Abstract
Excited state bond orders calculated in
the previous paper (McCoy and Ross 1962) are used in a necessarily approximate
attempt to predict the locations of intersections between potential energy
surfaces for all the lower excited states of benzene, naphthalene, azulene, and
anthracene, and thus to investigate the mechanism of degradation of electronic
excitation energy. It is concluded that excited states commonly, but not
invariably, intersect in such a way that there is no barrier to passage from the
zero-point level of one state to some state below it. Quite often, however,
tunnelling, as suggested by Robinson (1961), is necessary to effect the
passage. Diagrams are presented showing the preferred routes for descent
through the singlet and triplet states. The radiative properties of the
compounds considered are then successfully correlated with the distances
through which tunnelling needs to take place, and an approximate empirical
relationship emerges in which the tunnelling rate (multiplied by 106
if there is a spin change) decreases exponentially with barrier width. The
mechanism of tunnelling is briefly discussed.
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