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Synthesis and Photo- and Ionochromic and Spectral-Luminescent Properties of 5-Phenylpyrazolidin-3-one Azomethine Imines
Photochromic 5-phenylpyrazolidin-3-one-based azomethine imines containing 2-((1H-imidazol-2-yl)methylene) 1, 2-(pyridin-2-ylmethylene) 2, 2-(quinolin-2-ylmethylene) 3, and 2-((8-hydroxyquinolin-2-yl)methylene) 4 substituents were synthesized. All the compounds exist in the ring-opened O forms. Under irradiation with light of 365 nm, compounds 1–4 undergo thermally reversible isomerization into ring-closed bicyclic diaziridine isomers C. Azomethine imines 1–3 exhibit properties of ion-active molecular “off-on” switches of fluorescence when interacting with F− or AcO− anions. Compound 4 represents a bifunctional chemosensor demonstrating a colorimetric “naked-eye” effect for Ni2+ cation and complete fluorescence quenching in the presence of H+, F−, and CN− ions.
Bistable photochromic compounds capable of reversible transformation between two stable isomeric forms are widely used in the design of materials for molecular electronics, optical data storage, optical switching, molecular logic devices, photopharmacology, biovisualization, and chemo- and biosensors [1–7]. The most investigated classes of photochromes are spiropyrans and spirooxazines, diarylethenes, fulgides, and fulgimides  exhibiting positive photochromism with bathochromic spectral changes of the photoinduced isomers . To a lesser extent, the compounds with negative photochromism —norbornadienes [11, 12], acylotropic enaminoketones , and azomethine imines —are described. Azomethine imine molecules possess a polar 1,3-dipole N−-N+=C fragment, which makes these compounds to be valuable precursors in the combinatorial chemistry of heterocycles prepared by cycloaddition reactions [15–21]. The resulting products containing a pyrazolidine cycle annulated to other heterocycles find application in medicine as the multipurpose biologically active compounds , anti-HIV agents, inhibitors of NO synthase, and antidiabetic drugs [23–26], in agriculture [27, 28], and in other fields of science and technology . Previously, it was shown that N,N-cyclic azomethine imines are capable of photochromic transformation due to reversible intramolecular cyclization [14, 30, 31]; however, the ionochromic and chemosensor properties of azomethine imines remained to be virtually unexplored.
We have recently reported the synthesis and characterization of a series of novel 5-phenylpyrazolidin-3-one-based azomethine imines, including photochromic compounds and systems sensitive to cations and anions [32, 33]. The combination of photochromic and ionochromic properties in one molecular system opens the pathway to polyfunctional compounds that may be in demand for the design of ion-active fluorescent molecular switches and colorimetric “naked-eye” reagents [2, 4, 34, 35]. Especially interesting are multi- and bifunctional ion-active molecular switches capable of independent recognition of two and more types of “guest” ions owing to specific spectral response through the same or different channels .
Herein, with the aim of obtaining multifunctional photo- and ionochromic and fluorescent compounds, we report the synthesis of novel 5-phenylpyrazolidin-3-one-based azomethine imines containing 2-((1H-imidazol-2-yl)methylene) 1, 2-(pyridin-2-ylmethylene) 2, 2-(quinolin-2-ylmethylene) 3, and 2-((8-hydroxyquinolin-2-yl)methylene) 4 substituents and the investigation of their photochromic, fluorescent, and chemosensor properties. The choice of heterocyclic substituents was motivated by the possibility of metal ion binding and pH sensitivity due to the electron-donating pyridine nitrogen atom, whereas the presence of NH (OH) groups may lead to anion detection .
2. Experimental Section
The IR spectra were recorded on a Varian Excalibur 3100 FT-IR instrument using the attenuated total internal reflection technique (ZnSe crystal). The 1H and 13C NMR spectra in DMSO-d6 were recorded on a Varian Unity 300 spectrometer (300 MHz), the signals were referred with respect to the signals of residual protons of deutero-solvent (2.49 ppm), and values were measured with precision 0.01 ppm. Mass spectra were recorded on a Shimadzu GCMS-QP2010SE instrument with direct sample entry into the ion source (EI, 70 eV). The electronic absorption spectra were recorded on a Varian Cary 100 spectrophotometer. The irradiation of solutions in quartz cells (, ) with filtered light of a high-pressure Hg lamp (250 W) was performed on Newport 66941 equipment supplied with a set of interferential light filters. The intensity of light was photons·s−1 for the spectral line 365 nm. The electronic emission spectra were recorded on a Varian Cary Eclipse spectrofluorimeter. Acetonitrile of spectroscopic grade, d-metal perchlorates, and tetra-butylammonium salts (Aldrich) were used to prepare solutions. Melting points were determined on a PTP (M) instrument. The reaction progress and purity of the obtained compounds were controlled by TLC on Silufol U-254 plates using CHCl3/PrOH = 50 : 1 as eluent, and visualization was performed with iodine vapor in a moist chamber.
