Synthesis of nonsymmetrically N,N′-diaryi-substituted 4,4′-bipyridinium salts with redox-tunable and titanium dioxide (TiO2)-anchoring properties

A general method for the synthesis of so far unknown nonsymmetrically substituted N-aryl-N'-aryl'-4,4'bipyridinium salts is presented (Scheme 1). The common intermediate in all procedures is N-(2,4-dinitrophenyl)-4,4'-bipyridinium hexafluorophosphate (1 - PF6-) For the synthesis of nonsymmetric arylviologens, 6 1(.)PF(6)(-) was arenamine-exchanged by the Zincke reaction, and then activated at the second bipyridine N-atom with 2,4-dinitrophenyl 4-methylbenzenesulfonate. The detailed preparation of the six N-aryl-N'-aryl'-viologens 21 -26 is discussed (Scheme 2). The generality of the procedure is further exemplified by the synthesis of two nonsymmetrically substituted N-aryl-N'-benzyl- (see 11 and 12), and seven N-aryl-N'-alkyl-4,4'-bipyridinium salts (see 28-34) including substituents with metal oxide anchoring and redox tuning properties. The need for these compounds and their usage as electrochromic materials, in dendrimer synthesis, in molecular electronics, and in tunable-redox mediators is briefly discussed. The latter adjustable property is demonstrated by the reduction potential measured by cyclic voltammetry on selected compounds (Table).