Synthesis, Characterization and Fluorescent Property Evaluation of 1,3,5-Triaryl-2-pyrazolines

A series of 1,3,5-triaryl-2-pyrazolines was synthesized by dissolving the corresponding 4-alkoxychalcones in glacial acetic acid containing a few drops of concentrated hydrochloric acid. This step was followed by the addition of (3,4-dimethylphenyl) hydrazaine hydrochloride. Finally the target compounds were precipitated by pouring the reaction mixture onto crushed ice. The structures of the synthesized compounds were established by physicochemical and spectroscopic methods. The 1,3,5-triaryl-2-pyrazolines bearing homologous alkoxy groups were found to possess fluorescence properties in the blue region of the visible spectrum when irradiated with ultraviolet radiation. The fluorescent behavior of these compounds was studied by UV-Vis and emission spectroscopy, performed at room temperature.


Introduction
Due to the interesting properties of substituted pyrazolines as biological agents and optical materials, considerable attention has been focused on the study of this class of compounds. They have pharmaceutical importance as these can be effectively utilized as potential therapeutic agents against a variety of major ailments, due to their antibacterial and antifungal [1], antimicrobial [2,3] cytotoxic [4] anticancer [5] anti-inflammatory [6] and platelet aggregation inhibiting [7] properties. 1,3,5-Triaryl-2pyrazolines constitute an important class of conjugated nitrogen-containing fluorescent compounds emitting a blue fluorescence with high quantum yield [8][9][10][11] and have been found to act as hole transporting media in photoconductive as well as emitting materials [12][13][14][15][16] and in organic electroluminescent devices (OELDs) [17][18][19][20]. Organic electroluminescent devices find potential use in various displays [21][22][23], and have many advantages over inorganic ones, such as high luminous efficiency, low cost, wide range of emission colors via specialized molecular design of organic compounds, and easy processing. 1,3,5-Triaryl-2-pyrazolines are also utilized as optical brightening agents for textiles, fabrics, plastics, papers [24] fluorescent switches [25] and as fluorescent probes in many chemosensors [26].

