Study On The Synthesis And Fluorescent Performance Of Coumarin-based Dyes

When the compounds were excited by UV, electricity or chemicalenergy, the electrons occured ground-excited state transition, gave offphotons by radiation decay and returned to ground state as well asproduced fluorescence. The research about molecular structure andperformance of fluorescent materials became one of the advancing fronts inthe field of functional materials. Coumarin compounds were one kind ofspecial internal ester of cinnamic acid with good fluorescence performance.Based on the analysis and evaluation of bibliographies, the principleof fluorescence was discussed, the relation between fluorescence andmolecular structure was studied, and the performance parameters andmeasurement of fluorescent spectrum were analysed. Four novelflourescent dyes based on coumarin were prepared and one typicalheterocyclic coumarin was synthesized by chloroaluminate ionicliquids-catalyzed reaction. The structures of the compounds werecharacterized by IR, ¹HNMR.The four oxadiazol-5-yl-coumarin derivatives were synthesizedstarting from coumaric acid-3 and substituted benzoyl hydrazine by thereactions of cyclic, oxadiazole cyclisation in the presence of a dehydratingagent. Coumaric acid-3 was prepared starting from diethyl malonate and 4-diethylaminosalicylaldehyde by Knoevenael reaction. The yields of the products were about 21.35%, 29.23%, 32.36%, 37.19% respectively. Themechanism of the reactions was studied and the processing parameterswere evaluated experimentally.The environmentally benign process for preparing fluorescent dyebased on coumarin over room temperature ionic liquids catalyst and solventwas proposed. Ionic liquids 1-Butyl-3-methylimidazolium chloride([BMIm]Cl) and 1-Butyl-3-methylimidazolium bromide ([BMIm]Br) weresynthesized from 3-methylimidazole and chlorobutane or bromobutane;and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm]BF₄) wasprepared via metathesis of [BMIm]Cl and NaBF₄. Aluminium chlorideionic liquids containing imidazonium salts and anhydrous aluminiumchloride ([BMIm]X-A1Cl₃) were found to work well as Lewis acid catalystsin the synthesis of typical fluorescent dye 3-(2'-benzoxazolyl)-7-diethyl-amino-2H-Chromen- 2-one by one-pot synthesis of 4-diethyl-aminosalicyl-aldehyde and 2-aminophenol with ethyl cyanoacetate. Theinfluence of ionic liquids' structure and the processing parameters werediscussed, the optimum processing parameters were evaluated anddetermined as follows: n(4-diethylaminosalicylaldehyde): n(2-aminophenol):n(ethylcyanoacetate):n([BMIm]Br-AlCl₃)=1:1:1:1, reaction time 60 minand reaction temperature 100～110℃. The yield can reach 78.3%. Theprominent advantages of using ionic liquids were operationally simple,economizing energy and environmentally benign.The fluorescent spectra and absorption spectra of 10 coumarincompounds were measured in tetrahydrofuran and acetone respectively.Study of the spectra characteristics showed that, with the introduction ofbenzoxazolyl and oxadiazol at the position of 7-diethyiaminocoumarins,their absorption maxima bathochromicly shifted 40nm, emission maximabathochromicly shifted 47nm, stokes shift increased 30～78nm, and fluorescence quantum yields raised from 0.18 to 0.49. With the introductionof various substituents at the position of benzene ring of 3-benzoxazolylsubstituted coumarins, absorption maxima and emission maxima weredissimilar between each other. The fluorescent performances of thecomounds with -CH₃ and -Cl or without any substituent at the position ofbenzene ring are similar, and there is the intense quenching effect onfluorescence emission with the strong electron acceptor -NO₂ as well as-SO₂NH₂ substitutants. The compounds containing symmetrical structureshad larger conjugate system, which caused a significant bathochromic shiftof the absorption maxima and emission maxima; they also had higherfluorescence quantum yields. The electron-donor (-OH) or anelectron-acceptor (-Cl) at the position of benzene had little influence to thefluorescence quantum yields and the similar results were obtained when thepolarity of solvent increased.