Synthesis and Tunability of Highly Electron-Accepting, N‑Benzylated “Phosphaviologens”

We report a structure–property
study on phosphoryl-bridged
viologen analogues with a remarkably low reduction threshold. Utilizing
different benzyl groups for N-quaternization, we were able to confirm
the p-benzyl substituent effect on the electronic
tunability of the system while maintaining the characteristic chromic
response of viologens with two fully reversible one-electron reductions.
Due to the considerably increased electron-acceptor properties of
the phosphoryl-bridged bipyridine precursor, N-benzylation was found
to be very challenging and required the development of new synthetic
strategies toward the target viologen species. This study also introduces
a new and convenient way for the anion exchange of viologen systems
by utilizing methyl triflate. Finally, the practical utility of the
new species was verified in simplified proof-of-concept electrochromic
devices.