Shengmei Guo1, Jianting Pan1,2, Jianyan Huang1, Lin Kong1, Jiaxiang Yang1. 1. College of Chemistry & Chemical Engineering, Anhui University, Anhui Province Key Laboratory of Chemistry for Inorganic/Organic Hybrid Functional Materials Hefei 230601 P. R. China jxyang@ahu.edu.cn. 2. Anhui Province Institute of Product Quality Supervision & Inspection Hefei 230051 P. R. China.
During the past decade, aesthetic organic fluorophores with aggregation induced emission (AIE) or aggregation-induced enhanced emission (AIEE) properties have received intense attention by virtue of their extensive applications as imaging agents and sensors, components of light-emitting diodes, and data recording and storage, among others.[1-8] Developing functional AIE/AIEE materials can provide several advantages over conventional materials, such as their high quantum yields, controlled self-assembly effects, photostabilities, controllable emission colour, and flexible post-functionalization such as tetraphenylethene, α-cyanostilbene, silole, diphenyldibenzofulvene, organoboron, phosphindole oxide, pyrazine, phenothiazine derivatives, etc[9-13]. The twisted molecular conformations of organic fluorophores can emit strong fluorescence in the aggregated state by suppressing a dense face-to-face packing and immense π–π stacking interaction. Also, in the aggregated state, the molecules interlock themselves which restricts the intramolecular rotation (RIR) and vibration (RIV) and relaxes through radiative pathways.[14-21] Therefore, it is of great significance to design and understand the relationships between structure and property for their further applications.[22-24]For the purpose of obtaining eminent organic molecules with strong fluorescence in aggregated states, the molecular design according to organic fluorophores is critical for adjusting their photophysical properties. Triphenylamine (TPA) is strong electron-donating structural units with good luminescence properties, high thermal, photostability, good absorptivity, and high absolutely quantum yields.[25-27] Furthermore, an organoboron compound allowing trigonal-planar geometry, endows high extinction coefficients and high absolutely fluorescence quantum yields.[28-31,55] The combination of α-cyanostilbene modified with triphenylamine and BF2 groups were used to design AIEE-active compounds TPE-B and TPE-BN (Scheme 1).[32-34] In the present investigation, it was found that picric acid (PA) is a powerful explosive as TNT, only a few available PA sensing reports in aqueous medium. Meanwhile, PA is a seriously environmental pollutant, which consequently causes damage of human health and aquatic systems.[35-39] We expect two compounds to exhibit brilliant PA detection in aqueous medium.
Scheme 1
Synthetic routes of compounds TPE-B and TPE-BN.
Based on the above considerations, two α-cyanostilbene derivatives (named TPE-B and TPE-BN) were synthesized via facile reaction. The UV-vis absorption spectra and fluorescence spectra of compounds TPE-B and TPE-BN were systematically investigated in different polar solvents. Meanwhile, the aggregates emission behaviors of compounds were studied in THF/water mixture solution. In addition, the recognition properties of TPA-B and TPA-BN to PA were studied and the corresponding sensor mechanism was analyzed by density functional theory (DFT) calculations.
Experimental section
Materials and instruments
The chemicals and solvents were purchased from a commercial supplier and used directly without any further refinement. The 1H and 13C NMR spectra were identified by a Bruker Avance 400 MHz spectrometer with tetramethylsilane as the internal standard. FT-IR spectra were obtained by Nicolet 380 spectrometer (4000–400 cm−1, KBr pellets). UV-vis absorption spectra were measured using a TU-1901 spectrometer and the fluorescence spectra were collected by Hitachi FL-7000 spectrofluorimeter from Hitachi High Technologies Corporation (Tokyo, Japan). Photo images were obtained using a digital camera (Nikon D7000).
Synthesis
The synthetic routes of compounds TPE-B and TPE-BN are shown in Scheme 1.
