02261 Inorg. Chem. 2017, 56, 14533−14539

Article

Inorganic Chemistry Table 1. Optical Properties of 1 and 2 in THF/CH3OH (4:1 v/v) at 298 Ka absorption λmax, nm (log ε)

dyads 1 2

349 (4.90) 348 (4.99)

425 (5.87) 425 (5.89)

558 (4.39) 558 (4.46)

603 (4.50) 601 (4.63)

emission λmax, nm (Φf)

640 (4.57) 641 (4.74)

668 (5.09) 663 (5.17)

700 (5.14) 699 (5.23)

c

605 (0.0032 ) 605 (0.0010c)

ηEETb (%) d

703 (0.010 ) 700 (0.025d)

92 97

a

Because of the solubility reason, all of the absorption and emission spectra were measured in THF/CH3OH, 4:1 v/v. bEfficiency (%) of energy transfer (ηEET) obtained by the donor quenching method according to ηEET = (1 − ΦDA/ΦD) × 100;36 the fluorescence quantum yield of ZnTriBPP(OH) is 0.038 (ΦD); errors estimated as ±5%. cBy reference to ZnTPP in benzene (Φ = 0.033).34 dBy reference to Pc in toluene (Φ = 0.33).35

dyads 1 and 2. Notably, the energy transfer efficiency of dyad 2 is higher than that of dyad 1 by 5−8%, which can be interpreted by taking the Förster parameters into account. At first, the orientation factor (k2) of 2 is remarkably larger than that of 1 (0.940 vs 0.120). This is due to the different steric hindrance effect of the nonperipheral (α) and peripheral (β) connection of the dyads, which influences the relative orientation between the ZnPor donor and the H2Pc acceptor (Supporting Information Figure S5 and Table S1). Second, the spectral overlap integral (JDA) of 2 is also slightly larger than that of 1 (2.49 vs 2.25 cm6 mmol−1) because of the relative bathochromic shift of H2Pc Q absorptions for dyad 1. As a result, the Förster radius (R0) of 2 is significantly greater than that of 1 (31.0 vs 21.8 Å). Though the distance between the donor and acceptor (r, 15.0 Å for 1 vs 16.1 Å for 2) tends to a converse effect on the energy transfer efficiency (E), this cannot change the fact that the β-substituted dyad H2Pc-β-ZnPor (2) exhibits higher FRET efficiency than the α-substituted one H2Pc-αZnPor (1). Pb2+ Selectivity and Sensing Mechanism. Dyad 1 was first applied as a representative to test the selectivity toward metal ions. When 10 equiv of metal ions, including Ag+, Al3+, Ba2+, Ca2+, Cd2+, Co2+, Cr3+, Cu2+, Fe2+, Fe3+, Hg2+, K+, Mg2+, Mn2+, Na+, NH4+, Ni2+, and Zn2+, were added to dyad 1 (2 μM in THF/CH3OH, 4:1 v/v), respectively, no significant change was observed in either absorption or emission spectra. However, upon the addition of Pb2+ (10 equiv), the characteristic Q absorptions of H2Pc moiety (668 and 700 nm) disappeared completely, and a new intense absorption arose at 712 nm (Figure 2). A remarkable change was also detected in the emission spectrum of dyad 1, which includes an increase of the ZnPor emission (605 and 650 nm) and a decrease of the H2Pc emission (703 nm), leading to highly selective ratiometric fluorescent response (F605/F703) to Pb2+ (Figure 2). Dyad 2 possesses similar selectivity toward Pb2+ over other metal ions as indicated by the absorption and emission spectra and the ratiometric fluorescent responses (F605/F700) (Supporting Information Figure S6). The above spectral responses of dyads 1 and 2 toward Pb2+ suggest the transformation of the metal-free phthalocyanine moiety (H2Pc) to the metalated form of lead phthalocyanine [Pb(II)Pc].29,38 This was further supported by the matrixassisted laser desorption/ionization time-of-flight (MALDITOF) mass data of dyads 1 and 2 in the presence of Pb2+ (10 equiv), in which the signals at m/z 2304.1 and 2303.2 can be identified to the Pb(II) complexes of PbPc-α-ZnPor [1-Pb(II)] and PbPc-β-ZnPor [2-Pb(II)], respectively (Supporting Information Figure S7). The selective binding of Pb2+ in the Pc core induces a substantial bathochromic shift of the Q absorption from 660 to 700 to 710 nm region, leading to a decrease of the spectral overlap between the donor (ZnPor) emission and the acceptor (H2Pc or PbPc) absorption (Figure 3). This is known to

which is mainly due to the stabilization of the lowest unoccupied molecular orbital (LUMO) energy level of H2Pc unit by the nonperipheral (α) substitution in dyad 1.33 In addition, the Soret absorption of ZnPor is well-separated from the absorptions of H2Pc, which facilitates the selective excitation of the ZnPor donor. Upon excitation at 420 nm, where the ZnPor moiety absorbs most of the light, the phthalocyanine−porphyrin dyads give not only an emission of ZnPor centered at 605 nm but also an emission of H2Pc around 700 nm, indicating the occurrence of intramolecular energy transfer process from the ZnPor donor to the H2Pc acceptor in dyads 1 and 2. The efficiency (%) of energy transfer (ηEET) was first estimated by the donor quenching method according to eq 1:36 ηEET = (1 − ΦDA /ΦD) × 100

