ОПТИКА И СПЕКТРОСКОПИЯ, 2015, том 118, № 4, с. 594-603
^ СПЕКТРОСКОПИЯ ^^^^^^^^
КОНДЕНСИРОВАННОГО СОСТОЯНИЯ
УДК 543.42
SYNTHESIS, FLUORESCENCE, TGA AND CRYSTAL STRUCTURE OF THIAZOLYL-PYRAZOLINES DERIVED FROM CHALCONES
© 2015 г. T. Suwunwong*, S. Chantrapromma*, and Hoong-Kun Fun*****
* Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Prince of Songkla University, Hat-Yai, Songkhla 90112, Thailand ** Department of Pharmaceutical Chemistry, College of Pharmacy, King Saud University, P. O. Box 2457, Riyadh 11451, Saudi Arabia *** X-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, 11800 USMPenang, Malaysia
E-mail: suchada_3p@hotmail.com Received July 7, 2014; in final form, September 30, 2014
Thiazolyl-pyrazolines 3a-3d were synthesized in a three step procedure using chalcones as starting materials and characterized by FT-IR, UV-Vis, and 1H NMR techniques. The crystal structure of compound 3a was also determined by X-ray diffraction analysis. Compound 3a crystallized out in the orthorhombic P212121 space group with the unit cell dimensions: a = 5.2106(2) A, b = 12.4341(5) A, c = 33.3254(13) A, a = в = y = 90°, V = 2159.12(15) A3, Z = 4, Dcald = 1.372 Mg m-3 and F(000) = 928. Fluorescence of 3a-3d were studied in solid state and acetonitrile solution. It was found that these compounds exhibit the green fluorescence light (506-508 nm) in both solid and solution states. The pH stability on fluorescence property and the thermal gravimetric analysis of compound 3a were specifically carried out. It was revealed that 3a shows high thermal stability up to around 250°C and presenting high stability in various pH ranges in the acetoni-trile-water matrix.
DOI: 10.7868/S0030403415040212
INTRODUCTION
Pyrazoline derivatives are important nitrogen containing five-membered heterocyclic compounds that have been attracting increasing attention due to their pharmaceutical applications in antimicrobial [1, 2], anticancer [3, 4], HIV-1 protease inhibitory [5], antidepressant [6, 7], antinociceptive [8, 9], antiinflammatory [10, 11] and analgesic agents [12]. Furthermore, these molecules show high hole-transport efficiency, excellent blue emission, easy accessibility and high quantum yield [13—15]. In this respect, they also have been attracted considerable interest for study as fluorescent organic compounds as well as being widely used as whitening or brightening reagents for synthetic fibers [16, 17], hole transport materials in electrophotography [18], organic light emitting diodes (OLED) [19—21] and novel fluorescent materials [22—24]. The thiazole group has also been investigated extensively due to its interesting biological activities such as possessing antimicrobial [25, 26], antitubercular [27], antiinflammatory [28, 29], analgesic [29], and antitumor [30] properties as well as fluorescence properties [31— 35]. In view of the considerable importance of both pyrazoline and thiazole derivatives, we have been focus in our interest on the synthesis and optical properties of the thiazolyl-pyrazoline derivatives which were linked by C—N bond. Moreover, there are many pyra-
zolines that exhibit fluorescence both in solution and solid states even though most molecular organic chro-mophores aggregation in the solid state causes fluorescence quenching [36], and it is interesting to note that there are a few reports on fluorescence property in both solution and solid states of some thiazolyl-pyra-zoline moiety [24, 37]. Herein, we report the synthesis of this kind of compounds via a three-step procedure using chalcones as starting materials (Fig. 1) and study for their fluorescence properties in both solution and solid states.
EXPERIMENT
Reagents and Apparatus
Ethanol, acetone and acetophenone were purchased from Merck Ltd. and were used as received. Sodium hydroxide was purchased from Lab-Scan Analytical Sciences whereas 2-chloroacetophenone, benzaldehyde, p-anisaldehyde, 4-ethoxybenzaldehyde, 2-naphthalde-hyde, thiosemicarbazide and 2-bromo-4'-methylace-tophenone were obtained from Sigma—Aldrich and were used without purification. IR spectra (KBr pellets) were recorded in the range of 4000—400 cm-1 using a Perkin-Elmer FT-IR System Spectrum BX spectrophotometer. 1H NMR spectra (300 MHz) were recorded on a Bruker Ultra Shield™, using DMSO-d6
Fig. 1. The synthetic route of the thiazolyl-pyrazolines 3a—3d. 1a, 2a, 3a: R = CgH5; 1b, 2b, 3b: R = 4-OCH3C6H4; 1c, 2c, 3c: R = 4-OC2H5C6H4; 1d, 2d, 3d: R = 2-naphthyl.
as solvent and tetramethylsilane as the internal standard. UV-Vis spectra were recorded on a Shimadzu UV-2450 UV-Vis spectrophotometer. Fluorescence emission spectra were recorded on a Perkin—Elmer LS 55 luminescence spectrometer at the ambient temperature. TGA data was collected employing a TGA7, Perkin Elmer thermoanalyser in flow of N2, in the temperature range from 50 to 1000°C, with a heating rate of 10°C min-1. Crystallographic data were collected on a Bruker SMART APEXII CCD area detector diffractometer with a graphite monochromated Mo Ka radiation (X = 0.71073 A) at 100.0(1) K with the Oxford Cryosystem Cobra low-temperature attachment. The collected data were reduced using SAINT [38] and the empirical absorption corrections were performed using SADABS program [38]. The structure was solved by direct method and refined by least-squares using the SHELXTL [39] software package.
