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EPSRC Reference: EP/C536134/1
Title: Excited State Photoengineering: Virtual Crystallography - a New Approach to Spectroscopy, Molecular Dynamics and Structure
Principal Investigator: Bain, Professor AJ
Other Investigators:
Researcher Co-Investigators:
Project Partners:
Cancer Research UK Universite De Renne (CNRS)
Department: Physics and Astronomy
Organisation: UCL
Scheme: Standard Research (Pre-FEC)
Starts: 05 April 2005 Ends: 04 October 2009 Value (£): 625,471
EPSRC Research Topic Classifications:
Instrumentation Eng. & Dev.
EPSRC Industrial Sector Classifications:
No relevance to Underpinning Sectors
Related Grants:
Panel History:  
Summary on Grant Application Form
Excited State Photoengineering is the creation, by excitation with two ultrashort (0.1 and c.a. 1 picosecond) laser pulses, arrays of fluorescent (light emitting) molecules which, for a short time, behave as 'virtual' crystals. The lifetime of such crystalline states is determined by molecular tumbling (orientational averaging). For small molecules embedded in larger (host) structures such as proteins their internal motions are much faster than those of their host, typically hundreds of picoseconds compared to tens of nanoseconds. So, following laser excitation (PUMP), the host material remains ordered in the laboratory frame of reference in this latter time window. This however is 'slow' in terms of optical pulses and fast electronic detection. We then employ a second laser pulse, the DUMP, created at the same time as the excitation pulse but delayed in time by being made to follow a longer (and controllable) optical path. This is used to select a subset of the excited probe molecules by removing others using stimulated emission (the fundamental process operating in all lasers). We then observe the motion (dynamics) of this subset using either fast fluorescence measurements or via the absorption of a second time delayed (PROBE) laser pulse.Virtual crystallography allows us to probe details of molecular structure and dynamics (motion) that are otherwise hidden to current experiments. The preparation of molecular (protein) crystals is difficult and in such structures the molecules are removed from their natural environments.The PUMP and DUMP processes are the crucial preparative steps. Unlike the preparation of ordered samples (e.g. by crystallisation) these are based on the de-excitation of electronically excited molecules. This is achieved by the DUMP stimulating transitions between the initially excited (PUMPED) electronic state of the molecule and 'hot' levels in the ground state (where the molecule's electrons have relaxed but its atoms are vibrating). The DUMP laser pulse must therefore be tailored (i.e. stretched in time) to minimise 'reverse DUMPING' (i.e. repopulation of the excited state) while the DUMPED population cools. The cooling process is molecule (and environment) specific with lifetimes typically between 0.2 and 1 picosecond. In experiments in which there are multiple species of fluorescent molecule present in the host (e.g. two or more naturally occurring fluorescent molecules in a protein) it is crucial that the PUMP pulse differentiates between them. At present we are limited to a fixed wavelength two-photon PUMP (excitation via the simultaneous absorption of two near-infra red photons). Single- and two-photon excitation using an independently tuneable source, a second optical parametric amplifier (OPA), will permit this together with the probing of fast excited and ground state molecular properties in PUMP-DUMP-PROBE experiments. In practical terms this requires the installation of a larger laser to power two OPA systems, new fluorescence detection apparatus and delay lines to control the DUMP and PROBE laser pulses.The instrument, when constructed, will be used to implement virtual crystallography experiments in liquid crystals and fluorescent bio-proteins (in collaboration with Cancer Research UK). In addition PUMP-DUMP-PROBE experiments will be used to investigate the dynamics of new fluorescent molecules optimised for excited state photoengineering with applications to low power high resolution fluorescence imaging (a collaboration with the CNRS Institut deChimie, UMR 6510 at Rennes)
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