(1)

where ΦDA and ΦD are the fluorescence quantum yields of the donor (ZnPor) in the presence and absence of the acceptor (H2Pc), respectively. Because the emission of ZnTriBPP(OH) (ΦD = 0.038) was significantly quenched in dyads 1 (ΦDA = 0.0032) and 2 (ΦDA = 0.0010), the efficiencies of energy transfer (ηEET) were calculated to be up to 92% for 1 and 97% for 2. To correlate the FRET properties with the molecular structures, Förster theory was also applied to dyads 1 and 2. The theoretical efficiency of energy transfer (E) is given by eq 2:36,37 E=

R 06 R 06 + r 6

(2)

where r is the distance between the donor and acceptor, and R0 is the Förster radius determining the distance at which E = 50% and can be calculated by eq 3: R 0 = 0.211[k 2n−4 ΦDJDA ]1/6 (in Å)

(3)

where k2 is the orientation factor describing the relative orientation of the absorption and emission transition dipoles of the FRET pair; n is the refractive index; ΦD is the fluorescence quantum yield of the donor; and JDA is the spectral overlap integral of the donor emission and the acceptor absorption. The calculation details are included in the Supporting Information. The obtained energy transfer efficiency (E) and Förster parameters are listed in Table 2. The values of energy transfer efficiency based on Förster theory (E) are reasonably close to those from the donor quenching method (ηEET) for Table 2. Förster Parameters and Energy Transfer Efficiency dyads

k2

JDA (cm6 mmol−1)

r (Å)

R0 (Å)

E (%)

1 2

0.120 0.940

2.25 2.49

15.0 16.1

21.8 31.0

90.1 98.1 14535

DOI: 10.1021/acs.inorgchem.7b02261 Inorg. Chem. 2017, 56, 14533−14539

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Inorganic Chemistry

Figure 2. Electronic absorption (a) and emission (b) spectra and ratiometric fluorescence responses (c) of dyad 1 (2 μM in THF/ CH3OH, 4:1 v/v, λex = 420 nm) upon addition of Ag+, Al3+, Ba2+, Ca2+, Cd2+, Co2+, Cr3+, Cu2+, Fe2+, Fe3+, Hg2+, K+, Mg2+, Mn2+, Na+, NH4+, Ni2+, Pb2+, and Zn2+ (20 μM), respectively.

Figure 4. Electronic (a) absorption and (b) emission spectra of dyad 2 (2 μM in THF/CH3OH, 4:1 v/v, λex = 420 nm) upon addition of Pb2+ (0−20 μM). (inset) The relationship between F605/F700 and Pb2+ concentration in the range of 0−20 μM. (c) Fluorescence intensity ratio changes (F605/F700) of dyad 2 as a rectilinear function of Pb2+concentration in the range of 0−4.0 μM.

suppress the FRET process, thus leading to enhancing of the donor emission (ZnPor, 605 nm) and diminishing of the acceptor emission (H2Pc, 700 nm). Furthermore, the binding of Pb2+ can highly quench the emission of Pc moiety by heavymetal ion effects. Therefore, sensitive ratiometric fluorescent responses to the target ion Pb2+ are expected for dyads 1 and 2. Ratiometric Fluorescent Detection of Pb2+. The validity for the quantitative detection of Pb2+ was tested by UV−vis and fluorescence spectrophotometric titration method. Upon addition of Pb2+ (0−10 equiv) to 2, the characteristic absorptions of H2Pc moiety gradually disappeared with the simultaneous appearance of the PbPc absorptions, while the absorptions of ZnPor moiety were almost unchanged (Figure 4a). This observation is consistent with the binding of Pb2+ to

the Pc core. The concomitant changes in the fluorescence spectra are more significant. The emission of H2Pc moiety (700 nm) gradually decreased and was accompanied by a remarkable increase of the ZnPor emission (605 and 650 nm), and a clear iso-emission point was observed at 680 nm (Figure 4b). Impressively, the emission intensity ratio (F605/F700) experienced an increase from 0.12 to 80.44 along with the addition of Pb2+ (0−20 μM), corresponding to a 670-fold enhancement. The plots of F605/F700 against Pb2+ concentrations display a good linear relationship in the range of 0−4.0 μM (Figure 4c);

