Bipyridinium radical cation

In LB films not only the interaction of chromophores but also their orientation can be controlled at the molecular level. Molecular orientation of chromophores has been determined by several methods including polarized UV/vis or IR absorption, second harmonic generation (SHG), Electron Spin Resonance (ESR), or resonance Raman scattering. We have measured the incident angle and polarization angle dependencies of polarized UV/vis absorption to study the molecular orientation of alloxazine, porphyrin, and carbazolyl chromophores, or 4,4 -bipyridinium radical cations in LB films[3-12]. Usually in-plane components of transition dipoles of chromophores are... 

Upon irradiation of an IPCT band in degassed condition (Xex> 365 nm), the colour of both LB films changed from pale yellow to blue. The UV/vis absorption spectrum after irradiation is shown in Figure 25, which is characteristic of 4,4 -bipyridinium radical cation monomer[108]. Coloured species photogenerated in mixed LB films of AV2+/AA or HV2+/AA systems decayed almost exponentially in the dark in vacuo with a lifetime of about 4 h at 20 °C [93,94]. The lifetime of 4,4 -bipyridinium radical cations in LB films was almost... 

Figure 26 The p-polarized absorbance of 4,4,-bipyridinium radical cations in LB films at 400 ran after correction of the decay and optical path length for photoexdted (a) HV2+/AA and (b)AV2+/AA systems. The solid lines are calculated dependences.

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 fate of the bipyridinium radical cation in the mercaptan monolayer is not clear. The second reduction wave (Figure 3) possesses structure suggesting a precipitated phase similar sharp peaks are seen during the reductive precipitation of Cu.bpyMe2 ... 

Viologen salts act as one-electron phase-transfer agents and, in conjunction with sodium dithionite which regenerates the bipyridinium radical cation, they have been used for the debromination of 1,2-dibromoalkanes to yield alkenes in variable yields [13-15]. Nitroarenes are reduced to anilines in high yield (>90%) under similar conditions [16], whereas conjugated nitroalkenes are converted into the oximes of the saturated ketones [17] saturated aliphatic nitro compounds are not reduced by this process. 

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... 

Immediately upon excitation of an IPCT band with a fs laser at 400 nm, transient absorption was observed for both salts in solutions with a peak at about 600 nm, characteristic of 4,4/-bipyridinium radical cations. Figure 20 shows the transient absorption spectra of PV2+(I )2 in methanol solution. A marked increase in the absorbance of the 4,4/-bipyridinium radical cations took place within 1 ps after excitation. 4,4/-Bipyridinium radical cations were thus formed in a fs time scale by the photoinduced electron transfer from a donor I- to an acceptor 4,4/-bipyridinium upon IPCT excitation [48], The time profiles of transient absorption at 600 nm are shown in Fig. 21 for (a) PV2+(I )2 in a film cast from DME and (b) PV2+(TFPB )2 in DME solutions. Both of them showed a very rapid rise in about 0.3 ps, which was almost the same as the time resolution of our fs Ti sapphire laser measurement system with a regenerative amplifier. Similar extremely rapid formation of 4,4/-bipyridinium radical cations was observed for PV2+(I )2 salts in methanol and dimethylsulfoxide solutions upon IPCT excitation, respectively. These results demonstrated that the charge separated 4,4/-bipyridinium radical cations were formed directly upon IPCT excitation because of the nature of IPCT absorption bands (that the electrons correlated with the IPCT band are transferred partially at the ground state and completely at the excited state). Such a situation is very different from usual photochromism which is caused by various changes of chemical bonds mainly via the excited singlet state. No transient absorption was observed for PV2+(I )2... 

Artificial electron carriers are recognizable by the active sites of different redox enzymes and specifically biocatalysts containing Fe of Mo sulfur clusters as active sites. Bipyridinium radical cations, i.e. methyl viologen radical, MV+, exhibit proper electrical and size properties to penetrate into protein structures and to mediate reduction processes at the enzymes active sites. 

Hydrogenase, HyD, includes a Fe-S cluster as active site. Photogenerated N,A -dimethyl-4,4 -bipyridinium radical cation, MV+, mediates H2-evolution in the presence of HyD [201-203], see Fig. 40. Different photosensitizers such as Ru(bpy) +, Zn-TMePyP4+ or acridine orange and sacrificial electron donor, i.e. ethylenediamine tetraacetic acid, EDTA, mercaptoethanol or cysteine, have been... 

Fig. 40. Biocatalyzed photosynthetic systems mediated by an artificial electron carrier, JV,iV -dimethyl-4,4 -bipyridinium radical cation, MV+. Photogenerated MV+ mediates H2-evolution, C02 reduction to formate and sequential reduction of NO3 to ammonia, using Hyd, ForDH and a mixture of NitraR and NitriR, respectively...

