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Fluorescence quantum yields and chromatic properties of poly(azomethine)s containing pyridine ring
Materials Science & Engineering B 252 (2020) 114483
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Materials Science & Engineering B journal homepage: www.elsevier.com/locate/mseb
Fluorescence quantum yields and chromatic properties of poly(azomethine)s containing pyridine ring Kevser Temizkan, İsmet Kaya
T
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Çanakkale Onsekiz Mart University, Department of Chemistry, Polymer Synthesis and Analysis Laboratory, 17020 Çanakkale, Turkey
A R T I C LE I N FO
A B S T R A C T
Keywords: Poly(azomethine)s Poly(pyridine)s Multichromatic emissions Thermal characteristics Fluorescence quantum yield
The poly(azomethine)s containing pyridine ring were synthesized in two stages and analyzed by electrochemical, optical and thermal analysis techniques. Pyridine with centric dialdehyde (DA-Py) and poly(azomethine-pyridine)s (PAZ-Py)s were synthesized by elimination and condensation reactions in organic solvent medium, respectively. The structures of the synthesized dialdehyde and poly(azomethine-pyridine)s were verified by nuclear magnetic resonance spectroscopy (NMR), Fourier-transform infrared spectroscopy (FT-IR) and ultraviolet–visible spectroscopy (UV–Vis) analysis was performed and they were also characterized with cyclic voltammetry (CV), photoluminescence (PL) analysis, thermal analysis (TG-DTA) and Differential Scanning Calorimetric (DSC) analysis. According to thermal analysis, the initial degradation temperature (Ton) and glass transition temperature (Tg) values of PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3 were found to be 126, 185, 139 °C and 102, 153 and 125 °C, respectively. According to fluorescence analysis, PAZ-Py-1 emitted four different colors of blue, green, orange and red emissions when excited at different wavelengths such as 340, 360, 380, 400, 420, 440 nm; 460, 480 nm; 500 nm; and 580 nm, respectively, in dimethylformamide (DMF) solutions. Also, two different colors were observed when excited at different wavelengths (blue emissions at 280, 300, 320, 340, 360, 380, 400, and 440 nm and orange emissions at 460, 480, 500, 520, 540, and 560 nm) in ethanol (EtOH) solutions. In addition, fluorescence quantum yields of PAZ-Py were found to be 9.0% and 11.0%; and 9.0% and 9.5% when excited at 440 and 460 nm in EtOH solution and when excited at 380 and 400 nm in DMF solutions, respectively.
1. Introduction Poly(azomethine)s/poly(imine)s are known as a part of the group of applicable polymeric materials. Their important roles are derived from including nitrogen heteroatom (–CH]N–) and π conjugations. These specific properties provide them with different optical, electrical and thermal characteristics [1–6]. Usually, these macromolecules are synthesized from condensation reactions of di-amine and di-aldehyde compounds and are abbreviated as PAZ or PI [4]. Also, poly(azomethine)s have good ability for filming, emitting light when excited at specific wavelengths/colorful disposition, high quantum yields, sensitivity to electronic and photonic impulses, supramolecular interactions (other specific compounds or polymeric chains), and biocompatibility [7–13]. With all these specific properties, they are useful polymers in light emitting/storage-diode technologies, imaging drug release-tissue engineering and for designing-durable space materials [14–17]. Poly (pyridine)s are derivative poly(azomethine)s including aromatic azomethines and have some similar properties. Hopefully polymers
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simultaneously containing aliphatic azomethine and aromatic azomethine can be developed with new properties and improved known abilities [18–20]. The pyridine compound is a heteroaromatic molecule with rigidity and polarizability. Among different heterocyclic rings, the advantage of using pyridine is based on its high thermal stability derived from its molecular symmetry and aromaticity [21]. Poly(azomethine)s containing a pyridine unit show improved solubility due to the increased dipole–dipole interaction of the polymer–solvent system. There is some interesting research about poly(azomethine)s containing pyridine [21]. We synthesized three new poly(azomethine-pyridine)s containing both azomethine bonding and pyridine units in this study. Initially we synthesized dialdehyde (DA-Py) via elimination and then we synthesized poly(azomethine-thiophene)s containing pyridine and azomethine units via condensation reactions. Then, the structural confirmation and characterization of all compounds were clarified with 1H and 13C NMR, FT-IR, UV–Vis analysis and TG, DTA, PL and CV measurements, respectively. Surface images, molecular weight distributions
Corresponding author. E-mail address: [email protected] (İ. Kaya).
https://doi.org/10.1016/j.mseb.2019.114483 Received 6 February 2018; Received in revised form 9 October 2019; Accepted 29 November 2019 0921-5107/ © 2019 Elsevier B.V. All rights reserved.
Materials Science & Engineering B 252 (2020) 114483
K. Temizkan and İ. Kaya
Scheme 1. Synthesis procedures for dialdehyde and poly(azomethine-pyridine)s.
azeotropic mixture in order to remove water) and p-toluene sulfonic acid (p-TSA) (4 mg) as catalyst were added to each reaction mixture and purged with a stream of argon (Scheme 1) as in the literature [22]. Reaction mixtures were stirred for 2 h at room temperature and at 60 °C for 7 h under argon atmosphere. After cooling at room temperature, PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3 were precipitated into cold methanol, washed with methanol at room temperature to remove unreacted monomers and dried at 40 °C for 24 h in a vacuum oven [23]. The yields of PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3 were 65, 60 and 70%, respectively. For PAZ-Py-1, 1H NMR (DMS-d6, δ, ppm): 7.84 (t, 1H, Ha), 5.37 (d, 2H, Hb), 7.00 (d, 2H, Hc), 7.75 (d, 2H, Hd), 5.69 (d, 2H,-He), 7.40 (t, 1H,-Hf), 8.23 (s, –CH]N), 4.93 (s, terminal NH2), 10.93 (s, 1H,terminal-CHO). 13C NMR (DMS-d6, δ, ppm): 141.54 (C1-H), 97.33 (C2-H), 154.54 (C3-ipso), 159.07 (C4-H), 136.06 (C5-ipso), 138.68 (C6H), 128.12 (C7-H), 152.01 (C8-ipso), 156.24 (C9-ipso), 95.62 (C10-H), 142.62 (C11-H) and 186.74 (terminal –CHO). FT-IR (cm−1): 3030 ν(C]CH), 1627 ν(–CH]N), 1608 ν(–C]N, ring), 1592, 1564, 1542, 1450 ν(C]C), 1300 ν(C–O), 784 ν(C–H). For PAZ-Py-2, 1H NMR (DMS-d6, δ, ppm): 7.56 (t, 1H, Ha), 6.75 (d, 2H, Hb), 6.85 (d, 2H, Hc), 7.70 (d, 2H, Hd), 6.60 (d, 2H,-He), 6.50 (d, 2H,-Hf), 8.43 (s, –CH]N), 4.80 (s, terminal NH2), 9.19 (s, 1H,terminalCHO). 13C NMR (DMS-d6, δ, ppm): 140.38 (C1-ipso), 122.15 (C2-H), 126.64 (C3-H), 140.16 (C4-ipso), 151.67 (C5-H), 139.93 (C6-ipso), 132.93 (C7-H), 127.99 (C8-H), 142.57 (C9-ipso), 164.29 (C10-ipso), 115.37 (C11-H), 147.76 (C12-H), and 190.00 (terminal –CHO). FT-IR (cm−1): 3030 ν(C]CH), 1641 ν(–CH]N), 1619 ν(–C]N, ring), 1562, 1542, 1495, 1412 ν(C]C), 1213, 1282 ν(C–O), 784 ν(C–H). For PAZ-Py-3, 1H NMR (DMS-d6, δ, ppm): 7.46 (t, 1H, Ha), 6.63 (d, 2H, Hb), 7.23 (d, 2H, Hc), 7.83 (d, 2H, Hd), 6.99 (d, 2H,-He), 6.55 (d, 2H,-Hf), 8.18 (s, –CH]N), 5.55 (s, terminal NH2), 9.36 (s, 1H,terminalCHO). 13C NMR (DMS-d6, δ, ppm): 140.48 (C1-ipso), 122.22 (C2-H), 126.79 (C3-H), 140.20 (C4-ipso), 151.67 (C5-H), 139.75 (C6-ipso), 132.93 (C7-H), 128.10 (C8-H), 142.59 (C9-ipso), 163.08 (C10-ipso), 115.44 (C11-H), 147.76 (C12-H), and 197.31 (terminal –CHO). FT-IR (cm−1): 3030 ν(C]CH), 1633 ν(–CH]N), 1617 ν(–CH]N, ring), 1592, 1545, 1490, 1406 ν(C]C), 1277 ν(C–O), 784 ν(C–H), 818 ν(C–S) (Scheme 2).
