PHOTOCHEMISTRY OF STRUCTURAL ANALOGS OF PREVITAMIN D-3 - GENERALITY OF THE WAVELENGTH-DEPENDENT TRIENE PHOTOCYCLIZATION

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.