Multidimensional Spectroscopy Development for the Study of Energy Materials

The highly efficient energy collection machinery Nature developed over millions of years can serve as an inspiration and model upon which newly synthesized molecular structures can be created. New synthetic molecular structures will underpin energy efficient devices capable of harvesting and converting sunlight into other useful forms of readily available or stored energy. This is a key problem to be addressed by successful economies in the 21st century. The optical absorption and emission spectra of natural photosynthetic and artificially created molecular structures are the key observables into their operating mechanisms. Such spectra contain information about the molecular excited state dynamics, how molecules are affected by internal motions and by interactions with their surroundings. Therefore, information about how energy dynamically propagates (timescales and pathways) and how it is employed for functions such as chemical reactions can be obtained by studying optical light absorption and emission. However, aggregated molecular structures have complicated couplings presenting broad and congested linear absorption spectra from which it is impossible to recover any detailed dynamical information.

In order to unravel the physical mechanisms underlying broad and congested absorption spectra I will deploy innovative two-dimensional electronic spectroscopy methods. In this experiment, the correlation of the coherent excitation and emission frequencies in the visible region of the spectrum is measured revealing couplings between electronic transitions that are otherwise obscured in the linear absorption and fluorescence spectra. As an analogy, one could imagine that the molecular spectra are like the frequency distribution of the sound coming from an orchestra. The task here would be to identify all the instruments performing a given symphony through an analysis of the overall sound frequency distribution. Of course, the reality is much more complex because molecules interact between them (coupling) and their surroundings leading to changes of their spectra. In the orchestra/symphony analogy, a given instrument would produce a different tone (frequency) because it is closer or further away to another instrument or because the room is warmer or colder. The tools I will develop permit such a deconvolution.

My interest is focused on studying pi-conjugated porphyrin nanorings that show great potential as biomimetic light-harvesting molecular structures. One remarkable feature (among many others) of these nanorings is that they exhibit ultrafast excitation delocalization upon light absorption, similar to what is observed in naturally occurring light harvesting structures. However, it is not yet known how fast and what mechanism enables this fast delocalization to occur. Working with ultrafast two-dimensional electronic spectroscopy I plan to understand this and other features as well as the pathways and timescales of energy transfer and charge transport down to the quantum mechanical level, in this new range of porphyrin based nanoscale molecular structures.

The advanced research proposed here is a fundamental program primarily focused on developing innovative multidimensional ultrafast spectroscopy to study, down to the quantum mechanical level, the pathways and timescales of energy transfer and charge transport in a carefully selected suite of (mainly) porphyrin based nanoscale molecular structures. As such, it will have its most immediate impact in the academic research community through the production of new knowledge.

The UK ultrafast spectroscopy research community will benefit from the development of an innovative multidimensional electronic spectroscopy facility at the University of East Anglia (UEA). I will use my proven expertise in instrument design to construct homemade ultrafast ultra-broadband light sources. These will be combined in an ultrasensitive 2D ES spectrometer building on my established 2D technology expertise. This spectrometer will be constructed in the EPSRC supported ultrafast facility at UEA, and will be available to external users, naturally including collaborators, namely Anderson (Oxford) and Cammidge (UEA), but also to other researchers in the energy materials community. In addition, the optical and instrumentation tools that I will develop will be freely available and transferable to other ultrafast research groups through methodology papers I plan to publish. It is important to note that the construction of homemade light sources coupled to advanced multidimensional spectroscopy methods will enable the country to continue to be competitive in this highly advanced research area.

This proposal will also have impact on the careers of research personnel. Instrument design and development is a key skill in academia and a major hi-tech industry in Europe. I will work with, educate and transfer skills to PhD students (one is already allocated to the PI through a UEA funded studentship) and final year undergrad project students, influencing positively their careers through better job prospects. This will also provide societal and the economic impacts. Specifically these researchers will become skilled in the design, construction and control of advanced instrumentation including: lasers; ultrafast sources; linear and nonlinear optics; instrument control and analysis software. These are all key skills in instrument development.
Further impact will be felt in the materials research community. Research into the detailed electronic structure of recently synthesized pi-conjugated porphyrin aggregates is timely given their possible applications in, for example, organic solar cells. With the broad bandwidth 2D ES technique proposed here, simultaneous measurement of donor and acceptor spectral signatures in charge transfer reactions will be performed for the first time, providing a comprehensive study of the pathways and timescales of any intermediate states in electron transport. Also, the ultrahigh time resolution of the 2D ES method will allow the ultrafast excited charge delocalization (observed, for example, in synthesized pi-conjugated porphyrin nanorings) to be studied and its mechanisms unravelled. These results will feed into theory and modelling which then feeds back into the know-how to develop improved devices. Beyond this grant (in the long term), it is intended that these new materials become components of energy harvesting and charge separation devices. It is widely agreed that solar energy must make a major contribution to 21st century energy devices. Solar cells and fuel cells are among the most promising devices currently under development. The contribution that this research can make to the refinement of materials used in these devices thus has the potential for a very significant impact on the economy and society.