Eumelanin pigment precursor 2-carboxy-5,6-dihydroxyindole and 2-amino-6-methylbenzothiazole chromophore integration towards melanin inspired chemoresponsive materials: the case of the Zn2+ ion

Abstract

The 2-amino-6-methylbenzothiazole chromophore is introduced at the carboxyl group of the melanin precursor 2-carboxy-5,6-dihydroxyindole achieving a novel dihydroxyindole derivative with metal chelation properties not depending on the catechol moiety. In view of potential exploitation in charge storage systems, systematic investigation of the interaction of the new amide derivative with metal ions is carried out, in comparison with that of the parent 2-carboxy-5,6-dihydroxyindole, and the stoichiometry of the zinc-amide complex is determined.

Fig. 1. (A) Formation of DHICA-ABtz amide adduct. (B) Coupling agents investigated.

Fig. 2. Absorption intensity changes of 3 (68.5 μM) upon addition of Zn²⁺ (0–2.0 equiv.: 0 (pink) – 0.25 (brown) – 1.0 (turquoise) – 1.25 (purple) – 1.5 (blue) – 1.75 (red) – 2.0 (black)) in 2.0 mL of acetonitrile solution. Inset: variation of absorption at 362 nm.

Fig. 3. Structures of two amides containing respectively the acid (4) and amino (5) precursors of 3.

Fig. 4. Fluorescence intensity of 3 68.5 μM in acetonitrile (λ_ex = 325 nm) after the addition of different metal ions (n equiv. = 0.75). Inset: fluorescent changes of 3 (left) upon the addition of Zn²⁺ ions (equiv. = 0.75) (right).

Fig. 5. Fluorescence intensity changes of 3 (68.5 μM) upon addition of Zn²⁺ (0–2.0 equiv.: 0 (pink) – 0.25 (turquoise) – 1.0 (purple) – 1.25 (red) – 1.5 (teal) – 1.75 (brown) – 2.0 (black)) in 2 mL of acetonitrile solution. Inset: changes of emission intensity at 417 nm. λ_ex = 325 nm.

Fig. 6. Absorbance of a thin film of polymerized 3 at 387 nm (left axis, black squares) as prepared (cycle 0) and after alternating expositions to Zn²⁺ and EDTA solutions. Ratios of the absorbance at 387/359 nm (right axis, blue dots).