Facile synthesis of 2-(1,3-benzoxazol/benzothiazol-2-yl)-3H-benzo[f]chromen-3-one as blue fluorescent brighteners

RESEARCH ARTICLE H.N. Harishkumar, K.M. Mahadevan and J.N. Masagalli, S. Afr. J. Chem., 2012, 65, 5–9, <http://journals.sabinet.co.za/sajchem/>. 5 Facile Synthesis of 2-(1,3-Benzoxazol/benzothiazol-2-yl)3H-benzo[f ]chromen-3-one as Blue Fluorescent Brighteners Hosanagara N. Harishkumar, Kittappa M. Mahadevan* and Jagadeesh N. Masagalli Department of Post Graduate Studies and Research in Chemistry, School of Chemical Sciences, Kuvempu University, Shankaraghatta-577 451, Karnataka, India. Received 22 July 2011, revised 10 October 2011, accepted 8 December 2011. ABSTRACT A novel synthetic method was developed to prepare new fluorescent 2-(1,3-benzoxazol/benzothiazol-2-yl)-3H-benzo by the Knoevenagel condensation between 2-hydroxy-1-naphthaldehyde and [f]chromen-3-one derivatives benzothiazole-2-yl-aceatates or N-methyl benzoxazole-2-yl-acetates using choline chloride/urea ionic liquid as a green catalyst. The results of fluorescence studies revealed that all the compounds show moderate to low emission intensities and are expressed in the form of quantum yields. KEYWORDS Benzooxazolyl-3H-benzo[f]chromen-3-one, benzothiazolyl-3H-benzo[f]chromen-3-one, choline chloride/urea, fluorescent brighteners. 1. Introduction Coumarin dyes exhibit unique photochemical and photophysical properties, which make them useful in applications such as optical brighteners, laser dyes, non-linear optical chromophores, solar energy collectors, fluorescent labels and as two-photon absorption (TPA) materials.1,2 Coumarin dyes have also been used as blue, green and red dopants in organic light-emitting diodes (OLEDs).3–6 Fluorescent brighteners normally have a system of conjugated double bonds and electron-donating groups that render high efficient fluorescence properties.7 Typically, coumarin fluorescent dyes contain an electron donor at position-7 and an electron acceptor at position-3.8 The lasing range covered by coumarin dyes is appreciably extended when they contain a heterocyclic substituent at position-3.9,10 Further, it is well known that seminaphthofluoresceins (SNAFLs) and naphthofluoresceins have been recognized as annulated derivatives of fluorescein by one or two aromatic ring have longer emission wavelength when compared to fluorescein.11 Although, the application of various coumarin fluorescent labels has been extensively studied,12,13 their benzo counterparts, namely benzocoumarins, have been less studied. Recently, Akira et al., reported that coumarins and benzocoumarins coupled with different heterocycles at the 3-position, exhibited strong fluorescent properties.14 In our earlier investigation we studied the synthesis and florescent properties of benzocoumarin oxadiazolyl derivatives.15–17 Kidwai and Kumar18 have synthesized similar compounds by using 2-hydroxy-1-naphthaldehyde with 5-methyl-1,3,4-thiadiazol-2-ylsulfanyl-, 1H-1,2,3,4-tetrazol-1-yl-, 1H-indol-3-ylquinolin-8-yloxy- and 4-methylquinolin-2-yloxy-acetic acids in the presence of DCC–DMSO using microwaves. On the other hand, with increasing environmental concerns and regulatory constraints, the development of environmentally benign organic reactions has become mandatory. Although several methods have been reported for the synthesis of various * To whom correspondence should be addressed. E-mail: benzocoumarin dyes many suffer from disadvantages such as high-temperature requirements, the use of corrosive catalysts, longer reaction times, large solvent volumes, tedious work up methods and often produce low yield. In this context we used choline chloride/urea ionic liquid as the solvent and catalyst. They have gained attention in past few years because of their unique physical and chemical properties.19–21 Hence, in view of the above drawbacks and in continuation of our work on the synthesis of fluorescent brighteners15 and development of new compounds for other applications,22–29 in this paper we report an environmentally benign one-pot synthesis of 2-(1,3-benzoxazol/benzothiazol-2-yl)-3H-benzo[f]chromen-3ones 3a–p as new blue fluorescent brighteners by using choline chloride/urea ionic liquid as a green catalyst and as solvent (IL).19,20 Structure of choline chloride/urea ionic liquid Choline chloride/urea ionic liquid is easy to handle, miscible in water, nonhazardous, and is known to result in faster synthetic transformations. In addition, due to the ionic liquid properties the products can frequently also be isolated more easily. The synthesized products were purified by boiling with MeOH in which the unreacted starting materials dissolve completely and upon filtration, the products were isolated in high purity. Due to the ecofriendly and reusable nature of the catalyst, this procedure is an attractive option compared to existing methods.32–38 2. Results and Discussion 2-(1,3-Benzoxazol/benzothiazol-2-yl)-3H-benzo[f]chromen-3ones 3a–p (Scheme 1, Table 1) were synthesized via two component one-pot reaction of benzoxazol/benzothiazol-2-ylacetate 2a–p with 2-hydroxy-1-napthaldehyde 1a in presence of 50 mol% of the choline chloride/urea as catalyst, at reflux temperature on a hot plate for about 1 h. The reaction was quick and immediate product formation was observed in all cases. In RESEARCH ARTICLE H.N. Harishkumar, K.M. Mahadevan and J.N. Masagalli, S. Afr. J. Chem., 2012, 65, 5–9, <http://journals.sabinet.co.za/sajchem/>. 6 Scheme 1 Synthesis of 2-(1,3-benzoxazol/benzothiozol-2-yl)-3H-benzo[f]chromen-3-ones 3a-p. addition, all the synthesized compounds were confirmed by their spectral data. Various percentages of choline chloride/urea were employed in order to find out the effective concentration of the catalyst. In our observations 50 mol% of the choline chloride/urea was sufficient for the synthesis of 2-(1,3-benzoxazol/benzothiazol-2-yl)-3H-benzo[f]chromen-3-ones 3a–p with high yields. The increase in the amount of choline chloride/urea to 70 mol%, 90 mol% and 100 mol%, made only marginal differences in the yield and the reaction time. Hence, 50 mol%, choline chloride/urea was used in the synthesis of all compounds 3a–p. Further, the catalyst was recovered by removing water under reduced pressure and reused three times without any reduction in catalytic activity was noted. It was also observed that the electron-withdrawing group present in benzothiazole-2-yl-acetates and N-methyl benzoxazole-2-yl-acetates gave low yields, whereas electron-donating groups yielded more than 90 % with high purity of 2-(1,3-benzoxazol/benzothiazol-2-yl)-3H-benzo [f]chromen-3-ones 3a–p (Table 1). 2.1. Fluorometric Properties Absorption spectra of 2-(1,3-benzoxazol/benzothiazol-2-yl)3H-benzo[f]chromen-3-ones 3a–p were recorded in chloroform and the results of fluorescence studies were expressed in terms of Stoke’s shift and quantum yields as shown in Table 1. Zhou et al.8 reported that the fluorescent properties of 3-(2’-benzothiazolyl) coumarins, having electron donor substituents on the 7-position, resulted in red shif (...truncated)