Hexatriene-cyclohexadiene photoisomerization

Nonadiabatic transitions definitely play crucial roles for molecules to manifest various functions. The theory of nonadiabatic transition is very helpful not only to comprehend the mechanisms, but also to design new molecular functions and enhance their efficiencies. The photochromism that is expected to be applicable to molecular switches and memories is a good example [130]. Photoisomerization of retinal is well known to be a basic mechanism of vision. In these processes, the NT type of nonadiabatic transitions play essential roles. There must be many other similar examples. Utilization of the complete reflection phenomenon can also be another candidate, as discussed in Section V.C. In this section, the following two examples are cosidered (1) photochromism due to photoisomerization between cyclohexadiene (CHD) and hexatriene (HT) as an example of photoswitching molecular functions, and (2) hydrogen transmission through a five-membered carbon ring. 

In solution, an initial photoequilibrium is established between the Z- and -isomers, while the rearrangement products 117 and 118 are formed along with traces of cyclohexadiene (CHD) over much longer irradiation times (equation 46). In solution, the major products are 3-vinylcyclobutene (117) and bicyclo[3.1.0]hex-2-ene (118) Z-l,2,4-hexatriene (119), which is a major product in the gas phase176,211, is formed in relatively low yields. The quantum yields for ,Z-photoisomerization of Z- and -l,3,5-hexatriene in pentane solution (265 nm excitation) are /, r = 0.034 and E—Z = 0.016, respectively188. 

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