Natural bond orbital transferability property

It appears from the description of radical ions in Sects. 1 and 3 that redox reactions can significantly change the chemical and physical properties of conjugated 7r-systems. Whether the extended jc-species are treated within molecular orbital theory or within band-structure theory, the inherent assumption in these concepts is that an electron transfer is reversible and does not promote subsequent chemical reactions. While inspection of cyclic voltammetric waves and the spectroscopic characterization of the redox species provide reliable criteria for the reversibility of an electron transfer and the maintenance of an intact (T-frame, it is generally accepted that electron transfer, depending on the nature of the substrate and on the experimental conditions, can also initiate chemical reactions under formation or cleavage of er-bonds [244, 245],... 

The specific structural features of SPs affect the photochromic properties of these compounds. Since photophysical and photochemical properties of organic molecules depend essentially on their structure in the excited state, we shall examine the nature of the structural changes in these molecules upon photoexcitation. The nature of such changes, in turn, depends on the type of electronic transition that converts the molecule into a photochemically active state (PCAS). The converstion of SP1-SP4 molecules to a PCAS upon UV irradiation with k > 300 nm is known to be due to electronic transitions of the n-n type in molecules without EA substituents and of the nn -jt 7t type in molecules with EA substituents in the benzopyran moiety.29 These transitions are characterized by intramolecular charge transfer (ICT), mainly from the l orbital of the oxygen atom and the 7t orbital of the C(3)=C(4) bond to the phenyl group and the EA substitutents. 

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