Switching On and Powering Molecular Machines: Ultrafast Dynamics of Photoswitches

我们通常都熟悉交换机的概念，并且它们的操作对大多数用户来说是自然的。在微观层面上，我们也熟悉晶体管的合作行动，作为固态处理器中的开关，增强和控制我们日常生活中的许多功能。电子工程的一项成就是可以应用于硅芯片的晶体管的密度越来越大，从而提高了处理的速度和复杂性。多年来，至少从费曼1959年的演讲“底部有足够的空间”开始，一个重要的科学目标就是超越微观固态器件，创造真正的纳米级分子机器。在过去的十年中，这一领域取得了重大进展，2016年诺贝尔化学奖也承认了这一点。有许多重要的特征与这种纳米机器的设计相关。首先，它们将与宏观机器非常不同，因为它们必须在热噪声驱动分子运动的环境中工作：纳米机器机器将不断改变形状。其次，热噪声并不排除功能分子机器的构建，正如核糖体高效的蛋白质机器表达所证明的那样。第三，分子机器将需要分子开关来控制它们。最后，一般的分子机器，特别是开关，需要能量来源。针对分子机器设计的这一方面提出的解决方案是光驱动分子光开关。 分子光开关是一种分子，其在吸收光子后改变其与环境的相互作用（开启），并自发地或在吸收不同波长的第二个光子后恢复到其原始状态（关闭）。使用光激活分子开关有巨大的优点;首先，人们可以通过脉冲光源来控制开关何时发生，其次，人们可以将能量传递到开关，而无需将其连接到电源。用于光开关的经典分子基序包括烯键和应变环。以烯属双键为例，由π到π * 激发的键级降低诱导的光驱动异构化充当开关，并且，如果顺式和反式形式具有不同的吸收光谱，则异构化可以由第二光子可逆地驱动。由于光子吸收导致分子运动，这也是将光子能量转换为机械运动的一种简洁方式，即马达。经过一些复杂的合成，已经证明有可能将这种分子开关转化为分子马达，为纳米机器提供动力。这种烯键开关是合成模仿自然的一个例子，因为在我们的眼睛中检测光子并将其转换为电信号的光开关也是基于多烯视网膜中的顺式到反式异构化。值得注意的是，生物方法的效率非常高（异构化的产率大于60%）。相比之下，大多数光异构化和开环光开关反应仅以低产率（<20%）发生，其中大多数群体恢复到其初始状态，并且吸收的能量作为热降解。这是必要的，以提高这一产量的实际应用。在这项工作中，我们将应用一些最先进的工具，时间分辨光谱，以遵循在激发电子态，开关反应发生的光开关动力学。我们将观察哪些途径导致反应，哪些不，并研究分子或其环境的哪些特征优化了转换产率。通过这种方式，我们将开发分子开关的设计原理，为未来的机器照亮道路。

We are all generally familiar with the concept of a switch, and their operation comes naturally to most users. At the microscopic level we are also familiar with the cooperative action of transistors as switches in the solid state processors which enhance and control so many features of our daily lives. One of the triumphs of electrical engineering has been the ever greater density of transistors that can be applied to a silicon chip, with consequent increases in speed and complexity of processing. For many years, at least since Feynman's 1959 lecture 'Plenty of Room at the Bottom', an important scientific goal has been to move beyond microscopic solid state devices to create truly nanoscale molecular machines. Over the past ten years significant progress has been made in this area, as acknowledged in the 2016 Nobel Prize for Chemistry. There are a number of important characteristic features associated with the design of such nanomachines. First, they will be very different to macroscopic machines, as they will have to work in an environment where thermal noise drives molecular motion: nanomachines machines will keep changing shape. Second, thermal noise does not rule out the construction of functioning molecular machines, as demonstrated by the efficient machine-like expression of proteins by the ribosome. Thirdly, molecular machines will require molecular switches to control them. Finally, molecular machines in general, and switches in particular, require a source of energy. The solution proposed for this aspect of molecular machine design is the light driven molecular photoswitch. A molecular photoswitch is a molecule which modifies its interaction with its environment following absorption of a photon (turn-on) and reverts to its original state either spontaneously or after absorbing a second photon of a different wavelength (turn-off). There are enormous advantages to the use of light activated molecular switches; firstly one can control when switching occurs, through pulsing the light sources, and secondly one can get energy to the switch without the necessity of wiring it to the source. Classical molecular motifs for photoswitching include the ethylenic bond and the strained ring. Taking the ethylenic double bond as an example, light driven isomerization induced by bond -order reduction on pi to pi* excitation acts as the switch, and, provided the cis and trans forms have different absorption spectra, the isomerization can be driven reversibly by a second photon. Since photon absorption results in molecular motion this is also a neat way of converting photon energy into mechanical motion, a motor. After some complex synthesis it has proven possible to convert such molecular switches into molecular motors to power nanomachines. Such ethylenic switches are an example of synthesis mimicking nature, since the photoswitch which detects a photon and converts it to an electrical signal in our eye is also based on a cis to trans isomerization in the polyene retinal. Significantly, the efficiency of the biological process is very high (greater than 60% yield of the isomerization). In contrast most photoisomerization and ring opening photoswitch reactions happen with only a low yield (<20%) with most of the population reverting to its initial state and the absorbed energy being degraded as heat. It is essential to improve this yield for practical applications. In this work we will apply some of the most advanced tools of time resolved spectroscopy to follow the photoswitch dynamics in the excited electronic state, where the switching reaction occurs. We will observe which pathways lead to reaction and which do not, and investigate what features of the molecule or its environment optimise the switching yield. In this way we will develop design principles for molecular switches, lighting the way for the machines of the future.