Fluorescent pyridopyrimidine fused pyranones - design, synthesis, fluorescent whitening and DFT studies

Four novel fluorescent whitening agents based on 4-methyl-2H,5H-pyrano derivatives of 2H-pyrido[1,2- a ]pyrimidine-2,4(3H)-dione were synthesized using a simple two step route. The structures were characterized by spectroscopy techniques such as, 1 H, 13 C NMR, and HRMS. Photophysical properties in various microenvironments were evaluated experimentally and correlated with optical absorption and fluorescence emission using global hybrid (B3LYP) and long range separated functional (CAM-B3LYP) with the Pople's 6–31G(d) basis set and in combination with polarizable continuum model. The experimental and theoretical photophysical properties were found in good agreement with each other. The excited state calculations revealed that the excitation values were in the range of 350–370 nm and emission values were in the range of 400–425 nm suggesting their potential application as fluorescent whitening agents. Dyes 4a-d were applied to polyester fabric and their performance was evaluated by means of measuring the degree of whiteness, the degree of brightness, color values, and reflectance spectrum in comparison with a standard fluorescent whitening agent, Hostalux ESR. Interestingly our synthesized optical whitener, 4a showed pronounced enhancement in the whitening properties over the Hostalux ESR.