Theoretical Design of Near-Infrared Al3+ Fluorescent Probes Based

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Theoretical Design of Near-Infrared Al3+ Fluorescent Probes Based on Salicylaldehyde Acylhydrazone Schiff Base Derivatives Xiao Pan,† Jiamin Jiang,† Junfeng Li,‡ Wenpeng Wu,*,† and Jinglai Zhang*,† †

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Institute of Upconversion Nanoscale Materials, College of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, PR China ‡ College of Chemistry and Chemical Engineering, and Henan Key Laboratory of Function-Oriented Porous Materials, Luoyang Normal University, Luoyang 471934, PR China S Supporting Information *

ABSTRACT: The aim of this paper is to design near-infrared (NIR) Al3+ fluorescent probes based on a Schiff base to extend their applications in biological systems. By combining benzo[h]quinoline unit and salicylaldehyde acylhydrazone, we designed two new Schiff base derivatives. According to theoretical simulations on previous experimental Al3+ probes, we obtained the appropriate theoretical approaches to describe the properties of these fluorescent probes. By employing such approaches on our newly designed molecules, it is found that the new molecules have high selectivity toward Al3+ and that their corresponding Al3+ complexes can emit NIR fluorescence. As a result, they are expected to be potential NIR Al3+ fluorescent probes.

1. INTRODUCTION Aluminum is the most abundant metal in the Earth’s crust and extensively used in modern life. A high level of the soluble form of aluminum (Al3+), however, is toxic to plant growth.1 Excessive aluminum, especially when deposited in the brain even in small amounts, has also been shown to be toxic to humans and is believed to cause neurodementia such as Parkinson’s disease, Alzheimer’s disease, dialysis encephalopathy, and osteoporosis, and so on.2−5 For these reasons, the development of Al3+ sensors for its facile detection is of great importance in both environmental monitoring and biological assays.6 In recent years, many fluorescence sensors for detecting Al3+ have been synthesized. The fluorophores used in such probes usually include units such as quinoline,7,8 fluorescein,9,10 rhodamine,11−13 coumarin,14,15 naphthalene,16,17 pyrene,18−20 BODIPY,21,22 and so on. Most of these probes are incorporated with nitrogen and oxygen donor sites to coordinate with Al3+ as Al3+ is a hard acid.23,24 Meanwhile, a number of sensing mechanisms have been used to develop an efficient fluorescence system to detect Al3+, such as photoinduced electron transfer (PET),25−27 intramolecular charge transfer (ICT),28,29 fluorescent resonance energy transfer (FRET),30,31 rhodamine ring-opening reaction,11,12,32 aggregation-induced emission (AIE),33,34 CN isomerization,35−37 excited-state intramolecular proton transfer (ESIPT),38−40 chelationenhanced fluorescence (CHEF) mechanism,17,41 and dual interplaying sensing mechanisms.42,43 In spite of their simplicity, high sensitivity, and selectivity, the maximum emission wavelengths of almost all of these probes are less © XXXX American Chemical Society

than 550 nm (in the visible region), which hinders their applications in biological systems. Compared to fluorescent probes in the visible region, nearinfrared (NIR, 650−900 nm) fluorescent probes are known to be more suitable for living systems, because they produce fluorescence in the NIR region which results in less damage to living cells, better tissue penetration, and minimum interference from the background autofluorescence of biomolecules in living systems.44−46 To our knowledge, to date only two NIR fluorescent probes have been reported for the detection of Al3+,47,48 so the development of new NIR fluorescent probes with better performance for Al3+ detection is still urgently needed. Table 1 summarizes some fluorescent probes for detecting Al3+ reported in the experimental literature.49−51 All these probes have a salicylaldehyde acylhydrazone Schiff base moiety (e1). Every probe has a very small limit of detection (LOD) for Al3+, and each LOD is lower than the WHO standard in drinking water (7.4 μM).52 Furthermore, any other competitive metal cation, such as Na+, Mg2+, K+, Cr3+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Ba2+, and Pb2+, has almost no effect on the detection of Al3+. Thus, e1 is a good recognition unit for Al3+. However, the maximum emission wavelengths of these probes are in the visible region, and all of them are less than 500 nm. Considering that 10-aminobenzo[h]quinoline (ABQ)/ 10-hydroxybenzo[h]quinoline (HBQ) can emit NIR fluorescence Received: May 7, 2019

