Ultrafast Dynamics in Light-Driven Molecular Rotary Motors Probed by Femtosecond Stimulated Raman Spectroscopy
Ultrafast Dynamics in Light-Driven Molecular Rotary Motors Probed by Femtosecond Stimulated Raman Spectroscopy
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DOI：
10.1021/jacs.7b03599
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发表时间：
2017-05-31
影响因子：
15
通讯作者：
Meech, Stephen R.
中科院分区：
文献类型：
--
作者：
Hall, Christopher R.;Conyard, Jamie;Meech, Stephen R.
空间拥挤的手性烯烃中的光化学异构化是纳米级机器中分子旋转马达的驱动力。这里，通过飞秒受激拉曼光谱 (FSRS) 和瞬态吸收 (TA) 在超快时间尺度上跟踪原型光驱动旋转电机的激发态动力学和结构演化。 TA 揭示了由于沿反应势能表面的弛豫而产生的低于 100 飞秒的蓝移和弗兰克-康登亮态的衰变。衰变伴随着相干激发的振动动力学，该振动动力学在激发态结构演化中幸存下来。超快 FranckCondon 亮态弛豫到暗激发态，FSRS 显示与电子基态相比具有丰富的光谱，最强烈的拉曼活性模式转移到明显较低的波数。这是根据中心桥键的键序减少和暗态中键的整体弱化来讨论的，这得到了电子结构计算的支持。 FSRS 光谱中观察到的演变归因于振动冷却，同时伴随着产物异构体和原始基态之间暗态的划分。通过 FSRS 实时观察产物异构体的形成。它在几皮秒内以振动方式形成并冷却，从而完成了光驱动一半光循环的表征。
Photochemical isomerization in sterically crowded chiral alkenes is the driving force for molecular rotary motors in nanoscale machines. Here the excited-state dynamics and structural evolution of the prototypical light-driven rotary motor are followed on the ultrafast time scale by femtosecond stimulated Raman spectroscopy (FSRS) and transient absorption (TA). TA reveals a sub-100-fs blue shift and decay of the Franck-Condon bright state arising from relaxation along the reactive potential energy surface. The decay is accompanied by coherently excited vibrational dynamics which survive the excited-state structural evolution. The ultrafast FranckCondon bright state relaxes to a dark excited state, which FSRS reveals to have a rich spectrum compared to the electronic ground state, with the most intense Raman-active modes shifted to significantly lower wavenumber. This is discussed in terms of a reduced bond order of the central bridging bond and overall weakening of bonds in the dark state, which is supported by electronic structure calculations. The observed evolution in the FSRS spectrum is assigned to vibrational cooling accompanied by partitioning of the dark state between the product isomer and the original ground state. Formation of the product isomer is observed in real time by FSRS. It is formed vibrationally hot and cools over several picoseconds, completing the characterization of the light-driven half of the photocycle.