Azulene photochemistry

The photochemistry of a-methylstilbene (5) resembles stilbene photochemistry in many ways. However, as pointed out earlier, both the cis and trcms isomers are nonclassical acceptors of triplet excitation. This suggests that both the cis and trcms triplet states correspond to high-energy vibrational levels of the twisted or phantom triplet. Azulene does not alter the photo-... 

Exceptions to Kasha s rule can not only be found with azulene and other compounds [37], where emission from S2 is observed (typically because the energetic difference between the S2 and Si states is sufficiently large to reduce the S2-S1 internal conversion to values close to the S2-S0 radiative rate), but also when there is competition between vibrational relaxation and photochemistry, the so-called vibronic effect. This is an important concept that has been recently developed contrasting with the general wisdom in photochemistry, that only very few exceptions to Kasha s rule exist. The foundations of the vibronic effects were found in 1966 when Ralph Becker and Joseph Michl noticed that the fluorescence excitation spectrum of a photochromic compound, 2,2-diethylchromene (see Scheme 15.4), was significantly different from the absorption spectrum [38]. 

---