ESIPT-Active 8-Hydroxyquinoline-Based Fluorescence Sensor for Zn(II) Detection and Aggregation-Induced Emission of the Zn(II) Complex.

A D-π-A type quinoline derivative, 2-(((4-(1, 2, 2-triphenylvinyl)phenyl)imino)methyl)quinolin-8-ol (HL), was synthesized and structurally characterized. The five-membered ring formed by the O-H···N hydrogen bond in HL contributed to the excited-state intramolecular proton transfer (ESIPT) behavior of HL, which was further verified by theoretical computations. Upon coordination with Zn2+, the hydroxyl proton in HL was removed, resulting in the inhibition of ESIPT. In the meanwhile, the formed Zn 2 L 4 complex displayed aggregation-induced emission (AIE) character in THF/H2O mixtures, which is conducive to the fluorescence enhancement in aqueous media. Structure analysis suggested that the origin of the AIE characteristic was attributed to restriction of intramolecular rotations along with the formation of J-aggregates. Based on ESIPT coupled with AIE, HL could recognize Zn(II) in aqueous media via an orange fluorescence turn-on mode. Benefitting from the AIE property, chemosensor HL was successfully applied to fabricate test strips for rapid sensing of Zn(II) ions.

Introduction

A molecule with a five- or six- membered ring formed by intramolecular hydrogen bond is prone to undergo excited-state intramolecular proton transfer (ESIPT), which can affect its luminescent property.[1−6] In the ground state, an ESIPT molecule exists in the enol form. Upon photoexcitation, a proton is transferred from the proton donor (−OH, −NH2) to the acceptor (−C=O, −N=), generating the keto form. The chelation of metal cations could remove the proton of −OH or −NH2, then the ESIPT process would be prohibited with significant spectral changes.[1−3] Therefore, one of the most important applications of ESIPT is the fluorescence sensors for metal ions. Fluorescence sensors based on ESIPT usually exhibit abnormally large Stokes shifts to avoid self-absorption.[1,3,7,8] A strong intramolecular H-bond interaction was observed in 8-hydroxyquinoline with five-membered ring structure enhancing the probability of ESIPT. In the meanwhile, 8-hydroxyquinoline, as a strong metal chelator, exhibits good ability to coordinate with transition-metal ions due to the formation of a stable five-membered chelate ring. Therefore, 8-hydroxyquinoline derivatives have been employed to develop novel fluorescence sensors to visualize metal cations via ESIPT suppression.[3,9,10]

Over the last decade, a mass of fluorescence chemosensors based on ESIPT were reported,[11−13] but some influential factors such as aggregation-caused quenching (ACQ)[14] and complex synthesis are still bottlenecks for their practical applications. It is difficult for traditional fluorescent sensors to be utilized in aqueous media or strip test due to the ACQ effect. In 2001, aggregation-induced emission (AIE) was originally developed by Tang’s group.[15] AIE luminogens (AIEgens) can be protected from the ACQ effect. They display weak or non-emission in dilute solutions, but strong fluorescence in the solid state on account of several mechanisms, including restriction of intramolecular motions,[16−18]J-aggregates,[19,20] inhibition of TICT process,[21,22] and excimer formation.[23] For propeller-like AIEgens in the aggregation state, the π–π stacking interactions which are the main reason for ACQ can be suppressed by their propeller-like structure. In addition, intramolecular motions inhibit the non-radiative dissipation channels that resulted from the molecular motions, and the radiative decay is accelerated with fluorescence enhancement.[24] Tetraphenylethylene (TPE) and its derivatives are known as a prototype of propeller-like AIEgens,[25] and they have been used extensively for fluorescence sensing.[26−29] However, most of the reported TPE-based sensors display blue or green fluorescence responses to analytes,[28−30] limiting their applications in the field of bioimaging. In contrast, long-wavelength emissions are considered to be less harmful to tissues and exhibit higher penetration depth compared with blue or green emissions. The most effective way to create fluorophores with long-wavelength fluorescence is to reduce the energy gap (Eg) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Based on the molecular orbital theory, electron donor (D)−π–electron acceptor (A) structure is an ideal model to reduce the Eg because the electron donor could elevate the HOMO level and the electron acceptor could decrease the LUMO level.[31]

