Photochemical Ring-Opening and Ground State Relaxation in 伪-Terpinene with Comparison to Provitamin D3

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• 作者：Brenden C. Arruda ; Jian Peng ; Broc Smith ; Kenneth G. Spears ; Roseanne J. Sension
• 刊名：The Journal of Physical Chemistry B
• 出版年：2013
• 出版时间：April 25, 2013
• 年：2013
• 卷：117
• 期：16
• 页码：4696-4704
• 全文大小：501K
• 年卷期：v.117,no.16(April 25, 2013)
• ISSN：1520-5207

文摘

Ultrafast broadband UV鈥搗isible transient absorption spectroscopy is used to characterize the photochemistry of 伪-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 D₃) and the subsequent relaxation of previtamin D₃. The major experimental results are as follows: (1) Like DHC, but unlike 1,3-cyclohexadiene, 伪-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 2 脳 10^鈥?. 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 伪-terpinene. The gZg鈫抰Zg 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鈥? ps depending on solvent, but shows no particular trends within the solvent series. (4) The conformational relaxation of previtamin D₃ occurs on a similar time scale of ca. 5鈥? ps with a modest dependence on the solvent viscosity.