Subpicosecond ring opening of 7-dehydrocholesterol studied by ultrafast spectroscopy

The photoinitiated electrocyclic ring opening of 7-dehydrocholesterol (DHC) Co form previtamin D-3 was studied in methanol solution with femtosecond t ransient absorption spectroscopy. The molecules were excited at 265 nm and probed at 20 wavelengths ranging from 261 to 650 nm. A sizable absorption s ignal decaying on a 0.95 +/- 0.10 ps time scale was seen at all probe wavel engths from 470 to 650 nm. This absorption is assigned to an S-1 --> S-n tr ansition that peaks near 470 nm, Thus, the S-1 state has a lifetime of 0.95 ps, while the initially prepared St state has a lifetime less than or equa l to 0.1 ps, Data obtained with probe wavelengths in the ultraviolet yielde d a subpicosecond (0.4-1.0 ps) component corresponding to the lifetime of t he S-1 state. This data set also exhibits a 1-5 ps wavelength-dependent com ponent assigned to vibrational cooling, and a 100 +/- 20 ps component assig ned to rotational isomerization of the photoproduct to achieve a thermal di stribution of rotamers. No further dynamics are seen for time delays of up to 1 ns. Agreement between the static difference spectrum and the absorptio n difference observed in the kinetic data at 300 ps indicates that there ar e no significant longer-time dynamics.