Triplet-singlet crossing

The preceding discussion reveals a few of the complexities of the photochemistry that is typical of carbonyl and azo compounds. Results on the olefin-carbonyl Paterno-Buchi system,30 and on the photorearrangements of a, (i-enones,31 P, y-enones,32 azo compounds (diazomethane33 and cyclic diazoalkenes104), and acylcyclopropenes34 show similar features. In these examples, one encounters fourfold intersections as well as conical intersections and singlet-triplet crossings. Thus the potential surfaces are more complex than in hydrocarbon photochemistry. 

Location of Conical Intersections and Singlet-Triplet Crossing Points in Solution... 

Figure 6-2. Conical intersection and singlet-triplet crossing location scheme...

Figure 16-14. Computed energy levels for the ground state (S0) and the lowest triplet excited state (T) of the cytosine dimer in its triplet locally excited state 3(LE), step-wise intermediate 3(SWI), and ground-state cyclobutane cytosine (CBC) dimer. The main intermolecular geometric parameters (in A) for 3(SWI) are included. At the 3(SWI) optimized structure a singlet-triplet crossing, (T1/S0)x, takes place...

However, the azepine yield was low when a 5-OMe substituent was present, and no azepine was formed from the 5-N02 compound. This is possibly due to a rapid singlet-triplet crossing in the nitro compound the triplet nitrene then gives the corresponding azo compound.258 With the acyl group in meta or para position with respect to the nitrene, the azepine yields were lower. It was concluded that a reasonably electrophilic singlet nitrene is essential for azepine formation.258... 

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