Synthesis of 2-( 1-Benzofuran-2-yl )-4-( 1 , 3-benzoxazol-2-yl / 1 , 3-benzothiazol-2-yl ) Quinolines as Blue Green Fluorescent Probes

A series of novel 2-(1-benzofuran-2-yl)-4-(1,3 benzoxazol-2-yl/1,3-benzothiazol-2-yl) quinoline derivatives 4(a–d) were synthesized in one step by the reaction of 2-(1-benzofuran-2-yl) quinoline-4-carboxylic acids 3(a-b) with o-aminophenol and o-amino thiophenol, respectively, using polyphosphoric acid (PPA) as a cyclizing agent. e �uorescent properties of newly synthesized compounds were investigated in three different organic solvents like chloroform (CHCl3), tetrahydrofuran (THF), and dimethyl sulfoxide (DMSO). e photophysical constants such as quantum yield and stokes shi were determined. From the results of �uorescence study, it is evident that all synthesized compounds are �uorescent in solution. Compound 4a emitted green light (490.4 nm, 518.2 nm, and 522.4 nm) with high quantum yield in all the three solvents, while compounds 4b, 4c, and 4d emitted green light (512 nm, 499 nm, 510 nm) only in polar solventDMSO.All �uorescent probes exhibited a bathochromic shi on increase in polarity of the solvent.


Introduction
ere is an ever-increasing interest in the development of efficient photoluminescent materials especially those which emit light in the blue region of the visible spectrum.ese materials play a vital role in optoelectronic devices such as tunable lasers and ampli�ers, optical �bers, switches, or modulators with diverse applications in optical communications, photonics, medicines, optical spectroscopy, and organic electroluminescent diode (OLED) [1].
Among the various nitrogen-containing heterocyclic compounds, quinolines occur predominately in nature appreciable to their stability and ease of generation.ey exhibit pronounced biological activities [2], and many derivatives have been reported to possess �uorescent property, having wide application as organic electroluminescent devices [3,4], biosensors [5,6], detection of metal ions [7][8][9], and so forth.On the other hand, benzofuran chromophore with high photoluminescence (PL) and quantum efficiencies [10] found wide application as organic electroluminescent devices [11][12][13] and chemosensors [14,15].e present work describes the synthesis and characterization of six new dyes.Initially the two derivatives of 2-(1-benzofuran-2-yl) quinoline-4-carboxylic acid 3(a-b) were synthesized, and it was found that they emit light in the blue region of the spectrum.Further it was planned to modify the carboxylic acid functionality by electron donor benzoxazole and benzothiazole ring system.erefore, the present investigation involves the synthesis and �uorescent studies of new 2-(1-benzofuran-2-yl)-4-(1,3-benzoxazol-2yl/1,3-benzothiazol-2-yl) quinoline derivatives 4(a-d) with a motive of getting possible application as blue OLE materials.Here we selected an unique combination of three different fused heterocyclic ring systems, where quinoline ring acts as core moiety, benzofuran nucleus substituted at second position acts as chromophore, and 3-benzoxazol-2-yl /benzothiazol-2-yl rings substituted on the fourth position of quinoline nucleus act as an electron acceptor system.  1. �ll �uorescent probes exhibited a bathochromic shi of the emission peak with decreased intensity of emitted light on raising the polarity of solvents, and a related increase of the Stokes shi and decrease in quantum yield are observed.Fluorescent probes 3a and 3b emitted light in the blue region of the spectrum, and 2-(1benzofuran-2-yl)-6-chloroquinoline-4-carboxylic acid (3b) showed bathochromic shi by 11.5 nm when compared to 2-(1-benzofuran-2-yl) quinoline-4-carboxylic acid 3a.is may be due to the presence of electron releasing chloro substitution on the 6th position of quinoline ring system in compound 3b, which acts as a chromophore.On replacing carboxylic group of 2-(1-benzofuran-2-yl) quinoline-4carboxylic acid with benzoxazole 4(a-b) and benzothiazole 4(c-d), we found bathochromic shi in emission peaks and this may be attributed to the electron accepting tendency of benzoxazole and benzothiazole ring system, which decreases the energy gap between LUMO ( * ) and HOMO () leading to shi towards longer wavelength than compared to carboxylic acid.On increasing the solvent polarity, all compounds exhibited shi towards longer wavelength.e emission peaks are at lower energy or longer wavelength than compared to absorption peak; this loss of energy is due to a variety of dynamic processes, which occur following light absorption.e �uorophore is typically excited to the �rst singlet state (S 1 ), usually to an excited vibrational level within S 1 .e excess vibrational energy is rapidly lost to the solvent.If the �uorophore is excited to the second singlet state (S 2 ), it rapidly decays to the S 1 state in 10 −12 s due to internal conversion.Solvent effects shi the emission to still lower energy owing to stabilization of the excited state by the polar solvent molecules.Typically, the �uorophore has a larger dipole moment in the excited state than in the ground state.Following excitation, the solvent dipoles can reorient or relax at that state, which lowers the energy of the excited state.As the solvent polarity is increased, this effect becomes larger, resulting in emission at lower energies or longer wavelengths [16].

