Bifunctional terpyr

Elsevier Editorial System(tm) for Journal of Molecular Structure Manuscript Draft Manuscript Number: Title: Bifunctional terpyridine/o-hydroxyimine chemosensors Article Type: Research Paper Keywords: Terpyridine; Fluorescence; ESIPT; Chemosensor; "Naked-eye". Corresponding Author: Professor Alexander D. Dubonosov, Doctor of chemical science Corresponding Author's Institution: Southern Scientific Center of Russian Academy of Science First Author: Alexander D. Dubonosov, Doctor of chemical science Order of Authors: Alexander D. Dubonosov, Doctor of chemical science; Evgenii N Shepelenko, Doctor; Vitaly A Podshibyakin , postgraduate; Yurii V Revinskii , Doctor; Karina S Tikhomirova , Doctor; Leonid D Popov , Doctor; Igor N Shcherbakov , Doctor; Vladimir A Bren , Professor; Vladimir Minkin , Professor

Highlights (for review)

Highlights -

Terpyridines containing substituted o-iminophenolic groups were synthesized.

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Spectral, luminescent and ESIPT properties were studied.

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The obtained compounds selectively bind Cu2+/Ni2+ and Fe2+ with corresponding "nakedeye" effects.

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Bifunctional terpyridine/o-hydroxyimine chemosensors Evgenii N. Shepelenko a,Vitaly A. Podshibyakin b,Yurii V. Revinskii a, Karina S. Tikhomirova b, Leonid D. Popov c, Alexander D. Dubonosov

a,

*, Igor N. Shcherbakov c, Vladimir A. Bren b,

Vladimir I. Minkin a, b a

Southern Scientific Center of Russian Academy of Sciences, Rostov on Don 344006, Russian

Federation b

Institute of Physical and Organic Chemistry, Southern Federal University, Rostov on Don

344090, Russian Federation с

Department of Chemistry, Southern Federal University, Rostov on Don 344090, Russian

Federation _____________________________________________________________________________ ABSTRACT _____________________________________________________________________________ Terpyridines containing substituted o-iminophenolic groups were synthesized. The obtained compounds possess two emission bands: with normal (at 480-495 nm) and anomalous Stokes shifts (at 509-569 nm) generated by the fast intramolecular O→N proton transfer in the singlet excited state (ESIPT effect). They represent bifunctional chromogenic "naked-eye" chemosensors for Cu2+ and Ni2+ due to their complexation with o-hydroxyimine fragment (change of the solution color from colorless to bright yellow) and for Fe2+ - due to complexation with the terpyridine moiety (colorless to violet). ___________________________________________________________________________ Keywords: Terpyridine; Fluorescence; ESIPT; Chemosensor; “Naked-eye” ________________________ * Corresponding author. E-mail address: [email protected] (A.D. Dubonosov).

1. Introduction Design of effective organic chemosensors for express monitoring of vital (or toxic to living organisms) cations and anions in soil, atmosphere, waters, food products and biological objects represent an essential scientific area in organic chemistry, closely related to environmental sciences, biology and medicine [1-6]. Fluorogenic sensors possessing high sensitivity and selectivity allow to perform in situ and in vivo studies [7-9]. Colorimetric (chromogenic) chemosensors are capable of “naked-eye” detecting ions without using of cumbersome and expensive equipment [10-12]. Currently, multi- and bifunctional sensors capable of independent detection of several cations (anions) and/or cations and anions due to specific spectral responses through the same or different channels are extensively studied [13-16]. Herein, we describe the synthesis and investigation of spectral, luminescent and chemosensor properties of novel bifunctional chemosensors - terpyridine derivatives of imines of substituted salicylic aldehydes. When designing of these molecules it was taken into account that o-hydroxyimine fragment is possible to detect d-metal cations [2], while polypyridines are especially sensitive for iron (II) and cobalt (II) cations [17, 18].

2. Experimental 2.1. General The 1H and

13

C NMR spectra in DMSO-d6 were recorded on a Bruker DPX-250 (250 MHz

for 1H, 62.9 MHz for 13C) spectrometer, the signals were referred with respect to the signals of residual protons of deutero-solvent (2.49 ppm), δ values were measured with precision 0.01 ppm. The IR spectra were recorded on a Varian Excalibur 3100 FT-IR instrument using the attenuated total internal reflection technique (ZnSe crystal). 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. Electronic emission spectra were recorded on a Varian Cary Eclipse spectrofluorimeter. Toluene and

acetonitrile of the spectroscopic grade and d-metal perchlorates (Aldrich) were used to prepare solutions. Melting points were determined on a PTP (M) instrument.

