Vinylferrocene

Networks with labeled branch points have also been synthesized by anionic techniques. The crosslinks contain either ferrocene units 107) or lead, originating respectively from vinylferrocene or tetrakis[4(l-phenylvinyl)phenyl]plumbane 94). 

F. la-c. Cyclic voltammograms of dissolved and stance confined ferrcx ne in a< tonitrile/0.1 M TBAP. a. 4 X 10 M dissolved ferrocene at Pt. b. 4-ferrocenyl-phenylacetamid monolayer bound to Pt (ref. ). c. Poly-vinylferrocene dip coated on Pt,r = 1 x lO raolcm. Straight arrows indicate diffusional events. Curved arrows electron transfer events (from ref. ). 

Zinc sulfide, with its wide band gap of 3.66 eV, has been considered as an excellent electroluminescent (EL) material. The electroluminescence of ZnS has been used as a probe for unraveling the energetics at the ZnS/electrolyte interface and for possible application to display devices. Fan and Bard [127] examined the effect of temperature on EL of Al-doped self-activated ZnS single crystals in a persulfate-butyronitrile solution, as well as the time-resolved photoluminescence (PL) of the compound. Further [128], they investigated the PL and EL from single-crystal Mn-doped ZnS (ZnS Mn) centered at 580 nm. The PL was quenched by surface modification with U-treated poly(vinylferrocene). The effect of pH and temperature on the EL of ZnS Mn in aqueous and butyronitrile solutions upon reduction of per-oxydisulfate ion was also studied. EL of polycrystalline chemical vapor deposited (CVD) ZnS doped with Al, Cu-Al, and Mn was also observed with peaks at 430, 475, and 565 nm, respectively. High EL efficiency, comparable to that of singlecrystal ZnS, was found for the doped CVD polycrystalline ZnS. In all cases, the EL efficiency was about 0.2-0.3%. 

Kunitake and coworkers3 first reported the cationic polymerization of vinylferrocene, and Korshak and co-workers33 polymerized 1,1 -diisopropenylferrocene with cationic initiators. Recently,... 

In these styrene copolymers with isopropenylferrocene, the low inclusion of styrene units could be attributed to the greater reactivity of isopropenylferrocene, and vinylferrocene has been assumed to have an r value greater than styrene by Aso and Kunitake.30... 

All of these observations on the copolymerizations of isopropenylferrocene can again be attributed to the extreme stability of the intermediate carbenium ions. In order to obtain more conclusive evidence of this unreactivity, an attempt was made to destabilize the vinylferrocene by introducing a deactivating electron-withdrawing trifluoromethyl substituent at the a-position. For this purpose, the new monomer a-trifluoromethylvinylferrocene (TVF) was synthesized by the route shown below 0... 

The electrochemical and spectroscopic data indicates that sites on these polymers can communicate with each other, in the electron transfer sense, on a relatively short time scale and without the formation of stable mixed valence clusters. Electronic tranport via hopping or tunnelling and modulated by means of neighboring molecular group collisions would be consistent with these requirements. The relative molecular nonspecificity of this mechanism suggests that other polymeric materials would show similar effects and this has been seen for thin films of poly — (vinylferrocene) and poly — (nitrostyrene). 

The oxidation potential of the polymer is not very different from that of the vinylferrocene monomer, which in turn is not very different from that of ferrocene. 

The third approach has been to graft the redox couple by means of a covalent bond to the polyelectrolyte backbond as described early in 1965 in the book of Cassidy and Run [20]. Several of these systems are charged polymers in at least one oxidation state, like poly(viologen), poly(vinylferrocene), and so on. Examples of polyelectrolytes like polyacrylic acid with covalently bound viologen were reported by Fernandez, Katz and coworkers [21], hydroquinone [22] and Anson et al. with bound ferrocene [23]. 

During continuous redox cycling, the first cycle usually differs from the following ones. This effect is referred as break-in. In poly(vinylferrocene), PVF, films this has been related to the incorporation of solvent and ions into the film, decreasing its resistivity [132]. This effect has been observed for several polyelectrolyte and polymer-modified electrodes, for example, polyaniline [155]. 