and glass transition temperatures of poly(azomethine-pyridine)s were determined by SEM, GPC and DSC measurements, respectively. Both photochromatic properties and quantum yields of poly(azomethinepyridine)s were determined from PL measurements in DMF and EtOH solvents at different wavelengths. 2. Experimental 2.1. Chemicals 4-Hydroxy benzaldehyde, 2,6-dibromopyridine,4,4′-thiodianiline, pyridine-2,6-diamine, and 4,4′-oxydianiline were purchased from Sigma Aldrich. Ethanol (EtOH), methanol (MeOH), acetonitrile, dimethylformamide (DMF), and tetrahydrofuran (THF) solvents were supplied from Merck Chem. Co. (Germany). 2.2. Synthesis of dialdehyde Dialdehyde containing a pyridine unit was synthesized according to the procedure presented in Scheme 1. 4-hydroxybenzaldehyde (0.02 mol, 2.44 g) was dissolved in 25 mL of THF in a 250 mL threenecked flask equipped with a condenser and magnetic stir bar. Anhydrous sodium carbonate (2.653 g, 0.025 mol) was added to the flask. 2,6-dibromopyridine (0.02 mol, 4.74 g) was dissolved in 25 mL THF and added to the reaction mixture under argon atmosphere. The mixture was heated for 7 h at 60 °C under continuous stirring. After cooling, the product was poured into 250 mL cold water and ice (approximately 0–5 °C). The precipitate was washed with 3 × 250 mL water to separate mineral salts [22,23]. Then, the dialdehyde compound was filtered, dried and finally recrystallized in methanol. The yield of the dialdehyde compound was found to be 75%. For DA-Py, 1H NMR (DMS-d6, δ, ppm): 9.76 (s, 2H, –CHO), 7.82 (t, 1H, Ha), 6.90 (d, 2H, Hb), 7.57 (d, 4H, Hc), 7.69 (d, 4H, Hd). 13C NMR (DMS-d6, δ, ppm): 142.62 (C1-H), 116.12 (C2-H), 172.90 (C3-ipso), 170.01 (C4-ipso), 128.12 (C5-H), 140.46 (C6-H), 132.65 (C7-ipso), 191.45 (C8-H). FT-IR (cm−1): 3030 ν(C]CH), 1675 ν (CH]O), 1625 ν(–C]N, ring), 1561, 1542, 1406, 1401 ν(C]C), 1320 ν(C–O), 781 ν(C–H). 2.3. Synthesis of poly(azomethine-pyridine)s
2.4. Characterization techniques
For the synthesis of poly(azomethine)s containing thiophene units, 0.02 mol DA-Py was dissolved in THF (30 mL) in three different bottom flasks and then 0.02 mol pyridine-2,6-diamine (10 mL THF:MeOH, 1:1, v/v) solute mixture; 0.02 mol 4,4′-thiodianiline (10 mL THF:MeOH, 1:1, v/v) solution mixture; or 0.02 mol 4,4′-oxydianiline (10 mL THF) solution were separately added to initialize the reaction in the bottom flasks. Reaction systems were contained in 250 mL three-necked round bottom flasks equipped with a reflux condenser, a gas inlet–outlet, a Dean-Stark trap and a magnetic stirrer. Also, 2 mL of toluene (as an
Spectral analyses of compounds were taken using a PerkinElmer FTIR Spectrum (650–4000 cm−1), AnalytikJena Specord 210 Plus double beam spectrophotometer (260–800 nm) and Agilent 600 MHz Premium COMPACT NMR Magnet by using (DMSO‑d6) and DMSO for NMR and UV–Vis measurements. Fluorescence analyses of compounds were taken with a Shimadzu RF-5301PC spectrofluorophotometer. Thermal analyses of compounds were examined using a PerkinElmer Diamond Thermal Analysis TG-DTA and PerkinElmer Pyris Sapphire differential scanning calorimetric (DSC) between 20 and 1000 °C (heating rate 2
Materials Science & Engineering B 252 (2020) 114483
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Scheme 2. Structures of dialdehyde and poly(azomethine-pyridine)s.
10 °C min−1) and 20–450 °C (heating rate 10 °C min−1) in nitrogen atmosphere (200 mL min−1), respectively. Weight-average molecular weight (Mw), number-average molecular weight (Mn), Z-average molecular weight (Mz), peak molecular weight (Mp) and polydispersity (PDI) values for poly(azomethine-pyridine)s were determined with a Malvern Viscotek GPC Dual 270 max system for Gel Permeation Chromatography-Light Scattering (GPC-LS) analysis. SEM images of poly(azomethine-pyridine)s were taken by a Jeol JSM-7100F Scotty instrument.
for stretching vibration bands of these groups [24,25]. The stretching vibration bands of terminal aldehyde (–CH]O) groups in PAZ-Py-1, PAZ-Py-2, and PAZ-Py-3 were observed at 1710, 1676, and 1679 cm−1, respectively. 1 H and 13C NMR spectra of PAZ-Py-3 were exemplary and are given in Fig. 2. In reference to DA-Py, 1H NMR, aromatic and aldehyde (–CHO) protons were observed at 7.82–6.90 and 9.76 ppm, respectively. In reference to 13C NMR, aromatic and aldehyde (–CHO) carbon signals and imine carbon signal of the pyridine ring were observed at 142.62–116.12 and 191.45 ppm and 172.90 ppm, respectively. According to 1H NMR measurements, the signals for azomethine (–CH] N–) terminal amine (–NH2) and terminal aldehyde (–CHO) protons of PAZ-Py-1, PAZ-Py-2, and PAZ-Py-3 were seen at 8.23, 8.43 and 8.18; 4.93, 4.80 and 5.55 ppm; and 10.93, 9.19 and 9.36 ppm, respectively. According to 13C NMR measurements, the carbon signals of azomethine (–CH]N–) bonding and imine of the pyridine ring and terminal aldehyde (–CHO) for PAZ-Py-1, PAZ-Py-2, and PAZ-Py-3 were seen at 159.07, 151.67 and 151.67; 156.24, 164.29 and 163.08; and 186.74, 190.00 and 197.31, respectively. There are similar results in the literature about these functional groups [25,26]. UV–Vis spectral analysis data and curves for the compounds are given in Table 1 and Fig. 3, respectively. Two electronic transition bands were illustrated on the UV–Vis spectrum of DA-Py. The first band was observed at 260 nm due to π → π* transition of benzene rings, the second band was observed at 288 nm due to π → π* transition of the pyridine ring, and the third band is at 480 nm due to n → π* transition of aldehyde oxygen atom. The UV–Vis spectrum of PAZ-Py-1
3. Results and discussion 3.1. Solubility and spectral analysis of synthesized compounds The synthesized poly(azomethine-pyridine)s were soluble in DMF, DMSO, THF, acetonitrile, methanol, and ethanol solvents but these polymers were insoluble in apolar solvents such as heptane, hexane, and toluene. These soluble properties make them applicable in several areas. Fourier transform infrared (FT-IR) spectra of compounds are shown in Fig. 1. According to FT-IR spectra of DA-Py, stretching vibration bands of aldehyde carbonyl (–CHO) and imine (–C]N–) of pyridine rings were observed at 1675 and 1625 cm−1, respectively. The stretching vibration bands of azomethine (–CH]N–) and (–C]N–) of pyridine rings in PAZ-Py-1, PAZ-Py-2, and PAZ-Py-3 were observed at 1627 and 1608 cm−1; 1641 and 1619 cm−1; and 1633 and 1617 cm−1, respectively, on FT-IR spectra. Similar results are given in the literature
Fig. 1. FT-IR spectra of DA-Py, PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3 (4000–650 cm−1). 3
Materials Science & Engineering B 252 (2020) 114483
K. Temizkan and İ. Kaya
Fig. 2. 1H and
13
C NMR spectra of PAZ-Py-2.