2.2. 4'-(4-Nitrophenyl)-2,2':6',2''-terpyridine (1) and 4-([2,2':6',2''-terpyridin]-4'-yl)aniline (2) were prepared according to the procedure [24].

2.3. General procedure for the synthesis of terpyridines (3a-d, 4). Corresponding salicylic aldehyde or 2-methoxybenzaldehyde (2.6 mmol) was added with stirring to a solution of 4-[2,2':6',2''-terpyridin]-4'-yl)aniline 2 (0.81 g, 2.5 mmol) in BuOH (15 mL). The reaction mixture was heated at reflux for 15 min, p-toluenesulfonic acid (10 mg, 0.06 mmol) was added and the mixture was refluxed for another 45 min. The solution was cooled, the precipitate filtered and recrystallized from BuOH or BuOH/DMF mixture (4:1). 2.4. (E)-2-(((4-([2,2':6',2"-Terpyridin]-4'-yl)phenyl)imino)methyl)-4-methoxyphenol (3a). Yield 78%, yellow solid, mp 115-116 °C. IR max (cm−1): 1614 (C=C), 1603 (C=N), 1586 (CC). 1H NMR δ (ppm): 3.75 (s, 3H, Me), 6.91-6.92 (m, 1Н, arom. H), 7.04-7.05 (m, 1Н, arom. H), 7.28 (s, 1H, arom. H), 7.51 (br s, 2H, arom. H), 7.56 (br s, 2H, arom. H), 8.00-8.01 (m, 4H, arom. H), 8.64-8.65 (m, 2H, arom. H), 8.71-8.75 (m, 4H, arom. H), 9.03 (s, 1H, CH=N), 12.32 (s, 1H, OH). 13C NMR δ (ppm): 163.26, 155.72, 155.72, 155.32, 154.94, 154.94, 154.40, 151.92, 149.28, 149.28, 148.62, 137.38, 137.38, 135.66, 128.01, 128.01, 124.45, 124.45, 122.29, 122.29, 120.89, 120.89, 120.89, 119.24, 117.62, 117.62, 117.50, 115.04, 55.55. EIMS, 70 eV, m/z: 459 [M]+. Anal. Calcd (%) for C29H22N4O2: C, 75.97; H, 4.84; N, 12.22. Found: C, 75.87; H, 4.70; N, 12.33. 2.5. (E)-2-(((4-([2,2':6',2''-Terpyridin]-4'-yl)phenyl)imino)methyl)-4-bromophenol (3b). Yield 87%, yellow solid, mp 125-126 °C. IR max (cm−1): 1614 (C=C), 1604 (C=N), 1588 (CC). 1H NMR δ (ppm): 6.95-6.98 (m, 1Н, arom. H), 7.50-7.62 (m, 5H, arom. H), 7.92-8.05 (m, 5H, arom. H), 8.65-8.77 (m, 6H, arom. H), 9.03 (s 1H, CH=N), 12.92 (s, 1H, OH). EIMS, 70 eV,

