All isomers of a four stage rotary molecular motor, dimethyl-tetrahydro-bi(cyclopenta[α]napthal-enylidene), are studied with ultrafast transient absorption spectroscopy. Single and two pulse excitations (pump and delayed repump with a different wavelength) are used to optically probe the excited state dynamics. These measurements demonstrate that this motor is not only designed for unidirectional isomerization, but is also “primed” for efficient rotary motion. The yield for photoisomerization from the stable P-cis isomer to the metastable M-trans isomer is 85% ± 10%, while the yield for the undesired back reaction is ca. 0.08 (+0.02, −0.05). The yield for photoisomerization from stable P-trans to the metastable M-cis isomer is ca. 85% ± 3% and the yield for the back reaction is 15% ± 3%. Excitation of P-trans in the lowest singlet state results in formation of a dark state on a 3.6 ps time scale and formation of the M-cis isomer on a ca. 12 ps time scale. Excitation of P-cis in the lowest singlet state results in formation of a dark state on ca. 13 ps time scale and formation of the M-trans isomer on a 71 ps time scale. Excitation of either isomer at 269 nm, higher in the excited state manifold, accesses additional excited state pathways, but does not change the ultimate product formation. This result suggests that pulse sequences accessing higher excited states may provide a tool to manipulate the molecular motor. Pulse sequences using a 269 nm pump pulse and a 404 nm repump pulse are able to increase the yield of the P-cis to M-trans reaction but only decrease the yield of the P-trans to M-cis reaction. These pulse sequences are unable to access reaction pathways that bypass the helix inversion step, although other wavelengths and time delays might yet provide optical control of the entire reaction cycle. We propose intermediates and candidate conical intersections between all four isomers.

中文翻译：

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为高效运动而准备：四级旋转分子电动机的超快激发态动力学和光学操纵

用超快瞬态吸收光谱研究了四级旋转分子马达的所有异构体，即二甲基-四氢-双（环戊[α]萘-亚乙烯基）。单脉冲和两个脉冲激发（具有不同波长的泵和延迟的泵）用于光学探测激发态动力学。这些测量结果表明，该电动机不仅设计用于单向异构化，而且还针对有效的旋转运动进行了“启动”。从稳定的P-顺式异构体到亚稳态的M-反式异构体的光异构化的产率为85％±10％，而不希望的逆反应的产率为约5％。0.08（+0.02，-0.05）。从稳定的P-反式到亚稳的M-顺式异构体的光致异构化的产率约为1。85％±3％，并且后反应的产率为15％±3％。以最低的单重态激发P-trans会在3.6 ps的时间尺度上形成暗态，并在ca上形成M-顺式异构体。12 ps时间刻度。以最低的单重态激发P-顺式导致在ca上形成暗态。13 ps时标，并在71 ps时标上形成M-反式异构体。任一异构体在269 nm处的激发态在激发态歧管中的​​激发更高，可进入其他激发态途径，但不会改变最终产物的形成。这一结果表明，脉冲序列进入更高的激发态可能提供了一种操纵分子马达的工具。使用269 nm泵浦脉冲和404 nm再泵脉冲的脉冲序列能够提高P-顺式向M-反式反应的产率，但仅降低P-反式向M-顺式反应的产率。这些脉冲序列无法进入绕过螺旋反转步骤的反应路径，尽管其他波长和时间延迟可能仍提供了整个反应周期的光学控制。我们提出了所有四个异构体之间的中间体和候选圆锥形相交点。