Longer conjugated systems

Benzofluoran compounds develop more bathochromic color than the corresponding fluoran compounds because of the longer conjugated system. Thus, 9 -diethylaminobenzo[a]fluoran (27 R1, R2 = C2H5),20 9 -(A -ethyl-... 

In a molecule such as formaldehyde, the bonding and non-bonding orbitals are localized (like the bonds) between pairs of atoms. Such a picture of localized orbitals is valid for the a orbitals of single bonds and for the n orbitals of isolated double bonds, but it is no longer adequate in the case of alternate single and double carbon-carbon bonds (in so-called conjugated systems). In fact, overlap of the n orbitals allows the electrons to be delocalized over the whole system (resonance... 

When the -C=C- unit is directly bonded to the zinc porphyrin unit (polymer of type B), the Soret and Q-bands are shifted to a longer wavelength, suggesting the formation of a highly 71-conjugated system along the main chain, due to the lack of steric hindrance around the zinc porphyrin ring. 

The first intermediate to be generated from a conjugated system by electron transfer is the radical-cation by oxidation or the radical-anion by reduction. Spectroscopic techniques have been extensively employed to demonstrate the existance of these often short-lived intermediates. The life-times of these intermediates are longer in aprotic solvents and in the absence of nucleophiles and electrophiles. Electron spin resonance spectroscopy is useful for characterization of the free electron distribution in the radical-ion [53]. The electrochemical cell is placed within the resonance cavity of an esr spectrometer. This cell must be thin in order to decrease the loss of power due to absorption by the solvent and electrolyte. A steady state concentration of the radical-ion species is generated by application of a suitable working electrode potential so that this unpaired electron species can be characterised. The properties of radical-ions derived from different classes of conjugated substrates are discussed in appropriate chapters. 

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