Table 1 UV–Vis analysis results of DA-Py, PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3. Compounds
λmax (nm)
λonset (nm)
Eg (eV)
DA-Py PAZ-Py-1 PAZ-Py-2 PAZ-Py-3
270,348 270,320,444 270,330,480 268,320
450 680 575 600
2.76 1.82 2.16 2.07
demonstrated four different electronic transition bands. The first band was observed at 260 nm due to π → π* transition of benzene rings, the second band was observed at 285 nm due to π → π* transition of pyridine rings, the third band was between 395 nm due to π → π* transition of azomethine bonds, and the fourth band was observed at 680 nm due to n → π* transition of azomethine bonds. UV–Vis spectrum of PAZ-Py-2 had four different electronic transition bands. The first band was observed at 260 nm due to π → π* transition of benzene rings, the second band was observed at 292 nm due to π → π* transition of pyridine rings, the third band was between 350 nm due to π → π* transition of azomethine bonds, and the fourth band was observed at 575 nm due to n → π* transition of azomethine bonds. The UV–Vis spectrum of PAZ-Py-3 contained three different electronic transition bands. The first band was observed at 260 nm due to π → π* transition of benzene rings, the second band was observed at 340 nm due to π →
Fig. 3. The UV–Vis spectra of DA-Py, PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3.
π* transition of pyridine ring and azomethine bonds, and the third band was observed at 600 nm due to n → π* transition of azomethine bonds. The optic band gaps (Eg) values of DA-Py, PAZ-Py-1, PAZ-Py-2, and PAZ-Py-3 were calculated to be 2.76, 1.82, 2.16, and 2.07 eV by using λonset values 450, 680, 575, and 600 nm for DA-Py, PAZ-Py-1, PAZ-Py-
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Materials Science & Engineering B 252 (2020) 114483
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3.2. Electrochemical properties Electrochemical properties of the synthesized DA-Py, PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3 were examined with cyclic voltammetric (CV) analysis and CV curves and data are illustrated in Fig. 4 and Table 2, respectively. HOMO, LUMO and E′g values of DA-Py, PAZ-Py-1, PAZ-Py2, and PAZ-Py-3 were found to be −6.24, −5.97, −5.78, and −5.99 V; −2.78, −2.99, −3.28, -and 2.80 V; and 3.45, 2.97, 2.50, and 3.19 eV, respectively. HOMO, LUMO and E′g values of compounds were calculated using equations 1–3 [26,29].
Table 2 Cyclic voltammetric analysis results of DA-Py, PAZ-Py-1, PAZ-Py-2 and PAZ-Py3. Eox (V)
Ered. (V)
a
HOMO (eV)
b
DA-Py PAZ-Py-1 PAZ-Py-2 PAZ-Py-3
1.850 1.580 1.395 1.604
−1.602 −1.394 −1.109 −1.588
−6.24 −5.97 −5.78 −5.99
−2.78 −2.99 −3.28 −2.80
a b c
LUMO (eV)
(1)
ELUMO = −(4.39 + ERed)
(2)
E'g = ELUMO − EHOMO
(3)
The molecular weight distributions of poly(azomethine-pyridine)s were calculated using a Gel Permeation Chromatography-Light Scattering (GPC-LS) system. The weight-average molecular weight (Mw), number-average molecular weight (Mn), Z-average molecular weight (Mz), peak molecular weight (Mp) and polydispersity (PDI) values of (PAZ-Py-1, PAZ-Py-2, PAZ-Py-3) poly(azomethine-pyridine)s were calculated to be 9650, 9100, 10250, 7700 Da and 1.06; 10650, 10000, 11300, 8500 Da and 1.06; and 9800, 9400, 10200, 8800 and 1.04, respectively.
Fig. 4. CV cycles of DA-Py, PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3.
Compounds
EHOMO = −(4.39 + EOx )
c E′g (eV)
3.3. Fluorescence properties and quantum yields of obtained compounds
3.45 2.97 2.50 3.19
Emission properties with different wavelengths and chromatic variations of the synthesized compounds were clarified with fluorescence (PL) spectroscopy and spectra, emissions and details are given in Figs. 5 and 6. Also, quantum yields of compounds in DMF and EtOH solutions excited at different wavelengths were calculated to use as references in the literature [30,31]. The fluorescence quantum yields of poly(azomethine-pyridine)s were calculated by using fluorescein standard (0.1 M NaOH solution). The measurements of sample fluorescence were taken by putting one drop into solutions of DMF and EtOH, respectively. The excitation and emission slit width values for standard and sample solutions were calibrated as 3 and 5 nm. The obtained peak
Highest occupied molecular orbital. Lowest unoccupied molecular orbital. Electrochemical band gap.
2, and PAZ-Py-3, respectively. Similar results were calculated about π → π* and n → π* electronic transitions of poly (azomethine)s in the literature [27,28].
Fig. 5. PL spectra of DA-Py, PAZ-Py-1, PAZ-Py-2, PAZ-Py-3 in DMF, a- Blue emission spectra of DA-Py excited at 294, 300, 320, 340 and 360 nm (at slit 5) b- Blue emission spectra of PAZ-Py-2 excited at 280, 300, 320, 340, 360, 380, 400 nm (at slit 5) c- Blue emission spectra of PAZ-Py-3 excited at 340, 360, 380, 400, 420, 440 nm (at slit 5) d- Blue emission spectra excited at 340, 360, 380, 400, 420, 440 nm (at slit 3), green emission spectra excited at 460, 480 nm (at slit 5), orange emission spectra excited at 500 nm (at slit 5), red emission spectra (excited at 580 nm (at slit 3) of PAZ-Py-1compound (concentration: 0.025 mg mL−1 and under sunlight solution). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 5
Materials Science & Engineering B 252 (2020) 114483
K. Temizkan and İ. Kaya
Fig. 6. PL spectra of DA-Py, PAZ-Py-1, PAZ-Py-2, PAZ-Py-3 in EtOH, a- Blue emission spectra of PAZ-Py-1 excited at 280, 300, 320, 340, 360, 380, 400, 420 nm and orange emission spectra of PAZ-Py-1 excited at 460, 480, 500, 520, 540, 560 nm (at slit 3) b- Blue emission spectra of PAZ-Py-3 excited at 340, 360, 380, 400, 420, 440 nm (at slit 5) c- Blue emission spectra of PAZ-Py-2 excited at 280, 300, 320, 340, 360, 380, 400, 420, 440 nm (at slit 5) d- Emission spectra of DA-Py at excited 293, 300, 320 nm (at slit 5) (concentration: 0.025 mg mL−1). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
2.4%, 2.0%, 2.0%, 4.2%, 5.0% and 4.0%, respectively, when excited at 280, 300, 320, and 340 (for slit 5); and 360, 380, and 400 (for slit 3) in DMF solutions. According to Fig. 5c, blue emissions were observed at 340, 360, 380, 400, 420, and 440 nm in DMF solutions (0.025 mg mL−1) for PAZ-Py-3. Also, the quantum yields of PAZ-Py-3 were calculated to be 3.8%, 9.0%, 9.5%, 4.7%, 5.0%, and 2.3%, respectively, when excited at 340 (slit 5); and 360, 380, 400, 420, and 440 nm (for slit 3 nm). According to Fig. 5d, blue emissions was observed when excited at 340, 360, 380, 400, 420, ad 440 nm in DMF solutions (0.025 mg mL−1) for PAZ-Py-1. Green emissions were observed when excited at 460 and 480 nm in DMF solutions for PAZ-Py-1. Orange emission was observed when excited at 500 nm in DMF solutions (0.025 mg mL−1) for PAZ-Py-1. Red emission was observed when excited at 580 nm in DMF solutions (0.025 mg mL−1) for PAZ-Py-1. The quantum yields of PAZ-Py-1 were calculated as 7.0%, 9.0%, 15%, 6.4%, 5.3%, 5.8%, 3.9%, 7.4% and 6.8%, respectively, when excited at 340, 360, 380, 400, 420, 440, 460, 480, 500, and 580 nm (for slit 3 nm), respectively [34,35]. According to Fig. 6a, blue emissions in EtOH solutions for PAZ-Py-1 were observed at 280, 300, 320, 340, 360, 380, 400, and 440 nm (0.025 mg mL−1). Quantum yields of PAZ-Py-1 were calculated as 6.0%, 7.2%, 7.0%, 2.8%, 9.0%, 6.0%, 5.0% and 9.0%, respectively, when excited at 280, 300, 320, 340, 360, 380, 400, and 440 nm (for slit
areas were calculated from spectra taken after the measurement induced by the same excitation values. In the same exothermic substance, the absorbance values were observed in the UV–vis spectrum. The quantum yields of sample solutions were calculated by the following equation.
