Abstract
Based on the Hueckel method and simple model of long-range interelectron repulsion, the tendency for polymethine cations to form twisted structures in the excited state is elucidated. Changes in energy level positions and populations as well as the intramolecular charge transfer occurring on twisting are simulated in terms of π-decoupling of the corresponding conjugated system. The charge transfer between the fragments formed is shown to depend on the end-group nature and to alternate in direction for rotations of successive bonds in the polymethine chain. It is also reversed on switching from the ground to the excited state. The energy advantage of certain excited-state twistings over the planar form can be understood by taking into account the long-range Coulomb interaction of electrons in a quasi-one-dimensional system. On this basis, electron density transfer from the longer to shorter fragment is preferable and can compensate the general energetic disadvantage of π-decoupling upon twisting. Using the [Me 2N–(CH) 13–NMe 2] + cation as an example, it is inferred that the rotation around the 2–3 bond in the excited state is highly probable for long streptopolymethines, whereas twisting the 1–2 bond is improbable. The reverse predictions are found for boron-containing polymethines.
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