Abstract
Flavylium ions (6a–6e) were synthesized using Aldol condensation to compare the spectroscopic properties caused by the different numbers and locations of hydroxy groups on the flavylium cations (AH+). Without the addition of acid, increasing the concentration of flavylium ions to 10 mM in ethanol led to the following observation. The flavylium ions with the least number of OH groups (6a and 6b) showed a change in peak at higher concentrations, whereas 6c and 6d did not show the same degree of change in their 1H-NMR signals. This suggests an equilibrium that shifts the isomers B, CE, and Cz more towards the flavylium ion with more OH groups even at low concentrations. One possibility for the cause of this shift is that the flavylium ions become more stable through aggregation. In addition to the NMR results, the UV measurements confirmed that isomers with fewer OH groups showed a more dramatic shift towards the flavylium ion at higher concentrations. Using modeling data at DFT//B3LYP/6311**G(d) level, the self-association was investigated to show that the hydrogen bonding of OH groups is the main player but cannot stabilize entirely without the presence of the chloride ion in the complex.
Highlights
The flavylium cation ring (AH+) in anthocyanidin is arguably the most versatile natural dye molecule [1–18]
The optical property from change-responsive absorption and emissions within the visible range of the colored flavylium ion ring allows for light-harvesting applications such as dye-sensitized solar cells (DSSCs) and recent medical applications
Since the self-association property in aqueous solution has been reported as the key to lowering the potential energy in aggregation, we first aimed to systematically understand the effect of the number and position of hydroxy groups on flavylium ions without glucoside moiety at different concentrations, especially in a non-aqueous solution, ethanol
Summary
The flavylium cation ring (AH+) in anthocyanidin is arguably the most versatile natural dye molecule [1–18]. Since the self-association property in aqueous solution has been reported as the key to lowering the potential energy in aggregation, we first aimed to systematically understand the effect of the number and position of hydroxy groups on flavylium ions without glucoside moiety at different concentrations, especially in a non-aqueous solution, ethanol. It is usually studied in the following conditions, the application of the flavylium-containing dyes should not be limited to an aqueous solution or acidic condition. The effect of hydroxy groups on their molecular orbitals and self-assembly modes were correlated with modeling studies using the Gaussian 95 package (Wallingford, CT, USA) at DFT//B3LYP/6311**G(d) level
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