Novel conjugated azomethines consisting uniquely of thiophene units are presented. The highly conjugatedcompounds were synthesized by simple condensation of a stable diamino thiophene (
2) with itscomplementary thiophene aldehydes. These interesting nitrogen-containing thiophene units exhibit variablereactivity leading to controlled aldehyde addition. Because of the different amino reactivity, a one-potsynthesis of unsymmetric and symmetric conjugated azomethines with varying number of thiopheneunits was possible by judicious choice of solvent and careful control of reagent stoichiometry. The resultingcovalent conjugated connections are both reductively and hydrolytically resistant. The thermodynamically
E isomer is formed uniquely for all of the azomethines synthesized, which is confirmed by crystallographicstudies. These also demonstrated that the azomethine bonds and the thiophene units are highly planarand linear. The fluorescence and phosphorescence of the thiopheno azomethines measured are similar tothose of thiophene analogues currently used in functional devices, but with the advantage of low tripletformation and band-gaps as low as 1.9 eV. The time-resolved and steady-state temperature-dependentphotophysics revealed the thiopheno azomethines do not populate extensively their triplet manifold byintersystem crossing. Rather, their excited-state energy is dissipated predominantly by nonradiative meansof internal conversion. Quasi-reversible electrochemical radical cation formation of the thiophene unitswas found. These compounds further undergo electrochemically induced oxidative cross-coupling, resultingin conjugated products that also exhibit reversible radical cation formation.
