Scope and Limitations of Hula-Twist Mechanism of Photoisomerization

Authored by: Robert S.H. Liu , Yao-Peng Zhao , Lan-Ying Yang

CRC Handbook of Organic Photochemistry and Photobiology

Print publication date:  March  2012
Online publication date:  March  2012

Print ISBN: 9781439899335
eBook ISBN: 9781466561250
Adobe ISBN:

10.1201/b12252-25

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Abstract

Ever since the introduction of the torsional potential curves for the excited states of ethylene [1], it became commonly accepted that the mechanism of photoisomerization of substituted ethylenes is torsional relaxation [2], that is, turning over one-half of the molecule around the formal double bond (also known as one-bond-flip [OBF]) [3]. This appears to be logical because the excited state bond order of the formal double bond is considerably reduced in both the excited singlet and triplet states [1]. However, in the 1970s, the observation of facile isomerization of polyene chromophores in confined media raised the question whether the volume demanding torsional relaxation mechanism can indeed take place within the observed picosecond or subpicosecond timescale [4]. Therefore, several less volume-demanding mechanisms for photoisomerization were proposed.

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