Room Temperature Phosphorescence in Crystalline Iodinated Eumelanin Monomer

We report the room temperature phosphorescence upon iodination on a crystalline eumelanin monomer with shielded hydroxyl moieties, ethyl 5,6-dimethoxyindole-2-carboxylate (DMICE). Ultrafast intersystem crossing (ISC) is observed in the iodinated (IDMICE) as well as brominated (BDMICE) analogues of the eumelanin monomer derivative in solution. The triplet quantum yields (φ_T) and intersystem crossing rates (k_ISC) of the halogenated eumelanin derivatives are ${\phi }_T^{BDMICE}$ =25.4±1.1 %; $k_{ISC}^{BDMICE}$ =1.95×10⁹ s^-1 and ${\phi }_T^{IDMICE}$ =59.1±1.6 %; $k_{ISC}^{IDMICE}=$ 1.36×10¹⁰ s^-1, as monitored using transient absorption spectroscopy. Theoretical calculations based on nuclear ensemble method reveal that computed k_ISC and spin-orbit coupling matrix elements for eumelanin derivatives are larger for IDMICE relative to BDMICE. The halogen and π-π interactions, with distinct excitonic coupling and higher ISC rate promote phosphorescence in IDMICE molecular crystals. Accessing triplet excited states and resultant photoluminescence through structural modification of eumelanin scaffolds paves way for exploring the versatility of eumelanin-inspired molecules as bio-functional materials.