Thermodynamic and Kinetic Processes of Anthocyanins and Related Compounds and their Bio-Inspired Applications

This chapter revisits the reversible pH-dependent chemistry of anthocyanins and related compounds, proposing a step-by-step procedure by which to calculate all the rate and equilibrium constants of their respective networks of chemical reactions. Self-aggregation dramatically changes the values of the rate and equilibrium constants of a network. The equilibrium constant (K-a) increases and the hydration (K-h) and global (K-a') constants decrease with increasing anthocyanin concentration. The hydration rate constant (k(h)) decreases as the dehydration rate constant (k(-h)) increases. Evidence for the formation of flavylium cation dimers aggregating according to a left-handed helix has been obtained from circular dichroism, while indirect evidence for a stronger aggregation in quinoidal base than in flavylium cation has been found. The chapter discusses the photochromism of synthetic flavylium compounds lacking or possessing a high cis-trans isomerization barrier; in the latter case, the system illustrates the concept of an optical memory capable of write-lock-read-unlock-erase. Extended conjugation in flavylium-derived compounds such as styryl-1-benzopyrylium and naphtoflavylium permits access to photochromic systems that present new switching colors. Dye-sensitized solar cells based on anthocyanins and synthetic flavylium analogs show the potential of flavylium compounds in practical devices. Two final sections explain in detail the steps required to construct the energy-level diagram of the network and the mole-fraction distribution of the respective species in solution.