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-2ylmethylene) 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 Ni cation and complete fluorescence quenching in the presence of H, F, and CN ions.

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 ionactive 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 [36].

Experimental Section
2.1.General.The IR spectra were recorded on a Varian Excalibur 3100 FT-IR instrument using the attenuated total internal reflection technique (ZnSe crystal).The 1 H and 13 C NMR spectra in DMSO-d 6 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 δ 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 (l = 1 cm, V = 2 • 10 −3 L) 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 3 2 • 10 16 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 CHCl 3 / PrOH = 50 : 1 as eluent, and visualization was performed with iodine vapor in a moist chamber.(1)(2)(3)(4).A solution of 5-phenylpyrazolidin-3-one [33] (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.
The IR spectra of 1-4 exhibit characteristic spectral bands of C=O and C=N + groups at 1658-1673 cm −1 .The 1 H NMR spectra contain signals for the diastereotopic protons of the cyclic CH 2 groups (two doublets of doublets at 2.93-3.38 ppm) and CH groups (a doublet of doublets at 5.56-5.68ppm).Methine protons of СН=N + fragments are observed as singlet signals in the region of 7.10-7.30ppm. 1 H and 13 C 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 longwavelength bands with maxima in the range of 321-364 and 351-380 nm as shown in the example of 4 (Figure 1, Table 1).
Irradiation of azomethine imine 1-4 O solutions in acetonitrile with light of λ irr 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 τ = 1538 s.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 Zn 2+ , Hg 2+ , and Ni 2+ 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, H 2 PO 4 , CN, AcO, and ClO 4 ) to solutions of compounds 1-4 in acetonitrile results in changes in both absorption and fluorescence spectra (Figures 4 and 5, Table 2).International Journal of Photoenergy 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    5 International Journal of Photoenergy 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).

Comp
Compounds 2-4 possessing a pyridine-type nitrogen atom exhibit pronounced acidochromic properties [9], 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.
fl max : maxima of absorption and fluorescence bands, respectively; I fl : fluorescence intensity; A: optical density at the maximum of absorption band of form C after irradiation with light of 365 nm for 2 min.

Table 1 :
Electronic absorption and fluorescence spectra of isomeric forms of 1-4 in acetonitrile * .