Figure 3. Normalized spectral overlap between the donor (ZnPor) emission and the acceptor (Pc) absorption: before (A) and after (B) binding with Pb2+. 14536

DOI: 10.1021/acs.inorgchem.7b02261 Inorg. Chem. 2017, 56, 14533−14539

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Inorganic Chemistry the regression equation is F605/F700 = 0.120 + 0.863c (R2 = 0.994, c represents the Pb2+ concentration). The limit of detection (LOD = 3σ/slope) was calculated to be 2.2 × 10−8 M (4.6 ppb) in the present conditions,39 where σ (0.0065) is the standard deviation for 20 parallel determinations of the blank solution. The UV−vis and fluorescence titration profiles of dyad 1 with Pb2+ (Supporting Information Figure S8) are quite similar to those of dyad 2. The emission intensity ratio of dyad 1 (F605/ F703) underwent a remarkable increase from 1.01 to 65.4 along with the addition of Pb2+ (0−20 μM), corresponding to a 65fold enhancement. The F605/F703 of 1 also exhibited a linear function to the concentration of Pb2+ in the range of 0−4.0 μM (F605/F703 = 1.01 + 4.91c, R2 = 0.995), resulting in the LOD of 3.4 × 10−9 M (0.71 ppb) for Pb2+.39 On the basis of the above analysis, clearly the LOD values for Pb2+ are sufficiently below the critical Pb2+ level in drinking water (10 ppb) as reported by WHO,3 thus proving the capability of dyads 1 and 2 for sensitive ratiometric fluorescent detection of Pb2+. In particular, the LOD of dyad 1 for Pb2+ reaches sub-parts per billion level, exhibiting superior performance to that of dyad 2 in the linear range (0−4.0 μM of Pb2+). This is because dyad 1 has more suitable energy transfer efficiency (ηEET = 92%) than dyad 2 (ηEET = 97%), providing 1 with stronger residual emission of Por donor than that of 2.40 As a result, dyad 1 possesses two emissions from the ZnPor donor and the H2Pc acceptor with comparable intensity (F605/ F703 = 1.01), which is a proper starting point for the Pb2+ induced intensity ratio change. Therefore, a small standard deviation (σ = 0.0056) of the blank signals (F605/F703) and a large slope (4.91) in the linear range of F605/F703-[Pb2+] plot were obtained for 1. By contrast, for dyad 2, the emission of ZnPor donor is much weaker than that of the H2Pc acceptor (F605/F700 = 0.120), because of the relatively high energy transfer efficiency; however, this is undesired for examining the intensity ratio change.

mL) was added dropwise. The mixture was stirred for another 30 min. The precipitate obtained by filtration was thoroughly washed with methanol and further purified by silica gel and biobead column chromatography with CHCl3 as the eluent. After recrystallization from CHCl3/CH3OH, the target compound 1 was obtained as green powder (67 mg, 16%). 1H NMR (CDCl3, 400 MHz, 293 K): δ 9.18 (d, 1H, J = 8.0 Hz; Pc α-H), δ 8.86 (dd, 4H, J = 4.0 Hz; Por β-H), δ 8.80 (d, 2H, J = 4.0 Hz; Por β-H), δ 8.76 (d, 2H, J = 4.0 Hz; Por β-H), δ 8.57 (s, 1H; Pc α-H), δ 8.56 (s, 1H; Pc α-H), δ 8.48 (s, 1H; Pc α-H), δ 8.30 (s, 1H; Pc α-H), δ 8.29 (s, 1H; Pc α-H), δ 8.15 (t, 1H, J = 8.0 Hz; Pc β-H), δ 8.12 (s, 1H; Pc α-H), δ 8.09 (d, 6H, J = 8.0 Hz; PorPh-H), δ 8.04 (d, 2H, J = 8.0 Hz; Por-Ph-H), δ 8.00 (d, 1H, J = 8.0 Hz; Pc β-H), δ 7.70 (dd, 6H, J = 4.0 Hz; Por-Ph-H), δ 7.38 (d, 2H, J = 4.0 Hz; Por-Ph-H), δ 7.36 (m, 15H, Pc-Ph-H), δ 7.15−7.00 (m, 3H, Pc-Ph-H), δ 2.46−2.12 (m, 36H, 2,6-dimethylphenoxy, DMPO− CH3), δ 1.59−1.56 (m, 27H, tBu-H). MALDI-TOF-MS: m/z calcd for C136H116N12O7Zn (M+) 2095.9; found 2096.1. Anal. Calcd (%) for C136H116N12O7Zn·CHCl3: C, 74.28; H, 5.32; N, 7.59. found: C, 74.09; H, 5.08; N, 7.91. Synthesis of H2Pc-β-ZnPor (2). By using the procedure described above with 4 (198 mg, 0.2 mmol) instead of 3 as the starting material, compound 2 was obtained as green powder (88 mg, 21%). 1H NMR (CDCl3, 400 MHz, 293 K): δ 9.31 (d, 1H, J = 8.0 Hz; Pc α-H), δ 9.16 (s, 1H; Pc α-H), δ 8.97 (br, 4H, J = 8.0 Hz; por β-H), δ 8.86 (br, 4H, J = 8.0 Hz; por β-H), δ 8.55 (s, 1H; Pc α-H), δ 8.44 (s, 1H; Pc α-H), δ 8.41 (s, 1H; Pc α-H), δ 8.26 (br, 1H; Pc α-H), δ 8.25 (m, 2H; Por-PhH), δ 8.16 (br, 2H; Pc α-H), δ 8.13 (br, 6H; Por-Ph-H), δ 8.00 (d, 1H, J = 8.0 Hz; Pc β-H), δ 7.74 (m, 6H; Por-Ph-H), δ 7.63 (m, 2H; PorPh-H), δ 7.36 (br, 15H; Pc-Ph-H), δ 7.07 (d, 2H, J = 8.0 Hz; Pc-PhH), δ 6.86 (br, 1H; Pc-Ph-H), δ 2.45−2.37 (m, 36H; DMPO−CH3), δ 1.60−1.58 (m, 27H; tBu-H). MALDI-TOF-MS: m/z calcd forC136H117N12O7Zn (MH+) 2096.9; found 2097.2. Anal. Calcd (%) for C136H116N12O7Zn·CHCl3: C,74.28; H, 5.32; N, 7.59. found: C, 74.15; H, 5.13; N, 7.61.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02261. Chemicals and instruments; synthetic scheme of dyads 1 and 2; MALDI-TOF mass spectra, 1H NMR and 1H−1H COSY spectra of dyads 1 and 2; density functional theory and Förster theory calculations; the method for determining fluorescence quantum yields; Pb2+ selectivity of dyad 2; mass spectra of dyads 1 and 2 in the presence of Pb(NO3)2; ratiometric fluorescent detection of Pb2+ with dyad 1 (PDF)

