Polydopamine (PDA) is a black, biomimetic polymer synthesised through the self-polymerisation of dopamine. PDA possesses desirable physicochemical properties, including high biocompatibility, electrical conductivity, and antimicrobial activity, as well as a unique ability to form functional coatings independent of the substrate's surface chemistry. However, its polymeric structure and formation mechanism have puzzled the scientific community since 20071 due to its notorious insolubility in conventional aqueous and organic solvents.
We recently discovered that PDA can be solvated in select ionic liquids.2–4 Following this, we used solution-phase 1H NMR spectroscopy to finally elucidate the polymerisation mechanism of dopamine5 – a significant breakthrough in the field of materials science. In this work, we determined that PDA’s structure was dominated by aggregates consisting of π-stacked cyclised dopamine monomers and dimers. However, we were unable to rule out the formation of higher-order oligomers. A fundamental question also remained unanswered: the origin of PDA’s monotonic broadband absorption across the UV-Visible spectrum.
To address this, we monitored dopamine polymerisation in 1-ethyl-3-methylimidazolium acetate ([C₂mim][OAc]) using time-resolved UV–Visible spectrophotometry. Furthermore, vertical excitation energies for proposed intermediate species were calculated using time-dependent density functional theory (TD-DFT). We developed an analytical framework that reconstructs absorption spectra by representing TD-DFT vertical excitations as Gaussian functions and fitting their weighted sum to the experimental spectra, enabling progressive modelling of oligomer formation and stacking during polymerisation.
Our results demonstrate that PDA’s monotonic broadband absorption arises from the broad configurational and electronic landscape formed by π-stacked cyclised catecholamine oligomers, including monomers, dimers, and trimers spanning multiple oxidation states. Overlapping low-intensity electronic transitions across structurally diverse stacked assemblies lead to the characteristic broadband absorption spectrum. These findings link PDA’s molecular structure to its optical properties and establish a transferable modelling strategy for other structurally complex catecholamine-derived biopolymers, including eumelanin.