2.2. General Procedure for the Synthesis of Azomethine Imines (1–4)
A solution of 5-phenylpyrazolidin-3-one  (1.62 g, 10 mmol) and the corresponding aldehyde (10 mmol) in 2-PrOH (25 mL) was refluxed for 1.5 h. The reaction mixture was cooled to room temperature. The solvent was removed on a rotary evaporator, and the residue was recrystallized from the corresponding solvent.
2.3. 2-((1H-Imidazol-2-yl)methylene)-5-oxo-3-phenylpyrazolidin-2-ium-1-ide (1)
Yield 37%, colorless powder, m.p. 233–235°С (dec.) (BuOH). IR (/cm−1): 3262, 3113, 3031, 2984, 2943, 1658, 1612, 1506, 1495, 1425, 1348, 1163, and 1072. 1H NMR (DMSO-d6, 300 MHz, ppm) δ: 12.59 (s, 1Н, NH), 7.43–7.35 (m, 5Н, СHarom.), 7.25 (s, 1Н, С6H), 5.88 (d. d, 1Н, , 4.3, 9.4 Hz, C5H), 3.24 (d. d, 1Н, , 4.8, 11.7 Hz, C4H), 2.64 (d. d, 1Н, , 4.4, 16.4 Hz, C4H). 13C NMR (75 MHz, DMSO-d6, ppm) δ: 181.33 (C3), 138.87 (C7), 138.15 (С1), 132.05 (C6), 129.21 (C8, C12), 128.94 (C10), 126.66 (C9, С11), 122.48 (С3), 121.52 (С4), 70.71 (C5), and 38.76 (C4). EIMS, 70 eV, m/z (%): 240 (38) [M]+, 198 (6), 136 (8), 108 (12), 105 (10), 104 (100). Anal. Calcd for C13H12N4O: C, 64.99; H, 5.03; N, 23.22. Found: C, 64.78; H, 4.90; N, 23.11.
2.4. 5-Oxo-3-phenyl-2-(pyridin-2-ylmethylene)pyrazolidin-2-ium-1-ide (2)
Yield 65%, colorless solid, m.p. 185–186°С (2-PrOH). IR (/cm−1): 3053, 2986, 2958, 1679, 1665, 1579, 1570, 1557, 1495, 1454, 1433, 1394, 1282, 1259, 1239, 1195, 1091, and 1080. 1H NMR (DMSO-d6, 300 MHz, ppm) δ: 8.97 (d, 1Н, , С2Harom.), 8.64 (d, 1Н, , С5Harom.), 7.98 (t, 1Н, , С3Harom.), 7.45–7.42 (m, 5Н, СНarom.), 7.22 (s, 1Н, С6H), 6.00 (d. d, 1Н, , 9.6 Hz, C5H), 3.23 (d. d, 1Н, , 16.6 Hz, C4H), 2.64 (d. d, 1Н, , 16.6 Hz, C4H). 13C NMR (75 MHz, DMSO-d6, ppm) δ: 182.83 (C3), 149.97 (С5), 148.00 (С1), 138.83 (C7), 136.97 (С3), 131.44 (C6), 129.28 (C8, C12), 129.05 (C10), 126.79 (C9, С11), 125.95 (С2), 124.83 (С4), 73.19 (C5), and 37.86 (C4). EIMS, 70 eV, m/z (%): 251 (10) [M]+, 194 (6), 120 (41), 119 (41), 105 (10), 104 (100). Anal. Calcd for C15H13N3O: C, 71.70; H, 5.21; N, 16.72. Found: C, 71.59; Н, 5.29; N, 16.62.
