nπ-ct-π* Transitions: nπ* Transitions of the second kind

Two important classes of molecules, maleimides and syn-bimanes [syn-bicyclo[3.3.0]octadienediones], are shown to exhibit charge-transfer absorptions and emissions, mainly on the basis of solvent effects on emissions. The electronic excitation originates mainly in a “non-bonding” p-orbital substantially localized on N, and oriented parallel to the π-system, hence nπ. The label, nπ-ct→π*, is proposed for these transitions. For the first time, two nπ-ct→π* transitions for a molecule containing a pair of adjacent nitrogens adjacent to a π-acceptor system are reported.