Multidimensional Spectroscopy Development for the Study of Energy Materials

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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.