Ultrafast Multidimensional Spectroscopy for Photomolecular Science

Ultrafast laser technology has advanced to the extent that experiments of a complexity which was unimaginable only a few years ago now fall within the realms of the possible, and have the potential to become routine. Modern solid state laser sources produce ultrastable pulses a few million billionths of a second wide with extreme stability over most of the electromagnetic spectrum. This opens up almost any atomic or molecular process to real time interrogation. In this proposal we describe three experiments at the cutting edge of advanced laser spectroscopy. Our objective is to develop and apply these experiments to important problems in molecular and biomolecular science, with a view to demonstrating their utility in, and with the objective of establishing them as important new tools for, materials characterisation. To this end we have established collaborations with world leading laboratories in molecular and biomolecular materials science who will be the first users of the new methods.

The first experiment, 2D electronic spectroscopy(ES), is a unique tool for the study of electronic coupling and energy transport in (bio-)molecular assemblies. These processes are central to the collection and utilization of solar energy and in the operation of photoactivated nanomaterials. The experiment measures the correlation of the coherent excitation and emission frequencies in the visible region of the spectrum in a three pulse four wave mixing experiment. The measurement can be thought of as the optical analog of 2D NMR, in that it reveals couplings between electronic transitions that are obscured in the linear absorption spectrum. Such couplings are the underlying mechanism for energy and charge transport in both natural and artificial solar energy collectors, and thus need to be characterised and understood. In addition the same experiment resolves the temporal evolution of the energy flow in the molecular assembly with femtosecond resolution by varying the inter-pulse timings. An extension of this experiment to include polarization resolved data, will introduce a correlation between 2D spectra and molecular structure, and thus reveal the spatial arrangement of the chromophores. We will apply 2DES to elucidate excitation dynamics in multi-heme proteins and artificial porphyrin arrays, both of which figure prominently in solar energy conversion schemes and the latter can act as molecular wires in molecular electronics. The 2DES will provide the first direct measurement of the route and mechanism of energy transport in these molecular materials. How this correlates with structure will inform future designs strategies. In addition many heme proteins have unknown or disputed structures, so 2DES will provide new structural data. In short, 2DES has the power do for electronic structure what 2D NMR has done for nuclear structure.

The next two experiments report Raman and IR spectra of electronically excited molecules as a function of time after excitation. Excited state dynamics are a critical component of photoactivated molecular devices, where they act as transducer between optical and mechanical energy, by means of changes in shape or charge. Vibrational spectroscopy yields a detailed picture of the nuclear structure, and such measurements in real time allow us to track the structural changes which act as the driving force for motion in molecular machines. The time resolved coherent Raman experiment (FSRS) is well established. The transient IR measurement we will develop will permit IR detection in the visible region, using the same detection apparatus as Raman. This new method overcomes the limited spectral resolution of traditional IR detectors, and will permit the observation of subtle changes in bond lengths and angles which accompany structural change on a single electronic surface. These tools will be applied to investigate the mechanism of operation of molecular motors and molecular switches in a variety of environments.

We envisage impact in three areas beyond the realm of physical sciences:

(a) Personnel for instrument development - economic impact
(b) Development of design tools for solar energy devices - economic and societal impact
(c) Development of optical to mechanical energy transducers - economic and societal impact

The project involves the development of advanced spectroscopic methods for the characterisation of molecular dynamics in the condensed phase. In this sense it fits into the mainstream of academic experimental physical chemistry. The impact associated with this kind of fundamental research outside of the immediate subject area is mainly associated with contributions of the trained personnel produced. Students and postdocs are trained to the highest level of practical problem solving using the multidisciplinary skills developed during such a wide rangeing research project. These skills are highly transferable, but even the direct application of instrument development and data interpretation skills is of major importance to employers in the pharmaceutical and life sciences, defence, forensic science and industrial chemistry sectors. Further there are numerous though often smaller scale companies directly involved in the development and supply of instrumentation to these major employers, who require the kind of staff we train.

The particular problems to be addressed with the new techniques are energy/electron transport in molecular assemblies and the transduction of optical to electrical or mechanical energy. Both have the potential for major impact in the economic area and on quality of life. The 2DES we will develop is the only tool capable of directly measuring coupling and energy transfer pathways in assemblies of molecular chromophores, and their dependence on assembly structure. Such assemblies have a critical role as energy collectors (antennae) in many proposed solar energy conversion systems. They permit the efficient collection and transport of solar energy, usually to a centre for charge separation. Thus, 2DES will be an essential tool for understanding and optimizing such energy transport processes. To ensure that our apparently esoteric method is applied to systems of real importance in this area we have formed collaborations with leading groups with interests in both bioinspired and artificial antennae. Our demonstration of the utility of 2DES in designing materials for energy transport will reach a wide audience both through our collaborators, who are well integrated into these communities, and through the publication and presentation of our work in journals and at conferences. In this way this project will play a significant role in the development of materials for exploitation of solar energy, an area of the widest imaginable impact.

Light absorption is one way in which power can be efficiently supplied to 'molecular machines'. This requires the conversion of absorbed optical energy to mechanical or electrical energy, which is achieved by excited state structure change and charge redistribution. Molecules which undergo such changes have been designed, and have demonstrated the ability to act as molecular motors and switches. An ideal structure change is one in which the 'power stroke' occurs prior to relaxation to the ground state. In many molecules the first motion provides no power but leads to leakage of population. To optimise the efficiency of molecular machines requires correlation of excited state population with time dependent molecular structure. The time resolved vibrational experiments we will develop have precisely this capability. Thus we will provide the tools to be used to optimise performance of molecular machines, which will in turn influence areas such as photo drug release, artificial phototaxis, photocatalysis, etc. To ensure that these developments impact on the molecular machines community we have established collaborations with important well connected groups in the area.