Microwave-assisted potassium fluoride-catalyzed synthesis of coumarin derivatives via tandem Knoevenagel condensation and intramolecular cyclization was established under solvent-free conditions. Satisfactory product yields were obtained with 67.87 and 90.21% as the highest recorded yields for the target compounds 3a (ethyl 2-oxo-2H-chromen-3-carboxylate) and 3b (ethyl 2-oxo-2H-benzo[f]chromen-3-carboxylate), respectively. The changes in the optoelectronic properties of the synthesized coumarin derivatives relative to the parent coumarin (2H-chromen- 2-one) molecule were also characterized to study, to some degree, the structure-to-property relationship of the compounds. Generally, extending the π-conjugation at position-3 by adding the ethoxycarbonyl group and expanding the π-system of the coumarin molecule at position-f shifts the absorption maxima to longer wavelengths, decreases the HOMO-LUMO energy band gap, and increases the light absorptivity of the coumarin compound.