Photophysical, Crystallographic, and Electrochemical Characterization of Symmetric and Unsymmetric Self-Assembled Conjugated Thiopheno Azomethines

Novel conjugated azomethines consisting uniquely of thiophene units are presented. The highly conjugated
compounds were synthesized by simple condensation of a stable diamino thiophene (2) with its
complementary thiophene aldehydes. These interesting nitrogen-containing thiophene units exhibit variable
reactivity leading to controlled aldehyde addition. Because of the different amino reactivity, a one-pot
synthesis of unsymmetric and symmetric conjugated azomethines with varying number of thiophene
units was possible by judicious choice of solvent and careful control of reagent stoichiometry. The resulting
covalent 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 crystallographic
studies. These also demonstrated that the azomethine bonds and the thiophene units are highly planar
and linear. The fluorescence and phosphorescence of the thiopheno azomethines measured are similar to
those of thiophene analogues currently used in functional devices, but with the advantage of low triplet
formation and band-gaps as low as 1.9 eV. The time-resolved and steady-state temperature-dependent
photophysics revealed the thiopheno azomethines do not populate extensively their triplet manifold by
intersystem crossing. Rather, their excited-state energy is dissipated predominantly by nonradiative means
of internal conversion. Quasi-reversible electrochemical radical cation formation of the thiophene units
was found. These compounds further undergo electrochemically induced oxidative cross-coupling, resulting
in conjugated products that also exhibit reversible radical cation formation.