ABSTRACT
The NEER principle is based on the idea, inspired by Hückel molecular orbital (MO) theory, that excitation of conjugated polyenes and related molecules to their lowest singlet or triplet excited states tends to reverse double/single bond character, preventing equilibration of ground state conformers. Thus, the ground state conformer equilibrium compositions, the individual spectra and excited state energies of the conformers, and the conformer-specific photochemical properties of the conformers control observed product quantum yields and distributions. The dependence of acyclic 1,3diene photodimer distributions
and
trans
/
cis photostationary states
on the triplet excitation energy of the sensitizer demonstrated by Hammond, Saltiel, and co-workers, soon thereafter, provided dramatic confirmation of this principle (Figure 27.1 and Schemes 2 and 3). Ground state conformer controlled photochemical ring opening of a substituted 1,3-cyclohexadiene to isomeric conjugated trienes was first reported in the case of
α
-phellandrene (Scheme 4).
