Synthesis, structural characterization and chromatic features of new 2-phenyl-1-benzopyrylium and 2-phenyl-styryl-1-benzopyrylium amino-based blue dyes

Six new amino-based flavylium dyes (5,7OH(2)4'NMe2, 5,7OH(2)st4'NMe2, 7NEt(2)4'NMe2, 7NEt(2)st4'NMe2, 7NEt(2)4'NH2 and 7NEt(2)4'OH) were synthesized through acidic aldolic condensation reaction between 2,4,6-trihydroxybenzaldehyde or 4-diethylaminosalysaldehyde with different acetophenones (4-amino-, 4-dimethylamino-, 4-hydroxyl-acetophenone or p-dimethylstyrylmethylketone). The obtained amino-based dyes were structurally characterized by LC-MS and NMR analysis. It was demonstrated that the nature and position of the functional groups attached to the 2-phenyl-1-benzopyrylium core and the presence/absence of a 2-styryl linkage in the benzopyrylium moiety are responsible for a different electronic delocalization contributing to the different color displayed by each dye. Compounds presenting a 2-styryl linkage (5,7OH(2)st4'NMe2 and 7NEt(2)st4'NMe2) have a higher maximum absorption wavelength (636 nm and 675 nm, respectively), probably resulting from their extended electronic delocalization. On the other hand, the presence of stronger electron donating groups such as dimethyl- or diethyl-amino leads to a bathochromic shift in the maximum absorption wavelength of dyes when compared to the ones presenting hydroxyl groups or a primary amine. (C) 2021 Elsevier Ltd. All rights reserved.