Q MX = QYS × [Ax/As] × [fs/fx] × [η x/ηs]
(4)
QMX is the yield of matter whose quantum yield will be calculated, QYS is fluorescein standard yield (0.79), AX is area of the matter whose quantum yield will be calculated, AS is fluorescein standard area (22.362), fs is (1 − 10−D) and D is the absorbance value of the standard measured in UV–Vis 0.1506), fx is (1 − 10−D) and D is absorbance value of the measured substance in UV–Vis, ηx is the refractive index values of the matter solvents of DMF and ethanol (1.43 and 1.36, respectively), and ηs is the refractive index of the fluorescein standard solvent [32,33]. On Fig. 5a, blue emissions were observed at 294, 300, 320, 340, and 360 nm in DMF solutions with DA-Py (0.025 mg mL−1). Also, quantum yields of DA-Py were calculated as 2.6%, 3.0%, 2.0%, 1.6%, and 1.2%, respectively, when excited at 294, 300, 320, 340, and 360 nm (slit 5). According to Fig. 5b, blue emissions were observed at 280, 300, 320, 340, 360, 380, and 400 nm in DMF solutions (0.025 mg mL−1) for DAPy-2. Also, the quantum yields of PAZ-Py-2 were calculated as 1.0%, 6
Materials Science & Engineering B 252 (2020) 114483
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Fig. 7. TG-DTG-DTA curves of DA-Py, PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3.
Fig. 8. DSC curves of PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3.
and 480, 500, 520, 540, and 560 nm (for slit 5 nm). According to Fig. 6b, blue emissions in EtOH solutions for PAZ-Py-3 were observed at 340, 360, 380, 400, 420, and 440 nm (0.025 mg mL−1). Quantum yields of PAZ-Py-3 were calculated as 4.1%, 7.2%, 3.2%, 9%, 4.8% and 6.6%, respectively, when excited at 280 (for slit 5 nm), 300 (for slit 3 nm), 320 (for slit 5 nm), 340 (for slit 5 nm), 360 (for slit 5 nm), 380 (for slit 3 nm), 400 (for slit 5 nm), and 440 nm (for slit 3 nm). According to Fig. 6c, blue emissions in EtOH solutions for PAZ-Py-2 were observed at 280, 300, 320, 340, 360, 380, 400, 420, and 440 nm (0.025 mg mL−1). Quantum yields of PAZ-Py-2 were calculated as 1.5%, 6%, 7.5%, 1.8%, 4%, 3.2%, 3%, 5% and 5.4%, respectively, when excited at 280 (for slit 5 nm), 300 (for slit 3 nm), 320 (for slit 3 nm), 340 (for slit 5 nm), 360 (for slit 5 nm), 380 (for slit 5 nm), 400 (for slit 5 nm), 420 (for slit 5 nm), and 440 nm (for slit 5 nm). According to Fig. 6d, emission spectra in EtOH solutions for DA-Py were observed at 293, 300, and 320 nm (0.025 mg mL−1). Quantum yields of DA-Py were calculated as 2.0%, 1.8% and 1.0%, respectively, when excited at 293, 300, and 320 nm (for slit 5 nm), respectively. While the fluorescence quantum yields of poly(azomethine) containing thiophene and pyridine units were found to be between 1.6 and 8.0% for 280–470 nm in EtOH solution, the same values were between 4 and 18% for
Table 3 Thermal data of DA-Py, PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3. TG-DTG Compounds
a
DA-Py PAZ-Py-1 PAZ-Py-2
106 125 96
PAZ-Py-3
130
a b c d
Ton
b
DTA
DSC
c
d
% Char at 1000 °C
Endo (°C)
Endo (°C)
130 158 129, 240
110 134 108
125 158 128
2.60 8.25 2.25
118,138 163 186, 246
187, 269
185
278
19.20
197,271
– 105, 210 112, 172, 271 211, 265, 282
Tmax.
T20
T50
The onset temperature. Maximum weight temperature. 20% weight loss. 50% weight loss.
3 nm). Orange emissions in EtOH solutions for PAZ-Py-1 were observed at 460, 480, 500, 520, 540, and 560 nm (0.025 mg mL−1), while quantum yields for PAZ-Py-1 were calculated as 11.0%, 3.2%, 5.7%, 5.2% and 3.7%, respectively, when excited at 460 nm (for slit 3 nm);
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Fig. 9. SEM images of PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3.
3.5. Surface propertiesScheme 2
420–520 nm in DMF solution for this poly(azomethine) [36].
Images of the surface structure were obtained with SEM analysis techniques and photographs of PAZ-Py-1, PAZ-Py-2, and PAZ-Py-3 are given in Fig. 9. SEM images of PAZ-Py-1 appear to have bright and inhomogeneous form. Images of PAZ-Py-2 have fibrous and some sharp plates. Also, the surface image of PAZ-Py-3 was roughened and rigid form. There are similar surface morphologies in the literature [38].
3.4. Thermal properties Thermal characterization of the synthesized dialdehyde and PAZPy-1, PAZ-Py-2 and PAZ-Py-3 was completed with the thermogravimetry/differential thermal analysis (TG/DTA) and differential scanning calorimetry (DSC) techniques and thermal curves and data are shown in Figs. 7 and 8 and Table 3, respectively. The glass transition temperature (Tg) and ΔCp values onset temperatures for PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3 were found to be 85, 83 and 82 °C and 0.086, 0.016 and 0.019 J g−1 °C−1, respectively. Ton temperature values of DA-Py, PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3 were calculated to be 106, 125, 96 and 130 °C, respectively. While PAZ-Py-1 degraded in one step between 25 and 1000 °C, PAZ-Py-2 and PAZ-Py-3 degraded in two steps. The weight losses of PAZ-Py-2 were observed to be 65.12% and 32.60% between 50 and 150 and 150–1000 °C for first step and second step, respectively. The weight losses of PAZ-Py-3 were observed to be 34.72% and 46.08% between 50 and 237 and 237–1000 °C for the first step and second step, respectively. Thermal degradations of poly(azomethine)s occurred in a few steps, as in the literature [37]. Tonset and % char values for PAZ-Py-3 containing sulfur were higher than other poly (azomethine-pyridine)s. Both DTA and DSC curves for poly(azomethine-pyridine)s were observed to include a few endothermic degradation peaks.
4. Conclusion Poly(azomethine)s containing a pyridine unit were synthesized via condensation reaction and the structures were confirmed by 1H NMR, 13 C NMR, FT-IR and UV–Vis analyses. PAZ-Py-1, PAZ-Py-2, and PAZ-Py3 were characterized with CV, PL, TG/DTA, DSC, and SEM measurements. According to TG and DSC analyses, Ton, glass transition temperature (Tg) and % char values forPAZ-Py-1, PAZ-Py-2 and PAZ-Py-3 were 125, 96, and 130 °C; 85, 83, and 82 °C; and 8.25%, 2.25%, and 19.20%, respectively. Tonset and %char values for PAZ-Py-3 were higher than the other compounds. According to PL analyses, while PAZ-Py-1 showed multichromatic emissions, such as blue, green, orange and red in DMF solution, it had dichromatic emission, such as blue and orange in EtOH solution. Also, quantum yields of the compounds were calculated and examined with increases of about 20 nm at different excitation wavelengths. According to PL analysis of PAZ-Py-1, quantum yields 8
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were calculated as 7.0%, 9.0%, 15.0%, 6.4% and 5.3% for blue emissions when excited at 340, 360, 380, 400, 420 and 440, respectively. The quantum yields of PAZ-Py-1 were calculated as 5.8% and 3.9% for green emissions when excited at 460 and 480 nm, respectively. The quantum yield of PAZ-Py-1 was calculated as 7.4% for orange emission when excited at 500 nm. The quantum yield of PAZ-Py-1 was calculated as 6.8% for red emission when excited at 580 nm in DMF solvent. Also, the quantum yields of PAZ-Py-1 were calculated as 6%, 7.2%, 7%, 2.8%, 9%, 6%, 5% and 9% for blue emissions when excited at 280, 300, 320, 340, 360, 380, 400 and 440 nm, respectively, in EtOH solution. The quantum yields of PAZ-Py-1 were calculated as 11.0%, 3.2%, 5.7%, 5.2% and 3.7% for orange emissions when excited at 460, 480, 500, 520, 540 and 560 nm, respectively, in EtOH solutions. While DA-Py did not demonstrate any color emission in EtOH solution, PAZ-Py-1 and PAZ-Py-2 emitted only blue color in DMF and EtOH solutions.