A

DOI: 10.1021/acs.inorgchem.9b01335 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Fluorescent Probes for Sensing Al3+ Reported in Recent Literature

Figure 1. Optimized ball and bond model of e1: (a) top view (b) side view.

through the ESIPT process,53−55 we will combine e3 and ABQ/HBQ to design new NIR fluorescent probes. Taking into account that quantum chemistry is a convenient way to study the electronic properties, theoretical calculations will be carried out on such fluorescent probes. First, e1 was selected as the model molecule to search for proper theoretical approaches. Then, the spectroscopic properties of probes e2 and e3 were predicted by using these methods to testify their reliability. Last, new probes for detecting Al3+ were designed based on the salicylaldehyde acylhydrazone Schiff base structure, and their spectroscopic properties were calculated with the same methods as the former. It is found that our newly designed molecules can be used as NIR Al3+ fluorescent probes. The paper is organized as follows. In section 2, we briefly outline a general description of the computational methods used in this paper. Section 3 illustrates the results, including the geometries of the electronic ground state and the first excited singlet state, vertical excitation and vertical emission energies, and the interactions between probes and metal ions. Finally, our conclusions are given in section 4.

was applied to calculate the vertical excitation energies and vertical emission energies(Table S1). Meanwhile, a series of basis sets were also tested to calculate the vertical excitation and vertical emission energies, such as 6-31G(d,p), 6-31+G(d,p), 6-311G(d,p), 6-311+ G(d,p), 6-311++G(d,p), and 6-311++G(3df,2pd). It was found that 6-31+G(d,p) was the most appropriate basis set considering both the accuracy and cost. Taking these into account, the vertical absorption and fluorescence spectra were explored by the TD-M06/6-31+G(d,p) calculations under the S0 and S1 optimized geometries, respectively. The conductive polarizable continuum model (CPCM) was considered with water as solvent throughout the whole theoretical calculations.68 All these calculations were performed with the Gaussian 09 package.69

3. RESULTS AND DISCUSSION 3.1. Electronic Absorption Spectrum and Fluorescence Spectrum of e1. The optimized structure of e1 at the ground state is shown in Figure 1. It can be seen that all the atoms except two hydrogen atoms in methyl group are coplanar. There is an intramolecular hydrogen bond between hydroxyl group and imine nitrogen atom. If the hydrogen atom in hydroxyl group is transferred to the imine nitrogen atom, then the tautomer is formed. However, we cannot locate this structure, so only the normal structure is considered stable in the ground state. The simulated electronic absorption spectrum of e1 is illustrated in Figure 2, and the corresponding spectrum data are listed in Table 2. From Figure 2 and Table 2, we can see that the simulated spectrum agrees well with the experimental spectrum.49 There are two main absorption peaks in the region of 260−360 nm. The one at around 316 nm arises from the HOMO → LUMO transition, corresponding to the S0 → S1 absorption. The one at 277 nm can be ascribed to the HOMO−1 → LUMO transition, corresponding to the S0 → S2 absorption. Meanwhile, two new peaks are predicted to appear at 234 and 203 nm, due to absorption of higher-lying excited

