Photophysics and Electrochemistry of Conjugated Oligothiophenes Prepared by Using Azomethine Connections

Novel conjugated azomethines consisting of 1 to 5 thiophenes and up to 4 azomethine bonds prepared from a stable diaminothiophene are presented. The effect of the number of thiophene and azomethines bonds on the photophysics and electrochemistry was examined. A high degree of conjugation was confirmed by bathochromic shifts upward of 120 and 210 nm for the absorbance and fluorescence, respectively, relative to the diaminothiophene precursor. Acid doping with methanesulfonic acid resulted in further bathochromic shifts along with lowering of the HOMO-LUMO energy gaps to 1.3 eV. Moreover, the compounds are extremely stable as evidenced by the absence of decomposition products under acid conditions. The resulting heteroatomic covalent bonds are furthermore reductively and hydrolytically resistant. Increasing the degree of conjugation shifts the nonradiative mode of singlet excited state energy dissipation from internal conversion (IC) to intersystem crossing (ISC). The resulting triplet manifold produced by ISC was efficiently deactivated by intramolecular self-quenching from the azomethine bond leading to a nonemissive triplet. Cyclic voltammetry revealed unprecedented reversible radical cation formation of the azomethines. Both one-electron oxidations and reductions were found by electrochemical measurements demonstrating the azomethines' capacity to be mutually p- and n-doped. One of the azomethines exhibited reversible electrochromic behavior with the electrochemically generated radical cation absorbing in the NIR at 1630 and 792 nm. X-ray crystallography confirmed the thermodynamically stable E isomer was formed uniquely and that the thiophenes are coplanar adopting an antiparallel arrangement.