Tunable spectroscopic and electrochemical properties of conjugated push–push, push–pull and pull–pull thiopheno azomethines

Abstract

Novel azomethines consisting uniquely of thiophene units were examined. The highly conjugated compounds were prepared by condensing air stable aminothiophenes with 2-thiophene aldehydes, which were substituted with various electronic groups. The resulting azomethines are highly conjugated and are both reductively and hydrolytically resistant. Various electron donating and accepting groups placed in the 2-position of 5-thiophene carboxaldehyde lead to electronically delocalized push–push, pull–pull, and push–pullazomethines. These electronic groups affect both the HOMO and the LUMO levels, which influence the absorption and emission spectra. Colors spanning the entire visible spectrum ranging from yellow to blue are possible with these nitrogen containing conjugated compounds. Excited state deactivation of the singlet excited state occurs predominately by internal conversion while only a small amount of energy is dissipated by intersystem crossing to the triplet state and by fluorescence. The ensuing fluorescence and phosphorescence of the thiopheno azomethines are similar to those of their thiophene analogues currently used in functional devices, but with the advantage of a low triplet state and tunable HOMO–LUMO energy levels extending from 3.0 to 1.9 eV. Quasi-reversible electrochemical radical cation formation is possible while the oxidation potential is dependent on the nature of the electronic group appended to the thiophene. The crystallographic data of the electronic push–push system show the azomethine bonds are planar and linear and they adopt the E isomer.