The wavelength dependence of the photochemistry of previtamin D

Dauben et al. (J. Am. Chem. Sec. 113 (1991) 8367) have found that the quantum yield of the photochemical electrocyclic ring closure of previtamin D changes dramatically within a very narrow wavelength range. Two conflicting models have been proposed as explanations: control by conformers, or an excited state effect involving the crossing of S-1 and S-2. We measured the spectrum of the less stable cZc conformer by matrix spectroscopy and showed that it caused only a minor wavelength dependence. However, the second model also disagrees with part of this dependence. We suggest another mechanism which seems to be consistent with all observations: ring closure competes with double-bond isomerization (the latter process has a barrier so that its rate depends on the photon energy). Analysis of the data yields the branching ratio of reaction to internal conversion. Surprisingly, it is different from that of ring opening. This may be due to the existence of an additional decay channel from excited previtamin D: single-bond isomerization to the other conformer. Further properties of the potential energy surfaces are discussed. (C) 1997 Elsevier Science S.A.