Novel fluorophores based on imidazopyrazine derivatives: Synthesis and photophysical characterization focusing on solvatochromism and sensitivity towards nitroaromatic compounds

Dyes and Pigments 168 (2019) 248–256

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Novel fluorophores based on imidazopyrazine derivatives: Synthesis and photophysical characterization focusing on solvatochromism and sensitivity towards nitroaromatic compounds

T

Egor V. Verbitskiya,b,∗, Yuriy A. Kvashnina, Anna A. Baranovab, Konstantin O. Khokhlovb, Roman D. Chuvashovb, Yuliya A. Yakovlevab, Nadezhda I. Makarovac, Elena V. Vetrovac, Anatoly V. Metelitsac, Gennady L. Rusinova,b, Oleg N. Chupakhina,b, Valery N. Charushina,b a b c

I. Postovsky Institute of Organic Synthesis, Ural Branch of the Russian Academy of Sciences, S. Kovalevskaya Str., 22, Ekaterinburg, 620990, Russia Ural Federal University, Mira St. 19, Ekaterinburg, 620002, Russia Institute of Physical and Organic Chemistry, Southern Federal University, Stachki Av, 194/2, Rostov on Don, 344090, Russia

A R T I C LE I N FO

A B S T R A C T

Keywords: Pyrazine Imidazole Solvatochromism Fluorescence quenching Nitroaromatic compounds

Novel imidazopyrazine-based dyes of the D–A type, possessing (hetero)aryl electron-donating groups in the imidazole moiety, have been synthesized. Their photophysical properties have been investigated using absorption and emission spectral analyses, both in solution and in solid-state. All dyes exhibit strong emission solvatochromism (Δλem = 146–201 nm) and possess quantum yields up to 0.28, depending on their molecular structure and solvent polarity. Meanwhile, fluorescence studies have shown that emission of all fluorophores is sensitive towards different nitroaromatic compounds, either in solutions or in a vapor phase. Thus, these compounds can be regarded as promising multifunctional chemosensors.

1. Introduction Heterocyclic chromophores belong to one of the most investigated classes of optical sensing molecules due to their excellent spectral properties and ability to detect diverse analytes. Disubstituted (hetero) aryl fused imidazoles, such as N-aryl- and N-alkylbenzimidazoles, as well as fused imidazoles are gaining a growing attention as building blocks in molecular systems for different applications: optoelectronics and non-linear optics (NLO), photovoltaics, organic light-emitting diodes (OLEDs), sensing and bioimaging (Fig. 1) [1–7]. However, so far, there has been no systematic study dedicated to relationships between structural and photophysical properties of (hetero)aryl fused imidazoles due to a limited structural diversity of this chemical library. Recently, we have developed a convenient synthetic route to 5-aryl5H-imidazo[4,5-b]-[1,2,5]oxadiazolo[3,4-e]pyrazines based on the effective two-step procedure, involving treatment of 5,6-diamino [1,2,5] oxadiazolo[3,4-b]pyrazines with triethyl orthoformate followed by the reaction with a π-excessive arene in the presence of trifluoroacetic acid [8]. Furthermore, it has been shown that push-pull chromophores with electron-withdrawing azaaromatic units (e.g. pyridines [9–12],

pyrimidines [13–16] or [1,2,5]oxadiazolo[3,4-b]pyrazines [17]) are very promising sensing materials for recognition of a variety of nitroaromatic compounds (NACs) through fluorescence quenching. This paper is a further extension of our research studies that are focused on the design of novel fluorophores for chemosensors. Herein we wish to report the synthesis of a new series of D–A imidazopyrazine-based compounds 3a-c obtained through incorporation of the appropriate electron-donative substituents into the imidazole core, and systematic investigation of their photophysical properties, solvatochromism, structure-property relationships and applications, as fluorescent sensors for nitroaromatic explosives. 2. Experimental section General Information. All reagents and solvents were obtained from commercial sources and dried by using standard procedures before use. Nitroaromatic explosives, including 2,4-dinitroanisole (DNAN), picric acid (PA), styphnic acid (SA), 1,3,5-triethoxy-2,4,6-trinitrobenzene (TETNB), 2,4-dinitrotoluene (DNT), 2,4,6-trinitrotoluene (TNT), 2,4,6-triamino-1,3,5-trinitrobenzene (TATB) were of analytical

∗ Corresponding author. I. Postovsky Institute of Organic Synthesis, Ural Branch of the Russian Academy of Sciences, S. Kovalevskaya Str., 22, Ekaterinburg, 620990, Russia. E-mail address: [email protected] (E.V. Verbitskiy).

https://doi.org/10.1016/j.dyepig.2019.04.073 Received 27 March 2019; Received in revised form 30 April 2019; Accepted 30 April 2019 Available online 01 May 2019 0143-7208/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Representative compounds bearing benzimidazole and (hetero)aryl fused imidazole units.

