DFT characterization of the optical and redox properties of natural pigments relevant to dye-sensitized solar cells

Theoretical studies of the betalains (betanidin, betanin and geomphrenin) have been performed at the B3LYP/6-31G(d) level. As dye-sensitizers in DSSCs, only little is known about their molecular properties relevant to the electron injection process. Here, ground state geometries, electronic transition energies, oxidation potentials, and proton affinities are reported. The HOMO → LUMO transition describes all lowest singlet excited states. The ground and excited state oxidation potentials are calculated to be 1.0 V and −1.3 V (vs. NHE), respectively. Deprotonation order is determined by calculating proton affinities at different sites. The vibrational–electronic coupling and its role in the electron transfer process are analyzed.