Vibronic Coupling through the In-Phase, CC Stretching Mode Plays a Major Role in the 2Ag- to 1Ag- Internal Conversion of all-trans-β-Carotene

作者：Hiroyoshi Nagae, Michitaka Kuki, Jianping Zhang, Tokutake Sashima, Yumiko Mukai, Yasushi Koyama · 发表于：The Journal of Physical Chemistry A · 年份：2000 · DOI：10.1021/jp9924833 · 被引用次数：94 · 研究领域：Photosynthetic Processes and Mechanisms、Antioxidant Activity and Oxidative Stress、Porphyrin and Phthalocyanine Chemistry

The major role of vibronic coupling through the in-phase, C C stretching ( ν 1 ) mode in the 2A g - to 1A g - internal conversion of all - trans -β-carotene has been shown by the use of isotopic effects on the rate of internal conversion and on the strength of vibronic coupling as follows: (1) The rates of internal conversion for all - trans -β-carotene having natural abundance isotope composition [NA], along with 2 H-labeled [ 2 H], 13 C-labeled [ 13 C], and 2 H, 13 C-doubly labeled [ 2 H, 13 C] all - trans -β-carotenes, were determined, by subpicosecond time-resolved absorption spectroscopy, to be in the ratio [NA]/[ 2 H]/[ 13 C]/[ 2 H, 13 C] = 1:0.92:0.70:0.64. (2) The strength of vibronic coupling was estimated for each isotope species by using the frequency difference between the 2A g - and 1A g - states, which was determined, by picosecond Raman spectroscopy, to be in the ratio, [NA]/[ 2 H]/[ 13 C]/[ 2 H, 13 C] = 1:1.21:0.89:1.07. On the other hand, a theory was presented to show that the nonadiabatic vibronic-coupling constant that determines the rate of internal conversion is proportional to the adiabatic vibronic-coupling constant that determines the frequency difference. The application of the observed relative strength of vibronic coupling to the Englman−Jortner equation, for a single mode ν 1, predicted the relative rates of internal conversion to be 1:0.80:0.72:0.60, which are in good agreement with those observed above. (3) A theory showing that the adiabatic vi...