Zinc, as an essential metal element for the human body, plays significant roles in various biological processes such as gene expression,[32] neural signal transmission,[33] and regulation of enzymes.[34] The deficiency and dysregulation of Zn2+ are involved with a number of diseases, including epilepsy,[35] Alzheimer’s disease,[36] cerebral ischemia,[37] and so forth. Thus, the development of fluorescence sensors to monitor the concentration of Zn2+ has drawn much attention of scientists.[38,39]

This study aims to design and synthesize a D−π–A type fluorescent sensor to recognize Zn2+ based on ESIPT coupled with AIE. Quinoline is a highly electron-deficient molecule, resulting from the sp2 hybrid nitrogen atom. Therefore, we chose the ESIPT-active 8-hydroxyquinoline group to act as the electron acceptor for the D−π–A system. In the meanwhile, the TPE group could behave as the electron donor as well as an activator for AIE to avoid the ACQ effect. To simplify the difficulty of synthesis, a Schiff-base compound, 2-(((4-(1,2,2-triphenylvinyl)phenyl)imino)methyl)quinolin-8-ol (HL), was designed and prepared by aldimine condensation to incorporate 8-hydroxyquinoline and TPE group via −CH=N– (Scheme ). HL exhibited little fluorescence that resulted from the ESIPT process, which was further illustrated by the density functional theory (DFT) calculations using the wB97xD/6-31G(d,p) method. Upon coordination with Zn2+, fluorescence was greatly enhanced due to the prohibition of ESIPT and the AIE behavior of the formed Zn(II) complex. HL was also successfully utilized to prepare a test strip for detecting Zn2+. Comparisons of HL with the recently reported small molecule fluorescence sensors for Zn2+ in terms of detection media, association constant, detection limit, and test paper application are listed in Table S1. Aqueous media are more ideal than pure organic media, and only a few fluorescence sensors were applied to fabricate test strips. At the two points, HL performs better than some of the reported chemosensors. Moreover, we can find that the association constant and the detection limit of HL for Zn(II) are above medium level among the reported chemosensors.

Scheme 1

Reaction Scheme for the Synthesis of HL

Experimental Section

Materials and Apparatus

All the solvents and reagents (analytical grade) were used as received. All the materials for synthesis were purchased from Macklin Biochemical Co., Ltd. The solutions of metal ions were prepared from LiCl, NaCl, KCl, MgCl2·6H2O, Mn(ClO4)2·6H2O, FeCl3·6H2O, Co(ClO4)2·6H2O, CrCl3·6H2O, CaCl2, Ni(ClO4)2·6H2O, Cd(ClO4)2·H2O, Cu(ClO4)2·6H2O, Zn(Ac)2, Al(ClO4)3·9H2O, and Pb(ClO4)2·3H2O. Elemental analyses were conducted using a Vario EL elemental analyzer. 1H NMR and 13C NMR spectra were obtained using a Bruker Ascend 400 (400 MHz) spectrometer. UV–vis absorption spectra were obtained on a UV-2600 spectrophotometer and fluorescence spectra were acquired on a Thermo Scientific Lumina fluorescence spectrophotometer, with a quartz cuvette (path length = 1 cm). The absolute fluorescence quantum yields were measured on the Hamamatsu C9920 Quantum Efficiency Measurement. Fluorescence lifetimes were determined with an FLS920 spectrometer. Fourier transform infrared spectra were measured on a Bruker INVENIO Fourier transform infrared spectrometer using KBr pellets. Electrospray ionization (ESI) mass spectra were obtained on a Thermo Fisher Q Exactive mass spectrometer with an ESI interface. Dynamic light scattering (DLS) data were carried out on a Zetasizer Nano ZS90 laser particle size and zeta potential analyzer (Malvern). Scanning electron microscopy (SEM) images were obtained using a Hitachi SU8100 SEM. pH values were measured on a PHS-3E pH meter.

