Towards eumelanin@zeolite hybrids : pore-size-controlled 5,6-dihydroxyindole polymerization

5,6-Dihydroxyindole (1) and its N-methyl derivative (2), key eumelanin building blocks, were inserted into zeolite L by sublimation at 175 C for 5 days. At a 10 mg/300 mg indole/zeolite ratio, the resulting hybrids displayed a stable deep red coloration. CP/MAS 13C NMR and UV/Vis spectroscopy of the red species suggested the generation and accommodation of quinonoid biindole derivative(s) within the void space of the acidic zeolite channels. Removal of the zeolite matrix by treatment with HF gave a stable species that could be separated by HPLC and characterized by mass spectrometry as an oxygenated biindole derivative (or a mixture of isomers), suggesting addition of water to the original dimer and subsequent re-oxidation. The characterization was corroborated by optimized molecular geometries and simulated UV spectra with density functional calculations. Loading 1 or 2 into the larger pores of SBA-15 type mesoporous silica resulted in black eumelanin-type polymers, confirming channel size dependence over the polymerization process. Confinement in nanometer-sized channels of zeolite L allowed the formation and stabilization of the quinonoid 5,6-dihydroxyindole dimer(s) as a precursor step to synthetic eumelanin-like polymers. The entrapped species were characterized by an integrated spectroscopic and computational approach. In mesoporous silica, the larger diameter of the channels determines the formation of black eumelanin-type materials.