m/z: 507 [M]+. Anal. Calcd (%) for C28H19BrN4O: C, 66.28; H, 3.77; N, 11.04. Found: C, 66.39; H, 3.70; N, 11.12. 2.6. (E)-2-(((4-([2,2':6',2''-Terpyridin]-4'-yl)phenyl)imino)methyl)-4-nitrophenol (3c). Yield 97%, yellow solid, mp 131-133 °C. IR max (cm−1): 1614 (C=C), 1606 (C=N), 1592 (CC). 1H NMR δ (ppm): 6.75-6.78 (m, 1H, arom. H), 7.05-7.12 (m, 1H, arom. H), 7.48-7.75 (m, 4H, arom. H), 7.95-8.10 (m, 3H, arom. H), 8.25-8.45 (m, 4H, arom. H), 8.61-8.77 (m, 4H, arom. H), 9.27 (s, 1H, CH=N), 13.15 (s, 1H, OH). EIMS, 70 eV, m/z: 473 [M]+. Anal. Calcd (%) for C28H19N5O3: C, 71.03; H 4.04; N, 14.79. Found: C, 71.07; H 4.09; N, 14.62. 2.7. (E)-2-(((4-([2,2':6',2"-Terpyridin]-4'-yl)phenyl)imino)methyl)-5-methylphenol (3d). Yield 67%, yellow solid, mp 117-119 °C. IR max (cm−1): 1611 (C=C), 1603 (C=N), 1586 (CC). 1H NMR δ (ppm): 2.33 (s, 3H, Me), 6.81 (m, 2H, arom. H), 7.53-7.62 (m, 5H, arom. H), 8.02-8.04 (m, 4H, arom. H), 8.66-8.78 (m, 6H, arom. H), 9.02 (s, 1H, CH=N), 13.03 (s, 1H, OH). 13C NMR δ (ppm): 163.60, 160.42,155.75, 155.34, 154.96, 150.53, 150.44, 149.31, 149.19, 148.69, 144.31, 137.41,137.27, 135.56, 132.52, 128.02, 127.52, 124.48, 124.22, 122.30, 122.30, 120.92, 120.77, 120.34, 117.65, 116.99, 116.86, 116.11, 114.24. EIMS, 70 eV, m/z: 442 [M]+. Anal. Calcd (%) for C29H22N4O: C, 78.71; H, 5.01; N, 12.66. Found: C, 78.87; H, 4.90; N, 12.53. 2.8. (E-4-([2,2':6',2"-Terpyridin]-4'-yl)-N-(2-methoxybenzylidene) aniline (4). Yield 66%, yellow solid, mp 118-119 °C. IR max (cm−1): 1609 (C=C), 1601 (C=N), 1582 (CC). 1H NMR δ (ppm): 4.93 (s, 3H, Me), 7.26-7.38 (m, 5H, arom. H), 7.93-7.96 (m, 5H, arom. H), 8.68-8.77 (m, 8H, arom. H), 9.00 (s, 1H, CH=N). EIMS, 70 eV, m/z: 442 [M]+. Anal. Calcd (%) for C29H22N4O: C, 78.71; H, 5.01; N, 12.66. Found: C, 78.87; H, 4.90; N, 12.53.

3. Results and discussions As shown in Scheme 1, interaction of 1-(2-oxo-2-(pyridin-2-yl)ethyl)pyrydinium iodide with 3-(4-nitrophenyl)-1-(pyridin-2-yl)prop-2-en-1-one leads to the formation of nitro derivative 1,

which produces amine 2 upon reduction. Condensation of 2 with substituted salicylic aldehydes (SA) or 2-methoxybenzaldehyde in the presence of p-toluenesulfonic acid gives rise to terpyridines 3a-d and 4, respectively.

Scheme 1. Synthesis of terpyridines 3a-d and 4.

The structure of compounds 3a-d and 4 was confirmed by 1H, 13C NMR, IR, mass-spectra and elemental analysis. The IR spectra of 3a-d, 4 exhibit characteristic bands of the C = C and C = N bonds at 1609-1614 and 1601-1606 cm-1. In the downfield region of 1H NMR spectra of 3a-d at 12.32-13.15 ppm there are singlet signals of OH protons. Spectral absorption and spectral fluorescent properties of terpyridines 3a-d and 4 in solutions in isopropanol, acetonitrile and DMSO are summarized in Table 1.

Table 1 Spectral absorption and spectral fluorescent properties of 3a-d, 4. Absorption, max., nm (∙10-3)

Fluorescence, max., nm

Comp. 2-PrOH

CH3CN

ДМСО

2-PrOH

CH3CN

ДМСО

3a

377 (14.85)

373 (14.17)

377 (15.00)

489, 559

494, 563

495, 569

3b

358 (15.38)

355 (15. 61)

357 (15.70)

480, 551

480, 546

480, 544

3c

323 (20.23)

323 (19.28)

341 (21.04)

509

520

527

3d

348 (32.14)

354 (30.94)

355 (36.76)

480, 529

485, 519

480, 531

4

325 (28.69)

326 (26.32)

343 (31.14)

493

492

497

The electronic absorption spectra of maleimides 3a-d in solutions are characterized by the long wavelength bands in the range of 323-377 nm with the molar absorption coefficients (14.17-36.76).103 L mol–1 cm–1. The compounds 3a-d exhibit fluorescence at 480-559 nm, characterized by two emission bands (Table 1). The short wavelength emission peak at 480-495 nm with normal Stokes shift is well modeled by o-methoxy structure 4 in which the proton transfer is impossible (Fig. 1). The long wavelength band at 509-569 nm with anomalous Stokes shift value (ASS) is associated with ESIPT effect (Excited-State Intramolecular Proton Transfer) due to the fast intramolecular O→N proton transfer in the singlet excited state (Scheme 2) [19, 20]. The effect of dual fluorescence has previously been discussed in our studies on molecular switches with o-hydroxy imine component [see, e.g., 21, 22].