The study of photostability of the prepared dyes has been performed. Toluene solutions of the novel dyes were irradiated with UV light at wavelength 365 nm for 1 h. Initial optical density of all solutions at wavelength 365 nm was equal to the same value (0.102). Fluorescence spectra of the dyes were recorded with 10 min interval between exposures, and their intensity were fixed at the maximum of the band. Photolysis of solutions was carried out using “Newport” system on basis mercury lamp (200 W) with a set of interference filters. The intensity of the optical radiation determined by the optical power meter “Newport 2935” at 365 nm was equal to 3.2 × 1016 photon·s−1. The fluorescence quenching studies were carried out on a Hitachi F7000 fluorescence spectrophotometer at room temperature in chloroform. For each analyte, the typical test procedure was as follows: 2.5 mL of chloroform solution of one of the fluorophores (5.0 × 10−7 mol/L) was drawn and placed in a quartz cell of the standard size. Without analyte the fluorescence spectrum of pure fluorophore was first recorded. Subsequently, different amounts of analyte were added respectively in the cell. Each time after full mixing the analyte with fluophore, the fluorescence spectrum was registered. The plots of I0/I values of the quenching systems as functions of quencher concentrations ([Q]) were well described by the Stern–Volmer equation, I0/ I = 1 + Ksv[Q], where I0 and I are fluorescence intensities without and in the presence of analyte. The detection limits were calculated by use literature procedure (see details in Supporting Information). To investigate detection of nitroaromatic explosives in vapor phase the original device « Zaslon-M» (see Fig. S1 in Supporting Information) has been used. This device was produced by company « EnergoSpetsKomplektServis» (Moscow region, Mytischi, Russia). The instrument is based on registration of excited steady-state luminescence quenching. Sensors for this device were obtained by application of a dye solution on the cellulose matrix. Reduction of the luminescence intensity of the sensor takes place due to an interaction with explosive vapors contained in the air. The calibration of the device « Zaslon-M» was carried out by using the internal software. The fluorescence measurements (excited at 375 nm) for nitroaromatic vapors were performed with the device « Zaslon-M » connected with a computer. The sensor for detection of NACs in vapor phase has been designed as cartridge [13]. The cartridge has two parts that connected at the contour of the framework. In previous investigation, it has been shown that non-woven spunlace fabric (70% viscose, 30% polyester, Industrial

grade and used directly without further purification. (Caution: All nitrocontaining compounds used in the present study are high explosives and should be handled only in small analytical quantities). 1 H and 13C NMR spectra were recorded on a Bruker DRX-400 and AVANCE-500 instruments using Me4Si as an internal standard. Elemental analysis was carried on a Eurovector EA 3000 automated analyzer. High resolution mass spectrometry was performed using a Bruker maXis Impact HD spectrometer. Melting points were determined on Boetius combined heating stages and were not corrected. Flash-column chromatography was carried out using Alfa Aesar silica gel 0.040–0.063 mm (230–400 mesh), eluting with chloroform. The progress of reactions and the purity of compounds were checked by TLC on Sorbfil plates (Russia), in which the spots were visualized with UV light (λ 254 or 365 nm). UV/vis spectra were recorded for a 2 × 10−5 M solutions with Varian Cary 100 spectrophotometer. Photoluminescent spectra were recorded for a (1.0–3.0) × 10−6 M solutions on a Varian Cary Eclipse fluorescence spectrophotometer. UV/vis and fluorescence spectra were recorded using standard 1 cm quartz cells at room temperature. The ФF values were calculated using the established procedure with quinine sulfate in 0.05 M H2SO4 [18]. Stokes shifts were calculated considering the lowest energetic absorption band. IR spectra of samples (solid powders) were recorded on a Spectrum One Fourier transform IR spectrometer (PerkinElmer) equipped with a diffuse reflectance attachment (DRA) in the frequency range 4000 ÷ 400 cm−1. Spectrum processing and band intensity determination were carried out using the special software supplied with the spectrometer. The emission lifetimes have been measured using a time-correlated single-photon-counting picosecond spectrophotometer (FluoTime 200, PicoQuant). The sample has been excited by a 40 ps pulsed laser centered at 375 nm, and the emission signal has been collected at the magic angle. The instrument response function (IRF) has been recorded under described conditions by replacing the sample with a Ludox solution. The time decay data have been analyzed by nonlinear least-squares fitting with deconvolution of the IRF using the FluoFit software package [19]. The absorption and emission spectra for solid state and time-resolution study were recorded on Horiba FluoroMax-Plus Spectrofluorometer and Fluorolog-3 (Kyoto, Japan).

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Scheme 1. Synthetic route to 6-(het)aryl-5-phenyl-5H-imidazo-[4,5-b]-[1,2,5]oxadiazolo[3,4-e]pyrazines (3a-с).

