Extensive stacking of DHI-like monomers as a model of out-of-plane complexity in eumelanin protomolecules: Chemical and structural sensitivity of optical absorption spectra

In this work, we study the optical absorption spectra of extensive stacking forms of four molecules known to be among eumelanin basic building blocks: 5,6-dihydroxyindole (DHI), 5,6-dihydroxyindole-2-carboxylic acid (DHICA), indolequinone (IQ) and quinone-methide (MQ). Stacked monomers can be considered as a minimal model of out-of-plane complexity in eumelanin, as complementary to in-plane oligomerization. The choice of a plane wave density functional theory (DFT) approach allows us to address extensive, i.e. several-layer, stacking of these DHI-like monomers at a relatively low computational cost, by treating stacked systems as periodic along the stacking direction. Absorption spectra of stacked monomers display interesting trends in terms of chemical and structural sensitivity, which can shed light on the role of extensive stacking in the transition from the optical properties of small molecules such as DHI to the typical broadband and monotonic absorption spectrum of the eumelanin pigment.