Synthesis of Chemosensor HL

HL was synthesized along the reaction route depicted in Scheme . 4-(1,2,2-Triphenylethenyl)benzenamine[40] (0.0694 g, 0.20 mmol) in 1 mL of toluene was added into a solution of 8-hydroxyquinoline-2-carbaldehyde (0.0346 g, 0.20 mmol) in 1 mL of toluene. Next, the mixture was stirred and heated at 105 °C for 4 h. After cooling down, the resultant solution was evaporated at room temperature for 1∼2 days, then yellow crystals of HL were obtained. Yield: 0.0632 g, 62.8%. Anal. Calcd for C36H26N2O: C, 86.03; H, 5.21; N, 5.57. Found: C, 86.08; H, 5.50; N, 5.43. 1H NMR (400 MHz, DMSO-d6) (Figure S1a): δ 9.91 (s, 1H), 8.76 (s, 1H), 8.43 (d, J = 8.0 Hz, 1H), 8.25 (d, J = 8.0 Hz, 1H), 7.55 (t, J = 8.0 Hz, 1H), 7.48 (d, J = 8.0 Hz, 1H), 7.20 (m, 12H), 7.06 (m, 8H) ppm. 13C NMR (100 MHz, DMSO-d6) (Figure S1b): δ 160.71, 154.27, 152.67, 148.77, 143.61, 143.54, 142.60, 141.40, 140.50, 138.70, 137.30, 132.27, 131.19, 131.12, 129.89, 129.58, 128.40, 128.29, 127.13, 127.05, 121.36, 118.79, 118.27, 112.63 ppm. IR (KBr pellet, cm–1): 3682, 3031, 1562, 1498, 1457, 1375, 1324, 1236, 1193, 1077, 971, 841, 753, 696, 619, 546. HRMS (ESI) (Figure S2): m/z [M + H]+ Calcd for C36H26N2O, 503.2118; found, 503.2100.

Synthesis of Complex Zn2L4

Compound HL (0.0251 g, 0.050 mmol), Zn(Ac)2 (0.0045 g, 0.025 mmol), 5 mL of isopropanol, 1 mL of toluene, and 0.5 mL of DMF were sealed in a 25 mL Teflon-lined autoclave and heated at 70 °C for 72 h. After cooling down, the crude product was collected by filtration and then mixed with 2 mL of DMF in a closed 25 mL Teflon-lined autoclave, heated at 90 °C for 12 h. After cooling to room temperature, orange-red crystals of complex ZnL were collected by simple filtration. Yield: 0.0063 g, 11.7%. Anal. Calcd for C144H100N8O4Zn2: C, 80.93; H, 4.72; N, 5.24. Found: C, 81.03; H, 4.98; N, 5.13. IR (KBr pellet, cm–1): 3693, 3051, 1753, 1553, 1496, 1444, 1375, 1333, 1269, 1103, 846, 747, 700, 620.

Synthesis of HL + Zn2+ Aggregates

0.5 mL of Zn(Ac)2 (0.0367 g, 0.20 mmol) aqueous solution was added to 3 mL of HL (0.2010 g, 0.40 mmol) tetrahydrofuran (THF) solution and stirred for 5 h. Then, 6.5 mL of water was slowly added to obtain THF/H2O (3:7, v/v) mixed solution, and the solution gradually became turbid and finally formed sediment. After standing for 1 day, an orange powder of HL + Zn2+ aggregates were collected by filtering, washing with water, and air-dried. Yield: 0.1960 g, 45.9%. Anal. Calcd for C144H100N8O4Zn2: C, 80.93; H, 4.72; N, 5.24. Found: C, 81.07; H, 5.03; N, 5.17.

Molecular Structure Determination

Crystallographic data were collected on a Bruker D8 Venture diffractometer (Germany) with mirror optics monochromated Cu Kα radiation (λ = 1.54184 Å) at 298 K for HL and with graphite monochromated Ga Kα radiation (λ = 1.34139 Å) at 193 K for complex ZnL, respectively. The structures were solved by the direct methods and refined by the full matrix least-squares technique using the SHELX-2014/7 program.[41,42] Crystal data and the main bond lengths and bond angles for HL and complex ZnL are listed in Tables S2 and S3, respectively. CCDC 2132709 (HL) and CCDC 2132710 (ZnL) contain the supplementary crystallographic data for this paper.

Theoretical Calculation Methods

Gaussian 09 program[43] was employed for DFT calculations. Ground-state and S1-state geometry optimization was performed at the wB97xD/6-31G(d,p) level,[44−46] in combination with the SMD implicit solvation model of water. Because no TD-DFT analytic Hessian is available in Gaussian, the transition state for excited-state proton transfer was located at the gas phase and then a single point calculation was performed in solvation.