19.17. Found: C, 73.98; H, 3.53; N, 19.13. HRMS (APCI): m/z calcd for C27H15N6O: 439.1302 [M+H]+; found: 439.1296. ν (DRA, cm−1) 3061, 3018, 1614, 1597, 1575, 1560, 1539, 1515, 1503, 1458, 1437, 1417, 1392, 1385, 1334, 1317, 1289, 1238, 1217, 1197, 1171, 1146, 1097, 1060, 1024, 974, 963, 886, 856, 845,832, 821, 808, 771, 765, 754, 733, 723, 712, 695, 984, 675, 646, 624, 586, 494. 6-[9-(2-Ethylhexyl)-9H-carbazol-3-yl]-5-phenyl-5H-imidazo [4,5-b][1,2,5]oxadiazolo[3,4-e]pyrazine (3c). Yield 168 mg (65%), orange powder, mp 292–294°С. 1H NMR (400 MHz, DMSO‑d6) δ 8.62 (s, 1H), 8.00 (d, J = 7.7 Hz, 1H), 7.85 (d, J = 8.9 Hz, 1H), 7.73–7.65 (m, 7H), 7.55 (t, J = 7.6 Hz, 1H), 7.31 (t, J = 7.4 Hz, 1H), 4.31 (d, J = 7.4 Hz, 2H), 2.01–1.95 (m, 1H), 1.36–1.14 (m, 8H), 0.85 (t, J = 7.3 Hz, 3H), 0.75 (t, J = 6.8 Hz, 3H). 13C NMR (101 MHz, DMSO‑d6) δ 171.0, 158.5, 153.4, 153.0, 151.6, 143.5, 141.1, 134.6, 130.2, 128.4, 128.1, 127.1, 124.2, 122.2, 121.8, 120.6, 120.3, 116.8, 110.6, 110.2, 46.9, 38.5, 30.0, 27.8, 23.5, 22.4, 13.7, 10.6. Calcd. for C31H29N7O (515.62): C, 72.21; H, 5.67; N, 19.02. Found: C, 72.13; H, 5.48; N, 18.91. HRMS (APCI): m/z calcd for C31H30N7O: 516.2506 [M +H]+; found: 516.2503. ν (DRA, cm−1) 3067, 2958, 2928, 2873, 2857, 1624, 1594, 1562, 1513, 1495, 1456, 1438, 1395, 1341, 1284, 1247, 1219, 1185, 1146, 1126, 1113, 1025, 948, 920, 897, 878, 858, 818, 768, 754, 727, 719, 694, 679, 640, 619.

Spunlace purchased from Afalina Co., Ltd., Russia) is the best porous substrate for fabrication of sensors [14]. The parts of cartridge made from Plexiglas were connected to each other along the contour by means of acrylonitrile glue. A piece of spunlace (50 mm in diameter) was immersed in the fluorophore solution with a concentration 1.0 × 10−3 M in chloroform for 5 min. Then, spunlace with immobilized fluorophore 3a (3b or 3c) was removed from the solution and dried at 70–80 °C for 30 min. To demonstrate its application as a fluorescence sensor for NAC detection, the obtained spunlace was placed in cartridge. To detect vapors of nitroaromatic compounds, such as TNT (400 mg), 2,4-DNT (1.0 g) and nitrobenzene (1 mL) in open glass tubes (150 mm in diameter), they were placed in a hermetic glovebox (0.8 m × 0.6 m × 0.4 m). The explosive was kept in this box during 48 h at room temperature until saturated vapor was formed. Similar protocol was used for interferents (15 mL), such as ammonia, ethanol, ethylene glycol, acetone, acetic acid, 1,2-dichlorobenzene, phenol and toluene. General procedure for the synthesis of 6-(het)aryl-5-phenyl-5Himidazo-[4,5-b][1,2,5]oxadiazolo[3,4-e]pyrazines (3a-c). To a stirred solution of 6-ethoxy-5-phenyl-6,7-dihydro-5H-imidazo[4,5-b] [1,2,5]oxadiazolo[3,4-e]pyrazine (1) (142 mg, 0.5 mmol) in CF3COOH (5 mL) was added anthracene (2a) (89 mg, 0.5 mmol) [pyrene (2b) (101 mg, 0.5 mmol) or N-(2-ethylhexyl)carbazole (2c) (73 mg, 0.5 mmol)]. The reaction mixture was stirred at 50 °C for 48 h. After that, solvent was distilled off in vacuo, and the residue was purified by flash column chromatography (CHCl3) to afford the desired products (3a, 3b and 3c). 6-(Anthracen-9-yl)-5-phenyl-5H-imidazo[4,5-b][1,2,5]oxadiazolo[3,4-e]pyrazine (3a). Yield 126 mg (61%), dark red powder, mp 302–304°С. 1H NMR (400 MHz, DMSO‑d6) δ 8.88 (s, 1H), 8.24–8.22 (m, 2H), 8.19–8.16 (m, 2H), 7.62–7.57 (m, 4H), 7.25–7.23 (m, 2H), 7.20–7.18 (m, 3H). 13C NMR (101 MHz, DMSO‑d6) δ 172.7, 159.3, 153.1, 152.1, 151.7, 132.4, 131.3, 130.0, 129.6, 129.4, 129.1, 128.7, 127.8, 126.5, 126.0, 125.1, 121.2. Calcd. for C25H14N6O (414.43): C, 72.46; H, 3.41; N, 20.28. Found: C, 72.36; H, 3.25; N, 20.33. HRMS (APCI): m/z calcd for C25H15N6O: 415.1302 [M+H]+; found: 415.1301. ν (DRA, cm−1) 3056, 1618, 1594, 1557, 1508, 1483, 1456, 1438, 1398, 1385, 1341, 1317, 1307, 1288, 1266, 1203, 1176, 1165, 1153, 1076, 1059, 1029, 1011, 953, 903, 883, 860, 835, 813, 785, 759, 742, 733, 721, 688, 677, 666, 640, 604, 581, 543, 496. 5-Phenyl-6-(pyren-1-yl)-5H-imidazo[4,5-b][1,2,5]oxadiazolo [3,4-e]pyrazine (3b). Yield 149 mg (68%), dark orange powder, mp 300–303°С. 1H NMR (500 MHz, DMSO‑d6) δ 8.69 (d, J = 9.2 Hz, 1H), 8.45 (t, J = 7.9 Hz, 2H), 8.41–8.32 (m, 3H), 8.25 (d, J = 9.0 Hz, 1H), 8.20 (t, J = 7.6 Hz, 1H), 8.14 (d, J = 8.0 Hz, 1H), 7.46–7.42 (m, 2H), 7.40–7.31 (m, 3H); 13C NMR (126 MHz, DMSO‑d6) δ 172.6, 158.9, 153.1, 151.9, 151.5, 133.3, 133.2, 130.5, 130.1, 130.0, 129.7, 129.3, 129.2, 127.9, 127.5, 127.10, 127.07, 126.9, 126.7, 124.6, 124.2, 123.5, 123.0, 121.4. Calcd. for C27H14N6O (438.45): C, 73.96; H, 3.22; N,