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Fluorescence quantum yields and chromatic properties of poly(azomethine)s containing pyridine ring Kevser Temizkan, İsmet Kaya
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Çanakkale Onsekiz Mart University, Department of Chemistry, Polymer Synthesis and Analysis Laboratory, 17020 Çanakkale, Turkey
A R T I C LE I N FO
A B S T R A C T
Keywords: Poly(azomethine)s Poly(pyridine)s Multichromatic emissions Thermal characteristics Fluorescence quantum yield
The poly(azomethine)s containing pyridine ring were synthesized in two stages and analyzed by electrochemical, optical and thermal analysis techniques. Pyridine with centric dialdehyde (DA-Py) and poly(azomethine-pyridine)s (PAZ-Py)s were synthesized by elimination and condensation reactions in organic solvent medium, respectively. The structures of the synthesized dialdehyde and poly(azomethine-pyridine)s were verified by nuclear magnetic resonance spectroscopy (NMR), Fourier-transform infrared spectroscopy (FT-IR) and ultraviolet–visible spectroscopy (UV–Vis) analysis was performed and they were also characterized with cyclic voltammetry (CV), photoluminescence (PL) analysis, thermal analysis (TG-DTA) and Differential Scanning Calorimetric (DSC) analysis. According to thermal analysis, the initial degradation temperature (Ton) and glass transition temperature (Tg) values of PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3 were found to be 126, 185, 139 °C and 102, 153 and 125 °C, respectively. According to fluorescence analysis, PAZ-Py-1 emitted four different colors of blue, green, orange and red emissions when excited at different wavelengths such as 340, 360, 380, 400, 420, 440 nm; 460, 480 nm; 500 nm; and 580 nm, respectively, in dimethylformamide (DMF) solutions. Also, two different colors were observed when excited at different wavelengths (blue emissions at 280, 300, 320, 340, 360, 380, 400, and 440 nm and orange emissions at 460, 480, 500, 520, 540, and 560 nm) in ethanol (EtOH) solutions. In addition, fluorescence quantum yields of PAZ-Py were found to be 9.0% and 11.0%; and 9.0% and 9.5% when excited at 440 and 460 nm in EtOH solution and when excited at 380 and 400 nm in DMF solutions, respectively.
1. Introduction Poly(azomethine)s/poly(imine)s are known as a part of the group of applicable polymeric materials. Their important roles are derived from including nitrogen heteroatom (–CH]N–) and π conjugations. These specific properties provide them with different optical, electrical and thermal characteristics [1–6]. Usually, these macromolecules are synthesized from condensation reactions of di-amine and di-aldehyde compounds and are abbreviated as PAZ or PI [4]. Also, poly(azomethine)s have good ability for filming, emitting light when excited at specific wavelengths/colorful disposition, high quantum yields, sensitivity to electronic and photonic impulses, supramolecular interactions (other specific compounds or polymeric chains), and biocompatibility [7–13]. With all these specific properties, they are useful polymers in light emitting/storage-diode technologies, imaging drug release-tissue engineering and for designing-durable space materials [14–17]. Poly (pyridine)s are derivative poly(azomethine)s including aromatic azomethines and have some similar properties. Hopefully polymers
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simultaneously containing aliphatic azomethine and aromatic azomethine can be developed with new properties and improved known abilities [18–20]. The pyridine compound is a heteroaromatic molecule with rigidity and polarizability. Among different heterocyclic rings, the advantage of using pyridine is based on its high thermal stability derived from its molecular symmetry and aromaticity [21]. Poly(azomethine)s containing a pyridine unit show improved solubility due to the increased dipole–dipole interaction of the polymer–solvent system. There is some interesting research about poly(azomethine)s containing pyridine [21]. We synthesized three new poly(azomethine-pyridine)s containing both azomethine bonding and pyridine units in this study. Initially we synthesized dialdehyde (DA-Py) via elimination and then we synthesized poly(azomethine-thiophene)s containing pyridine and azomethine units via condensation reactions. Then, the structural confirmation and characterization of all compounds were clarified with 1H and 13C NMR, FT-IR, UV–Vis analysis and TG, DTA, PL and CV measurements, respectively. Surface images, molecular weight distributions
Corresponding author. E-mail address: [email protected] (İ. Kaya).
https://doi.org/10.1016/j.mseb.2019.114483 Received 6 February 2018; Received in revised form 9 October 2019; Accepted 29 November 2019 0921-5107/ © 2019 Elsevier B.V. All rights reserved.
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Scheme 1. Synthesis procedures for dialdehyde and poly(azomethine-pyridine)s.
azeotropic mixture in order to remove water) and p-toluene sulfonic acid (p-TSA) (4 mg) as catalyst were added to each reaction mixture and purged with a stream of argon (Scheme 1) as in the literature [22]. Reaction mixtures were stirred for 2 h at room temperature and at 60 °C for 7 h under argon atmosphere. After cooling at room temperature, PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3 were precipitated into cold methanol, washed with methanol at room temperature to remove unreacted monomers and dried at 40 °C for 24 h in a vacuum oven [23]. The yields of PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3 were 65, 60 and 70%, respectively. For PAZ-Py-1, 1H NMR (DMS-d6, δ, ppm): 7.84 (t, 1H, Ha), 5.37 (d, 2H, Hb), 7.00 (d, 2H, Hc), 7.75 (d, 2H, Hd), 5.69 (d, 2H,-He), 7.40 (t, 1H,-Hf), 8.23 (s, –CH]N), 4.93 (s, terminal NH2), 10.93 (s, 1H,terminal-CHO). 13C NMR (DMS-d6, δ, ppm): 141.54 (C1-H), 97.33 (C2-H), 154.54 (C3-ipso), 159.07 (C4-H), 136.06 (C5-ipso), 138.68 (C6H), 128.12 (C7-H), 152.01 (C8-ipso), 156.24 (C9-ipso), 95.62 (C10-H), 142.62 (C11-H) and 186.74 (terminal –CHO). FT-IR (cm−1): 3030 ν(C]CH), 1627 ν(–CH]N), 1608 ν(–C]N, ring), 1592, 1564, 1542, 1450 ν(C]C), 1300 ν(C–O), 784 ν(C–H). For PAZ-Py-2, 1H NMR (DMS-d6, δ, ppm): 7.56 (t, 1H, Ha), 6.75 (d, 2H, Hb), 6.85 (d, 2H, Hc), 7.70 (d, 2H, Hd), 6.60 (d, 2H,-He), 6.50 (d, 2H,-Hf), 8.43 (s, –CH]N), 4.80 (s, terminal NH2), 9.19 (s, 1H,terminalCHO). 13C NMR (DMS-d6, δ, ppm): 140.38 (C1-ipso), 122.15 (C2-H), 126.64 (C3-H), 140.16 (C4-ipso), 151.67 (C5-H), 139.93 (C6-ipso), 132.93 (C7-H), 127.99 (C8-H), 142.57 (C9-ipso), 164.29 (C10-ipso), 115.37 (C11-H), 147.76 (C12-H), and 190.00 (terminal –CHO). FT-IR (cm−1): 3030 ν(C]CH), 1641 ν(–CH]N), 1619 ν(–C]N, ring), 1562, 1542, 1495, 1412 ν(C]C), 1213, 1282 ν(C–O), 784 ν(C–H). For PAZ-Py-3, 1H NMR (DMS-d6, δ, ppm): 7.46 (t, 1H, Ha), 6.63 (d, 2H, Hb), 7.23 (d, 2H, Hc), 7.83 (d, 2H, Hd), 6.99 (d, 2H,-He), 6.55 (d, 2H,-Hf), 8.18 (s, –CH]N), 5.55 (s, terminal NH2), 9.36 (s, 1H,terminalCHO). 13C NMR (DMS-d6, δ, ppm): 140.48 (C1-ipso), 122.22 (C2-H), 126.79 (C3-H), 140.20 (C4-ipso), 151.67 (C5-H), 139.75 (C6-ipso), 132.93 (C7-H), 128.10 (C8-H), 142.59 (C9-ipso), 163.08 (C10-ipso), 115.44 (C11-H), 147.76 (C12-H), and 197.31 (terminal –CHO). FT-IR (cm−1): 3030 ν(C]CH), 1633 ν(–CH]N), 1617 ν(–CH]N, ring), 1592, 1545, 1490, 1406 ν(C]C), 1277 ν(C–O), 784 ν(C–H), 818 ν(C–S) (Scheme 2).
