46833-AC7
1,3-Dipolar Cycloaddition for the Creation of Conjugated Polymers
We have investigated the click reaction, i.e. the copper catalyzed 1,3-dipolar cycloaddition of alkynes to azides, to generate novel chromophores. While our goal is to make conjugated polymers that are built up solely from 1,3-dipolar cycloaddition reactions, we have undertaken the necessary pre-studies in this grant period and investigated the synthesis of the simple cycloadducts from suitable alkynes and azides. The cycloadducts form in excellent yields and are colorless with an absorption maximum around 290 nm. A pyridine-adductis the most interesting one as it forms a natural binding pocket for metal cations that is composed from the basic nitrogen in the triazole ring and the pyridine nitrogen in the acceptor substituted arene. Exposure of 3 to either metal cations or to acid leads to a significant response and an increase of the fluorescence quantum yield for manganese, barium, zinc, and copper salts. In the case of magnesium salts or calcium salts a significantly attuned response is recorded.
The first protonation of the pyridine product happens at the pyridine center lone pair leading to a dramatic increase in fluorescence and a slight change in absorption. Upon addition of 400 or so equivalents of trifluoroacetic acid the fluorescence is somewhat red-shifted, while the emission displays a significant red shift from 300 to 330 nm. Upon addition of more acid, the fluorescence shifts from blue to green and the absorption changes again, now it is slightly blue shifted. One should note that the titration of the pyridine adduct with TFA produces changes in the absorption spectrum that are very similar to those observed for the addition of metal cations. However, upon addition of a large excess of TFA we observed the appearance of a second, blue-shifted absorption band, which we attribute to a species in which both the pyridine and the triazole are protonated. .
The lack of fluorescence some of the adducts is difficult to explain, as there is no easily visible mechanism for fluorescence quenching. The most probable reason would be charge transfer, leading to a charge separated excited state that would undergo radiation-less deactivation. Alternatively, the excited state could be coupled to a vibrationally excited S0, state experiencing effective quenching. Some 1,3-diploar cycloadducts are metallochromic and show large fluorescence turn-on in the case of divalent ions.
