Photoisomerization of 2,2"-Ethano-Bridged m-Terphenyl Derivatives: Ring Constraint Activates an Unreactive Chromophore

Article abstract of DOI:10.1021/jo00027a059

The photochemistry of 2,2"-ethano-bridged m-terphenyl derivatives 4, 8, and 9 was investigated.These compounds are chiral and may be suitable photoresolvable additives to discotic liquid crystals.Unlike their unbridged analogues, 4, 8, and 9 rearrange rapidly when irradiated with UV light to the o-terphenyl compounds 10,11, and 12, respectively.This rearrangement is likely to proceed through an intermediate benzvalene valence-bond isomer.The high reactivity of the 2,2"-ethano-bridged compounds is attributed to distortion in their lowest excited singlet states and to the relief of strain when the ortho isomers are formed.