Results and Discussion

Crystal Structure of HL

The asymmetric Schiff-base HL was synthesized by the condensation of 1-(4-aminophenyl)-1,2,2-triphenylethene and 8-hydroxyquinoline-2-carboxaldehyde and structurally characterized by single-crystal X-ray diffraction. HL crystallizes in the triclinic space group P1̅. The molecular structure of HL in an asymmetric unit is presented in Figure S3a. There exists an intramolecular hydrogen bond (O–H···N: 0.820 Å, 2.236 Å, 116.13°) between the hydroxyl group and the quinoline nitrogen atom in HL, forming a five-membered ring which is prone to the ESIPT process and contributes to little fluorescence of HL. In the asymmetric unit of HL, there are two molecules arranged in a head-to-head mode, which is also unfavorable to luminescence.[47] Moreover, Figure S3b reveals that there exist intermolecular π–π interactions with a distance of 3.596 Å between the adjacent pyridine rings of different asymmetric units. The face-to-face π–π interactions could further strengthen the head-to-head stacking pattern in HL solids.

Selectivity of HL to Metal Cations

Compound HL in THF/H2O (3:7, v/v) displayed no fluorescence emission, which could be assigned to the ESIPT behavior of HL. The sensing properties of HL for metal ions were investigated via fluorescence emission measurement. Figure displays fluorescence spectra of HL mixed with various cations (Zn2+, Al3+, Ca2+, Co2+, Cu2+, Cd2+, Cr3+, Fe3+, K+, Li+, Na+, Ni2+, Mn2+, Mg2+, and Pb2+) in THF/H2O (3:7, v/v). Apparently, Zn2+ is the only cation that caused a significant fluorescence enhancement upon excitation at 420 nm. The fluorescence titration experiment was carried out by varying the Zn2+ concentration in THF/H2O (3:7, v/v). As depicted in Figure , the emission intensity at 596 nm was enhanced by degrees as the amount of Zn2+ increased. The corresponding plot of fluorescence intensity versus the concentration of Zn2+ was constructed and a good linear relationship was observed over the range of 2∼15 μM (Figure S4). Based on the rule of 3σ/slope,[48−50] the detection limit of HL to Zn2+ in THF/H2O (3:7, v/v) was estimated to be 1.07 × 10–7 M. The UV–vis absorption spectra of HL at variable concentrations in THF/H2O (3:7, v/v) were also measured and are presented in Figure S5a. There are two peaks centered at 265 and 316 nm corresponding to π–π* and n–π* transitions, respectively. Accordingly, the molar attenuation coefficients ε265 and ε316 were calculated to be 54740 and 18440 L·mol–1·cm–1, respectively (Figure S5b). Figure S6 displays the UV–vis absorption spectra of HL upon addition of different amounts of Zn2+. As the concentration of Zn2+ increased, the absorption peak at 265 nm decreased, a new absorption band in the range of 400–530 nm emerged and gradually increased. There existed an isosbestic point at 274 nm. All the results demonstrated that HL was coordinated with Zn2+ to form a new complex.

Figure 1

Fluorescence responses of HL (20 μM) in THF/H2O (3:7, v/v) to 1.0 equiv Zn2+, Al3+, Ca2+, Co2+, Cu2+, Cd2+, Cr3+, Fe3+, K+, Li+, Na+, Ni2+, Mn2+, Mg2+, and Pb2+, respectively. λex = 420 nm.

Figure 2

Changes of fluorescence spectra of 20 μM HL upon addition of Zn2+ ions (0, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.90, 1.0, and 1.2 equiv Zn2+) in THF/H2O (3:7, v/v). Inset: fluorescence intensity at 596 nm as a function of [Zn2+]/[HL]. λex = 420 nm.

For the purpose of validating complexation stoichiometry, Job’s plot analysis was conducted on the basis of emission intensity at 596 nm (Figure S7). The result suggested that the complex ratio of HL with Zn2+ was 2:1, which was further confirmed by the ESI mass spectrum. As shown in Figure S8, the intense peak at m/z = 1067.33052 corresponding to [ZnL2]H+ (Calcd m/z = 1067.33032) proved the existence of complex HL + Zn2+ with 2:1 stoichiometry. Additionally, on account of the fluorescence titration profile, the binding constant K of HL combined with Zn2+ in THF/H2O (3:7, v/v) was calculated to be 1.60 × 109 M–2 by the following equation 1[51,52] (Figure S9).

