On the fluorescence anomaly of azulene

Azulene (symmetry C-2v) is the only known example of a compound in which the fluorescence transition originates from the second electronically excited singlet state (S-2 -> S-0) rather than from the lowest one (S-1 -> S-0). It is shown that single annelation. lowering of symmetry (from C-2v, to C-5 or C-2). increase of the energy gap ratio (E-S1 - E-S0)/(E-S2 - E-S1). inclusion of a "loose bolt" substituent or an increase in spinorbit coupling does not remove this anomaly.