When two (or more) chromophores are brought sufficiently close together in a single molecule, it is commonly observed that the absorption spectrum of the coupled system is not merely a superposition of the spectra of the separate species. For specifying the nature of the transitions of the composite system, a very definite interpretative convenience is achieved through the approach of Longuet-Higgins & Murrell (1955). In their formulation, the basis states employed for a description of the states of the combined system are classified as either ‘locally excited’ or ‘charge transfer’. Locally excited states are those associated with electronic promotions within a single chromophore, and charge transfer states are those that arise from the promotion and transfer of an electron from a filled orbital on one chromophore to an unfilled orbital on another. The electronic states of the coupled system can then be characterized as some linear combination of the interchromo-phoric charge transfer states and the locally excited states characteristic of the separate chromophores. Therefore, the extent to which charge transfer and/or local excitations contribute to transitions of the coupled system is readily given by the coefficients in the linear combination of charge transfer and locally excited states that go to specify a state of the polychromophoric species. Of course, such information is also available, in principle, in an orbital approach. However, it is not nearly so manifest there. The Longuet-Higgins & Murrell method has been applied to the following series of compounds (Moscowitz
et al
. 1964; Hansen 1964; Mislow 1962): ( + )-(1R)-5-methylenebicyclo[2. 2. 1]hept-2-ene (I), ( + )-(1R)-bicyclo[2. 2. 2]oct-5-ene-2-one (II), and bicyclo[2. 2. 1]hept-2-7-dione (III). The motivation, in part, was to test the extent of validity of a fundamental assumption commonly employed in theoretical treatments of optically active helical polypeptides (Tinoco 1961), namely, that charge transfer is of negligible importance for the near ultraviolet optical properties of the helical polypeptides, and that electrostatic coupling (without charge transfer) of the locally excited states of the ordered amide monomers is primarily responsible for the enhanced optical activity in the helical conformations relative to the random coil forms.
Addressing Charge‐Transfer and Locally‐Excited States in a Twisted Biphenyl Push‐Pull Chromophore