Synthesis of compound 1
The 4-N,N-diphenylaminobenzaldehyde (410 mg, 1.50 mmol) and p-nitrophenylacetonitrile (240 mg, 1.50 mmol) were dissolved in absolute ethanol (40 mL). After adding 2 drops of piperidine, the reaction mixture was refluxed for 5 h. After cooling to room temperature, the red precipitate was filtered, washed with ethanol for three times. The final product was dried in vacuum and collected as red powder 1 (290 mg, 0.69 mmol, yield: 46%). 1H NMR (DMSO-d6, 400 MHz) δ (ppm): 6.95 (d, J = 7.9 Hz, 2H), 7.17–7.23 (m, 6H), 7.42 (t, J = 7.9 Hz, 4H), 7.92 (d, J = 8.4 Hz, 2H), 7.98 (d, J = 8.0 Hz, 2H), 8.15 (s, 1H), 8.32 (d, J = 8.4 Hz, 2H). 13C NMR (DMSO-d6, 100 MHz) δ (ppm): 103.70, 118.01, 119.21, 124.29, 125.10, 125.22, 125.96, 126.30, 129.94, 131.52, 140.84, 145.56, 145.69, 146.75, 150.37.
Synthesis of compound 2
Compound 1 (250 mg, 0.60 mmol) and SnCl2·2H2O (680 mg, 3.01 mmol) were reflux under absolute ethanol (40 mL) for 2 hours. After cooling to room temperature, the mixture was poured into a saturated K2CO3 aqueous solution and extracted with dichloromethane. The organic layer was dried with MgSO4 and concentrated by vacuum evaporation. The crude product was purified by column chromatography (petroleum ether : ethyl acetate = 2 : 1). The final product was dried in vacuum and collected as orange powder 2 (180 mg, 0.46 mmol, yield: 76%). 1H NMR (DMSO-d6, 400 MHz) δ (ppm): 5.57 (s, 2H), 6.55 (d, J = 8.0 Hz, 1H), 6.63 (d, J = 8.4 Hz, 1H), 6.73 (d, J = 7.9 Hz, 1H), 6.96 (d, J = 8.3 Hz, 2H), 7.03–7.08 (m, 2H), 7.10–7.19 (m, 5H), 7.35–7.40 (m, 4H), 7.57 (s, 1H), 7.76 (d, J = 8.4 Hz, 2H). 13C NMR (DMSO-d6, 100 MHz) δ (ppm): 107.71, 113.90, 118.78, 120.84, 121.25, 124.15, 125.09, 126.45, 127.40, 129.73, 129.95, 136.21, 146.31, 148.35, 149.67. MALDI-TOF calcd for C27H21N3, 387.170, found, 387.947.
The compound 3 or 4 (1.0 mmol) and N,N-diisopropylethylamine (DIPEA, 1 mL) were dissolved in CH2Cl2 (10 mL) and the reaction was stirred at room temperature for 2 h. Then, BF3·OEt2 (1.5 mL) was dropwise added into the mixture solution. The reaction mixture was stirred and monitored by TLC until completed. The mixture was poured into cold water (80 mL), extracted with CH2Cl2 and dried with MgSO4. After the solvent is evaporated under reduced pressure. The solid was obtained by chromatography on a silica gel column using DCM as the eluent.Orange red powder TPA-B (400 mg, 0.75 mmol, yield: 75%). FT-IR (cm−1, KBr) v: 3056, 2923, 2853, 2217, 1627, 1583, 1556, 1534, 1459, 1389, 1331, 1329, 1270, 1216, 1179, 1153, 1047, 758, 698, 530, 512. 1H NMR (CDCl3, 400 MHz) δ (ppm): 5.53 (s, 3H), 6.83 (d, J = 7.5 Hz, 1H), 7.03–7.18 (m, 8H), 7.29–7.35 (m, 3H), 7.48 (s, 1H), 7.53 (t, J = 7.4 Hz, 1H), 7.59 (s, 1H), 7.63 (d, J = 7.9 Hz, 1H), 7.69 (t, J = 8.0 Hz, 1H), 7.76–7.81 (m, 3H), 8.48 (s, 1H). 13C NMR (CDCl3, 100 MHz) δ (ppm): 105.76, 118.35, 120.53, 120.61, 124.12, 124.42, 124.66, 125.54, 125.76, 125.90, 126.83, 129.47, 129.54, 129.63, 130.22, 131.02, 132.26, 136.09, 139.58, 143.02, 146.42, 150.50. MALDI-TOF calcd for C34H24BF2N3O, 538.202, found, 538.595.Yellow powder TPA-BN (410 mg, 0.66 mmol, yield: 66%). FT-IR (cm−1, KBr) v: 3035, 2975, 2929, 2204, 1626, 1589, 1509, 1459, 1352, 1313, 1297, 1278, 1241, 1217, 1192, 1145, 1075, 1030, 971, 961, 898, 842, 750, 729, 529. 