Stockholm University, AlbaNova University Center, Roslagstullsbacken 21, SE-10961 Stockholm, Sweden.
J Phys Chem A. 2013 Apr 25;117(16):3340-7. doi: 10.1021/jp400616c. Epub 2013 Apr 15.
Dihydroazulenes are interesting because of their photoswitching behavior. While the ring-opening to vinylheptafulvalene (VHF) is light induced, the back reaction is known to proceed thermally. In the present paper, we show the first gas phase study of the ring-opening reaction of 2-phenyl-1,8a-dihydroazulene-1,1-dicarbonitrile (Ph-DHA) by means of time-resolved photoelectron spectroscopy which permits us to follow the ring-opening process. Moreover, we investigated s-trans-Ph-VHF in a series of transient absorption experiments, supported by ab initio computations, to understand the origin of the absence of light-induced ring-closure. The transient absorption results show a biexponential decay governed by a hitherto unknown state. This state is accessed within 1-2 ps and return to the ground state is probably driven through a cis-trans isomerization about the exocyclic C1═C2 double bond. The rapid decrease in potential energy disfavors internal rotation to s-cis-Ph-VHF, the structure that would precede the ring-closure reaction.
二氢薁因其光致开关行为而备受关注。虽然环开至乙烯庚富瓦烯（VHF）是光诱导的，但已知其逆反应是热诱导的。在本文中，我们通过时间分辨光电子能谱首次研究了 2-苯基-1,8a-二氢薁-1,1-二腈（Ph-DHA）的环开反应，该方法使我们能够跟踪环开过程。此外，我们通过从头算计算支持的一系列瞬态吸收实验研究了 s-trans-Ph-VHF，以了解光诱导闭环缺失的原因。瞬态吸收结果显示出双指数衰减，由一个迄今未知的状态控制。该状态在 1-2 ps 内被访问，回到基态可能是通过环外 C1═C2 双键的顺式-反式异构化驱动的。势能的快速下降不利于 s-顺式-Ph-VHF 的内旋转，而 s-顺式-Ph-VHF 是环闭合反应之前的结构。
