Molecular Photonics Laboratory

Spectroscopy

Fluorescence spectroscopy is the main tool used to interrogate the photophysical properties of designer molecules. A range of commercial and purpose-built spectrometers is available for making such measurements, each of which can be used in conjunction with low temperature cryostats, pulsed magnets or high-pressure equipment. Data are transferred from the instrument for detailed curve fitting using a battery of software. Samples can be prepared as solutions, spin coated films of predetermined thickness, optical glasses or compressed solids. Occasionally, fluorescence spectral measurements are made with single crystals (REF 1) where comprehensive structural information is available. Measurements are aimed at (1) understanding the photophysics of individual molecules under isolated conditions and (2) resolving energy-transfer pathways in elaborate molecular architectures. A recent example of the former case is our observation of how subtle changes in the molecular topology affect excimer emission from face-to-face boron dipyrromethene dyes (REF 2).


The main objective of the second type of spectroscopic investigation is to develop artificial light-harvesting arrays with which to drive photochemical processes, such as a solar cell. An example of our work in this area is shown opposite where a C60 sphere is decorated with different organic dyes (REF 3). Electronic energy transfer occurs from yellow to blue dyes on the same sphere and also between yellow dyes. In a plastic film, energy transfer also occurs between spheres. As a consequence, the exciton is effectively delocalised throughout a large volume without being dissipated as heat. The terminal blue dye is selected for its suitability to sensitise an amorphous silicon solar cell.




Our spectroscopic studies are supported by calculations performed at various levels of sophistication. Much of this theoretical research is carried out in collaboration with Dr Jerry Hagon, who manages the MPL Computer Resource. Two main lines of enquiry are being pursued at present. Firstly, it has become clear that minor changes in molecular conformation can cause significant variations in the rate of Förster-style electronic energy transfer across molecular dyads. This realisation is the stimulus for much of the on-going pressure- dependence work. In order to gauge the scale and range of such minor structural changes, we are developing models to compute the electronic coupling matrix element, VDA, governing coulombic interactions between donor and acceptor as a function of nanoscopic change. Many geometric changes, most notably rotations around the molecular axis, have little effect on the magnitude of VDA but, in many molecular dyads, slight torsional motion can perturb the coupling to such a degree that energy transfer is switched on or off. Freezing out these particular motions, especially in cases where they disturb otherwise orthogonal couplings, allows control over the dynamics of intramolecular energy transfer in extreme examples. For more detail see Ref 4, where a sliding motion between the appendages has been identified as being critical for Förster-type energy transfer. This work provides a rare opportunity to separate through-bond and through-space mechanisms.


Secondly, it has been shown by other research groups that the ideal dipole approximation underpinning much of Förster theory breaks down under conditions where the distance between the centres of the reactants is comparable to the sum of the respective transition dipole moment vectors. Now, the simplicity of Förster theory is lost and calculations of VDA for coulombic interactions become more challenging. In collaboration with Dr Ata Amini, we are examining how best to treat cases where highly conjugated molecules are separated by relatively short distances. On-going research is looking into aspects of intramolecular electronic energy transfer within the molecular dyad shown opposite - a close analogue of this molecule was synthesized in Strasbourg by the group of Raymond Ziessel - which is at the upper size limit for computational chemistry. Here, the transition dipole moment vector associated with the acceptor is two-fold degenerate and spread over much of the chromophore. Our work aims to compute VDA for comparison with experiment and then to allow for the effects of torsional motion as above.