2.5. 5-Oxo-3-phenyl-2-(quinolin-2-ylmethylene)pyrazolidin-2-ium-1-ide (3)
Yield 64%, colorless powder, m.p. 202–204°С (dec.) (BuOH). IR (/cm−1): 3064, 2990, 1672, 1575, 1495, 1452, 1406, 1301, 1278, and 1072. 1H NMR (DMSO-d6, 300 MHz, ppm) δ: 9.05 (d, 1Н, , С2Harom.), 8.54 (d, 1Н, , С3Harom.), 8.06 (d, 1Н, , С7Harom.), 7.97 (d, 1Н, , С10Harom.), 7.78 (t, 1Н, , С9Harom.), 7.66 (t, 1Н, , С8Harom.), 7.50–7.42 (m, 5Н, СНarom.), 7.39 (s, 1Н, С6H), 6.05 (d. d, 1Н, , 9.6 Hz, C5H), 3.27 (d. d, 1Н, , 16.4 Hz, C4H), and 2.69 (d. d, 1Н, , 16.6 Hz, C4H). 13C NMR (75 MHz, DMSO-d6, ppm) δ: 182.89 (C3), 148.66 (С5), 147.42 (С1), 138.73 (C7), 136.90 (С3), 131.49 (C6), 129.33 (C8, C12), 129.13 (C10), 128.95 (С10), 130.37 (С9), 127.35 (С8), 127.74 (С7), 126.95 (C9, С11), 128.14 (С4), 122.15 (С2), 73.55 (C5), and 37.84 (C4). EIMS, 70 eV, m/z (%): 301 (54) [M]+, 300 (26), 257 (12), 230 (8), 174 (12), 173 (100), 143 (31), 142 (25), 131(6), 116 (24), 115 (38), and 104 (16). Anal. Calcd for C19H15N3O: C, 75.73; H, 5.02; N, 13.94. Found: C, 75.89; Н, 5.09; N, 13.82.
2.6. 2-((8-Hydroxyquinolin-2-yl)methylene)-5-oxo-3-phenylpyrazolidin-2-ium-1-ide (4)
Yield 35%, yellow powder, m.p. 183–184°С (dioxane). IR (/cm−1): 3048, 3028, 2974, 2922, 2856, 1671, 1652, 1627, 1576, 1502, 1456, 1397, 1316, 1286, 1241, 1196, 1166, and 1085. 1H NMR (DMSO-d6, 300 MHz, ppm) δ: 9.96 (s, 1Н, ОН), 9.06 (d, 1Н, , С2Harom.), 8.45 (d, 1Н, , С3Harom.), 7.50–7.37 (m, 6Н, СНarom.), 7.37 (s, 1Н, С6H), 7.10 (d, 1Н, , С9Harom.), 6.08 (d. d, 1Н, , 5.4, 8.8 Hz, C5H), 3.26 (d. d, 1Н, , 16.6 Hz, C4H), 2.67 (d. d, 1Н, , 16.6 Hz, C4H). 13C NMR (75 MHz, DMSO-d6, ppm) δ: 182.80 (C3), 153.69 (С10), 146.51 (С1), 138.86 (C7), 138.71 (С5), 136.71 (С3), 131.74 (C6), 129.48 (С8), 129.46 (C8, C12), 129.21 (C10), 128.39 (С4), 127.08 (C9, С11), 122.43 (С2), 117.50 (С7), 111.99 (С9), 73.48 (C5), and 38.18 (C4). 15N NMR (75 MHz, DMSO-d6, ppm) δ: 259.41 (N1), 273.71 (N2), 308.24 (N6). EIMS, 70 eV, m/z (%): 317 (64) [M]+, 160 (14), 159 (100), 158 (89), 131 (15), 130 (24), 129 (20), 104 (10). Anal. Calcd for C19H15N3O2: C, 71.91; H, 4.76; N, 13.24. Found: C, 72.02; Н, 4.89; N, 13.32.
3. Results and Discussion
The IR spectra of 1–4 exhibit characteristic spectral bands of C=O and C=N+ groups at 1658–1673 cm−1. The 1H NMR spectra contain signals for the diastereotopic protons of the cyclic CH2 groups (two doublets of doublets at 2.93–3.38 ppm) and CH groups (a doublet of doublets at 5.56–5.68 ppm). Methine protons of СН=N+ fragments are observed as singlet signals in the region of 7.10–7.30 ppm. 1H and 13C NMR data corresponds to the ring-opened azomethine imine structure O (Scheme 2).
The electronic absorption spectra of ring-opened isomers 1–4 O in acetonitrile are characterized by long-wavelength bands with maxima in the range of 321–364 and 351–380 nm as shown in the example of 4 (Figure 1, Table 1).
and : maxima of absorption and fluorescence bands, respectively; : fluorescence intensity; : optical density at the maximum of absorption band of form C after irradiation with light of 365 nm for 2 min.
Compounds 1 and 2 are not fluorescent. Quinoline containing azomethine imine 3 possesses low-intensity fluorescence at 411 nm, while 8-hydroxyquinoline derivative 4 demonstrates more intense emission with a larger Stokes shift at 514 nm () (Table 1).