Material and Preparation. General Procedure for the Synthesis of Compounds 3a—3d
Chalcone derivatives 1a-1d were prepared as starting materials according to the published literature [40]. The synthesis of pyrazolines 2a-2d which served as the starting materials for further synthesis is shown in Fig. 1. To a stirred solution of appropriated chalcone (2.0 mmol) in ethanol (15 mL) was added thiosemicarbazide (3 mmol) and NaOH (6.0 mmol). The reaction mixture was refluxed for around 4-5 h. The progress of the reaction was monitored by TLC. After completion, the reaction mixture was cooled to room temperature and diluted with distilled water. The solid products which precipitated out were filtered, washed with water and ethanol, and diethyl ether, re-
spectively, and then recrystallized from ethanol to afford 2a—2d. The synthetic route of the proposed thiazolyl-pyrazolines 3a—3d is shown in Fig. 1. Pyrazolines 2a—2d (1 mmol) was dissolved in ethanol (10 mL). A solution of 2-bromo-p-methoxyacetophe-none (1 mmol) in ethanol (10 mL) and NaOH in ethanol were added to the reaction. The reaction mixture was refluxed for around 3—4 h. The progress of the reaction was monitored by TLC. After completion, the reaction mixture was cooled to room temperature. The solid products which precipitated out were filtered, and washed with hexane and cooled ethanol, respectively, and then recrystallized from ethanol to afford 3a—3d.
Absorption and Fluorescence Measurement
Absorption and fluorescence emission spectra of compounds 3a—3d in acetonitrile (2 ^M) were studied at room temperature. The Stokes shift (Av) and oscillator strength f) are important characteristics for the fluorescent compounds [41]. In this study, the Stokes shift (in cm-1) was calculated by equation (Aa and AF are the absorption and fluorescence emission maxima in Â, respectively):
Av = (1/Aa - 1/Af) x 107. (1)
The oscillator strength f represents the effective number of electrons whose transition from ground to excited state giving the absorption area in the electron spectrum [42]. The oscillator strength was calculated by the following equation, where £max is molar extinction coefficient and Av1/2 is the width of the absorption band (cm-1) at 1/2 (£max):
f = 4.32 x 10-9 Av1/2e
1/2°max'
(2)
Table 1. UV-Vis absorption and fluorescence data for thiazolyl-pyrazolines 3a-3d in acetonitrile (2 ^M) and in solid state
Compound acetonitrile solution Solid state
^abs (e), nma Xem, nmb Av, cm 1c O F fe EfF Xem, nmb RIg
3a 3b 3c 3d 276 (24300), 362 (16600)* 276 (38100), 362 (18400)* 276 (16700), 363 (16100)* 276 (38600), 363 (16800)* 508 508 508 507 7939 7939 7863 7824 0.076 0.066 0.065 0.068 0.36 0.45 0.37 0.43 0.054 0.047 0.046 0.049 507 507 507 506 0.061 0.077 0.048 0.080
absorption maximum wavelength (* selected absorption wavelength for fluorescent study), b fluorescent emission wavelength, c stokes
shift, d fluorescent quantum yield (used coumarinl as standard, Of cence, g relative intensity of fluorescence in solid state.
: 0.73 in EtOH), e oscillator strength, 1 the energy yield of fluores-
Fluorescent quantum yield (Of) was determined by the relative comparison procedure, using coumarinl as the fluorescence standard. The corrected emission spectra were measured for the coumarinl standard (Aex = 370 nm, A (absorbance) < 0.01, Of = 0.73 in EtOH). For all the measurements of fluorescence spectra scan speed was 300 nm min-1 using a quartz cell of 1 cm optical path length. The UV-Vis absorption spectra were recorded in a standard 1 cm path length quartz cell in range 200-500 nm with spectral resolution 1 nm. The general equation used in the determination of fluorescent quantum yields from earlier research was given in [43] as
O x = 0 ST
Sx S
ST
*ST
( ^2 ^ 'Ix
V ^st y
(3)
where O and S are fluorescence quantum yield and integrated area under the corrected emission spectrum, respectively, A is absorbance at the excitation wavelength, n represents the refractive index of the solution, and the subscripts X and ST refer to the unknown and the standard, respectively. Furthermore, the energy yield of fluorescence (EF) also can be used instead of the fluorescence quantum yield (OF) [42] which is calculated by
E F = 0 f(K a/À F).
(4)
For solid state fluorescent the ability to emit the fluorescent light was measured as relative fluorescence intensity compared with the emission intensity of cou-marin1 standard in solid state which can be calculated with the proportion of sample fluorescent intensit
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