and glass transition temperatures of poly(azomethine-pyridine)s were determined by SEM, GPC and DSC measurements, respectively. Both photochromatic properties and quantum yields of poly(azomethinepyridine)s were determined from PL measurements in DMF and EtOH solvents at different wavelengths. 2. Experimental 2.1. Chemicals 4-Hydroxy benzaldehyde, 2,6-dibromopyridine,4,4′-thiodianiline, pyridine-2,6-diamine, and 4,4′-oxydianiline were purchased from Sigma Aldrich. Ethanol (EtOH), methanol (MeOH), acetonitrile, dimethylformamide (DMF), and tetrahydrofuran (THF) solvents were supplied from Merck Chem. Co. (Germany). 2.2. Synthesis of dialdehyde Dialdehyde containing a pyridine unit was synthesized according to the procedure presented in Scheme 1. 4-hydroxybenzaldehyde (0.02 mol, 2.44 g) was dissolved in 25 mL of THF in a 250 mL threenecked flask equipped with a condenser and magnetic stir bar. Anhydrous sodium carbonate (2.653 g, 0.025 mol) was added to the flask. 2,6-dibromopyridine (0.02 mol, 4.74 g) was dissolved in 25 mL THF and added to the reaction mixture under argon atmosphere. The mixture was heated for 7 h at 60 °C under continuous stirring. After cooling, the product was poured into 250 mL cold water and ice (approximately 0–5 °C). The precipitate was washed with 3 × 250 mL water to separate mineral salts [22,23]. Then, the dialdehyde compound was filtered, dried and finally recrystallized in methanol. The yield of the dialdehyde compound was found to be 75%. For DA-Py, 1H NMR (DMS-d6, δ, ppm): 9.76 (s, 2H, –CHO), 7.82 (t, 1H, Ha), 6.90 (d, 2H, Hb), 7.57 (d, 4H, Hc), 7.69 (d, 4H, Hd). 13C NMR (DMS-d6, δ, ppm): 142.62 (C1-H), 116.12 (C2-H), 172.90 (C3-ipso), 170.01 (C4-ipso), 128.12 (C5-H), 140.46 (C6-H), 132.65 (C7-ipso), 191.45 (C8-H). FT-IR (cm−1): 3030 ν(C]CH), 1675 ν (CH]O), 1625 ν(–C]N, ring), 1561, 1542, 1406, 1401 ν(C]C), 1320 ν(C–O), 781 ν(C–H). 2.3. Synthesis of poly(azomethine-pyridine)s
2.4. Characterization techniques
For the synthesis of poly(azomethine)s containing thiophene units, 0.02 mol DA-Py was dissolved in THF (30 mL) in three different bottom flasks and then 0.02 mol pyridine-2,6-diamine (10 mL THF:MeOH, 1:1, v/v) solute mixture; 0.02 mol 4,4′-thiodianiline (10 mL THF:MeOH, 1:1, v/v) solution mixture; or 0.02 mol 4,4′-oxydianiline (10 mL THF) solution were separately added to initialize the reaction in the bottom flasks. Reaction systems were contained in 250 mL three-necked round bottom flasks equipped with a reflux condenser, a gas inlet–outlet, a Dean-Stark trap and a magnetic stirrer. Also, 2 mL of toluene (as an
Spectral analyses of compounds were taken using a PerkinElmer FTIR Spectrum (650–4000 cm−1), AnalytikJena Specord 210 Plus double beam spectrophotometer (260–800 nm) and Agilent 600 MHz Premium COMPACT NMR Magnet by using (DMSO‑d6) and DMSO for NMR and UV–Vis measurements. Fluorescence analyses of compounds were taken with a Shimadzu RF-5301PC spectrofluorophotometer. Thermal analyses of compounds were examined using a PerkinElmer Diamond Thermal Analysis TG-DTA and PerkinElmer Pyris Sapphire differential scanning calorimetric (DSC) between 20 and 1000 °C (heating rate 2
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Scheme 2. Structures of dialdehyde and poly(azomethine-pyridine)s.
10 °C min−1) and 20–450 °C (heating rate 10 °C min−1) in nitrogen atmosphere (200 mL min−1), respectively. Weight-average molecular weight (Mw), number-average molecular weight (Mn), Z-average molecular weight (Mz), peak molecular weight (Mp) and polydispersity (PDI) values for poly(azomethine-pyridine)s were determined with a Malvern Viscotek GPC Dual 270 max system for Gel Permeation Chromatography-Light Scattering (GPC-LS) analysis. SEM images of poly(azomethine-pyridine)s were taken by a Jeol JSM-7100F Scotty instrument.
for stretching vibration bands of these groups [24,25]. The stretching vibration bands of terminal aldehyde (–CH]O) groups in PAZ-Py-1, PAZ-Py-2, and PAZ-Py-3 were observed at 1710, 1676, and 1679 cm−1, respectively. 1 H and 13C NMR spectra of PAZ-Py-3 were exemplary and are given in Fig. 2. In reference to DA-Py, 1H NMR, aromatic and aldehyde (–CHO) protons were observed at 7.82–6.90 and 9.76 ppm, respectively. In reference to 13C NMR, aromatic and aldehyde (–CHO) carbon signals and imine carbon signal of the pyridine ring were observed at 142.62–116.12 and 191.45 ppm and 172.90 ppm, respectively. According to 1H NMR measurements, the signals for azomethine (–CH] N–) terminal amine (–NH2) and terminal aldehyde (–CHO) protons of PAZ-Py-1, PAZ-Py-2, and PAZ-Py-3 were seen at 8.23, 8.43 and 8.18; 4.93, 4.80 and 5.55 ppm; and 10.93, 9.19 and 9.36 ppm, respectively. According to 13C NMR measurements, the carbon signals of azomethine (–CH]N–) bonding and imine of the pyridine ring and terminal aldehyde (–CHO) for PAZ-Py-1, PAZ-Py-2, and PAZ-Py-3 were seen at 159.07, 151.67 and 151.67; 156.24, 164.29 and 163.08; and 186.74, 190.00 and 197.31, respectively. There are similar results in the literature about these functional groups [25,26]. UV–Vis spectral analysis data and curves for the compounds are given in Table 1 and Fig. 3, respectively. Two electronic transition bands were illustrated on the UV–Vis spectrum of DA-Py. The first band was observed at 260 nm due to π → π* transition of benzene rings, the second band was observed at 288 nm due to π → π* transition of the pyridine ring, and the third band is at 480 nm due to n → π* transition of aldehyde oxygen atom. The UV–Vis spectrum of PAZ-Py-1
3. Results and discussion 3.1. Solubility and spectral analysis of synthesized compounds The synthesized poly(azomethine-pyridine)s were soluble in DMF, DMSO, THF, acetonitrile, methanol, and ethanol solvents but these polymers were insoluble in apolar solvents such as heptane, hexane, and toluene. These soluble properties make them applicable in several areas. Fourier transform infrared (FT-IR) spectra of compounds are shown in Fig. 1. According to FT-IR spectra of DA-Py, stretching vibration bands of aldehyde carbonyl (–CHO) and imine (–C]N–) of pyridine rings were observed at 1675 and 1625 cm−1, respectively. The stretching vibration bands of azomethine (–CH]N–) and (–C]N–) of pyridine rings in PAZ-Py-1, PAZ-Py-2, and PAZ-Py-3 were observed at 1627 and 1608 cm−1; 1641 and 1619 cm−1; and 1633 and 1617 cm−1, respectively, on FT-IR spectra. Similar results are given in the literature
Fig. 1. FT-IR spectra of DA-Py, PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3 (4000–650 cm−1). 3
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Fig. 2. 1H and
13
C NMR spectra of PAZ-Py-2.