3. Results and discussion 3.1. Synthesis All compounds used in this study were obtained in good yields by the reaction of readily available 6-ethoxy-5-phenyl-6,7-dihydro-5Himidazo[4,5-b][1,2,5]oxadiazolo[3,4-e]pyrazine (1) [8] with equal amount of the corresponding electron-donating (hetero)arene [anthracene (2a), pyrene (2b), and 9-(2-ethylhexyl)-9H-carbazole (2c)] under oxidative conditions in CF3COOH at room temperature. The synthetic route is shown in Scheme 1. The structural evidence was accrued by 1H and 13C NMR, mass spectrometry, elemental analysis, and corresponded well with their expected structure. The corresponding 1H and 13C NMR spectra are shown in the supporting information. 3.2. Photophysical studies of the obtained fluorophores The UV–vis and photoluminescence (PL) spectroscopic data for compounds 3a-c have been measured in six aprotic solvents with different Dimroth-Reichardt polarity parameters [ET(30)] [20], such as cyclohexane (CyHex) (30.9), tetrachloromethane (TCM) (32.4), toluene (33.9), chloroform (39.1), dichloromethane (DCM) (40.7), acetone (42.2) and dimethyl sulfoxide (45.1) at room temperature. The photophysical parameters are summarized in Table 1. (Supplementary Material, Figs. S2–S7). The aim of this study was to explore the effects of solvent polarity on photophysical properties of novel imidazopyrazine250

251

c

b

a

DMSO 45.1

Acetone 42.2

DCM 40.7

Chloroform 39.1

Toluene 33.9

455 (6.0), 381 (9.3), 362 (10.8), 345 (7.7), 313sh (14.9), 301 (16.9), 259 (55.4) 460 (22.7), 390 (10.7), 340 (25.1), 327 (29.9), 314sh (23.7), 275 (27.4) 450 (43.8), 372sh (8.3), 336 (16.7), 281 (23.4) 457 (6.2), 382 (8.9), 364 (10.3), 346 (8.0), 298 (13.9) 459 (22.6), 391 (10.7), 340 (25.1), 326 (28.4) 449 (37.8), 377sh (8.4), 335 (15.8) 482 (5.4), 385 (10.0), 366 (12.3), 348 (9.08), 313sh (15.8), 301 (17.6), 256 (118.8) 478 (23.5), 392 (10.3), 342 (30.3), 328 (31.6), 276 (34.7), 265 (27.0), 244 (58.6) 467 (37.1), 381sh (7.4), 339 (16.6), 280 (24.4), 242 (27.1) 476 (5.9), 385 (11.0), 365 (13.5), 348 (10.1), 312sh (17.9), 301 (20.2), 255 (128.9) 473 (22.8), 392 (10.8), 341 (30.9), 326 (32.3), 275 (31.7), 265 (24.8), 242 (64.2) 466 (36.7), 381sh (7.4), 339 (16.4), 279 (24.1), 238 (31.0) 442 (4.7), 380 (10.4), 361 (11.8), 348 (9.2) 443 (20.5), 386 (13.9), 337 (33.5), 326 (31.1) 448 (39.1), 371sh (9.7), 334 (18.8) 446 (4.0), 385 (11.0), 366 (12.6), 348sh (9.7), 293 (19.2) 450 (15.7), 389 (10.9), 341 (32.5), 326 (32.0), 276 (36.9) 457 (32.1), 381sh (8.9), 337 (16.6), 294 (18.5), 278 (24.2)

444 (9.4), 378 (13.7), 359 (15.1), 343 (10.7), 312 (21.7), 300 (24.6), 253 (131.9)

Absorption λmax (nm)/ε (103 M−1 сm−1)

Dimroth-Reichardt polarity parameter, kcal mol−1 [20]. Fluorescence quantum yield was determined relative to quinine bisulfate in 0.05 M H2SO4 as standard (ФF = 0.52); excitation at 365 nm [18]. No signals were detected.

3c 3a 3b 3c 3a 3b 3c 3a 3b 3c 3a 3b 3c 3a 3b 3c 3a 3b 3c

Cyclohexane 30.9

3a 3b

457 (22.2), 386 (11.7), 338 (23.1), 325 (29.5), 273 (21.1), 262 (23.0), 242 (54.0), 237 (51.2) 443 (30.0), 420 (19.8), 371sh (14.4), 333 (16.8), 291sh (18.1), 271 (24.2), 234 (33.3) TCM 32.4

Solvent ET(30)a (kcal mol−1)

Dye

Table 1 UV/vis and Photoluminescence (Pl) data.

