Subpicosecond Ring Opening of 7-Dehydrocholesterol Studied by Ultrafast Spectroscopy

The photoinitiated electrocyclic ring opening of 7-dehydrocholesterol (DHC) to form previtamin D 3 was studied in methanol solution with femtosecond transient absorption spectroscopy. The molecules were excited at 265 nm and probed at 20 wavelengths ranging from 261 to 650 nm. A sizable absorption signal decaying on a 0.95 ± 0.10 ps time scale was seen at all probe wavelengths from 470 to 650 nm. This absorption is assigned to an S 1 → S n transition that peaks near 470 nm. Thus, the S 1 state has a lifetime of 0.95 ps, while the initially prepared S 2 state has a lifetime ≤ 0.1 ps. Data obtained with probe wavelengths in the ultraviolet yielded a subpicosecond (0.4−1.0 ps) component corresponding to the lifetime of the S 1 state. This data set also exhibits a 1−5 ps wavelength-dependent component assigned to vibrational cooling, and a 100 ± 20 ps component assigned to rotational isomerization of the photoproduct to achieve a thermal distribution of rotamers. No further dynamics are seen for time delays of up to 1 ns. Agreement between the static difference spectrum and the absorption difference observed in the kinetic data at 300 ps indicates that there are no significant longer-time dynamics.