Table 1 UV–Vis analysis results of DA-Py, PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3. Compounds
λmax (nm)
λonset (nm)
Eg (eV)
DA-Py PAZ-Py-1 PAZ-Py-2 PAZ-Py-3
270,348 270,320,444 270,330,480 268,320
450 680 575 600
2.76 1.82 2.16 2.07
demonstrated four different electronic transition bands. The first band was observed at 260 nm due to π → π* transition of benzene rings, the second band was observed at 285 nm due to π → π* transition of pyridine rings, the third band was between 395 nm due to π → π* transition of azomethine bonds, and the fourth band was observed at 680 nm due to n → π* transition of azomethine bonds. UV–Vis spectrum of PAZ-Py-2 had four different electronic transition bands. The first band was observed at 260 nm due to π → π* transition of benzene rings, the second band was observed at 292 nm due to π → π* transition of pyridine rings, the third band was between 350 nm due to π → π* transition of azomethine bonds, and the fourth band was observed at 575 nm due to n → π* transition of azomethine bonds. The UV–Vis spectrum of PAZ-Py-3 contained three different electronic transition bands. The first band was observed at 260 nm due to π → π* transition of benzene rings, the second band was observed at 340 nm due to π →
Fig. 3. The UV–Vis spectra of DA-Py, PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3.
π* transition of pyridine ring and azomethine bonds, and the third band was observed at 600 nm due to n → π* transition of azomethine bonds. The optic band gaps (Eg) values of DA-Py, PAZ-Py-1, PAZ-Py-2, and PAZ-Py-3 were calculated to be 2.76, 1.82, 2.16, and 2.07 eV by using λonset values 450, 680, 575, and 600 nm for DA-Py, PAZ-Py-1, PAZ-Py-
4
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3.2. Electrochemical properties Electrochemical properties of the synthesized DA-Py, PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3 were examined with cyclic voltammetric (CV) analysis and CV curves and data are illustrated in Fig. 4 and Table 2, respectively. HOMO, LUMO and E′g values of DA-Py, PAZ-Py-1, PAZ-Py2, and PAZ-Py-3 were found to be −6.24, −5.97, −5.78, and −5.99 V; −2.78, −2.99, −3.28, -and 2.80 V; and 3.45, 2.97, 2.50, and 3.19 eV, respectively. HOMO, LUMO and E′g values of compounds were calculated using equations 1–3 [26,29].
Table 2 Cyclic voltammetric analysis results of DA-Py, PAZ-Py-1, PAZ-Py-2 and PAZ-Py3. Eox (V)
Ered. (V)
a
HOMO (eV)
b
DA-Py PAZ-Py-1 PAZ-Py-2 PAZ-Py-3
1.850 1.580 1.395 1.604
−1.602 −1.394 −1.109 −1.588
−6.24 −5.97 −5.78 −5.99
−2.78 −2.99 −3.28 −2.80
a b c
LUMO (eV)
(1)
ELUMO = −(4.39 + ERed)
(2)
E'g = ELUMO − EHOMO
(3)
The molecular weight distributions of poly(azomethine-pyridine)s were calculated using a Gel Permeation Chromatography-Light Scattering (GPC-LS) system. The weight-average molecular weight (Mw), number-average molecular weight (Mn), Z-average molecular weight (Mz), peak molecular weight (Mp) and polydispersity (PDI) values of (PAZ-Py-1, PAZ-Py-2, PAZ-Py-3) poly(azomethine-pyridine)s were calculated to be 9650, 9100, 10250, 7700 Da and 1.06; 10650, 10000, 11300, 8500 Da and 1.06; and 9800, 9400, 10200, 8800 and 1.04, respectively.
Fig. 4. CV cycles of DA-Py, PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3.
Compounds
EHOMO = −(4.39 + EOx )
c E′g (eV)
3.3. Fluorescence properties and quantum yields of obtained compounds
3.45 2.97 2.50 3.19
Emission properties with different wavelengths and chromatic variations of the synthesized compounds were clarified with fluorescence (PL) spectroscopy and spectra, emissions and details are given in Figs. 5 and 6. Also, quantum yields of compounds in DMF and EtOH solutions excited at different wavelengths were calculated to use as references in the literature [30,31]. The fluorescence quantum yields of poly(azomethine-pyridine)s were calculated by using fluorescein standard (0.1 M NaOH solution). The measurements of sample fluorescence were taken by putting one drop into solutions of DMF and EtOH, respectively. The excitation and emission slit width values for standard and sample solutions were calibrated as 3 and 5 nm. The obtained peak
Highest occupied molecular orbital. Lowest unoccupied molecular orbital. Electrochemical band gap.
2, and PAZ-Py-3, respectively. Similar results were calculated about π → π* and n → π* electronic transitions of poly (azomethine)s in the literature [27,28].
Fig. 5. PL spectra of DA-Py, PAZ-Py-1, PAZ-Py-2, PAZ-Py-3 in DMF, a- Blue emission spectra of DA-Py excited at 294, 300, 320, 340 and 360 nm (at slit 5) b- Blue emission spectra of PAZ-Py-2 excited at 280, 300, 320, 340, 360, 380, 400 nm (at slit 5) c- Blue emission spectra of PAZ-Py-3 excited at 340, 360, 380, 400, 420, 440 nm (at slit 5) d- Blue emission spectra excited at 340, 360, 380, 400, 420, 440 nm (at slit 3), green emission spectra excited at 460, 480 nm (at slit 5), orange emission spectra excited at 500 nm (at slit 5), red emission spectra (excited at 580 nm (at slit 3) of PAZ-Py-1compound (concentration: 0.025 mg mL−1 and under sunlight solution). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 5
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Fig. 6. PL spectra of DA-Py, PAZ-Py-1, PAZ-Py-2, PAZ-Py-3 in EtOH, a- Blue emission spectra of PAZ-Py-1 excited at 280, 300, 320, 340, 360, 380, 400, 420 nm and orange emission spectra of PAZ-Py-1 excited at 460, 480, 500, 520, 540, 560 nm (at slit 3) b- Blue emission spectra of PAZ-Py-3 excited at 340, 360, 380, 400, 420, 440 nm (at slit 5) c- Blue emission spectra of PAZ-Py-2 excited at 280, 300, 320, 340, 360, 380, 400, 420, 440 nm (at slit 5) d- Emission spectra of DA-Py at excited 293, 300, 320 nm (at slit 5) (concentration: 0.025 mg mL−1). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
2.4%, 2.0%, 2.0%, 4.2%, 5.0% and 4.0%, respectively, when excited at 280, 300, 320, and 340 (for slit 5); and 360, 380, and 400 (for slit 3) in DMF solutions. According to Fig. 5c, blue emissions were observed at 340, 360, 380, 400, 420, and 440 nm in DMF solutions (0.025 mg mL−1) for PAZ-Py-3. Also, the quantum yields of PAZ-Py-3 were calculated to be 3.8%, 9.0%, 9.5%, 4.7%, 5.0%, and 2.3%, respectively, when excited at 340 (slit 5); and 360, 380, 400, 420, and 440 nm (for slit 3 nm). According to Fig. 5d, blue emissions was observed when excited at 340, 360, 380, 400, 420, ad 440 nm in DMF solutions (0.025 mg mL−1) for PAZ-Py-1. Green emissions were observed when excited at 460 and 480 nm in DMF solutions for PAZ-Py-1. Orange emission was observed when excited at 500 nm in DMF solutions (0.025 mg mL−1) for PAZ-Py-1. Red emission was observed when excited at 580 nm in DMF solutions (0.025 mg mL−1) for PAZ-Py-1. The quantum yields of PAZ-Py-1 were calculated as 7.0%, 9.0%, 15%, 6.4%, 5.3%, 5.8%, 3.9%, 7.4% and 6.8%, respectively, when excited at 340, 360, 380, 400, 420, 440, 460, 480, 500, and 580 nm (for slit 3 nm), respectively [34,35]. According to Fig. 6a, blue emissions in EtOH solutions for PAZ-Py-1 were observed at 280, 300, 320, 340, 360, 380, 400, and 440 nm (0.025 mg mL−1). Quantum yields of PAZ-Py-1 were calculated as 6.0%, 7.2%, 7.0%, 2.8%, 9.0%, 6.0%, 5.0% and 9.0%, respectively, when excited at 280, 300, 320, 340, 360, 380, 400, and 440 nm (for slit
areas were calculated from spectra taken after the measurement induced by the same excitation values. In the same exothermic substance, the absorbance values were observed in the UV–vis spectrum. The quantum yields of sample solutions were calculated by the following equation.