Results and Discussion
Twelve alkoxybenzaldehydes were synthesized by refluxing 4-hydroxybenzaldehyde with twelve alkyl bromides (CH 3 -Br -C 12 H 25 -Br) in the presence of potassium carbonate and butanone. The resulting 4-alkoxybenzaldehydes 1a-12a were reacted with acetophenone in the presence of sodium hydroxide and ethanol to yield corresponding 4-alkoxychalcones 1b-12b. 4-Alkoxychalcones were dissolved in glacial acetic acid containing a few drops of concentrated hydrochloric acid. This step was followed by the addition of (3,4-dimethylphenyl)hydrazaine hydrochloride. Finally the target compounds 1h-12h were precipitated by pouring the reaction mixture onto crushed ice as outlined in Scheme 1.
The FT−IR spectra of synthesized 4-alkoxybenzaldehydes 1a−12a showed two strong bands at stretching frequencies in the range of 1260-1245 cm −1 and 1050-1035 cm −1 , which are characteristic of Ar−O−R groups. No peak appeared in the range of 3400-3300 cm −1 , which indicated the disappearance of the -OH group of the 4-hydroxybenzaldehyde. A strong carbonyl group (C=O) band appeared in the 1700-1660 cm −1 range.
The 4-alkoxychalcones 1b−12b were synthesized by base catalysed Claisen−Schmidt aldol condensation and were obtained as pale yellow crystalline solids. The FT-IR spectra of all the synthesized 4-alkoxychalcones showed absorption bands in the range of 1700-1675 cm −1 for carbonyl groups (C=O) and 1644-1617 cm −1 for olefinic double bonds (−CH=CH−). In addition the two strong characteristic Ar−O−R group bands were observed.
The chemical shifts of doublets corresponding to the α-hydrogen atoms were found to be 7.37-7.43 ppm, with coupling constants of 15.6 Hz. The chemical shifts of doublets corresponding to the β-hydrogen atoms appeared at 7.80-7.81 ppm, with coupling constants of 15. Gas chromatograms (GC) displayed single peaks with different retention times for every compound, confirming the purity of the compounds. Mass Spectrometry (MS) was used to determine the molecular weights of these compounds. A molecular ion peak (M + ) was observed for the 4-alkoxychalcones 1b−12b, in their respective mass spectra. The fragmentation pattern of synthesized compounds matched the typical pattern of chalcone fragmentation, further confirming the structures of the compounds.
The FT-IR spectra of the 1,3,5-triaryl-2-pyrazoline compounds 1h−12h, showed absorption bands in the range of 1690-1640 cm −1 confirming the presence of carbon-nitrogen (C=N) double bonds, and at 1350-1000 cm −1 for carbon-nitrogen single bonds (C-N). A strong band for arylcarbon-alkylcarbon (Ar-R) appeared in the 1500-1490 cm −1 range. Two strong bands at stretching frequencies in the range of 1260-1245 cm −1 and 1050-1035 cm −1 were also observed, which confirmed the presence of Ar−O−R groups.
The formation of the five membered pyrazoline cycle was confirmed by the presence of two methylene protons (H a and H b ) and one methine proton (H x ) for the pyrazoline ring. The 1 H-NMR spectra of the compounds 1h−12h showed three doublet of doublets corresponding to the H a , H b and H x atoms of the pyrazoline ring, respectively.  . + .
The chemical shifts of a doublet of doublets for H a of the compounds 1h−12h were observed at 3.11 ppm, with coupling constants in the range of 7.5-7. 8  The proton-decoupled 13 C-NMR spectra for all the synthesized 2-pyrazolines 1h−12h were recorded at 75 MHz in CDCl 3 and peak assignments were made on the basis of calculated and reported values [27,28]. The aromatic carbon resonances of the compounds are assigned on the basis of signal intensities and comparing with the calculated values. The methylene carbons attached to the oxygen atom of aromatic ring (ArO−CH 2 −) were observed around 60-70 ppm. In compounds 1h−12h, the methylene carbon (−CH 2 −) and methine carbon (−CH−) of the five membered pyrazoline rings were assigned chemical shift values in the range of 41-44 ppm and 55-65 ppm, respectively.
A molecular ion peak (M + ) which was also the base peak was observed for all the synthesized 2-pyrazolines in their respective mass spectra. The fragmentation pattern of the synthesized compounds (exemplified by that of compound 6h) matched the typical pattern of pyrazoline fragmentation [29] ( Figure 1), confirming the structure of compounds. On the basis of the combined physical and spectral data, compounds 1,3,5-triaryl-2-pyrazolines were assigned the structures shown in Figure 2. The synthesized 1,3,5-triaryl-2-pyrazolines 1h−12h bearing homologous alkoxy groups were found to possess fluorescence properties in the blue region of the visible spectrum when irradiated with ultraviolet radiation. The fluorescence behavior of these compounds was studied by UV-Vis and emission spectroscopy performed at room temperature.
The UV-Vis spectra ( Figure 3) of 1,3,5-triaryl-2-pyrazolines 1h−12h exhibit an intense absorption band which is attributed to the π − π* transition of the conjugated backbone. In case of absorption spectra of compounds 1h−12h, the absorption maxima (λ max abs ) wavelength was observed in the range of 367−377 nm (Table 1) with the corresponding absorbance. The absorption spectra of the 1,3,5-triaryl-2-pyrazolines 1h−12h showed that the spectral shapes are very similar because all these compounds possess the same (1,3,5-triaryl)-2-pyrazoline central nucleus. The difference of absorption wavelengths and absorption intensities is due to the effect of the presence of 4-alkoxyphenyl groups at the 5-position of the 2-pyrazoline moiety. In the emission spectra of 2-pyrazolines 1h−12h ( Figure 3) it was observed that all these compounds exhibited blue emission with variable emission intensity at constant concentration. The emission maxima (λ max em ) wavelengths of all the synthesized compounds lie in the blue region of visible spectrum and therefore show blue fluorescence.  The geometry of the 1,3,5-triaryl-2-pyrazoline nucleus has a configuration such that the aryl groups at the 1 and 3 positions are in conjugation with the pyrazoline ring. However, the aryl group at position 5 is not part of the conjugated moiety of the 2-pyrazoline nucleus. The alkoxy substitution at the aryl group at the 5 position can be expected to have some effect on the physico-chemical properties due to its electron donating nature. From emission spectra of the reported compounds, it can be concluded that the introduction of electron-donating 4-alkoxyphenyl groups at the 5-position strongly affects the emission intensity without causing any major blue-or red-shift in the emission wavelength.