2. COMPUTATIONAL DETAILS Geometries of the electronic ground state (S0) were optimized by using the density functional theory (DFT) method along with Becke’s three-parameter hybrid exchange functional with Lee−Yang−Parr gradient-corrected correlation (B3LYP density functional),56,57 using the 6-31G(d,p) basis set for nonmetal, Al, and Mg atoms and LANL2DZ for other metal atoms.58−60 Frequency calculations were also done at the same level to ensure that the optimized structure was the minimum on the potential energy surface (PES). Structures of the first electronic excited state (S1) were optimized by the TD-B3LYP approach with 6-31G(d,p) basis set.61−63 In the calculations of electronic vertical excitations, a series of DFT functionals were tested, including B3LYP, PBE0,64 M06,65 M06-2X,66 and CAM-B3LYP.67 Finally, M06 proved the most suitable one and B


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Figure 2. Simulated electronic absorption spectrum of e1 together with the experimental spectrum.49

Figure 3. Energy levels and profiles of frontier molecular orbitals for e1 and e1-Al3+.

states. The related molecular orbitals and their energy levels are depicted in Figure 3. 3.2. Electronic Absorption Spectrum of e1-Al 3+ Complex. In order to investigate the interaction between e1 and Al3+, we first examined the electrostatic potential surface (ESP) of e1, which is presented in Figure 4. It can be seen that ESP around oxygen atoms in hydroxyl group and carboxyl group are more negative. This means that this area has a strong attractive force to a cation like Al3+, whose ESP is positive. As a result, oxygen atoms are the most probable sites to coordinate with Al3+, which is in agreement with the experiment.49 According to the experimental Job plot and high-resolution mass spectroscopy, the complex was assumed to have a ligandto-metal ratio of 2:1 with the hydrogen atom in −OH group deprotonated,49 so the complex e1-Al3+ was designed and its geometry optimized, as displayed in Figure 5. e1-Al3+ adopts a slightly twisted octahedron configuration, and the two ligand planes are nearly perpendicular to each other, with oxygen atoms and imine nitrogen atom as binding sites. The simulated electronic absorption spectrum of e1-Al3+ is illustrated in Figure 6, and the corresponding spectrum data are listed in Table 3. From Figure 6 and Table 3, it can be seen that the simulated spectrum is consistent with the experimental one.49 There are two main absorption peaks in the region of 260−400 nm. The weaker one at around 374 nm arises from the S0 → S1 and S0 → S2 transitions. The stronger one at 292 nm can be ascribed to the S0 → S5 and S0 → S6 transitions. Compared with e1, the electronic absorption spectrum of e1-Al3+ shifts to red. This can be easily explained by the corresponding molecular orbitals (Figure 3). After e1 is coordinated with Al3+, the LUMO energy level decreases significantly, while the HOMO energy level changes only a little, leading to a smaller energy gap. 3.3. Fluorescence Spectra of e1 and e1-Al3+. In order to obtain the vertical emission energy, the geometry of the first singlet electronic excited state (S1) should be optimized.

Figure 4. ESP of e1.

Figure 5. Optimized ball and bond model of e1-Al3+ from different directions.

Because of the O−H···N intramolecular hydrogen bond in e1, two possible isomers may exist in the first excited state: the normal isomer (N) and its tautomer (T). During the optimization of S1, we could not locate the normal isomer and only obtained the tautomer configuration (Figure 7). All attempts to optimize the normal configuration lead to the tautomer configuration. The normal form in the S1 state is thus considered unstable. The proton in −OH is transferred to the

Table 2. Calculated Electronic Absorption Spectrum Data of e1 excited states

ΔEv/eV

λ/nm

f

main transition

peak/nm

expt./nm49

S1 S2 S5 S17

3.93 4.47 5.27 6.14

316 277 235 202

0.3350 0.4260 0.2050 0.3160

HOMO → LUMO (0.944) HOMO−1 → LUMO (0.852) HOMO → LUMO+2 (0.792) HOMO → LUMO+8 (0.516)

316 277 234 203

317 277

C


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Table 4. Calculated Fluorescent Spectrum Data of e1 and e1-Al3+ compound

ΔEemi/eV

λ/nm

main transition

expt./nm49

e1 e1-Al3+

2.37 2.54

524 489

LUMO → HOMO (0.941) LUMO → HOMO (0.990)

