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.