ФFb

0.15 0.23 0.08 0.08 0.18 0.28 0.04 0.13 0.23 0.01 0.06 0.08 0.002 0.02 0.03 0.0002 0.001 0.002 – – –

Emission λmax (nm) 499, 530sh 486, 516sh 455, 480sh 524 511 477 564 544 505 629 612 586 658 635 588 700 640 601 -c -c -c

Photoluminescence

595 2894 2170 1258 4151 3404 2470 4849 4581 4348 5811 5394 4452 8338 6948 5682 – – –

2482 1305

Stokes shift ΔνSt (cm−1)

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Fig. 2. Absorption and emission spectra of compounds 3a-c in chloroform.

based dyes 3a-c of the D–A type, bearing (hetero)aryl electron-donating groups in the imidazole ring, and to correlate these effects with their structures. UV–vis spectra of dyes 3a-c demonstrate long wavelength absorption maxima at 442–482 nm (3a), 457–478 nm (3b), 443–467 nm (3c) respectively, that can be attributed to intramolecular charge-transfer (ICT) excitation from (hetero)aryl electron-donating fragments to the imidazopyrazine ring system (acceptor) (Table 1). UV–vis spectra of 3ac in various solvents are shown in Figs. S2–S4. ICT bands of 3a-c are similar in shape and position, but differ significantly in intensity. As representative example, the spectra of dyes 3a-c in chloroform are shown in Fig. 2. There is a significant increase in intensity of the ICT band in series 6-anthracen-9-yl substituted imidazopyrazine 3a (ε = 4000-9400 M−1 сm−1), 6-pyrenyl substituted imidazopyrazine 3b (ε = 15700-30000 M−1 сm−1), and 6-carbazolyl substituted imidazopyrazine 3c (ε = 32100-43800 M−1 сm−1). A bathochromic shift of the charge transfer absorption band has been observed in the following order 3c to 3b and 3a. It has been found that polarity of solvents exerts a weak influence on the long-wavelength absorption maxima of compounds 3a-c. A small bathochromic shift of the charge transfer band has been observed with increase of solvent polarity in the following order: cyclohexane, tetrachloromethane, toluene, dichloromethane. Compounds 3a-c exhibit photoluminescent properties in aprotic solvents with the exception of DMSO. Solvent polarity can have a dramatic effect on emission spectra. An increase of solvent polarity leads to bathochromic shifts of emission maxima along with a consistent decrease of fluorescence intensity (Table 1, Fig. 3, Figs. S5–S7).

Fig. 4. Solutions photographs of dyes 3a-c in cyclohexane (1), tetrachloromethane (2), toluene (3), chloroform (4), dichloromethane (5), acetone (6) and DMSO (7) under UV light (Photoluminescence, λex = 365 nm) at room temperature.

Depending on the structure and solvent the emission bands are located in the range of 455–700 nm (Table 1, Fig. 3). The color shifts from deep blue in cyclohexane to red in DCM and acetone. The change in the emission color can be seen easily by naked eye, as shown in Fig. 4 for compounds 3a-c. As seen from the spectra of 3b, the emission wavelength maximum at λem = 486 nm observed in the nonpolar solvent (cyclohexane) is redshifted by about Δλem = 154 nm (Δνem = 4976 cm−1) when using acetone as a solvent (λem = 640 nm). The change in the emission color for compounds 3a, and 3c is 201 nm (Δνem = 5754 cm−1), and 146 nm (Δνem = 5378 cm−1), respectively. The excitation spectra of fluorescence are in good agreement with the absorption spectra for all dyes (Figs. S4–S6). The observed correlation of the emission band wavelength with the solvent-dependent ET(30) Dimroth-Reichardt polarity parameter (Fig. 3) is typical for compounds which undergo the intramolecular photoinduced electron transfer leading to a high polarity state which is stabilized by solvent [15,20,21]. Fluorescence quantum yields of compounds 3a-c are strongly dependent on their molecular structure and polarity of solvents (Table 1, Fig. 5). The quantum yields of the photoluminescence are reduced when increasing the polarity of the solvent. The fluorescence efficiency of 6-carbazolyl-substituted imidazopyrazine 3c (ФF = 0.002–0.28) is higher than those obtained for related compounds 6-aryl substituted imidazopyrazines 3a (ФF = 0.0002–0.15) and 3b (ФF = 0.001–0.23). For compounds 3a-c the highest fluorescence quantum yields have been found in nonpolar aprotic solvents, such as cyclohexane, tetrachloromethane and toluene. So, novel imidazopyrazine-based dyes of the D–A type, bearing (hetero)aryl electron-donating groups in the imidazole ring, exhibit a strong emission solvatochromism. By changing the solvent, the fluorescence maximum may be shifted from blue (green) to the red region and the quantum yield can be varied in the range of 0.0002–0.28. Following trends of the experimental emission data, the Stokes shifts for dyes 3a-c proved to increase from 595 to 2482 cm−1 in cyclohexane up to 5682–8338 cm−1 in acetone (Table 1, Fig. S7),

Fig. 3. Absorption and emission wavelength (λmax) as a function of the Dimroth-Reichardt polarity parameter for dyes 3a-c. 252