486 441

Table 5. Calculated Thermodynamic Data (kcal/mol) for e1-M

Figure 6. Simulated electronic absorption spectrum of e1-Al3+ together with the experimental spectrum.49

ΔE

ΔH

ΔG

e1-Al3+ e1-Fe3+ e1-Cu2+ e1-Ni2+ e1-Co2+ e1-Fe2+ e1-Mg2+ e1-Zn2+

−178 37 182 203 218 217 288 310

−188 26 170 192 205 206 276 298

−172 42 181 205 218 222 290 310

From the aspect of thermodynamic, the reaction occurs easily. For the other metal ions, ΔE, ΔH, and ΔG are all positive, and these reactions occur with difficulty. At this point, e1 is selective to Al3+. 3.5. Reliability Test of Above Approach. To test the reliability of above approach, the same method was applied to e2 and e3. The electronic absorption spectra and fluorescence spectra were obtained; see Figures S1−S10 and Tables S2−S7. They all agree well with the experiment spectra.50,51 For example, the absorption wavelength of S1 state for e2 is 329 nm, consistent with 325 nm in the experiment.50 For e2-Al3+ complex, the absorption wavelength of S1 state red-shifts to 388 nm, consistent with 367 nm in the experiment.50 The emission from e2 arises from the tautomer, and the wavelength is 459 nm. The experimental value is 450 nm.50 For e2-Al3+ complex, the emission wavelength was calculated to be 473 nm, consistent with 452 nm in the experiment.50 Then, the interaction between e2 and e3 and different metals were also computed, and ΔE, ΔH, and ΔG were obtained, as listed in Tables 6 and 7. The detailed thermodynamic data are summarized in Tables S9 and S10. It was found that for Al3+, ΔE, ΔH, and ΔG of the corresponding reaction are all negative, while for the other metal ions considered here, ΔE, ΔH, and ΔG are all more positive than those for Al3+. Hence, e2 and e3 have selectivity toward Al3+.

imine nitrogen atom. ESIPT reaction occurs in the S1 state of e1. For the S1 state, the side chain twists to some extent, and the dihedral angle between the side-chain plane and the phenyl ring is about 60°. The calculated fluorescent emission wavelengths of e1 and e1-Al3+ are shown in Table 4. The calculated values agree with the experimental ones.49 The fluorescence of e1 arises from the ESIPT tautomer. When Al3+ is added to e1, the hydrogen atom in −OH group is ionized and they coordinate with each other. As a result, the spectrum blue-shifts 35 nm, consistent with the 45 nm blueshift in the experiment.49 3.4. Selectivity: The Interaction Between e1 and Different Metal Ions. In a previous study, Li et al. examined the selectivity of CN− probes by simulating the interaction between the functional group and CN− ion.70 Inspired by this point, we calculated the thermodynamic data of the reaction: e1 + M → e1‐M + H+

compound

(1)

including the reaction energy ΔE, the reaction enthalpy ΔH, and the reaction Gibbs free energy ΔG. Therein, M is metal ions, and e1-M is the metal complex. The corresponding data are given in Table 5, and the detailed thermodynamic data for e1-M are listed in Table S8. From Table 5, it can be seen that when M = Al3+, ΔE, ΔH, and ΔG of reaction 1 are all negative, indicating that the reaction is exothermal and spontaneous.

