Improved one-pot synthetic conditions for synthesis of functionalized fluorescent coumarin-thiophene hybrids: Syntheses, DFT studies, photophysical and thermal properties

We have synthesized coumarin-thiophene hybrids using stepwise and one-pot methods under MWI and CM conditions, and then report the findings and conclusions: one-pot is better than stepwise in terms of high yields and fastness of getting products, and MWI is better than CM in terms of purity of compounds, better yields, and reaction period. Even though both protocols were found to be efficient, the microwave-assisted irradiation reactions were identified to be better, environmentally friendly, cost-effective, mild, more efficient, less time consuming but with high purity. The two protocols were employed for the synthesis of the target coumarin-thiophene hybrids via the Gewald reaction in stepwise and in one-pot three-component. The dyes 3a-j were prepared in good to excellent yields. Although, we recorded better yields via the stepwise protocol (80-96%), for the synthesis of the target molecules, as compared to the one-pot (82-90%), the stepwise condition is fraught with many changes such as long reaction conditions, extra use of solvents for purification of intermediates, and long reaction durations. Photophysical activities of all the target derivatives were investigated using a combination of UV vis and fluorescence spectroscopies in different solvents. The umbelliferon-like derivative 3g was further investigated for possible use as a pH sensor via deprotonation (using TBAOH) and reverse protonation (using TFA), and was found to be applicable as pH sensor both in organic medium (DMSO) and partial aqueous binary (DMSO/H2O, viv, 9:1) medium. Again, the ground and the excited state properties of the umbelliferon-like derivative 3g and its deprotonated form were also studied using the quantum chemical calculations. Finally, the thermal properties of all the target compounds were investigated using TGA in order to test their potency and applicability as optical dyes. All the dyes 3al have high thermal stability therefore they can be applicable as optical dyes. (C) 2019 Elsevier Ltd. All rights reserved.