Knowledge Bank

The interpretation of the energies of excimer luminescence of some polyacenes is considered and it is shown that neither simple molecular exciton attitudes nor charge resonance attitudes can provide satisfactory agreement with experiment. The $D2h$ excimer formed from $D2a$ aromatics is investigated and it is shown that the zeroth-order dimeric $B3u$ state of $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}La$ parentage may configurationally interact with the zeroth-order dimeric $B2g$ state of charge resonance nature. The interaction of the zeroth-order dimeric $B2u$ states of $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}La$ and charge resonance parentages is shown to be negligible. It is further suggested that the $B3g$ interaction is of sufficient magnitude to make experiment and theory compatible. A review of the data thereby provided as well as other data in the literature, necessitate that the criteria relevant to excimer formation and luminescence given by Forster be discarded. A number of other topics including a few rather general group theoretic theorems relating to excimer formation, are briefly discussed.