Wg. Dauben et al., PHOTOCHEMISTRY OF STRUCTURAL ANALOGS OF PREVITAMIN D-3 - GENERALITY OF THE WAVELENGTH-DEPENDENT TRIENE PHOTOCYCLIZATION, Journal of organic chemistry, 62(26), 1997, pp. 9005-9008
Two structural previtamin Da analogues (R = H, CH3) cyclized photochem
ically with a 1.4-1.8-fold increase in quantum yields over a 3-nm chan
ge in excitation wavelength. The sudden increase in quantum yields is
due to the participation and mixing of both 2A and 1B excited states.
At lambda less than or equal to 306 nm, the 1B state is initially exci
ted and then (a) partitions between isomerization to the corresponding
trans triene isomers, (b) decays to the 2A state to give the correspo
nding cyclohexadienes, and (c) decays to the ground state. At lambda g
reater than or equal to 309 nm, the 2A state is directly excited to gi
ve the corresponding cyclohexadienes and the relaxation path from 1B s
tate to the ground state or isomerization in the 1B state is diminishe
d.