Investigation of Dopamine Analogues: Synthesis, Mechanistic Understanding, and Structure-Property Relationship.

Dopamine, perhaps the simplest molecule that covalently links catechol and amine, together with its derivatives, has shown impressive adhesive and coating properties with its polymers. However, the scope of the molecules is rather limited, and the polymerization mechanisms are still elusive. We designed a general synthetic scheme and successfully synthesized a series of dopamine analogues with different alkyl chain lengths between the catechol and amine. Taking these new dopamine analogues, together with the molecular systems that have separate catechol and alkyl amine, we show that having both catechol and amine in the molecular system, whether covalently linked via an alkyl chain or not, is sufficient to polymerize under a similar reaction condition to that of dopamine polymerization. However, the time-dependent UV-vis characterization of the individual polymerization indicates that the polymerization for individual molecular systems likely proceeds via different reaction intermediates, depending on the length of the alkyl chain and whether there is a covalent linkage. Interestingly, whereas the covalent linkage via an alkyl chain is not necessary for showing the adhesive property, it is required to achieve the impressive coating property. Our results offer new insights into the design and synthesis of dopamine analogues for future applications, as well as a further mechanistic understanding of the polymerization of these dopamine analogues.