1H NMR (CDCl3, 400 MHz) δ (ppm): 1.23–1.27 (m, 6H), 3.49–3.44 (m, 4H), 6.25 (s, 1H), 6.38 (d, J = 9.2 Hz, 1H), 6.82 (d, J = 8.6 Hz, 1H), 7.04–7.17 (m, 9H), 7.28–7.34 (m, 4H), 7.43 (s, 1H), 7.51 (s, 1H), 7.56 (d, J = 8.4 Hz, 1H), 7.23 (s, 1H), 7.69 (d, J = 8.5 Hz, 1H), 7.79 (d, J = 8.6 Hz, 1H), 8.06 (s, 1H).13C NMR (CDCl3, 100 MHz) δ (ppm): 12.69, 45.24, 98.03, 106.40, 106.97, 107.17, 118.58, 120.75, 123.49, 124,51, 125.79, 126.22, 126.57, 129.62, 130.86, 134.17, 134.20, 142.03, 143.11, 146.55, 150.16, 156.48, 157.52, 161.96. MALDI-TOF calcd for C38H33BF2N4O, 609.271, found, 609.830.
Results and discussion
Solvatochromic effect
The UV-vis absorption and photoluminescence (PL) spectra of compounds TPA-B and TPA-BN were measured in various polar solvents and demonstrated in Fig. S1† and Fig. 1, respectively, and the corresponding photophysical data were summarized in Table 1. All the spectroscopic studies were performed at room temperature and the concentration of the solutions were 1.0 × 10−5 mol L−1. The UV-vis absorption spectra of TPA-BN showed two characteristic absorption bands at 297 nm and 420 nm in various polarity solvents, respectively. The absorption peak at 297 nm is ascribed to the π–π* transition in TPA moiety corresponding to locally excited (LE) state,[27,40,41] while the latter peak at 420 nm denotes intramolecular charge transfer (ICT) process. The UV-vis absorption of TPA-B has a similar peak shapes. And the emission wavelengths of two compounds revealed larger shift with increasing polarity. For compound TPA-B, the emission peak was hypochromatic-shifted from 541 nm in benzene to 530 nm in ethanol, indicated that TPA-B molecule possibly had a larger polarity in the ground state than in the excited state. However, the fluorescent emission peak of compound TPA-BN was bathochromic-shifted from 516 nm in benzene to 540 nm in DMF, indicated that TPA-BN molecule might have a larger polarity in the excited state than in the ground state.[42-44] Meanwhile, the evident shift of emission peaks manifested the existence of ICT process. The slopes of the fitting line in Lippert–Mataga plots (Fig. S2†), which were −646.5 and 2887.8, respectively, indicated that the compound TPA-BN introduced into the electron-donating group significantly possessed the solvatochromic effects. And, Fig. 5 shows that the TPA group has a larger density in the highest occupied frontier molecular orbital (HOMO) of TPA-B molecular. As the electron density of the lowest unoccupied molecular orbital (LUMO) of the TPA unit decreases, the other moieties achieve larger electron density. For TPA-BN, the HOMO and LUMO were mostly distributed over the molecular and TPA groups, respectively, which further manifested that the ICT process might occur in TPA-B and TPA-BN.[60] It was indicated that two compounds were characteristic of polarity-dependent solvatochromic effects.
