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 pi-system, hence n(pi). The label, n(pi)-ct ->pi*, is proposed for these transitions. For the first time, two n(pi)-ct ->pi* transitions for a molecule containing a pair of adjacent nitrogens adjacent to a pi-acceptor system are reported. (c) 2005 Published by Elsevier B.V.