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fluorescent lifetimes in a molecule is realized. It should be noted that the excitation and emission spectra of the two fluorescent states of dyes 3a-c are highly overlapping and inseparable at room temperature, so that the transition between them can only be resolved by using fluorescence lifetime measurements [24]. The photostability of new fluorophores 3a-c has been studied (Figs. S9–S12). The dependence of the fluorescence intensity of the investigated dyes on irradiation time at steady-state excitation is shown in Fig. S12. Fluorescence intensities of the solutions of 3c under UV irradiation after 1 h and 2.5 h were found to be 21% and 40%, respectively. On the other hand, the fluorescence intensity of 3a and 3b under the same conditions decreased only up to 2%. Therefore, 6-arylsubstituted imidazopyrazines 3a and 3b exhibit a significantly higher resistance to photodegradation than their 6-carbazolyl-substituted analogues 3c. 3.3. Detection of nitroaromatic compounds in chloroform solution To evaluate a sensitivity of fluorophores 3b,c towards nitroaromatics and 2,3-dimethyl-2,3-dinitrobutane (DDBu), a common tag required by law for all commercial NATO plastic explosives, fluorescence quenching measurements in chloroform solutions containing measured quantities of different nitroaromatic compounds have been carried out using a well-known procedure [13–17] (Fig. 6). It should be noted that the fluorescence measurements of compound 3a in chloroform solution in the presence of nitro-explosives have not been carried out due to a relatively poor quantum yield 0.01 (Table 1). However, compound 3a has exhibited the best intensity of fluorescence in solid state in comparison with 3b,c (see Table S1, Figs. S13 and S14 in Supplementary Material) and it has also been used for detection of nitroaromatic vapors. Figs. S15 and S16 (see Supplementary Material) show the fluorescence emission spectra of 3b,c in the presence of various concentrations of analytes with 478 or 467 nm, as the excitation wavelength. All nitroaromatics, as well as DDBu, appear to act as fluorescence quenchers for compounds 3b,c (Table 3). Quenching efficiency was calculated by using the Stern-Volmer equation (I0 – I)/I0 = 1+ Ksv[Q], where I0 and I are the fluorescence intensities of fluorophores 3b and 3c before and after exposure to a particular analyte at the quencher concentration [Q] and Ksv SternVolmer's constant, respectively. Fig. 7 shows quenching efficiency of various nitroaromatics and DDBu towards fluorophores 3b,c. It can be seen that fluorophore 3c exhibits the highest values of quenching constants Ksv and detection limits (DL) (see Table 3), which are in a good agreement with increasing values of quantum yields from 3a to 3c (see Table 1). The linear relationship for the Stern-Volmer plot (in the range of concentrations from 0 to 4 × 10−6 M) (Figs. S17 and S18 in Supporting Information) and similarity of structures with linear push-pull pyrimidines, bearing pyrene and carbazole as electron-donating fragment, suggests a high role of static type interactions for the quenching process [13,16]. The mechanism of fluorescence quenching is explained with the frontier molecular orbital. Fluorophores 3a-c exhibit fluorescence quenching upon coordination of nitroaromatics, presumably due to a

Fig. 5. Fluorescence quantum yield as a function of the Dimroth-Reichardt polarity parameter for dyes 3a-c.

following rise of polarity of solvents. Anomalous Stokes shifts of fluorescence of 3а-с in polar solvents testify that relaxation processes take place in excited states. The latter is related to rotational isomerization of the molecules of the D-A type (the so-called structure relaxation) and relaxation of the solvent molecules (orientational relaxation of the solvent). These two types of relaxation are interdependent [22,23]. Following excitation there is an increase in charge separation within the molecule. If the solvent is polar, then species with charge separation (the ICT state) are transformed into the lowest energy state. The fluorescence decays of 3a-c have been studied in cyclohexane, tetrachloromethane, toluene, dichloromethane and acetone in order to examine effects of solvent polarity and structure of imidazopyrazinebased dyes on relaxation of photoinduced ICT state. The excitation wavelength (λexc) was 375 nm and the emission wavelengths (λem) proved to correspond to the maxima of the fluorescence bands of the dyes at 293 K (Table 1). Bi-exponential function has been employed to fit the fluorescence intensity decay of the compounds 3a-c in most solvents, with the exception for compound 3a in cyclohexane. The fitted parameters are listed in Table 2. All dyes 3a-c show similar photophysical behaviour in solutions of different polarities. The chromophores of 3a-c exhibits two fluorescent states with lifetimes of (τ1 = 1.08–3.77 ns) and (τ2 = 0.18–0.92 ns) (Table 2). The presence of two components in the description of fluorescence decay of compounds 3a-c indicates that there are fluorophores under two conformation states. The ratio of them is strongly dependent on the solvent polarity. It is important that in non-polar cyclohexane and TCM the short component (τ2) is minor. While in high polar solvents, such as DCM and acetone, the situation is inversed for the dyes 3a-c, because the long component (τ1) becomes minor. As the solvent polarity is increased, transition between states of different Table 2 The fluorescence decay lifetime (τ) of the dyes 3a-c in different aprotic solvents. Solvent

CyHex TCM Toluene DCM Acetone

3a

3b

3c

τ1 (ns)

α1 (%)

τ2 (ns)

α2 (%)

τ1 (ns)

α1 (%)

τ2 (ns)

α2 (%)

τ1 (ns)

α1 (%)

τ2 (ns)