Q MX = QYS × [Ax/As] × [fs/fx] × [η x/ηs]
(4)
QMX is the yield of matter whose quantum yield will be calculated, QYS is fluorescein standard yield (0.79), AX is area of the matter whose quantum yield will be calculated, AS is fluorescein standard area (22.362), fs is (1 − 10−D) and D is the absorbance value of the standard measured in UV–Vis 0.1506), fx is (1 − 10−D) and D is absorbance value of the measured substance in UV–Vis, ηx is the refractive index values of the matter solvents of DMF and ethanol (1.43 and 1.36, respectively), and ηs is the refractive index of the fluorescein standard solvent [32,33]. On Fig. 5a, blue emissions were observed at 294, 300, 320, 340, and 360 nm in DMF solutions with DA-Py (0.025 mg mL−1). Also, quantum yields of DA-Py were calculated as 2.6%, 3.0%, 2.0%, 1.6%, and 1.2%, respectively, when excited at 294, 300, 320, 340, and 360 nm (slit 5). According to Fig. 5b, blue emissions were observed at 280, 300, 320, 340, 360, 380, and 400 nm in DMF solutions (0.025 mg mL−1) for DAPy-2. Also, the quantum yields of PAZ-Py-2 were calculated as 1.0%, 6
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Fig. 7. TG-DTG-DTA curves of DA-Py, PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3.
Fig. 8. DSC curves of PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3.
and 480, 500, 520, 540, and 560 nm (for slit 5 nm). According to Fig. 6b, blue emissions in EtOH solutions for PAZ-Py-3 were observed at 340, 360, 380, 400, 420, and 440 nm (0.025 mg mL−1). Quantum yields of PAZ-Py-3 were calculated as 4.1%, 7.2%, 3.2%, 9%, 4.8% and 6.6%, respectively, when excited at 280 (for slit 5 nm), 300 (for slit 3 nm), 320 (for slit 5 nm), 340 (for slit 5 nm), 360 (for slit 5 nm), 380 (for slit 3 nm), 400 (for slit 5 nm), and 440 nm (for slit 3 nm). According to Fig. 6c, blue emissions in EtOH solutions for PAZ-Py-2 were observed at 280, 300, 320, 340, 360, 380, 400, 420, and 440 nm (0.025 mg mL−1). Quantum yields of PAZ-Py-2 were calculated as 1.5%, 6%, 7.5%, 1.8%, 4%, 3.2%, 3%, 5% and 5.4%, respectively, when excited at 280 (for slit 5 nm), 300 (for slit 3 nm), 320 (for slit 3 nm), 340 (for slit 5 nm), 360 (for slit 5 nm), 380 (for slit 5 nm), 400 (for slit 5 nm), 420 (for slit 5 nm), and 440 nm (for slit 5 nm). According to Fig. 6d, emission spectra in EtOH solutions for DA-Py were observed at 293, 300, and 320 nm (0.025 mg mL−1). Quantum yields of DA-Py were calculated as 2.0%, 1.8% and 1.0%, respectively, when excited at 293, 300, and 320 nm (for slit 5 nm), respectively. While the fluorescence quantum yields of poly(azomethine) containing thiophene and pyridine units were found to be between 1.6 and 8.0% for 280–470 nm in EtOH solution, the same values were between 4 and 18% for
Table 3 Thermal data of DA-Py, PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3. TG-DTG Compounds
a
DA-Py PAZ-Py-1 PAZ-Py-2
106 125 96
PAZ-Py-3
130
a b c d
Ton
b
DTA
DSC
c
d
% Char at 1000 °C
Endo (°C)
Endo (°C)
130 158 129, 240
110 134 108
125 158 128
2.60 8.25 2.25
118,138 163 186, 246
187, 269
185
278
19.20
197,271
– 105, 210 112, 172, 271 211, 265, 282
Tmax.
T20
T50
The onset temperature. Maximum weight temperature. 20% weight loss. 50% weight loss.
3 nm). Orange emissions in EtOH solutions for PAZ-Py-1 were observed at 460, 480, 500, 520, 540, and 560 nm (0.025 mg mL−1), while quantum yields for PAZ-Py-1 were calculated as 11.0%, 3.2%, 5.7%, 5.2% and 3.7%, respectively, when excited at 460 nm (for slit 3 nm);
7
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Fig. 9. SEM images of PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3.
3.5. Surface propertiesScheme 2
420–520 nm in DMF solution for this poly(azomethine) [36].
Images of the surface structure were obtained with SEM analysis techniques and photographs of PAZ-Py-1, PAZ-Py-2, and PAZ-Py-3 are given in Fig. 9. SEM images of PAZ-Py-1 appear to have bright and inhomogeneous form. Images of PAZ-Py-2 have fibrous and some sharp plates. Also, the surface image of PAZ-Py-3 was roughened and rigid form. There are similar surface morphologies in the literature [38].
3.4. Thermal properties Thermal characterization of the synthesized dialdehyde and PAZPy-1, PAZ-Py-2 and PAZ-Py-3 was completed with the thermogravimetry/differential thermal analysis (TG/DTA) and differential scanning calorimetry (DSC) techniques and thermal curves and data are shown in Figs. 7 and 8 and Table 3, respectively. The glass transition temperature (Tg) and ΔCp values onset temperatures for PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3 were found to be 85, 83 and 82 °C and 0.086, 0.016 and 0.019 J g−1 °C−1, respectively. Ton temperature values of DA-Py, PAZ-Py-1, PAZ-Py-2 and PAZ-Py-3 were calculated to be 106, 125, 96 and 130 °C, respectively. While PAZ-Py-1 degraded in one step between 25 and 1000 °C, PAZ-Py-2 and PAZ-Py-3 degraded in two steps. The weight losses of PAZ-Py-2 were observed to be 65.12% and 32.60% between 50 and 150 and 150–1000 °C for first step and second step, respectively. The weight losses of PAZ-Py-3 were observed to be 34.72% and 46.08% between 50 and 237 and 237–1000 °C for the first step and second step, respectively. Thermal degradations of poly(azomethine)s occurred in a few steps, as in the literature [37]. Tonset and % char values for PAZ-Py-3 containing sulfur were higher than other poly (azomethine-pyridine)s. Both DTA and DSC curves for poly(azomethine-pyridine)s were observed to include a few endothermic degradation peaks.
4. Conclusion Poly(azomethine)s containing a pyridine unit were synthesized via condensation reaction and the structures were confirmed by 1H NMR, 13 C NMR, FT-IR and UV–Vis analyses. PAZ-Py-1, PAZ-Py-2, and PAZ-Py3 were characterized with CV, PL, TG/DTA, DSC, and SEM measurements. According to TG and DSC analyses, Ton, glass transition temperature (Tg) and % char values forPAZ-Py-1, PAZ-Py-2 and PAZ-Py-3 were 125, 96, and 130 °C; 85, 83, and 82 °C; and 8.25%, 2.25%, and 19.20%, respectively. Tonset and %char values for PAZ-Py-3 were higher than the other compounds. According to PL analyses, while PAZ-Py-1 showed multichromatic emissions, such as blue, green, orange and red in DMF solution, it had dichromatic emission, such as blue and orange in EtOH solution. Also, quantum yields of the compounds were calculated and examined with increases of about 20 nm at different excitation wavelengths. According to PL analysis of PAZ-Py-1, quantum yields 8
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were calculated as 7.0%, 9.0%, 15.0%, 6.4% and 5.3% for blue emissions when excited at 340, 360, 380, 400, 420 and 440, respectively. The quantum yields of PAZ-Py-1 were calculated as 5.8% and 3.9% for green emissions when excited at 460 and 480 nm, respectively. The quantum yield of PAZ-Py-1 was calculated as 7.4% for orange emission when excited at 500 nm. The quantum yield of PAZ-Py-1 was calculated as 6.8% for red emission when excited at 580 nm in DMF solvent. Also, the quantum yields of PAZ-Py-1 were calculated as 6%, 7.2%, 7%, 2.8%, 9%, 6%, 5% and 9% for blue emissions when excited at 280, 300, 320, 340, 360, 380, 400 and 440 nm, respectively, in EtOH solution. The quantum yields of PAZ-Py-1 were calculated as 11.0%, 3.2%, 5.7%, 5.2% and 3.7% for orange emissions when excited at 460, 480, 500, 520, 540 and 560 nm, respectively, in EtOH solutions. While DA-Py did not demonstrate any color emission in EtOH solution, PAZ-Py-1 and PAZ-Py-2 emitted only blue color in DMF and EtOH solutions.
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