Fluorescence The Azulene Anomaly

The molecule often cited as the exception that proves Kasha s Rule is azulene, that fluoresces preferentially to So from its second excited singlet, [4, pp. 8,22-23], [6, 147-148]. The anomaly has been ascribed to a rather large S1-S2 energy gap and to a remarkably weak fluorescence from Sj, that cannot compete with vibronically induced internal conversion to So and subsequent relaxation to its vibrational ground-state. It is clear, however, that orbital symmetry cannot be an insignificant factor. 

The photophysical properties of 9,10-dioxabimanes (l,5-diazabicyclo[3.3.0j-octadienediones) were extensively investigated by Kosower and his coworkers [llj. The most striking feature of their results is the stereoselectivity of the relaxation processes from 81. The 5yn-dioxabimanes fluoresce so well that they can 

Stereoisomer Symmetry point group MTf (kcal/mol) Configuration [...HOMO iLUMO ] 

The 57/n-bimane has a substantial permanent dipole moment, fi = lAbD [12], but Si has B2 symmetry it is polarized along x, at right angles to the dipole axis, so fluorescence - though allowed - is not an overwhelmigly favored process. Intersystem crossing from Si to Ti is strictly forbidden in the planar 5yn-bimane, because both have the same space symmetry, and conservation of overall symmetry would require crossing to a triplet component with a totally symmetric spin function that cannot exist in C2V Rx is totally symmetric in so production of Tx(a ) is weakly allowed in the non-planar 5yn-bimane, but would hardly be expected to compete with fluorescence and internal conversion to Sq. 

The anh -bimane is, of course, non-polar in the planar conformation, and acquires a weak dipole along 2 (/i = 1.77D) when it bends out of plane to C. Si has the same irrep in as x and y Bu) so fluorescence is allowed in this isomer as well. Here, however, ISC is also allowed is totally symmetric in 

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