The competitive experiments were performed to investigate the interference of other metal ions to the detection of Zn2+. Figure S10a shows that a number of metal ions had a negative influence on the response of HL to Zn2+ in THF/H2O (3:7, v/v). However, when the competitive experiments were conducted in THF/H2O (3:7, v/v, HEPES buffer, 20 mM, pH = 7.4), other competitive metal ions except Co2+ and Cu2+ did not have significant interference for the detection of Zn2+ (Figure S10b).

Sensing Mechanism of HL to Zn(II)

To further investigate the species of HL and Zn2+ formed in the sensing system and the sensing mechanism of HL to Zn2+, the Zn(II) complex, ZnL was synthesized by a direct reaction and the crystal structure was determined. The crystal structure determination showed that complex ZnL belonged to the triclinic space group P1=. As presented in Figure , complex ZnL is dinuclear and consists of two Zn(II) and four L– ligands. One Zn(II) was five-coordinated with two quinoline nitrogen atoms and three hydroxyl oxygen atoms [two hydroxyl oxygen atoms bridge two Zn(II) ions], respectively. The 1:2 coordination ratio of Zn2+ with L– in the complex is in agreement with Job’s plot and ESI–MS results (Figures S5 and S6) of the HL + Zn2+ sensing system. In addition, the UV–vis absorption spectra of HL, HL + Zn2+, and ZnL in THF/H2O (3:7, v/v) were also measured. As presented in Figure S11a, the absorption spectra of HL + Zn2+ are very different from that of HL and are the same as that of complex ZnL. The fluorescence spectrum of HL + Zn2+ is also in accord with that of complex ZnL in THF/H2O (3:7, v/v) (Figure S11b). Furthermore, HL + Zn2+ aggregates were collected under the detection condition (THF/H2O 3:7, v/v) and the elemental analysis was determined. The elemental analysis result of HL + Zn2+ aggregates is in agreement with that of complex ZnL (Experimental Section). These phenomena could further validate that the species formed in the HL + Zn2+ system was complex ZnL. Additionally, as depicted in Figure S11a, there is almost no spectral overlap between the absorption spectrum of HL and the emission spectrum of HL + Zn2+. However, a little spectral overlap in the range of 500–580 nm between the emission and absorption spectra of the HL + Zn2+ system could be observed. The concentration of HL used in the fluorescence detection was 20 μM. Accordingly, the concentration of the formed ZnL complex in the detection system was about 5 μM. The UV–vis spectra of complex ZnL in THF/H2O (3:7, v/v) at variable concentrations were measured and are displayed in Figure S12a. Molar attenuation coefficients of ZnL at 430 and 550 nm were calculated to be 49,400 and 9700 L·mol–1·cm–1, respectively (Figure S12b). Thus, the absorbance of ZnL (5 μM) in THF/H2O (3:7, v/v) at 550 nm was estimated to be only 0.0485 (<0.05) based on the Lambert–Beer law (A = εbc, ε550 = 9700 L·mol–1·cm–1), indicating that the inner-filter effect barely presents at this concentration.

Figure 3

Crystal structure of the complex ZnL.

For HL, almost no fluorescence could be observed in THF (Figure S13) and in the solid state (Figure ). This can be attributed to the ESIPT process from OH to the quinoline N atom. Figure S13 shows that the emission intensity was enhanced obviously upon the addition of Zn2+, and the fluorescence spectrum consisted with that of ZnL in THF. Therefore, the fluorescence enhancement can be attributed to the coordination of Zn(II) which led to the removal of the hydroxyl proton and blocked the ESIPT process originally existing in HL. In the solid state, complex ZnL also displayed a much stronger emission than HL for the same reason (Figure ). Additionally, we explored the fluorescence properties of ZnL in THF/H2O (from 9:1 to 1:9, v/v) and found it exhibited obvious AIE character, which would be further discussed in the next section. The emission intensity was further amplified as the water fraction increased to 70%, which resulted from the formation of ZnL aggregates. Consequently, based on the discussion above, it is easy for us to draw the conclusion that the sensing mechanism of HL to Zn2+ in THF/H2O could be attributed to ESIPT coupled with AIE. The synergy of the AIE effect also can be confirmed by the fluorescence titration profiles (Figure ), which showed that the emission intensity did not get saturated after the addition of 0.5 equiv Zn2+. The excess Zn2+ ions contribute to the aggregation process together with the formation of ZnL, activating the AIE property of the formed Zn(II) complex.[53]

Figure 4

Fluorescence emission spectra of HL and ZnL in the solid state, λex = 470 nm. Inset: fluorescence photographs of HL and ZnL in the solid state under 365 nm UV lamp irradiation.

