Irradiation of ethyl cis-o-fluorocinnamate and related compounds in organic glass led to two HT-isomerization processes that exhibit a strong preference at C-beta than at C-alpha as shown by low temperature UV absorption spectroscopy and supported by ab initio calculations.

The photoreactions of 1,4-diphenyl-1,3-butadienes (DPB) fused with a bicyclo[2.2.1]heptano ring under direct irradiation were examined in solution at room temperature and in organic glass at liquid nitrogen temperature. Photocyclization yielding a phenylnaphthalene compound was shown to be preceded by facile E,E to E,Z photoisomerization. The reverse E,Z to E,E isomerization took place with equal ease in low temperature organic glass and in solution at room temperature. The pattern of reaction at low temperature is consistent with the involvement of the Hula-twist mechanism. However, complexity in conformational population, suggested by ab initio calculated data, made the experimental evidence less clear-cut than in previously reported examples of HT.