Result and Discussion
From the absorption data (Table 1), it is evident that the absorption wavelengths are slightly varied or no variations are observed on increasing polarity of the solvents.is may be because absorption of light occurs in about 10 −15 s, a time that is too short for motion of the �uorophore or solvent.Absorption spectra are not affected by the decrease in the excited-state energy, which occurs aer absorption has occurred [16].
Among the tested compounds, compound 4a emitted light in the green region in all three solvents CHCl 3 , THF, and DMSO at wavelength of 490.4 nm, 518.2 nm, and 522.4 nm, respectively.Compound 4b, 4c, and 4d emitted light in the green region only in polar solvent DMSO.

Experimental
All the chemicals used were of analytical grade.Melting points were determined in open capillary tubes and are uncorrected.Purity of the compounds was checked by TLC on silica gel.e IR spectra were recorded on Nicolet-Impact-410 FT-IR spectrometer, using KBr pellets. 1 H NMR spectra were recorded on a Bruker Supercon FT NMR (400 MHz) Compound spectrometer in CDCl 3 or DMSO- 6 using TMS as an internal standard.e chemical shis are expressed in  units.Mass spectra were recorded on a JEOL SX 102/DA-6000 (10 kV) FAB mass spectrometer.

Synthesis of 2-(1-Benzofuran-2-yl) Quinoline-4carboxylic Acid 3(a-b)
3.1.1.General Procedure.e compounds 3a-b were synthesized by the P�tzinger method [17], with the slight modi�cation in procedure.Aer completion of the reaction, the reaction mixture was cooled to room temperature, diluted with cold water and extracted using ethyl acetate to suspend the impurities in organic layer.e aqueous layer was acid-i�ed with dilute hydrochloric acid and the resulting yellow solid mass was �ltered and dried to get pure compound.
SolventAbsorption  max nm Emission  max nm Molar absorpivity Mol −1 cm −1 Stokes shi nm Quantum yield Φ 413 (M + 1), 414.9 (M + 2).Anal.Calc.(in%)for C 24 H 13 ClN 2 OS: C, 69.8; H, 3.14; N, 6.78.Found: C, 69.4; H, 3.22; N, 6.67.3.3.FluorescentStudies. e �uorescence spectra were recorded on F-7000 spectro�uorometer (Hitachi) with Xenon �ash lamp.e spectra were recorded by preparing the solutions of test compounds at the concentration of 5 × 1 −6 mole/mL in three different solvents CHCl 3 , THF, and DMSO.e solutions of each sample were scanned aer keeping it at room temperature for about 1 h.e emission and excitation wavelength data are recorded using glass cuvettes.e emission quantum yields were determined at  exc corresponding to the maximum of the absorption band ( max ) of quinine sulphate in 0.1 M H 2 SO 4 , which was used as a �uorimetric standard (Φ  = 54).