Fig. 1
UV-vis absorption spectra (A) and PL spectra (B) of the compound TPA-BN in different solvent.
Photophysical properties of compounds TPA-B and TPA-BN in different polar solvents
Compounds
Solvents
λabsa
εmaxb
λflc
Δνd
Φe (%)
TPA-B
Benzene
419
4.59
541
5382
8.8
DCM
427
7.10
538
4832
1.0
Ethyl acetate
420
6.64
570
6262
2.3
THF
420
6.76
531
4977
1.5
Ethanol
408
6.06
530
5642
1.2
Acetonitrile
420
7.19
535
5118
0.1
DMF
423
7.07
540
5122
0.1
TPA-BN
Benzene
423
7.78
516
4260
9.1
DCM
418
7.93
533
5161
6.1
Ethyl acetate
415
7.81
519
4828
8.8
THF
416
7.75
520
4807
4.2
Ethanol
414
7.59
526
5143
6.9
Acetonitrile
422
8.76
542
5246
1.2
DMF
425
7.66
540
5010
3.1
Absorption peak position in nm (1 × 10−5 mol L−1).
Maximum molar absorbance in 104 mol−1 L cm−1.
Peak position of fluorescent (1.0 × 10−5 mol L−1), excited at the absorption maximum.
Stokes shift in cm−1.
Quantum yields determined (RhB as the standard).
Fig. 5
HOMO and LUMO energy–level diagram of TPA-B/TPA-BN and PA.
Absorption peak position in nm (1 × 10−5 mol L−1).Maximum molar absorbance in 104 mol−1 L cm−1.Peak position of fluorescent (1.0 × 10−5 mol L−1), excited at the absorption maximum.Stokes shift in cm−1.Quantum yields determined (RhB as the standard).
Aiming to investigate the AIEE characteristics of TPA-B and TPA-BN, their UV-vis absorption spectra and PL spectra with different water fraction were performed. The concentrations were kept at 1.0 × 10−5 mol L−1. For UV-vis absorption spectra of compound TPA-BN (Fig. S3b†), with the increasing water fraction, the solubility of the compound gradually decreased and the molecules gradually shifted from the monomolecular state to the aggregation state, forming the molecular aggregated particles. The level-off tails attributed to Mie scattering were responsible for the aggregated forms in the UV-vis absorption spectra.[45-50] A similar change trend was also observed from the UV-vis absorption spectra of TPA-B with the change of water contents. For compounds TPA-B and TPA-BN, dynamic light scattering (DLS) experiment displays the formation of aggregates (the hydrodynamic diameter of 267.2 nm and 390.2 nm, respectively). For PL spectra (Fig. 2), in pure THF solution, compound TPA-BN emitted weak yellow-green fluorescence. Compound TPA-BN started gradually aggregate with the increasing water fraction, the PL intensity showed a remarkable enhancement accompanied with a bathochromic-shift of fluorescent emission peak. This emission behavior may be accounted for the restriction of intramolecular rotation (RIR) and forms of J-aggregation,[51] which blocks the channel of non-radiative decay in the aggregates. Furthermore, when the water fraction is higher than 80%, the PL intensities gradually decreased, because the settlement of large aggregated particles resulted in a lower fluorescence intensity. Moreover, we found that the PL spectra of the compound TPA-B (Fig. S4†) displayed a similar behavior. The PL intensity of the compounds TPA-B and TPA-BN in the aggregated state were obviously stronger than that of the THF solution, the PL intensities were 1.6 and 2.0 fold, respectively. It was easily concluded that compounds TPA-B and TPA-BN were typical AIEE-active molecules.
