Fluorescence benzenoid aromatics, higher excited

The raditionless processes S wS, and T a -T are usually so fast that lifetimes of higher excited states are very short and quantum yields of emission from higher excited states are very small. In the vast majority of cases luminescence is observed exclusively from the lowest excited state. This so-called Kasha s rule is of course relative in that what is observed depends on the sensitivity of the detector. For benzenoid aromatics, fluorescence from higher excited states in addition to fluorescence from the lowest excited state was observed for the first time in 1969 (Geldorf et al., 1969), whereas the fluorescence from the S2 state of azulene had been known for quite some time. (See below.)... 

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... 

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