Azulene, second excited singlet

Azulene displays the fluorescence originating from the second excited singlet which is very much stronger than that from... 

Control of Photophysical Properties of Polyatomic Molecules by Substitution and Solvation The Second Excited Singlet State of Azulene. 

The absorption and fluorescence spectra of azulene (1) and cycl[3.3.3]azine (2) are shown in Figures 4.22 and 4.23. The fluorescence emissions are from the second excited singlet state S2, in violation of Kasha s rule (Section 2.1.8), due to the large energy gap between the Si and S2 states in these compounds. 

The phosphorescence and fluorescence spectra of compound (23) have been recorded <89CBi 119>. The electronic and fluorescence spectra have been measured for azuleno[l,2-f>]furan (144), which is an iso-/7-electron system of benz[a]azulene and presents anomalous fluorescence from the second excited singlet state, as does azulene <87SA(A)1377). Fluorescence spectral properties of a series of 2-(4-substituted phenyl)benzofurans have been investigated to determine their applicability as organic reagents for analysis <85bcj2192>. 

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. 

Azulene is the best-known exception to Kasha s rule and serves as a model for other nonaltemant aromatic compounds, which also exhibit anomalous fluorescence from their second excited singlet states. This anomalous anission of the Sj state was first observed unambiguously by Beer and Longuet-Higgins [2] and has been confirmed many times in more recent studies [15,16]. The second excited singlet state of azulene has a lifetime of ca. 1 to 2 ns in both the gas phase and in solution, and exhibits dual emission, decaying radiatively to S, with a minute quantum yield (< 10" ) [17,18] and to So with a quantum yield most recently determined to be 0.041 (in ethanol at room temperature) [16]. Earlier studies placed the Sj - Sq fluorescence quantum yield near 0.03 [19,20]. Small et al also recently measured the quantum yield of azulene s Sj-Sq nonradialive decay using a completely independent... 

Fujimori and coworkers [42 4] studied several azulene derivatives, including azulene[l,2-b] furan, azulenopyridines, and azulenofuran, all of which exhibit anomalous fluorescence from the second excited singlet state. The emission quantum yields observed for these compounds were quite small, in the 10" to 10" range. 

The participation of higher excited singlet states (Sn, n > 1) of molecules in photophysical (Sn SQ fluorescence (FL)) or photochemical (photoinduced electron transfer (PET), isomerization, etc.) processes, which compete with radiationless deactivation, manifests itself in the dependence of the quantum yield (q>) and FL spectra on the wavelength of the exciting light (the violation of the Vavilov law). Such processes were first shown for the FL of azulene solutions due to the transition from the second excited level to the ground state S2 -> S0. ... 

---