Charge localisation

The concept of the oxidation state of an element in a chemical compound has a long and confusing history. In the most pretentious form of the concept the oxidation state or oxidation number is the electrical charge localised on the concerned atom in the compound. Confusion arises when we realise that the definition of the atomic domain is arbitrary and the experimental determination of the electrical charge in the once chosen domain is often problematic. 

The initial removal of electrons (following the oxidation, p-doping process) leads to the formation of a positive charge localised in the polymer chain (radical cation), accompanied by a lattice distortion which is associated with a relaxation of the aromatic structural geometry of the polymer chain towards a quinoid form. This form extends over four pyrrolic rings ... 

The photoluminescence quenching of semiconductor CdSe/ZnS quantum dots (QDs) caused by suifacely attached porphyrins increases with QDs size decrease. Porphyrin-QD interaction induces a pertubation of the excitonic wave function leading to a net charge localisation of the electron-hole pair accompanied by an increase of radiationless decay rates being proportional to the calculated radial probability of the confined exciton wave function at QD surface. 

In addition, the polarity of the surface modes probably increases at a defect (either charge localisation in the midgap state or oxide bond formation would greatly increase the polarity of the modes (ctp) associated with the surface defect). The increase in polarity naturally leads to a larger modulation of the electron energy within the band and mixing of states for the same rms displacement. 

Figure 1.46). ° The monocation 97 can be considered as a push-pull chromophore and exhibits a / i.o6 (HRS) value twice that of its neutral precursor 97 and the fully oxidised compound 97. Complex 97 is a class II mixed-valence complex, retaining charge localisation, but with enough electronic coupling between the redox centres to permit intervalence charge transfer transitions. As the redox couples 97/97" and... 

Hard nucleophiles are those with a negative charge localised on one small atom (e.g. MeLi). These tend to react at the carbonyl carbon, which is known as a hard electrophilic centre. 

Building on the ideas of Wheland and Mann, Kuhn supposed that the (constant) parameters co, co and co" represented the electrostatic potentials created at vertex (atom) r of the conjugated system by the charge localised on atoms r,f and s, respectively. Kuhn adopted the conventional value for co of 1.4 and then considered co and co" to be proportional to co and inversely proportional to the distance between the pairs of centres, r and /, and r and s, respectively. ThusR21R22... 

Compounds of form fra s-M2(TiPB)2(02CR)2 (M = Mo or W) have been proven to be very effective testbeds for the study of state dynamics and charge localisation in their photo-excited states. Examples of the R groups used include furan selenophene (L ), nitrophenyl... 

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