α2 (%)

1.72 1.57 1.84 3.55 1.08

100 92.00 77.01 0.18 0.08

– 0.74 0.92 0.88 0.18

– 8.00 22.93 99.82 99.92

1.70 1.80 2.17 3.77 3.24

83.86 89.69 86.55 2.18 1.03

0.91 0.72 0.83 1.36 0.19

16.14 10.31 13.45 97.82 98.97

0.30 1.21 1.96 3.73 1.25

65.07 78.41 65.74 26.86 20.02

0.52 0.63 0.91 0.62 0.054

34.93 21.59 34.26 73.14 79.98

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Fig. 6. Structures of used quenchers.

lower than half studied NACs (NB, 1,3-DNB, 2-NP, 4-NP, 4-NT, 2,4-DNT and DNAN). These data indicate that photo-induced electron transfer is unlikely between both fluorophores 3a,b and quencher molecules and thus exhibit no fluorescence quenching. The energy gap between the LUMO of excited state of 3c and LUMO of the most NACs (except for NB, 2-NP, 4-NP, 4-NT and DNAN) may provide driving force for the electron transfer in the fluorescence quenching process (Fig. S19 in Supporting Information) [15]. Nonetheless, calculations cannot fully explain all experimental findings, due to the contribution of other factors.

Table 3 Quenching constants (KSV) and detection limits (DL) of NB, 1,3-DNB, 1,3,5TNB, 2-NP, 4-NP, 2,4-DNP, PA, SA, 4-NT, 2,4-DNT, TNT, DNAN, TNAN, TATB and DDBu towards fluorophores 3b and 3c in CHCl3. Nitro-compound

NB 1,3-DNB 1,3,5-TNB 2-NP 4-NP 2,4-DNP PA SA 4-NT 2,4-DNT TNT DNAN TNAN DDBu

Ksv, M−1/DL, mol × L−1 Fluorophore 3b

Fluorophore 3c

515/7.29 × 10−2 1126/9.19 × 10−2 375/1.03 × 10−1 526/1.33 × 10−1 518/1.36 × 10−1 1889/6.80 × 10−2 1625/4.63 × 10−2 296/1.29 × 10−1 2390/5.72 × 10−2 662/7.60 × 10−2 2555/4.29 × 10−2 759/6.92 × 10−2 1642/3.41 × 10−3 375/1.80 × 10−1

1182/7.64 × 10−3 5526/4.35 × 10−3 2661/2.74 × 10−3 3297/3.71 × 10−3 9591/2.74 × 10−3 5353/3.43 × 10−3 5565/3.62 × 10−3 2809/3.15 × 10−3 22625/3.32 × 10−3 2051/4.83 × 10−3 3340/2.53 × 10−3 2696/6.55 × 10−3 1672/2.95 × 10−3 9300/4.18 × 10−3

3.4. Application of sensors in detecting of model explosive vapors Nitroaromatic compounds, such as 2,4,6-trinitrotoluene (TNT), 2,4dinitrotoluene (DNT) and their derivatives, are well known explosives and environmental toxic pollutants. NACs are easily distributed in air, water and soil and can cause severe adverse effects on human health due to their high toxicity [26]. In addition, nitrobenzene is known to be a toxic, organic pollutant, which is frequently discharged into the environment by industrial production processes, and detection of this material is of great importance [27]. Moreover, many home-made explosive devices are fabricated by using impure TNT, in which the major impurity is 2,4-DNT, used as the starting material for the synthesis of TNT. Therefore, the fabrication of rapid and reliable sensors for detection of NB, DNT and TNT vapors is very important for public safety. We have fabricated sensors with each of fluorophores 3a-c based on non-woven spunlace fabric according to the description given in the Experimental Part. The sensors with each of the tested compounds 3a-c were placed and kept in the glove-box containing saturated NB (DNT or TNT) vapors during 30 min. After that, the fluorescence spectra of sensors were recorded (Fig. 8, S20 and S21 in Supporting Information). As can be seen, the significant fluorescence quenching is observed only for the sensors based on fluorophore 3a, that is in a good agreement with the most fluorescence intensity in the solid state (Fig. S14 in

photoinduced electron transfer mechanism (PET). To achieve the maximum efficiency of the PET process the fluorogenic fragment must be appended close to the electron-rich receptor. In this case, upon fluorophore excitation, the electron in the LUMO of the fluorophore (donor) can be transferred to the LUMO of the nitroaromatic analyte (acceptor). As a result, the excited state of the sensor does not relax with emission of light, that is, its fluorescence is quenched. Geometry optimization and energy calculation for 3a-c and thirteen nitroaromatic compounds were performed by using density-functional theory at the B3LYP/6-31G* level with the ORCA 4.0.3 program (Table S2 and Fig. S19 in Supporting Information) [25]. Despite of fluorophores 3a-c have similar values of the HOMO–LUMO gap of (2.74, 2.77 and 3.04 eV, respectively), the energies of LUMO of 3a and 3b are −3.21 eV that 254

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Fig. 7. The plot of quenching efficiencies of NB, 1,3-DNB, 1,3,5-TNB, 2-NP, 4-NP, 2,4-DNP, PA, SA, 4-NT, 2,4-DNT, TNT, DNAN, TNAN and DDBu relative to fluorophores 3b (a) and 3c (c) at mM level in CHCl3.

