Photochemical Ring-Opening and Ground State Relaxation in alpha-Terpinene with Comparison to Provitamin D-3

Ultrafast broadband UV-visible transient absorption spectroscopy is used to characterize the photochemistry of alpha-terpinene, a 1,4-disubstituted-1,3-cyclohexadiene natural product. These results are compared with experiments probing the analogous ring-opening reaction of 7-dehydrocholesterol (DHC, provitamin D-3) and the subsequent relaxation of previtamin D-3. The major experimental results are as follows: (1) Like DHC, but unlike 1,3-cyclohexadiene, a-terpinene exhibits a broad excited state absorption (ESA) spectrum in the visible. The lifetime of the excited state is ca. 0.16 ps in 1-butanol and 0.12 Ps in hexane. (2) The state responsible for the ESA is the initially excited state. Fluorescence from this state has a quantum yield of similar to 2 x 10(-5). The fluorescence quantum yield is an order of magnitude smaller, and the excited state lifetime is an order of magnitude shorter than that observed for DHC. (3) The initial gZg-triene photoproduct absorbs to the red, and the relaxed tZg-triene product absorbs to the blue of alpha-terpinene. The gZg -> tZg reaction of the vibrationally hot photoproduct requires ca. 6.5 ps with no significant dependence on solvent polarity or viscosity. Thermalization occurs on a time scale of 2-4 ps depending on solvent, but shows no particular trends within the solvent series. (4) The conformational relaxation of previtamin D-3 occurs on a similar time scale of ca. 5-8 ps with a modest dependence on the solvent viscosity.