Comparing molecular photofragmentation dynamics in the gas and liquid phases
Comparing molecular photofragmentation dynamics in the gas and liquid phases
复制标题
DOI：
10.1039/c3cp50756d
复制
发表时间：
2013-01-01
影响因子：
3.3
通讯作者：
Ashfold, Michael N. R.
中科院分区：
文献类型：
--
作者：
Harris, Stephanie J.;Murdock, Daniel;Ashfold, Michael N. R.
本文探讨了在何种程度上的见解收集到的详细研究的分子在气相中的光解（即在孤立的分子条件下）可以告知我们的理解，在溶液中相应的光致碎裂过程。选择用于比较的系统包括苯硫酚（对甲基苯硫酚）、苯硫醚（对甲基苯硫醚）和苯酚，在真空和环己烷溶液中。气相中的UV激发导致RX-Y（X = O，S; Y = H，CH 3）键裂变，但在时间尺度上变化了近似4个数量级-所有这些行为都可以基于通过UV光激发获得的相关结合和解离激发态势能表面（PES）来合理化，以及有助于这些PES之间的无辐射传输的锥形交叉。时间分辨紫外泵浦宽带紫外/可见光探测和/或紫外泵浦宽带红外探测研究环己烷溶液中的相应系统揭示了额外的过程，是独特的凝聚相。因此，例如，数据清楚地揭示了以下证据：（i）光激发分子在其解离之前的振动弛豫和X-Y键裂变时形成的自由基片段的振动弛豫，以及（ii）RX和Y产物的成对重组（导致基态母体和/或异构加合物的重组）。尽管如此，数据还表明，在每种情况下，在孤立分子条件下发生的初始键裂过程的特征（和时间尺度）几乎不会因存在弱相互作用溶剂（如环己烷）而改变。这些凝聚相的研究，然后扩展到醚类似物的苯酚（烯丙基苯基醚），其中UV光诱导的RO-烯丙基键裂变构成的光克莱森重排的第一步。
This article explores the extent to which insights gleaned from detailed studies of molecular photodissociations in the gas phase (i.e. under isolated molecule conditions) can inform our understanding of the corresponding photofragmentation processes in solution. Systems selected for comparison include a thiophenol (p-methylthiophenol), a thioanisole (p-methylthioanisole) and phenol, in vacuum and in cyclohexane solution. UV excitation in the gas phase results in RX-Y (X = O, S; Y = H, CH3) bond fission in all cases, but over timescales that vary by similar to 4 orders of magnitude - all of which behaviours can be rationalised on the basis of the relevant bound and dissociative excited state potential energy surfaces (PESs) accessed by UV photoexcitation, and of the conical intersections that facilitate radiationless transfer between these PESs. Time-resolved UV pump-broadband UV/visible probe and/or UV pump-broadband IR probe studies of the corresponding systems in cyclohexane solution reveal additional processes that are unique to the condensed phase. Thus, for example, the data clearly reveal evidence of (i) vibrational relaxation of the photoexcited molecules prior to their dissociation and of the radical fragments formed upon X-Y bond fission, and (ii) geminate recombination of the RX and Y products (leading to reformation of the ground state parent and/or isomeric adducts). Nonetheless, the data also show that, in each case, the characteristics (and the timescale) of the initial bond fission process that occurs under isolated molecule conditions are barely changed by the presence of a weakly interacting solvent like cyclohexane. These condensed phase studies are then extended to an ether analogue of phenol (allyl phenyl ether), wherein UV photo-induced RO-allyl bond fission constitutes the first step of a photo-Claisen rearrangement.