Scheme 2. ESIPT effect in 3a-d.

Fig. 1. Electronic absorption and fluorescence spectra of terpyridines 3a (1 and 1') and 4 (2 and 2') in acetonitrile (c = 2.4.10-5 M).

Chemosensing properties of terpyridines 3a-d and 4 in acetonitrile for cations were studied using absorption and fluorescence spectroscopy. The addition of d-metal cations Ni2+, Cd2+, Cu2+, Co2+, Zn2+, and Hg2+ to solutions of compound 4 possessing o-methoxy sustituent does not lead to appreciable changes in the absorption and emission spectra. Apparently, in this case the terpyridine fragment also does not exhibit chemosensor activity. In contrast, the addition of dmetal perchlorates to solutions of 3a-d leads to the appearance of new absorption maxima in the visible region at ~440 nm (Fig. 2).

Fig. 2. Changes in the absorption intensity of terpyridines 3a-d in acetonitrile at 440 nm after the addition of metal perchlorates (c3a-d == 2.10-5 M, cM = 5.10-5 M).

In the presence of Cu2+ and Ni2+ a visual distinct "naked eye" color change of acetonitrile solutions from almost colorless (pale yellow) to bright yellow is observed (Fig. 3a, b).

Fig. 3a. Electronic absorption spectra of terpyridine 3b in acetonitrile before (1) and after addition of Cu2+ (2) and Fe2+ (3) (c = 2.10-5 М).

Fig. 3b. Visual color change in acetonitrile with 3b and Cu2+.

Simultaneously the initial fluorescence of the same solutions is almost completely quenched, whereas with other ions the emission quenching is sufficiently specific (Fig. 4, 5). In ohydroxyimine structures the observed CHEQ effect (Chelation-Enhanced Fluorescence Quenching) is associated with complete or partial substitution of the OH protons by metal cations resulting in significant deactivation of ESIPT [2, 19, 20]. According to the data of the isomolar series method and spectrophotometric titration, the compounds 3a-d form 2:1 complexes with Cu2+ and Ni2+ (Scheme 3), and their detection limit for copper (II) is 2.6-3.1 µМ.

Fig. 4. Fluorescence spectra of 3a in acetonitrile before (1) and after addition of Co2+ (2), Cd2+ (3), Hg2+ (4), Zn2+ (5), Ni2+ (6), and Cu2+ (7) (c3a 2.10-5 M, cM 5.10-5 M).

Fig. 5. Fluorescence spectra of 3b in acetonitrile before (1) and after addition of Co2+ (2), Cd2+ (3), Hg2+ (4), Zn2+ (5), Ni2+ (6), and Cu2+ (7) (c3a 2.10-5 M, cM 5.10-5 M).

Selective chromogenic activity of terpyridines 3a-d for iron (II) cations is characterized by appearance of a new intense absorption band at 573 nm (Fig. 3, 6). In this case, a color of acetonitrile solutions changes from almost colorless (pale yellow) to violet.

Fig. 6. Visual color change in acetonitrile with 3b and Fe2+.

The binding of Fe2+ by terpyridine moiety is clearly confirmed by similar spectral transformations of model compound 4 with o-methoxy group. According to spectrophotometric titration data, the adding of equimolecular amounts of Fe(ClO4)2 to 3a-d in acetonitrile lead to formation of 1:1 complexes (Scheme 3), however, an excess of Fe2+ results in a more complicated equilibrium with the participation of 2:1 complexes (cf. with [23]); the detection limits are quite high - 1.5-1.8 µM. In all cases complete quenching of the initial fluorescence is occurred.

Scheme 3. A tentative scheme of sensing of Cu2+, Ni2+ and Fe2+ by the bifunctional chemosensors 3a-d.

4. Conclusions Thus, the synthesized terpyridines represent bifunctional chemosensors exhibiting chemosensing activity for both copper (II) and nickel (II) cations (due to the o-hydroxyimine fragment) and iron (II) cations (due to the terpyridine moiety).

Acknowledgments The work was supported by State Assignment in the field of scientific activity of Russian Federation (Initiative research - project No. 4.6497.2017/8.9; Leading researchers on an ongoing basis - Bren V.A., assignment No. 4.5593.2017/6.7).

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