Abstract

The photochemical ring opening of 1,3-cyclohexadiene to 1,3Z,5-hexatriene is a prototype of the orbital-symmetry controlled electrocyclic reactions. In previous work [1], we studied by transient absorption spectroscopy (frequency-tripled Ti:sapphire laser around 267 nm, pulse length 250 fs, identical pump and probe wavelength) 7-dehydrocholesterol, which contains the cyclohexadiene moiety. The excited state (1B₂) relaxes very rapidly (10 fs, deduced from the fluorescence quantum yield) to the 2A₁ surface, which is a dark excited state and has a much lower minimum. From there, the product previtamin D is formed with a temperature independent rate within 5.2 ps. Subsequently, this primary product relaxes by rotation around a formally single bond to its stable conformer within 125 ps at room temperature (activation energy 15 kJ/mol).

© 1996 Optical Society of America