Switching On and Powering Molecular Machines: Ultrafast Dynamics of Photoswitches

点击翻译按钮获取中文摘要

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.