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| - Photophysics and Electrochemistry of Conjugated OligothiophenesPrepared by Using Azomethine Connections
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| - Novel conjugated azomethines consisting of 1 to 5 thiophenes and up to 4 azomethine bonds preparedfrom a stable diaminothiophene are presented. The effect of the number of thiophene and azomethinesbonds on the photophysics and electrochemistry was examined. A high degree of conjugation wasconfirmed 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 resultedin 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 acidconditions. The resulting heteroatomic covalent bonds are furthermore reductively and hydrolyticallyresistant. Increasing the degree of conjugation shifts the nonradiative mode of singlet excited state energydissipation from internal conversion (IC) to intersystem crossing (ISC). The resulting triplet manifoldproduced by ISC was efficiently deactivated by intramolecular self-quenching from the azomethine bondleading to a nonemissive triplet. Cyclic voltammetry revealed unprecedented reversible radical cationformation of the azomethines. Both one-electron oxidations and reductions were found by electrochemicalmeasurements demonstrating the azomethines' capacity to be mutually p- and n-doped. One of theazomethines exhibited reversible electrochromic behavior with the electrochemically generated radicalcation absorbing in the NIR at 1630 and 792 nm. X-ray crystallography confirmed the thermodynamicallystable E isomer was formed uniquely and that the thiophenes are coplanar adopting an antiparallelarrangement.
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