Molecules, properties intersystem crossing

Biphenylene and trisdehydro[12]annulene are representatives of conjugated hydrocarbons with a 4 -membered ring. The pattern of their absorption spectra is completely different from that of benzenoid aromatics. Their HOMO and LUMO are derived from the two NBMOs of an ideal perimeter and both the lowest excited singlet and triplet state can be described by the configuration A ho lu- The transition is symmetry forbidden in molecules of D2h symmetry or higher. Nonradiative decay usually dominates their photophysical properties. Quantum yields of fluorescence and intersystem crossing are low.307 The LCAO version of Platt s perimeter model has been extended to treat conjugated systems with AN jt-electrons derived from [ ]annulenes.308,309... 

The fluorescence of aromatic molecules is quenched (diminished in intensity) partially or completely by heavy-atom substituents such as —As(OH)2, Br, and I, and by certain other groups such as —NH2, —CHO, —COR, and nitrogen in six-membered heterocyclic rings (e.g., quinoline). Each of these substituents has the ability to cause mixing of the spin and orbital electronic motions of the aromatic system. Spin-orbital coupling destroys the concept of molecular spin as a well-defined property of the molecule and thereby enhances the probability or rate of singlet —> triplet intersystem crossing. 

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