Control of molecular geometries using new photo-electro-switchable azobenzenes

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University of Alabama Libraries

There has been a recent growing interest in azobenzene derivatives because of their ability to isomerize when visible light is absorbed. This unique property allows azobenzenes to undergo reversible conformation change between the trans and cis forms upon absorbing light and slowly thermally relaxing from the cis to the trans form. By adding a redox-active (RA) group to the azobenzene skeleton, we have developed a series of new redox appended azo switches capable of efficient photo “folding” and catalytic electron transfer mediated “opening” on exposure to an oxidant or a small amount of voltage as an electrical trigger. We utilized an aryl amino group as our redox auxiliary moiety because of its excellent redox properties- it is chemically stable in both neutral and radical cation forms. The basic premise of our work is that the cationic state of the RA-azo will be much more reactive toward azo isomerization (cis-to-trans) than its neutral form. Thus, upon one electron removal from the cis-RA-azo to make cis-RA•+-azo, the azo group will immediately unfold. How well the redox activation of azo unfolding works may depend on where the RA group is located. Thus, we have prepared RA-azo substrates with the RA group linked at the para and meta positions of the azobenzene ring to test their relative abilities to support redox activation of azo unfolding. Because the oxidized radical cation state of the RA group (RA•+) is so chemically stable, it should be able to exchange an electron with other neutral cis-RA-azo molecules thereby generating a catalytic cycle for azo unfolding. Based on this study, we hypothesized a new photo-electro isomerization catalysis mechanism that accounts for the fast relaxation from “folded” cis to “open” trans. To date, we have successfully synthesized six different RA-azo in which the placement of the RA moiety is varied on the azobenzene ring and we observed that redox activation of the RA-azo unfolding process works catalytically with rate increases relative to thermal azobenzene unfolding of ≥ 105 and turnover numbers in the 100-1000 range.

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Chemistry, Organic chemistry, Physical chemistry