Fig. 2
PL emission (a) spectra changes of TPA-BN (5.0 × 10−5 M) in THF/H2O mixtures with different water volume fractions; plots of PL intensity determined in THF–H2O solutions versus water fractions (b). Insets: photos of TPA-BN in THF–H2O mixtures (fw = 0%, 80% and 99%) taken under 365 nm UV lamp.
Furthermore, PL spectra of compounds TPA-B and TPA-BN were investigated to explore the difference of solid emission. As was shown in Fig. 3, TPA-B exhibited orange emission, whereas TPA-BN containing an additional electron-donating group has a obvious influence on hypsochromic shift of fluorescent peaks, emitting yellow fluorescence. The solid state fluorescent quantum yields of TPA-B and TPA-BN were measured as 13.5% and 19.4%, respectively in the solid state. The possible fluorescence emission discrepancy was caused by the role of intermolecular arrangement and stacking in solid state. Thus, the introduction of additional electron-donating group could effectively affect their fluorescence emission behavior and result in different spectral shifts and absolute quantum yields.
Fig. 3
Normalized PL spectra of two solids. Inset: the corresponding fluorescence images of TPA-B and TPA-BN under illumination.
The detection for picric acid
Based on brilliant AIEE behavior of compounds TPA-B and TPA-BN in mixtures of THF/H2O. We next explore whether compounds TPA-B and TPA-BN can detect common nitroaromatic compounds (p-methyl phenol (p-MP), o-nitrophenol (o-NP), 2,4,6-trinitrotoluene (TNT), m-dihydroxybenzene (m-DOB), p-nitrophenol (p-NP), p-dihydroxybenzene (p-DOB), phenol (PhOH), p-nitroaniline (p-NA), nitrotoluene (NT), nitrobenzene (NB), 2,4,6-trinitrophenol (PA)) in aqueous medium, the PL spectra were measured with addition of 10 equivalents nitroaromatic compounds. As shown in Fig. 4a and S6a,† compared with other analytes, it was clearly observed that the positive interactions of TPA-B and TPA-BN with PA showed significant fluorescent quenching effect. Experimental results exhibited that other nitroaromatics exhibited relatively little effect on emission quenching compared with that of PA, suggesting the selective sensing behaviors of TPA-B and TPA-BN towards PA. Furthermore, it is worth noting that further additions of 10 equivalents PA into other nitroaromatic systems leads to remarkable quenching efficiency of fluorescence intensity (Fig. 4b and S6b†), means that other compounds have negligible effect on the detection of PA by TPA-B and TPA-BN. According to the above results, compounds TPA-B and TPA-BN are regarded as promising candidates for PA detection in aqueous medium.
Fig. 4
(a) PL spectra obtained for different analytes; (b) quenching percentages of compound TPA-BN (10.0 μM) with different analytes in THF/water (v/v = 2 : 8) mixtures before (black) and after (red) the addition of 10 equiv. a: p-MP, b: o-NP, c: TNT, d: m-DOB, e: p-NP, f: p-DOB, g: PhOH, h: p-NA, i: NT, j: NB; (c) PL spectra of TPA-BN (10.0 μM) in THF/water (v/v = 2 : 8) containing different amounts of PA (100.0 μM); (d) corresponding Stern–Volmer plot for PA detection. Inset: Stern–Volmer plot obtained at a lower concentration of PA.
