Tuning properties of 3,6-disubstituted-s-tetrazine by changing the chemical nature of substituents

Organic electroactive compounds fulfil high requirements such as low production cost and flexibility. Organic molecules with electron withdrawing character are smaller in number than those of electron-rich compounds. One such material is Tetrazine, a six – membered ring with four nitrogen atoms and possibly having three isomers. This material acts as building block for n-type and ambipolar materials due to its properties like high electron affinity, being electroactive and reversible electrochemical reduction at very high potential. They are also used in optoelectronics since they possess property of fluorescence.

Characterization of six 3,6-distributed-s-tetrazine is done both electrochemically and spectroelectrochemically. This process later evolved into various processes like examination of effect of halogen when replaced with ethoxy and phenoxy group, investigating the influence of heterocyclic substituents and the analysis of reduction process of compound with two s-tetrazine rings. This reduction process in two s-tetrazine rings is a two-step process which involves formation of two radicals with different g-factors, one localizing on one s-tetrazine ring and the other localizing on another ring in first and second step of reduction respectively.

Reduction of s-tetrazine electrochemically leads to anion-radical generation. A study was proposed Sandra Pluczyk and colleagues to reveal the electrochemical behavior of the compound with two s-tetrazine rings. The research is now published in peer-reviewed journal, Electrochimica Acta.

Their studies showed that the compounds were proved to be electroactive in the negative range of potential and exhibited quasi – reversible electrochemical reduction. The reduction potential of this compound highly depends on the type of 3, 6-substitution. It is found that 3, 6-dichloro-s-tetrazine shows high reduction potential whereas the derivative with two s-tetrazine rings have low reduction potential. Hence 3, 6-dichloro-s-tetrazine is easily reducible. There occurs a decrease in the reduction potential by replacing chlorine by either phenoxy or ethoxy substituent due to electron donating effect. This lead to an increase in the  electron density at the tetrazine ring thereby increasing the difficulty to reduce. This decrease in the reduction potential is also correlated with increase in the electron affinity.

On reducing the two s-tetrazine rings, two non-separated peaks occur which proves the fact that simultaneous reduction of these rings is not possible and hence two steps are involved. First electron is induced with strong localization causing disparition of their degeneracy. Injection of another electron into second tetrazine ring requires high energy. This step by step reduction leads to the formation of dianion radical of two s-tetrazine rings. This later interacts with the four nuclei of nitrogen atoms of s-tetrazine rings producing spectra. However, the spectra obtained is complex in case of halogen derivatives as the spin density due to reduction of the compound interacts with the chlorine nuclei.

The analysis done revealed the impact of substituents on the reduction potential along with electrochemical and spectroelectrochemical characteristics of six 3, 6-distributed-s-tetrazine derivatives. It was also inferred that the electrochemical reduction occurs at high reduction potential for 3,6-dichloro-s-tetrazine and at low potential for two s-tetrazine rings. Investigation done on two s-tetrazine rings showed the reduction process happening at two steps leading to the formation of two anion-radicals with different g-factors.