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CONCLUSION In summary, we have designed FRET-based phthalocyanineporphyrin(Zn) heterodyads for the ratiometric fluorescent detection of Pb2+. The dyads H2Pc-α-ZnPor (1) and H2Pc-βZnPor (2) feature a highly efficient FRET process that then can been suppressed by selective binding of Pb2+ in the Pc core, leading to enhancing of the donor (ZnPor) emission and diminishing of the acceptor (Pc) emission. Especially, the binding of Pb2+ can further quench the emission of Pc by heavy-metal ion effects; thus, the synergistic coupled functions afford remarkable ratiometric fluorescent readout in response to the target ion Pb2+ with LOD values down to sub-parts per billion level. Moreover, we have demonstrated that, by altering the connecting position of the ZnPor moiety onto the Pc moiety, the distance and relative orientation between the donor and acceptor fluorophores can be readily changed to adjust the energy transfer efficiency and to improve the ratiometric fluorescent sensing performance.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (Y.B.) *E-mail: [email protected]. (J.J.) ORCID

Yongzhong Bian: 0000-0003-0621-3683 Jianzhuang Jiang: 0000-0002-4263-9211 Notes

The authors declare no competing financial interest.

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EXPERIMENTAL SECTION

Synthesis of H2Pc-α-ZnPor (1). Lithium metal (35 mg, 5.0 mmol) was dissolved in n-pentanol (8 mL) at 80 °C under an atmosphere of dry N2. To this solution, 3 (198 mg, 0.2 mmol) and 4,5bis(2,6-dimethylphenoxy)phthalonitrile (516 mg, 1.4 mmol) were added and reacted at 155 °C for 24 h. After it cooled, the reaction mixture was poured into methanol (50 mL), and glacial acetic acid (1

ACKNOWLEDGMENTS Financial support from the Natural Science Foundation of China (21471015, 21631003, and 21671017), National Ministry of Science and Technology of China (Grant No. 2013CB933402), Fundamental Research Funds for the Central 14537

DOI: 10.1021/acs.inorgchem.7b02261 Inorg. Chem. 2017, 56, 14533−14539

Article

Inorganic Chemistry

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Universities, and Beijing Natural Science Foundation is gratefully acknowledged.

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DOI: 10.1021/acs.inorgchem.7b02261 Inorg. Chem. 2017, 56, 14533−14539

Article

Inorganic Chemistry Porphyrin-Appended Mixed (Phthalocyaninato)(Porphyrinato) Yttrium(III) Double-Decker Complexes. J. Phys. Chem. B 2010, 114, 13143−13151.

14539

DOI: 10.1021/acs.inorgchem.7b02261 Inorg. Chem. 2017, 56, 14533−14539