Table 3. Calculated Electronic Absorption Spectrum Data of e1-Al3+ excited states

ΔEabs/eV

λ/nm

f

main transition

S1 S2 S5 S6

3.25 3.32 4.20 4.28

382 373 295 289

0.1162 0.1239 0.6023 0.4069

HOMO → LUMO (0.936) HOMO−1 → LUMO (0.748) HOMO−2 → LUMO (0.836) HOMO−3 → LUMO (0.752)

peak/nm

expt./nm49

374

352

292

290

Figure 7. Optimized ball and bond model of S1 state of e1: (a) top view (b) side view. D


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3.6.1. Absorption Spectrum. Because of the two intramolecular hydrogen bonds in ABQ-Th, there are four kinds of proton transfer isomers. We tried our best to optimize these isomers. For the ground state, we only obtained the normal isomer, i.e., no hydrogen atom is transferred (Figure S11). On the basis of the geometry, the electronic absorption spectrum was simulated (Figure 9). Two absorption peaks are clearly seen: One at around 475 nm arises from the HOMO → LUMO transition, corresponding to S0 → S1 absorption, and another one at 327 nm can be ascribed to the HOMO−1 → LUMO transition, corresponding to S0 → S4 absorption. When Al3+ is added to ABQ-Th, the absorption peak at 475 nm redshifts to 591 nm. 3.6.2. Emission Spectrum. For the first excited state, we obtained four stable isomers (Figure 10). The first and second N/T in the name of each isomer represents the normal/ tautomer form of −O1H and −N1H2, respectively (Figure 8a). Their relative energies and fluorescent spectra data are listed in Table 8. Therein, ABQ-Th-T-N isomer has the lowest energy, i.e., the proton in −O1H group transferred and the proton in −N1H2 is untransferred. ABQ-Th−N-N isomer with neither proton transferred has a little higher energy. Their energy difference is within 0.04 eV. Both of them may emit fluorescence. The other two isomers with the proton in −N1H2 transferred have relatively higher energies, and they are less likely to emit fluorescence. After ABQ-Th is coordinated with Al3+, the isomer with the proton in −N1H2 being untransferred has much lower energy than the other (Table 8). The normal isomer is likely to emit fluorescence. As a result, ABQ-Th is likely to emit double fluorescence at 579 and 697 nm. After ABQ-Th is coordinated with Al3+, a new peak at 782 nm takes the place of the former peaks. Therefore, ABQ-Th-Al3+ can emit near-infrared fluorescence.

Table 6. Calculated Thermodynamic Data (kcal/mol) for e2-M compound

ΔE

ΔH

ΔG

e2-Al3+ e2-Ga3+ e2-Fe3+ e2-Cu2+ e2-Pb2+ e2-Mg2+ e2-Zn2+ e2-Cd2+

−388 −335 −166 −34 52 81 96 136

−387 −335 −165 −34 51 81 95 135

−356 −304 −133 −6 74 109 123 162

Table 7. Calculated Thermodynamic Data (kcal/mol) for e3-M compound

ΔE

ΔH

ΔG

e3-Al3+ e3-Ga3+ e3-Fe3+ e3-Cu2+ e3-Ni2+ e3-Mg2+ e3-Zn2+

−346 −295 −135 5 33 121 137

−345 −294 −134 1 30 118 134

−340 −291 −130 9 40 126 142

However, for Ga3+, Fe3+, and Cu2+, ΔG values are also negative. They may disturb the detection of Al3+ to some extent. As a result, these approaches can be considered reliable, and they have some applicability in such systems. Hence, we will apply these approaches to our newly designed molecules to check whether they can be used as an Al3+ probe. 3.6. ABQ-Th. Combining probe e3 and ABQ,53 we designed a new molecule, ABQ-Th (Figure 8a).

Figure 8. Molecular structures of ABQ-Th (a) and HBQ-Th (b).

Figure 9. Simulated electronic absorption spectra of ABQ-Th (a) and ABQ-Th-Al3+ (b). E


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Inorganic Chemistry

Figure 10. Four stable isomers of S1 state of ABQ-Th: (a) ABQ-Th-T-N, (b) ABQ-Th−N-N, (c) ABQ-Th-T-T, and (d) ABQ-Th-N-T.