Irradiation of azomethine imine 1–4 O solutions in acetonitrile with light of 365 nm results in spectral changes in the characteristic of negative photochromism due to intramolecular photocyclization into diaziridines 1–4 C (Scheme 2), accompanied by a decrease in the intensity of long-wave absorption maxima and the appearance of absorption bands in the short-wave region of the spectrum [10, 30, 31] as shown in Figure 2(a) in the example of compound 1 and in Table 1.
Thermal reopening of the diaziridine cycle in dark conditions after the end of irradiation is clearly observed only for 1 (Figure 2(b)); the lifetime of the photoproduct 1 C was calculated as . In other cases, the photoproducts 2–4 C were stable for 3–4 days.
The addition of metal cations (in the form of perchlorates) to solutions of 1–3 in acetonitrile does not lead to noticeable spectral changes. In contrast, the interaction of 4 with Zn2+, Hg2+, and Ni2+ cations results in the appearance of novel long-wave absorption bands at 486–582 nm (Figure 3). A particularly distinct “naked-eye” effect (change of solution color from pale yellow to dark blue) was observed for Ni (II). These color changes are accompanied by almost complete quenching of the fluorescence of the initial solution (Figure 3 (inset)).
The addition of tetra-butylammonium salts (TBAX: X=F, Cl, H2PO4, CN, AcO, and ClO4) to solutions of compounds 1–4 in acetonitrile results in changes in both absorption and fluorescence spectra (Figures 4 and 5, Table 2).
Binding of fluoride anion with quinoline containing azomethine imine 3 causes almost complete diminishing intensity of the 364 nm band. At the same time, the emission maximum shifts to the longer-wavelength region at 460 nm and its intensity sharply increases (Figure 4).
On the contrary, coordination of fluoride anion with 8-hydroxyquinoline containing azomethine imine 4 is accompanied by a pronounced “naked-eye” effect (change in solution color from yellow to bright purple) due to the appearance of a new absorption band at 555 nm (Figure 5(a)). However, in this case, the initial emission of 4 is almost completely quenched (Figure 5(b)).
Interaction of nonfluorescent compounds 1 and 2 with tetra-butylammonium salts in acetonitrile does not significantly affect the absorption spectra but causes “on-off” switching of fluorescence at 425 and 460 nm in the presence of fluoride and acetate anions, respectively (Table 2, Figure 6).
Compounds 2–4 possessing a pyridine-type nitrogen atom exhibit pronounced acidochromic properties , whereas 1 is practically not sensitive to pH of the solution. For example, a decrease in the pH when adding trifluoroacetic acid to acetonitrile solution of 4 results in the appearance of wavelength absorption band at 419 nm and a decrease in the absorption intensity of the initial 374 nm band (Figure 7).
New maxima of acid-induced colored form of compounds 2 and 3 are located at 385 and 424 nm, respectively. Azomethine imines 3 and 4 are also рН-controllable “on-off” molecular switches of fluorescent properties. During acidification, the intensity of initial emission at 411 (3) and 514 nm (4) is gradually reduced until complete quenching.
To sum up, the synthesized 5-phenylpyrazolidin-3-one azomethine imines containing 2-((1H-imidazol-2-yl)methylene) 1, 2-(pyridin-2-ylmethylene) 2, 2-(quinolin-2-ylmethylene) 3, and 2-((8-hydroxyquinolin-2-yl)methylene) 4 substituents exist in the ring-opened O forms. Irradiation of their acetonitrile solutions with UV light of 365 nm results in thermally reversible transformation into the ring-closed bicyclic diaziridine isomers 1–4 C. Nonfluorescent compounds 1 and 2 exhibit selective “off-on” emission switching in the presence of F− and AcO− anions that cause an increase in fluorescence intensity in 30 and 20 times (1) and in 80 and 60 times (2), respectively. Azomethine imines 3 and 4 are bifunctional molecular switches capable of both inflaming (with F− and AcO− anions) or complete quenching of emission (with H+ cation). 2-((8-Hydroxyquinolin-2-yl)methylene)-5-oxo-3-phenylpyrazolidin-2-ium-1-ide 4 represents a bifunctional chemosensor demonstrating a pronounced “naked-eye” colorimetric effect for Ni2+ cation detection and fluorescence quenching in the presence of H+, F−, and CN− ions.
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was carried out in the framework of the basic part of State Task in the Sphere of Scientific Activity (nos. 4.6497.2017/8.9 and 4.5593.2017/6.7) and State Task of SSC RAS no. 01201354239.
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