Switching On and Powering Molecular Machines: Ultrafast Dynamics of Photoswitches

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.

The primary objectives of this research are academic in nature. We conduct very high quality measurements of excited state dynamics, demonstrably competitive with the best in the world, and in so doing contribute to the general quality and development of experimental physical chemistry in the UK.

The secondary effects of our research are more broadly distributed, but certainly include the following:

(1) Theoreticians can test and validate their methods on the challenging problem of dynamics and structural evolution in excited electronic states. These theoretical methods are by no means only applicable to problems found in academic institutions - high quality quantum chemical and molecular mechanical calculations are vital in many areas of industry, most notably drug development. Thus the fundamental advances we make will influence directly the calculation methods which are then applied to optimise processes such as light activated drug delivery or photodynamic therapy, for example.

(2) Life sciences in both academic and industrial/public laboratories will make good use of better designed photoswitches for both super-resolution microscopy and optogenetics. The power of these tools to investigate cellular processes is considerable and can make a major contributions in extra-academic settings.

(3) Materials science already makes use of photoswitching in photochromic shape memory polymers, photogelation, light induced phase transitions, etc. Our fundamental work aims to both understand and improve the light driven mechanisms operating, and so optimise these processes.

This impact can only be realized if influential people in those fields are aware of our research. The primary route for dissemination is through high quality research publications. Most major technology and life science companies have access to scientific literature. In addition all of our published work appears immediately on acceptance in a publicly searchable database hosted by UEA. Further, we will present our ongoing and preliminary work at key laser and photonics conferences such as CLEO, as well as at academic conferences attended by instrument suppliers and manufacturers (e.g. Ultrafast Phenomena, International Conference of Photochemistry). In addition we will remain alert to the possibilities of exploiting some of our own technical innovations in instrument development. The possibility of licencing such developments is actively discussed with the Research and Enterprise services group at UEA.

Finally, a key impact will be through the people trained in the project, which will include the PDRA appointed, PhD students and Masters students. We anticipate high quality outputs in an important and dynamic field, which will directly
benefit the PDRA. Career development is an important part of the 'The Concordat to Support the Career Development of Researchers' to which UEA subscribes. In this case we will provide detailed advice and assistance to the PDRA in the development and authoring of research fellowships, typically an important next step for PDRAs. We expect that all researchers trained in our laboratory will be skilled in the design and control of novel instrumentation and its exploitation in challenging problems. They are thus able to make important contributions in a diverse range of scientific positions.