C02-fixation to formate is catalyzed by formate dehydrogenase, ForDH. Photogenerated MV+ mediates the reduction of C02 to formate [200]. Other bipyridinium radicals, such as JV,j V -dimethyl-2,2 -bipyridinium or JV,Ar -trime-thylene-2,2 -bipyridinium radical cation act also as charge carriers for ForDH. The photosystem that Was utilized for generation of MV+ and 002-fixation includes Ru(bpy)f+ as photosensitizer, cysteine as sacrificial electron donor and MV2+ as electron acceptor. The net photosynthetic process accomplished in this photosystem (Fig. 40) corresponds to the reduction of 0O2 to formate by cysteine, see Eq. (70). This is an endoergic transformation by ca. 12.5 kcal mol-1. 

Ru(II)tris(bipyridine) [Ru(bpy)3 +] as a photosensitizer, triammonium ethylene-diaminetetraacetic acid [(NH4)3EDTA] as a sacrificial electron donor and the enzyme ferredoxin NADP+ reductase (FDR) [215, 216]. Oxidative electron-transfer quenching of the excited Ru(bpy)3 + yields the A,A -dimethyl-4,4 -bipyridinium radical cation (reduced methylviologen, MV+), which mediates the reduction of NADP+ in the presence of FDR as a biocatalyst (Figure 32A). The quantum efficiency for NADH production corresponds to = 1.9 x 10 . A related system that includes Zn(II)wc50-(A-tetramethylpyridinium)porphyrin (Zn-TMPyP +) as a photosensitizer, mercaptoethanol as a sacrificial donor and lipoamide dehydrogenase (LipDH) as a biocatalyst has been applied for the photochemical reduction of NAD+ to NADH (Figure 32B). 

The reduction of carbon dioxide to formate in the presence of FDH represents a photochemical C02-fixation process. In addition to MV+, the enzyme recognizes other electron carriers such as 2,2 -bipyridinium radical cations [184]. Reduction of nitrate (N03 ) to nitrite (NO2 ) and subsequently the reduction of nitrite to ammonia in the presence of NitraR and NitriR, respectively, allows the sequential 8e ... 

Figure 46. Spectra of 1,1 -dibenzyl-4.4 -bipyridinium radical cation taken after equal incremental reductions of the corresponding dication at an SnOi OTE. (From Ref. 322.)...

Fig. 16. A photo- and electrochemically controllable molecular shuttle. The unperturbed rotaxane 116+ exists preferentially in the translational isomer in which the BPP34C10 crown ether resides around the bipyridinium unit, a Photochemical excitation of the Ru(bipy)3 unit results in PET to the bipyridinium site, and consequent translation of the crown ether to the 3,3dimethylbipyridinium unit, which is a less efficient recognition site for the cyclophane CBPQT4+ than a bipyridinium system. This process occurs only in the presence of a sacrificial reductant which reduces the Ru(III) center back to its Ru(II) state in order to prevent charge recombination, b Conversely, upon electrochemical reduction of the bipyridinium unit, the crown ether takes up residency around the 3,3 -dimethylbipyridi-nium site. This process is reversed through electrochemical oxidation of the bipyridinium radical cation back to the dication...

The rationale behind this design was justified upon electrochemical investigation of the [2]catenane 184+. This catenane - synthesized in 43% yield (Fig. 26) from crown ether BPP34C10, the bipyridinium dibromide derivative 192+ and ( )-l,2-bis(4,4 -bipyridyl)ethylene - was demonstrated to consist, in solution, of mainly co-conformer A, with the more powerful n-electron-accepting bipyridinium unit located inside the cavity of the crown ether. Upon electrochemical reduction of this bipyridinium unit, the cyclophane undergoes a circumrotational movement with respect to the crown ether such that the profoundly more electron-deficient 7t-extended bipyridinium unit resides inside the cavity of the crown ether, affording co-conformer B. When the bipyridinium radical cation is oxidized back down to its dicationic state, the opposite circumrotational process occurs and the system reverts back to co-conformer A, its ground state [49]. 

Fig. 26. When the dibromide salt 19-2PF6 is reacted with bis (pyridine) ethylene in the presence of BPP34C10 in acetonitrile, the resultant [2]catenane 18-4PF6 is formed in 43% yield. The crown ether resides preferentially around the bipyridinium site in a 92 8 ratio with respect to occupancy around the bis(pyridinium) ethylene site - co-conformer A. a Upon electrochemical reduction, the best electron donor - the bipyridinium site - is reduced first. This reduction leads b to the unfavorable situation in which the bipyridinium radical cation is located within the cavity of the crown ether, and so the cyclophane circumrotates c to locate the bis(pyridinium)ethylene site within the crown ether cavity - co-conformer B. This process is reversible, in that d reoxidation of the bipyridinium radical cation leads to e circumrotation of the cyclophane to yield the [2]catenane in its original state - co-conformer A...

Figure 22. Biocatalyzed photosynthetic system mediated by iV,N -dimethyl-4,4 -bipyridinium radical cation, MV. ...

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