Aggregation-Induced Emission of Complex Zn2L4

Complex ZnL is soluble in THF and insoluble in water. To demonstrate the AIE characteristic of ZnL, the fluorescence spectra and quantum yields (ΦF) of ZnL were measured in pure THF and THF/H2O mixtures with different volume percentages of water (fw). As depicted in Figure , complex ZnL in THF displayed very faint light with a quantum yield as low as 0.017. As water was added into THF, the emission intensity at about 620 nm decreased by degrees with increasing fw from 10 to 50%. The decrement in fluorescence intensity can be ascribed to the electron-driven proton transfer from protic solvent H2O to complex ZnL.[54,55] When fw reached 60%, an orange fluorescence emission band in the range of 525–725 nm appeared. Then, fluorescence intensity was enhanced gradually as fw increased and reached the intensity maximum in THF/H2O mixture with fw = 75%. The inset photograph of Figure b, taken under 365 nm UV lamp irradiation, clearly displays that the solution fluorescence changes from darkness to a strong orange light. In the meanwhile, the quantum yield of ZnL in THF/H2O increased from 7.7 to 10.2% as fw increased from 65 to 75% (Table ). The enhancement of the fluorescence emission with the increase of fw might have resulted from the aggregation of ZnL in a poor solvent. To confirm the formation of ZnL aggregates after adding water to THF solution, DLS and SEM measurements were conducted. As shown in DLS results (Figure ), nanoaggregates with average hydrodynamic diameters of 423, 317, 271, and 227 nm were detected in THF/H2O mixtures with 70, 75, 80, and 85% H2O, respectively. Both DLS results (Figure ) and SEM images (Figure ) revealed that the particle size decreased with the increase in fw, this is consistent with the appearances of many other reported AIEgens.[56,57] The abovementioned phenomenon revealed the AIE characteristic of complex ZnL. Moreover, complex ZnL exhibited a reduction of emission intensity in THF/H2O mixtures as fw increased from 75 to 90%, but the fluorescence intensity was still much stronger than in pure THF (Figure b). This could have resulted from the “concentration quenching” effect. Along with the increase of fw, the particle size decreased and the particle concentration increased, resulting in a higher frequency of particle collisions. Therefore, the non-radiative transition processes were greatly accelerated accompanied with a fluorescence attenuation.[10]

Figure 5

(a) Emission spectra of ZnL (10 μM) in THF/H2O with different water fractions and (b) emission intensity of ZnL (10 μM) at 596 nm in THF/H2O. λex = 420 nm. Inset: fluorescence photographs of ZnL in THF/H2O with different water fractions (fw).

Table 1

Quantum Yields (ΦF) and Lifetimes (τ) of ZnL in THF/H2Oa

fw	65%	70%	75%	80%	85%
ΦF (%)	7.7	9.5	10.2	9.5	7.2
τ (10–9 s)	5.50	5.39	5.44	5.37	5.08
kr (107s–1)	1.40	1.76	1.88	1.77	1.42
knr (107s–1)	16.78	16.79	16.51	16.85	18.27

Quantum yields were determined by a calibrated integrating sphere.

Figure 6

DLS data of ZnL in THF/H2O with different water fractions (a: fw = 70%, b: fw = 75%, c: fw = 80%, d: fw = 85%).

Figure 7

SEM images of ZnL in THF/H2O with different water fractions (a: fw = 75%, b: fw = 80%, c: fw = 85%).

To gain an in-depth interpretation for the photoluminescence property of ZnL in THF/H2O, we also determined fluorescence lifetimes (τ) of ZnL in THF/H2O with different fw (Figure S14) and estimated the decay rate constants based on kr = ΦF/τ as well as knr = (1 - ΦF)/τ using the experimental profiles (ΦF and τ). kr and knr are radiative and non-radiative transition rate constants, respectively. As shown in Table , the knr was remarkably larger than kr, suggesting that non-radiative transition was dominant for ZnL in THF/H2O mixtures. The higher quantum yield of ZnL in THF/H2O with fw = 75% can be primarily ascribed to the suppression of non-radiative decay. As fw increased from 75 to 85%, the kr was decreased slightly, and the knr was enhanced obviously (Table ), implying that high-frequency collisions of particles promote non-radiative transition processes effectively with a decrement in quantum yield.

