1,4-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. 

The products obtained upon direct excitation of butadiene,291 norbornadiene,292 cyclohexadiene,293 and other dienes294 are also completely different from those obtained by photosensitization, again demonstrating quite different chemical reactivity of excited singlet and triplet dienes. However, 1,l -bicyclohexenyl295 and 1,3-cycloocta-diene298 represent exceptions, undergoing both direct and sensitized photoisomerizations to cyclobutene derivatives. 

Intramolecular heavy atom effects influence the photoisomerization derivatives of 5,5-diphenyl-1,3-cyclohexadiene The homogeneous acid catalysis of the photoisomerization of trans-3-(2-hydroxy-benzylidene)-4,5-dihydrofuran-2(3H)-one and model mechanisms for isomerization of carbocyanines have both been analyzed. The process of photoisomerization of the biologically important rhodopsin and bacteriorhodopsin has been examined by a theoretical ab initio study of retinal analogues. ... 

The cyclohexadiene-triene photoisomerization as well as cis-trans isomerization have been studied in impressive detail for compounds related to vitamin D (Havinga, 1962). No evidence could be obtained for the intermediacy of triplet states in the observed transformations and for this reason all the mechanisms considered do not include triplet states. 

This effect is attributed to the increased microenvironmental polarity around the sensitizer chro-mophore that stabilizes the exciplex or contact ion pair in nonpolar solvents. As a result of this effect, the stereochemical interaction between the sensitizer and the substrate is more intimate. Because significant enantioselectivities were only observed for dimer 44, an independent cyclodimerization pathway to 44 via an exciplex or contact ion pair of cyclohexadiene and the chiral sensitizer was suggested. Dimer 45 gave much lower ee values even at low temperatures, but the product chirality was inverted within the tested temperature range in favor of enantiomer ent-A5. Similar temperature switching of product chirality has been reported in the enantiodifferentiating photoisomerization of cycloalkenes and in the polar addition of alcohols to 1,1-diphenylalkenes. This effect has been rationalized by a non-zero differential activation entropy of the same sign as the differential activation enthalpy. 

---