Novel Conjugated s-Tetrazine Derivatives Bearing a 4H-1,2,4-Triazole Scaffold: Synthesis and Luminescent Properties

Abstract

A series of new symmetrical s-tetrazine derivatives, coupled via a 1,4-phenylene linkage with a 4H-1,2,4-triazole ring, were obtained. The combination of these two rings in an extensively coupled system has significant potential applications, mainly in optoelectronics. The methodology used turned out to be useful regardless of the type of five-membered ring or the nature of the individual substituents. All the products were identified by spectroscopic methods, and the target compounds were tested for luminescent properties. This study showed that all the synthesized highly-conjugated triazoles exhibited luminescence; in particular, one derivative, 3,6-bis(4-(5-(4-methoxyphenyl)-4-phenyl-4H-1,2,4-triazol-3-yl)phenyl)-1,2,4,5-tetrazine (13b), showed strong fluorescence emission and ahigh quantum yield close to 1.

Synthesis and Luminescent Properties of s-Tetrazine Derivatives Conjugated with the 4H-1,2,4-Triazole Ring

“…The quantum yield (Φ) is directly related to the fluorescence intensity for the studied compounds ( Figure S66 ). Generally, the compounds with Ph as R 2 exhibit higher Φs than those with n -Bu, which is in agreement with previous findings [ 26 ]. However, most of the studied compounds are not efficient fluorescent materials, because their Φs do not exceed 0.3 ( Table 3 ).…”

“…The location of emission maximum (excitation wavelength—λ ex and emission wavelength—λ em ) is dependent on substituents R 1 , R 2 , and R 3 , which indicates that both tetrazine and triazole rings are involved in the orbitals from which the excitation occurs. The influence of substituents on λ ex and λ em is the same as in previously reported symmetrically substituted analogs of the 10a – l series [ 26 ]. The R 2 affects the λ ex , whereas R 1 and R 3 affect the λ em .…”

“…This prompted us to change the reaction path by synthesizing other imidoyl chlorides ( 8a – h ) from the corresponding amides. These intermediates were treated with hydrazide ( 3 ), resulting in the target precursors ( 6a – h ) in satisfactory yields [ 26 ].…”

“…Under the conditions of the Pinner method, the treatment of the precursors 6a – h with hydrazine hydrate, in the presence of an activating agent, leads to the formation of unoxidized derivatives of the assumed products 9a – l . One of the popular activating agents is sulfur, with the help of which we have successfully obtained symmetrical s -tetrazine derivatives connected via a 1,4-phenylene linker with a 4 H -1,2,4-triazole ring, and extended systems containing 1,3,4-oxadiazole and 1,3,4-thiadiazole cores [ 24 , 25 , 26 ]. Therefore, we also began to research the synthesis of unsymmetrical compounds with the use of this methodology, which allowed us to obtain the product 10a with a yield of 42% (Entry 1, Table 1 ).…”

“…Our research to date proves that, among its other uses, it is perfect for the preparation of complex conjugated systems that contain additional five-membered rings. In recent years, we have successfully synthesized s -tetrazine conjugated via a 1,4-phenylene linker with a range of 1,3,4-oxadiazoles, 1,3,4-thiadiazoles and 4 H -1,2,4-triazoles; however, in the latter case, we have so far only obtained symmetrical systems [ 24 , 25 , 26 ]. In a continuation of our research, we decided to use the Pinner method to prepare unsymmetrical ones.…”