Due to the presence of the propeller-like TPE group, the AIE property of ZnL would have resulted from the restriction of intramolecular rotations. When ZnL was dissolved in pure THF, ZnL existed in a monomer form. The phenyls of the TPE group could rotate freely, thus the energy of the excited state was consumed by intramolecular rotations in a non-radiation way, generating faint luminescence. The addition of water caused the formation of ZnL aggregates, hence the intramolecular rotations were greatly suppressed by space constraints. Then, the non-radiation decay was impeded and the emission intensity increased obviously. Moreover, the arrangement of molecules in aggregates would affect the AIE character of complex ZnL to some extent.[47,58] Therefore, the crystal structure of ZnL was further investigated. As displayed in Figure , in ZnL, the four ligands L– complexed with Zn2+ were arranged in a head-to-tail pattern. Additionally, Figure S15 shows that pairs of adjacent ZnL molecules adopted face-to-face conformations connected by π–π interactions with a distance of 3.607 Å between the adjacent quinoline rings in different molecules. These π–π interactions reinforced the head-to-tail arrangement to a certain extent, and ligands L– experienced a J-type stacking in the solid state of ZnL. Thus, J-aggregates of ZnL were formed in THF/H2O mixtures, favoring fluorescence enhancement.[47] Consequently, the origin of AIE character for complex ZnL can be ascribed to RIR along with the formation of J-aggregations.

Theoretical Exploration of ESIPT and Orbital Energies in HL

DFT calculations were conducted to better illustrate the charge distributions in the molecules and the ESIPT behavior of HL (Figure a). The optimized Cartesian coordinates of the species are listed in Table S4. The energies and frequencies are summarized in Table S5. The calculated HOMO and LUMO distributions of HL and HL′ at the ground state are demonstrated in Figure S16. As expected of HL, the TPE group plays a significant role in adjusting the HOMO energy level due to the stronger electron-donating ability, and the electron-withdrawing effect of the 8-hydroxyquinoline group would influence the LUMO energy level. For HL, the HOMO is mainly distributed on the TPE group, and the LUMO is spread primarily over the 8-hydroxyquinoline group and the −C=N– bridging unit. As shown in Figure a, when the hydroxyl proton of HL transfers to the quinoline N atom, HL′ would be formed. The conjugation of HL′ is different from that of HL. Therefore, the calculated energy levels of them are totally different: −7.29 eV (HOMO) and −0.38 eV (LUMO) for HL and −6.95 eV (HOMO) and −0.94 eV (LUMO) for HL′, respectively (shown in Figure S16).

Figure 8

(a) Enol form (HL) and the Zwitterionic form (HL′). (b) Energy profile of the proton-transfer reaction of HL → HL′ in the ground state and the excited state.

Figure b illustrates the probable reaction route and relative energy of the ESIPT process. In the ground state, there exists mainly the enol form (HL) because the enol form (HL) is about 9.8 kcal/mol lower than that of the zwitterionic form (HL′). The proton-transfer process from OH to the quinoline N atom needs to overcome the energy barrier of 18.8 kcal/mol, but the activation energy of its reverse process is only 9.0 kcal/mol. Upon the photoexcitation, HL can be excited to HL(ex) state. The calculated absorption wavelength (331 nm) and the measured absorption wavelength (316 nm) of HL (Figure S5a) have good consistency, which certifies the effectiveness of the theoretical method utilized in the present work. In the excited state of HL, the proton-transfer process is more likely to happen due to a lower energy barrier (13.5 kcal/mol), and the relaxation pathway of fluorescence emission for HL(ex) to return to the ground state would be prevented. When the geometric change from HL(ex) to HL′(ex) is completed, HL′(ex) would be relaxed to the ground state by fluorescence emission, and the emission wavelength and oscillator strength are calculated to be 642 nm and 0.1912, respectively. This oscillator strength value (0.1912) is much smaller than that of radiative relaxation of HL(ex) (0.4612), indicating that little fluorescence emission can be observed. Consequently, the ESIPT process could give rise to faint fluorescence for HL when excited. The coordination with Zn2+ would form complex ZnL, resulting in the deprotonation of OH, then the ESIPT process is inhibited with large fluorescence enhancement.