Fig. 8. Change of fluorescence spectra for the sensor based on compound 3a after exposure in saturated NB, DNT or TNT vapors during 30 min (excited at 372 nm).

Fig. 9. Fluorescent recovery cycles for sensor on the basis of compound 3a in device « Zaslon-M» with exposure to saturated NB, DNT and TNT vapors.

Supporting Information). Therefore, further investigation of fluorescence responses to saturated vapors of NB, DNT and TNT by using the portable sniffer « ZaslonM » have been performed only for the sensors based on fluorophore 3a (Fig. 9). It has been shown that the quenching efficiency of fluorophore 3a towards NB was higher than that to DNT and TNT at the same temperature, because the vapor pressure of NB was much higher than that of DNT and TNT under such conditions. To evaluate the selectivity of 3a, the fluorescence response towards vapors of other quenchers have been performed. Indeed, sensor 3a shows a good selectivity to NACs in comparison with other interferents, such as ammonia, ethanol, ethylene glycol, acetone, acetic acid, 1,2dichlrobenze, phenol and toluene (see Fig. 10), since their quenching sensitivities are weaker in these vapors that in NB. Notably, that concentration of vapor for interferent liquids is several times higher than that for solid NACs. All the above features make 3a a practical and reliable detection method for NACs in air samples. 4. Conclusion Fig. 10. Fluorescent quenching (%) on the basis of compound 3a in first cycle in device « Zaslon-M » towards various nitro-explosives and volatile interferents at room temperature.

We have described the synthesis and photophysical properties of novel imidazopyrazine-based dyes of the D–A type, bearing (hetero)aryl electron-donating groups in the imidazole ring. The compounds obtained demonstrate a strong influence of electronic nature of substituents on their fluorescence properties, thus showing that the 255

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fluorescence with quantum yields up to 0.28 can be reached in aprotic solvents. One of the main conclusions that can be drawn is that incorporation of (hetero)aryl substituent at С(6) position has a significant effect governing the optical properties; in particular, this fragment changes solvatochromic behaviour of imidazopyrazine derivatives. In addition, our studies have shown that this group enhances sensitivity of the fluorophores to polarity of solvents and to traces of NACs both in organic solutions and air; therefore, they are proposed for use as multifunctional chemosensors. Further studies on enhancement of optical properties of imidazopyrazines are currently in progress.

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Acknowledgments This work (synthetic part and sensory properties) was supported by the Russian Foundation for Basic Research (Research Project No. 17-0300011-A). VEV is grateful to the financial support for the synthetic part from the Ministry of Education and Science of the Russian Federation within the framework of the State Assignment for Research (project No. АААА-А19-119011790132-7). N.I. Makarova, E.V.V. and A.V.M. would like to acknowledge financial support for the absorption and fluorescence studies from the Ministry of Education and Science of the Russian Federation within the framework of the State Assignment for Research (project № 4.6759.2017/8.9). The authors are grateful to Grigory A. Kim for carrying out the DFT calculations which were performed by using « Uran » supercomputer of the Institute of mathematic and mechanics of the Ural Brach of the Russian Academy of Sciences. NMR experiments were carried out by using equipment of the Center for Joint Use « Spectroscopy and Analysis of Organic Compounds » at the Postovsky Institute of Organic Synthesis of the Ural Branch of the Russian Academy of Sciences. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.dyepig.2019.04.073. References [1] Horak E, Kassal P, Murković Steinberg I. Benzimidazole as a structural unit in fluorescent chemical sensors: the hidden properties of a multifunctional heterocyclic scaffold. Supramol Chem 2017:838–57https://doi.org/10.1080/10610278. 2017.1403607. [2] Shan T, Gao Z, Tang X, He X, Gao Y, Li J, Sun X, Liu Y, Liu H, Yang B, Lu P, Ma Y. Highly efficient and stable pure blue nondoped organic light-emitting diodes at high luminance based on phenanthroimidazole-pyrene derivative enabled by triplei-triplet annihilation. Dyes Pigments 2017;142:189–97https://doi.org/10.1016/j. dyepig.2017.03.032. [3] Tavgeniene D, Krucaite G, Baranauskyte U, Wu J-Z, Su H-Y, Huang C-W, Chang C-H, Grigalevicius S. Phenanthro[9,10-d]imidazole based new host materials for efficient red phosphorescent OLEDs. Dyes Pigments 2017;137:615–21https://doi.org/10. 1016/j.dyepig.2016.11.003. [4] Xiong J-F, Li J-X, Mo G-Z, Huo J-P, Liu J-Y, Chen X-Y, Wang Z-Y. Benzimidazole derivatives: selective fluorescent chemosensors for the picogram detection of picric acid. J Org Chem 2014;79:11619–30https://doi.org/10.1021/jo502281b. [5] Zhong K, Cai M, Hou S, Bian Y, Tang L. Simple benzimidazole based fluorescent sensor for ratiometric recognition of Zn2+ in water. Bull Korean Chem Soc 2014;35:489–93https://doi.org/10.5012/bkcs.2014.35.2.489. [6] Zhao D, Hu J, Wu N, Huang X, Qin X, Lan J, You J. Regiospecific synthesis of 1,2disubstituted (hetero)aryl fused imidazoles with tunable fluorescent emission. Org Lett 2011;13:6516–9https://doi.org/10.1021/ol202807d.

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