Table 8. Calculated Fluorescent Spectra Data of ABQ-Th and ABQ-Th-Al3+ Complexes compound

ΔEr/eVa

ΔEemi/eV

λ/nm

ABQ-Th-T-N ABQ-Th-N-N ABQ-Th-T-T ABQ-Th-N-T ABQ-Th-Al3+-N ABQ-Th-Al3+-T

0 0.04 0.32 0.35 0 0.38

1.78 2.14 1.17 1.67 1.59 1.12

697 579 1060 741 782 1108

main transition LUMO LUMO LUMO LUMO LUMO LUMO

→ → → → → →

HOMO HOMO HOMO HOMO HOMO HOMO

(1.000) (0.973) (0.986) (0.992) (0.987) (1.000)

ΔEr is relative energy.

a

where M is a metal ion and ABQ-Th-M is the metal complex. The corresponding data are listed in Table 9, and the detailed thermodynamic data for ABQ-Th-M are listed in Table S11. From Table 9, it can be seen that when M = Al3+, ΔE, ΔH, and ΔG of reaction 2 are all negative, indicating that the reaction is exothermal and spontaneous. For the other metal ions, ΔE, ΔH, and ΔG are much more positive than those for Al3+. Thus, ABQ-Th is selective to Al3+. However, for Fe3+ and Cu2+, ΔE, ΔH, ΔG of reaction 2 are also negative, so Fe3+ and Cu2+ may disturb the detection of Al3+. From the above, it can be concluded that in the first excited state, the proton in −N1H2 of ABQ-Th-Al3+ is not transferred. In order to make the proton transfer and to obtain longer emission wavelength, −O2H is introduced to substitute for −N1H2 as it may form a stronger hydrogen bond which is

Table 9. Calculated Thermodynamic Data (kcal/mol) for ABQ-Th-M compound 3+

ABQ-Th-Al ABQ-Th-Fe3+ ABQ−Th-Cu2+ ABQ-Th-Ni2+ ABQ-Th-Mg2+ ABQ-Th-Zn2+

ΔE

ΔH

ΔG

−346 −140 −5 34 122 138

−345 −139 −9 32 119 134

−340 −136 −16 41 128 143

3.6.3. Selectivity toward Al3+. In order to evaluate the selectivity of ABQ-Th toward Al3+, we calculated the thermodynamic data of the reaction: ABQ‐Th + M → ABQ‐Th‐M + H+

(2)

Figure 11. Simulated electronic absorption spectra of HBQ-Th (a) and HBQ-Th-Al3+ (b). F


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Figure 12. Four stable isomers of S1 state of HBQ-Th: (a) HBQ-Th-T-T, (b) HBQ-Th-T-N, (c) HBQ-Th-N-T, and (d) HBQ-Th−N-N.

3.7.3. Selectivity toward Al3+. In order to judge the selectivity of HBQ-Th toward Al3+, we also calculated the thermodynamic data of the reaction:

advantageous to ESIPT. Another new molecule HBQ-Th is designed (Figure 8b). 3.7. HBQ-Th. 3.7.1. Absorption Spectrum. For the ground state, based on the normal isomer geometry (Figure S16), the electronic absorption spectrum was simulated (Figure 11). It is clear to see two absorption peaks. The one at around 438 nm arises from the HOMO → LUMO transition, corresponding to S0 → S1 absorption. The one at 324 nm can be ascribed to the HOMO−1 → LUMO transition, corresponding to S0 → S4 absorption. When Al3+ is added to HBQ-Th, the peak at 438 nm red-shifts to 530 nm. 3.7.2. Emission Spectrum. For the first excited state, we also obtained four stable isomers. The first and second N/T in the name of each isomer represents the normal/tautomer form of -O1H and -O2H, respectively (see Figure 12). Their relative energies and fluorescent spectra data are listed in Table 10.