pH Influence on Sensing of HL to Zn(II)

To evaluate sensor performance in practical applications, the influence of acidity on the fluorescence response of HL to Zn(II) was investigated via the acid titration experiments, which were carried out at a pH range from 2 to 13, with the concentration of HL fixed at 20 μM and Zn2+ at 10 μM, respectively. The pH was regulated using hydrochloric acid or sodium hydroxide solution. As displayed in Figure , free sensor HL exhibited little fluorescence in the acidic and neutral pH range (2∼7) as a result of ESIPT. With the increase of the pH value to 11, the emission intensity at 596 nm gradually increased, which could have been resulted from the deprotonation of HL in alkaline solution. This phenomenon could further confirm the existence of ESIPT in HL. However, HL still showed weak fluorescence in alkaline solutions although HL was deprotonated. This can be attributed to the rotations of the TPE group. Upon the addition of Zn2+, the fluorescence intensity was largely enhanced upon excitation at 420 nm over a comparatively wide pH range of 6∼12 (Figure ). This can be ascribed to the deprotonation of HL together with the formation of ZnL aggregates. The ESIPT process and intramolecular rotations were suppressed at the same time. Consequently, chemosensor HL is convenient for practical applications of Zn2+ detection in complex biological or environmental samples and there is no need to strictly control the pH value of sample solutions for recognizing Zn2+.

Figure 9

nfluence of pH on fluorescence intensities of HL and HL in addition of 0.5 equiv Zn2+ in THF/H2O (3:7, v/v). λex = 420 nm, λem = 596 nm.

Test Paper-Based Application

Considering that the test paper strip applications have attracted much attention due to its low-cost and simplicity, test paper strips were fabricated by immersing filter papers in THF solution containing sensor HL (1 mM) for 5 min, then dried in air. Next, the dried HL-loaded test papers were dipped into THF/H2O (3:7, v/v) solutions containing different cations (1 mM) for 1 min, respectively. As shown in Figure a, under 365 nm UV light, only Zn2+ resulted in obvious orange luminescence among all cations. Moreover, another batch of HL-loaded test papers were immersed in THF/H2O (3:7, v/v) solutions with different concentrations of Zn2+ (0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.1, 0.3, 0.5, 1, 3 mM), respectively. An obvious change from non-fluorescence to strong orange fluorescence was observed under 365 nm UV lamp irradiation (Figure b). When the concentration of Zn2+ reached as low as 0.01 mM, the fluorescence turn-on response could be discerned. These experimental results demonstrate that HL can be utilized to construct handy test paper strips for the detection of Zn2+, benefitting from the AIE character of the formed ZnL complex.

Figure 10

Under 365 nm UV light irradiation, photographs of HL-loaded test papers treated with different cation-containing analytes (1 mM) (a) and different concentrations of Zn2+ (b), respectively.

Conclusions

In summary, we have synthesized a D−π–A type Schiff-base (HL) via aldimine condensation of 8-hydroxyquinoline-2-carbaldehyde with 4-(1, 2, 2-triphenylvinyl)benzenamine. It can be used to identify Zn(II) via orange light turn-on in the THF/H2O system. The structures of HL and the ZnL complex were determined by X-ray crystallography. The ESIPT behavior of HL was studied and verified by experiments together with theoretical calculations. Upon coordination with Zn2+, the dinuclear complex ZnL was formed and the hydroxyl was deprotonated, resulting in the suppression of the ESIPT process. The fluorescence intensity of ZnL in THF/H2O increased as fw increased from 60 to 75%. Furthermore, the formation of aggregates was confirmed by DLS and SEM. Thus, ZnL displayed a typical AIE property which was attributed to the restriction of rotational freedom of the TPE group. The structure analysis suggested that the coordination of Zn2+ and π–π interactions resulted in J-mode stacking of ligands L– in the solid state of ZnL, hence the formation of J-aggregates is another reason for the fluorescence enhancement. The AIE property of complex ZnL formed in the detection system enables HL to be constructed into test paper strips for recognizing Zn2+. This study provides an insight into the sensing mechanism of HL for Zn2+ and provides a way for chemosensor design based on ESIPT along with AIE, so as to develop a strip test technique with low cost and simplicity..