Asymmetric annellation effects—IV Dibenzacenes

The asymmetric annellation effects and the shift difference rule are explained on the basis of the two schemes A and B. The asymmetry of the annellation effects in passing from acenes to 1:2-benz-, and 1:2,3:4-dibenzacenes support the scheme B involving the view that the transition of a cyclic conjugated polyene into a benzenoid ring produces a violet spectral shift. Another set of asymmetric annellations is found in the heteronuclear dibenzacenes. This series can be started from ethylene. The merits of a symplified valence bond theory are considered for the interpretation of asymmetric annellation effects. The assumption of two transferable electrons capable of migrating through the rings of an acene leads to two π-electron levels for benzene with 4 π-electrons in two mobile double bonds in the lower level and 2 π-electrons moving freely about the ring and not belonging to any double bond.