It is also worthwhile to compare the ferrocenyl ethylene (vinylferrocene) anion-and cation-radicals. For the cyano vinylferrocene anion-radical, the strong delocalization of an unpaired electron was observed (see Section 1.2.2). This is accompanied with effective cis trans conversion (the barrier of rotation around the -C=C- bond is lowered). As for the cation-radicals of the vinylferrocene series, a single electron remains in the highest MO formerly occupied by two electrons. According to photoelectron spectroscopy and quantum mechanical calculations, the HOMO is mostly or even exclusively the orbital of iron (Todres et al. 1992). This orbital is formed without the participation of the ethylenic fragment. The situation is quite different from arylethylene radical cations in which all n orbitals overlap. After one-electron oxidation of ferrocenyl ethylene, an unpaired electron and a positive charge are centered on iron. The —C=C— bond does not share the n-electron cloud with the Fe center. As a result, no cis trans conversion occurs (Todres 2001). 

Unexpectedly, the analysis of the MOs of the cation-radical from (5-cyano vinylferrocene reveals the possibility for cis trans conversion if more than one-electron oxidation takes place. Namely, the cation-radical has an MO, which is four levels higher in energy than the one occupied by the single electron, that is centered on the cyanoethylene fragment (Todres et al. 1992). 

The high electron richness of vinylferrocene as a monomer is illustrated in its copolymerization with maleic anhydride, where 1 1 alternation copolymers are formed over a wide range of monomer feed ratios and ri -2 = 0.003. Subsequently, a large number of detailed copolymerization studies have been undertaken using metal-containing vinyl monomers. 

Cuadrado et al. reported that hydrosilylation of 1,3,5,7-tetramethylcy-clotetrasiloxane (1) with four equivalents of vinylferrocene (2) in the presence of catalytic bis(divinyltetramethyldisiloxane)platinum(0) yielded tetraferrocenyl compound 3 in 92% yield (Scheme 1). The cyclic voltammogram of 3 exhibited a single reversible oxidation wave and coulometry established that this wave corresponded to the removal of four electrons per molecule, suggesting that the four ferrocenyl units act as independent, non-interacting redox centers. ... 

Both typical and exceptional examples of the polymerization of a vinyl monomer containing a transition-metal ion are provided by the radical polymerization of vinylferrocene31. Vinylferrocene and its derivatives are polymerized by a radical or a cationic initiator to form a polymer of high molecular weight. The high polymeriz-ability is based on the property that the ferrocene compounds are extraordinarily stable against chemical reactions. 

A number of observations have been made which qualitatively suggest that carbonium ions adjacent to metallocene systems possess unusual stability. Ferro-cenecarboxaldehyde, for example, is soluble in dilute hydrochloric acid (5), ferrocenyl carbinols such as ferrocenyl phenyl carbinol form ethers with great ease (124), and ferrocenylmethylcarbinol can be dehydrated to vinylferrocene under exceedingly mild conditions (114). The concept of stabilizations of this type has also been used to explain certain anomalous ring substitution reactions. 

Buell, McEwen, and Kleinberg have observed that weak acids such as hydrogen azide and acetic acid add readily across the double bond of vinylferrocene (XLI, M = Fe) (8). They have postulated that the mechanism of addition proceeds via intermediate formation of the a -ferrocenylcarbonium ion (X-LII, M = Fe), followed by conversion to the acetate (XLIII, M = Fe). Stabilization of carbonium ions of this type can result from overlap of filled metal orbitals with the vacant p -orbital of the carbonium ion. 

The extraordinary reactivity of vinylferrocene under these conditions prompted a study of the rates of addition of absolute acetic acid to vinylmetallocenes (XLI, M = Fe, Ru, Os). The relative rates are given in Table III. 

An enormous number of polymers have been used to prepare chemically modified electrodes. Some examples are given in Table 13.2 Albery and Hillman provide a more extensive list [8]. As indicated in Table 13.2, these polymers can be divided into three general categories—redox polymers, ion-exchange and coordination polymers, and electronically conductive polymers. Redox polymers are polymers that contain electroactive functionalities either within the main polymer chain or in side groups pendant to this chain. The quintessential example is poly(vinylferrocene) (Table 13.2). The ferrocene groups attached to the polymer chain are the electroactive functionality. If fer-... 

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