Stabilizing a cis viologen via co-crystal engineering: electric and magnetic fields are in action to confirm no π-mer formation

Abstract

Stabilizing a cis isomer is challenging due to the higher energy barrier than the trans isomer. However, external stimuli can cross this energy barrier to achieve the cis isomer. We report conformationally modulated electrical conductivity in anthracene-substituted viologen dications (1,1′-disubstituted-4,4′-bipyridinium salts, V²⁺·2X⁻). The AnV²⁺ adopts either a trans or cis conformation, the latter stabilized through non-covalent interactions with dimethylamine (DMA). While solution-state AnV²⁺ exhibits rapid rotation about the methylene linker, single-crystal X-ray diffraction confirms distinct packing modes for cis and trans conformers. Two-terminal electronic devices, ITO/AnV²⁺/ITO prepared from respective crystal forms, exhibit dramatically different charge transport properties, with the trans-AnV²⁺ showing a 33-fold higher electrical conductivity, which was further supported by relatively higher activation energy needed for cis than trans isomer (129.4 meV vs. 102.5 meV). Computational studies corroborate the thermodynamic stability of the trans form in the absence of DMA, and periodic DFT reveals that DMA inclusion stabilizes the cis crystal. External and varied magnetic fields applied during electrical measurements further confirm that viologens remain in the native dicationic state. Our present study on tuning electrical conductivity in cis and trans viologens emphasizes the importance of solid-state molecular conformation in designing organic electronic materials for nanofabrication towards practical applications.