The impressive colors of the bis-aminostyrylbenzopyrylium system. Evidence by stopped-flow for the B4 formation at pH>13

Abstract

The kinetics and thermodynamics of the compound 7-diethylamino-2-(4′-dimethylaminostyryl)-1-benzopyrylium was studied in ethanol:water (1:1) and in microheterogenous media (SDS and CTAB micelles) and compared with its corresponding flavylium (7-diethylamino-4′-dimethylaminoflavylium) also known as 7-diethylamino-2-(4′-dimethylaminophenyl)-1-benzopyrylium. The introduction of a double bond between the benzopyrylium system and the phenyl group gives rise to vivid color changes resulting from the formation of violet protonated styrylbenzopyrylium cation (AH22+), green styrylbenzopyrylium cation (AH+), orange trans-chalcone (Ct) and magenta anionic trans-chalcone (Ct−). All species except protonated styrylbenzopyrylium cation are stable for long periods. In ethanol:water the pH domain of the styrylbenzopyrylium cation and trans-chalcone are respectively ΔpH = 4.5 and 4.4 and the anionic trans-chalcone for pH > 10.5. In the presence of SDS micelles, there is an increase of the pH domain of the styrylbenzopyrylium cation ΔpH = 5.9 at the expense of the trans-chalcones, ΔpH = 2.95, while in CTAB it is the trans-chalcone the dominant species, ΔpH = 6.2, with concomitant decreasing of the styrylbenzopyrylium cation pH range ΔpH = 1. This can be due to the electric charge present on the surface of each micelle, for instance the negative charge surface of SDS micelles stabilizes the positive charge of styrylbenzopyrylium cation and its protonated form, while the positive charge surface of CTAB micelles stabilizes the negative and neutral charge of the trans-chalcones species. The peculiar behavior of the kinetics in basic medium was investigated by stopped-flow. It was verified that in ethanol:water, SDS and CTAB micelles a transient species resulting from the OH− attack to position 4 of the styrylbenzopyrylium cation (B4) behaves as a kinetic product delaying the kinetic toward the equilibrium.