General
Melting points of the synthesized compounds were recorded on a MFB-595-101M Gallenkamp digital melting point apparatus in open-end capillary tubes and are uncorrected. Thin layer chromatography was carried out on pre-coated silica gel plates (0.2 mm, E. Merck, 20 × 20 cm, 60F 254 ). FT-IR spectra were recorded on a Bio-Red Merlin Spectrophotometer using KBr discs. 1 H-NMR (300 MHz) and 13 C-NMR (75.43 MHz) spectra were recorded on a Bruker AM-300 spectrometer in DMSO and CDCl 3 solutions using TMS as internal standard. EIMS were recorded on a GCMS Agilent VG: 70 SE Mass Spectrometer. Purity of the synthesized compounds was also monitored by the same instrument.

Procedure for the Preparation of 4-Alkoxybenzaldehydes 1a-12a
4-Hydroxybenzaldehyde (0.1 mole, 12.2 g) and anhydrous K 2 CO 3 (0.1 mole, 13.8 g) were added to 2-butanone (150 mL) in a 250 mL two neck round bottom flask. The reaction mixture was heated gently at 60-65 °C for 30 minutes, then 1-bromoalkane (Br-CH 3 to Br-C 12 H 25 , 0.1 mole) was added drop wise and the reaction mixture was heated to reflux for 5-8 hours. The progress of the reaction was monitored by TLC. After completion of the reaction, the solvent was evaporated under reduced pressure on a rotary evaporator. The reaction mixture was transferred to a 1 L separatory funnel and diethyl ether (250 mL) was added. The K 2 CO 3 was removed by washing the reaction mixture repeatedly with distilled water. Traces of unreacted 4-hydroxybenzaldehyde were removed by washing the ether layer with 5% NaOH solution (25 mL). The ether layer was washed with distilled water till it became neutral to pH paper and was dried on anhydrous MgSO 4 . The diethyl ether was evaporated to give the 4-alkoxybenzaldehydes 1a−12a as transparent pale yellow liquids.

Procedure for the Preparation of 4-Alkoxychalcones 1b-12b
In a 500 mL beaker acetophenone (0.1 mole, 12.0 g) was dissolved in ethanol (20 mL). To this mixture, ice cold 10% NaOH solution (200 mL) was added and the mixture stirred for 30 minutes at room temperature. Then 4-alkoxybenzaldehyde 1a-12a (0.1 mole) was added to the reaction mixture that was further stirred for 5-8 hours till pale yellow precipitates appeared. Crushed ice was added to the solid mass and the reaction mixture was neutralized with dilute HCl. The products 1b-12b were obtained as yellow solids, which were filtered and recrystallized from ethanol.

Conclusions
A series of twelve 1,3,5-triaryl-2-pyrazolines 1h-12h containing homologous alkoxy groups (CH 3 -C 12 H 25 ) were synthesized by converting benzaldehyde into 4-alkoxybenzaldehydes with the corresponding alkyl halides. The 4-alkoxybenzaldehydes yielded 4-alkoxychalcones on reaction with acetophenone. The target compounds were finally obtained by the reaction of the 4-alkoxychalcones with (3,4-dimethylphenyl) hydrazaine hydrochloride. Their structures were confirmed by IR, 1 H-and 13 C-NMR, mass spectrometric and elemental analysis. The synthesized 1,3,5-triaryl-2-pyrazolines were found to possess fluorescence properties in the blue region of the visible spectrum when irradiated with ultraviolet radiation. The comparison of emission intensities of pyrazoline derivatives revealed that the alkoxy chains have remarkable effect on emission intensity of the 1,3,5-triaryl-2pyrazolines. The absorption intensities of 6h, 10h 11h, 12h and 8h are much higher than other derivatives. Approximately, it was the same case in emission intensities. It can thus be concluded that variation in alkoxy chains can be used to tune the emission intensity that can find potential applications in controlling/optimizing the optoelectronic and luminescence properties of Organic Light Emitting Diodes (OLEDs) based on such types of fluorescent compounds.