Photoisomerization by hula-twist: A fundamental supramolecular photochemical reaction

The volume-conserving hula-twist cis/trans isomerization process has been i ncorporated in a general scheme for photoisomerization of polyenes, applica ble to small organic molecules as well as to protein-bound polyene chromoph ores. The main theme is that in solution the conventional one-bond-flip mec hanism dominates, while in frozen media (or under other forms of supramolec ular constraint) the hula-twist mechanism takes over. Literature examples o f photoisomerization obtained under confined conditions have been criticall y reviewed, the applicability of I-IT has been examined, and new systems un ambiguously testing this volume-conserving process are proposed.