4,4 -bipyridinium anions

Figure 27 Schematic representation of the orientation of 4,4 -bipyridinium radical cations in (a)HV2+/AA and (b) AV2+/AA LB films. Counter anions (TFPB ) and AA are not shown for simplicity.

The electron spin resonance (ESR) spectra of the radical anion of 2,2 -bipyridine, sometimes in the form of its alkali metal com-plgx, 71.175,177.299-304 radical anion of 3,3 -bipyridine, ° and the radical anion of 4,4 -bipyridine, ° ° usually obtained by reduction of the bipyridines with an alkali metal, have been measured, and hyperfine splitting constants were assigned. Related biradical species have also been investigated. The ESR spectrum of the 4,4 -bipyridinium radical cation, of which... 

Figure 7.10 Electrons travel from the electrode to the ruthenium centers through the bipyridinium dications (a), when the gold voltage (V versus Ag/AgCl) matches the reduction potential of the bipyridinium dications, and return from the ruthenium centers through the electroactive anions to the electrode (b), when the gold voltage matches the oxidation potential of the iron centers.

The far IR spectra of the [Znd4]2- anion in salts with the 1,10-phenanthrolinium and 2,2-bipyridinium cations have been reported.1039 The anion in the former salt appears to be tetrahedral, while that in the latter is believed to be halogen-bridged. 

Amphiphilic binding also occurs for molecular anionic substrates [4.31, 4.32, 4.34]. Charged heterocyclic ring systems, such as those derived from the pyridinium group, represent an efficient way to introduce simultaneously electrostatic interactions, hydrophobic effects, structure, and rigidity into a molecular receptor in addition, they may be electroactive and photoactive. 4,4 -Bipyridinium groups have... 

Bifurcated hydrogen bonds between dipyridinium cations and coordinated dithiooxalates have been used to assemble tapes [130] with similar geometry to those in which coordinated halides act as acceptors [15]. Replacement of dipyridinium with 4-carboxypiperidinium gave a structure in which the anions are linked by carboxylic acid dimer assembled dications. Similar bifurcated hydrogen bonds were observed in the structures of bipyridinium salts of anilic acids [131]. 

Bipyridinium cation 9 [58], phosphonium [59-62], guanidinium [63], and vina-midinium [64] cations were used to form thermotropic ionic liquid crystals. For example, the mixture of diheptyl and dioctyl viologens 9 (20 80 by wt%) with TFSI anion forms a smectic A phase between 22° and 132°C. It was reported that the viologens show fluorescence in both non-polar and polar organic solvents, which can be useful for the development of biological and chemical sensors [65]. 

Typical examples are the CT ion pairs of bipyridinium di-cations (viologens, with metal (M = Zn, Cd, Hg) dithiolene (DT) anions [ 155], which form reduced viologen (V+ ) and oxidized metal dithiolene complex upon CT excitation in dime-thylsulfoxide, (Eq. 16) ... 

Specific examples of electron transfer studies made include a time resolved spectroscopic investigation of CT complexes of 2-naphthol with triethylamine in polar and non-polar solvents , fluorescence quenching of carbazole and indole by ethylene thiodicarbonate which forms ground state complexes , and the luminescent charge transfer complex of 4,4 -bipyridinium ion with tetrakis [3.5-bis(trifluoromethyl)phenyl]borate anion . 

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