The key to performing better practical application, the experiment was carried out to better explore the sensitivity of compounds TPA-B and TPA-BN towards PA in aqueous medium. As shown in Fig. 4c and Fig. S6c,† the fluorescent quenching efficiencies of TPA-B and TPA-BN were 82.7% and 81.6% in the presence of 10 equivalents PA, respectively. In order to evaluate the quenching efficiency of TPA-B and TPA-BN in response to the PA in solution, a quenching process can be analyzed by Stern–Volmer equation,[52] the equation was as follow:where I0 and I are the initial and final PL intensity after addition of PA, respectively; [A] is the molar concentration of analytes; and Ksv is the Stern–Volmer quenching constant. By fitting the linear curves, the slopes of the plots gave the Ksv of TPA-BN and TPA-B, which were 1.28 × 104 M−1 and 1.07 × 104 M−1, respectively. The limit of detection (LOD)[53,54] was obtained from the equation of LOD = 3σ/k, wherein σ is the standard deviation of the blank test, k represents the slope of the linear calibration curve, the LOD were calculated as 1.26 × 10−6 M for TPA-B and 1.51 × 10−6 M for TPA-BN, respectively (Fig. S7†). Compounds exhibiting brilliant PA detection in aqueous medium are comparable with organoboron chemosensors in the Table S1.†[56-59]Density functional theory (DFT) calculation was used to investigate possible PA sensing mechanism. As shown in Fig. 5, the LUMO level (−2.79 eV) of TPA-B is higher than that of PA (−3.90 eV), which suggested when excited TPA-B was exposed to PA, the excited electron was transferred from the LUMO of fluorophores to that of the electron-deficient PA. The radiative electronic transition was blocked due to the electron transfer from the chromophore to PA, resulting in fluorescence quenching. Moreover, the other driving force is the energy difference between the LUMO of the fluorophore and the LUMO of analytes.[61-65] The LUMO orbital energy typically indicates how easily an electron can be transferred from excited fluorophore to the electron-deficient analytes. As the LUMO orbital energies of PA and TNT calculated by DFT is −3.90 eV and −3.33 eV, respectively. As expected, because of its small LUMO value, the PET driving force of PA is larger than that of TNT. This may partly explain the sensitivity and selectivity of TPA-B for PA detection. Similarly, for TPA-BN, the LUMO energy level of PA is between the LUMO energy level and the HOMO energy level of TPA-BN, indicating that the fluorescence quenching of TPA-BN with PA may suggest that the possibility of electron transfer from the LUMO of the chemosensor to the lower LUMO of PA. Based on the above results, the main driving force was attributed to the photo-induced electron transfer (PET) process, resulting in fluorescence quenching. Meanwhile, as was showed in the Fig. S8,† the UV-vis absorption spectra of PA and the PL spectra of TPA-B and TPA-BN were recorded and presented to examine whether fluorescence resonance energy transfer (FRET) occurs in detection process. The absorption spectrum of PA has no overlap with the PL spectra of the two chemosensors, indicating that FRET was neglectable in the quenching process. Therefore, the PET process may be main reason for recognition mechanism between the chemosensors and PA.
Conclusions
In conclusion, two novel α-cyanostilbene derivatives (TPA-B and TPA-BN) containing triphenylamine and BF2 units are synthesized and characterized that displayed obvious solvatochromic effect. Their ICT behaviors are certified by DFT analysis on molecular orbitals. Meanwhile, the incorporation of α-cyanostilbene unit in the backbone endows molecules an obvious AIEE behaviour in THF–water mixture solvent. Compounds showed excellent fluorescence emission in the solid state with high absolute quantum yield. Most impressing, TPA-B and TPA-BN were used as highly selectively and sensitively chemosensor to detect PA in aqueous medium and the LODs were calculated as 1.26 × 10−6 M and 1.51 × 10−6 M, respectively. The efficient sensing mechanism of two chemosensors in the presence of PA contributed to the PET process, verified by DFT calculations on molecular orbitals. This research provides two novel compounds for the rational design of AIEE-active materials for sensing application in aqueous medium.
Conflicts of interest
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Authors: J Luo; Z Xie; J W Lam; L Cheng; H Chen; C Qiu; H S Kwok; X Zhan; Y Liu; D Zhu; B Z Tang Journal: Chem Commun (Camb) Date: 2001-09-21 Impact factor: 6.222
Scheme 1
Fig. 1
Fig. 5
Fig. 2
Fig. 3
Fig. 4