HBQ‐Th + M → HBQ‐Th‐M + H+

wherein M is a metal ion and HBQ-Th-M is the metal complex. The corresponding data are listed in Table 11, and Table 11. Calculated Thermodynamic Data (kcal/mol) for HBQ-Th-M

Table 10. Calculated Fluorescent Spectra Data of HBQ-Th and HBQ-Th-Al3+ Complexes compound HBQ-Th-T-T HBQ-Th-T-N HBQ-Th-N-T HBQ-Th−N-N HBQ-Th-Al3+-T HBQ-Th-Al3+-N

ΔEr/eV ΔEv/eV a

0 0.05 0.22 0.31 0 0.15

1.42 1.92 2.04 2.44 1.41 1.89

λ/nm 874 645 607 509 878 655

→ → → → → →

HOMO HOMO HOMO HOMO HOMO HOMO

compound

ΔE

ΔH

ΔG

HBQ-Th-Al3+ HBQ-Th-Fe3+ HBQ-Th-Cu2+ HBQ-Th-Ni2+ HBQ-Th-Mg2+ HBQ-Th-Zn2+

−346 −135 −2 34 122 137

−345 −134 −6 32 118 134

−340 −130 −11 41 126 142

the detailed thermodynamic data are listed in Table S12. From Table 11, it was found that for Al3+, ΔE, ΔH, and ΔG of the corresponding reaction are all negative, while for the other metal ions considered here, ΔE, ΔH, and ΔG are all more positive than those for Al3+. Hence, HBQ-Th has selectivity toward Al3+.

main transition LUMO LUMO LUMO LUMO LUMO LUMO

(3)

(0.990) (0.991) (0.998) (0.975) (0.997) (0.983)

4. CONCLUSION Density functional theory and time-dependent density functional theory were employed to explore the geometries and electronic absorption and emission spectroscopic properties of three salicylaldehyde acylhydrazone Schiff base derivatives which were used as fluorescent probes for detecting Al3+ in previous experiments. It is found that the predicted electronic absorption and emission spectra agree well with the experimental ones, either before or after the probe reacting with Al3+. The corresponding reaction energy, reaction enthalpy, and reaction free energy were also calculated, indicating that this is a feasible way to evaluate the selectivity of the probe. On the basis of above method, two new molecules with a combination of a salicylaldehyde acylhydrazone Schiff base unit and a benzo[h]quinoline unit were designed, and their corresponding properties were calculated. It shows that the two newly

ΔEr is relative energy

a

Therein, HBQ-Th-T-T, in which both protons in −OH groups transferred, has the lowest energy. HBQ-Th-T-N isomer with one proton transferred has a little higher energy. Their energy difference is within 0.05 eV. Both of them may emit fluorescence. The other two isomers have relatively higher energies, and they are less likely to emit fluorescence. After HBQ-Th is coordinated with Al3+, the isomer with a proton in −O2H transferred has a much lower energy than the other. The tautomer is likely to emit fluorescence. As a consequence, HBQ-Th is likely to emit double fluorescence at 874 and 645 nm. After HBQ-Th is coordinated with Al3+, a new peak at 878 nm replaces the former peaks. Therefore, HBQ-Th-Al3+ can also emit NIR fluorescence. G


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Inorganic Chemistry designed molecules not only can emit NIR fluorescence but also have high selectivity toward Al3+. They are potential NIR Al3+ fluorescence probes and may be applied in biological systems.

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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.inorgchem.9b01335.

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Geometries, electrostatic potential surfaces, electronic absorption spectra, frontier molecular orbitals of e2, e3, ABQ-Th, HBQ-Th, and detailed thermodynamic data for their metal complexes (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.W.). *E-mail: [email protected] (J.Z.). ORCID

Jinglai Zhang: 0000-0002-2728-0511 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the National Natural Science Foundation of China (NSFC) (Grant Nos. 21703053, 21376063, 21476061, 21503069, and 21676071), Program for Henan Innovative Research Team in University (Grant No. 15IRTSTHN005). We thank the State Key Laboratory of Physical Chemistry of Solid Surfaces (Xiamen University) and the National Supercomputing Center in Changsha (Changsha Cloud Computing Center) for providing computational resources.

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Theoretical Design of Near-Infrared Al3+ Fluorescent Probes Based

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